phD Thesis Aerogels-M.Moner

UNIVERSITAT DE BARCELONA

DEPARTAMENT DE FÍSICA APLICADA I ÒPTICA

SILICA AEROGELS: SYNTHESIS

AND CHARACTERIZATION

Magda Moner i Gerona

INSTITUT DE CIÈNCIA DE MATERIALS DE BARCELONA-CSIC

ii

UNIVERSITAT DE BARCELONA

DEPARTAMENT DE FÍSICA APLICADA I ÒPTICA

SILICA AEROGELS: SYNTHESIS

AND CHARACTERIZATION

Magda Moner i Gerona

INSTITUT DE CIÈNCIA DE MATERIALS DE BARCELONA-CSIC

Programa de doctorat: Física i Tecnologia de Materials

Bienni: 1997-1999

Tutor: Joan Esteve i Pujol Directors: Elies Molins i Grau Anna Roig i Serra

Memòria presentada per optar al grau de Doctor en Ciències Físiques

Barcelona, Juny 2002

iii

Aquesta tesi ha estat realitzada a l‟Institut de Ciència de Materials de

Barcelona-CSIC en el departament de Cristal.lografia i Química de

l‟Estat Sòlid, sota la direcció del doctor Elies Molins i la doctora Anna

Roig. El treball ha esta realitzat dins el marc dels projectes d‟investigació

de la CICYT MAT97-0688 i MAT2000-2016, i de varis projectes R+D

entre l‟empresa Carburos Metálicos S.A i l‟Institut de Ciència de

Materials de Barcelona. Durant els dos últims anys, la doctorant ha

gaudit d‟una beca 2001TDOC011 de la Direcció General de Recerca de

la Generalitat de Catalunya.

iv

v

AGRAÏMENTS

He d’agrair al Dr. Elies Molins, cap del grup d’aerogels de l’ICMAB i co-director d’aquesta tesi,

l‘oportunitat que em va donar d’iniciar la tesi amb un material tan sorprenent i atractiu com és

l’aerogel. Per descomptat agrair-li les seves constants noves idees, la il·lusió que m’ha transmès per

està obert a qualsevol proposta, i la seva accessibilitat i respecte pel meu treball.

De manera especial, m’agradaria transmetre el meu agra ïment a la Dra. Anna Roig, co-directora

d’aquesta tesi, per oferir-me un continu i ferm suport tant científic com personal en la realització

d’aquesta tesi. Agreixo la seva dedicació i interès constant per la meva feina, i sobretot pel seu tracte

com a persona.

La meva gratitud al Dr. Carles Miravitlles director de l’Institut de Ciència de Materials de Barcelona

per posar a la meva disposició els mitjans necessaris per dur a terme aquesta tesi i al Dr. José-Luis

Morenza, director del departament de Física Aplicada i Òptica pels seus consells en la logística de

presentació de la memòria.

Durant el temps de realització d’aquesta tesi he tingut la sort de treballar en un grup que sempre

m'ha ofert la seva ajuda i amb moltes ganes per treballar en equip tant dins com fora de l'institut. A

tots els companys que heu participat en la realització d'aquesta tesis gràcies pel vostre ajut i amistat: a

l’Ignasi (pel seu constant ajut i per l’esforç de millorar la situació dels becaris), l’Elisenda (per la seva

amistat i la seva voluntat de donar sempre un cop de mà), en Lluís (com no, per fer-nos compartir la

seva passió pel món occità), el Raul (sense el seu treball constant i la seva paciència no hauria estat

possible l’obtenció dels aerogels presentats en aquesta tesi), la Joana (el seu ajut ha estat

imprescindible alhora d’organitzar aquesta tesi), en Martí, la Mònica, la Mihaela, l’Agnes, la

Stephanie, en Fabian i en Ramon (a tots ells per haver aportat part d’aquest treball) .

A molts d’altres companys de l’ICMAB, la Karina (por su original y apasionante visión de la ciencia),

el Ramon, l’Oriol, l’Andrea, l’Imma, el Manu, en Jerôme, en Felip, la Sílvia, per ajudar-me a passar

molt bons moments a l’Institut. I especialment en Lluís Balcells per tots els seus consells i les

discussions que hem mantingut. To Sari for introducing me in the IR analysis and to share a very nice

time in Gràcia.

Aquesta tesi ha estat realitzada gràcies a un conctracte d’investigació per part de Carburos Metalicos,

i al suport científic de Joan Llibre, Joaquim Torras, i d’Emili de la Serna

He d’agrair tot l’equip d’infrastructura de l’Institut, administració, documentació, servei informàtic i

manteniment per la seva ajuda en aquella part del treball que no queda reflectit en la tesi però que és

imprescindible alhora de desenvolupar-la.

vi

Així mateix, el fet de treballar amb un material poc conegut i amb unes propietats tan sorprenents

com les de l’aerogel ajuda a que molt sovint m’hagi trobat amb l’entusiasme d’altres investigadors

per a ‘provar’ noves iniciatives i ‘jugar’ amb la seva peculiar estructura. Principalment he tingut la

sort de treballar conjuntament amb l’Elena Martínez i en Joan Esteve en la caracterització mecànica

dels aerogels. Ha estat una experiència molt positiva de treballar en col·laboració d'un equip que em

rebien amb les portes ben obertes cada vegada que els portava noves mostres per caracteritzar.

Gràcies Elena per l'ajuda i per l’afecte que sempre m’has mostrat i a en Joan per aquella energia que

transmets per gaudir amb la investigació.

Sense l'ajuda de Yves Maniette i Joaquim Portillo (i la visualització dels nostres ‘núvols’ per TEM),

Núria Ferrer i Pau Gorostiza, tots ells dels serveis científico-tècnics de la UB, no hauria estat possible

la caracterització dels aerogels. Agraeixo en Miquel-Àngel Cuevas per haver-me acollit en el seu grup

i per haver-me donat suport al llarg dels anys de la realització d’aquesta tesi.

I would like to kindly express my gratitude to Arlon Hunt for his guidance during the realization of

the stages in his Microstructure Group at the Lawrence National Berkeley Laboratory. I have learned

a lot not only about these surprising aerogels but also about the sweet manner to appreciate science.

Of course to Mike Ayers, thanks a lot to teach me so many fruitful ‘tricks’ about how to handle the

aerogels, for his interest and nice suggestions. And in particular, this wonderful decision to bike

always to everywhere and under any kind of weather, and to repair my ‘vintage’ bike so many

times…To Ian Shepherd for his interest and pleasant discussions and to try (without much results) to

improve my English, and to Paul Berdahl and Gary for his valuable contributions to the development

of the nephelometer set-up and analysis of the data. I nicely thank to Giovanni Dietler, Cynthia

Lopez and Steven Grover to advice and help me so many times.

D’altres persones que no han col·laborat directament en la meva feina però han estat imprescindibles

perquè fos complerta: l’Àlex, per la seva paciència constant durant el temps de la realització de la tesi,

i per haver-me donat suport sempre que l’he necessitat. Els meus avis Núria, Antonio i Roser per

encoratjar-me sempre tan dolçament i pel vostre suport incondicional. L'Alba, l'Enric, el Josep-Lluís i

la Mercè per aguantar a la tipàtica força sovint. I com no, per ensenyar-me uns valors tan importants

com són el respecte a la natura i la passió per la muntanya. A totes/ts les jugadores/rs i companyes

del Sitari i del Bonanova que sense saber-ho hi han col·laborat ajudant-me a desconnectar totalment

dels aerogels. A tot aquest cercle d’amics que m’han encoratjat i que han tingut la paciència d'escoltar

el procés d'aquesta tesi: A l'Aracel.li, la Marta Pérez (per desenboirar-me amb les correlacions i per

mantenir la nostre amistat a distància), la Maria Serra, la Marta Janeras, l’Oriol, la Betlem, l’Albert, i el

Fidel.

INDEX

PRÒLEG ....................................................................................................................................................................... 1

GLOSSARY OF TERMS ......................................................................................................................................... 4

C h a p t e r I . AEROGELS: AN INTRODUCTION

1. GENERAL OVERVIEW ON AEROGELS .....................................................................................12

1.1. COMMERCIAL SILICA AEROGEL .................................................................................. 14

2. SOL-GEL METHOD.............................................................................................................................15

2.1. ANTECEDENTS OF GEL SYNTHESIS ......................................................................... 16

2.2. PREPARATION OF SILICA GELS.................................................................................... 16

3. DRYING PROCEDURE OF GELS.................................................................................................. 20

4. AEROGEL APPLICATIONS ............................................................................................................. 22

4.1. POROSITY AND SURFACE AREA APPLICATIONS ................................................. 22

Electrodes in capacitors ................................................................................. 22

Capacitive deionization ................................................................................. 23

Studies of superf luid transitions and phase separation of 3He-4He...................... 23

4.2. OPTICAL PROPERTY APPLICATIONS ........................................................................ 23

Çerenkov (particle detection and counters)........................................................ 23

4.3. THERMAL INSULATOR APPLICATIONS .................................................................. .24

4.4. ACOUSTICAL AND MECHANICAL APPLICATIONS ............................................. 24

Shock compression experiments ...................................................................... 25

4.5. ELECTRICAL AND ELECTRONIC APPLICATIONS .............................................. 25

4.6. SPACE APPLICATIONS........................................................................................................ 25

5. REFERENCES....................................................................................................................................... 25

C h a p t e r I I : SYNTHESIS OF SILICA AEROGELS

1. INTRODUCTION ................................................................................................................................ 32

2. PREPARATION OF AEROGELS .................................................................................................... 33

2.1. USED REACTIVES ................................................................................................................. 33

2.2. SYNTHESIS PROCEDURE ................................................................................................. 34

2.3. DRYING PROCEDURE ........................................................................................................ 36

Index ii

2.3.1 Supercritical drying at high temperature ................................................................ 36

2.3.2 CO2 supercritical drying ................................................................................................ 38

3. SYNTHESIS ROUTES: EFFECT OF DIFFERENT ALKOXIDE PRECURSORS ........... 39

3.1. TETRAMETHOXYSILANE: TMOS AEROGELS ........................................................ 40

3.1.1 The effect of the TMOS concentration...................................................................... 45

3.1.2 The effect of the nature of the solvent ...................................................................... 46

3.1.3 The effect of the hydrolysis solution .......................................................................... 47

Water amount.............................................................................................. 47

3.2. TETRAETHOXYSILANE: TEOS AEROGELS ............................................................. 49

3.2.1 The effect of the TEOS concentration ................................................................... 53

Neutral ....................................................................................................... 53

Acid catalyst ................................................................................................ 54

Base-catalyst ................................................................................................ 56

3.2.2 The effect of hydrolysis solution ............................................................................... 58

Water amount.............................................................................................. 58

Influence of the amount of the catalyst ............................................................. 61

Influence of the nature of the catalyst ............................................................... 64

3.3. ‘TWO-STEP’ SYNTHESIS .................................................................................................... 66

Effect of precursor concentration...................................................................... 67

Effect of water amount .................................................................................. 69


4. SUMMARY AND CONCLUSIONS...................................................................................................71

5. REFERENCES....................................................................................................................................... 72

C h a p t e r I I I : BULK SILICA AEROGEL CHARACTERIZATION

1. MONOLITHICITY, BULK SHRINKAGE, DENSITY AND POROSITY............................ 75

1.1. TMOS AEROGELS .................................................................................................................. 76

Skeletal density ............................................................................................ 76

Bulk density ................................................................................................ 76

1.1.1 Supercritical drying at CO2 conditions.................................................................... 80

1.2. TEOS AEROGELS................................................................................................................... 81

1.2.1 TEOS aerogels without presence of catalyst.......................................................... 82

1.2.2 Base-catalyst ................................................................................................................. 84

1.2.3 Acid-catalyst .................................................................................................................. 87

Fluorhydric acid ........................................................................................... 88

Citric acid.................................................................................................... 88

1.3. TWO-STEP METHOD H5 AEROGELS .......................................................................... 91

Index iii

2. SURFACE AREA MEASURENTS BY BET (BRUNAUER, EMMET AND TELLER).... 94

3. INFRARED SPECTROPHOTOMETRY, IR ................................................................................ 99

3.1. METHANOL SERIES .......................................................................................................... 100

3.2. ACETONE SERIES ............................................................................................................... 102

4. ULTRA VIOLET-VISIBLE SPECTROSCOPY ............................................................................103

4.1. AEROGEL TRANSPARENCY .......................................................................................... 103

4.2. RAYLEIGH SCATTERING................................................................................................ 107

4.2.1 A model to interpret the porous aerogel structure using Rayleigh scattering110

5. LIGHT SCATTERING MEASUREMENTS OF AEROGELS BY A POLARIZATION-

MODULATED NEPHELOMETER .............................................................................................. 113

5.1. INTRODUCTION TO LIGHT SCATTERING VERSUS ANGLE

EXPERIMENTS......................................................................................................................113

5.1.1 Description of the polarization-modulated nephelometer ................................. 114

5.2. EXPERIMENTAL RESULTS .............................................................................................115

5.3. STRUCTURAL INFORMATION FROM THE LIGHT SCATTERING

MEASUREMENTS .................................................................................................................118

5.3.1 Inhomogeneous media .............................................................................................. 118

Short range correlations: Rayleigh scattering................................................... 120

Long range correlations: departures f rom Rayleigh scattering ............................ 121

5.4. COMPARATIVE STUDY BETWEEN EXPERIMENTAL MEASUREMENTS

AND THEORY ....................................................................................................................... 122

5.5. CONCLUSIONS AND FUTURE WORK ....................................................................... 125

6. DIRECT METHODS: ELECTRON MICROSCOPY ...............................................................127

6.1. STRUCTURAL STUDIES BY SCANNING ELECTRON MICROSCOPY ........ 127

6.1.1 Acetone series ..............................................................................................................127

6.1.2 Effect of the solvent .................................................................................................... 131

6.1.3 Drying procedure ........................................................................................................132

6.1.4 TMOS aerogels in CO2 as solvent ...........................................................................133

6.2. TRANSMISSION ELECTRON MICROSCOPY .......................................................... 134

Sample preparation..................................................................................... 134

TEM set-up .............................................................................................. 134

6.2.1 Imaging the acetone-series silica aerogels ...........................................................135

6.2.2 Methanol series............................................................................................................135

6.2.3 Replicas visualization.................................................................................................138

Replicas visualization.................................................................................. 138

7. REFERENCES...................................................................................................................................... 141

Index iv

C h a p t e r I V : MECHANICAL CHARACTERIZATION OF SILICA AEROGELS

1. INTRODUCTION ...............................................................................................................................146

1.1. MICROINDENTER DESCRIPTION............................................................................. 147

1.2. MECHANICAL CHARACTERIZATION...................................................................... 149

2. MECHANICAL PROPERTIES OF SILICA AEROGELS AS A FUNCTION OF

DENSITY................................................................................................................................................152

2.1. SAMPLE PREPARATION .................................................................................................. 152

2.2. EFFECT OF THE ALKOXIDE......................................................................................... 152

3. INFLUENCE OF SOLVENT AND SUPERCRITICAL DRYING METHOD ON THE

MECHANICAL PROPERTIES .......................................................................................................156

3.1. EFFECT OF THE DRYING PROCEDURE................................................................. 156

3.2. EFFECTS OF THE SOLVENT......................................................................................... 158

4. MICROMECHANICAL PROPERTIES OF CARBON-SILICA AEROGEL

COMPOSITES ......................................................................................................................................159

4.1. INTRODUCTION ................................................................................................................. 159

4.2. EFFECT OF CARBON ADDITION................................................................................ 160

5. VISCOELASTICITY OF SILICA AEROGELS AT ULTRASONIC FREQUENCIES .....169

5.1. INTRODUCTION ................................................................................................................. 166

5.2. EXPERIMENTAL SET-UP: AIR-COUPLED BROAD-BAND

PIEZOELECTRIC TRANSDUCERS ............................................................................. 167

5.3. DETERMINATION OF THE VISCOELASTICITY OF SILICA AEROGELS AT

ULTRASONIC FREQUENCIES ..................................................................................... 169

5.4. DISCUSSION............................................................................................................................171

6. CONCLUSIONS ...................................................................................................................................173

7. REFERENCES......................................................................................................................................175

C h a p t e r V : SILICA AEROGEL MICROPARTICLES

1. SOL-GEL ROUTE TO DIRECT FORMATION OF SILICA AEROGEL

MICROPARTICLES USING SUPERCRITICAL ETHANOL/ACETONE........................ 181

1.1. ‘IN SITU’ PARTICLE PROCESSING ............................................................................. 182

1.2. AEROGEL MICROPARTICLE CHARACTERIZATION ........................................ 184

1.2.1 Scanning Electron Microscopy ................................................................................185

Independent solutions .................................................................................. 189

Index v

1.2.2 Transmission Electron Microscopy ........................................................................190

1.2.3 Atomic Force Microscopy ......................................................................................... 191


MICROPARTICLES USING SUPERCRITICAL CARBON DIOXIDE ...............................195

2.1. ‘IN SITU LOW TEMPERATURE MICROPARTICLES: TEOS, HCOOH, AND

SUPERCRITICAL CO2 AS SOLVENT ............................................................................196

2.2. ‘PRECURSOR DIRECTLY INJECTED IN CO2 SUPERCRITICAL

CONDITIONS AT LOW TEMPERATURE.................................................................. 198

2.2.1 Injection of hydrolysis and precursor solution independently ..........................199

2.2.2 Injection of sol ............................................................................................................ 203

2.3. IN SITU’ INJECTION IN LIQUID CO2........................................................................ 203

2.4. INJECTION OF PREPOLYMERIZED PRECURSOR IN SUPERCRITICAL

CO2 .............................................................................................................................................. 205

3. CONCLUSIONS .................................................................................................................................. 208

4. REFERENCES..................................................................................................................................... 208

C h a p t e r V. SILICA AEROGEL FILMS

1. APPLICATIONS OF AEROGEL FILMS.......................................................................................212

1.1. ELECTRONIC........................................................................................................................ 212

1.2. OPTICAL .................................................................................................................................. 213

1.3. THERMAL ............................................................................................................................... 214

1.4. ACOUSTIC ............................................................................................................................... 214

1.5. ENVIRONMENT AND OTHERS................................................................................... 214

2. SOL-GEL COATING METHODS...................................................................................................215

2.1. DIP COATING........................................................................................................................ 215

2.2. SPIN COATING ..................................................................................................................... 216

2.3. SPRAY COATING.................................................................................................................. 217

2.4. SURFACE TENSION COATING..................................................................................... 217

2.5. SUBCRITICAL DRYING BY SURFACE DERIVATION ......................................... 217

3. REFERENCE EXPERIMENTAL RESULTS..............................................................................217

3.1. DIP COATING........................................................................................................................ 217

a) Low-temperature dip coating .................................................................... 218

b) High-temperature dip coating ................................................................... 219

3.2. SPIN COATING ..................................................................................................................... 221

a) Spin coating with High-Temperature drying .............................................. 222

Index vi

4. PROPOSED NEW METHODS: ‘IN SITU’ PREPARATION AT HIGH PRESSURE

AND INJECTION AT SUPERCRITICAL CONDITIONS..................................................... 224

4.1. ‘IN SITU’ PREPARATION AT HIGH PRESSURE.................................................... 224

4.1.2 ‘In situ’ high temperature......................................................................................... 225

4.1.3 ‘In Situ’ low-temperature coating method............................................................ 225

4.2. SPRAY COATING BY DIRECT INJECTION IN SUPERCRITICAL CO2 AT

LOW TEMPERATURE........................................................................................................ 227

5. CONCLUSIONS .................................................................................................................................. 230

6. REFERENCES......................................................................................................................................231

CONCLUSIONS ....................................................................................................................................................233

ARTICLES PUBLISHED RELATED TO THIS THESIS ........................................................................241

ANNEX I: SUPERCRITICAL FLUIDS..........................................................................................................243

ANNEX II: TECHNICAL DESCRIPTION OF THE JOIN ICMAB-CM HIGH PRESSURE-HIGH TEMPERATURE LABORATORY ....................................................................................................247

ANNEX III: INTRODUCTION TO ADSORPTION ANALYSIS AND THE BET MODEL ...257

REsum de la tesis. AEROGELS de sílice: SÍNTESIS i caracterització 265

AGRAÏMENTS ............................................................................................................................................... 297

PRÒLEG

Durant el temps de realització d‟aquesta tesi he tingut la sort de treballar amb un material tan

sorprenent i atractiu com és l‟aerogel. El resultat ha estat aquesta tesi estructurada en 5

capítols:

El primer capítol („Aerogels: An Introduction’), és una introducció amb apunts històrics

dels aerogels i de les seves aplicacions. El capítol està estructurat en 5 apartats: 1) mètode sol-gel,

una recapitulació bàsica de la tècnica sol-gel, 2) assecat supercrític dels gels, on s‟expliquen els

conceptes bàsics dels fluids supercrítics i els diferents mètodes d‟assecat supercrític, 3)

tècniques de caracterització, breu introducció a les tècniques aplicades per la caracterització dels

aerogels, 4) aplicacions, un resum de les aplicacions més comunes dels aerogels i, finalment 5)

bibliografia, on es presenta un ampli llistat de referències.

La motivació per escriure aquesta llarga introducció és el fet de que aquesta tesis doctoral és

„la primera‟ del nostre grup, i probablement de l‟estat espanyol, centrada en l‟estudi,

caracterització i síntesi d‟aerogels de sílice. La idea principal és que serveixi d‟ajut a futurs

projectes que impliquin aquest material.

El segon capítol, (Synthesis and optimization of silica aerogels: Influence of molar

ratios of precursor, solvent and water) és bàsicament descriptiu i el seu objectiu es centra

en fer un seguiment del procés usat per a l‟optimització de la síntesi i del cicle d‟assecat dels

aerogels. En aquest capítol s‟estableix quin és el camí de síntesi més adequat per obtenir les

característiques requerides per cada tipus d‟aerogel. Aquesta secció també conté una

bibliografia àmplia per a que es faciliti l‟oportunitat d‟obtenir més informació d‟una

determinada síntesi. En definitiva el segon capítol inclou bàsicament informació per a la

reproducció de les síntesis que he seguit en el nostre laboratori. Aquesta informació la trobem

esbossada en taules que contenen la majoria de paràmetres importants de la preparació de les

mostres.

En l‟últim apartat d‟aquest capítol s‟inclouen les conclusions on es resumeix les síntesis més

adequades per a cada procés, per tant la lectura completa d‟aquest capítol serà essencialment

necessària només en el cas que es vulgui reproduir algun dels resultats.

Prefaci 2

En el tercer capítol (Aerogel Characterization) s‟exposa la caracterització realitzada en les

diverses sèries d‟aerogels obtingudes amb diferent transparència, densitat i porositat, com a

conseqüència de la seva microestructura. Un dels aspectes més importants en la preparació

d‟aerogels és la possibilitat de control de les propietats físiques del material: densitat, porositat

i àrea superficial. Aquestes propietats estan directament relacionades amb l‟estructura del

material: la distribució i mida de porus, la distribució i mida de partícules, la mida de

„clusters‟,...

Per determinar la microestructura porosa d‟aquest material s‟ha desenvolupat un model que

utilitza els resultats de varies tècniques ja que l‟ús d‟una sola no permet la caracterització

completa de tot el rang de porositats i de mida de partícules. Les tècniques utilitzades en

aquest treball per a obtenir informació estructural dels aerogels són les tècniques de BET,

microscopia electrònica (SEM, TEM) o microscopia de forces atòmiques AFM). Per a

obtenir informació complementaria de la microestructura d‟aquests materials tan altament

porosos -determinació de la mida de porus i partícula- és necessària la utilització d‟altres

tècniques (l‟anàlisi de la dispersió de la llum).

Un dels inconvenients dels aerogels és la seva fragilitat, és per aquest motiu que un dels

objectius d‟aquesta tesi s‟ha centrat en la caracterització mecànica d‟aquests materials

mitjançant una tècnica no destructiva: la microindentació. El quart capítol („Mechanical

properties of silica aerogels’) mostra els resultats obtinguts explicant els resultats publicats

en una sèrie de quatre articles, el primer relaciona la dependència de les propietats

mecàniques dels aerogels de sílice amb la seva densitat, el segon article analitza l‟influencia de

varis paràmetres de síntesi (solvent utilitzat i procediment de assecat supercrític) amb les

propietats mecàniques dels aerogels de sílice, finalment el tercer article descriu com, amb la

finalitat de millorar les propietats mecàniques dels aerogels es varen sintetitzar nous aerogels

compostos amb carbó actiu. Aquests composites presenten un augment molt accentuat de

l‟elasticitat de l‟aerogel de sílice. En l‟últim article es caracteritzen les propietats

viscoelàstiques dels aerogels de sílice.

En el cinquè capítol (Aerogel Microparticles) es descriu el procés d‟obtenció de

micropartícules d‟aerogels mitjançant una nova tècnica. També s‟hi descriu la caracterització

de les diferents micropartícules obtingudes.

Prefaci 3

En el sisè capítol (Aerogel Films) es desenvolupa un estudi d‟obtenció de capes d‟aerogels

mitjançant una nova tècnica. També es porta a terme la caracterització de les diferents capes

obtingudes.

GLOSSARY OF TERMS1:

Aerogel

Defined as a group of extremely light and porous solid materials; the lightest is less

than four times as dense as dry air. Aerogels are derived via a sol-gel process in

combination with a subsequent drying step (most often achieved by supercritical

extraction) the result are monolithic, open porous materials with a backbone

morphology that can be modeled in terms of three dimensionally interconnected

strings of microscopic pearls.

Aerosol

A colloidal suspension of particles in a gas (the suspension may be called a fog if the

particles are liquid and a smoke if the are solid).

Aging

The term aging is applied to the process of change in structure and properties after

gelation. Bond formation does not stop at the gel point. In the first place, the

network is initially compliant, so segments of the gel network can still move close

enough together to allow further condensation (or other bond-forming processes).

Moreover, there is still a sol within the gel network, and then those smaller polymers

or particles continue to attach themselves to the network.

Alcohol

A molecule formed by adding a hydroxyl (OH) group to an alkyl molecule, as in

methanol (CH3OH) or ethanol (C2H5OH).

1 Their literal definitions were taken from http://eande.lbl.gov/ECS/aerogels/satoc.htm, Academic Press Dictionary of

Science and Technology and from C.J. Brinker, G.W. Sherer, Sol-Gel Science. Physics and Chemistry of Sol-Gel

Processing, Academic Press, New York, 1990.

Alkane

A molecule containing only carbon and hydrogen linked exclusively by single bonds,

as in methane (CH4) and ethane (C2H6); the general formula is CnH2n+2.

http://eande.lbl.gov/ECS/aerogels/satoc.htm

http://www.harcourt.com/dictionary/



Glossary of terms 5

Alkoxy

A ligand formed by removing a proton from the hydroxyl on an alcohol, as in methoxy

( OCH3) or ethoxy ( OC2H5). Where the dot, , indicates an electron that is available

to form a bond.

Alkyl

A ligand formed by removing one hydrogen (proton) from an alkane molecule

producing, for example, methyl ( CH3) or ethyl ( C2H5)

Brownian motion

In a colloid, the inertia of the dispersed phase is small enough to exhibit Brownian

motion (or Brownian Diffusion), a random walk driven by momentum imparted by

collisions with molecules of the suspending medium.

Ceramic

A nonmetallic and inorganic material; included all metal oxides, nitrides, and carbides,

both crystalline and noncrystalline.

Colloid

A mixture in which one substance is divided into minute particles (called colloidal

particles) and dispersed throughout a second substance. The dispersed phase is so

small ( 1–1000 nm) that gravitational forces are negligible and interactions are

dominated by short-range forces, such as van der Waals attraction and surface

charges. The mixture is also called a colloidal system, colloidal solution, or colloidal

dispersion. Familiar colloids include fog, smoke, and homogenized milk.

Glossary of terms 6

Condensation

A condensation reaction occurs when two metal hydroxides (M-OH + HO-M)

combine to give a metal oxide species (M-O-M). The reaction forms one water

molecule.

(OR)3Si-OH + HO-Si(OR)3 (OR)3Si-O-Si(OR)3 + H2O

or

(OR)3Si-OR + HO-Si(OR)3 (OR)3Si-O-Si(OR)3 + ROH.

By definition, condensation liberates a small molecule, such as water or alcohol. The

R represents a proton or other ligand (if R is an alkyl, then OR is an alkoxy group),

and ROH is an alcohol; the bar (-) is sometimes used to indicate a chemical bond.

Critical point

State at which two phases of a substance first become indistinguishable. For example,

at pressures higher than 217.6 atm and temperatures above 374°C, the meniscus

between steam and liquid water will vanish; the two phases become indistinguishable

and called supercritical fluid.

Crosslinkage/branching

If a polyfunctional unit with 2 is present, the chains can be joined by crosslinks to

form a three-dimensional structure. Polymerization of silicon alkoxide, for instance,

can lead to complex branching of the polymer because a fully hydrolyzed monomer

Si(OH)4 is tetrafunctional. On the other hand, under certain conditions (e.g., low

water concentration) fewer than four ligands will be capable of condensation, so

relatively little branching will occur.

Emulsion

Is a suspension of liquid droplets in another liquid. These types of colloids can be

used to generate polymers or particles from which ceramic materials can be made.

Glossary of terms 7

Functionality

The number of bonds that a monomer can form is called its functionality, . Typical

oxide monomers are bifunctional ( = 2), trifunctional ( = 3), or tetrafunctional ( = 4),

any of which may be called polyfunctional ( arbitrary).

Gel Point

If a monomer can make more than two bonds, then there is no limit on dimensions

so that it extends throughout the solution. The point in time at which the network of

linked oxide particles spans the container holding the sol. At the gel point, the sol

becomes a gel. The gel point corresponds to the percolation threshold, when a single

cluster (called the spanning cluster) appears that extends throughout the sol; the

spanning cluster coexists with a sol phase containing many smaller clusters, which

gradually become attached to the network

Gel

A gel consists of two parts, a solid part and a liquid part. The solid part is formed by

the three-dimensional network of linked oxide particles. The liquid part (the original

solvent of the sol) fills the free space surrounding the solid part. The liquid and solid

parts of a gel occupy the same apparent volume. Thus, a gel is a substance that

contains a continuous solid skeleton enclosing a continuous liquid phase. The gel can

be removed from its original container and can stand on its own. The continuity solid

structure gives elasticity to the gel (as in the familiar gelatin dessert).

Hydrolysis

The reaction of a metal alkoxide (M-OR) with water forms a metal hydroxide (M-

OH). A hydroxyl ion becomes attached to the metal atom, as in the following

reaction:

Si(OR)4 + H2O HO-Si(OR)3 + ROH

Glossary of terms 8

Depending on the amount of water and catalyst present, hydrolysis may go to

completion (so that all of the OR groups are replaced by OH).

Si(OR)4 + 4H2O H2 O-Si(OH)4 + 4ROH

or stop while the metal is only partially hydrolyzed, Si(OR)4-n(OH)n.

Meniscus

A phase boundary that is curved because of the surface tension.

Miscible

Two liquids are considered miscible or mixable if shaking them together results in a

single liquid phase, with no meniscus visible between layers of liquid.

Metal alkoxides

Are members of the family of metalorganic compounds, which have an organic

ligand attached to a metal o metalloid atom. The most thoroughly studied example is

silicon tetraethoxide (or tetraethoxy-silane, or tetraethyl orthosilicate, TEOS),

Si(OC2H5)4. Organometallic compounds are defined as having direct metal-carbon

bonds, not metal-oxygen-carbon linkages as in metal alkoxides; thus, alkoxides are

not organometallic compounds. Metal alkoxides are popular precursors because they

react readily with water.

Monolith

Gelation can occur after a sol is cast into a mold, in which case it is possible to make

objects of a desired shape. If the smallest dimension of the gel is greater than a few

millimeters, the object is generally called a monolith.

Polymer

A polymer (“many member”) is a huge molecule (also called a macromolecule) formed

from hundreds o thousands of units called monomers that are capable of forming at

least two bonds.

Glossary of terms 9

Porosity:

Microporosity

IUPAC definition: pores with mean diameter lower than 2 nm.

Mesoporosity

Pores with mean diameter between 2 nm and 50 nm.

Macroporosity

Pores with mean diameter larger than 50 nm.

Precursors

In the sol-gel process, the precursors (starting compounds) for preparation of a

colloid consist of a metal or metalloid element surrounded by various ligands

(appendages not including another metal or metalloid atom). The latter is an example

of an alkoxide, the class of precursors most widely used in sol-gel research.

Shrinkage

Shrinkage of a gel, either during syneresis or as liquid evaporates during drying,

involves deformation of the network and transport of liquid through the pores.

Sol

A colloidal suspension of solid particles in a liquid. A solution of various reactants

that are undergoing hydrolysis and condensation reactions. The molecular weight of

the oxide species produced increases continuously. As these species grow, they may

begin to link together in a three-dimensional network.

Supercritical fluid

A substance that is above its critical pressure and critical temperature. A supercritical

fluid possesses some properties in common with liquids (density, thermal

Glossary of terms 10

conductivity) and some in common with gases (fills its container, does not have

surface tension).

Syneresis

Some gels exhibit spontaneous shrinkage; called syneresis, as bond formation or

attraction between particles induces contraction of the network and expulsion of

liquid from the pores.

Xerogel

Drying by evaporation under normal conditions by evaporation gives rise to capillary

pressure that causes shrinkage of the gel network. The resulting dried gel is named

"xerogel", a word issued from the Greek word "xeros" and which means dry. The

shrinkage during drying is often extreme (~90%) for xerogels. The no collapse

requirement distinguishes aerogels from xerogels.

C h a p t e r I

AEROGELS: AN INTRODUCTION

SECTION OUTLINE

1. GENERAL OVERVIEW ON AEROGELS .....................................................................................12

1.1. COMMERCIAL SILICA AEROGEL...................................................................................... 14

2. SOL-GEL METHOD.............................................................................................................................15

2.1. ANTECEDENTS OF GEL SYNTHESIS .............................................................................. 16

2.2. PREPARATION OF SILICA GELS ....................................................................................... 16

3. DRYING PROCEDURE OF GELS.................................................................................................. 20

4. AEROGEL APPLICATIONS ............................................................................................................. 22

4.1. POROSITY AND SURFACE AREA APPLICATIONS...................................................... 22

Electrodes in capacitors ................................................................................. 22

Capacitive deionization ................................................................................. 23

Studies of superf luid transitions and phase separation of 3He-4He...................... 23

4.2. OPTICAL PROPERTY APPLICATIONS ............................................................................. 23

Çerenkov (particle detection and counters)........................................................ 23

4.3. THERMAL INSULATOR APPLICATIONS........................................................................ .24

4.4. ACOUSTICAL AND MECHANICAL APPLICATIONS ................................................... 24

Shock compression experiments ...................................................................... 25

4.5. ELECTRICAL AND ELECTRONIC APPLICATIONS..................................................... 25

4.6. SPACE APPLICATIONS .......................................................................................................... 25

5. REFERENCES....................................................................................................................................... 25

Chapter I. Silica aerogel: An Introduction 12

1. GENERAL OVERVIEW ON AEROGELS

This section surveys the literature and summarizes the historical background of aerogels

development, their production by the sol-gel process, several drying methods, and various

structural investigations.

Aerogel is defined as a group of extremely light and porous solid materials. Silica-based

aerogels are among the lightest ones, can be less than four times as dense as dry air, and

some are nearly transparent, its nickname is “solid smoke” or “frozen smoke”.

Since this definition is good for most porous materials, the term aerogels became reserved

for the porous gels obtained by removing solvent from highly swollen gels at the conditions

that no or minimal collapse occurs, which causes the liquid in the gel to become supercritical

(in a state between a liquid and a gas) and lose its surface tension. The result is an open

porous material with a backbone morphology that can be modeled in terms of three

dimensionally interconnected strings of nanoscopic pearls. The length scale of both the

“pearls” as well as the interconnected voids can be independently tailored over a wide range,

i.e. from a few nanometers to several microns.

Figure I.1 The basic blocks of the structure of silica aerogel are spherical primary particles with five nanometers in diameter and with the same density as bulk silica. These spheres cluster into secondary particles that are linked in chains to create the porous aerogel skeleton (on the range 20-50 nm).

One of the striking advantages of aerogels compared to other porous materials is that both

porosity and inner surface area can be tuned independently. Porosities of up to 99.9 % are

achievable; when microporosity is present, the specific surface area can exceed 1500 m2/g.


Because of their unique properties, i.e., large surface area, very small pores and very low bulk

density, aerogels are potentially important candidates for a wide range of applications. Table

I.1 gathers some of the remarkable properties of silica aerogels.

Table I.1 Physical properties of silica aerogels.

Property Value Comments

Apparent density 0.003-0.5 g/cm3 Most common density is 0.1g/cm3 ( air = 0.001g/cm3)

Inner surface area 500-1500 m2/g As determined by nitrogen adsorption/desorption A cubic centimeter of an aerogel has about the same surface area as one soccer field)

Solid percentage in volume

0.13-15 % Typically 5 % (95 % free space)

Mean pore diameter 20-150 nm As determined by nitrogen adsorption/desorption (varies with density)

Primary particle diameter

2-5 nm Determined by transmission electron microscopy

Index of refraction 1.007-1.24 Very low for solid material (nair= 1.004)

Thermal tolerance Up to 500 C Shrinkage begins slowly at 500 C, increases with increasing temperature. Melting point is ~1200ºC

Poisson’s ratio 0.2 Independent of density, similar to dense silica. Determined using ultrasonic methods.

Young’s modulus 0.1-300 MPa Very small (<104) compared to dense silica

Tensile strength 16 kPa For density of 0.1 g/cm3

Fracture toughness 0.8 kPa.m1/2 For density of 0.1 g/cm3. Determined by 3-point bending

Dielectric constant 1.1 For density of 0.1 g/cm3, very low for a solid material (kair= 1)

Acoustic impedance 104 Kg/m2.s Determined using ultrasonic methods al KHz frequency.

Sound velocity through the medium

20-800m/s 100 m/s for density of 0.07 g/cm3, one of the lowest velocities for a solid material

Optical property Transmittance>90%

(630nm) Transparent-blue haze

Thermal conductivity 0.02 W/mK (20 C) Very low thermal conductivity. 2 cm slab provides the same insulation as 30 panes of glass

Aerogel is an extremely adaptable material: aerogels have been prepared from many metal

oxides, including tin, tungsten and iron and also, alumina, zirconia, titanic, and magnesia , as

well as from organic gels: carbon, gelatin, organic polymers, proteins, and cellulose.


The laboratory of the Aerogel Research Group at the Material Science Institute of Barcelona

(ICMAB) is equipped to produce aerogels in a variety of shapes and configurations (from

bulk monoliths to thin films or microspheres), in small to medium-sized batches.

1.1 COMMERCIAL SILICA AEROGEL

A sign of growth of the technology is the increasing number of companies producing

aerogels and the increasing number of patents involving aerogels. Some works have been

published about aerogel commercialization, technology, markets and costs [29]. In 1930, S.S.

Kistler invented the aerogels by supercritically drying of gels, and Montsanto Corp. produced

thousands of tons of silica aerogels during 1940s and 1950s, using the substance as an

additive in cosmetics and toothpaste. Aerogel research was largely abandoned for the next 50

years. Then, in the 1980s, newer safer production processes to create aerogels were

developed under the leadership of A.J. Hunt at the Lawrence Berkeley Laboratory

eetd.lbl.goc/ecs/aerogels/kistler/inde.htm, and J. Phalippou at the University of Montpellier,

leading to the identification of applications for aerogels as insulators for rocket fuel storage

and later as cosmic dust collectors on two shuttle missions. Airglass, in Sweden,

(www.airglass.se) is the only current large producer of aerogel for thermal insulation, but

Cabot Corp. (www.cabot-corp.com/cabot.nsf) and Aspen Systems Inc.

(www.aspensystems.com/aerogel.html) will reach the market in the next months. Nanopore

(www.nanopore.com) explores other applications for nanoporous solids.

Silica aerogels, attracted international attention early in the 1990s after Livermore Lab.

Scientist created a silica aerogel 10 time less dense than the lightest precious version.

(http://www.llnl.gov/).

Aerogels are also the best thermal insulators ever discovered. NASA used aerogels to insulate

the electronics on the intrepid Sojourner from the cold of the Martian night

www.science.nasa.gov/aerogel. Aerogels have amazing thermal dissipation properties.

Aerogels can also be either electrically conductive (i.e. carbon aerogels) or insulators (silica

aerogels). Electrical insulators fabricated with aerogels may double the actual computer

speeds. Contemporary circuits boards have dielectric constants, k, between 2.5 and 4 (air has

k=1). Decreasing k of the insulating film can increase the speed of the computer by allowing

engineers to place components closer together. Researchers have already successfully created

aerogel films -made mostly of air- with dielectric values ranging between 2.3 and 2.01.

Other aerogels are organic, made of carbon and hydrogen atoms. Organic aerogels are stiffer

and stronger than silica aerogels and are measurably better insulators. Organic aerogels have

eetd.lbl.goc/ecs/aerogels/kistler/inde.htm

http://www.airglass.se/

http://www.cabot-corp.com/cabot.nsf

http://www.aspensystems.com/aerogel.html

http://www.nanopore.com/

http://www.llnl.gov/

http://www.science.nasa.gov/aerogel


extremely high thermal resistance (six times higher than fiberglass) and can be converted to

pure carbon aerogels with still retaining many properties of the original aerogel, and at the

same time becoming electrically conductive. Carbon aerogels have been used as electrode in

energy storage devices known as double-layer capacitors. Such devices are able to deliver

power faster than conventional batteries and thus have potential application in electric

vehicles, “pure power” stations, telecommunications, and microelectronics. Carbon aerogels

capacitors are already in the electronics shops by Cooper Electronics Technologies

(www.cooperelectech.com/power/indexIntro.htm).

One of the applications uses aerogels as catalysts to reduce nitrous oxide emissions from cars

exhaust systems. Some scientists expect aerogels will be used as catalysts within a few years

because of their high surface area (a cubic centimeter of an aerogel has about the same

surface area as one soccer field). In addition, metallic atoms or metal-oxide particles can be

placed in aerogels to cause reactions.

Lots of information about aerogels can be obtained from Internet. A good starting point can

be the NASA aerogel Web site at the http://www.science.nasa.gov/aerogel. A nice and

documented work on the history of aerogels has been prepared by Mike Ayers (LBNL) and

can be navigated at eetd.lbl.goc/ecs/aerogels/kistler/index.htm.

2. SOL-GEL METHOD

The formation of aerogels, in general, involves two major steps, the formation of a wet gel,

and the drying of the wet gel, avoiding major shrinkage, to form an aerogel.

2.1 ANTECEDENTS OF GEL SYNTHESIS

Kistler [1, 2] was the first researcher who formulated the idea of replacing the liquid phase by

a gas with only a slight shrinkage of the gel back in the 1930s. Eventually, he obtained silica

aerogels by a technique known as the „water-glass process‟ outlined below:

http://www.cooperelectech.com/power/indexIntro.htm)

http://www.science.nasa.gov/aerogel

eetd.lbl.goc/ecs/aerogels/kistler/inde.htm

http://www.sciencedirect.com/#b1



Figure I.2 Left recipient contains a sol /transparent at visible range). Right recipient contains a gel (blue shading).

1. Preparation of a hydrogel (gels with water as a solvent) in reaction of sodium silicate

with hydrochloric acid.

2. Removal of sodium and chlorine ions. This step involves a long and tedious soaking of

the gel.

3. Converts the hydrogel into alcogel by replacing water with ethyl alcohol in a lengthy

process of solvent replacement.

4. Drying at above critical conditions for ethyl alcohol.

When these steps were followed, an extremely light solid remained; however, the work on

solid aerogels was mostly forgotten until 1970s.

An improved method of preparing gels took place in the Teichner and Nicholaon group [3]

[4] at the Claude Bernard University in Lyon. The procedure was substantially simplified by

carrying out the sol¯gel transition directly in the solvent (that was removed at supercritical

conditions) through the use of relatively new class of compounds called metal alkoxides.

Alkoxide-based sol-gel chemistry avoids the formation of undesirable salt by-products, and

allows a much greater degree of control over the final product.

2.2 PREPARATION OF SILICA GELS

A common way to synthesize gels at room temperature corresponds to a chemical reaction

implying metal alkoxides and water in an alcoholic solvent. The majority of silica aerogels

prepared utilizes silicon alkoxide precursors. The most common of these are tetramethyl

orthosilicate (TMOS or Si(OCH3)4), and tetraethyl orthosilicate (TEOS or Si(OCH2CH3)4).

However, many other alkoxides, containing various organic functional groups, can be used to




impart different properties to the gel [5, 6]. The first reaction is a hydrolysis which induces

the substitution of OR groups linked to silicon by silanol Si-OH groups. A condensation

reaction occurs when two silanol groups (Si-OH + HO-Si) react together to form Si-O-Si

(siloxane) bonds, which lead to the silica network formation.

Three reactions are generally used to describe the sol̄ gel process [7]:

(Eq. I.1)

(Eq. I.2)

(Eq. I.3)

where R is an alkyl group, CxH2x+1

The hydrolysis reaction (Eq. I.1) replaces alkoxide groups (OR) with hydroxyl groups (OH).

Subsequent condensation reactions (Eq. I.2, Eq. I.3) involving the silanol groups produce

siloxane bonds (Si¯O¯Si) plus the by-products alcohol (ROH) or water.

Because water and alkoxysilanes are immiscible, a mutual solvent such as alcohol is normally

used as a homogenizing agent. The final density of the aerogel depends on the concentration

of silicon alkoxide monomers in the solution.

The balanced chemical equation for the formation of a silica gel is:

Si(OR)4 (liq.) + 2H2O (liq.) = SiO2 (solid)+ 4HOR (liq.) (Eq. I.4)

The stoichiometry of the reaction requires two moles of water per mole of alkoxysilane. In

practice, this amount of water leads to incomplete reaction, and weak, cloudy aerogels.

Therefore, most aerogel recipes use a higher water ratio than is required by the balanced

equation (anywhere from 4-30 equivalents).

The gel point is the time at which the network of linked oxide particles spans the container

holding the sol. At the gel point, the sol becomes a gel. This two-phase material, a solid part

and a liquid part, consists of shaped solid exhibiting specific properties.

The solid part is formed by the three-dimensional network of linked oxide particles. The

liquid part (the original solvent of the sol and a small amount of water) fills the free space




surrounding the solid part. The liquid and solid parts of a gel occupy the same apparent

volume.

Figure I.3 Scheme of silica aerogel sol-gel synthesis by condensation of silica alkoxide precursor on alcohols.

As condensation reactions progress the gel will gain rigidity. At this point, the gel is usually

removed from its mould. However, the gel must be kept covered by alcohol to prevent

evaporation of the liquid contained in the pores of the gel. Evaporation causes severe damage

to the gel and will lead to poor quality aerogels.

Catalysts

The kinetics of the above reaction is impractically slow at room temperature, often requiring

several days to reach completion. For this reason, acid or base catalysts are added to the

formulation. The amount and type of catalyst used play key roles in the microstructural,

physical and optical properties of the final aerogel product (sections 3.5, 3.5).

Acid catalysts can be any protic acid, such as HCl. Base-catalysis usually uses ammonia, or

ammonia buffered with ammonium fluoride. Aerogels prepared with acid catalysts often

Alcohol TMOS/ TEOS

Catalyst + Water

Start of reaction

Sol

Alcogel

Gelation

Skeleton

Solvent


show more shrinkage during supercritical drying and may be less transparent than base

catalyzed aerogels. The microstructural effects of various catalysts are harder to describe

accurately, as the substructure of the primary particles of aerogels can be difficult to image

with electron microscopy (section 3.7). All have small (2-5 nm diameter) particles that are

generally spherical or egg-shaped. With acid catalysis, however, these particles may appear

less defined than those in base-catalyzed gels.

Two-Step Aerogels

Typical acid or base catalyzed gels are often classified as "single-step" gels, referring to the

"one-pot" nature of this reaction. A more recently developed approach uses pre-polymerized

TEOS as the silica source. Pre-polymerized TEOS is prepared by heating an ethanol solution

of TEOS with a sub-stoichiometric amount of water and an acid catalyst. The solvent is

removed by distillation, leaving a viscous fluid containing higher molecular weight silicon

alkoxy-oxides. In a second step, this material is redissolved in ethanol and reacted with

additional water under basic conditions until gelation occurs. Gels prepared in this way are

known as "two-step" acid-base catalyzed gels. Pre-polymerized TEOS is available

commercially from Silbond Corp. (Silbond H-5).

These slightly different processing conditions impart important changes to the final aerogel

product. Single-step base catalyzed aerogels are typically more brittle than two-step aerogels.

Moreover, two-step aerogels have a smaller and narrower pore size distribution and are often

optically clearer than single-step aerogels (Chapter II, section 3).

Aging and Soaking

At the gel point the silica backbone of the gel contains a significant number of unreacted

alkoxide groups. Sufficient time must be given for the strengthening of the silica network.

This can be enhanced by controlling the pH and water content of the covering solution.

Common aging procedures for base catalyzed gels typically involve soaking the gel in an

alcohol/water mixture of equal proportions to the original sol. The gels are soaked in this

solution for up to 24 hours. This step, and all subsequent processing steps, is diffusion

controlled. Diffusion is affected by the thickness of the gel. Then, the time required for each

processing step increases dramatically as the thickness of the gel increases.


After aging the gel, all water still contained within its pores must be removed prior to drying.

This is simply accomplished by soaking the gel in pure alcohol several times until all the water

is removed. Again, the length of time required for this process is dependent on the thickness

of the gel. Any water left in the gel will not be removed by supercritical drying, and will lead

to an opaque, white, and dense aerogel.

Chapter II shows that variations in synthesis conditions (for example, ratio H2O/Si, the

catalyst type and concentrations, the type of solvent, temperature and pressure of

supercritical drying) cause modifications in the structure and properties of the obtained silica

aerogels [8]. Thus, porous structure of silica aerogels strongly depends on preparation and

drying parameters [9 -18].

3. DRYING PROCEDURE OF GELS

An aerogel results from a supercritical drying process. This is where the liquid within the gel

is removed, leaving only the linked silica network. The difference between classical drying

and supercritical drying is shown in Figure I.4. From point 1 to 5, (purple arrow) the liquid is

depressurized isothermally (classical drying). Consequently, we can say that xerogels refer to

gels dried at temperature close to room temperature and under atmospheric pressure. The

supercritical drying is performed inside an autoclave, which allows to overpass the critical

point (PC, TC) of the solvent, as shown in Figure I.4 (Path 1-2-3-4-5); or by prior solvent

exchange with liquid CO2 followed by supercritical CO2 venting (lower temperature drying).

The supercritical fluid is a substance that is above its critical pressure and critical

temperature; it possesses some properties in common with liquid (density, thermal

conductivity) and some in common with gas (fills its container, does not have surface

tension). A more detailed description of supercritical fluids can be found in Annex I). Strong

inorganic solids are commonly dried using alcohol (or acetone) as solvent or dried using CO2

as solvent.




Figure I.4 Scheme of the pressure and temperature variation on the solvent phase diagram during a gel supercritical drying process. The shaded area represents the supercritical region (SCF), where C is the critical point, Tr represents the triple point, and 1 to 5 are random points in the phase diagram.

Under ambient conditions, during the evaporation of the solvent, a liquid-vapor interface is

formed within the pores of the gel. The surface tension of the liquid creates a concave

meniscus in each capillary. By evaporation, the meniscus recedes and the compressive force

on the wall of the pores produces the collapse of the initial gel framework, or shrinkage

(Figure I.5). A liquid-vapor interface or the presence of a liquid in equilibrium with the vapor

is only observed below the critical temperature and pressure of the solvent. Above the critical

point (Figure I.4) the liquid no longer existed. Supercritical fluid is a gaseous like-phase, so

the liquid meniscus and its interfacial tension would not form in these conditions.

r

cos2p , capillary pressure

is the surface tension of the liquid

r is the pore radius

Figure I.5 The liquid-vapor interface formed in the gel capillary during drying.

Using the high temperature drying procedure some problems may arise from the

combination of high pressures and high temperatures (methanol critical parameters: Pc=81

bar, Tc=240 C) , i.e. flammability of the solvents. Alternatively, supercritical drying with CO2

has been developed by substituting, under pressure, alcohol in the gel by liquid carbon

Liquid

Meniscus

r, pore radius

Vapor


dioxide and then drying the aerogel with carbon dioxide at supercritical conditions. CO2 is of

particular interest due to its low critical temperature (31 C), non-flammability, and non-

toxicity. The process results in a reduction of the temperature and pressure required for

drying aerogels.

4. AEROGEL APPLICATIONS*

Aerogels were discovered more than 70 years ago. During these years, many potential

applications were described [19-22] and more new applications are mentioned in recent

reviews [23-28]. This section reviews some of the aerogels applications.

In addition to being the best thermal insulators ever discovered, aerogels have the lowest

dielectric constant, and the lowest sound velocity of any known solid material, sound

propagates more slowly through aerogels than through air. Other possible applications

include, substrates for chemical catalysis, ultrafilters, seawater desalinization, battery

electrodes, solar collector covers, acoustic delay lines, refrigerator insulation, replacements for

the air between the panes of double-glazed windows, to detect high-energy subatomic

particles emitted by particle accelerators, micrometeoroid collectors, and supercapacitors,

adsorbents, sensors and fuel cells, and even safe insecticides.

The following applications for aerogels are associated with certain properties of aerogel

materials. In many cases, the application is associated with a single property even if the

aerogels have a combination of properties appropriate to the given application.

4.1 POROSITY AND SURFACE AREA APPLICATIONS

Due to their high porosity, their very large inner surface area (easily accessible because of the

open porosity), and the controllable dispersion of the active component, they are especially

active catalysts or catalytic substrates [30-36]. There are numerous references of this

application for various aerogels and doped aerogels [37-43].

Moreover, the high porosity and large surface areas lead to applications as filters [44],

absorbing media for desiccation [45,46,47], filters, reinforcement agents, pigments, gellifying

agents [48], waste containment [49], encapsulation media [50], and pesticides [23].

* Lawrence W. Hrubesh April 1998 Journal of Non-Crystalline Solids Volume 225, Issue 1 Pages 335-342 and from C.J.

Brinker, G.W. Sherer, Sol-Gel Science. Physics and Chemistry of Sol-Gel Processing, Academic Press, New York, 1990.






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Electrodes in capacitors

The carbon aerogels have been used as electrodes capacitors in energy storage devices known

as double layer capacitor because they are electrically conductive with a very large surface area

[51-53]. The stored energy in these devices can be released faster than conventional batteries

with high power densities. Thus, have potential application in electric vehicles,

microelectronics, and hydrogen fuel storage [54].

Capacitive deionization

One of the promising new applications for aerogels is in a cost-effective purification process

[55]. The carbon aerogel capacitive deionization process works by sending solutions with

various positively and negatively charged ions through an electrochemical cell consisting of

numerous electrodes containing carbon aerogels in the form of sheets. The aerogel process

can have a variety of uses ranging from extracting harmful contaminants from industrial

waste water [56] to desalinizing seawater.

Studies of superfluid transitions and phase separation of 3He¯4He

Low-density silica aerogels are used to study the superfluid transition of 4He and phase

separation of the 3He¯4He mixture. The aerogels provide a random disordered structure that

modifies the normal superfluid and phase separation behavior with helium. [57- 62].

4.2 OPTICAL PROPERTY APPLICATIONS

Aerogel is transparent when its microstructural components are very small compared with

the wavelength of light. Transparent aerogels, together with their exceptional thermal

insulation ability, have been considered for use as super-insulating sheets in double walled

window systems because help considerably to reduce thermal losses in windows [63- 69].

Translucent aerogels have been proposed to improve the efficiency of solar thermal energy

storage devices [69-75]. Moreover, the ultra-low density aerogels can be used as lightweight

mirror backings [76].

Aerogels have been used to prepare ultra-pure, full-density silica glass by sintering at

temperatures below the melting temperature of silica [77- 80].

Silica aerogels with silicon exhibits strong photo-luminescence (luminescence stimulated by

visible or ultraviolet radiation). Silica aerogel, doped with radioactive tritium and phosphor,

makes an efficient radio-luminescent light source [81]. There is also evidence for quantum

confinement in nanoparticle-loaded silica aerogels [82] for producing blue light emission.

















Çerenkov (particle detection and counters)

The first modern application, in the early 1980s, involved the use the silica aerogels in

detectors called Çerenkov counters. The Çerenkov detector measures the velocity of

elementary particles and cosmic rays [83-84]. Since the speed of light depends on the

refraction index of the medium, media with different refraction indices were searched. Silica

aerogels with refraction indices between 1.007 and 1.024 offer specific refractive indexes for

range threshold detectors. [86-92]. Aerogel are used for particle detectors and counters

continue in space, at accelerators around the world [93]. A low-density silica aerogel was used

in radiation detection vacuum tubes to support the high voltage wire [94].

4.3 THERMAL INSULATOR APPLICATIONS

Aerogel materials, known for exhibit the lowest thermal conductivity of any of the solid or

porous materials, are excellent for applications requiring thermal insulation [95]. These

include development of electric automobiles equipped with batteries that operate at high

temperatures and that need heat storage [95- 97], and insulation for architectural purposes

[98]. A layer of transparent aerogel allows the penetration of the sun radiation to the wall,

but not the escape of the heat generated [99]. Then, aerogels can be used in heat and cold

storage devices [100], automotive exhaust pipes, transport vehicles, and vessels.

4.4 ACOUSTICAL AND MECHANICAL APPLICATIONS

Aerogels may also have acoustic and mechanical applications. Because of their unusual

structure, aerogels have low sound velocities, as low as 30 meters per second.

Another important acoustic property of aerogel is its mechanical impedance. The impedance

is the product of density and the sound velocity of the material. Since both are low, silica

aerogel has the lowest impedance of all solid material. This allows the aerogels to be used for

coupling sound waves in air to a transducer (device that converts energy from one form to

another), this may be useful either to generating or detecting sound. Therefore, they should

be efficient ultrasonic devices as acoustic impedance matching [101-103], and sound

absorption (anechoic chambers) [104, 105].

in Ardon (Switzerland) a house was built using aerogel granulate as a translucent insulating material








Shock compression experiments

Aerogels have also been proposed as a shock absorbing material. One of the earliest

experiments was to measure shock compression in silica aerogels [106]. The low density of

the silica aerogel allowed more internal energy could be deposited in it.

4.5 ELECTRICAL AND ELECTRONIC APPLICATIONS

Silica aerogel is an electrical insulator with a low dielectric constant, k (k is the measure of the

ability of a material to store electrical potential energy under the influence of an electric field).

The velocity of signal propagation in a chip is dependent on the dielectric constant of the

surrounding electrical insulation. The lower the dielectric constant, the higher the velocity.

Therefore, thin aerogel films are almost ideal dielectrics for ultra-fast integrated circuits [107-

109].

The bulk aerogels can be used for microwave electronics and high voltage insulators [110].

The pure carbon aerogels are quite electrically conductive, so they have applications as

electrodes for batteries, fuel cells, and capacitors [111]. Other metal oxide aerogels have been

made, which exhibit super-conducting behavior [112], thermoelectric behavior [113], and

piezoelectric properties [114].

4.6 SPACE APPLICATIONS

Aerogels have already captured cosmic dust while on the European Retrieval Carrier

(EURECA) satellite and in Space Shuttle experiments [115], and will capture cometary‟s dust

in NASA's STARDUST project. Lightweight silica aerogels have also been proposed as a

contaminant collector, to protect space mirrors from volatile organics [116]. Aerogels were

used to insulate the Mars Rover, where its lightness and strength were established as ideal.

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C h a p t e r I I

SYNTHESIS OF SILICA AEROGELS

SECTION OUTLINE

1. INTRODUCTION ................................................................................................................................ 32

2. PREPARATION OF AEROGELS .................................................................................................... 33

2.1. USED REACTIVES ................................................................................................................. 33

2.2. SYNTHESIS PROCEDURE ................................................................................................. 34

2.3. DRYING PROCEDURE ........................................................................................................ 36

2.3.1 Supercritical drying at high temperature................................................................... 36

2.3.2 CO2 supercritical drying ................................................................................................ 38

3. SYNTHESIS ROUTES: EFFECT OF DIFFERENT ALKOXIDE PRECURSORS ..... 39

3.1. TETRAMETHOXYSILANE: TMOS AEROGELS ........................................................ 40

3.1.1 The effect of the TMOS concentration...................................................................... 45

3.1.2 The effect of the nature of the solvent ...................................................................... 46

3.1.3 The effect of the hydrolysis solution .......................................................................... 47

Water amount.............................................................................................. 47

3.2. TETRAETHOXYSILANE: TEOS AEROGELS ............................................................. 49

3.2.1 The effect of the TEOS concentration ...................................................................... 53

Neutral ....................................................................................................... 53

Acid catalyst ................................................................................................ 54

Base-catalyst ................................................................................................ 56

3.2.2 The effect of hydrolysis solution ................................................................................. 58

Water amount.............................................................................................. 58


Influence of the nature of the catalyst .............................................................. 64

3.3. ‘TWO-STEP’ SYNTHESIS .................................................................................................... 66

Effect of precursor concentration...................................................................... 67

Effect of water amount .................................................................................. 69


4. SUMMARY AND CONCLUSIONS.............................................................................................71

5. REFERENCES................................................................................................................................. 72

Chapter II. Synthesis of silica aerogels 32

1. INTRODUCTION

The aim of this section is to present detailed description of the conditions at which silica

aerogels were prepared through out the thesis and the influence of the synthesis routes on

some of the silica aerogel final properties. Hydrolysis and polycondensation of a silicon

alkoxide is a well-known route to prepare silica gels [1]. Hydrolysis and condensation

reactions compete with each other during all the stages of the sol-gel process, and are

additionally influenced to a different degree by many parameters. The influence of the

different parameters on the network formation is very complex, since many parameters

change progressively as polycondensation proceeds. Synthesis of silica gels has been carried

out by hydrolysis of several silicon alkoxide precursors in different solvent solutions.

To tailor the structure and properties of the final material the influence of different

parameters have been studied. The varied parameters were:

1. The type of metal alkoxide(s)

2. The type of solvent

3. The relative and absolute concentration of the metal alkoxide and the solvent

4. The concentration of alkoxy group to water ratio

5. The type of catalyst

6. The pH of hydrolysis

A data–base computer file containing all the information since 1995 to these days was used

with the aim to classify synthesis and sample information of all products prepared in our

laboratory. Twenty characterization parameters were used together with a code name to

identify each sample. The Database allowed us to develop a statistical study of different

parameters of all the synthesis prepared until now.

A simple code name was used in order to name the samples from different experimental

conditions. As an example TE01AA00 means:

TE TEOS precursor, (TM when TMOS was used, H5 when Prepolymerized TEOS)

01 2001 year,

AA name of the synthesis process, in a chronological order

01 number of the sample with AA synthesis.

Another example: H501AB97 meaning H5 Prepolymerized TEOS precursor, 97 1997

year, AB name of the synthesis process, 02 number 02 of the sample with AB synthesis.


Few aerogels that have been thoroughly characterized and that will appear ‘recurrently’ on

the thesis are labeled in a short form as acetone series A1, A2, A3, and A4 and methanol as

M. These syntheses are described in section 4.

2. PREPARATION OF AEROGELS

2.1 USED REACTIVES

Sols were prepared by using three different silicon alkoxide precursors: (a) tetraethoxysilane

(TEOS) (b) tetramethoxysilane (TMOS), and (c) prepolymerized tetraethoxysilane (H5).

Their purity was above 98% (GC) and they were used as supplied. The solvents used were

ethanol, methanol, and acetone. The hydrolysis and catalyst solutions were done using one of

the following catalysts: HCl, C6H8O7, HNO3, NH3, CH3COOH, KOH, C2H2O4+ NH4OH,

NH4F+NH3, NH4OH+CH3COOH in the form of solutions in demineralized H 2O. Table

II.1 summarizes the properties of the reactives used in the sol-gel synthesis, molar density, M,

density , purity, melting and boiling points (mp and bp) and commercial trade.

Table II.1 Properties of the synthesis reactive: molar density, M,

density , purity, melting and boiling points (mp and bp) and commercial trade.

Reactive M

(g/mol)

(g/cm3) Purity

(%) mp-bp

( C) Trade

Tetraethoxysilane, TEOS

Si(OCH2CH3)4 208.33 0.933 98+ 163-167 Fluka

Prepolymerized

Tetraethoxysilane, H5 98+ Silbond

Tetramethoxysilane, TMOS

Si(OCH3)4 152.22 1.027 98+ 118-122 Fluka

Acetone

(CH3)2CO 58.08 0.789 99.5 - Panreac

Citric Acid

C6H8O7 192.12 - 99.5 - Fluka

Clorhidric Acid

HCl

36.46

1.18

1.017

35

0.9834N -

Panreac

Aldrich

Formic Acid

HCOOH 46.03 1.220 95-97 100-101 Aldrich

Phosphoric Acid

H3PO4 98.00 1.685 85 - Aldrich

Nitric Acid

HNO3 63.01 1.38 60 - Panreac

Ammonia

NH3 17.03 0.88 32 - Merck

Ethanol

CH3CH2OH 46.070 0.789 99.8 v/v 78.5 Carlo Erba

Hydroxide sodic

NaOH 39.996 - 97+ - Carlo Erba


CH3COOH

Acetic acid 60.05 1.05 99 Panreac

Methanol

CH3OH 32.04 0.790 99+ - Aldrich

2.2 SYNTHESIS PROCEDURE

The sol-gel syntheses of the silica aerogels comprises three steps: (i) hydrolysis and

condensation of alkoxide precursor, (ii) gelation and aging, and (iii) drying. The gels were cast

into several sized and shaped recipients (summarized in Table II.2): Pyrex® test tubes with a

different inner diameter and height, and disks with different inner diameter and height were

used in order to obtain aerogels with several size and shape. The specific shape (rod, plate or

square) of the gels was needed for some of the measurements of the aerogel properties [2, 3]

(Chapter III. Bulk characterization).

Table II.2 Characteristics of the recipients

Recipient

Material

Volume (cm3)

High (mm)

Diameter (mm)

Glass tube Pyrex® 50 10 30

Glass tube, g Pyrex® 25 140 15

Glass tube, p Pyrex® 15 12

Quimiboro tube Pyrex® 750 60 32

Culture tube Polystyrene 5 75 12

Petri Dish Polystyrene 38.5 10 35

The following steps were common to all routes. A solution was firstly prepared by adding the

solvent to metal precursor drop-wise while stirring. After five minutes, the water was added,

eventually containing the catalyst. The solutions of precursors in alcohol were brought to

desired pH by introducing the catalyst with a precision micropipette (Gilson P1000). pH of

reaction mixture during hydrolysis was monitored in some of the cases and values ranged

from 2.3 to 7.8. After stirring for fifteen additional minutes, the solution was distributed in

recipients. The recipients were tightly closed and kept either at room temperature or in a

constant temperature chamber (40°C) until gelification took place, then the gels were covered

with their respective solvents and left to age until supercritical drying. Gelation time depends

strongly on the initial conditions, it ranges from a few minutes to months. Some of the

obtained alcogels were aged at 40°C to accelerate the aging rate. In some synthesis, before the

supercritical drying, the alcogel was washed 3 times for 24 h, at room temperature, in a fresh

pure alcohol bath to remove water and oligomers that were still present in the liquid phase


[4]. In order to control the reproducibility, at least two samples were prepared at identical

conditions. Next figure shows a photo of the sample holder for those samples that were

soaked in the washing solution. This holder configuration allowed to facilitate the washing

process.

Figure II.1 Sample holder formed by six independent shelves. The net will allow that the gels follow a washing process.

Table II.3 gathers the experimental runs for gel synthesis with TMOS as alkoxide precursor

and Table II.4 with TEOS:

Table II.3 Synthesis experimental conditions: TMOS as alkoxide precursor, two different kind of solvent (methanol and acetone), with several acid and base catalysts

Solvent Catalyst

Methanol HCl

Methanol KOH

Methanol NH4OH+CH3COOH

Methanol NH4OH

Acetone NH4OH Acetone KOH Acetone -

- HCOOH

Sample-holder

Gel samples

Etanol bath


Table II.4 Synthesis experimental conditions: TEOS and H5 as precursor, three different kind of solvent, with several acid and base catalysts

Precursor Solvent Catalyst

TEOS Ethanol - TEOS Ethanol C6H8O7 TEOS Ethanol HCl TEOS Ethanol HNO3 TEOS Ethanol NH4OH TEOS Ethanol C2H2O4+NH4OH

TEOS Ethanol CH3COOH TEOS Acetone - TEOS Methanol HNO3

H5 Ethanol NH3

2.3 DRYING PROCEDURE

2.3.1 Supercritical drying at high temperature

Supercritical extraction took place in a high-temperature and high-pressure plant [5,6]. See

Annex II for the technical details of the equipment. Silica gels were placed inside the vessel

filled with the corresponding solvent. It allowed to maintain the solid part of the gel covered

with solvent until the two phases (gas and liquid) were no longer distinguishable. Supercritical

drying with alcohol requires a cautious manipulation of the autoclave because the alcohol is

flammable and has a high critical temperature. Alcohol supercritical drying has some

advantages in producing silica aerogels such as, good transparency and hydrophobicity

(characterization, Chapter II). As an example, Figure II.2 shows a process of a supercritical

drying cycle for a real experiment of the methanol aerogels. Each step of the process is

labeled. The methanol liquid-gas interface has been depicted (Pc=77bar, T c=240°C). Every

point of the graph corresponds to an elapsed time of five minutes.


0 50 100 150 200 250 300 350 400

0

50

100

150

200

Supercritical Cycle

Temperature (oC)

Pre

ssur

e (a

tm)

Methanol

Liquid

Gas

Supercritical Region

Figure II.2 Supercritical drying cycle used for a set of alcogels with methanol as a solvent.

Before starting the drying sequence, the reactor was sealed and flushed for a few minutes with

CO2 gas at 20¯25°C. Then, the pressure was raised up until the pressure reached 150 bars (100

bar for the acetone series) (step i). The pressurization was accomplished with compressed CO2

and the pressure was monitored and kept stable with great accuracy. Once the final pressure

was reached, the chamber was slowly heated from room temperature to a final temperature

to avoid thermal shock, varying between 230°C and 280°C (which is above the critical

temperature of alcohol) (step ii). Care must be taken not to reach too high temperatures to

avoid solvent decomposition or even its autoignition. The rate of heating was about 100°C/h.

The autoclave was flushed with CO2 for 2 hours after reaching the predetermined conditions

(step iii) keeping the high temperature and high pressure. The samples were kept in the

reactor for one more hour. Then, the solvent was removed by slow depressurization over a

period of 2h at a constant rate of 10bar/h (flow about 6ml/min) while the temperature was

maintained (step iv). After the pressure inside the autoclave reached atmospheric pressure,

autoclave was cooled down to room temperature at 40 °C /h (step v), before that, the reactor

was again flushed with CO2 for a few minutes. Finally, the vessel was opened and the aerogels

were removed [8- 10].

Figure II.3 shows a photograph of the sample holder for those samples that were dried at

high temperature without solvent washing. In that case, the gels were not subtracted from its

respective recipients.

(i)

(ii) (iii)

(iv) (v)


Figure II.3 Sample holder for the gels dried under, ethanol, methanol or acetone supercritical conditions without washing process.

Table II.5 shows the critical temperature and critical pressure of the used solvents, ethanol,

methanol, acetone, CO2, and other supercritical fluids frequently used.

Table II.5 Critical pressure, Pc, and critical temperature, Tc, of various solvents used as pore liquid

Solvent

Pc (atm)

Tc (°C)

H20 216 374

NH3 110 132

CO2 70 31

CH3CH2OH 62 243

CH3OH 77 240

CH3CN 47 275

Few experiments were not successful due to that the drying pressure was below the threshold

pressure (below which the gel network begins to shrink) [14] and consequently the dried gels

showed a significant shrinkage, even up to 70%.

2.3.2 CO2 supercritical drying

Supercritical drying with CO2 is widely used because it is a safer and cheaper method than

alcohol supercritical drying: CO2 is non-flammable and has a low critical temperature [15, 16,

17]. The gels were placed in a 2000 ml vessel and covered with the corresponding solvent.

Sample-holder

Pyrex tubes

Gel


The vessel was sealed and liquid CO2 was pumped inside at room temperature until the

pressure reached about 100 bars. Then the micrometer valve at the autoclave bottom outlet

was opened to allow the ethanol to flow out. The flow was about 6ml/min. The

displacement of ethanol, methanol or acetone by liquid CO2 took from about 4h to 12h,

which allowed the liquid CO2 to diffuse into the gel pores displacing the solvent. The solvent

was collected in another reactor for evaluation. After a complete substitution of the solvent

for liquid CO2, the outlet valve was closed, the pump was turned off, and the temperature

was raised up to 45°C (above the critical temperature of CO2). The ramping took about

30min, after which the vessel remained closed for another hour. Then the micrometer valve

was opened, the pump was turned on, and the system was flushed for 30min. The CO2 flow

was stopped and the system was slowly depressurized, over a period of 1 hour at a constant

rate of 10bar/h while the temperature was maintained at 45°C. After the autoclave reached

atmospheric pressure, the vessel was opened and the aerogels were removed [18]. The

characterization of the aerogels is shown in Chapter III (Bulk characterization).

3. SYNTHESIS ROUTES: EFFECT OF DIFFERENT ALKOXIDE

PRECURSORS

The effect of various precursors such as (a) tetraethoxysilane (TEOS) [19- 23], (b)

tetramethoxysilane (TMOS) [24- 32], and (c) prepolymerized tetraethoxysilane (H5) [33- 35]

will be presented and discussed in this section. For each type of alkoxide precursor, sol -gel

reactions were performed under acidic, neutral or basic conditions, in alcohols (methanol or

ethanol) or acetone. The use of these precursors allowed synthesis of a very large variety of

monolithic transparent, translucent, and opaque silica aerogels. A systematic and detailed

study was undertaken regarding the comparison of physical properties such as monolithicity,

bulk density, optical transmission, total surface area, porosity, pore size distribution, and total

pore volume (see chapter III: Bulk characterization). It was shown that these values are

strongly correlated.

The wet gels were also characterized by means of the gelification time, tg. The gelification

time is a parameter that characterizes the sol gelification velocity, and at the same time

determines the aerogel microstructure, resulting in a more polymeric aerogel for longer

gelification times.



3.1 TETRAMETHOXYSILANE: TMOS AEROGELS

The first series of silica gels were prepared from TMOS (tetramethoxysilane) in different

concentrations, different combinations of solvent (methanol and acetone) and catalyst (none,

HCl, potassium hydroxide, ammonium hydroxide, and NH4OH+CH3COOH). Tables II.6,

and II.8 gather the synthesis experimental conditions for the TMOS aerogels, when the used

solvent were methanol and acetone, respectively.

Table II.6 Synthesis experimental conditions, TMOS as precursor, methanol as solvent and different type of catalyst, several TMOS molar concentration (m=solvent/TMOS) versus water concentration (h=H2O/TMOS), and several catalyst concentrations (c).

Catalyst Label m h c

HCl TM99AQ 8 4 0.05

KOH TM99AR 8 4 0.05

NH4OH+CH3COOH TM96C-D 12.25 4 0.065

NH4OH M TM96B-E-F-H-97F-O-

98A-H-I-99AU 12.25 4 0.065

NH4OH TM00AR 3 2 NH4OH TM00AS 4 2

NH4OH TM00AU 4 4

The gels synthesized with TMOS and methanol as solvent under NH4OH catalyst

TMOS/EtOH/H2O= 1/12.25/4 were labeled as M gels. Those samples offered a good

reproducibility and very good quality.


Moreover, the gels series synthesized with TMOS with H2O/TMOS fixed at 4 and a variable

concentration of acetone as solvent was labeled as A-series gel. The gels were labeled as A1,

A2, A3, A4, when m= EtOH/TMOS= 1.22, 0.54, 0.32, 0.20. Those samples offered a good

reproducibility and good quality. Next Table II.7 lists these experiments.

Table II.7 Synthesis experimental conditions, TMOS as precursor, acetone as solvent and different type of catalyst, several precursors (m) versus water concentration (h), and several catalyst concentrations (c).

Catalyst Label m h c

NH4OH TM97E 12.25 4 0.065 KOH TM96A-I-J 12.25 4 0.065

- TM00AB-AW 0.1 4 -

- A05TM98F-G 3.87 4 -

- TM98O 3.87 2 -

- A1TM96N-97A-N-98E-99AO 1.22 4 -

- A2TM96K-R-97B-G-H-I-J-K-

M-P-Q-98A-B-99AJ-AN-AP-D-

00AC-AE

0.54 4 -

- A3TM96M-Q-97C-99AL-AQ 0.32 4 -

- A4TM96L-97D-L-P-98C-N-S-

AC-99Q-AM-AR-AS-00AD 0.20 4 -

- TM99O 0.54 2 - - TM98J-Q 0.20 2 - - TM98L-T 0.20 6 -

- TM98M-U 0.20 8 - - TM98V 0.20 16 - - TM98K-R 0.20 3 -

Another type of sol-gel syntheses route was performed at low temperature without needing

to use water. In these syntheses, supercritical CO2 was used as solvent, and HCOOH as the

condensation agent. Because the use of supercritical CO2 as solvent, it was necessary to

synthesize the gels directly inside the reactor. Next Figure II.4 shows a scheme of this

process.


Si (OR)4 + HCOOH 4SiO2 + 4HCOOR

Figure II.4 Aerogels produced with TMOS as alkoxide precursor, supercritical CO2 as solvent, and HCOOH as the condensation agent.

In parallel to this synthesis, some experiments were performed out of the reactor in order to

have an estimation of the gelification rate inside the reactor. Table II.8 lists several

HCOOH/TMOS molar ratios used.

Table II.8 Synthesis experimental conditions, TMOS as precursor and with HCOOH as condensation agent

Gel label HCOOH/TMOS

TM99F 6.67

TM99G 6 TM99H 4 TM99I 3

Two TMOS one-step routes were selected because of the quality and reproducibility of the

resulting products. The first set consisted of methanol as solvent and ammonium hydroxide

(NH4OH at 32%) as catalyst, M labeled samples. The total molar ratio of the reagents was

kept constant precursor/solvent/water/catalyst = 1/12.25/4/6.5·10 -2 [36]. In the second set,

no catalyst was added, being acetone the solvent [37]. In this case, different concentrations of

solvent were used. The water molar ratio, TMOS/water, was kept at four: A-series. The

scheme below shows these two synthesis routes:


Figure II.5 The two selected synthesis routes for the TMOS aerogels, M-series and A-series.

Moreover, a second route was prepared to compare two different drying cycles: some of the

gel samples were submitted to supercritical drying in acetone while the others were dried by

CO2 exchange (see supercritical drying section).

The two selected synthesis routes and a list of the gelation times is collected in Table II.9, the

given values were averaged for a number of synthesis processes.

Table II.9 Effect of synthesis conditions on gelation time for M and A-series: type of solvent, molar ratio, volume ratio and gelation time.

Label Solvent Solvent

Precursor

V

V

TMOS

(TMOS+solvent)

Gelation time (Mean value)

M Methanol 12.25 0.08 5 minutes A05 Acetone 3.874 0.05 180 days

A1 Acetone 1.224 0.10 90 days A2 Acetone 0.544 0.20 16 days

A3 Acetone 0.317 0.30 7 days A4 Acetone 0.204 0.40 2 days

TMOS

+

MetOH

NH 4 OH

+

H 2 O

Hydrolisis Polymerization

Gelling

Supercritical drying

Evaporation

TMOS

+ Acetone

H 2 O

Hydrolisis Polymerization

Gelling

Supercritical drying

Evaporation

M A-series


Figure II.6 shows gelification times for all the gels synthesized following the same A2 route.

Two groups can be differentiated. The first one shows a longer gelification time

corresponding to the gels with larger volume, 750 ml. Its mean gelification time was 30 days

and the standard deviation was of 4 days. The other group corresponding to 50 and 25 ml

presented a mean gelification time of 16 days and its standard deviation was of 4 days.

0 10 20 300

5

10

15

20

25

30

35

40

45

50

Small tubes

Large tubes

16 days

30 days

4 days

4 days

t g

elif

ication (

da

ys)

A2 samples

Figure II.6 Gelification time for the samples with 0.2 and h=4. The mean value for the gelification time was of 20 days for large tubes and 18 days for small tubes.

These results show that the size of the recipient it is an important factor influencing the

gelification time. The gel point was defined as the point in time at which the network of

linked silica oxide particles spans the container holding the sol. Then, it can be observed that,

larger recipient implied longer gelification time.

A similar study was done comparing samples that remained either at 40 C or at room

temperature. At 40 C, the mean gelification time was reduced for the 50 ml gels from 18 to 7

days (with a standard deviation of 30 hours) verifying that increasing temperature accelerates

the gelification rate.

The sol-gel reactions lead to a progressive decrease of the solution pH with time because the

hydrolysis reaction is generally more rapid and complete under acidic conditions, [ 38]

whereas the average condensation rate is generally maximized near pH=4. Figure II.7 shows

a decrease of pH during the gelation process in A1 gels.


Figure II.7 pH versus time during gelation process of TMOS at room temperature in acetone solution (A1 type gel).

3.1.1 The effect of the TMOS concentration

In the TMOS set, when acetone was used as solvent, different concentrations of acetone

were used. The water molar ratio, TMOS/water, was kept at value 4. The concentration of

the alkoxide was given by ,

acetoneTMOS

TMOS

VV

Vv (Eq. II.1)

The value was varied from 0.05 to 0.4. The samples were labeled as A05, A1, A2, A3 and

A4 corresponding to = 0.05, 0.1, 0.2, 0.3 and 0.4, respectively. In turn, these values

correspond to molar ratios of Acetone/TMOS of 3.874, 1.224, 0.5434, 0.317 and 0.204.

The dependency of the gelification time with the volume concentration, , was studied.

Figure II.8 shows the gelification time versus the concentration of TMOS in acetone. The

gelification times ranged from an average of three months for A1 to three days for A4, longer

times as expected, for more diluted sols [39]. The gelification time decreases exponentially as

a function of the TMOS/Acetone volume concentration. Table II.10 gathered several

characteristics of the different samples. The obtained values were averaged for a number of

synthesis processes.

0 20 40 60 80 100

5,4

5,6

5,8

6,0

6,2

6,4

6,6

6,8

7,0

7,2

7,4

A1 gel

pH

time (hours)


Figure II.8 Gelification time versus concentration, v for TMOS/acetone gels. The exponential fitting results in tgeli fication=180.e(-v/0.06).

The influence of the TMOS/acetone concentration of the sol on the aerogels can be assume

that when value decreases, i.e. decreasing the concentration of the precursor, the excess of

acetone increases the distance between the reacting species and hinder the progress of cross-

linkage of the siloxane chains (Si-O-Si) leading to the separation of SiO2 clusters in the sol.

This process leads to a decrease in the reaction rate and hinders the formation and growth of

the gel network particles, resulting in smaller particles and larger pore sizes [40] (see chapter

III: Silica aerogel characterization).

3.1.2 The effect of the nature of the solvent

The TMOS series of silica gels were prepared under different conditions by using various

solvents as pore liquids: methanol, ethanol, acetone, and CO2. Table II.5 showed their critical

pressure and temperature: Pc, and T c [41- 44].

Because water and alkoxysilanes are immiscible, a mutual solvent is normally used as a

homogenizing agent. The alcohol is not simply a solvent since, as indicated in equations I.2

and I.3, it can participates in esterification or alcholysis reactions.

The gelification times for methanol were of few minutes, for the acetone series they ranged

from an average of three months for A1 to three days for A4, longer times as expected for

longer solvent chains. The effects of the solvent, methanol or acetone, on the differences in

gelation time of the two solvents can be explained by considering two retarding factors: i) the

0,0 0,1 0,2 0,3 0,4 0,5

0

30

60

90

120

150

180

Fit y0+Ae

(-x/t)

y0=0, A=180, t=0.06

t g

elif

icat

ion

(d

ays)

v


hydrogen bonding and ii) the steric hindrence [36]. Steric (spatial) factors exert a great effect

on the hydrolytic stability of organoxylanes [45]. Any obstacle of the alkoxy group retards the

hydrolysis of alkoxysilanes, but the hydrolysis rate is mainly lowered by branched alkoxy

groups. If one compares the gel time under the same conditions, the retarding effect of the

ethoxide group in TEOS is evident. In the case of methanol, these two factors have very little

effect and hence the shortest gelation time was observed. Complexes are formed between

silicic acid and esters through hydrogen bonding [46] which retard the polymerization process.

The relative effectiveness of hydrogen bonding activity is the shortest for methanol, and the

highest for acetone, therefore the gelation time using methanol is shorter than for acetone.

Steric hindrence increases when acetone is used as solvent which will lead to larger pores and

hence a decrease in transparency of the aerogels (see chapter III: characterization).

With respect to supercritical drying, an amount of solvent was added into the autoclave. This

additional solvent can take part in the further dissolution of the silica gel [47].

Summarizing the effect of the solvent in TMOS gels:

i) Gelification time for methanol samples is shorter than for acetone samples due to the

effect of the hydrogen bonding and the steric hindrence of the solvents.

iii) Methanol gels are very transparent while acetone aerogels had white shading; their

opacity decreases with increasing TMOS content.

iv) When ethanol was used as a solvent (using TMOS as a precursor) cracked gels were

obtained.

v) Two good quality routes were selected for obtaining acetone monolithic structures

without cracks. The M gels were very transparent and strong. The A1 gels were especially

fragile.

Further study on the effects of the supercritical drying media on aerogel properties and

microstructures is essential for developing means of quality control in creating aerogels.

3.1.3 The effect of hydrolysis solution

Water amount

The amount of water used at the hydrolysis step was found to have a significant effect on the

texture of aerogels. The effect of water concentrations was examined for the samples

prepared at acetone/TMOS molar concentrations of 0.204 (v=0.4).


The dependency of the gelification time with the molar ratio h, TMOS

OHh 2 , was studied.

The reaction time increases very quickly with the water molar ratio, h. With an increase from

h=2 to a water excess over the stoichiometric amount (h=16), the time of gelation was

reduced from 3 days to 56 min. This effect is reflected in the exponential decreases of the

gelification time as a function of the water molar ratio (Figure II.9).

Figure II.9 Gelification time as a function of the hydrolysis constant, h, for gels with acetone/TMOS molar ratio of 0.204. tgel decreases exponentially as a function of water molar ratio.

Figure II.10 shows the variation of pH in a gel with acetone/TMOS molar ratio of 0.204

when water molar ratio, h, was increased from 2 to 16. The pH variation will be controlled by

the variation in hydrolysis and condensation rates.

0 2 4 6 8 10 12 14 16 18

0,0

2,0x103

4,0x103

6,0x103

8,0x103

1,0x104

1,2x104

1,4x104

1,6x104

Chi2 = 248014

y0= 398.1±309.0

A= 15770±1493

t = 3.1±0.38

t gelif

icació

n (

min

)

[H2O]/[TMOS]

tgelification=400+16.103.e-h/3.1.


Figure II.10 pH versus water amount for gels with acetone/TMOS molar ratio fixed at 0.204

3.2 TETRAETHOXYSILANE: TEOS AEROGELS

As the fumes from TMOS are toxic and may cause blindness, other esters of orthosilicic acid

like TEOS were used to obtain silica aerogels. TEOS is not only less toxic when compared to

TMOS but it is cheaper too. Hence, TEOS is a suitable precursor for the commercial

production of silica aerogels.

The experimental results on the influence of molar ratios of precursor, solvent and water of

TEOS silica aerogels are reported in this section. Alcogels were prepared by hydrolysis and

polycondensation of tetraethoxysilane (TEOS) in ethanol using either C6H8O7, HCl,

NH4OH, or NH4F+ NH4OH as a variable catalyst solution [48 - 50]. In order to identify the

optimal condition for producing the best quality TEOS silica aerogels in terms of

monolithicity, density and transparency, the molar ratios of EtOH/TEOS (0.15 m 9) and

H2O/TEOS (0.23 h 10) were systematically varied. The synthesis conditions are

summarized in Table II.10, 11, 12, and 13.

0 2 4 6 8 10 12 14 16 18

5,4

5,6

5,8

6,0

6,2

6,4

6,6

6,8

7,0

7,2

Acetone/TMOS 0.204/1

pH

h (H2O/TMOS)


Table II.10 Synthesis experimental conditions, type of precursor TEOS, as solvent EtOH, without presence of catalyst, precursor (m) and water concentration (h)

Label m h

TE99M07-N17 7 8

TE00AA-AF-AK-N5 5 7

TE99BS 9 6

TE99BG-M05-N13 7 6

TE99BU-M-N 5 6

TE99BW 4 6

TE99BV 3 6

TE00AA-W-CD-BZ-99M03-N7 7 5

TE00AM5b 6.9 5

TE99BI 7.6 5.4

TE00AMa 3.8 3.3

TE99C1 4 3.17

TE99C2 3.45 2.72

TE99C3 2.87 2.26

TE98W 2.9 2.2

TE99B 2.8 2.2

TE99C6 2.3 1.81

TE99C7 0.15 0.91

TE99C8 0.57 0.45

TE99C8 0.29 0.23

Table II.10 shows m and h values of TEOS gels produced without catalyst. m (EtOH/TEOS

molar ratio) has been changed from 0.3 to 7.6 and h (H2O/TEOS molar ratio) from 0.23 to

8. The most reproducible samples were TEOS/EtOH/ H2O molar ratio equal to 1/7/5 and

1/5/7.


Table II.11 Synthesis experimental conditions when using TEOS as alkoxide precursor, EtOH as solvent, several type of acid-catalyst, with a variable precursor and water concentration (m/h), and catalyst concentration (c).

Catalyst Label m/h c

C6H8O7 TE98X07bb 3/5 0.0001 C6H8O7 TE98X07 5/5 0.0009 C6H8O7 TE98X07bc 6/5 0.0001

C6H8O7 TE98X07bd 9/5 0.0001 C6H8O7 TE98X09-12 5/6 0.0001 C6H8O7 TE98X13-18 5/7 0.0001 C6H8O7 TE98X19-20 5/8 0.0001 C6H8O7 TE00AT 7/5 0.003 C6H8O7 TE00AV-99AM 7/5 0.0005

C6H8O7 TE98X01-99S1 7/5 0.0009 C6H8O7 TE98X01-99S2 7/5 0.0004 C6H8O7 TE99AO 7/5 0.01 C6H8O7 TE99AR 7/6 0.01 C6H8O7 TE00AA-99AN-AQ 7/6 0.003 C6H8O7 TE99AP 7/6 0.0005

C6H8O7 TE98X03 7/6 0.0001 C6H8O7 TE98X03bb 7/7 0.0001 C6H8O7 TE98X05 7/8 0.0001 C6H8O7 TE99AS 7/8 0.0005 C6H8O7 TE99AT 7/8 0.003 C6H8O7 TE99AU 7/8 0.01

HCl TE99AB 6.4/7.5 0.51 HCl TE99AF 6.8/4.6 0.058 HCl TE99BE 6.9/4 0.13 HCl TE99BF 6.9/4 0.2 HCl TE99AI00-02 6.9/5.2 0.21 HCl TE99AI03-04 6.9/5.7 0.23

HCl TE99AI05-06 6.9/6.2 0.25 HCl TE99BK 6.9/6.2 0.51 HCl TE99AI07-08 6.9/6.7 0.27 HCl TE99AI09-10 6.9/7.23 0.3 HCl TE99AI11-12 6.9/7.75 0.32 HCl TE99BK 2.3/6.5 0.53

HCl TE99BH 7/4.8 0.1 HNO3 TE99AG 6.8/5.5 0.03 HNO3 TE99AH01-02 2.7/0.4 0.23 HNO3 TE99AH03-04 2.7/0.5 0.23 HNO3 TE99AH05-06 2.7/0.6 0.23 HNO3 TE99AH07-08 2.7/0.7 0.23

HNO3 TE99AH09-10 2.7/0.9 0.23 HNO3 TE99AH11-12 2.7/1 0.23 HNO3 TE99AJ 3/10 0.3 HNO3 TE99AK 3/9.6 0.5


Table II.12 shows TEOS gels synthesized with ethanol as solvent and three types of acid-

catalysts: C6H8O7, HCl, and HNO3. When C6H8O7 was used, m (EtOH/TEOS) was ranged

from 3 to 9 and h (H2O/TEOS) from 5 to 8, the concentration of C6H8O7 was ranged from

1.10-3 to 1.10-2..

Table II.12 Synthesis experimental conditions when using TEOS as alkoxide precursor, EtOH as solvent, several base-catalyst, and a variable precursor and water concentration (m, h), and catalyst concentration (c).

Catalyst Label m/h c

NH4OH TE99AV 5/4 0.01 C2H2O4+NH4OH TE99AW 5/4 0.01 C2H2O4+NH4OH TE99BA 5/5 0.05

C2H2O4+NH4OH TE99AY 5/5 0.01 C2H2O4+NH4OH TE99AZ-BB 5/5 0.005

CH3COOH TE99AX 5/4 0.01

Table II.13 Synthesis experimental conditions when using TEOS as alkoxide precursor, acetone or methanol as solvent, several catalyst, precursor and water concentration (m, h), and catalyst concentration (c).

Solvent Catalyst Label m/h c

Acetone - TE97A0 0.317/4 - Acetone - TE97B0 0.204/4 - Methanol HNO3 TE00AM'-AN-AÑ-AP 4.3/ 5 0.05

The solutions were kept either at room temperature, 25°C or at 40°C for gelation. In some

cases, to accelerate the aging time in base-catalyzed gels, the gels were first soaked in an aging

solution, that is produced by water and catalyst diluted in ethanol (of equal proportions to the

original sol), for 24 hours at room temperature. In addition, in a second step were soaked in a

washing bath of ethanol for 3 times from 6 to 72 hours at room temperature. This step is

diffusion controlled, then the time required for washing increases strongly whit the gel

thickness. Generally, the volume of the washing and aging solution was approximately three

times the volume of the gels. It is observed that during aging, the wet gels gain weight.

A statistical study was performed on the variation of the gelation time on the aerogels

obtained using the same synthesizing route, TEOS/EtOH/H2O=1/7/5, to ensure

reproducibility of the series.


3.2.1 The effect of the TEOS concentration

Neutral

A study was done using the EtOH/TEOS series without presence of catalyst. The molar

ratio water/TEOS, h, was fixed at 6 and the EtOH/TEOS molar ratio was varied from 3 to

9. Monolicithy and homogeneity of the gels obtained were good except for samples with too

high TEOS concentration, which had cloudy parts in the aerogel center as reported in

chapter III. Next Figure studies the dependency of gelation time with the molar ratio

EtOH/TEOS when h was fixed at 6. The gelation time was large compare to those gels

synthesized when using catalyst.

Figure II.11 Gelation time versus EtOH/TEOS molar ratio for water/TEOS=6 and without using catalyst.

The gelation time showed an exponential dependency versus EtOH/TEOS molar ratio.

Summarizing the effect of ethanol content on the TEOS gels, it can be concluded, that an

increase in solvent content reduces the probability of mutual collisions of hydrolyzed

alkoxides molecules (Si-OH), resulting in a decrease in the rate of polymerization reaction.

Moreover, although ethanol acts as homogenizing agent to promote hydrolysis of TEOS, an

increase of the ethanol content reversed the hydrolysis and polymerization processes and

promoted esterification (Eq. II.2 and Eq. II.3):

Si-(OH) + C2H5OH Si-(OC2H5) + H2O Esterification ( Eq. II.2)

Si-O-Si + C2H5OH Si-(OC2H5) + (HO)-Si Alcholysis (Eq. II.3)

3 4 5 6 7 8 9

0

48

96

144

192

240

288

336

y0

0 0

A 0.37 0.03

t0

1.33 0.02

y = y0 + Ae

x/t0

Water/TEOS= 6

no catalyst

Gel

atio

n t

ime

(ho

urs

)

molar ratio EtOH/TEOS


Acid catalyst

To establish the effect of precursor concentration in alcohol the gels were prepared by fixing

the molar ratio of H2O/TEOS ,h, constant at h=5 (excess over the stoichiometric amount)

and the catalyst (citric acid) concentration at 0.0001M. The molar ratio of EtOH/TEOS (m)

was changed from 3 to 9. All samples were treated in the same way.

As the ethanol content increased, the gelling process was significantly retarded, as shown in

Figure II.12. This was due to the fact that excess of ethanol separates the molecular species

formed and hinders the progress of cross-linkage of the siloxane bond chains (Si-O-Si),

leading to the separation of sol clusters.

Figure II.12 Gelation time versus EtOH/TEOS molar ratio at h=5 and 0.0001M of citric acid. The gelation time increased exponentially with the EtOH/TEOS molar ratio.

The time of gelation increased from 30 to 60 hours when increasing the EtOH/TEOS molar

ratio from 3 to 9. The same study was repeated adding water concentration at 6, 7, 8, and 10

or at lower values from 3 to 4.

Figure II.13 shows the tg dependency on EtOH/TEOS molar ratio when h (water/TEOS

molar ratio) was fixed at 8.

3 4 5 6 7

35

40

45

50

55

60

y0

36.02 ±0.16

A1

0.44 ±0.03

t1 1.88 ±0.03

y = y0 + A

1e

x/t1

Water/TEOS= 5

0.0001M

Gel

atio

n t

ime

(ho

urs

)



Figure II.13 Gelation time versus EtOH/TEOS molar ratio for samples with water/TEOS ratio of 8 and acid citric 0.0001M as catalyst

When h was fixed at 8 the rate of gelation when increasing TEOS concentration increased

faster than for h=5, so increasing the water concentration the gel rate of reactions are more

sensitive to the rise of ethanol concentration.

The acid-catalyst used in the following series was 52%HF. The acid TEOS series was

obtained by first synthesizing the more concentrated solution, AHF1, and then diluting it in

different volumes of ethanol, AHF2, to AHF5. Next table shows the acid-catalyzed gels (HF)

with water/TEOS fixed at 25.

Table II.14 Experimental parameters of TEOS gels with acid-catalyst (HF), with water/TEOS molar ratio fixed at 25.

Label EtOH/TEOS Optical

transparency

AHF1 12.23 Transparent



AHF 33.65 Transparent


Label TEOS H2O EtOH 52% HF

A1 25ml 50 ml 80ml 0.4ml

3 4 5 6 7

12

16

20

24

28

y0 13.25 ±0.41

A1

0.23 ±0.08

t1

1.83 ±0.16

y = y0 + Ae

x/t

Water/TEOS= 8

0.0001M

Gel

atio

n t

ime

(hours

)



All the series of acid aerogels obtained with TEOS, acid-catalyst (HF), and with water/TEOS

molar ratio fixed at 25 were monoliths very transparent and without cracks.

Figure II.14 Synthesis scheme for the TEOS gels with acid-catalyst (HF). In this case, the alkoxide solution was dissolved in ethanol and then mixed with the hydrolysis solution dissolved in ethanol.

Base-catalyst

The base-catalyzed gels were obtained using a base-catalyst of 30%NH3 +NH4F 0.5M. The

series was obtained by first synthesizing the more concentrated solution, B1, and then

diluting it in different concentrations.

Figure II.15 Synthesis scheme for the TEOS gels with base-catalyst (NH3+NH4F)

TEOS + EtOH EtOH + H2O

+52% HF

SOL

Gelification

time

GEL

TEOS + EtOH EtOH + H2O +

30%NH3+ 0.5M NH4F

SOL

Gelification

time

GEL


Table II.15 summarizes the basic TEOS gels produced at several dilutions with a fixed water

ratio (h= 10) and with h=25.

Table II.15 Experimental parameters of TEOS gels with water/TEOS molar ratio fixed at 10 and 25 and base-catalyst (30%NH3+0.5MNH4F) concentration of 3.3.10-3.

Label EtOH/TEOS Optical

transparency

B1 17 Transparent

B2 25 Transparent

B3 66 Translucent

B4 91 Translucent

B5 116 Opaque

B1’ 12.23 Transparent





Label TEOS H2O EtOH 30%NH3 0.5MNH4F

B1’ 25ml 50 ml 80ml 0.055ml 0.055ml

B1 25ml 20 ml 110ml 0.055ml 0.055ml

The obtained samples presented different shrinkage and transparency. Some of the aerogels

with highest density were cracked (B1 and B2). In order to obtain more transparent aerogels

the water/TEOS molar ratio was increased up to 25. When h=25, the gels were slightly more

transparent than when water amount was 10. All gels were monolithic.

In some samples, acetone was used as a solvent and non-monolithic gels were obtained. The

acetone amount was fixed at two and the water amount was varied from 0.017 to 0.2

(TE97A). The initial solution appeared translucent and, in the cases with lowest h, divided in

two phases. Upper phase was more opaque and lower phase was transparent. For the largest

h, appeared as translucent solution including a white precipitated (indicating that the amount

of water was too large). Therefore, no monolithic gels were produced when synthesizing

TEOS gels with acetone as solvent.

In conclusion, the aerogels had better quality when TEOS was diluted with i ts respective

parent alcohol (EtOH) because it avoided trans-esterification.


3.2.2 The effect of hydrolysis solution

Water amount

The amount of water used at hydrolysis step was found to have a significant effect on the

texture of aerogels. Water directly participates in the hydrolysis reaction, and can be described

by three successive steps:

(OH)nSi(C2H5O)4-n+4H2O (OH)n+1Si(C2H5O)3-n +C2H5OH (Eq. II.4)

where n=0, 1, 2, 3, and 4.

From Equation II.4, an increased value for h is expected to promote hydrolysis reaction.

Depending on the water/TEOS molar ratio, the following two condensation reactions

occurred after the initial hydrolysis:

(Eq. II.5)

(Eq. II.6)

Then, the initial relative amount of water determined the distribution and number of the

hydrolyzed monomers formed. The amount of water formed during the condensation

reactions (Eq. II.6) is half to the water amount used for the formation of silanols (Si-OH)

(Eq. II.5).

The effect of water concentrations was examined by keeping the EtOH/TEOS molar

concentration constant at 7. At the same time, the catalyst concentration was kept constant at

0.001M. The dependency of the gelification time with the molar ratio h, H 2O/TEOS, was

studied (Figure II.16).


Figure II.16 Gelation time versus water/TEOS molar ratio for gels with EtOH/TEOS molar ratio fixed at 7 and acid catalyst 0.0001M.

The same dependency was observed for gels with acid citric as a catalyst but with smaller

gelation time. The most obvious effect of increasing h value is the acceleration of hydrolysis

reactions (Eq. II. 4), the reaction time increased very quickly with the water molar ratio, h.

The same study was performed in EtOH/TEOS fixed at 7 without the presence of catalyst.

The tg dependency is more pronounced than with acid catalyst but the gelation time was

much slower.

4 5 6 7 8 9

24

30

36

42

48

54

60

66

72

78

84

y=y0+Ae

-x/t0

y0

22.77 0.66

A 5623.9 1661.0

t0

0.97 0.05

EtOH/TEOS = 7

acid citric 0.0001MG

elat

ion t

ime

(hours

)

molar ratio water/TEOS


Figure II.17 Gelation time versus water/TEOS molar ratio for gels with EtOH/TEOS molar ratio fixed at 7 and no use of catalyst

In Figures II.16 and 17, gelation time, tg, was shown as a function of water/TEOS molar

ratio, h. As the h value increase, gelation time first decreased sharply, then for h values

greater than 6 there was a gradual decrease in gelation. This means that for a complete

hydrolysis, h should be at least 6. This higher value over the theoretic stoichiometric value is

because of the steric hindrance of Si-OC2H5 groups. Then, adding water for ratios h=6, the

corresponding value for the complete consumption of the initial water, will not decreases

very strongly the gelification time. It was found that at low h values (h 4) the tg was very

large. For h values 4, the hydrolysis reaction proceeded rapidly to the complete

consumption of the initial water. This observation indicates that the rate of hydrolysis

reaction (Eq. II.1) was much faster than the sum of the rates of the water and ethanol

producing condensation reactions (Eq. II.2)+ ( Eq. II.3). Then, the t g was very large because

the low number of condensation reactions. Also, in this h range, was observed that gelation

time decreased when h increased. With h 4 an increase of tg is more pronounced because

the hydrolysis and the resulting condensation reactions were not completed, leading to a very

limited cross-linking of the silanols.

For larger h values (4<h<6) it was observed a general decreased in tg. This indicated that

when adequate amount of water is present, tg was probably governed primarily by

condensation. For 4 h 6, the ethanol producing reaction (Eq. II.2) dominated, while water-

producing condensation (Eq.II.3) dominated for h 6 [51]. The consequence of these

4 5 6 7 8 9

24

48

72

96

120

144

168

192

y0

46.67

A 157478.31

t0

0.64

y = y0 + Ae

-x/t0

EtOH/TEOS= 7

no catalyst

Gel

atio

n t

ime

(ho

urs

)



reactions was a decrease in the concentration of monomeric molecules and an increase in the

concentration of oligomers.

On the contrary, at h 6, the rate of hydrolysis increased, enhancing the gelation process that

increased the cross-linkage of siloxane bonds chains (Si-O-Si) leading to a three-dimensional

gel network. This explains the general decrease in tg at larger h values.

Influence of the amount of the catalyst

The influence of the amount and the type of the catalyst on the properties of aerogels was

studied. Tetraethoxysilane easily hydrolyzes in the presence of water, the rate of hydrolysis

and condensation reactions depends on pH of the solution [52, 53].

The conditions that were considered the optimal concentrations of reagents were for 7 initial

molar concentration of TEOS in ethanol, and with excess of stoichiometric amount of water

5, 6, and 8. Therefore, the influence of catalyst on these three ratios was studied. The

following range of acid citric ratio was tested:

0.0005 < Ccitr ic acid < 0.1

TEOS/EtOH/H2O

1 / 7 / 5, 6, 8

The variation of citric acid concentration leaded to change in pH as shown in Figure II.18

and then, induced changes in the rate of reaction (Figure II.19).



Figure II.18 pH of the sol versus the citric acid concentration in TEOS/EtOH/H2O gels 1/7/8.

Increasing the citric acid concentration from 0.0005 to 0.1, the pH was changed from 5 to

3.5.

Table II.16 Synthesis experimental conditions for TEOS as precursor, ethanol as solvent, citric acid as catalyst, and several precursor and water concentration (EtOH/H2O), and catalyst concentration (Ccitr ic acid). Gelation time (tg) and transparency of the resulting gels.

EtOH/H2O Ccitric acid tg (h) transparency

7/5

0.0005 44-141 translucent

0.003 47 transparent 0.01 138 very transparent

7/6

0.0005 44 translucent

0.003 41 transparent

0.01 69 transparent

7/8 0.0005 23 opaque 0.003 23 opaque

0.01 46 very transparent

1E-3 0,01 0,1

3,4

3,6

3,8

4,0

4,2

4,4

4,6

4,8

5,0

pH 1/7/8

pH

log10

(catalyst concentration)


Gelification was quite rapid for all the set samples mostly for 7/8. Between 0.01 and 0.0005

M, gels were monolithic. The gels obtained at EtOH/H2O = 7/5, 7/6, 7/8 have no cracks

and give a good quality aerogels.

Figure II.19 Gelation time as a function of the citric acid concentration for 1/7/6 TEOS/EtOH/water molar ratio

It was found that the role of catalyst concentration was very important in the formation of

silica alcogels for a mixed molar ratio of TEOS/EtOH/H2O at 1/7/6 (Figure II.19). In the

case of lower catalyst, ( 0.005M) only turbid gels were obtained. This may be due to the

formation of silica precipitates. On the other hand, catalyst concentrations 0.001M lead to

the formation of transparent alcogels. Figure II.20 shows the gelation time as a function of

catalyst concentration. It is clear that a maximum gelation time was obtained for 0.001 M

citric acid whereas at lower and higher concentrations of the citric acid the gelation time

decreased.

Hydrolysis (substitution of OR for OH) or condensation (substitution of OSi for OR or OH)

decreases the electron density on silicon (see Figure II.21). This line of reasoning leads to the

hypothesis that under acidic conditions, the hydrolysis rates decreases with each subsequent

hydrolysis step (electron withdrawing), whereas under basic conditions each subsequent

hydrolysis step occurs more quickly as hydrolysis and condensation proceed [54]. Then, the

decrease of tg at lower concentrations of acid catalyst may be due to the fact that at lower

catalyst concentrations the rate of condensation overcompensates the increased surface

charge of all ions in the sol and provided for a faster gelation [55].

10-4

10-3

10-2

30

40

50

60

70

80

G

elat

ion

tim

e (h

ours

)

log10

(concentration of the catalyst (citric acid))


On the other hand, at higher catalyst concentrations hydrolysis and condensation rates are

high which lead to decrease surface charge of all ions in the sol and hence a faster gelation

[56].

Figure II.20 Inductive effects of substitution attached to silicon, R, OR, OH or OSi [56].

Figure II.21 Gelation time versus water/TEOS molar ratio for several catalyst concentrations. Gels with EtOH/TEOS fixed at 7 and citric acid as catalyst

Influence of the nature of the catalyst

Although the hydrolysis and condensation can proceed without the involving catalyst, their

use is quite useful. The understanding of catalytic effects is often complicated. There are two

interlinked effects that play an important role: the acidity of silanol groups (Si-OH) increases

with the extent of the hydrolysis and polymerization, and the effects of reverse reactions that

become increasingly important with greater concentration of water and /or base. Hence, to

4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0

20

40

60

80

100

120

140

0.0001M

0.0005M

0.001M

0.003M

EtOH/TEOS = 7

acid citric

Gel

atio

n t

ime

(hours

)


-OSi

Si−OH

-OR

- R

Increasing acidity

(electron withdrawn) OR

RO


facilitate the study of the catalyst in the synthesis, sol-gel parameters were kept constant and

then the type of catalyst was varied.

Previous work indicated that using base-catalyzed in the synthesis with large water ratio (h)

produced highly condensed ‘particulate’ aerogels whereas acid catalyzed with low h produced

branched ‘polymeric’ sols. Intermediate conditions produce structures intermediate to these

extremes. See Figure II.22.

Figure II.22 Scheme of gel syntheses routes either under basic or acid conditions

The understanding of catalytic effect is complicated because with continue hydrolysis and

condensation the acidity of silanol groups (Si-OH) increased, and because the effects of

reverse reactions become increasingly important with greater concentrations of water or base.

In this section, numerous acid or base catalysts were used. The basic catalysts used were

NH4OH (weak basic catalyst), and NH3+NH4F. The acid catalyst used were HF, HCl, HNO3

and C6H8O7 and the mixture of some of these acids with NH4OH, NH4OH+C2H2O4. Their

syntheses have been described in the previous sections of this chapter. Table II.18 gathers

the gel time (tg) and transparency of the experimental synthesis that resulted in monolithic

gels.

monomer

dimer

cycle

particle

.

Base

conditions

Sols

10 nm

5 nm

Acid

conditions

100

nm

1 nm

Gel: three

dimensional

network

30 nm

monomer

dimer

cycle

particle

.

Base

conditions

Sols

10 nm

5 nm

Acid

conditions

100

nm

1 nm

Gel: three

dimensional

network

30 nm


Table II.17 Synthesis experimental conditions for TEOS as precursor, ethanol as solvent, and several kind of catalyst, gel time (tg) and transparency of the resulting gels.

Gel label Catalyst tg Gel

TE99AV

(1/5/4) NH4OH 0,01M 2 days

Transparent monolith,

opaque gel

TE99AW (1/5/4)-

AY(1/5/5)

C2H2O4+NH4OH

0.01M 6 days

Transparent, monolith

transversal crack.

TE99AZ-BA-BB

(1/5/5)

C2H2O4+NH4OH

0.005M 23 hour Turbid monolith

TE99AZ? NH3+NH4F

2 days


opaque gel

TE99AW?

C2H2O4

1 day


and opaque gel

0.03M TE99AG HNO3 (1/7/5) 19 days White gel

0.01M TE99AX CH3COOH (1/5/4) 69 hours Monolith, transparent

TE99BL,TE99BJ HCl (1/7/5) 12days+2 h Cracked gel

For all the catalysts when the gels were set with low catalyst concentration (c<10 -4 M), gels

were turbid, with c > 1M they were transparent. Turbid alcogels were obtained for the

mixtures of acids and bases at lower concentrations (c<10 -2 M).

1. Strong acid + weak basic mixture used as catalysts gives transparent but cracked gels.

It is reported that the hydrolysis rate is faster under strongly acidic conditions [57].

2. Weak acid + weak basic mixture gives shrunk and semi-transparent gels.

3. Weak basic gives the best gel.

3.3 ‘TWO-STEP’ SYNTHESIS

This section deals with the achievement in the optimization of the two-steps preparation

process roughly corresponding to previous reported recipes [42, 58, 59]. This process was

performed using prepolymerized TEOS obtained by the reaction of tetraethoxysilane

(TEOS) and sub-stoichiometric amount of water in the presence of acid catalyst. This acid

step involves the formation of partially hydrolyzed and partially condensed silica mixture in

ethanol, leaving a viscous fluid containing higher molecular weight silicon alkoxy-oxides. In a

second part, this viscous mixture is re-dissolved in ethanol with additional water under basic

conditions [60]. Gels prepared in this way are known as ‘two-step’ acid-base catalyzed gels.

The chemical reactions carried out by this two-step process can be summarized as:


(1) The first step under acid conditions, which operated in concentrated medium, enhanced

hydrolysis and formation of dimmers and oligomers.

(2) The second step under basic conditions that enhanced polycondensation and allowed

dimmers and oligomers to stick in order to form large clusters involving very large pores.

This route permitted us to obtain uncracked transparent aerogels with low densities. Specific

surface area values were in the range of usual aerogels. (See chapter III, bulk

characterization). To establish the effect of several synthesis routes all gels were achieved

following the same receipt. The first step was replaced for using the commercially available

Prehydrolyzed Ethyl Silicate (H5) as prepolymerized precursor from Silbond Corp. (Silbond,

H5).

The second step was performed mixing:

i) The silica solution containing a variable volume of precondensed silica (v H5) diluted in

ethanol.

ii) With the catalyst solution, that was containing EtOH/H2O/NH3 or NH4F+NH3 with

variable amounts according to EtOH/H2O ratio. Nevertheless, the volume ratio vNH3/vH5

was varied from 0.1 to 0.02.

Then after gelification, for enhancing the aging process, the gels were soaked in an

alcohol/water/catalyst mixture of equal proportions to those of the original sol. The gels

were maintained in this solution up to 24 hours.

Figure II.23 Aging bath including H2O, EtOH and catalyst

After aging the gels, all water still contained within the pores was removed prior drying by

soaking the gels in pure ethanol several times.

Effect of precursor concentration

The effect of precursor concentrations was examined by keeping the H 2O/H5 volume

concentration constant at 1.5 and the catalyst concentration was kept constant at 7.10 -3 M.

H2O+EtOH+ NH3

18ml 20ml 0.08ml


In order to study the dependency of the gel time with the volume ratio the vEtOH/ vH5

concentration was changed from 0.8 to 2.5.EtOH/H5. (see Figure II.22). Table II.19

summarizes the two-steps conditions: volume amounts and volume ratios for precursor,

ethanol, and NH3

Table II.18 Volume amounts and volume ratios for two-steps aerogels with H5 as precursor, ethanol as solvent, and NH3 as base-catalyst

H5 H2O EtOH NH3

Volume

(ml) 12 18 VETOH 0.08

Volume

ratio 1 1.5

Variable

VETOH/VTEOS 7.10-3

Table II.20 gathers the gelation time and transparency of a series of ‘two-steps’ aerogels

synthesized at vH5/vH2O/vNH3 1/1.5/7.10-3 with a variable amount of ethanol, VETOH/VTEOS

Table II.19 Gelation time and transparency of a series of ‘two-steps’ aerogels synthesized at vH5/vH2O/vNH3 1/1.5/7.10-3 with a variable amount of ethanol, VETOH/VTEOS

Label VETOH/VH5 tgel Observations

H501AF00-05 0.8 - White precipitate

H501AG00-05 1.25 30’ Translucent gels

H501AA00-05 1.7 1h Transparent gels

H501AH00-05 2.1 1h30’ Transparent gels

H501AI00-05 2.5 2h45’ Transparent and very soft gels

H501AJ00-05 3.3 4h45’ Transparent gels

The 36 gels (six samples for each molar ratio) were soaked in an aging solution for 24 hours,

and after soaked 4 times in pure ethanol for 10 days. It should be pointed out that the

washing process increased strongly the quality of the aerogels and decreased the shrinkage

during supercritical drying.


Figure II.24 shows the exponential dependency of gelation time versus volume ratio

EtOH/H5 for the two-steps gels with EtOH/H5 molar ratio fixed at 1.5 and base-catalyst

(NH3).

Figure II.24 Gel time versus water/H5 volume ratio for gels with EtOH/H5 molar ratio fixed at 1.5 and base-catalyst (NH3).

It was observed the same gel time dependency versus EtOH/TEOS volume ratio that one-

step gels with TEOS and acid citric as a catalyst. The reaction time increased sharply with the

ethanol/H5 volume ratio. For volume ratios lower than one a white precipitate was

produced, gels were not formed.

Effect of water amount

As the same effect that one-step gels, the amount of water used at hydrolysis step was found

to have a significant effect on the texture of aerogels. The effect of water concentrations was

examined by keeping the catalyst concentration constant at 8.7.10 -5.

1,0 1,5 2,0 2,5 3,0

20

40

60

80

100

120

140

160

180

200

y = y0 + A

1e

x/t1

y0

9.12367

A1

6.11881 4.9821-144

t1

0.65685

vH2O

/vH5

= 1.5

G

elat

ion

tim

e (m

in)

volume ratio EtOH/H5


Table II.20 Synthesis experimental conditions for H5 as precursor, ethanol as solvent, and NH3 catalyst.

H5 H2O EtOH NH3

Volume (ml) 14 32 14 0.01

Volume ratio 1 2.3 1 7x10-4

molar ratio - 1 0.13 8.7x10-5

molar ratio - 7.4 1 6.5x10-4

The gel time was too short because water was added in excess, then the amount of water was

decreased and homogeneous clear and gels were produced.

Table II.21 Synthesis experimental conditions for H5 as precursor, ethanol as solvent, and NH3 catalyst

H5 H2O EtOH NH3

Volume (ml) 14 21 24 0.01

Volume ratio 1 1.5 1.7 7.10-4

In this synthesis, the gels were not diluted because the quality of the alcogels was very good

resulting in non-cracked and transparent samples.

Influence of the amount of the catalyst

The influence of the amount of the catalyst on the properties of aerogels was studied. The

conditions that were considered the optimal concentrations of reagents were for

vH5/vEtOH/vH2O 1/1.5/1.7 initial volume concentration of H5 in ethanol with presence of

water. Therefore, the influence of the amount of catalyst was studied. The following range of

NH3 was tested:

0.002 < xNH3 < 0.1 vH5/vEtOH/vH2O

1/1.5/1.7

Table II.22 Synthesis experimental conditions for H5 as precursor, ethanol as solvent, and a variable amount of NH3 catalyst, c.

H5 H2O EtOH NH3

Volume(ml) 14 21 24 x (volume ratio)

Volume ratio 1 1.5 1.7 c (molar ratio)


Table II.23 H5 aerogels synthesized with vH5/vEtOH/vH2O fixed at 1/1.7/1.5 and with a variable amount of NH3 catalyst, XNH3.

Label XNH3 tgel

H501AA00-05 0.08 1 h

H501AB00-05 0.04 1h 30’

H501AC00-05 0.06 1 h

H501AD00-05 0.10 <1h

H501AE00-05 0.02 12h 10’

All the 30 gels (6 for each catalyst concentration) obtained in the series of two-steps aerogels

synthesized with vH5/vEtOH/vH2O fixed at 1/1.7/1.5 were monolithic and very transparent.

4. SUMMARY AND CONCLUSIONS

For aerogels using TMOS as alkoxide precursor

To obtain transparent aerogels the best synthesis was by using methanol as solvent with the

presence of base catalyst at low concentrations.

For aerogels using TEOS as alkoxide precursor

The best quality TEOS aerogels, in terms of monolithicity and transparency, without much

shrinkage were obtained by using weak base or acid catalyst:

1. Tetraethoxysilane concentration in alcohol: 10¯40 volume%,

2. Presence of low concentration catalyst.

3. Excess of stoichiometric amount of water (1mol/mol of oxyethylene group).

It was found that the gel time varied widely from few minutes to several days, depending on

the type of solvent and catalyst combinations, with shortest tg being for methanol solvent and

sodium hydroxide catalyst. This effect is a result of the shortest chain length and branching

of the solvent, and the effect of the catalyst. Strong acidic catalyst gave transparent but

cracked aerogels, whereas weak acids yielded monolithic, transparent aerogels.

For all the precursors, it was observed a decrease of gel time either increasing the amount of

water or decreasing the concentration of the precursor. With the presence of low

concentration of catalyst, the reaction was more easily controlled.

The conditions that were considered optimal concentration of reactive were for 1/5/7 and

1/7/5, 6 or 8 with low concentration of acid citric catalyst (0.01<c<0.03). But, summarizing

one can state that from the point of view of the texture, the most favorable condition of the

preparation were when the ‘two-step’ method was followed and water was removed by


soaking the gels in an ethanol solution. In that case, non-cracked and very transparent

aerogels were obtained with a high surface area and a varied density (depends on the

vEtOH/vH5 from 0.03 to 0.1).

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Materials Chemistry and Physics Volume 53,

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Venkateswara Raoa, and G. M. Pajonkb

Materials Chemistry and Physics Volume 50,

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Chapter III. Bulk silica aerogel characterization 75

C h a p t e r I I I

BULK SILICA AEROGEL CHARACTERIZATION

SECTION OUTLINE

1. MONOLITHICITY, BULK SHRINKAGE, DENSITY AND POROSITY .................................... 75

1.1 TMOS AEROGELS ........................................................................................................................ 76

Skeletal density......................................................................................................................... 76

Bulk density.............................................................................................................................. 76

1.1.1 Supercritical drying at CO2 conditions ............................................................................ 80

1.2 TEOS AEROGELS ......................................................................................................................... 81

1.2.1 TEOS aerogels synthesized without presence of catalyst ............................................ 82

1.2.2 Base-catalyst .......................................................................................................................... 84

1.2.3 Acid catalyst ........................................................................................................................... 87

Fluorhydric acid ....................................................................................................................... 88

Citric acid ................................................................................................................................. 90

1.3 TWO-STEPS SYNTHESIS ........................................................................................................... 91

2. SURFACE AREA MEASUREMENTS BY BET (BRUNAUER, EMMET AND TELLER) ..... 94

3. INFRARED SPECTROPHOTOMETRY, IR......................................................................................... 99

3.1 METHANOL SERIES ................................................................................................................. 100

3.2 ACETONE SERIES ..................................................................................................................... 102

4. ULTRAVIOLET-VISIBLE (UV-VIS) SPECTROSCOPY ...................................................................103

4.1 AEROGEL TRANSPARENCY ................................................................................................. 103

4.2 RAYLEIGH SCATTERING ...................................................................................................... 107

4.2.1 A model to interpret the porous aerogel structure using Rayleigh scattering ........ 110

5. LIGHT SCATTERING MEASUREMENTS OF AEROGELS BY A POLARIZATION-

MODULATED NEPHELOMETER. ................................................................................................................ 113

5.1 INTRODUCTION TO LIGHT SCATTERING VS ANGLE EXPERIMENTS .........113

5.1.1 Description of the polarization-modulated nephelometer.......................................... 114

5.2 EXPERIMENTAL RESULTS ....................................................................................................115

5.3 STRUCTURAL INFORMATION FROM THE LIGHT SCATTERING

MEASUREMENTS .......................................................................................................................118

5.3.1 Inhomogeneous media....................................................................................................... 118

Short range correlations: Rayleigh scattering....................................................................... 120

Long range correlations: departures from Rayleigh scattering ........................................... 121

5.4 COMPARATIVE STUDY BETWEEN EXPERIMENTAL MEASUREMENTS AND

THEORY ......................................................................................................................................... 122

5.5 CONCLUSIONS AND FUTURE WORK.............................................................................. 125

6. DIRECT METHODS: ELECTRON MICROSCOPY ........................................................................127

6.1 STRUCTURAL STUDIES BY SCANNING ELECTRON MICROSCOPY .............. .127

6.1.1 Acetone series .......................................................................................................................127

6.1.2 Effect of the solvent............................................................................................................. 131

6.1.3 Drying procedure.................................................................................................................132

6.1.4 TMOS aerogels in CO2 as solvent....................................................................................133

6.2 TRANSMISSION ELECTRON MICROSCOPY ................................................................ 134

Sample preparation .................................................................................................................... 134

TEM set-up.............................................................................................................................. 134

6.2.1 Imaging the acetone-series silica aerogels.....................................................................135

Direct visualization .................................................................................................................... 135

6.2.2 Imaging the methanol silica aerogels ........................................................................... .138

Replicas visualization ................................................................................................................. 138

7. REFERENCES ............................................................................................................................................. 141


Previous studies have shown that it is possible to control such physical properties as the

porosity and transparency of the aerogels by adjusting the so-called sol-gel parameters. These

parameters include the type and concentration of the alkoxide precursor, acid or base catalyst,

and water content [1-3]. These parameters affect the structure of the initial gel and, in turn, the

properties of the resulting aerogel. In this chapter, it has been taken up a detailed study

regarding the comparison of physical properties of silica aerogel monoliths synthesized by

different recipes (see chapter II synthesis optimization). Then, each section containing the

characterization of a different physical property is arranged in subsections with the different

series of silica aerogels. The studied physical properties were:

i) Monolithicity: monolithic structure or presence of cracks (Section 1),

ii) Shrinkage: linear, diametric and volumetric (Section 1)

iii) Density of the bulk aerogel (Section 1) and density of the skeleton (Section 1)

iv) Volume porosity (Section 1)

v) Specific surface area and pore size distribution (Section 2)

vi) Optical transmission and transparency (Section 3)

vii) Silica nanoparticle size distribution (Section 6)

It will be shown that all these properties are strongly correlated.

1. MONOLITHICITY, BULK SHRINKAGE, DENSITY AND POROSITY

Depending on the synthesis and drying conditions of the gels, the aerogels samples were

completely cracked, monolith with presences of cracks, or entirely monolithic. Table III.1

summarizes the experimental observations.

In order to measure the shrinkage ( V/Vgel) produced during supercritical extraction (see

Table III.1) the aerogels and the wet gel dimensions should be compared.

Volume shrinkage is defined as: gel

aerogelgel

gel V

VV

V

V, (Eq. III.1)

Longitudinal shrinkage: gel

aerogelgel

gel L

LL

L

L, (Eq. III.2)

Diametric shrinkage: gel

aerogelgel

gel

(Eq. III.3)



Bulk aerogel density was determined by weighing samples of known dimensions and by

dividing the aerogel mass by its volume. A maximum error in the density of about 10% was

estimated.

The volume porosity was calculated from the density using the equation:

100*(%)

2

2

SiO

aerogelSiOP (Eq. III.4)

where: SiO2 = 2.19 g/cm3 is the density of pure silica

aerogel is the bulk aerogel density

A more accurate value of the aerogel porosity can be obtained using the aerogel skeletal

density, S, instead of SiO2. For few samples, s was measured by helium pycnometry at room

temperature (Micrometrics, AccuyPyc 1330) [4, 5]. For each aerogel, at least four

measurements of porosity and aerogel density were carried out.

1.1 TMOS AEROGELS

During gel formation, hydrolyzed TMOS forms the silica gel network and the solvent fills the

gel pores. In this section, depending on the solvent used in the gel synthesis, two TMOS

routes were characterized: methanol, labeled as M, [6] and acetone [7] labeled as A-series (the

corresponding sol-gel syntheses are described in chapter II). The A-series consisted in four

aerogels, A1, A2, A3, and A4 with an increasing density.

Skeletal density

The skeletal density for each of A-series was obtained by helium pycnometry. All the samples

showed a similar s (around 2.45 g/cm3) slightly larger than that of vitreous silica ( SiO2 =2.19

g/cm3).

Bulk density

In order to study the batch-to-batch reproducibility a statistical study was performed for a

series of aerogels samples obtained using the identical synthesis and drying process for

methanol synthesis. Figure III.1 shows the density for each produced M aerogel, mean

density value, standard deviation and a density distribution histogram.





Figure III.1 Study of the reproducibility of the aerogel densities obtained using methanol as solvent. Density histogram (left part of the figure) the mean density value was

of =0.122 g/cm3 with a standard deviation of 0.010 g/cm3.

To study the dispersion of the aerogel density the same reproducibility studies were

performed on the acetone series. Figure III.2 shows the densities values for each acetone

aerogel sample, the density mean value, the standard deviation and the histogram of the

density distribution.

0 10 20 30

0,09

0,10

0,11

0,12

0,13

0,14

0,15

0,16

0.010

0.122

Den

sity

(g/

cm3 )

TMOS aerogels, methanol as solvent

0,09

0,10

0,11

0,12

0,13

0,14

0,15

0,16

0 2 4 6 8

Density (g/cm3)Number of samples


Figure III.2 Aerogel densities for each series obtained when using acetone as gel solvent. 3/01.006.0 cmg for the A1C

series, 3/01.008.0 cmg for the A1 series, 3/02.015.0 cmg for the A2 series, 3/02.024.0 cmg for

the A3 series, 3/02.025.0 cmg for the A2CO2 series, and 3/03.027.0 cmg for the A4 series. For each series, the

density histogram is shown at the right of the figure.

It was observed in the Figure III.2 that the samples classified themselves in six different

densities groups depending on the different followed synthesis: The first four groups A1, A2,

A3 and A4, correspond to the different precursor dilution. The fifth density group is the A2

dried under carbon dioxide supercritical conditions labeled as A2CO 2. It can be observed that

A2CO2 samples have the same density than of A3 aerogels. Finally, the group with the lowest

density corresponds to A1C samples that are the A1 samples with added activated carbon.

The A-series was chosen to study the dependency of the bulk aerogel density and volume

porosity with the concentration of precursor on the initial synthesis solution. As it is shown in

Figure III.3, the densities and porosities of the acetone series aerogels presented a linear

relationship versus volume concentration of the precursor, acetoneTMOS

TMOS

VV

Vv .

=0.0099 + 0.7229 v (Eq. III.5)

0,0 0,1 0,2 0,3 0,40

2A4

A1C

0,0 0,1 0,2 0,3 0,4

0

2

4

A1

0,0 0,1 0,2 0,3 0,4

0

2

A2

0,0 0,1 0,2 0,3 0,4

0

2

4

6

8

A2CO2

0,0 0,1 0,2 0,3 0,4

0

2

0,0 0,1 0,2 0,3 0,4

0

2

A3

A4

0 10 20 30 40 50

0,05

0,10

0,15

0,20

0,25

0,30

0,35

A4

A3

A2CO2

A2

A1

A1C

A1C 0,06g/cm3 0,007

A1 0,08g/cm3 0,009

A2 0,15g/cm3 0,010

A2CO2 0,25g/cm3 0,002

A3 0,24g/cm3 0,017

A4 0,29g/cm3 0,026

den

sity

(g/

cm3 )

Acetona seriesAcetone series


Figure III.3 Porosity and apparent silica aerogel density versus v (precursor volume ratio) for the acetone samples. A

linear fitting gives, =0.0099 + 0.7229 v.

Varying the TMOS concentration in acetone gels proves to be a very easy way for controlling

density and porosity of the resulting aerogels. Following table gathers the mean values of

bulk density aerogels, porosity and volume shrinkage of the different TMOS samples.

Table III.1 Volume concentration of TMOS,

acetoneTMOS

TMOS

VV

Vv

,

apparent density, , porosity and volume shrinkage, V/Vgel, of the different TMOS aerogels.

Sample

(g/cm3)

Porosity

(%) V/Vgel

M 0.23 0.13 0.01 93 0.27

A1 0.10 0.08 0.01 96 0.43

A2 0.20 0.15 0.02 93 0.42

A3 0.30 0.25 0.02 88 0.47

A4 0.40 0.27 0.03 86 0.46

It was observed that the methanol-synthesized aerogels, M, presented a smaller shrinkage

( 27 %) than the acetone-series, A, ( 45 %) in which, volume shrinkage was independent on

the precursor concentration within the experimental error. Shrinkage differences could be

0,0 0,1 0,2 0,3 0,4 0,5

0,05

0,10

0,15

0,20

0,25

0,30

0,35

Porosity

v

Den

sity

(g

/cm

3)

82

84

86

88

90

92

94

96

98

100

DensityP

oro

sity (%

)


related with the use of a basic catalyst for the methanol aerogel while for the acetone series

no catalyst was used (sol polymerization conditions of pH between 5 and 6). It is interesting

to note that the pH values for both sols are in the acidic side. This is due to the fact that the

pH values of TMOS and methanol are on the acidic side (pH 4) and hence the pH of

NH4OH incorporated sol has a value typical of weak-acidic conditions. It is also possible that

a number of micro- and/or macropores in the acetone synthesized gels collapsed during

supercritical extraction and this resulted in a larger shrinkage.

1.1.1 Supercritical drying at CO2 conditions

To observe the effect of supercritical drying process at low-temperature on the physical

properties of the TMOS silica aerogels the acetone gels were used since liquid CO 2 is highly

miscible in acetone. The gels used were the A2 synthesis [7, 8] and the resulting aerogels were

called A2CO2.

In this first low-temperature drying set, monolithic aerogels free of cracks were obtained. The

aerogels obtained had different characteristics that those dried under conventional drying by

hypercritical evacuation of the solvent. The main differences between the two drying

procedure were:

i. The mean aerogel density for the acetone dried gels was = 0.141 g/cm3 while for the

CO2 dried aerogels was = 0.245 g/cm3. The aerogels with CO2 exchange were denser,

then, lower porosity: 85% for aerogels with exchange and 93% for acetone dried

aerogels.

ii. The mean diametric shrinkage was larger for CO 2 aerogels / =0.30 than for

acetone dried aerogels / =0.21.

iii. The low- temperature silica aerogels were more opaque.

iv. The A2CO2 aerogels were less fragile.

Is important to remark that in the synthesis of the A2CO2-series the gels were not washed

with ethanol after gelation. The soaking solvent was not exchanged, and then, the remaining

water and catalyst were still present in the liquid when A2 were supercritcally dried. It may be

the main reason for the dramatic shrinkage observed, and consequently the higher density

and opacity.



Previous work [9] reported that the silica aerogels dried at low temperature were hydrophilic

while the dried ones at high temperature were hydrophobic. Because the different drying

temperatures the composition of the aerogel surface was different. The CO 2 dried aerogels

presented OH groups very polar that can react with the water. In the surface of conventional

aerogels, there exists alkoxy groups that are not polar and do not react with water. However,

in the synthesized aerogels samples presented in this section, the water penetrated in the

porous of the aerogels dried conventionally, whiles the CO 2 dried aerogels are hydrophobic.

More extended work is necessary to understand this behavior.

Conclusions for TMOS aerogels

The properties of aerogels are modified according to the conditions of their preparation.

i) An easy way for controlling density and porosity of the resulting aerogels is by the

variation of TMOS concentration in acetone gels.

ii) Methanol aerogels are very transparent while acetone aerogels had a white shading;

their opacity decreases with increasing TMOS content.

iii) All the aerogels have monolithic structure without cracks. The A1 aerogels were

especially fragile.

iv) Volume shrinkage for all acetone aerogels is about 45 %, independently of the

reactive concentrations. This shrinkage is much larger than that observed for

methanol aerogels 27 %.

v) Methanol aerogels presented densities and porosities similar to those for A2. For

acetone aerogels, it has been observed that density increases, and porosity is reduced,

when increasing TMOS content (Figure III.3), the variation could be adjusted to a

straight line. This is the behavior that one should expect: denser material needs more

precursors, leading to reduced porosities.

These results will be supported and discussed by considering the particle and pore sizes

observed by scanning electron microscopy (section SEM), by transmission electron

microscopy (section TEM) and the optical transmission of aerogels (section UV-VIS-NIR

spectroscopy) [10].

1.2 TEOS AEROGELS

A large number of silica aerogels were prepared following, neutral, basic, and acid synthesis.

The samples were dried either under ethanol [11-14] or CO2 supercritical conditions [15, 16].



The bulk density, porosity and shrinkage of optimized TEOS aerogels (see chapter II for

synthesis optimization) are determined in this section.

1.2.1 TEOS aerogels synthesized without presence of catalyst

A TEOS-series of aerogels was synthesized without catalyst. Several EtOH/TEOS molar

ratio, m, and several H2O/TEOS molar ratio, h, were tested. Two different series of TEOS

aerogels were prepared under the same synthesis conditions. After gelling, the gels of the first

series were not washed with an ethanol bath, Table III.2, while the gels of the second series

were washed and aged in an ethanol soaking (Table III.3). Table III.2 gathers the shrinkages

for all the TEOS samples produced without catalyst and at different TEOS/EtOH/H2O

molar ratios.

Table III.2 Ethanol/TEOS and water/TEOS molar ratios,

density, porosity, volume shrinkage, V

V , diametric shrinkage,

and linear shrinkage, L

L for a series of TEOS aerogels without

ethanol washing.

EtOH/

TEOS

H2O/

TEOS

(g/cm3)

Porosity

(%) V/V / L/L Observations

5 5 0.2515 88.5 0.545 0.22 0.26 Cracks

5 6 0.2333 89.3 0.515 0.21 0.22 Cracks

5 7 0.1932 91. 2 0.444 0.18 0.17 Cracks

5 8 0.2215 89.9 0.535 0.23 0.23 Cracks

7 5 0.1625 92.6 0.41 0.15 0.18 Monolithic

7 6 0.1620 92.6 0.425 0.16 0.18 Monolithic

7 8 0.1645 92.5 0.480 0.19 0.21 Monolithic

The quality of the set of samples without ethanol exchange was not very good because these

aerogels presented some cracks. The shrinkage is similar in all the samples with different

TEOS concentration: the volume shrinkage ranged from 0.42 to 0.53, the diametric

shrinkage from 0.17 to 0.23, and the linear shrinkage from 0.16 to 0.23.

At fixed H2O/TEOS ratio, when increasing the EtOH/TEOS ratio the densities appeared to

be smaller.


Table III.3 EtOH/H2O molar ratio, density, volume shrinkage, and linear shrinkage for a series of TEOS aerogels with ethanol washing.

EtOH/

TEOS

H2O/

TEOS (g/cm3) V/V / L/L Observations

5 6 0.123 0.1192 0.365 0.058 Monolithic

5 7 0.121 0.1481 0.502 0.056 Monolithic

7 5 0.116 0.1321 0.360 0.0673 Monolithic

7 6 0.122 0.1191 0.161 0.0826 Monolithic

7 8 0.113 0.1552 0.530 0.0569 Monolithic

For the washed gels, all the produced silica aerogel were monolithic and with similar smaller

volume shrinkage (0.11-0.12). No significant differences of density (0.133 g/cm3< 0.123

g/cm3) were obtained when varying the TEOS/EtOH/ H2O molar ratio.

There was a significant difference between the gels not washed and those washed in an

ethanol solution [16]. For the same synthesis and drying conditions, gels washed in ethanol

exhibited a lower bulk density and the incidence of cracking or fracture of aerogels was

significantly lower. This difference in bulk density is attributed to the presence of water

during the drying that caused a larger shrinkage giving a denser bulk structure.

It was found that the optimum aerogels were produced in the range of 5>h>8, lower (h< 5)

and higher (h> 8) values resulted in opaque and cracked aerogels. On the contrary taking the

m precursor concentration lower than 5 (m< 5) and higher than 9 (m> 9) values resulted in

opaque, high density as well as cracked aerogels. Moreover, the density of aerogels decreases

with an increase in m values up to 9. Further increases in m values (m > 9) lead to an increase

in the bulk density of the aerogels (because the bad quality of the aerogels). Summarizing,

monolithic, low density and transparent TEOS silica aerogels were found for h values

between 5 and 8 (5<h<8) and m values between 5 and 9 (5<m<9) when no catalyst was


used. As a final point, the effect of washing solution was found to be a very important factor

to control the quality of the aerogels.

As mentioned before, some of the aerogels dried conventionally were hydrophobic (see

Figure III.4), one possible explanation of this effect may be that in the surface of

conventional aerogels, there exists alkoxy groups that are not polar and do not react with

water. However, in the synthesized aerogels samples presented in this section, the water

penetrated in the porous of some aerogels indistinctly of the followed drying process. Figure

III.4 shows a picture of a hydrophobic silica aerogel floating on water.

Figure III.4 Photograph of a hydrophobic aerogel produced by using TEOS as metal alkoxide precursor with a molar ratio of TEOS/EtOH/H2O=1/5/6 and dried under ethanol supercritical conditions.

1.2.2 Base-catalyst

One-step gels were prepared from TEOS under base-catalyzed conditions. In this case, a

small amount of ammonium with ammonium fluoride solution (0.03M NH3 + 0.5M NH4F)

was added as a supplementary catalyst to accelerate the gel formation. Gels were poured in

Petri disks ( =30mm, h= 5mm) and dried under CO2 conditions.

Two series of TEOS base-catalyzed aerogels with a variable ethanol concentration were

prepared, the first series with H2O/TEOS fixed at 10, and the second series with

H2O/TEOS fixed at 25. The obtained samples presented different shrinkage and

transparency. The aerogel densities for the two series are gathered in Tables III.4 (h=10) and

III.5 (h=25). From the aerogels listed in Table III.4, those ones with highest density

presented some cracks and those with lowest density presented an opaque appearance. A


large variation in aerogel density was produced in that series from 0.0190 g/cm3 up to 0.124

g/cm3.

Table III.4 Bulk density and porosity of TEOS aerogels with base-catalyst (NH3+NH4F) and H2O/catalyst/TEOS fixed at 10/3.3.10-3/1.

EtOH/TEOS (g/cm3)

Porosity

(%)

16.82 0.1244 94.3

24.73 0.0602 97.2

66.29 0.0408 98.1

91.27 0.0295 98.6

115.75 0.0190 99.1

The most important factor to obtain more transparent aerogels in the base-catalyzed aerogels

is to increases the H2O/TEOS molar ratio [17]. Then, in order to improve the transparency

of the base silica aerogels the H2O/TEOS molar ratio was increased at 25.

Table III.5 Bulk density and porosity of TEOS aerogels with base-catalyst (NH3+NH4F) and H2O/catalyst/TEOS =25/3.3.10-3/1.

EtOH/TEOS (g/cm3)

Porosity

(%)

12.23 0.053 97.6

18.85 0.047 97.8

26.25 0.037 98.3

33.65 0.036 98.4

When H2O/TEOS molar ratio was fixed at 25 (Table III.5) all the aerogel samples were

monolithic without presence of cracks and more transparent than when H2O/TEOS molar

ratio was fixed at 10. In this series, the dependency of density and porosity versus

EtOH/TEOS molar ratio was studied. Figure III.4 shows the linear dependency of density

and porosity versus EtOH/TEOS molar ratio when H2O/TEOS=25.


Figure III.5 Porosity and apparent silica aerogel density versus molar EtOH/TEOS ratio for the TEOS aerogels with base-catalyst 3.3.10-3 (NH3+NH4F) and TEOS/H2O fixed at 25. Solid lines are the linear fitting.

If the two density-molar ratio dependencies are compared, one can observe that for larger

H2O/TEOS the linear variation of the density versus the EtOH/TEOS was faster. See

Figure III.6.

10 15 20 25 30 35 40

0,035

0,040

0,045

0,050

0,055

Density

EtOH/TEOS

den

sity

(g/

cm3 )

97,4

97,6

97,8

98,0

98,2

98,4

98,6

water/TEOS = 25

base-catalyst (NH3+NH

4F)

PorosityP

oro

sity (%)


Figure III.6 Comparison of the dependency of density versus EtOH/TEOS ratio for two different samples, one with H2O/TEOS fixed at 10 at the other fixed at 25. Linear fittings.

It should be pointed out that the gels with base-catalyst were soaked in water, catalyst, and

ethanol solution in the same proportions than the initial sol in order to accelerate the aging of

the gels. There was a significant difference between the gels soaked only in ethanol and those

soaked in an aging solution. Gels washed only in ethanol exhibit a more fragile skeleton. This

difference is attributed to the presence of water during the aging solution. The presence of

water causes solution/reprecipitation giving a smoother network. In addition, and probably

more important, continued hydrolysis and condensation reactions occur giving a stronger

network [18, 19].

1.2.3 Acid catalyst

The influence of two acid-catalysts (acid fluorhydric and acid citric) on the density, porosity,

and shrinkage is studied in this section. All samples were dried under CO 2 supercritical

conditions. Several EtOH/H2O/TEOS ratios were used in order to optimize the quality of

the acid-catalyzed aerogels.

20 40 60 80 100 120

0,01

0,02

0,03

0,04

0,05

0,06

0,07

water/TEOS = 25


4F)

water/TEOS = 10


4F)

den

sity

(g

/cm

3)

molar concentration EtOH/TEOS


Fluorhydric acid

The acid catalyst used was 52%HF. The recipe followed was fixing the H2O/TEOS ratio at

10 and then varying EtOH/TEOS concentration from 12 to 42. Gels were washed with

ethanol for several days; Table III.6 shows some of the obtained aerogel densities.

Table III.6 Density and porosity of TEOS aerogels with acid-catalyst (52%HF) and H2O/TEOS fixed at 25.

EtOH/TEOS Density

g/cm3

Porosity

(%)

12.23 0.102 95.3

18.85 0.068 96.9

26.25 0.043 98.0

33.65 0.024 98.9

41.05 0.018 99. 2

In that case, the produced TEOS aerogels with HF catalyst were very transparent samples.

Figure III.7 shows a linear decrease in the bulk density and linear increase in the bulk

porosity with an increase of EtOH/TEOS ratio, which is due to the smaller SiO 2

concentration in the gels when larger ethanol concentrations.

15 20 25 30 35 40 45

0,01

0,02

0,03

0,04

0,05

0,06

0,07

Density

EtOH/TEOS

den

sity

(g/

cm3 )

96,5

97,0

97,5

98,0

98,5

99,0

99,5 Porosity

Po

rosity (%

)

Figure III.7 Porosity and apparent silica aerogel density versus EtOH/TEOS ratio for the TEOS aerogels acid-catalyst (HF) and H2O/TEOS fixed at 25. Linear fitting Porosity(%)=95.1+0.1m with R=0.993.


Another synthesis using HF was tried with a lower H2O/TEOS molar ratio (fixed at 12) and

with a variable EtOH/TEOS molar ratio (in polystyrene tubes of =16mm). Table III.7

gathers the densities obtained.

Table III.7 TEOS aerogel densities and porosities of acid series with a variable EtOH/TEOS molar ratio. Acid-catalyst (52% HF) and H2O/TEOS fixed at 12.

EtOH/TEOS Density

(g/cm3)

Porosity

(%) Transparency

8.55 0.0192 99.1 Most opaque

6.67 0.0294 98.7 Opaque

6.71 0.0410 98.2 Translucent

5.43 0.0602 97.2 Most transparent

4.52 0.1244 94.3 Transparent but opaque inside

In that case, it was found that EtOH/TEOS molar ratio strongly affects the transparency due

to the structural change of pore and particle size of the aerogel. A larger density results in

more transparent aerogel. Next photography compares a TEOS silica aerogel with an

ambient evaporated gel (xerogel). In the case of xerogel sample, it was observed a transparent

sample but with a very large shrinkage.

Figure III.8 Photograph comparing two citric acid TEOS silica dried gels samples, on the top a CO2 supercritically dried sample, and on the bottom, an ambient evaporated gel.


Citric acid

As a comparison, gels were prepared from TEOS under acid-catalyzed conditions at molar

ratios of the starting components of the sol: TEOS/EtOH/H2O=1/7/5. In this case, a

variable amount of acid citric (from 0.0005 to 0.1) was added as a supplementary catalyst to

speed up the gel formation. The gels were dried under ethanol supercritical conditions.

Table III.8 TEOS aerogel, density, shrinkage, and monolicithy of acid series with a variable H2O/EtOH/TEOS molar ratio. Acid-catalyst (0,0001M acid citric).

Label m h

(g/cm3)

Porosity

(%) V/V

d/d

(%) L/L Monolithicity

TE98X01-02b 7 5 0,160 92,7 0,40 14,52 0,18 Monolithic

TE98X03-04b 7 6 0,162 92,6 0,43 16,94 0,18 Monolithic

TE98X05-06b 7 8 0,164 92,5 0,48 19,2 0,21 Monolithic

TE98X07-08b 5 5 0,249 88,6 0,54 21,77 0,23 Monolithic with crack


TE98X13-18b 5 7 0,189 91,4 0,43 16,94 0,18 Monolithic


At fixed water concentration, the bulk density is larger when larger TEOS concentrations.

The gels experienced a typical linear shrinkage of 0.18-0.23 during the supercritical drying.

The density as a function of water molar ratio is shown in Figure III.9.


4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5

0,18

0,20

0,22

0,24

0,26

H2O/TEOS

den

sity

(g/

cm3 )

Figure III.9 Variation of density from silica aerogel versus H2O/TEOS molar ratio of acid series at fixed concentration of citric acid (0,0001M).

It was found that for h values lower than seven an increase in h value leads to a decrease of

the density of the aerogels. Moreover, for higher h values, h>7, the density increases with h.

Lowest (h<5) and highest values (h<8) lead to the cracking due to the high shrinkage.

1.3 TWO-STEPS SYNTHESIS

As described in chapter II, in this thesis the two-steps synthesis was achieved by replacing the

first acid-step for the commercially partially hydrolyzed silica precursor, H5 obtained by

Silbond corp. Then, it will be necessary to identify the desirable processing conditions in the

second step. In that second step, the prepared precursor, mixed with appropriate amounts of

H2O, EtOH and NH4OH underwent further hydrolysis and condensation reactions formed a

clear gel usually within few hours. Then after aging for 24 h in a solution with the same

amount of water and catalyst, they were soaked for several days in a pure ethanol bath for

removing the interstitial water. They were dried using the CO 2 substitution method. All of

the aerogels obtained were hydrophobic.


Figure III.10 Photograph comparing the quality of two silica aerogel samples obtained with TEOS as alkoxide precursor. The aerogel on the top is a one-step silica aerogel and on the bottom a two-step aerogel. The two-step aerogel shows a higher transparency and lower density.

A first series of two-steps silica aerogels were prepared with different amounts of water and

ethanol. A second series was prepared to show the effect of the amount of catalyst used in

the gel preparation [20]. Table III.9 gathers some of the densities and transparencies of the

two-step series obtained by changing the H5 dilution and the catalyst amount.

Table III.9 Density and transparency of the two-step aerogel series obtained with NH3 catalyst in the second step.

Label Catalyst

amount

VETOH/

VH5

VH2O/

VH5

Density

(g/cm3)

Porosity

(%) Transparency

H501AE 0.02 1.7 1.5 0.0299 98.6 Opaque

H501AB 0.04 1.7 1.5 0.0330 98.5 Opaque

H501AC 0.06 1.7 1.5 0.0466 97.9 Opaque

H501AA 0.08 1.7 1.5 0.0550 97.5 Transparent

H501AD00 0.1 1.7 1.5 0.0823 96.2 Translucent

H501AD01 0.1 1.7 1.5 0.0800 96.3 Translucent

H501AH01 0.08 2.1 1.5 0.0734 96.6 Translucent

H501AG01 0.08 1.25 1.5 0.0841 96.2 Translucent

H501AI01 0.08 2.5 1.5 0.0748 96.6 Translucent

H501AJ01 0.08 3.3 1.5 0.0672 96.9 Translucent

The two-step aerogel densities ranged from 0.03 to 0.08 g/cm3. For samples with the same

water and catalyst concentration increasing EtOH/H5 ratio from 1 to 1.7, the aerogel density


decreases from 0.0786 to 0.0550 g/cm3. In addition, it was observed that the most

transparent and with low-density samples were produced when a catalyst amount of 0.08 and

water/EtOH/H5=1.5/1.7/1. The dependence of silica aerogel density on the base-catalyst

amount for a fixed molar ratio of TEOS/EtOH/H2O is plotted in Figure III.11.

Figure III.11 Density of two-step silica aerogel versus the concentration of base catalyst

It is seen from the Figure III.11 that the aerogel density increases as the concentration of

catalyst increases. This is due to the fact that at higher concentrations the colloidal particles

and pores are smaller and therefore the gels tends to shrink and become denser [23, 24]. It

was observed that a very important factor to produce non-cracked two-step aerogels is to

soak the gels in a solution with water and basic catalyst dissolved in ethanol. This solution

enhances the aging of the gels. When skipping this step, highly cracked aerogels were

obtained. In conclusion, two-step process appears to be the best synthesis method to obtain

low-density, non-cracked, and transparent aerogels.

Figure III.12 Transparent ‘two-step’ silica aerogels.

10-2

10-1

0,02

0,03

0,04

0,05

0,06

0,07

0,08

den

sity

(g/

cm3 )

log10

(concentration of the catalyst (NH3))


2. SURFACE AREA MEASUREMENTS BY BET (BRUNAUER, EMMET

AND TELLER)

One important aspect of the aerogel pore network is its open nature. In a closed-pore

material, gases or liquids cannot enter the pore without breaking the pore walls. Instead, with

an open-pore structure, gases or liquids flow through the entire material. The IUPAC

classification for the pore size is given by: Macropores, when pore diameter, pore, is larger than

50 nm (0.05 m). Mesopores: when 2 nm pore 50 nm, and Micropores when pore is smaller

than 2 nm.

Silica aerogels possess a distribution of pores sizes in the micro, meso and macropores

regimes. However, the majority of the pores fall in the mesopore range, with relatively few

micropores.

To get an inside knowledge of the pore structure of aerogels several techniques are used:

Small angle X-ray scattering, SAXS, maybe the most useful one for such characteristics and a

lot of articles deal with it [25, 26]. Unfortunately, we were not able to have access to this

technique.

BET (Brunauer, Emmet and Teller) is the most widely available and utilized method for

determining aerogel porosity [27]. A detailed description of this method can be found in

annex III. This technique successfully accounts for pores below about 100 nm. Thus, in

aerogels with typical pore sizes in the 1-1000 nm range, only a fraction of the total available

pore space is detected. However, microporosity information can be obtained through

mathematical analyses of BET technique such as t-plots or the Dubinin-Radushevich method

(Annex III). Gas adsorption can not effectively determines macroporosity.

In this technique, nitrogen at its boiling point is adsorbed on the solid sample. The amount

of gas adsorbed depends of the size of the pores within the sample and on the partial

pressure of the gas relative to its saturation pressure. By measuring the volume of gas

adsorbed at a particular partial pressure, the Brunauer, Emmet and Teller equation gives the

specific surface area of the material and the pore size distribution of the sample. The pore

size distribution used in this work was determined using the Kelvin equation in the analysis

of the nitrogen adsorption/desorption curves and the parameter C is obtained by analyzing

the standard 2 parameter BET isotherm gives the amount of gas adsorbed as a function of

the relative pressure of the adsorbing gas:


)P/P()1c(1)P/P1(

)P/P(c

V

V

00

0

m

(Eq. III.6)

where:

V = Volume of gas adsorbed at pressure P, Vm = Volume of gas covering the surface with a

monomolecular layer, n/nmono is the ratio of the moles adsorbed to the moles adsorbed in a

single monolayer. Po = Saturation pressure of the gas (vapor pressure), i.e. the pressure of the

gas in equilibrium with bulk liquid at the temperature of the measurement and C is a constant

for the gas/solid combination, C=(slope+Yintercept)/Yintercept.

The isotherm can be converted to a linear form for ease of extracting the values of Vm and C.

The constant C represents the relative strengths of adsorption to the surface and

condensation of the pure adsorbate (see annex III).

BET porosity characterization of the aerogel samples was performed with a Micromeritics

ASAP 2000 instrument. The BET surface area, SBET, pore volume, VBET, mean pore diameter,

< pore>BET, and C parameter were obtained from the adsorption isotherm of N2 at 77K. The

samples were outgassed overnight at 300 C.

The porosity of several TMOS aerogels, A1, A2, A2CO2, A3, A4, M, and M+2%C was

characterized by BET method following the protocol described in Annex III. Surface area,

SBET, total pore volume, VBET, and mean pore diameter, < pore>BET, measured by BET, are

presented in Table III.10. It can be observed that all samples show surface area between 410

and 630 m2g-1.

Table III.10 BET measurements of A-series and M aerogels. Apparent density, porosity, BET surface area, SBET, total volume of

pores, VBET, mean pore diameter measured by BET, < pore>BET =

4VBET / SBET, and C parameter.

Sample

(g/cm3)

Porosity

(%)

SBET

[m2/g]

VBET

[cm3/g]

< pore>BET

(nm)

C

A1 0.08 0.01 96 470 1.0 10 57

A2 0.15 0.02 93 420 2.7 30 46

A2CO2 0.24 0.02 89 551 2.3 17 113

A3 0.25 0.02 88 560 2.1 20 51/58

A4 0.27 0.03 86 414 3.2 20 56

M 0.13 0.01 93 589 5.7 50 37

M+2%C 0.12 0.01 94 632 5.7 34 40


The C value showed in Table III.10 gives a quantitative method to evaluate the

hydrophobicity of the aerogel samples, smaller C values means larger hydrophobicity.

The pore size distributions of the aerogels are estimated by applying the Pierce method [32]

to the measured absorption isotherms. Some examples of the pore size distributions obtained

by BET are shown in Figure III.13.

Figure III.13 Incremental pore volume for A-series, A1, A2, A3, A4 and M aerogels obtained by BET.

From the pore size distribution of Figure III.13 it was observed that for the samples A1

(lowest density, largest opacity), no peak was observed in the mesopore region. In the aerogel

prepared at the composition of A4 (largest density, most transparent from A-series), a large

fraction of pores has a size in the range of 10 - 100 nm. A2 and A3 aerogels shows that only a

fraction of the pores is accounted by BET. For the methanol aerogels, a strong and narrow

increment in the range of 40 - 90 nm was observed. It should be noticed that the mean pore

diameter values shown in Table III.10 were calculated using de VBET, which only measures

100 1000

0

A1

A2

A3

A4

M

Incr

emen

tal

Po

re V

olu

me

(cm

3/g

)

Mean Pore Diameter (Å)

10


the pores in the mesopore range, so in the case of BET measurements the mean value

< pore>BET will be working successfully only for aerogels with a porosity included exclusively

below 100nm [34, 35].

By plotting the incremental surface area versus pores diameter, we can obtain more

information about microporosity. A clear presence of micropores is shown in the incremental

surface area plot (Figure III.14). For example, in A1 aerogel surface area are slightly increases

when we are close to the micropore range. In this sample, there was no presence of peak in

the mesopore range and increased very quickly when close to micropore range, meaning that

some pores are out of the measurable BET range.

100 1000

0

50

100

150

200

250

Incr

emen

tal su

rfac

e ar

ea (

m2/

g)

Mean Pore Diameter (A)

Figure III.14 Surface area increases for A1 aerogel. The value when close to micropore range was increasing.

If the pore volume is calculated based in the silica aerogel density:

2SiOAerogel

pore

11V (Eq. III.6)

Most of the aerogels showed larger pore volume values than that obtained by BET. Then the

V= Vpore -VBET will indicate approximately the volume of porous not measured by BET

(pores diameter smaller than 2 nm and larger than 50nm), as well as the percentage of micro

and/or macropores. The obtained results are also shown in Table III.11. In addition, using

the pore volume obtained by density, the mean pore diameter can be calculated by:

BET

pore

densityporeS

V4 (Eq. III.7)


Table III.11 Comparison of pore volume and mean pore diameter obtained by BET analysis and by density measurements for the A-series silica aerogels (TMOS).

Sample Vpores

[cm3/g]

VBET

[cm3/g]

Volume of

micro+macropores*

< pore>density

(nm)

< pore>BET

(nm)

A1 12.0 1.0 90% 110 10

A2 6.2 2.7 60% 60 30

A3 3.5 2.1 40% 30 20

A4 3.2 3.2 0% 20 20

M 7.2 5.7 20% 60 50

* Excluding mesopores, pore range being measured by the BET method.

Main discrepancies were observed when the obtained mean pore diameters are compared

with the real one obtained from the own density of each sample. For A1 and A2 aerogels the

differences are associated at the presence of macropore because the opaque aspect of the

samples (Rayleigh effect). This phenomenon appears when the scattering center, the pore,

has a diameter close to the visible wavelength, around 500nm.

From these data, one might conclude the following:

i) The opacity in A1 could be explained by the large value of V which would correspond

approximately to the macropores not measured by BET (~90%).

ii) A2 and A3 have a pore size distribution with an important number of micropores and/or

macropores, but the mean pore diameter obtained is within the mesopore range.

iii) A4 presents a mesopores distribution, which is totally accounted by the BET technique,

and agrees with the mean pore value obtained by optical measurements.

vi) M have a pore size distribution that is mostly accounted by BET (~80%).

On the other hand, the clear transparency of monolithic aerogels gives an upper limit for their

pore size of about 100 nm [33]. The translucence of some aerogel samples indicates the

presence of macropores, although they are not accounted by the BET technique. In order to

evaluate the effect of the macropores on the transparency, light transmission (LT) experiments

were performed (see section Rayleigh scattering).

The porosity of several silica aerogels produced with TEOS as metal alkoxide precursor, by

one and two-step synthesis was characterized by BET method following the protocol


described in Annex III. Next Table gathers the surface area; total pore volume, and mean

pore diameter measured by BET. It can be observed that all samples show larger surface area

than TMOS aerogels, between 700 and 1012 m2g-1. These differences can be originated by the

addition of the washing step in those TEOS aerogels.

Table III.12 BET measurements of TEOS aerogels by one and two-step synthesis. Apparent density, porosity, BET surface area, SBET, volume of pores, VBET, means pore diameter and C parameter measured by BET.

Sample

(g/cm3)

Porosity

(%)

SBET

[m2/g]

VBET

[cm3/g]

< pore>BET

(nm) C

TE00AA

(1/7/5)

0.115 94.7 780 4.8 22 48

TE00AF

(1/7/5)

0.107 95.1 812 4.8 18 60

H501AI

(0.08)

0.075 96.9 946 5.7 24 90.8

H501AD

(0.1)

0.080 96.2 899 4.9 21 95.2

It is observed that two-step aerogels shows a larger surface area (between 890 and 950 m2g-1)

than one-step aerogels.

3. INFRARED SPECTROPHOTOMETRY, IR

The infrared spectroscopy is generally used for the characterization and identification of

organic compounds or functional groups of those. Infrared spectroscopic methods have

provided considerable information about the surface and structure of silica [36-39].

In this section, the acetone series (A1, A2, A3 and A4) and the methanol aerogel series (M

synthesis) were characterized using IR spectrophotometry, a Fourier Transform Infrared

Spectrometer Nicolet 710 was used. The IR spectrometer settings were 50 scans, 16.0 cm-1

resolution, and ratio mode. The spectra were registered in a wavelength range of 400-4000

cm-1. A beam path background spectrum was recorded without sample. Then, the

background was subtracted for each IR spectrum to eliminate the vibrations due to air. The

IR samples were prepared by making a KBr tablet, approximately 1% weight mass of aerogel

mixed with KBr powder were placed inside a cylindrical stainless steel die. The tablet (wafer)

was compacted with a pressure of 1000 Kg/cm2, for 20 s in a Carver press. The wafer was

inspected for cracks or holes before fit the tablet in the IR holder.


Figures III.15, III.17 and III.18 show the IR spectra of M, A1 and A2 aerogels. The main

common features of these spectra are: two bands around 3500 cm-1 and 1600 cm-1

representing adsorbed water, and the bands around 2950 cm-1 associated to C-H groups. The

1080, 800, and 460 cm-1 bands are attributed to different modes of Si-O or O-Si-O vibrations

and the band around 960 cm-1 correspond to stretching vibrations of Si-OH.

3.1 METHANOL SERIES

Figure III.15 shows IR spectrum for the methanol aerogels, M, synthesized using TMOS, Si

(OCH2)4, water and base-catalyst (NH4OH).

Si-O-Si symmetric

464.8

559.3

808.1 958.6

1633.62854.4

2950.9

3438.8

Si-OH

Si-O-Si bending

C-O

OH- fromH2O

CH- frommethanol

M

20

40

60

%T

500100015002000300040001/cm

1091.6O-Si-O asymmetric

Figure III.15 IR spectrum of methanol aerogel (M).

The curve in Figure III.15 is the infrared spectrum of aerogel M. Several important bands

were observed. The adsorption bands identified characterizing the SiO2 aerogel were:

1200-1000 (1092) cm-1 : O-Si-O asymmetric stretching mode.

808 cm-1 :Si-O-Si symmetric stretching mode.

465 cm-1 : Si-O-Si bending mode.

These three IR adsorption peaks correspond to the different modes of silica [40].

959 cm-1 : Si-OH: stretching mode. Si-OH is a quite visible peak, indicating the

hydrophilic nature of the M silica aerogel. This result can be confirmed by the

C value obtained for this sample by BET.

Other identified adsorption bands are:

atmospheric CO2

atmospheric H2O


3439 cm-1: A broad band appears corresponding to water

1634 cm-1: Atmospheric H2O band that overlaps with the SiO-H band.

Both of these bands are related to adsorbed molecular water, indicating

again the hydrophilic nature of the sample.

2850 -2950 cm-1: C-H (O-CH2) methanol bonds. The bands of adsorbed

methanol are assigned to the symmetric and antisymmetric stretching

vibrations of the C-H bonds of residual methanol.

An IR spectrum complement is obtained using UV-VIS- near IR spectroscopy technique. In

a similar way of IR spectroscopy, absorption bands were obtained in the range of visible,

ultraviolet and near infrared. Figure III.16 shows a complete UV-VIS- near IR spectrum

from M aerogel.

500 1000 1500 2000 2500 3000

0

20

40

60

80

100

Tra

nsm

issio

n (

%)

Wavelength (nm)

Figure III.16 UV – VIS - Near IR Transmission spectrum of base-catalyzed TMOS aerogel M (sample 1 cm thick)

The intrinsic absorbance of silica is low in the visible region. It can be observed the low

intrinsic adsorption in the visible range (300-900 nm) resulting in a 100% of transmission in

the visible range. As wavelengths become progressively shorter, scattering increased,

eventually cutting off transmission near 300 nm. Weak absorbance begins to appear in the

near infrared, and again cut off transmission around 2700-3200 nm. There is then a "visible

UV VIS Near-IR IR


window" of transmission through silica aerogel that is an attractive feature of this material for

day lighting applications. As the spectrum moves into the infrared, scattering becomes less

important, and standard molecular vibrations account for the spectral structure. The

observed bands in the frequency range of the infrared and near infrared were:

vibrations of the ‘stretching’ modes of the SiO-H bonds at 7390 cm-1 (2700-

3200 nm).

hydrogen bond of the water at 5476 cm-1

SiO-H stretching and bending modes 4598 cm-1. The analysis of the UV-VIS

spectrum shows that the formation of SiO2 is not completed because the

presence of SiO-H groups.

C-H stretching and bending modes at 4233 cm-1

There is a region of high infrared transparency between 3300 and 2000 cm-1.

This allows a certain amount of thermal radiation to pass through silica aerogel

and lower its thermal insulative performance. Addition of additives that absorb

radiation in this region can remedy this problem

3.2 ACETONE SERIES

Figure III.17 shows IR spectrum of the A2 aerogel:

Si-O-Si symmetric

Si-OH

O-Si-O asymmetric

0

25

50

75

100

%T

50010001500200030004000

1 / c m

470.6

804.3

968.2

1103.2

1639.42358.8

3425.3

A2

OH- fromH2O

contamination C-O

Si-O-Si bending

Figure III.17 IR spectrum of A2 acetone aerogel.

atmospheric CO2

atmospheric H2O


No large differences were observed compared to M aerogels meaning that the A-series silica

skeleton composition is not much different from that one from M aerogels. The C-H peak at

2950 cm-1for A2 sample, present at M samples, did not appear indicating that no acetone

remains in the A2 aerogels.

Figure III.18 compares the spectrum for each of the acetone-series aerogel.

0

25

50

75

100

%T

500100015002000300040001/cm

A2

A1

A4

A3

Figure III.18 IR spectra for all the aerogels from the acetone-series, A1, A2, A3 and A4.

The four spectra did not present remarkable qualitative differences. The analysis shows that

the formation of SiO2 is not completed because the presence of SiO-H groups at 3820 cm-1.

The CO2 atmospheric band at 2360cm-1 (A2 and A4) depends on the quality of the

background extraction.

4. ULTRAVIOLET-VISIBLE (UV-VIS) SPECTROSCOPY

4.1 AEROGEL TRANSPARENCY

Depending on the preparation conditions silica aerogels may appear transparent, translucent

and opaque. They are transparent when the sizes of pore and particle are smaller than the

wavelength of light, and they are homogeneously distributed. The key to control the

inhomogenities (implying pore and particle size) of the aerogels lies in the sol-gel stage and in

the supercritical extraction process. The amount of light scattered from an aerogel depends

mainly on structural inhomogeneities smaller than the wavelength of visible light [41].

Therefore, transparent aerogel transmits rather than scatter light.


Figure III.19 shows a photograph of the aerogels produced in our laboratory, ordered from

the most opaque to the most transparent ones depending on the different sol compositions.

These aerogels have been optically characterized in this section by using UV-VIS

spectroscopy.

Figure III.19 Photograph of silica aerogels obtained under different sol-gel conditions. The acetone series from A1 to A4, and the methanol sample, M. Its degree of transparency is usually related with the presence of inhomogeneities, amount and size of macropores (section Rayleigh scattering).

In this section, with a view to understand the opacity and transparency of the aerogels, the

light transmission in the ultraviolet-visible (UV-VIS) wavelength was measured using a

Shimadzu UV/VIS UV-2102 spectrometer. The apparatus was equipped with light sources

covering the ultraviolet-visible wavelength range from 300-900 nm. The light transmission

(LT) is the amount of light with a fixed wavelength that is transmitted through the aerogel

without being scattered in other directions. Then, the maximum of LT corresponds to a

completely transparent aerogel where all the incident light is transmitted in the same direction

that was incidented. See Figure III.20

Figure III.20 UV/VIS spectrometer scheme used to measure light transmittance.

Sample

Detector

Lens Lens

Ligth

source


Aerogel tiles were carefully cut into samples with thickness of 1 cm, and with parallel sides.

Then, the incident, Ii, and transmitted, It, intensity of a monochromatic beam with a fixed

wavelength were measured. The optical transmittance, T, is defined as the ratio between

transmitted and incident intensities (with a fixed wavelength):

T = It/ Ii (Eq. III.8)

The transmittance, T, is related to the aerogel thickness, x, as:

Where the constant k is related to the sample structure. Therefore, from the k value obtained

it may be possible to extract some structural information. The optical absorbance, A, is

defined as:

Figure III.21 shows the experimental absorbance obtained for different synthesized samples.

Figure III.21 Absorbance versus wavelength at UV-VIS range for four acetone aerogels, A-series, and a methanol aerogel, M.

)10.III.Eq(kxTlnA

)9.III.Eq(eT kx

0

1

2

3

4

5

375 475 575 675 775 875

wavelength (nm)

Ab

so

rvan

ce

A1

A4

A3

A2

M

Ab

sorb

ance

(a.

u)


For all aerogel samples it was observed that the absorbance was larger at blue wavelength

(358nm) than at red (635nm). It may be used to quantify the red shading of aerogels as the

transmitted light is passed through a silica aerogel and the slight bluish haze when an

illuminated piece is viewed against a dark background. The optical transmission of the

aerogels was measured at a wavelength of 900 nm in order to compare quantitatively the

degree of transparency of the samples at the visible range. The results are presented in Table

III.13.

Table III.13 Optical transmission at 900nm for 1 cm thick aerogels.

Sample Transmission

at 900nm [%] Transparency

A1 23 Opaque

A2 28 Translucent

A3 30 Translucent

A4 40 Transparent

M 70 Very transparent

It was observed that the aerogels prepared using methanol as a solvent and under basic

conditions are more transparent than other solvents. This may be due to the fact that as size

of the alkoxy group increases, steric hindrance occurs, then when acetone was used as a

solvent instead of methanol leaded to larger pores, less homogeneity and hence a decrease in

transparency of the aerogels.

In Figure III.22 was observed that the amount of precursor strongly affects the optical

transmission due to the structural change of pore and particles of the aerogel. The lowest

transmission was obtained when more diluted sol conditions (A1), the percentage of

transmission was of 23% for a sample with a thickness of 1 cm. In this plot the thickness of

the aerogels are not normalized. A1 (x=0.75cm) thickness is smaller than A2, A3, A4 and M

(x=1cm). The large size pores may be responsible for the opaque nature of A1 aerogel.


Figure III.22 Percentage of optical transmission (at 900 nm) vs. TMOS volume ratio, v, for the acetone series with H2O/TMOS ratio fixed at 4.

In next section, the percentage of transmitted light will be related to the diameter size of the

porous, larger porous leads to lower transmission.

4.2 RAYLEIGH SCATTERING

Most of the light that our eyes receive comes not directly from the light source but comes

through the scattered light (light that reaches our eyes in an indirect way). The phenomenon

of scattering leads to several well-known natural effects, such as blue skies, red sunsets, the

white (or gray) color of clouds, and poor visibility on foggy days [42, 43]. Mie developed a

very complete theory for scattering spheres of arbitrary size using electromagnetic theory

[44].

As mentioned before, in silica aerogels the Rayleigh effect is observe by the reddening of

transmitted light (red light has a longer wavelength, and is scattered less by the fine structure

of aerogels) and the blue appearance of the reflected light of silica aerogels.

Scattering results from the interaction of light with inhomogeneities in aerogel structure. The

actual entity that causes scattering, called the scattering center, can be as small as a single large

molecule (with an inherent inhomogeneity) or clusters of small molecules arranged in a non-

uniform way. However, scattering becomes more effective when the size of the scattering

center is similar to the wavelength of the incident light. This occurs in small particles (~400-

0,1 0,2 0,3 0,4

20

25

30

35

40

vTMOS

/vTMOS+acetone

Tra

nsm

issi

on

at

900n

m [

%]

A1

A2

A3

A4


700 nm in diameter for visible light) that are separated from another, or by larger,

macroscopic, particles with inherent irregularities. When scattering centers are smaller in size

than the wavelength of the incident light, scattering is much less effective. In silica aerogels,

the primary particles have a diameter of ~2-5 nm, and do not contribute significantly to the

observed scattering. However, scattering does not necessarily arise from solid structures.

There is in silica aerogels, a network of pores, which can act, themselves, as scattering

centers. The majority of these are much smaller (~20 nm) than the wavelength of visible light

(see section 1 on Porosity). There are, however, invariably a certain number of larger pores

that scatter visible light. Control of the number and size of these larger pores is, to a certain

degree, possible by modifying the sol-gel chemistry used to prepare the aerogel. As scattering

efficiency is dependent on the size of the scattering center, different wavelengths will scatter

with varying magnitudes.

In this section, a method is proposed to quantitatively measure the relative contributions of

Rayleigh scattering (21-23) and the wavelength-independent transmission factor (due to

surface damage and imperfections) for silica aerogels prepared with different recipes and/or

drying procedures.

The transmission spectrum of an aerogel slab of known thickness is given by:

4

Cx

AeT (Eq. III.11)

Where:

T = transmittance

A = wavelength independent transmission factor,

C = intensity of Rayleigh scattering,

x =sample thickness,

= wavelength of the incident light

Then, the transmission is plotted against the inverse fourth power of the wavelength and A

and C parameters can be determined by fitting to the equation III.11. Aerogels with a high

value of A and a low value of C will be the most transparent.


Figure III.23 Light UV/VIS wavelength transmission and Rayleigh scattering fitting. The data are fitted by lnT=lnA-

C.x/ 4.

Table III.14 lists A and C values of A-series and M aerogels obtained from the optic

transmission at ultraviolet-visible range, wavelength between 300 and 900 nm.

Table III.14 A and C parameters from the analysis of light transmission

Sample A C (nm-3)

A1 3.62 1.70 1011

A2 3.81 4.41 1010

A3 3.96 3.73 1010

A4 3.67 3.51 1010

M 4.17 1.27 1010

1,0x10-11

2,0x10-11

3,0x10-11

4,0x10-11

0

2

4M

A4

A3

A2

A1

A1

A2

A3

A4

M

ln (

Tra

nsm

issi

on

)

nm4


The fit of the absorbance data in Figure III.23 confirms that Rayleigh scattering contributes to

the extinction of the light through an aerogel. The fitting parameter C is a measure of the

transparency of the aerogel, for lower values of C, more transparent are the aerogels. The

most transparent aerogel, M, has a C value of 1.27 1010 while the most opaque aerogel, A1, has a C

value of 1.7 1011. A is the wavelength-independent transmission factor that gives the

contribution of the surface effects on the optical transmission of visible light (due to surface

damage and imperfections). Besides the wavelength factor, the scattering intensity depends of

the pore size distribution. It is the absence of macropores, which is primarily responsible for

the lower light scattering and therefore higher visible transmission for M sample. However,

the light scattering measurements do not provide much information about the size of primary

particles, which are too small compared to the light wavelength. Section 5 will use the

dependency of visible light versus scattering angle to get further structural information.

4.2.1 A model to interpret the porous aerogel structure using

Rayleigh scattering

In this section, the correlation of scattering measurements with other structural investigations

(BET) has been attempted. The principal idea is to correlate the pore information obtained

by using the BJH pore size distribution (that determines the mesoporosity), with the Rayleigh

scattering measurements (that determines the macroporosity). Making use of both results,

one can obtain information on a wider range of pore sizes.

Silica absorbs only slightly in the visible and near ultraviolet, so most of the optical

attenuation results from Rayleigh scattering. For a material with pore structures in the 1 to

100nm size range, the strong Rayleigh scattering is expected towards the blue and ultraviolet

spectral region. The amount of scattered light in aerogels mainly depends on the number and

size of the pores. Thus, the optical transmission curve (see Figure III.21) is analyzed for

Rayleigh scattering and the data are fitted to the equation:

x).(eAT (Eq. III.12)

Where:

T = transmittance at the wavelength ,

A = wavelength independent transmission factor,

= wavelength of the incident light

( ) is the volumetric coefficient of scattering.


One might assume independent Rayleigh scattering from spheres with index of refraction n.

A characteristic size for the scatter centers (porous) can be derived from the C value using

the expression for the volumetric coefficient of scattering valid for an isolated spherical

scatter center with diameter < pore>Rayleigh:

2

2

2

4

43

Rayleigh

2n

1n)(8)1()( (Eq. III.13)

where is the volume fraction of air. The index of refraction for low-density aerogels is close

to one and was estimated with [22]:

)/(21.01 3cmgn (Eq. III.14)

Mean porous diameter derived from this calculation, < pore>Rayleigh, for the different samples

are shown in Table III.15. In order to show the discrepancies between mean pore diameters

obtained by the different analysis, the obtained mean pore diameters obtained by Rayleigh

scattering (Eq. III.13) were compared with the one obtained from the density of each sample

and from the BET analysis.

Table III.15 Mean pore values evaluated by three different

methods are compared, by BET, < pore>BET, by Rayleigh scattering

analysis, < pore> Rayleigh, and by bulk aerogel density, < pore>density.

Sample < pore>BET

(nm)

< pore>Rayleigh

(nm)

< pore>density

(nm)

A1 10 150 110

A2 30 50 60

A3 20 15 30

A4 20 20 20

M 50 40 60

The most astonishing feature is the difference between the mean pore diameters for the A1

sample as being determined by BET and LT techniques. For the most opaque sample, A1,

the mean pore diameter, < pore>Rayleigh, derived from this calculation was of 150 nm. This

value was very different of that obtained by BET, < pore>BET =10 nm. The reason of these

discrepancies may be due that BET accounts only in mesopore range, and A1 aerogel have

most of their pores in the macropore range (calculated by Rayleigh scattering). From


analyzing the differences in the data obtained using the different techniques one might obtain

the mean pore size distribution proposed in Figure III.24.

Figure III.24 Proposed pore size distribution for A1, A2, A3, A4 and M aerogels obtained by BET, Rayleigh scattering and density analysis.

The model accounts for the observed differences in the optical transparency of the aerogels:

the opacity in A1 is explained by the pore size distribution at macropore range (confirmed by

UV-VIS measurements). In addition, that clarifies why the A1 porosity was not measured by

BET. A2 and A3 have a bimodal pore size distribution with an important number of

micropores and/or macropores, but the mean pore diameter obtained is within the mesopore

range. That explains that BET measures only a part of the porosity. A4 presents a mesopores

distribution, which is totally accounted by the BET technique, and agrees with the mean pore

value obtained by optical measurements. M, completely transparent, have a pore size

M

A4

A3

A2

A1

Pore size (nm)

BET range

Mesopore range

1 10 100 1000


distribution that is mostly accounted by BET (~80%). Its mean value is similar for both

techniques, BET and optical measurements.

5. LIGHT SCATTERING MEASUREMENTS OF AEROGELS BY A

POLARIZATION-MODULATED NEPHELOMETER.

5.1 INTRODUCTION TO LIGHT SCATTERING VS ANGLE

EXPERIMENTS

Light scattering is a noninvasive and remote indirect method to derive structural information.

The experimental and theoretical development was carried out at the ‘Microstructured

Materials Group’ of the Lawrence Berkeley National Laboratory under the supervision of

Prof. Arlon Hunt and Dr. Michel Ayers. The objective of this work was to find the

correlation of the angular dependence of light scattering at visible regions with the structural

nature of the silica aerogel medium. In the previous section (Rayleigh scattering), it has been

proved that intensity of light measurements at fixed angle provide information about the size

of scattering centers. Few previous works on angular measurements at different wavelength

have been carried out for aerogels characterization. This section will demonstrate the

necessity of performing light scattering vs. angle measurements to obtain information outside

of the Rayleigh regime and to extract information about the inhomogeneities of the aerogel

microstructure. Since the nephelometer set up was not prepared to measure aerogels, and

very few articles about this field are published, the work in this section presented on

nephelometer characterization shows preliminary results indicating the advantage of using

this technique on supplementary structural information. Further studies should be

undertaken in order to optimize the use of this technique.

This section was structured in four sections:

1. Synthesis and drying of several transparent aerogel samples by the two-steps method

(explained in detail in chapter I and II).

2. Light scattering measurements of synthesized aerogels by a polarization-modulated

nephelometer (section 5.2)

3. Development of a model that gives structural information from the dependency of the

scattered light versus angle and wavelength (section 5.3).

4. And, finally the comparatives study between experimental measurements and theoretical

model (section 5.4).


Optical characterization of the aerogels was performed using scattering apparatus in which a

laser beam interacts with the specimen and a detector is rotated around the irradiated volume

section. Structural units comparable to the wavelength of the laser light cause a strong

isotropic scattering, and entities, which are smaller than the wavelength, cause more or less

isotropic scattering depending of the homogeneity of the microstructure.

5.1.1 Description of the polarization-modulated nephelometer

The polarization-modulated angle-scanning nephelometer apparatus, illustrated in Figure

III.25, was constructed to measure the wavelength, angular, and polarization dependence of

the scattered light intensity at visible regions. Three lasers were used, a first one was tunable

from 450 to 540 nm (from green to violet), a second one provides a fixed 635 nm red beam,

and the last one a ultra-violet beam at 355 nm. The wavelengths used for the scattering

measurements were of 355 nm, 458 nm, 488 nm, 514 nm and 635 nm provided the source of

light.

Figure III.25 Photograph of the nephelometer apparatus when silica aerogel light scattering is measured at blue wavelength

It was critical for correct measurements, the alignment of the detection optics and to ensure

that the scattering volume seen by the detector was at the center of rotation.

A cylindrical aerogel sample was correctly placed in the nephelometer taken attention to the

correct alignment with the incoming laser beam in order to diminish the shape and

geometrical effects on the detected light scatter intensity. Then, the incoming beam polarized

at an angle of 45 to the scattering plane, was focused into the aerogel, with plano-convex

aerogel sample

polarizer

laser beam

laser

Photomultiplier tube


lens. A rotatory detector was used to pick up the intensity of the scattered light at each angle

from 10 to 170 . The rotating arm carries collimation optics, a photomultimeter, and

polarizers oriented either parallel or perpendicular to the scattering plane. The rotating stage

is controlled by a personal computer. A scheme of the experimental apparatus of this set-up

is shown in Figure III.26.

Figure III.26 Schematic diagram of the angle-scanning nephelometer.

The formalism used to describe the polarization states of scattered light is based on the

Stokes vector [48]. The Stokes vectors describing incident and scattered light are connected

by a 4x4-element Mueller matrix, Si,j [48, 49]. I’=MI where the M is a 16-element Mueller

matrix, I is the Stokes vector of the incident light and I’ is the Stokes vector of the scattered

light. The information that can be gleaned from each element is dependent on the scattering

system. Depending on the polarizer’s configuration different elements of the Mueller matrix

can be obtained. In our experiments only S11 were analyzed because are those related with

the scattering intensity [48].

5.2 EXPERIMENTAL RESULTS

Scattering measurements were performed on aerogels following the above-described

conditions. In this section, it is proposed that a fraction of the aerogel scattering might be

due to the density fluctuations in the material on a scale range larger than the pore size [50,

51]. The predicted scattering from inhomogeneous two-phase materials will be described by

following the proposed hypothesis. Next figure shows the intensity of scattered light versus

scattering angle from a methanol aerogel sample, M, at different wavelength.


Figure III.27 The total intensity S11 on a log10 scale as a function of scattering angle for M aerogels at 4 different wavelengths. In each measure, the scattering angle varies from

10 to 170 . Intensity was arbitrary normalized at =90 and the geometrical factor for horizontally polarized incident light

was subtracted, 2

11

cos1

s, for each intensity.

The results given in Figure III.27 are the experimental results of the dependency of scattered

intensity versus scattering angle, , for a M aerogel. Each curve is the average of three

reproducible experimental data. The laser beam have different incident intensity for each

different wavelength, in the case of methanol sample was arbitrary normalized at 90 . The

angular dependence of the scattered intensity varied when using different wavelength. This

dependency is the most fundamental measurement for sizing and will be used in section 5.3

to obtain structural information of the aerogels.

The measurements of intensity versus h, )2/(sin4h , are called optical structure factor

measurement and leads to remark the variations in scattered intensity and quantify the

analysis. Figure III.28 shows the experimental s11 data for M aerogel plotted as a function of

h.

0 20 40 60 80 100 120 140 160 180

100

1000

10000

M aerogel

nm514

nm488

nm635

nm458

s 11/(1

+co

s2)


Figure III.28 Scattered intensity versus h, )2/(sin4h ,

from M silica aerogel for four different wavelength.

This experimental dependency will be used in next sections to calculate the density-density

correlation function. The intensity versus angle was also studied in samples with different

density and porosity using a fixed wavelength. Figure III.29 shows the dependency of the

scattered light for M, A4 and A3 for 635 nm wavelengths. The intensity of the scattered light

for A1, A2 aerogels was too large to get any useful data.

Figure III.29 Scattered intensity versus h for M, A3, and A4 silica aerogels at fixed wavelength: 635 nm. The intensities are not normalized.

0,0000 0,0005 0,0010 0,0015 0,0020 0,0025

1E-3

0,01

0,1

M

A3

A4

s 11

h

0,0000 0,0005 0,0010 0,0015 0,0020 0,0025 0,0030

100

1000

10000

M aerogel

nm458

nm488

nm514

nm635

s 1

1

h=4 sin( /2)


As treated in the Rayleigh scattering section, the intensity versus wavelength gave us some

information about pore size distribution, but if we want to get additional characterization of

the density inhomogeneities in the aerogels, the angular Rayleigh effect caused by the scatter

centers should be normalized, sin

1 .

5.3 STRUCTURAL INFORMATION FROM THE LIGHT SCATTERING

MEASUREMENTS

A model to interpret the experimental results may allow extracting structural information of

the aerogels.

5.3.1 Inhomogeneous media

In treating the scattering by a particle, it is assumed that the medium where the particle is

comprised is optically homogeneous. However, extended continuous media scatters light,

indicating that the solid material contains inhomogeneities. The problem of characterizing

inhomogeneities in solids and of relating these to the scattered intensity has been treated by

Debye using the Rayleigh-Debye theory [52].

In this section, the Rayleigh effect caused by the presence of ‘discrete’ scatter centers will be

normalized. By the scattering, we will obtain information about the inhomogeneities of the

aerogel media by treating the case assuming that the inhomogeneity of the medium is due to

a continuous variation of the dielectric constant. The macroscopic property of the solids

appears to be uniform. However, the dielectric constant varies from point to point, then the

dielectric constant at point A is given by [53]:

AA (Eq. III.15)

Where is the average values and A a local variation at the point A.

The study of the local variation A is related to the determination of the inhomogeneities of

the media. The correlation distance, BA

, is the average extension of the inhomogeneities.

In order to visualize the correlation distance, one can consider the product of the fluctuations

A and B at two points A and B separated by a distance r. The average of this product for all

points of the solid will depend upon the distance r. For r=0, BA is obviously equal to the

mean square value of the fluctuation 2 . If the scattering medium is statistically uniform and


isotropic, the correlation function will depend only upon the magnitude r, and will vanish for

sufficiently large r, 0BA.

The correlation function is defined by:

2

BA)r( (Eq. III.16)

Figure III.30 shows a scheme that may help to understand the behavior of the correlation

function in a two-different medium as aerogels are. Aerogels can be considered a physical

mixture of two media: the silica solid skeleton, medium A, and the pore network: medium B.

Figure III.30 Sketch describing the two aerogels media: the silica solid skeleton, medium A, and the pore network: medium B. To understand the variation on density-density correlation function the two figures may be progressively superposed, then density-density correlation function may account for the ‘self-similitude’ of the aerogel microstructure.

The density-density correlation function can be visualized with the aid of Figure III.30 that

represents the variations in density when left and right figures were superposed giving a

scheme about the ‘self-similitude’ of the microstructure. Then, an idea of the average

extension of the inhomogeneities is the steepness of the correlation function from 1 (r=0) to

0 (large r). A narrow shape (continuous line in Figure III.32) means short correlations: large

porous and more irregular structure. A broad shape (discontinuous line in Figure III.31)

means long correlation: smaller pores and structure that is more regular. If the pore size

distribution is very wide, then, the correlation density of the pore structure is too small to

contribute.

r

r

'

Medium A:

Silica skeleton

Medium B: Pore


Figure III.31 Density-density correlation function for two types of aerogel microstructure: the discontinuous line means long correlation (smaller pores and regular structure) and the continuous line means short correlations (large porous and more irregular structure).

The presence of the correlation function (r) in the scattered intensity equations permits one

to use the angular distribution of the scattered light to determine density-density correlation

in the aerogels medium and then, when analyzing this correlation function, allows to obtain

structural information. Following the Rayleigh-Debye mode the scattered light is defined by

[52]:

I 4

0

rr2

r( )sin h r( )

h rd

h 4

sin2

(Eq. III.16)

Where:

is the scattering angle.

is the wavelength of the laser beam.

(r) is the density-density correlation function.

Short range correlations: Rayleigh scattering

If the medium contains only short-range correlations in comparison with the wavelength of

the light (discrete scatter centers with a size similar of the wavelength). Then (r) vanishes as r

increases, in this case, the angular and wavelength dependence of the scatter intensity is the

described for the Rayleigh scattering (section 4), symmetrical about 90 and proportional to

-4.

r 5nm 200nm


Long range correlations: departures from Rayleigh scattering

When correlation (r) does not vanish for values of r comparable to the wavelength then, if

the correlation function (r) is known, the scattering can be predicted. For example, P.

Berdhald and A. Hunt assumed that the correlation function consists of a short-range

exponential part, 1(r), and a long-range gaussian part, 2(r):

22

2

1 a

r

a

r

wee)w1()r(2)r(1)r( (Eq. III.17)

Figure III.32 Density-density correlation used in the predictions for silica aerogels, consisting of a short-range exponential and a long-range gaussian part function

In aerogels, it can be assumed that short-range correlations will be related to the silica

nanoparticles, a1, and large-range correlation to the pore structure, a2. The parameter w is the

fraction of short to range correlation.

Substitution of the proposed correlation function (Eq. III.16) into the intensity equation (Eq.

III.17) and performing the Fourier transform of the intensity dependency on and , the

intensity resulted in the two components:

i i1 i2 (Eq. III.17)

Where:

i1 8 1 w( )a1

3

1 162

sin1

2

2

2a1

2

2

0 1 104

2 104

3 104

0

1.1

0.0

1

r( )

300000 r

a1 4000 a1

part icle radius

a2 porous radiusa2 500

22

2

1 a

r

a

r

wee)w1()r(


i2

3

2w a2 a2

2exp 4

2sin

1

2

2

2a2

2

Figure III.33 illustrates the dependency of the predicted scattered intensity for the proposed

model versus the scattering angle for each of the used laser. Various wavelengths will cause

different scattering intensity in the same aerogel.

Figure III.33 Theoretical angle dependency of the scattered

light intensity, i( , ), for each of the 5 laser wavelength.

Where: 1 = 355 nm ultraviolet wavelength, 2 = 458 nm

violet wavelength, 3 = 488 nm purple wavelength, 4 = 514

nm green wavelength, and 5 = 635 nm red wavelength.

5.4 COMPARATIVE STUDY BETWEEN EXPERIMENTAL

MEASUREMENTS AND THEORY

The experimental measurements of the angular distribution of the scattered light compared

with the theoretically predicted values of intensity. The experimental data have been fitted to

the obtained theoretical intensity function, i( , ), by the variation of the parameters a1, a2

and w. Rayleigh angular dependence and a wavelength-dependent intensity have already been

removed from both the experimental and theoretical expressions. The following procedure

was used to fit the proposed model to the experimental data:

0 50 100 1500

1 1010

2 1010

i 30 4580( ) 1.2

0

i 4880( )

i 5145( )

i 6350( )

i 4580( )

i 3550( )

1800

1

2

3

4

5


i) The experimental intensity/angle output file was appended into a matrix for posterior

fitting process. In the fitting process, the function readprint reads a data matrix into the

document from i( , ) file for each of the aerogel samples measured.

Figure III.34 Experimental data file for =514.5 nm of a M aerogel sample.

ii) Then, The MathCAD program has been used to find a linear combination of i1 and i2

functions that best fits to the experimental data. The proposed functions i1 and i2 were

entered in the vector F ion order to be fitted:

iii) For each wavelength, an independent file is obtained, so the same procedure to fit a1, a2,

and w parameters was repeated in for each of the wavelength experimentally used.

Figure III.35 compares the experimental curve with the curves fitted from the model

proposed for a M silica aerogel at 635 nm with a final values of a 1=50 nm, and a2 =400 nm.

The measured scattered intensity is fairy well predicted by the calculations, even is incorrectly

predicted for angles close to 10º. This disagreement cannot be remedied by calculations

0 60 120 180

0.02

0.04

.051

1.61 103

int1

1800 int( )0

Scat

tere

d in

tensi

ty

F

3

2w a2 a2

2exp 4

2sind

1

2

2

2a2

2

1 w( ) 8a1

3

1 162

sind1

2

2

2a1

2

2


assuming any reasonable a1, a2 and w values. A "better" fit could be obtained ignoring

several of the data points near = 0º and 180º. Further measurements of s11 may be

performed in order to diminish this effect.

Figure III.35 Intensity of scattered light versus angle for a M sample with red wavelength. Solid line shows fitted curve and cross curve gives experimental measurements. The correlation parameters obtained were: a1=50nm, a2=400nm, w=310-6

For the same aerogel sample, the intensity function was fitted for each of the wavelength

resulting in a similar a1, a2, and w values for the different wavelength. The fitted values are in

agreement with those values obtained by TEM (for particle size) and pore size (150 nm by

the model presented in previous section).

Figure III.36 compares the experimental curve with the curves fitted from the model

proposed for a two-step silica aerogel at 635 nm with a final values of a 1=150 nm, and a2

=2000 nm. The residual plot, q

jmin X( ) j

max X( ) min X( )( )

npoints , shows the differences

between theoretical and measured angle distribution of the scattering.

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120 140 160 180

Serie1

Serie2

Serie3

Aerogel

H5

red

purple

green

M

i/


Figure III.36 Intensity of scattered light versus angle for a H5 sample with red wavelength. Solid line shows fitted curve and cross curve gives experimental measurements. The residual plot, qj, shows the differences between theoretical and measured angle distribution of the scattering. The correlation parameters obtained were: a1=50nm, a2=400nm, w=310-6

The total intensity results of Figure III.36 shows that the predicted calculations are quite

good approximations to the experimentally measured data. Moreover, the fitted parameters:

a1=50nm, a2=400nm shows poor agreement with the pore and particle values obtained by

other techniques. A possible reason is that the two-step analyzed sample was extremely

transparent so the effects that caused scattering are exclusively the surface damage

(microcracks in the range of 2 microns, a2 value). In this case, the nephelometer technique

does not give any further information.

Further studies may be developed in order to improve the fitting of the data with the aim of

extract more accurate structural information from the scattered intensity. Also to normalize

the intensity for each laser beam.

5.5 CONCLUSIONS AND FUTURE WORK

The nephelometer has been used to measure the angular and polarization dependence of

light scattered at visible regions. It has been proved that intensity measurements at variable

angle provide structural information outside the Rayleigh scattering regime. The model

proposed for a correlation function 22

2

1 a

r

a

r

wee)w1()r( has fitted the experimental data

50 100 1500

0.02

0.04

X-Y data

Least-squares fit

.051

0.001

Yi

fit q j

max X( ) 1min X( ) 1 Xi q j

i

50 100 150

0

Residual Plot

qj


by the variation of the a1, a2 and w parameters. The model has also been fitted with five or

four different wavelengths. New correlation functions should be proposed trying to provide a

more accurate fits to the experimental scattering data.

The normalization of the experimental curves should be changed from 90º to 20º. Further

studies may find a better physical interpretation. The useful part of the curves should be

normalized with different scale factors because the incident intensity was different for each

wavelength.

For the short range e -r/a1 correlation function, since a1<<1/h, the Fourier transform can be

taken independent of h. In addition, in some places it may be possible to ignore w as small

compared to one.

Assuming that the present correlation function (r) could be improved to fit the data well, we

should plot i( , ) as a function of h alone. It should be possible to get the data for all three

wavelengths to plot on the same curve, by adjusting the normalization. Having constructed

such a function, it will be needed to extrapolate it to h = infinity (there may be more than

one way to do this).

Attention should be taken in the interpretation of the analyzed curves. Strong backscattering

was observed for values close to 10º, this should account the strong inhomogeneities of the

surface. Therefore, when these curves were fitted large a2 values were obtained explaining the

larger pores of the surface.

Chapter III Bulk silica aerogel characterization 127

6 DIRECT METHODS: ELECTRON MICROSCOPY

Scanning and Electron microscopy (SEM and TEM) techniques yield direct images of the

aerogel structure. Thus, morphological features, such as particle shapes and particle

arrangements, can be recognized. An estimation of the particle size can also be obtained,

although the acquisition of enough data as to evaluate the particle size distribution is rather

tedious.

6.1 STRUCTURAL STUDIES BY SCANNING ELECTRON MICROSCOPY

A microstructural investigation has been taken up using scanning electron microscopy (Leica

360 scanning electron microscope).

Preparation of the samples

A special problem with the aerogels is the image degradation due to the charging of the

samples during the sample exposure to the electron beam. This charge is caused by the

strong electrical isolating nature of the silica aerogels. This problem is solved by depositing a

thin, conductive 20 nm gold coating by sputtering, thus apparently the gold penetrates in the

pores, making the aerogel more conductive avoiding electrostatic charge during the SEM

observations. In such case, the tenuous aerogel skeleton may be slightly distorted by the gold

coating. A second coating with silver contacts was done to facility the electrical contact

between sample and holder. Thus, SEM pictures must be interpreted with caution [54].

6.1.1 Acetone series

When silica aerogel samples were observed by SEM at low amplification, a porous structure

was observed. Figure III.37 compare, at the same scale, the morphology of two different

types of aerogels, A3 and A4. In both cases, it was observed a cracked surface, probably

caused by the preparation process of the samples: samples were carefully cut but it did not

avoid the formation of some cracks of some microns in size.


Figure III.37 Morphology of A3, and A4 silica aerogels imaged by SEM.

SEM micrographs of A-series aerogels were compared at the same enlargement in order to

know the effect of TMOS/acetone molar ratio (directly related with the aerogel density) on

the microstructures of silica aerogels. Figure III.38 shows the A1, A2 microstructure, and

Figure III.39 the A3 and A4 microstructure at the same magnification.

Figure III.38 SEM micrograph of A1, the lowest density aerogel from A-series, and A2 silica aerogel. Parallel arrows

mark the constituting particles, particle, and pores, pore.

5.0m 5.0m

A3 A4

1.0m

A2

1.0m

A1

pore

pore

particle particle


Figure III.39 SEM micrograph of A3, and A4 silica aerogels.

Parallel arrows mark the constituting particles, particle, and the

pores, pore.

In all the SEM images at 1-micron range, the aerogels had a granular appearance composed

by spherical particles and some pores. SEM images show that the A1 sample is built by

smaller interconnected particles than the denser A4 sample. The A4 particles showed the

smallest pores, although particles were larger than those of A1. The constituting particle size

distributions of 300 particles for each of the aerogel samples calculated from the

corresponding micrographs are shown in Figures III.40, III.41, III.42 and III.43,

corresponding to A1, A2, A3 and A4, respectively. The slash indicates the corresponding

magnification.

SEM images show that A1 sample is built by smaller interconnected particles (with a mean

size of 48 nm) than the denser A4 sample (with a mean particle size around 90 nm).

Figure III.40 SEM micrographs of A1 silica aerogel and particle-size distribution of 300 particles seen in micrographs.

20 40 60 80

Øp=48 nm

σ=8 nm

500 nm

500 nm Particle size (nm)

1 m

A3

1 m

A4

pore

pore

particle

particle




A4

500 nm 20 40 60 80 100 120

Mean 88,1

sd 11,4

Øp =88 nm

σ=11 nm

Particle size (nm)

Particle size

(

μm) 20 40 60 80 0

Ø=55 nm σ=8 nm

Particle size (nm) 500 nm


Figure III.43 SEM micrographs of A3 silica aerogel and particle-size distribution of 300 particles seen in micrographs: samples.

The average diameter for 300 particles calculated from a similar SEM micrograph is

summarized in Table III.16. The A1 sample showed the smallest particle size. Considering

mean particle size and density, one could assume that in A1 aerogel there are more particles

per unit volume and a larger number of contacts among them.

Section 2.3 of chapter IV shows a model that correlates mechanical properties and

microstructure.

6.1.2 Effect of the solvent

To investigate the effect of the solvent; SEM measurements have also been performed on two

samples with similar density but different solvent, acetone (A2) and methanol (M). Figure

III.45 compares the SEM micrographs from A2 and M aerogels.

A variation of particle size is observed; the more opaque aerogel (A2) contains closely bound

spherical particles of 30-80 nm in diameter and larger interstitial pores whilst the particle size

of the transparent aerogel (M) is smaller. M aerogel shows an interconnected band structure

with smaller particle size (less than 50 nm). These microstructures are consistent with the

observed differences in optical transparency of the obtained samples.

20 40 60 80 100

Øp=62 nm σ=11 nm

200 nm Particle size (nm)


Figure III.44 SEM micrograph of M aerogel shows interconnected band microstructure.

Figure III.45 SEM micrographs comparing microstructures of a) A2, and b) M silica aerogels.

6.1.3 Drying procedure

A2 has been dried in two different manners. First, by a supercritical extraction of the solvent

and secondly by exchanging the acetone by liquid carbon dioxide, samples are labeled as A2

and A2CO2 respectively. The resulting aerogel microstructures have been compared. Figures

III.46a) and III.46b) compare the morphology of A2 and A2CO2 aerogels.

Figure III.46 SEM micrographs comparing microstructures from A2, and A2CO2 aerogels.

500 nm 500 nm

A2 M

1 m 1 m

A2 A2CO2


A2CO2 presents a slightly more cracked structure and larger particles. See TableIII.16.

Shrinkage during drying is larger in A2CO2 than in A2 samples and their final density is similar

to A3 sample.

6.1.4 TMOS aerogels in carbon dioxide as solvent

The microstructure of the aerogels obtained at low temperature using supercritical carbon

dioxide as solvent was compared to the aerogel obtained by the classical method.

Figure III.47 SEM of HCOOH aerogel obtained at low temperature.

The aerogel microstructure for samples obtained without presence of water was more

polymeric than a sample with similar density obtained by one –step method. Moreover, the

presence of porous was hardly detected in HCOOH aerogels compare to one-step aerogels.

Table III.16 Mean constituting particle diameter, p of silica

aerogels measured by SEM.

Sample

(g/cm3)

p

(nm)

A1 0.080.01 483

A2 0.150.02 554

A3 0.230.02 622

A4 0.260.03 886

M 0.140.01 401

A1 + C 0.060.01 463

A2CO2 0.240.03 604


It should be pointed at that this values are slightly larger than those expected for constituting

particles. Probably, this effect is caused by the agglomeration of the nanoparticles by the gold

treatment. Although this coating distortion, this technique was useful in order to account for

the differences in microstructure, i.e. in A-series increasing density implies increasing particle

size.

Further studies may be done in order to study the dependency of the microstructure with the

variation of the so-called sol-gel parameters.

6.2 TRANSMISSION ELECTRON MICROSCOPY

In general, the counting of particles for obtaining a distribution of particle sizes is tedious

and, usually only two-dimensional projections are available. In this section, some three-

dimensional images were obtained by the acquisition of stereographic micrographs. The main

effect of the electron beam is to charge the small particles because are made of so strong

insulating material. Therefore, some instability occurred (spatial drift) which affect the

resolution. Other effect of the radiation was an overall deformation of the structure after

long irradiation time so special attention was taken to control this effect.

Sample preparation:

The specimen preparation is important if one wants to preserve initial structure of the dried

aerogels. Small pieces of material were directly produced by crushing the solid in the agate

mortar with the presence of some small pieces of glass to improve the crushing effect. The

fine powder was directly deposited on the grid. In the aerogels with replicas visualization, the

replicas were obtained loading the aerogels specimens onto the stage of a freezing

microtome, evacuated to 1.10 -6 Pa, and cooled to –185C. The aerogels were platinum-

carbon replicated at an almost vertical angle (80) and backed with a rotary deposited carbon

film at a 100 angle. The silica replica was removed from the aerogel with diluted acid. The

replicas were deposed on copper grids. It should be account that the Pt-C film increases the

average of particle size (approx. 0.5nm).

TEM set-up

Electron microscope observations were performed either, directly on the aerogel samples at

100kV, 200 kV or 400 kV, or imaging the aerogel replicas at 20k-80k magnifications. All

replicas samples were examined in stereo.

Low voltages give high contrast, but the resolution is limited to 5 Å. High voltage gives low

contrast but very good resolution (aprox. 2 Å at 400kV). However, this theoretical resolution

level has never been obtained due to the charging effect. Practically the best resolution at


400kV was of the order of 4 to 5 Å. Stereopairs were taken to study and visualize the three-

dimensional arrangement: a tilt angle of ±5º is usually sufficient. Stereopairs were examined

to give an idea about the three-dimensional arrangement and the number of chains at the

crossing points. The effect of radiation damage by the electron beam was studied in some

aerogel samples.

6.2.1 Imaging the acetone-series silica aerogels

Direct visualization

The structure of A-series was confirmed to be a three-dimensional network with an open

structure of small chains of diameter a, interconnected at average distances, d. Figure III.48

shows an example of A4 microstructure. The structure is defined by chains with connecting

points with a number of departing branches (either 2 or 3). The optical diffractographs

exhibit the usual rings due to an amorphous phase indicating no sign of crystallinity.

Figure III.48 TEM image of A4 silica aerogel shows length chains of 70nm, marked with arrows, with average particle

diameter of a=14.0 3.19nm, marked with parallel arrows, and

distance of d=30.8 3.9nm.

a

0 5 10 15 20 25 300

20

40

60

A46

Gaussian fit

Mean SD

--------------

14,0 3,1

a d


Figure III.49 TEM image of a detail of the particles of A4

silica aerogel showing a mean particle diameter of 16.8 2.9nm.

The comparison of several aerogels with different densities, A1 and A4 from series-A,

(Figures III.50-53) indicated that the basic elements of the 3D network are dependent on the

aerogel density.

Figure III.50 TEM image of A4 silica aerogel shows length

chains of 70 nm with average particle diameter of 2.1 0.6 nm

and distance of 30.8 3.9 nm

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 260

10

20

A4151 (A4+15%C)

Gaussian fit

Mean SD

------------

16,8 2,9

1,0 1,5 2,0 2,5 3,0 3,5 4,0 0

20

40

60

80

Mean 2,10109 SD 0,59944

Particle size (nm)

Ø p =10.1nm σ=0.6nm

5 7.5 10 12.5 15 17.5 20 Particle size (nm)


Figure III.51 TEM image of A4 81K zoom silica aerogel.

Figure III.52 TEM image (x160K) of a detail of the particles of A1 silica aerogel showing a mean particle diameter of 8 nm.

Figure III.53 Zoom of A1 silica aerogel TEM image.

0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 0

20

40

60

Particle size (nm)

Mean = 0,00104 SD = 1,61623

Ø p = 8.0 nm σ= 0.1 nm

2.5 5 7.5 10 12.5 14.5 Particle size (nm)


The chain diameter is smaller by 30% for a density variation of 30% and the average distance

increased of 15%. At very low magnification, the organization of the different densities was

hardly observed. The structure of A4 was shown more compact. The chain diameter is

smaller and the network was much tighter.

6.2.2 Imaging the methanol silica aerogels

Transmission electron microscopy was used to study the morphology of the most

transparent one-step silica aerogel. The samples were fractured, vertically replicated with 0.95

nm Pt-C and backed with approximately 12 nm of rotary evaporated carbon. The silica

aerogel was then removed from the replica with dilute acid and the replicas were studied by

TEM. The stereoscopic TEM images reveal that particles in methanol aerogels are slightly

smaller than acetone aerogels and that their structure is markedly more polymeric. This

morphology results from side-chain formation on a nearly linear structure. For M, the

particles have diameters ranging from 1.7 to 14.2 nm with an average of 6.4 ± 0.5 nm and the

chains lengths averaged 62 ± 21 nm with some as long as 132 nm. Pore sizes ranged from 12

to 277 nm with an averaged 61 ± 56 nm. The pores were slightly larger than pores in A-

series, which ranged from 13 to 240 nm with an average of 74 ± 43 nm.

Replicas visualization

In Figure III.54, the surface of a M silica aerogel with TMOS/acetone/water 1/12.25/4 is

shown at 80k magnification. If this image is visualized with a 10x magnifier, the silica chains

can be observed. In Figure III.55, it is possible to observe Figure III.54 in three dimensions

using stereo glasses. This aerogel, when viewed in stereo, demonstrates that the „airy‟

appearance of the aerogel structure. It can be observed that the chains contain many small

spherical particles. The range in size is from 5 to 15 nm with an average diameter of 10±1nm.


Figure III.54 TEM micrograph of base-catalyzed silica aerogel. 50z M-aerogel: low temperature replica. In order to see the detail of the structure of the aerogel it should be viewed with an eye loop (x10)

Figure III.55 Stereoscopic images (tilt angle of 10 ). Aerogel skeleton is formed by interconnected particles in a three-


dimensional structure (average diameter of 6.4±1nm). Low temperature M-aerogel replica.

Figure III.56 Stereoscopic image of detail M-aerogel replicas at low temperature.

Figure III.57 Stereoscopic image of M-aerogel replicas. On the right, a stereoscopic image of M replicas is shown.

Summarizing TEM was used to examine a series of acetone silica aerogel (A-series) and a

base catalyzed aerogel with methanol as solvent (M). This technique provided molecular

information about the aerogels and enables to distinguish the different parts of the structure

of the aerogel: individual chains and crosslinking junctions were visualized. TEM microscopy

was used to examine either directly the silica aerogel or the surface replicas. The stereoscopic

images with a tilt series at 20k-80k magnifications have made possible a three-dimensional

visualization of the aerogel structure.


7 REFERENCES

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6. Dependence of monolithicity and

physical properties of TMOS silica

aerogels on gel aging and drying

conditions G. M. Pajonk Journal of

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7. Comparison of some physical

properties of silica aerogel monoliths

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Materials Chemistry and Physics

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8. Drying of aerogels in different solvents

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pressures

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Crystalline Solids Volume 225, Issue 1

April 1998 Pages 14-18

10. S.S. Kistler. J. Phys. Chem. 36 (1928),

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12. G.M. Pajonk, A.V. Rao, B.M. Sawant

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13. K. Tajiri, K. Igarashi and T. Nishio. J.

Non-Cryst. Solids 186 (1995), p. 83

14. A. Emmerling and J. Fricke. J. Non-

Cryst. Solids 145 (1992), p. 113

15. Supercritical drying media modification

for silica aerogel preparation. Satoshi

Yoda Journal of Non-Crystalline Solids

Volume 248, Issues 2-3 2 June 1999

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16. Influence of molar ratios of precursor,

solvent and water on physical

properties of citric acid catalyzed

TEOS silica aerogels. P. B. Wagha, A.

Venkateswara Rao, and D. Haranatha

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Volume 53, Issue 1 April 1998 Pages

41-47

17. Structural development of silica gels

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18. W,. Cao, and A.. J. Hunt, J. Non-Cryst.

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19. Physical properties of silica gels and

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and N.N. Paravathy. J. Non-Cryst.

Solids 209 (1997), p. 40

21. G.W. Scherer, J. Non-Cryst. Solids 145,

33 (1992).

22. J.D. Mackenzie, Applications of the

sol¯gel method: some aspects of initial

processing, in: L.L. Hench, D.R. Ulrich

(Eds.), Science of Ceramic Processing,

vol. 113, Wiley, New York, 1986

23. Smith D.M, Hua D.W. and EarlW.L.,

MRS Bulletin, 44 - 48 (1994).

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Area and Porosity, Chapman and Hall,

London, 1984.

25. J. Fricke, R. Caps, D. Buttner, V.

Heinemann, E. Himmer, G.

Reichenamer, Structural, elasto-

mechanical and thermal properties of

silica aerogels, in: K.K. Kruger et al.

(Eds.), Characterization of Porous

Structure, vol. 629, Elsevier,

Amsterdam, 1988 ll

26. S. Brunauer, P.H. Emmet and E.

Teller. J. Am. Chem. Soc. 60 (1938), p.

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27. Mercury porosimetry: applicability of

the buckling intrusion mechanism to

low-density xerogels, Journal of Non-

Crystalline Solids, Volume 292, Issues

1-3, November 2001, Pages 138-149

Christelle Alié, René Pirard and Jean-

Paul Pirard

28. C. Pierce , J. Phys. Chem. 57 (1953) 149.

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30. S. Yoda, S. Ohshima and F. Ikazaki. J.

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"Better ceramics through chemistry II"

Ed. Brinker, C.J., Clark, D.E., and

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33. Nitrogen sorption in aerogels, Journal

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G. Reichenauer and G. W. Scherer

34. Extracting the pore size distribution of

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39. B.E. Yoldas, J. Non-Cryst. Solids 63

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40. .Fricke and T. Tillotson, Thin Solid

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41. H. C. Van de Hulst, Light scattering by

small particles, Wiley, New York, 1957.


42. M. Kerker, The scattering of the light

and other electromagnetic radiation,

Academic Press, New York, 1969.

43. G. Mie, Ann. Physik 25 (1908) 377.

44. Rubin M. and Lampert C.M., Solar

Energy Materials 7 (1983) 393-400.

45. Born M. and Wolf E., “Principles of

optics” 6th ed.

46. Kistler S.S and Caldwell A.G., Ind. Eng.

Chem., 26 (1953) 658.

47. H. Mueller J.Opt. Soc. Am. 38 661

(1948).

48. A.J. Hunt, Proceedings of the intern.

conf. on ultrastructure processing of

ceramics, glasses, and composites,

(1983)

49. A. J. Hunt and P. Berdahl Mat. Res.

Soc. Symp. Proc. 32 (1984), 275

50. P. Debye, Ann. Phys. 30, 59 (1909).

51. Sorense C.M., Aerosol Science and

technology 35 (2001) 648-687. A

52. M. Kerker, The scattering of light,

Acacemic Press, New York, 1969

53. Bourret, A. Europhys. Lett. 6, 731

(1988)

54. Schaefer, D. MRS Bulletin, 49 (1994)

55. G. C. Ruben J. of Non-Crystalline

Solids 186, 209 (1995)

/science?_ob=IssueURL&_tockey=%23TOC%235594%231995%23998139999%23153967%23FLT%23display%23Volume_186,_Pages_1-427_(June_1995)%23tagged%23Volume%23first%3D186%23Pages%23first%3D1%23last%3D427%23date%23(June_1995)%23&_auth=y&_acct=C000000152&_version=1&_urlVersion=0&_userid=635986&md5=a749b2564894e56ecfc1194a7afdc492


SD = 1,61623 Mean = 0,00104 Particle size (nm) 60 40 20 0 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5

C h a p t e r I V

MECHANICAL CHARACTERIZATION OF SILICA AEROGELS

SECTION OUTLINE

1. INTRODUCTION ............................................................................................................................... 146

1.1 MICROINDENTER DESCRIPTION .................................................................................................147

1.2 MECHANICAL CHARACTERIZATION ...........................................................................................149

2. MECHANICAL PROPERTIES OF SILICA AEROGELS AS A FUNCTION OF

DENSITY ............................................................................................................................................................ 151

2.1 SAMPLE PREPARATION ................................................................................................................151

2.2 EFFECT OF THE ALKOXIDE CONCENTRATION..........................................................................152

2.2.1 Relationship between silica aerogel microstructure and mechanical

properties ....................................................................................................................................... 154

3. INFLUENCE OF SOLVENT AND SUPERCRITICAL DRYING METHOD ON THE

MECHANICAL PROPERTIES .................................................................................................................... 156

3.1 EFFECT OF THE DRYING PROCEDURE........................................................................................156

3.2 EFFECT OF THE SOLVENT IN MECHANICAL PROPERTIES .........................................................158


COMPOSITES ................................................................................................................................................... 159

4.1 INTRODUCTION............................................................................................................................159

4.2 EFFECT OF THE CARBON ADDITION ..........................................................................................160

5. VISCOELASTICITY OF SILICA AEROGELS AT ULTRASONIC FREQUENCIES ...... 166

5.1 INTRODUCTION .....................................................................................................................166

5.2 EXPERIMENTAL SET-UP: AIR-COUPLED BROAD-BAND PIEZOELECTRIC TRANSDUCERS ........167

5.3 DETERMINATION OF THE VISCOELASTICITY OF SILICA AEROGELS AT ULTRASONIC

FREQUENCIES. ..............................................................................................................................169

5.3.1 Normal incidence ...................................................................................................... 169

5.3.2 Oblique incidence ..................................................................................................... 171

6. MECHANICAL CHARACTERIZATION CONCLUSIONS ................................................... 173

7. REFERENCES...................................................................................................................................... 175

Chapter IV Mechanical characterization of silica aerogels 146

1. INTRODUCTION

The mechanical properties of the aerogels are crucial for almost any application because the

need of handling and machining the material, as well as its durability in rough environments.

In addition, there is fundamental interest in understanding the variation in elastic properties

with density, and to compare experimental results with theoretical models. The aim of this

chapter is to present a description of the mechanical characterization of the silica aerogels

which were prepared through out the thesis (chapter II). The mechanical measurements of

such samples were carried out by Elena Martínez at the „Departament de Física Aplicada i

Òptica‟ of the Universitat de Barcelona.

The mechanical characterization of silica aerogels presents some difficulties. Aerogel is a

fragile material, difficult to handle and brittle (very small loads are sufficient to crack them).

Efforts are being made on improving their mechanical properties and on overcoming the

difficulty of measuring such properties. Several testing methods for mechanical

characterization have been previously reported, namely; Young‟s modulus and toughness are

usually obtained by using ultrasonic sound velocity, [1-2-3], direct longitudinal compression

methods, [2, 4], and measurements by the three-point bending beam technique [5,6]. It has

been reported that tensile and bend specimens can give extraneous deformations under

estimating of the true modulus [7]. AFM in contact mode has also been used to measure

Young‟s modulus of aerogels [8]. Mechanical studies of reinforced aerogels using Vickers and

Knoop indentation techniques have also been published [1, 9] but are not sensitive enough to

measure the mechanical response of such materials. On the other hand, the Young‟s modulus

of aerogels is a factor 102 to 104 smaller than that of silica glass [10] and, therefore, they can be

easily compressed. It is a major challenge to improve the mechanical properties of such

materials without sacrificing others and also, to find a suitable non-destructive method for

their mechanical characterization.

Dynamical microindentation is a powerful technique, initially developed for thin film

mechanical characterization, where low applied loads and small penetration depths are required

[11-12-13]. When it is applied to aerogels, it presents the added advantage of recording the

continuous measurement of load-penetration curves, which makes the direct observation of the

indentation marks unnecessary. An asset of this technique is that the specimens do not need any


machining for the test to be performed, since the machining process may alter the sample

microstructure. Young‟s modulus (E), elastic parameter (EP) and indentation hardness (H)

can be measured by applying very small loads (~1mN), small enough to slightly deform the

composite aerogels while preventing crack formation in these brittle materials. This technique

allows elastic and plastic behavior to be differentiated.

The results are presented in five different sections corresponding to the mechanical

characterization of different aerogel series that were synthesized by a variety of the sol-gel

processes previously described in chapter II.

i) First section is an introduction to the mechanical characterization of silica aerogels

and a description of the microindenter.

ii) Second section shows the results obtained in a series of samples with different

densities. Results were analyzed as functions of density, , morphology and pore size

distribution. A relation of the type, E with ~2.9 was found for the acetone-

synthesized series. As a function of aerogel density, two different regimes of

mechanical behavior were observed. The lowest density aerogels are elastic but the

denser aerogels are elasto-plastic materials.

iii) Third section presents the mechanical characterization of different series of silica

aerogels using different supercritical drying procedure and different synthesis solvent.

iv) Fourth section presents the results obtained in a series of silica-carbon composite

aerogel obtained with a variable amount of activated carbon (Annex articles: article

III).

v) Fifth section reports the relationship between mechanical properties and the

microstructure of the silica aerogels and carbon-silica aerogel composites.

1.1 MICROINDENTER DESCRIPTION

The microindentation measurements were carried out on a Nanotest 550 (Micro Materials

Ltd., U.K.) provided with a Berkovich diamond indentation tip [14].


Figure IV.1 Photography of Nanotest 550 (Micro Materials Ldt.)

The Nanotest apparatus consists in two pendulums (1), and (2), and an indenter (3)

integrated in one platform (on the right of the photo). A second platform integrates the

motor (4), which allowed moving the sample (on the left of the photo). This system is placed

in an isolating chamber. The measured signal (load and penetration depth) is processed by a

control unit connected to a computer. The computer allows to control the process of

indentation, order the displacement of the motors and record the data. Figure IV.2 shows a

schematic design of the system.

Figure IV.2 Schematic design of Nanotest 550 system

weight to balance

the pendulum

Scheme of

Nanotest 550

weight to balance

the pendulum

weight to balance

the pendulum

Scheme of

Nanotest 550

•Non-destructive technique

• Doesn’t need sample preparation•

Dynamic microindentation

• Extremely small applied loads

• Diamond Berkovich tip

• Maximum loads up to 500 mN

• Maximum penetration

up to 10 mm

• Resolution: 0.1 nm, 0.1 mN

(1)

(2)

(3) (4)


The Nanotest consists in a diamond tip attached to a ceramic pendulum. When a current is

applied along the coil, it is attracted towards the magnet moving the arm until the diamond

tip penetrates on the sample surface. The displacement of the tip is measured using a

variable capacitor. The limit stop defines which will be the maximum penetration for each

type of sample. Continuous loads and penetrations are automatically recorded.

1.2 MECHANICAL CHARACTERIZATION

The microindentation system allows load and depth resolutions better than 1 N and 1 nm,

respectively [11]. The maximum applied load was 1 mN or 0.5mN for the most elastic

aerogels. These maximum loads led to maximum penetration depth in the range of 2 -7 m.

Each loading-unloading cycle was repeated at least 10 times at different points in each

sample to check the consistency of the results. Before measuring the unloading branch of the

cycle, the hold time at maximum load was varied between 0 to 600 seconds to determine if

aerogels were prone to stress relaxation or creep. Figure IV.3 illustrates a typical behavior for

the sample A2. It can be seen from Figure IV.3 that there is creep penetration when the

diamond tip is being held during some time at the maximum load.

Figure IV.3 Penetration creep as a function of time for the A2

sample under maximum indentation load.

0 200 400 600

Creep time (s)

2

4

6

8

Cre

ep p

ene

tratio

n (

m

)


The creep rate strongly increases for creep times less than 200 s and becomes almost

constant for longer creep times. This feature, if not considered, could lead to apparent higher

hardness values [15].

Figure IV.4 Scheme of penetration of the Berkovich indenter in

the sample surface.

Hardness values, H, were obtained from:

H = Pmax/A (Eq. IV. 1)

where Pmax is the maximum load and A is the projected indentation area for the maximum

penetration depth. For a Berkovich diamond tip, A is given by the expression A=24.5h t2,

where ht is the maximum penetration depth measured from the load-penetration curves (see

Figure IV.4).

Hardness values calculated by this method correspond to the total elasto-plastic

deformation. It should be pointed out that this is not the most usual hardness definition,

although is convenient in the case of very elastic materials, as aerogels are.

Young‟s modulus, E, is obtained from the analysis of the unloading branch of the

penetration vs. load curve by applying the method proposed by Oliver and Pharr [16] and

using Sneddon‟s relationship:

AE

dh

dP21

2, (Eq. IV. 2)

where is the Poisson‟s ratio ( =0.2 for a silica aerogel) [17,18] and dP/dh is obtained from

a polynomial fit of the unloading curve (see Figure IV.5).

P=0ht

hp

he

P

Pmax

P=0ht

hp

he

P

Pmax

P=0htht

hp

he

hp

he

PP

Pmax

ht: total penetration

depth

hp: plastic depth

he: elastic depth

Pmax: maximum load


Figure IV.5 Hysteresis curve penetration depth versus load

Another interesting parameter that can be derived from these measurements is the elastic

parameter, EP, which is similar to the one previously defined [19]. EP allows quantitative

comparison of the elastic behavior of the samples. EP [15], is defined as:

t

pt

h

hhEP (Eq. IV. 3)

where ht is the total penetration depth during the indentation process and hp is the non-recovery

depth (plastic depth) of the diamond tip inside the aerogel sample.

2. MECHANICAL PROPERTIES OF SILICA AEROGELS AS A FUNCTION

OF DENSITY

2.1 SAMPLE PREPARATION

In order to investigate the changes in the mechanical properties of the aerogels when varying

the concentration of the silicon alkoxide in the initial sol a series of acetone aerogels (samples

A1 to A4) was synthesized and characterized. Aerogels with different density, particle and pore

size were obtained using different molar ratio of TMOS/acetone. The molar ratio of TMOS

to water was always kept to four. Several volume ratios, v, defined as acetoneTMOS

TMOS

VV

Vv ,

were used. For v = 0.1, 0.2, 0.3 and 0.4, samples were labeled as A1, A2, A3 and A4 (chapter

II). All the obtained aerogels were monolithic, in the form of cylindrical rods of 1cm diameter

and lengths from 5 to 12 cm.

ht

h

Pmax

Loading

he

Unloading

P

hp

ht

h

Pmax

Loading

he

Unloading

P

hp

ht

h

Pmax

Loading

he

Unloading

P

hp

Pmax

Loading

he

Unloading

P

hp


2.2 EFFECT OF THE ALKOXIDE CONCENTRATION

Figure IV.6 shows the load-penetration hysteresis curves for each sample „as measured‟ without

any data smoothing or averaging.

Figure IV.6 Hysteresis loading-unloading curve penetration depth

versus load for the samples: A1 (+), A2 (∆), A3 (○) and A4 (□).

From those curves, we have obtained the three characteristic mechanical parameters,

Hardness, H, Young‟s modulus, E, and elastic parameter, EP. The results are shown in Table

IV.1, which also includes dilution and density for each sample preparation.

Table IV.1 Several volume ratios, v, were used to prepare the

aerogel samples, their densities, , mean particle diameter, p ,

and mean pore diameter, 0 , are listed below together with their mechanical properties: Hardness, H, Young‟s modulus, E, and elastic parameter, EP. Standard deviations are shown in parenthesis

and refer to the last digit. p / 0 is related with Young‟s modulus (see text).

Sample

label v

(g/cm3)

p

(nm)

0

(nm)

p

0

H

(MPa)

E

(MPa)

EP

(%)

A1 0.1 0.08 (1) 48 (7) 110 0.44 0.5 (1) 7.0 (9) 78 (1)

Load (mN)

.2 .4 .6 .8 1

A4

A1

A3

A2

0

1

2.

3

4

5

6

7

A1

Pen

etr

ati

on

depth

(m

)

Load (mN)

.2 .4 .6 .8 1

A4

A1

A3

A2

0

1

2.

3

4

5

6

7

A1

Pen

etr

ati

on

depth

(m

)

Load (mN)Load (mN)

.2 .4 .6 .8 1

A4

A1

A3

A2A2

0

1

2.

3

4

5

6

7

A1

Pen

etr

ati

on

depth

(m

)


A2 0.2 0.15 (2) 55 (8) 60 0.92 2.4 (3) 50 (2) 30 (1)

A3 0.3 0.23 (2) 70 (9) 30 2.3 4.8 (3) 153 (3) 10 (2)

A4 0.4 0.26 (3) 88 (11) 20 4.4 5.7 (3) 346 (6) 6 (2)

It can be observed that as the dilution of the sol changes from 0.1 to 0.4, the density of the

aerogels increases, so it does the hardness and Young‟s modulus. The EP values show a

decrease, as the samples become more plastic.

In order to analyze the evolution of Young‟s modulus, hardness and elastic parame ter as a

function of the aerogel densities, the experimental points were fitted to power law functions.

The resulting scaling exponent for the Young‟s modulus is = 3.0 0.2 (E ). This is in

agreement with other previous results reporting values within 2.5 and 3.8 [20, 21, 22, 23].

The scaling exponents for hardness, H, and elastic parameter, EP, were 2.0 0.1 and -1.7

0,4 respectively.

Figure IV.7 Plots of Young‟s modulus (a), hardness (b) and EP (c)

as a function of the aerogels density. Fittings are shown as solid

-3.0 -2.5 -2.0 -1.5 -1.0

log(density (g/cm-3))

-1

0

1

2

log

(H (

MP

a))

-3.0 -2.5 -2.0 -1.5 -1.0

log(density (g/cm-3))

0

2

4

6

log

(E

(M

Pa)

)

0.05 0.10 0.15 0.20 0.25 0.30

density (g/cm-3))

0.0

0.5

1.0

EP

H

E

EP


lines. The graphs of H and E are presented with the logarithm

values of the data.

From the load-displacement curves of Figure IV.6 and the results given on Table IV.1 it can

be observed that the aerogel samples investigated cover a wide range of mechanical

responses. In particular, lowest density samples are almost completely elastic materials, this is

seen because the loading and unloading branches do not show hysteresis, and they have a

very low Young‟s modulus and high EP value. While for larger density aerogels deformation

is almost entirely plastic and presents a much higher Young‟s modulus value, high modulus

values could be expected in a plastic material because Young‟s modulus accounts solely for

the elastic behavior of materials.

Hardness and Young‟s modulus increase and EP decrease when increasing the density of the

samples. Notice that hardness, as defined in Equation IV.1, is derived from the total elasto-

plastic penetration depth. If we had used the more common hardness definition, which

involves only the plastic penetration depth, then we would have obtained an unrealistically

high hardness value for the very elastic samples such as A1.

2.2.1 Relationship between silica aerogel microstructure and

mechanical properties


Microstructural observations were performed with a Leica 360 scanning electron

microscope. The analysis of such micrographs allowed us to assess the particle size

distribution for all the samples. Brunauer-Emmet-Teller (BET) porosity characterization of the

aerogel samples was performed with a Micrometrics ASAP 2000 instrument. The BET surface

area, SBET, was obtained from the adsorption isotherm of N2 at 77K. The mean pore diameter

was calculated by 0 = 4Vpore / SBET where Vpore = (1/ aerogel - 1/ Si lica). The results are shown

in Table IV.3.

Figure IV.8 Scanning Electron Micrographs and particle size

histograms for (a) A1 and (b) A4 aerogels. < p> mean particle

diameter, and σ standard deviation, calculated by SEM histogram

analysis.

SEM images (see Figures IV.8a) and b)), show that the A1 sample is built by smaller

interconnected particles than the denser A4 sample. The mean particle diameter obtained, p ,

for all samples were shown in Table IV.1. Assuming a similar fractal dimension between the

sample series [24] Young‟s modulus is related to the degree of necking among contiguous

particles. Necking contact increases with increasing particle size, p, but its density decreases

with pore size, 0. In fact, Young‟s modulus behaves linearly with the p / 0 quotient,

following E=k p / 0 , where k=3.4MPa.

20 40 60 80 100 120

Mean 88,1

sd 11,4 Ø =88nm σ=11nm

500 nm

20 40 60 80 100

48.5 nm

7.6 nm Ø

p

=48nm σ=7.6nm

a)

b)


3. INFLUENCE OF SOLVENT AND SUPERCRITICAL DRYING

METHOD ON THE MECHANICAL PROPERTIES

Silica gels were prepared by hydrolysis of TMOS under different conditions to evaluate the

different responses of the aerogels to indentation. The A2 sample ( 0.15 g/cm3) was used as

reference to compare the different syntheses conditions. A2 was compared in the previous

section to an aerogel series with different densities (A1, A3, and A4). In this section, A2 sample

will be compared with different drying procedure, A2CO2, and with a sample with the similar

density but different solvent, M. The solvent and the drying procedure used may cause

variations in the aerogel hardness and Young‟s modulus.

3.1 EFFECT OF THE DRYING PROCEDURE

The first sample was dry with a conventional high temperature supercritical extraction of

acetone while the second sample by exchanging the acetone for liquid carbon dioxide. The

corresponding samples were labeled as A2 and A2CO2 respectively. The microindentation

responses of both aerogels can be compared in Figure IV.9.


Figure IV.9 Penetration depth versus load loading and unloading

curves for A2 (∆) and A2CO2 (●) samples.

In the case of differently dried samples, differences can be due to sample shrinkage during

drying, which is more important in A2CO2 samples than in A2 samples. It may be remarked that

A2CO2 gels were not washed in an ethanol bath and then the presence of remaining water in the

gel solvent causes the noticeable shrinkage. As a result, the density of the A2 CO2 samples is

higher and similar to that of A3 samples. Therefore, the A2CO2 samples show a mechanical

behavior similar to A3 samples. This shrinkage might have shortened the length of the bonding

necks and increased connectivity between the primary silica particles, resulting in a slight

increase in the density of the silica aerogels, and consequently an increase in hardness and

Young‟s Modulus.

It can be pointed out that A2CO2 stills follow the scaling laws mentioned earlier in the previous

section. It is found that the A2 sample presents a much more plastic response than the A2CO2

sample, as it can be observed from the elastic recovery in Figure IV.9. Moreover, the A2CO2 is

harder than A2 and presents a much higher Young‟s modulus (values are shown in Table IV.2).

0 .2 .4 .6 .8 1.0

Load (mN)

0

1

2

3

4

5

6

A2 + CO2

A2

Pen

etr

ati

on

depth

(m

)

0 .2 .4 .6 .8 1.0

Load (mN)

0

1

2

3

4

5

6

A2 + CO2

A2

Pen

etr

ati

on

depth

(m

)

Label(g/cm3)

H(MPa)

E(MPa)

EP(%)

A2 0.15 2.4 50 30

A2CO2 0.24 4.9 197 8


Table IV.2 Several solvent volume ratios, v, and different solvent and drying procedures were used to prepare the aerogel samples, their densities are listed below together with their mechanical properties: Hardness, H, Young‟s modulus, E, and elastic parameter, EP. Standard deviations are shown in parenthesis and refer to the last digit.

Sample v

(g/cm3) H

(MPa) E

(MPa) EP (%)

A2 0.2 0.15 (2) 2.4 (3) 50 (2) 30 (1)

A3 0.3 0.23 (2) 4.8 (3) 153 (3) 10 (2)

M 0.23 0.14 (1) 1.4 (1) 29 (2) 49 (1)

A2CO2 0.2 0.24 (3) 4.9 (3) 197 (3) 8 (1)

v = VTMOS / (VTMOS + Vsolvent)

3.2 EFFECT OF THE SOLVENT IN MECHANICAL PROPERTIES

To investigate the effect of the solvent, measurements were performed on two different solvent

samples of similar density, acetone (A2) and methanol (M), see Table IV.2. When using

methanol as a solvent, samples labeled as M, the molar ratio of the reactives were:

TMOS:CH3OH:H2O:NH4OH = 1:12.25:4:6.5·10-2 [25]. For the syntheses with acetone [26], no

catalyst was used. Figure IV.10 shows the load-penetration curves obtained from A2 and M

aerogels.

Figure IV.10 Penetration depth versus load loading and unloading

curves for A2 (∆) and M (●) samples.

0 .2 .4 .6 .8 1.0

Load (mN)

0

1

2

3

4

5

6

M

A2

Pen

etr

ati

on

depth

(m

)

0 .2 .4 .6 .8 1.0

Load (mN)

0

1

2

3

4

5

6

M

A2

Pen

etr

ati

on

depth

(m

)

Label(g/cm

3)

H(MPa)

E(MPa)

EP(%)

M 0.14 1.4 29 49

A2 0.15 2.4 50 30


It can be observed that at equal maximum load, the maximum penetration depth is lower for

A2 than for M aerogel. However, both samples show similar residual penetration depths after

indenter unloading, so the M sample has a higher elasticity than the A2 sample while exhibiting

the same density. Consequently, the elastic parameter values increase and the Young‟s modulus

values decrease for the M sample. Thus, the A2 sample is harder than the M sample as can be

seen Table IV.2.

It is known that initial conditions for the TMOS sol-gel polymerization process, such as the

solvent, have a pronounced influence on the microstructure of the aerogels [18]. The

differences between elastic and plastic behavior in M and R samples must have their origin in

the differences in the aerogel microstructure. Base catalysts such as NH4OH, as in the M

sample, lead to polymeric samples with high cross-linking therefore increasing the elasticity

of the sample. Nevertheless, further structural and modeling efforts are needed for a

complete understanding of these dependencies.

In conclusion, the effects of the solvent, being methanol or acetone, on the silica aerogels have

been studied. It was found that for samples with similar density values, the ones obtained using

acetone have higher hardness and Young‟s modulus values than those obtained from methanol,

but with less elastic recovery. Moreover, the effect of the drying process has been studied. It has

been shown that the process with CO2-acetone exchange causes a slight improvement in

hardness and a relevant increase in the elastic modulus, mostly due to larger shrinking effects.

This shrinkage might have shortened the length of the bonding necks and increased

connectivity between the primary silica particles, resulting in a slight increase in the density, and

consequently an increase in hardness and Young‟s Modulus.


COMPOSITES

4.1 INTRODUCTION

To fit engineering needs, some potential aerogel applications may require materials with a

higher elastic recovery than that inherent to pure silica aerogels. As will be shown, this can

be done by incorporating activated carbon particles within the silica framework. Contrary to


conventional composites, in the resulting carbon-silica aerogel the most rigid phase

corresponds to the silica aerogel matrix. Such systems are called inverted composites [7].

Carbon-silica aerogel composites may have a broad range of applications [27]. For instance,

the thermal conductivity of silica aerogels can be reduced by absorbing infrared radiative

component of the heat transfer. Carbon-loaded silica aerogels increase the thermal resistance

due to carbon broad-band absorption. [28, 29].Another possible application for carbon-silica

aerogel composites is as low temperature-black materials, for example, as opaque or low-

reflectivity monoliths and coatings. Sometimes, the addition of carbon creates an electronic

path through the aerogel network [30].Carbon-silica aerogel composites can be used as a

catalyst support, which facilitates access of reagents to the electrocatalyst. 12 These examples

illustrate the versatility of carbon-silica aerogel composites in a wide range of applications

where their unique combination of mechanical, thermal, electrical, microstructural, and

chemical properties have opened up new possibilities.

4.2 EFFECT OF THE CARBON ADDITION

It has been previously reported that by incorporating active carbon powder in the silica

network, the aerogel elasticity increases [[31]. Carbon-silica aerogel composites were obtained

by adding powdered activated carbon (supplied by Norit, Darco KB) to the sol in some A1

and A4 samples just before gelation. The colloidal mixture was stirred for some minutes,

poured into cylindrical molds and covered with Parafilm. Activated carbon powders are

produced from carbonized graphite-like plates, followed by the steam activation process

where the carbonized material is reacted with steam at 1000 C. The carbon particle

aggregates have a diameter ranging from 10 to 45 m. This carbon shows a tamped bulk

density of 0.45g/cm3, high specific surface area (1500m2/g), and large elastic recovery. The

mass fraction of carbon in the aerogel,

aerogelcomposite

carbon

m

mx , was varied from 0 to 0.5. The A1

carbon-silica samples were labeled as A1c. In the composite A4 samples, the mass fraction of

carbon,

aerogelcomposite

carbon

m

mx , was 0.02, 0.15 and 0.5. Samples were accordingly labeled as C2, C15 and

C50, respectively. Supercritical extraction of the solvent took place in a high-temperature

process. All aerogel samples obtained were monolithic, in the shape of 5 - to 12-cm-long

cylindrical rods and 1cm in diameter. The apparent density was determined by accura tely

weighing samples of well-defined dimensions. The final densities of carbon-silica aerogel


composites are significantly lower than those corresponding to conventional silica aerogels.

The volume shrinkage, gel

gelaerogel

gel V

VV

V

V, produced during supercritical extraction was

determined by comparing the dimensions of the dry aerogels and the dimensions of the original

gel. Shrinkage during drying decreases dramatically as the mass fraction of carbon increases.

The cause of the lower densities is a consequence of the lower shrinkage observed for the

aerogel composites with high carbon content.

Figure IV.11 Photography comparing a silica aerogel sample with

a carbon-silica aerogel composite sample.

The most plastic aerogel sample (A4) and the most elastic sample (A1) have been used to

analyze this effect. In both cases, identical samples but with added carbon showed a much more

indentation elastic response than pure silica samples. This feature is illustrated in Figure IV.12,

which shows the load vs. penetration depth curves for the A4 sample and for samples with

carbon mass fractions of 0.02, 0.15 and 0.5. It can be seen that the addition of small amounts

of activated carbon to pure silica aerogel induces great changes in the shape of the indentation

curves. They change from curves characteristic of elastoplastic materials to those corresponding

to highly elastic materials, as reported in [31]. This increase in elasticity results, in higher EP

values of the composite aerogel samples, as it can be observed from the values listed in Table

IV.3, which also includes the carbon mass fraction, the density and the shrinkage corresponding

to all the listed samples. In particular, A1 and A1C are almost totally elastic materials, so they

have a very low Young‟s modulus and a high EP value. It should also be noticed that

experimental values of Young‟s modulus strongly decrease with slightly increasing carbon

mass fraction (Table IV.3).


Table IV.3 Several carbon mass fractions were used to prepare

the composite aerogel samples. Their densities, , and shrinkage,

V/V are listed below together with their mechanical properties: Hardness, H, Young‟s modulus, E, and elastic parameter, EP. Standard deviations are shown in parenthesis and refer to the last digit.

Sample label

Carbon percentage (mass %)

(g/cm3)

V/V (%)

H (MPa)

E (MPa)

EP (%)

A4 0 0.24 (1) 34.3 5.1 (2) 190 (5) 6 (1)

C02 2 0.21 (2) 29.8 3.4 (2) 52 (3) 29 (1)

C15 15 0.17 (2) 19.8 2.1 (2) 28 (2) 45 (2)

C50 50 0.15 (3) 7.2 1.7 (2) 23 (2) 54 (2)

A1 0 0.08 (1) - 0.5 (1) 7.0 (9) 78 (1)

A1C - 0.06(1) - 0.4(1) 9.0(8) 89(1)

The maximum applied load was 1 mN. For the A1 and A1C samples, the maximum load was

lowered to 0.5 mN due to the materials extreme softness.

Figure IV.12 shows applied load vs. penetration depth indentation curves „as measured‟ for each

sample for a loading–unloading cycle.


Figure IV.12 Penetration depth versus load indentation curves for

a pure SiO2 sample AC0 (▲) and composite aerogels with 2% of

carbon, AC2 (●), 15% of carbon AC15 (◊), and 50% of carbon

AC50 (□). The edge values are indicated for the non-recovery

penetration depth, hp, the elastic penetration depth, he, and the total

penetration depth, ht for the pure SiO2 sample (AC0).

It can be seen that the addition of small amounts of activated carbon to pure silica aerogel

induces great changes in the shape of the indentation curves. They change from those

characteristics of elastoplastic materials to those corresponding to highly elastic materials. From

these curves we have calculated three mechanical parameters; Young‟s modulus, E, the elastic

parameter, EP, and the hardness, H. Results are listed in Table IV.3, which also includes the

carbon mass fraction, the density and the shrinkage corresponding to all the studied samples.

Experimental values of Young‟s modulus strongly decrease with slightly increasing carbon

mass fraction (Table IV.3).

The dependence of the experimental Young‟s modulus on the carbon content of our aerogel

composites will be discussed by using the regular model for composite materials [32, 33].

The mechanical properties of composite materials depend on the mechanical properties of

the components, on the morphology of the phases and on the nature of the interface

0,0 0,2 0,4 0,6 0,8 1,0

0

1

2

3

4

5

Pe

ne

tra

tio

n d

ep

th (m

)

Load (mN)

AC0

AC50

AC2

AC15

0,0 0,2 0,4 0,6 0,8 1,0

0

1

2

3

4

5

Pe

ne

tra

tio

n d

ep

th (m

)

Load (mN)

AC0

AC50

AC2

AC15


between the phases. Figure IV.13 shows a schematic picture that illustrates two extreme

phase distribution models that can be considered to estimate Young‟s modulus of composite

materials.

(b)

rigid phase

elastic phase

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

0

20

40

60

80

100

120

140

160

180

200

E

(M

Pa)

x

(a)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Figure IV.13 Composite Young‟s modulus values obtained for

various mass fractions of silica, showing the upper and lower limits

for the two extreme layered ideal composites. Hollow arrows

indicate the direction of the external stress

The maximum possible value for the composite modulus is described by a linear

combination of the Young‟s modulus for the filler, E filler, and the matrix, Ematrix. This is

expected when the two components of the composite are distributed in parallel, E‖, in the

way schematically shown in Figure IV.13(a). In this case, the Young‟s modulus value, E‖, is

described by equation (4) as a function of the carbon mass fraction, x, see Figure IV.13 [31].

matrixfiller ExxEE )1( (Eq. IV.4)

The lowest possible modulus for a composite material results when the two phases of the

composite are connected in series Figure IV.13 (b). Then, the Young‟s modulus E is

described by equation (5).

matrixfiller E

x

E

x

E

11 (Eq. IV.5)

This model also predicts that when the composite is particulate, then the expected values for

the Young‟s modulus will be close to the equation IV.5, but do not follow the exact


hyperbolic function. Experimental Young‟s modulus values listed in Table IV.3 are also

included in Figure IV.12. It is found that the incorporation of small amounts of carbon into

the silica aerogels causes a large decrease in the modulus of the composite. Modulus values

for higher concentrations of carbon do not follow Equation IV.5, but lie between equations

IV.4 and IV.5 as predicted for a particulate composite. The measured modulus values are

closer to the lowest curve of Figure IV.12. We understand it in the following way: for low

carbon concentration, the carbon particles could act as anchorage centers for silica branches

and then be incorporated within the silica network. This may be the cause of the strong

diminution in Young‟s modulus when a small amount of carbon is added. When the carbon

mass fraction increases, the percentage of the carbon particles not incorporated into the

silica network rises, forming a particulate composite with Young‟s modulus values that differ

from the hyperbolic behavior of equation IV.5.

The elastic parameter values listed in Table IV.3 allow us to quantify the improvement in the

elastic response to indentation previously illustrated in Figure IV.11. In particular, the EP values

are dramatically increased by incorporating low carbon amounts in the silica aerogel and they

become saturated for higher carbon concentrations. A possible explanation for the important

changes observed in the elastic behavior of the carbon silica aerogel composites is that the filler

can also disrupt the cross-linking of the silica aerogel network and lead to a less rigid structure.

This consequently reduces the brittle failure typical of fragile materials such as silica aerogels and

thus, the indentation plastic deformation.

The measured hardness values of carbon-silica composites show a slight decrease when carbon

content increases (Table IV.3). These experimental hardness values are close to those previously

reported for silica aerogels [15]. Hardness as a function of aerogel density can be fitted to a

power law function: H with a scaling exponent = 2.3 0.1, very similar to the one

expected for pure silica aerogels [15]. Therefore, hardness of the composite is mainly due to the

silica phase and is much less affected by the addition of carbon than the elastic parameter or

Young‟s modulus values are.


5. VISCOELASTICITY OF SILICA AEROGELS AT ULTRASONIC

FREQUENCIES

This section is included for completeness and shows the measurements of viscoelastic

properties using a technique based on the analysis of thickness resonance of air-surrounded

aerogel plates at ultrasonic frequencies. The measurements and analysis of the results were

performed by T. E. Gómez Álvarez-Arenas and F. R. Montero de Espinosa from the

„Instituto de Acústica-CSIC‟ [34]. The aerogels samples were prepared at the ‟Institut de

Ciència dels Materials de Barcelona‟. This technique allow us to measure the density, Young

Modulus and Poisson ratio of the samples, these results can be compared to those ones

obtained by microindentation.

5.1 INTRODUCTION

Specially designed air-coupled, high-sensitivity and broad-band piezoelectric transducers [35,

36, 37], operating in the frequency range of 0.3-2 MHz, were used in order to excite and

sense the resonance by airborne ultrasonic waves. This technique allows us to obtain precise

and simultaneous measurements of velocity and attenuation of longitudinal and shear waves

in silica aerogels at different frequencies as well as aerogel density. From these results, it has

been achieved a full characterization of the viscoelastic properties of silica aerogels [38,15],

over a significant frequency range, i.e. to obtain complex-valued and frequency-dependent

data for all the elastic constants.

Figure IV.14 Specially designed air-coupled, high-sensitivity and

broad-band piezoelectric transducers.

The technique is based on a silica aerogel plate embedded in a continuum medium (air),

which is insonificated by a broad-band airborne ultrasonic pulse. The frequency spectrum of

the ultrasonic pulse comprises several eigenfrequencies of the plate [39]. Eigenvibrations of

the aerogel plate excited by the incident wave are recorded and analyzed. The theoretical

air

Aerogel

plate

Piezoelectric

transducers


approaches to this problem is to impose boundary conditions to stress and strains fields at

the surfaces of the aerogel plate and solve the system of equations for field amplitudes [40].

By this procedure, overlapping resonance can be discerned. The viscoelastic nature of the

silica aerogel plate is introduced into the theoretical analysis by means of the correspondence

principle (i.e. introducing complex and frequency dependent elastic constants) [41].

The use of aerogels as acoustic impedance matching layers for air-coupled piezoelectric

transducers in the kilohertz frequency range has previously been reported [42, 43, 44].

Operating over 0.5 MHz, air-coupled transducers could be used for particular applications

that require airborne signals and high spatial resolution: non-destructive testing [45], medical

applications [46], surface characterization [47], etc. Therefore, new techniques for

generation and detection of airborne ultrasonic signals are under very active research (e.g.

micromachined capacitive transducers) [48]. Recently, it has been analyzed the possibility of

using aerogel matching layers for the frequency range of 0.5-2 MHz. [49]. The use of

piezoceramics together with a stack of matching layers where the outer layer is made of

aerogel may give rise to a transducer whose performance may be better to that achieved

from other techniques. [50]. For this purpose, velocity and attenuation of ultrasonic waves in

the aerogel must be accurately measured. While methods to determine ultrasonic longitudinal

velocity are well established, attenuation data are limited. Attenuation was measured by

Debye-Sears diffraction for the frequency range of 0.8-8 MHz [51]. At lower frequencies (20-

200 kHz) a reverberation method was used [52]. Experimental data about shear wave

propagation are very limited.

5.2 EXPERIMENTAL SET-UP: AIR-COUPLED BROAD-BAND

PIEZOELECTRIC TRANSDUCERS

The synthesis of the aerogel used in this work was described in more detail in chapter II

(TE00AT page 49, 60, and 62.). The molar ratios between reagents were TEOS/EtOH/H2O

= 1/7/5 and 0.003M citric acid. The gel was washed several times in pure ethanol. The silica

aerogel slab (2.4 cm diameter and 0.3 cm height) had a density of 0.160 g/cm3. Before the

measurements the sample was placed in a vacuum chamber and heated to 100 ºC to remove

any absorbed moisture

A pair of specially designed air-coupled piezoelectric transducers facing each other was used

for the experimental set-up. The Insertion Loss (IL) for the aerogel plate is defined as:

refsample AAIL 10log20 (Eq. IV.6)


where:

sampleA is the amplitude of the FFT (Fast Fourier Transform) of recorded ultrasonic

waves with aerogel plate in between the transducers

refA is the amplitude of the FFT of recorded ultrasonic waves without the aerogel.

In addition, the transmission coefficient, T, is defined by the ratio transmitted to incident

energy fluxes:

ref

sample21

A

AT (Eq. IV.7)

Details of the technique can be found elsewhere [53, 54]. Two cases were analyzed: normal

and oblique incidence. At normal incidence (angle of 19 ), shear waves are not generated in

the aerogel. In this particular case, and following the quantum-mechanical theory of

resonance scattering [55], a simple analytical expression for T can be derived:

122

41

~sin11 tkmmT l (Eq. IV.8)

where:

m is the ratio between complex acoustic impedances of aerogel and air complex

acoustic impedance.

t is the thickness of the plate.

lk~

is defined as llllll cicikk ~~, the complex wave vector in the

aerogel, where:

lk is the wave vector,

l the longitudinal wave attenuation,

lc the longitudinal phase-velocity

is the angular frequency

lc~ the complex longitudinal velocity of sound.

Resonance of the plate appear at maximum values of transmission coefficient, T, which take

place at nkt , n = 0, 1, 2… On the contrary, no simple analytical expression for T can be

derived for oblique incidence.


5.3 DETERMINATION OF THE VISCOELASTICITY OF SILICA

AEROGELS AT ULTRASONIC FREQUENCIES.

5.3.1 Normal incidence

Figure IV.15 shows the measured insertion loss, IL (dots), for normal incidence in a

frequency range from 0.3 to 1.3 MHz. Separation between resonance is almost constant and

equal to 59 1 kHz. First order resonance might be located at 59 kHz and the first peak in

Figure IV.15 (354.5 kHz) should then correspond to the sixth order peak (n=6).

Figure IV.15 Insertion loss versus frequency for a silica aerogel

plate. Dots: experimental measurements. Solid line: theoretical

results. Normal incidence.

Longitudinal velocity, lc , longitudinal attenuation, l, and density, are used as fitting

parameters to match the theoretical calculations of IL to the measured values. The results for

the fitted IL (solid line) are compared with experimental values (dots) in Figure IV.16).

Figure IV.16 shows the obtained values of lc and l at different resonance.

0.3 0.5 0.7 0.9 1.1 1.3frequency (MHz)

-45

-35

-25

-15

insert

ion loss (

dB

) (a)

n=6

59 KHz


Figure IV.16 Attenuation versus frequency of longitudinal ( ) and

shear () waves. Solid lines: power fitting. Velocity versus

frequency of longitudinal () and shear (▲) waves. (50‟).

The density, , obtained by IL fitting is 0.220 0.020 g/cm3. While longitudinal phase

velocity, lc , is rather constant (324 m/s), longitudinal attenuation, l follows a frequency (f)

power law:

,y1ll fαfα (Eq. IV.9)

where: y = 1.1 0.05 and )./(1016.3 5 y

1lHzmNpα

It is of interest to compare these measurements with results obtained from different

techniques. Up to now, lc and l were also measured following a conventional “through

transmission” technique. High-frequency (1.4 MHz) and broad-band transducers were

employed in order to be able to separate, in the time domain, the different echoes due to

multiple reverberations within the sample [56]. Measured values at transducer center

frequency (1.4 MHz) are: lc = (325 3) m/s and l = (210 20) Np/m. These results are

consistent with those obtained from the resonance technique.

In addition, results obtained here are consistent with data reported by other authors.

Attenuation data shown in References 10 and 11 for the frequency range 0-200 kHz and 2.5-

8 MHz follow a linear frequency law with: Hz.m

Np102.1

Hz.m

Np101.2 4

1l

5 .

0.0 0.4 0.8 1.2 1.6frequency (MHz)

0

75

150

225

300

375

attenuation (

Np/m

)

185

210

235

260

285

310

335

velo

city

(m/s

)

Longitudinal

wave

Shear wave

Shear wave


Measurements for different aerogel samples having density values close to 0.2 g/cm3 provide

values of lc in the range 200-500 m/s [57, 58].

5.3.2 Oblique incidence

Figure IV.17 shows the measured IL (dots) for oblique incidence (19º). Interferences due to

the overlap of longitudinal and shear resonance are clearly appreciated.

Figure IV.17 Insertion loss versus frequency for a silica aerogel

plate. Dots: experimental measurements. Solid line: theoretical

results for incidence angle of 19º.

Using the values of llc and obtained in the analysis of IL for normal incidence, shear

wave phase-velocity ( tc ) and shear wave attenuation ( t ) were used as fitting parameters to

match theoretical calculations of IL to experimental values. The higher accuracy for

ttc and are obtained at frequencies were interferences between longitudinal and shear

resonance appear. The result for IL is shown in Figure IV.17 (solid line). Obtained results

for ttc and are shown in Figure IV.17. While tc is rather constant (197 m/s), results for

t can be adjusted by a power law:

5.0

1t

y

1tt Hz/m/Np226.0,15.05.0ywhere,ffα (Eq. IV.10)

For frequencies higher than 0.9 MHz, the uncertainty in the determination of t becomes

very high. Therefore, the error in the determination of the y exponent is also very high.

Using thinner samples and/or larger incidence angles might solve this problem.

0.3 0.5 0.7 0.9 1.1 1.3frequency (MHz)

-45

-35

-25

-15

insert

ion loss (

dB

)

(b)

Interaction longitudinal share

waves


Complex elastic constants and related viscoelastic parameters can be calculated from the

analysis of the exponential curves. Results are gathered in Table IV.4.

The complex shear modulus G is obtained from: 2~tcG . The complex elastic modulus M

is defined as: M=K+4/3G 2~lc , where K is the bulk modulus, and tc

~ is the complex shear

wave velocity. E is the Young Modulus defined by GK3

GK9E , with real values of the

same order as the ones found using microindentation [15]. Poisson coefficient, which is

difficult to obtain from other techniques, is calculated from: 1r2

1r2, where

2

l

2

t

c

cr .

Table IV.4 Poisson coefficient, , and the three complex elastic

modulus: complex shear modulus, G, shear loss, Gtan , complex

elastic modulus, M, and Young Modulus E are measured at different frequencies, f, from 0.44 MHz to 1.2 MHz.

f

(MHz)

G

(MPa) real(G)

imag(G)

Gδtan 1 M

(MPa) real(M )

imag(M )

M

1δtan E

(MPa) real(E)

imag(E)

E

1δtan

0.44 0.214 8.36+0.18i 0.0212 22.98+0.32i 0.0140 20.30+0.38i 0.0185

0.63 0.209 8.49+0.15i 0.0179 23.01+0.28i 0.0124 20.53+0.32i 0.0158

0.82 0.207 8.54+0.14i 0.0164 23.01+0.31i 0.0132 20.60+0.31i 0.0151

1.0 0.208 8.54+0.14i 0.0166 23.16+0.36i 0.0155 20.62+0.33i 0.0162

1.2 0.204 8.58+0.13i 0.0151 23.09+0.38i 0.0165 20.67+0.32i 0.0156

A detailed analysis of data shown in Table IV.4 reveals that none of the calculated elastic

moduli follows the frequency dependence predicted by any of the simple and basic

viscoelastic single relaxation models (Voigt and Maxwell). On the contrary, bearing in mind

that attenuations follow a frequency power laws (Equations. IV.9 and IV.10), a possible

alternative to single relaxation models could be a time causal model [59, 60].

The real value of E ranged from 20.3 to 20.7 Mpa, this values are of the same order as the

ones found using microindentation where a sample with a density similar to that one

(0.23g/cm3) was of 30Mpa (M aerogel). Moreover, Poisson ratio ranged from 0.204 to 2.214,

this values confirms that one used in microindentation, =0.2. In some aspects, the behavior

of the aerogel observed here resembles that of some well investigated materials. These

similarities permit to gain an insight into the underlying physics. For example, the shear


modulus (G) and shear loss ( Gtan ) exhibit a similar behavior to those reported for some

kind of polymers. For such polymers, the interpretation given is based on non-local

cooperative interactions of large molecules, which could also be applied for aerogels [60, 61].

Another interesting feature is provided by the fact that fctetanQ lM

1

M .

This relationship has also been observed, for some kind of aerogels, at higher and lower

frequencies [51, 52]. A similar behavior has been found in marine sediments (water-saturated

and dry sediments). Theoretical predictions of l in fluid-filled porous media (as sediments)

are based on the interaction between the fluid in the pores and the solid skeleton. This

provides a dependence of the attenuation with the frequency that varies at

or , 212 fff depending on the frequency range involved [62]. On the contrary,

experimental measurements over a wide frequency range suggest a linear frequency

dependence for l. This is the object of a long lasting argument between experimentalists

and theoreticians [62]. To explain this experimental behavior, different sources of dissipation

leading to an attenuation proportional to ƒ (e.g. friction at the contact area between particles

of the frame) were introduced into the theoretical modeling. The understanding of sound

attenuation in aerogels may benefit from this. In its turn, low-density aerogels exhibit

2fl , and some authors consider that this behavior is due to the influence of the fluid in

the pores [51, 63].

6. MECHANICAL CHARACTERIZATION CONCLUSIONS

In conclusion, the microindentation technique has proved to be a non-destructive, suitable

method to assess the parameters that characterize the mechanical behavior of extremely

porous materials such as aerogels, despite their brittleness and softness.

Silica aerogels of different mechanical responses have been obtained by varying the initial

parameters in the TMOS sol-gel polymerization process, such as alkoxide concentration,

solvent, drying process, as well as the carbon addition. Creep effects are very important for

these samples and should be taken into account during measurements and in the interpretation

of the microindentation results. Initial conditions have a pronounced influence on the density

of the aerogels and also on their microstructure, which is reflected by their mechanical behavior.

The A-series samples show two different types of mechanical behaviors; the low-density

aerogels are elastic, while the denser aerogels behave as elasto-plastic materials. Young‟s


modulus, hardness and the elastic parameter have been evaluated for these aerogel samples.

It is shown that the evolution of the parameters describing the mechanical behavior as a

function of the bulk density follows power-scaling laws. The evaluated exponents are 3.0, 2.0

and -3.2 for Young‟s modulus, hardness, and Elastic Parameter, respectively .

The large dependency of Young‟s modulus on the density as well as the change from elastic to

plastic behavior has its origin on the aerogel microstructure. Further structural and modelization

efforts are needed to a deeper understanding of these dependencies.

The effects of the solvent, being methanol or acetone, on the silica aerogels have also been

studied. It was found that for samples with similar density values, the ones obtained using

acetone have higher hardness and Young‟s modulus values than those obtained from

methanol, but with less elastic recovery. Moreover, the effect of the drying process has been

studied. It has been shown that the process with CO 2-acetone exchange causes a slight

improvement in hardness and a relevant increase in the elastic modulus, mostly due to larger

shrinking effects. From the results presented in the previous section, we conclude that the

addition of small amounts of powdered carbon as filler in silica aerogels increases the

elasticity of the composite and keeps the hardness similar to silica matrix values. It has been

found that including small amounts of activated carbon inside the fragile silica network

dramatically increases its elastic indentation recovery. This is reflected in the shape of the

indentation curves as well as in the increase of the elastic parameter value, which evaluates

the percentage of elasticity versus plasticity. Young‟s modulus values obtained for carbon-

reinforced aerogels show a similar variation to the carbon mass fraction as that predicted by

a commonly used model for composite materials. The measured hardness values

corresponding to the total elastoplastic deformation do not show such a prominent

dependency on the carbon mass fraction as the elastic parameter and Young‟s modulus do

and they are similar to those measured for the pure silica aerogel.

Moreover, we present an experimental technique that do not require any sample machining

to simultaneously measure velocity and attenuation of longitudinal and shear waves in

aerogels. A fully viscoelastic characterization of the aerogel is obtained and a deeper insight

on aerogel basic properties is gained.


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C h a p t e r V

SILICA AEROGEL MICROPARTICLES

SECTION OUTLINE


MICROPARTICLES USING SUPERCRITICAL ETHANOL/ACETONE........................ 181

1.1. ‘IN SITU’ PARTICLE PROCESSING .................................................................................. 182

1.2. AEROGEL MICROPARTICLE CHARACTERIZATION ................................................ 184

1.2.1 Scanning Electron Microscopy ............................................................................185

Independent solutions .................................................................................. 189

1.2.2 Transmission Electron Microscopy ....................................................................190

1.2.3 Atomic Force Microscopy ..................................................................................... 191


MICROPARTICLES USING SUPERCRITICAL CARBON DIOXIDE ...............................195

2.1. ‘IN SITU’ LOW-TEMPERATURE MICROPARTICLES: TEOS, HCOOH, AND

SUPERCRITICAL CO2 AS SOLVENT.................................................................................. 196

2.2. PRECURSOR DIRECTLY INJECTED IN CO2 SUPERCRITICAL CONDITIONS

AT LOW TEMPERATURE ..................................................................................................... 198

2.2.1 Injection of hydrolysis and precursor solution independently ......................199

2.2.2 Injection of sol ........................................................................................................ 203

2.3. IN SITU’ INJECTION IN LIQUID CO2............................................................................... 203

2.4. INJECTION OF PREPOLYMERIZED PRECURSOR IN SUPERCRITICAL CO2 ..... 205

3. CONCLUSIONS...................................................................................................................... 208

4. REFERENCES ........................................................................................................................ 208

Chapter V Silica aerogel microparticles 180

This chapter focuses on the preparation of silica porous microspheres directly in the high-

pressure reactor. This process allows the use of supercritical fluids in manufacturing fine

particles with high porosity.

Different approaches were undertaken to obtain silica aerogel microparticles by this method

and two routes were used depending on the supercritical fluid chosen. High temperature

method when acetone or ethanol were used as solvents, and low temperature method if using

carbon dioxide.

Silica in the form of microspheres is a material of interest in several fields. The Stöber [1]

method for the synthesis of nonporous silica spheres via the hydrolysis and condensation of

tetralkoxysilane has been well recognized for producing particles with very narrow

distributions. Moreover, several methods have been attempted for obtaining silica

microparticles. These include nozzle-reactor systems (spray drying or pyrolysis) [2-4], and

emulsion/phase separation techniques with sol-gel processing [1-2, 5-10].

Aerogel microspheres offer an attractive alternative for microparticle applications. In general,

porous microparticles provides an advantage over nonporous materials when large surface

areas can be an advantage. The resulting silica aerogel particles would exhibit much of the

surface area characteristic of a silica particle, while maintaining the narrow size distribution of

Stöber-type particles. Silica aerogel microparticles can be used for a range of applications,

including: as additives for conventional foaming operations, controlled release agents,

encapsulation of products, gas separations, advanced thermal and acoustic insulation,

microelectronics, optics, catalysis, high surface-area adsorbents [2], and chromatography

materials. The use of small (i.e., <5.0 μm) diameter particles in ultra -high pressure liquid

chromatography provides both an increase in resolution and a decrease in analysis time over

more traditional column packing materials [11-12].

Supercritical fluids have received considerable attention as solvents or reaction media for the

processing of powders, particles, fibers and coatings [13-17], since they offer a combination

of gas like properties (viscosity, diffusion coefficient) and liquid like properties (density).

Moreover, the synthesis of aerogel microparticle process at low temperature uses supercritical

carbon dioxide. CO2 is an attractive medium because, is a non-toxic, non-flammable and


inexpensive fluid with a low critical temperature. A simple and versatile method to obtain

silica aerogel particles based on the hydrolysis and subsequent condensation of silicon

alkoxides in several supercritical fluids is developed in this chapter. This microparticle

production route reduces the number of steps of traditional microparticle sol-gel processing.

The proposed method is based on two factors: the solvent power of the supercritical fluid

and the sol-gel reaction under these specific conditions.

Finally, efforts were aimed on understanding the relationship between the structure and the

synthesis conditions of these types of materials, the particle formation mechanisms and on

the conditions to tailor the particle morphology, size and porosity.


MICROPARTICLES USING SUPERCRITICAL ETHANOL/ACETONE

In this section monodisperse, microspherical aerogel and fiber silica morphologies powders

were produced inside an autoclave by sol-gel process by using supercritical ethanol or

acetone as solvent. The influence of different synthesis parameters and the conditions of the

synthesis media on the structure and morphology of the obtained aerogel silica particles were

investigated.

Figure IV.1 Scheme of the silica aerogel particle production by performing the sol-gel process under supercritical conditions.

Particle size was mainly controlled by varying the relative amounts of alkoxysilane, water and

acetone. Other important parameters in order to control the particle size are the relative time,

t/tgelification, and the order of introducing the reactants in the autoclave. It has been observed

that a relevant factor to control the particle morphology is the venting rate of the supercritical

Sol in process of

gelification

Solvent at

supercritical conditions

Colloidal particles


fluid. The morphology of the particles was characterized by electron microscopy (SEM and

TEM) and Atomic Force Microscopy (AFM). Coulter technique was also used to account for

the particle size distribution. In addition, BET surface area, pore volume and mean pore

diameter, < pore>BET, of the particles were obtained from nitrogen sorption isotherms.

Reagent grade alkoxides and solvents were used without further purification. Reactions were

carried out either in a specially designed high-pressure 300-ml cell or in a 2000-ml reactor.

(See annex II: Facilities). The basic component of the system is a syringe pump that is able of

pumping fluids inside the autoclave through a nozzle.

1.1 ‘IN SITU’ PARTICLE PROCESSING

The ‘in situ’ process consisted in the following steps:

i) A sol in a colloidal initial stage (containing acetone as a solvent, tetramethoxysilane

as a precursor and water) was placed inside the reactor.

ii) The reactor was then driven above supercritical values of the acetone, Pc = 47 Bar,

Tc = 235 .

iii) The sol was solubilized in the supercritical acetone distributing it over the entire

reactor leading to the formation of gel particles.

iv) Finally, after a remaining time the solvent was extracted from the pores of the gel

particles at supercritical conditions.

Two different experiments were tested: in the first experiment, a sol (formed by acetone as a

solvent, tetramethoxysilane (TMOS) as a precursor and water) was placed inside the reactor:

Samples A, B, and C, (See Table IV.1). In the second experiment, the alkoxide precursor,

the solvent and water were introduced in two independent vessels. One with the hydrolysis

solution (water dissolved in acetone) and the precursor solution (TMOS dissolved in acetone)

in the other vessel: Sample D (See Table IV.2). Figure IV.2 shows a simplified scheme of the

‘in situ’ process for the two set-ups either as single vessel with containing the sol (1a path in

Figure IV.2) or two independent solutions (1b path in Figure IV.2).


Figure IV.2 Illustration of the synthesis scheme for aerogel porous silica spheres by 'in situ' process either with sol (1b path): sample D or with independent solutions (1a path): samples A, B, and C.

In all the experiments the v volume ratio, acetoneTMOS

TMOS v , was fixed at 0.05. This

highly diluted sol was used to avoid the gelification of the sol before the supercritical

conditions were reached. The hydrolysis parameter, h, is defined, as in previous chapters, as

the molar ratio TMOS

OHh 2 . The h value was varied from 2 to 8 in order to observe the

effect of water concentration in the morphology of the aerogel microparticles. The obtained

samples were: A (h=2), B (h=4) and C (h=8).

H2O

+ acetone

silica

particles

TMOS

+

acetone

autoclave

sol

supercritical

fluid

Acetone

supercritical conditions a

)

b

)

Venting the

autoclave

1a)

2)

3)

4)

Sample D

Samples A, B, C


Table IV.1 Mean particle size obtained by varying the hydrolysis parameter, h.

Sample h Mean Particle Size Distribution width

A 2 1.2 m 0.2 m

B 4 1.7 m 0.7 m

C 8 2.2 m 0.6 m

In a second set of the experiments the same ratio were reproduced but the solvent and water

were placed in the reactor independently in two vessels, one with the hydrolysis solution

(water + acetone) and in the other the precursor solution (TMOS + acetone), avoiding the

premature gelification because the temperature effect. See process in Figure IV.2. A bimodal

distribution in particle sizes was found as seen in Table IV.2.

Table IV.2 Mean particle size obtained by independent solutions and v volume ratio 0.05

Sample h Particle Size

D 4 630nm 1.2 m

1.2 AEROGEL MICROPARTICLE CHARACTERIZATION

Particles were collected and characterized by a Leica 360 Scanning Electronic Microscope

(SEM), a JEM 100CX Transmission Electron Microscope (TEM), a Nanoscope III Digital

Instruments Atomic Force Microscope (AFM), N2 adsorption-desorption (Micromeritics

ASAP 2000 instrument), and He-pycnometry.

In all the ‘in situ’ experiments, a white dry powder was uniformly distributed all over the walls

and base of the reactor with a deposition thickness from 10 to 100 m depending on the

initial parameters. No liquid was found, indicating a complete reaction of the initial reactives

of the sol.

a

)


Figure IV.3 Several microparticles were collected depending on the initial conditions

An approximate value of the apparent density of the powder was obtained by measuring the

mass of a known volume of powder. The apparent powder density was between 0.06g/cm3

and 0.08g/cm3 depending on the initial parameters. The surface area of the particles was

characterized by nitrogen absorption (Brunauer, Emmet and Teller method). The technique

allows us to obtain the surface area of a material using the adsorption and desorption of an

inert gas, usually nitrogen. Gas adsorption measurements reveal the microspheres to have

surface areas from 400 to 600 m2/g, similar values to those found on their monolithic

counterparts. By pycnometry technique, it was checked that the skeleton density has the same

value to that of the silica (2.2 g/cm3) [18]. The particle surface microstructure was

characterized by several microscopy techniques (SEM, TEM and AFM) and it is shown in

the following sections.

1.2.1 Scanning Electron Microscopy

An example of a SEM micrograph from A experiment (h=2) is shown in Figure IV.4. The

formation of two kinds of morphologies: interlinked fibers and isometric spherical particles

are evidenced when a zoom is performed in this image (see Figure IV.5).

Initial

conditions

=400nm

=75nm

=1.8 m

=2.2 m


Figure IV.4 Porous deposition of silica aerogel microparticles

for A sample with 100 m of thickness.

As already mentioned, the type of morphology depends on the rate of depressurization a

high venting rate implied the presence of interlinked fibbers.

Figure IV.5 a. Referring to sample A: Scanning electron micrograph of aerogel spherical shape silica particles narrowly

distributed in size, particle diameter between 0.50 and 2 m. b. Fiber-like structure with a diameter of 75 nm and a length of some microns.

100 m100 m100 m

7.5 m7.5 m7.5 m 750 nm750 nm750 nm


The mean particle diameter and the particle size distribution could be modified by changing

the values of the h parameter. See from Figure IV.6) to 8). Table IV.1 shows the variation for

the high temperature experiments of the mean particle diameter , and the width of the

distribution 2 , when the TMOS dilution in acetone was fixed at 0.05 and the hydrolysis

parameter, h, was changed.

Figure IV.6 SEM micrograph and particle size distribution for sample A: v = 0.05, h=2.

Figure IV.7 SEM micrograph of the spherical particles and its particle size distribution for sample B: v = 0.05, h=4.

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60

5

10

15

Gaussian f it (0.3-2.6 0.1)

MeanSDAr ea

---------------------------------------------

1,182930,246498,4838

=1.2 m

2 =0.2 m

m

m

Particle diameter ( m)

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60

10

20

30Gaussian f it

Mean SD

-----------------------

1,676 0,359


=1.7 m

2 = 0.7

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60

10

20

30Gaussian f it

Mean SD

-----------------------

1,676 0,359


0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60

10

20

30Gaussian f it

Mean SD

-----------------------

1,676 0,359


0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60

10

20

30Gaussian f it

Mean SD

-----------------------

1,676 0,359

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60

10

20

30Gaussian f it

Mean SD

-----------------------

1,676 0,359


=1.7 m

2 = 0.7

m

m


Figure IV.8 SEM micrograph and particle size distribution for Sample C. Where v = 0.05 and h=8.

The size of the collected particles was very sensitive to the water/alkoxide concentration

ratio: the higher the ratio, the smaller the particles, maintaining the width of dispersion

constant.

The size distribution and mean size value were measured by Coulter in order to compare the

results by those obtained by SEM. Figure IV.9a-b compares the Coulter technique and image

analysis size distribution for A sample =0.05, h=2.

Figure IV.9 a) Coulter size distribution

for A sample = 0.05, h=2.

Figure IV.9 b) SEM size distribution for

A sample = 0.05, h=2.

Both techniques show a similar particle size distribution.

Nu

mb

er

part

icle

s (

%)

2

4

6

8

SEM

Particle

Distribution

0,0 0,5 1,0 1,5 2,0 2,5 3,0

Particle diameter (um)

0,0 0,5 1,0 1,5 2,0 2,5

0

1

2

3

4

5

6

Particle diameter (um)

Coulter

Particle

Distribution

Nu

mb

er

part

icle

s (

%)

Mean particle diameter

= 1.2 ± 0.4 m

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,80

10

20

30

40

Gaussian f it

MeanSD

------------------------------

2,176 0,309


=2.2 m

2 = 0.6 m

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,80

10

20

30

40

Gaussian f it

MeanSD

------------------------------

2,176 0,309


=2.2 m

2 = 0.6 m


Independent solutions

In the case of independent solutions, sample D, a differentiated morphology was observed.

This experiment allows us to observe the effect of water in the interlinkage of the

microstructure. Two different structures were observed, one placed inside the water recipient,

it is structured as a spider's web. This feature was formed by interlinkage of nanometric

spheres giving place to a fibrilar structure (Figure IV.10).

Figure IV.10 Spider’s web with a fiber diameter of 60 nm collected from the recipient where initially the water was dissolved in acetone.

The second structure was found overall the reactor as a 'snow' powder formed by

micrometric spheres, distributed as a bimodal function.

Figure IV.11 Bimodal particle size distribution for D sample.

0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,60

2

4

6

8

10

Gauss(2)

Peak Center Width Height

-----------------------------------------------------------

1 0,6320,352 6,34

2 1,244 0,182 3,72

-------------------------------------------------------

No

mb

re p

art

ícu

les

Diàmet re par t ícula (um)

1 =1.2 m

2 =0.6 m

2 1=0.2 m

2 2=0.1 m

Particle diameter

( m)


1.2.2 Transmission Electron Microscopy

The preparation method that was employed was by grounding a small amount of silica

aerogel into a fine powder in a mortar. The powder was ultrasonically suspended in inert

liquid (methyl alcohol) and the finest particles are selected after decantation. A droplet of the

suspension is then put on the usual electron microscope copper grid.

TEM analysis was useful as to visualize with more detail the contact region between the

microparticles as can be seen in Figure IV.12, although the resolution was not good enough

as to visualize the fine nanostructure of the microspheres.

Figure IV.12 Sample A, TEM microphotograph shows the contact area between particles.

The degree of necking between particles was mainly related to the remaining time of the

dispersed sol at supercritical conditions. Further studies should be undertaken in order to

better determine this phenomena.

1.2.3 Atomic Force Microscopy

To visualize the roughness and microstructure of the particle’s surface an Atomic Force

Microscope (AFM) in non-contact mode was used. The AFM probes the surface of a sample

with a sharp tip, a couple of microns long and less than 100Å in diameter. The tip is located

at the end of a cantilever that is 100 to 200µm long. Forces between the tip and the sample

surface cause the cantilever to deflect. A detector measures the cantilever deflection as the tip

is scanned over the sample, or the sample is scanned under the tip. The measured cantilever

deflections allow a computer to generate a map of surface topography. AFM can be used to

250 nm


study insulators. Non-contact AFM is one of several vibrating cantilever techniques in which

an AFM cantilever is vibrated near the surface of a sample. The spacing between the tip and

the sample for non-contact AFM is on the order of tens to hundreds of angstroms. In non-

contact mode, the system vibrates with amplitude of a few tens of angstroms for a stiff

cantilever near its resonant frequency (typically from 100 to 400 kHz). Changes in the

resonant frequency or vibration amplitude as the tip comes near the sample surface are

detected. There exists a relationship between the resonant frequency of the cantilever and

variations in sample topography. Thus, changes in the resonant frequency of a cantilever

reflect changes in the tip-to-sample spacing so allows to image sample topography.

In order to visualize the microstructure of the A particle’s surface an AFM was used. Before

the observation the particles were dispersed. A small quantity of powder (0.5 mg) was

dispersed in distilled water (50 ml) using ultrasounds (5 min). AFM was used in a high

amplitude resonant mode (‘tapping’ mode). The scale rate was around 1Hz. Squared images

were acquired by using a digitalization rate of 256 points per line.

The set of AFM images (Figures IV.13, 14, and 15) show the process of the high-resolution

images taken at the superior part of the curvature of the sphere. Figure IV.13 (14 m x 14

m) shows two localized microspheres.

Figure IV.13 AFM micrograph (14 m x 14 m) for A silica aerogel powder

These images are represented in dual mode. The left microphotography shows a height image

(z range from 0 m to 3 m). At the right part is a phase image; therefore it is possible to

obtain some differences in the material (in this case only from substrate to microparticle).


When the feature is sharper than the tip, the shape of the tip will dominate the image, in

these images this effect is reflected in the ‘shadow’ on the spheres.

Figure IV.14 Sample A, Surface of a microsphere by AFM.

Lateral dimensions: 1 m x 1 m. The colored scale corresponds to vertical amplitude of 15 nm.

The images were taken at the upper part of the microsphere (2-3 m of diameter). Figure

IV.14 (1 m x 1 m). Because many samples have features with steep sides, tip imaging is a

common occurrence in images, in this case because the surface convexity the roughness is

not clearly observed. A plane fitting was applied (Nanoscope III; Digital Instruments) on the

data type images. The plane fit calculates a single polynomial of a selectable order for our

selected image and subtracts it from the image. A second order has been used to correct the

distortion. The image’s distortion was removed almost entirely reflecting ‘a flat surface’ A

flatten portion of one particle were taken at the superior part of the curvature of one sphere

as shown in Figure IV.15.


Figure IV.15 Sample A, Surface of a microsphere by AFM. Lateral dimensions: (500nm x 500nm). The colored scale corresponds to vertical amplitude of 15 nm.

Figure IV.16 shows the roughness of the surface microspheres at high resolution (500nm x

500nm). The scale mark in z is of 0-15 nm. From those micrographs was possible to make

the image treatment for the surface analysis and roughness study. Figure IV.16 shows the

surface analysis study:

Figure IV.16 Micrographs of the microspheres surface by AFM: Roughness analysis.

It was observed that the surface of the microspheres had a micro-roughness only detectable

using this technique. The z range controls the vertical range of the image, corresponding to

the full extent of the color table and the scanning length of the 500 nm. It was also possible

to observe the existence of interlinked particles of few nanometers of diameter (5-30 nm) and


the presence of pores of few nanometers (5-50 nm) of diameter. This structure is very similar

to that of an aerogel [19-20].

Using the AFM technique, with the ’contact mode’, a trial to calculate the density of the

particle and determining if the spheres can be assimilated to an aerogel or xerogel was

attempted. By the roughness analysis of the surface the Rms parameter can be obtained, it is

defined by:

22

izzRms zi is la height in each point and z is the mean height value. In our sample

the value was of 1 nm. This value is considered micro-roughness, that was the reason that it

was not observed neither by SEM nor TEM. Figure IV.17 shows the surface of the sphere in

three dimensions that will help us for interpreting the obtained images.

Figure IV.17 Three-dimensional AFM micrograph.

Figure IV.18 shows the results of a cross-section analysis of one of the microspheres.


Figure IV.18 Sample A. Cross-section analysis of the surface of one microsphere by AFM. Lateral dimensions (500nmx500nm). The colored scale corresponds to vertical amplitude of 15nm.


MICROPARTICLES USING SUPERCRITICAL CARBON DIOXIDE

Alternatively, a low-temperature synthesis was performed by using supercritical carbon

dioxide as solvent and formic acid as condensation agent. In previous works, sol -gel

reactions were directly performed in supercritical carbon dioxide as a reactive medium by

using formic acid as condensation agent to obtain aerogel monoliths [21].

We named this approach as low temperature synthesis to differentiate it from the first

method where it was needed to reach the critical temperature of the solvent >200ºC

(ethanol) while the critical temperature of CO2 is 32ºC.

Four types of low temperature experiments were performed and are described in the four

sections:

Horitzontal distance: 20 nm Horitzontal distance: 50 nm


Section 2.1: ‘In situ’ low-temperature microparticles. The metal-organic precursor and the

hydrolysis solution (HCOOH) were placed inside the reactor in different ways either as a sol

or as independent solutions.

Section 2.2: Injection of precursor and hydrolysis solution at supercritica l conditions. This

approach consisted in injecting the precursor (TEOS/TMOS) and condensation solutions

(formic acid) in supercritical CO2, under several critical conditions. The injection of liquids

was achieved through a very fine nozzle using a syringe pump.

Section 2.3: Injection of precursor and hydrolysis solution in liquid CO 2: An alternative route

to this one was to inject the reactive in liquid CO2 and dissolving the remaining water and

alcohol from the sol reaction in a continuous flow of liquid CO2, then reach the CO2

supercritical parameters and vent the autoclave.

Section 2.4: Injection of prepolymerized precursor in supercritical CO 2. The prepolymerized

reactive was inject in supercritical CO2, then the temperature was reduced until liquid CO 2

and the remaining water and alcohol from the sol reaction were dissolved in a continuous

flow of liquid CO2, then reach the CO2 supercritical parameters and vent the autoclave.

2.1 ‘IN SITU’ LOW-TEMPERATURE MICROPARTICLES: TEOS, HCOOH,

AND SUPERCRITICAL CO 2 AS A SOLVENT

A process for making monolithic silica gels at low temperature is by reaction of tetralkoxy

orthosilicates (TEOS), with a strong carboxylic acid (p.e HCOOH) [22]. In this process, the

water does not need to be present as a reactant because the hydrolysis reaction is caused by

the carboxylic acid. In that case, supercritical CO2 can be used as solvent for the sol-gel

reactions. The supercritical extraction of the silica gel is performed under supercritical CO 2

conditions (temperature and pressure of 31 C and 71 Bar, respectively). Figure IV.19 shows a

sketch of an ‘in situ’ low-temperature process. The metal-organic precursor and the

hydrolysis solution (HCOOH) were placed inside the reactor in different ways either as a sol

or as independent solutions. Then, the temperature and pressure were raised over CO2

supercritical conditions waiting a certain period of time. In this case, supercritical CO2 was

expected to dissolve the precursor and formic acid (formic acid is miscible in supercritical

CO2 and water is immiscible) leading to the formation of a fine and uniformly aerogel

powder dispersed overall the reactor. Figure IV.19 shows a scheme of one of the ‘in situ’ low

temperature microparticles processing.


Figure IV.19 Sketch of an 'in situ’ low-temperature microparticle production process.

Several experiments were developed in order to optimize the quality of the silica aerogel

microparticles. It is important to point out that these experiments were changed mainly by

varying the following four parameters:

TABLE IV.3 Experimental parameters to obtain microparticles by 'in-situ' low-temperature process

Parameter Range

HCOOH/precursor ratio From 1 up to 24

P and T of the reactor during

the dissolving step

P from 90 to

200 bar

T from 40 to

55 C

P and T of the reactor during

the drying step

P from 90 to

200 bar

T from 40 to

55 C

Time at supercritical conditions From 1 to 24 hours

Figure IV.20 shows an experiment where HCOOH/TMOS was fixed at four. This solution

when is synthesized outside of the reactor gels in 4 hours.

a

)

SCCO2

HCOOH

TMOS

+ EtOH


Figure IV.20 SEM micrograph of microparticles obtained by ‘in situ’ low-temperature process in two vessels, one containing EtOH+TEOS the second one with water+NH3+NH4F.

Again, as in the ‘in situ’ at high temperature process, two kinds of morphologies were

collected, spherical particles and microfibers. In this case, the shape of the microparticles was

not as defined as in the high temperature method. Moreover, the diameter of the fibers was

much larger (a diameter of 750 nm and a length of 5 microns).

2.2 PRECURSOR DIRECTLY INJECTED IN CO2 SUPERCRITICAL

CONDITIONS AT LOW TEMPERATURE

Our approach for preparing aerogel powders at low temperature by direct injection of the

precursor at CO2 supercritical conditions mainly concerned the dissolution of the injected

precursor and formic acid in supercritical CO2 (the sol or the individual solutions), followed

by the sol-gel process and drying under supercritical conditions. The injection of liquids was

achieved through a very fine nozzle using a syringe pump. The reactor conditions were in the

temperature range of 313-368 K and pressure range of 100-150 Bar. The contact time

between precursor and hydrolysis solution at supercritical CO 2 conditions was varied from 1

to 24 hours and the duration of the pressure release in all the experiments was of 30 min.


2.2.1 Injection of hydrolysis and precursor solutions independently

This section describes the results of the process for the processing of silica aerogel particles

by the independent injection of the sol components in the reactor using supercritical CO2 as

solvent. To avoid clogging of the nozzles the TEOS was diluted in ethanol. Next figure

shows the steps followed for one of the experimental procedures:

Figure IV.21 Schematic design of the injection process. Step i) Drive the reactor at CO2 supercritical conditions in order to obtain supercritical fluid before the injection of the sol. Step ii) Inject HCOOH in the reactor using a nozzle. It acts as hydrolysis reactive in the sol-gel reactions. Step iii) Injection of TEOS/TMOS. Step iv) Remaining certain time at supercritical conditions until gel microparticles were formed. Step v) Slow depressurization in order to avoid the damage of the aerogel microstructure

Several experiments were performed under different injected ratios of HCOOH/precursor.

Figure IV.22 shows the SEM characterization for one of the processing particles process with

HCOOH/TEOS=12.

Particles of SiO2

HCOOH

SC-CO2

SC-CO2 TEOS

Injection of HCOOH and TEOS/TMOS in supercritical CO2

Step i) +ii)

Step iii)

Step iv)+v)


Figure IV.22 SEM micrograph obtained from the powder at low temperature process. Injection of TEOS and HCOOH with molar ratio=12. The injected TEOS was diluted in EtOH

with vTEOS/vethano=1. Remaining 3 hour at 150Bar and 85 C.

The powder was formed by microspheres aggregates of spheres with diameters between

100nm and 1 micron. In this case, the particle size distribution was not as narrow as those

obtained at high temperature. The resulting particles are less spherical in shape due to neck

forming between two of more particles.

In order to obtain better-defined shape microspheres by reducing the necking between

particles, the HCOOH/precursor molar ratio was reduced from 12 to 6 and the remaining

time at supercritical conditions was reduced from 3 hours to one hour.

Next SEM micrograph shows a closer up of the particle powder collected under these last

conditions.


Figure IV.23 SEM micrograph obtained from the aerogel powder following a low temperature process. Injection of TMOS and HCOOH with 1/6 molar ratio and vTMOS/vacetone=1.

Remaining 1 hour at 150Bar and 55 C

The microparticle had a better-defined spherical shape (smaller necking) but still a very wide

size distribution, the particle size range from 100nm to 1 m. To better understand the

process that occurs inside the reactor a cartoon is shown in Figure IV.24.

Figure IV.24 Scheme of what happens inside the reactor while the formation of silica aerogel microparticles by injection at supercritical conditions.

CO2HCOOH CO2CO2CO2HCOOHHCOOH

TEOSEtOH TEOSEtOH TEOSEtOH TEOSEtOH TEOSEtOH


When the autoclave was full of CO2 in supercritical conditions (marked in blue) the HCOOH

(marked in yellow) was injected (volume of the autoclave is 2 liters). The amount of the

injected HCOOH was of the order of 10 ml; depending on the number of CO 2 molecules

outnumber the HCOOH molecules by few order of magnitude. Then, each HCOOH

molecule was surrounded by CO2. Gel microparticles scattered all over the reaction. At

supercritical conditions the gel porous was filled with supercritical CO 2, therefore no capillary

forces collapsed the pores when the fluid was vented. In the reactor, there also were the

molecules resulting from the reaction, water and alcohol in very small quanti ties marked in

orange.

2.2.2 Injection of sol

Aerogel powder was obtained by a new approach injecting the sol at carbon dioxide

supercritical conditions. It is important to remark that the sol was constituted by HCOOH

since it is miscible in supercritical CO2. Next figure shows a scheme of the aerogel powder

processing by injection of a sol at supercritical conditions.

Figure IV.25 Scheme of ‘in situ’ injection of the sol in supercritical carbon dioxide

Next SEM images show interlinked microspheres with neck diameters between 100 and

500nm and particle diameters between 1 and 3 m.

SiO2 particles

P=150Bar

T= 80C

3 h at SC- CO2 conditions

Depressurization 30min

SC-CO2

Sol 50ml

TEOS : EtOH : HCOOH

60ml : 60ml: 12ml


Figure IV.26 SEM image of the silica aerogel powder collected

with in jection of a sol with HCOOH/TEOS=5, and vTEOS/vEtOH=1.

Remaining three hours at supercritical conditions 150Bar and 80 C.

The particles obtained by the injection of the sol did not show large differences with those

obtained by the injection of the precursors separately. The later one presents the advantage

that avoids the obstruction of the nozzle.

2.3 ‘IN SITU’ INJECTION IN LIQUID CO2

An alternative method to the ones injecting at CO2 supercritical conditions (described in

previous sections), was to inject the reactants in liquid CO2 and dissolve the remained water

and alcohols formed during the reactions by continuous flow of liquid CO 2. Then, reach the

CO2 critical parameters and after a given time, vent the autoclave. The experiment were

performed following these steps:

i) Preparation of the sol with variable HCOOH/TEOS ratios

ii) Reactor full with liquid CO2, that acts as solvent media.

iii) Inject HCOOH in the reactor using one nozzle.

iv) Injection of TMOS by using a second nozzle.

v) Increase of P, T until supercritical CO2 conditions.

vi) Slow depressurization.


Step ii) and iv) can be substituted by one injection step spraying directly the sol. Next figure

shows the steps followed by an 'in situ’ injection in liquid CO2 experiment.

Figure IV.27 Scheme of the route pursued for the processing of

silica aerogel microparticles using liquid CO2 as solvent. Step ii)

Reactor full with liquid CO2. Step iii)+iv) Injection of the sol. Step v)

autoclave under supercritical condit ions. Step vi) Vent the autoclave

and the silica aerogel microparticles are collected.

Depending on the experimental concentration ratios of TEOS/HCOOH/SCCO 2, diverse

particle agglomerations were obtained.

Figure IV.28 show the SEM micrographs of aerogel microparticles collected when the

injection of TEOS and HCOOH onto liquid carbon dioxide.

SC-CO2 liquid CO2

Sol TEOS + HCOOH

Aerogel SiO2

microparticles

Step ii)

Step iii)+iv)

Step v)

Step vi)


Figure IV.28 Scanning electron micrograph of aerogel silica

particles obtained drying at supercritical carbon dioxide at 45 C, 100 bar when injecting the reactive at liquid carbon dioxide: HCOOH/TEOS=6, and TEOS dissolved in ethanol at vTEOS /vEtOH =1.

In this picture, the neck forming between particles is evidenced compared to the method by

injection at supercritical CO2. A closer up of a broken neck allowed to observe the porous

structure inside the microspheres. The shape of the particles was much better defined and the

size distribution was very narrow. The particle size distribution is also narrow and, in general,

by this method the mean particle size is smaller (<1 m) than in the previous explained one

where the particles were between one and two microns.

2.4 INJECTION OF PREPOLYMERIZED PRECURSOR IN

SUPERCRITICAL CO2

Finally, in order to obtain particles with larger surface area, prepolymerized reactive was

injected in supercritical CO2 After the injection of the sol, the temperature was reduced to

15 C maintaining the pressure at 100 Bar meaning that liquid CO 2 filled the reactor. The

liquid CO2 dissolved the remaining water and alcohol from the sol reaction. An exchange of

liquid CO2 was done in a continuous flow, until all the water and alcohol was substituted.

Then the CO2 supercritical parameters were reached and after a while the autoclave was

vented.


Figure IV.29 Scheme of silica aerogel microparticles at low temperature by the injection of prepolymerized precursor and by addition of liquid CO2 exchange. Step i) Drive the reactor at CO2

supercritical conditions in order to obtain supercritical fluid before the injection of the sol. Step ii) Injection of H5. Step iii) Filling the reactor with liquid CO2. Step iv) Remaining certain time at supercritical conditions until gel microparticles were dried. Step v) Slow depressurization in order to avoid the damage of the aerogel microstructure.

Figure IV.30 shows some of the particles obtained following the above described process.

Step i)

Step ii)

Step iii)

Step iv)

Step v)


Figure IV.30 Alkoxide precursor injected at supercritical CO2.

One hour at 45 C, 100Bar. 1 hour of CO2 liquid exchange.

A more porous surface was observed by SEM compared to the previous injection methods.

That may be caused by the CO2 liquid - ethanol exchange process that facilitated to reduce

the shrinkage of the solid skeleton in the gel microparticles. The size of the collected particles

ranged from 200nm to 600nm. BET method was used to determine the surface area of those

particles, SBET= 520 m2/g. It is a large value compare to that of pure silica (SBET=35 m2/g) but

not very large compare to H5 aerogels (1000 m2/g).

Figure IV.31 Two-steps precursor injected at supercritical CO2 One hour at 45C, 100Bar. 1hour CO2 liquid exchange.


4. CONCLUSIONS

Reactions inside an autoclave open a whole range of possibilities for one step synthesis of

microparticles. The sol was solubilized in supercritical acetone to obtain powders of small

particles with narrow size distribution. Indeed, these microspheres syntheses were based on

hydrolysis and condensation in the supercritical fluid. The size of the collected particles was

very sensitive to the water/alkoxide concentration ratio: the higher the ratio, the smaller the

particles. Moreover, small changes on the depressurization rates results on very differentiated

morphologies, a slow and controlled venting of the autoclave leads to the formation of

monodispersed particles, spherical in shape, and narrowly distributed in the range of 0.5 to 2

m, otherwise fiber-like structures are present.

In the case of using supercritical carbon dioxide as a solvent, the resulting particles are less

spherical in shape due to necking formation between two or more particles. The particle size

distribution is also narrow and, in general, the mean particle size is smaller (<1 micron) than

in the previously described method.

Classical ways for obtaining aerogel particles involve high-temperature and high-pressure

reactions. However there are other ways to obtain aerogel particles based on the use of

supercritical CO2 medium and a condensation agent sufficiently soluble in CO2 (formic acid

has been proved to be a suitable one). In this case, the working temperature is lower. Aerogel

particle formation in the supercritical phase under the conditions employed is dependent on

the solubility of the reactive. The most porous microparticles were obtained when using two-

steps synthesis by injection of the precursors in CO 2 supercritical conditions and adding a

solvent exchange step ethanol- liquid CO2.

5. REFERENCES

1. W. Stöber, A. Fink, and E. Bohn, J.

Colloid Interface Science 26, 62

(1968).

2. D. L. Wilcox, M. Berg, T. Bernat, D.

Kellerman, J.K. Cochran, in Hollow

and Solids Spheres and

Microspheres: Science and

Technology Associated with their

fabrication and applications (Material

Research Society Proceedings,

Pittsburg, 1995), p. 372

3. M. Nogami, J. Mater. Sci. 17, 2845

(1982).

4. H. T. Blair, R.B. Mattews, Annual

Meeting Abstracts. Am. Ceram. Soc.

, 355 (1991)


5. C. J. Brinker, and G. Scherer, in Sol-

Gel Science (Academic Press, New

York, 1990).

6. A. Van Blaader, and A. P. M.

Kentgens, J. Non-Cryst. Solids 149,

161 (1992).

7. J. G. Liu, D. L. Wilcox, J. Mater. Res.

10, 84 (1995).

8. K. J.Pekarek, J. S. Jacob, E.

Mathiowitz, Nature 367, 258 (1994).

9. Unger, K., et al., US Patent No.

4,775,520 (1988)

10. Unger, K., Kaiser, C. German Patent

DE 195 30 031 A1

11. MacNair, J.E., Patel, K.D., Jorgens,

J.W. Anal. Chem., 71, 700 (1999)

12. MacNair, J.E., Lewis, K.C., Jorgens,

J.W. Anal. Chem., 69, 983 (1997)

13. M. Barj, J.F. Bocquet, K. Chhor, and

C. Pommier, J. Mater. Sci. 27, 2187

(1992).

14. K. Chhor, J.F. Bocquet, and C.

Pommier, Mater. Chem. Phys. 32,

249 (1992).

15. X. Y . Zeng, Y. Arai, and T. Furuya,

Trends Chem. Eng. 3, 205 (1996).

16. D. W. Matson, R. D. Smith, J. Am.

Ceram. Soc. 72, 871 (1989).

17. P.G. Debenedetti, in 3·rd

International symposium on

supercritical fluids, (Proceedings,

Strasburg, 1994), p. 213

18. N. Tohge, G.S. Moore, and J.D.

Mackenzie, J. Non-Cryst. Solids 63,

95 (1984).

19. R. w. Stark, T. Drobek, M. Weth, J.

Fricke, and W.M. Heckel,

Ultramicroscopy 75, 161 (1998).

20. C. Marlière, F. Despetis, P. Etienne,

T. Woignier, P. Dieudonné, and J.

Phalippou, J. Non-Cryst. Solids 285,

148 (2001).

21. ‘Process for making inorganic gels’

Patent US5558849, issued 1996-09-

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Yamanaka, and B.M. Baugher,

Chem. Mater 4, 749 (1997).

C h a p t e r V I

SILICA AEROGEL FILMS

SECTION OUTLINE

1. APPLICATIONS OF AEROGEL FILMS ...................................................................................... 212

1.1 ELECTRONIC .............................................................................................................................. 212

1.2 OPTICAL ...................................................................................................................................... 213

1.3 THERMAL.................................................................................................................................... 214

1.4 ACOUSTIC.................................................................................................................................... 214

1.5 ENVIRONMENT AND OTHERS................................................................................................. 214

2. SOL-GEL COATING METHODS .................................................................................................. 215

2.1 DIP COATING ............................................................................................................................. 215

2.2 SPIN COATING............................................................................................................................ 216

2.3 SPRAY COATING......................................................................................................................... 217

2.4 SURFACE TENSION COATING ................................................................................................. 217

2.5 SUBCRITICAL DRYING BY SURFACE DERIVATION................................................................. 217

3. REFERENCE EXPERIMENTAL RESULTS ............................................................................. 217

3.1 DIP COATING ............................................................................................................................. 217

a) Low-Temperature dip coating .................................................................. 218

b) High-Temperature drying ........................................................................ 219

3.2 SPIN COATING............................................................................................................................ 221

a) Spin coating with High-Temperature drying .............................................. 222

4. PROPOSED NEW METHODS: ‘IN SITU’ PREPARATION AT HIGH PRESSURE AND

INJECTION AT SUPERCRITICAL CONDITIONS ............................................................................. 224

4.1 ‘IN SITU’ PREPARATION AT HIGH PRESSURE......................................................................... 224

4.1.1 ‘In situ’ high temperature .......................................................................................... 225

4.1.2 ‘In Situ’ low temperature coating method ............................................................. 225

4.2 SPRAY COATING BY DIRECT INJECTION IN SUPERCRITICAL CO2 AT LOW TEMPERATURE

227

5. CONCLUSIONS .................................................................................................................................. 230

6. REFERENCES ..................................................................................................................................... 231

Chapter VI Silica aerogel films 212

Sol-gel processing has proved to be an important method for producing amorphous porous

silica films. A variety of processing techniques has been developed to minimize shrinkage and

prevent cracking of the films. One of the most successful methods is to avoid altogether the

capillary forces by drying the gel at temperature and pressure above the critical point of the

solvent. The resulting aerogel film retains most of the original volume of the wet gel and is

potentially useful as a wide range of applications due to its low density, high surface area and

low thermal conductivity.

This chapter is organized in four sections. Section 1 summarizes some of the aerogel film

applications. The most used techniques for obtaining films in the sol-gel processes and its

most used characteristics are summarized in section 2. In addition, dip coating and spin

coating were used as a pattern methods to be compared with the results from the „in-situ‟

method developed in our laboratory. Section 3 shows some of the silica aerogels films

obtained by spin and dip coating methods. Section 4 shows the proposed „one-pot‟ methods

to process silica aerogel films and the achieved experimental results. The acquired

„Laboratory Scale Plant‟ provided the possibility to better rationalize the number of free

parameters in the experiments and make simpler the realization of those „one-pot‟ tests.

1. APPLICATIONS OF AEROGEL FILMS

The most used applications for aerogel film either in thick or thin type are described in this

section.

1.1 ELECTRONIC

Thick organic aerogel films ( 0.5mm) are formed by capillary fill, and then pyrolyzed to

carbon aerogel films for use in aerocapacitors.

Thin aerogel films ( 2 m) are formed on silicon wafers to provide a low dielectric constant

in integrated circuits. SiO2 aerogel films are a promising material because it has a low

dielectric constant due to its inherent high porosity that is controllable in the fabrication

process. The dielectric constant for aerogels is around 2.0 and its value depends on the

porosity. The basic technological trend in ultra large-scale integration of electronic

circuits is the realization of higher speed devices with closer packing density, which

results in a multilevel interconnection structure. Improving a chip's insulation is one way


to avoid the problem. Good insulators let chip designers place interconnects close

together without slowing down the electrical signals. Air, the perfect insulator, has a

dielectric constant of 1.0 but it is not possible to hold chips together with air. Silicon

dioxide, the material now used on most chips, rates at about four. Therefore, aerogel‟s

dielectric constant may be as low as 2.5. The aerogel chip insulator could more than

double computing speeds 1, 16 .

Figure VI. 1 Aerogel film as a chip insulator.

1.2 OPTICAL

Thick aerogel films ( 0.2mm) are used as cover slips on solar cells and on optical fibers. A

method for fabricating a lightweight solar cell 2 is provided by preparing a low density

silica aerogel substrate at densities between 0.01-0.1 g/cm3, then a dielectric planarization

layer of SiO2 is applied to the substrate surface and one or more photovoltaic thin film

layers are deposited on the planarization. These solar cells, with their low-density aerogel

substrates, are much lighter than prior solar cells. This is advantageous in satellite

applications since the solar array weight is substantial. Such lightweight solar cells of the

invention also find further advantage on the ground, e.g. for solar-powered vehicles in

which weight is a primary concern. Silica aerogel films are also used in optical fiber 3 .

The problem to be solved is to enable transmission of more light at a larger light

receiving angle by fixing a silica aerogel via a resin to the surface of a film-like base

material.

Thin aerogel films ( 30 m) are coated on the outside of glass laser pump tubes, on solar energy

collectors 4 , and on thin film detectors as low mass optical coating. The aerogel acts as a

refractive index match between the pump tube glass and air, to minimize reflection of

internally generated light. Some thermal detector have an optical coating comprising an

aerogel film with greater porosity than 80% 5 . An optional optical impedance matching

layer may be deposited over the porous film.


1.3 THERMAL

Thick sheets ( 1mm) of silica aerogels are coated with thin film layers of metal, and then

laminated together to form a super-insulating thermal heat shield block 6 .

The aerogel layer may also contain infra-red (IR) opacifier and/or fibers. It is sandwiched

between two films.

Thin aerogel films ( 25 m) have been formed on glass substrates for use in „cool‟ infrared

(IR) detectors. The aerogel film serves as a thermal barrier to shield the IR detector elements

from the heat radiated by the substrate materials 7 .

1.4 ACOUSTIC

Thick aerogel films ( 0.5mm) are formed on the surface of ceramic transducers to serve as

acoustic impedance matching layers 8-12 . Thin films of silica aerogel on silicon have

acoustic waveguide properties. The large mismatch between the acoustic properties of the

silica aerogel film and substrate in creates pronounced dispersion in the velocities and leads

to unusual acoustic behavior: over a relatively large range of wavelengths, the group velocities

of certain modes are less than 50 m/s-nearly ten times slower than the intrinsic velocities of

the nanoporous silica and more than one hundred times slower than those of silicon.

Acoustic impedance matching between a transducer and the irradiated medium can be

achieved using quarterwave or impedance gradient layers 13-14 . SiO2 aerogels are suitable

materials for both purposes. To investigate matched transducers for air applications, sound

intensity measurements were performed with and without aerogel quarterwave layer for

different types of piezoceramic transducers. Ways to employ aerogel layers for the acoustic

matching of piezoceramics are discussed.

1.5 ENVIRONMENT AND OTHERS

Thick aerogel films ( 0.5mm) are formed on the surface a metal mesh matrix for getters in

decontamination. A porous lightweight getter that collects particulate and molecular

contaminates. Such composite getters are useful in decontamination in semiconductor

manufacturing processes and storage and in decontaminating optical systems including a

space-based telescope. In other embodiments, the getter can be mounted in air conducts to


serve as a filter therefore, can be mounted in a photocopier for capturing of toner fog, it can

be mounted in areas of semiconductor manufacturing for collecting contaminates 15 .

2. SOL-GEL COATING METHODS

Some important applications require the aerogels to be as films. To do that, certain

experimental details should be taken in account. Since there is a large surface area in contact

with the atmosphere, the solvent evaporation rate should be slowed down. Thus, the coating

is usually done in a saturated solvent vapor chamber. A second step is required to

supercritcally dry the coating. Another important requirement is the substrate surface

preparation previous to coating because gels do not stick well to unprepared surfaces and any

kind of grease must be eliminated. Bonding to the surface is enhanced by etching the

substrate surface with a mild alkaline solution and then raising it with an alcohol immediately

prior to film deposition.

Next subsections summarize the most used techniques for obtaining silica aerogel films in

the sol-gel processes (2.1 to 2.5) together with two new methods developed in our group.

These „one-pot‟ techniques present the great advantage to perform the whole process inside

the autoclave. In addition, dip and spin coating were used as pattern methods to compare the

results with those of the „one-pot‟ methods.

2.1 DIP COATING

Substrates are dipped into a precursor solution and slowly withdrawn from it. They are then

placed edgewise and vertical in a holder that is located within the solvent vapor saturated

chamber. The disadvantage of this method is the low homogeneity of the obtained films,

since the solvent is evaporating and gravidity draining the film acquires a wedge-like shape.

The main characteristics of dip coating films are: i) thickness lower than few microns

depending on the viscosity of the precursor and withdrawal rate. ii) Gelation time of the film

took only a few minutes because the rapid evaporation of the solvent. iii) The coated

substrates can be either Pyrex glass or silicon wafers. iv) The coated films are immersed in

the corresponding solvent until ready for supercritical drying. Some of the resulting films are

shown in section 3.


FigureVI. 2 shows the main steps of the dip-coating process.

Figure VI. 2 Stages of the dip coating process: (a-e) batch; (f)

continuous. (17)

2.2 SPIN COATING

The most used procedure to form gel films is by dropping the precursor solution onto a

spinning substrate while its spin rate increases. Spinner is then turned off and stopped with a

brake (causing minimal loss of solvent during gelation). The main problem is the low

homogeneity of the films and the need to work in a solvent saturated chamber, also it is

necessary to find the balance between the two main forces, centrifugal and viscous (friction).

The main characteristics of spin coating films are: i) thickness lower than two microns. ii)

The gel is formed within a few minutes. iii) The substrates used are silicon wafers up to 1 cm

of diameter. iv) The substrate coated with the gel film is immersed in solvent to age until the

supercritical drying.

Figure VI. 3 Stages of the spin-coating process 17 .


Different type of catalyst and several TEOS/EtOH ratios were attempted with the purpose

of processing spin-coating films with a wide variety of porosity. Some of the results are

shown in section 3.

2.3 SPRAY COATING

Usually this technique is used to coat thicker single layer. films as thick as 80 m In this

method, the solution precursor is directly sprayed onto the substrate (supported in a vertical

position within the chamber). Excess solution drains by gravity leaving a thick film that gels

within a few minutes. The surface of the gel is smooth and continuous. In the same manner

that in the previously mentioned methods, the coated substrate is immersed in solvent after

gelation until ready for supercritical drying.

2.4 SURFACE TENSION COATING

In this coating method, surface tension is used to draw the liquid onto the solid surfaces. A

feature is made by forming a space between the substrate to be coated and another flat

surface (treated to prevent bonding to the gel so that should be removable after drying

process). Liquid precursor fills the available volume by capillarity when the element is dipped

into the solution. The spacers are used to separate the substrates by the desired film

thickness. Common film thicknesses are from 2 to 50 m.

2.5 SUBCRITICAL DRYING BY SURFACE DERIVATION

In this technique, aerogels are prepared by dip-coating at ambient pressure 18-19 without

needing of supercritical drying. To achieve that, surface groups (organosilyl-terminated

surfaces) are added to the gel, making drying shrinkage reversible: as the solvent is

withdrawn, the gel springs back to a porous state. Final pore volume of the ambient pressure

aerogel is a result of competing effects: the capillary stress induced collapse, and

condensation /polymerization reactions which tend to stiffen the matrix resisting the

collapse. The surface organic groups reduce capillary stresses due to contact angle

modification and offer resistance to pore collapse.

3. REFERENCE EXPERIMENTAL RESULTS

3.1 DIP COATING

This technique has been used as a guide to compare with the „one-pot‟ experiments. Film

thicknesses were of few microns depending on the initial molar ratios of the precursor sol.


The gel film can be supercritically dried at CO2 conditions (Section 3.1.a) or under the

supercritical conditions of the gel solvent (Section 3.1.b).

a) Low-Temperature dip coating

A low temperature test comparing dip coating and „in situ method‟ (see section 4) has been

performed under the same supercritical drying experiment. The sol-gel synthesis is the same

for the two type of coating process: equivalent molar ratio,

TEOS/EtOH/H2O=1/12.6/34.7 and equivalent catalyst concentration (citric acid 0.03M).

Acid catalyst has been used because in acid conditions the contact surfaces of necks in the gel

increases and may facilitate the film formation.

Figure VI. 4 Scheme of comparat ive experiment, the d ip coated

samples were dried under the same experimental conditions that the ‘low

temperature’ coated samples.

In the case of dip-coating, the sol is prepared outside of the reactor and the substrates are

dipped in the sol before gelling. They are immediately placed in a saturated atmosphere

avoiding the evaporation of the solvent. For the in situ process, the two independent

solutions were placed inside the autoclave. When the substrates were ready for the

supercritical extraction P and T were increased until supercritical CO 2 conditions 150bar

(Pc=73bar) and 80 C (Tc=31 C). The autoclave remains 5 hours under these conditions. In

this step is when the sol-gel process occurs; in addition, some longer time was required to age

the film. Figure VI.5 shows the inhomogeneous but comparable films that were obtained by

the dip-coating and on the „in situ‟ method.

SCCO 2

2mlTEOS+20ml EtOH 1ml H 2 O+

catalitzador Dip coated substrate

In situ + Dip coating SCCO 2

2mlTEOS+20ml EtOH 1ml H 2 O+

ac cítric

In situ + Dip


Figure VI. 5 SEM of the inhomogeneous films, dip coating at the left

and ‘in situ’ experiment at the right.

It should be remarked that the surfaces of the substrates were untreated so the homogeneity

of the films could be improved making a surface treatment on the substrates. Comparing

both experimental results, the morphology of the films seems to be very similar indicating

that in the „in situ‟ method the sol-gel process has been preceded in a similar way than for the

classical method.

b) High-Temperature drying

In order to optimize the synthesis parameters of the aerogels films several dip-coating tests

have been attempted. The sol-gel dip coatings following a high temperature route were dried

under supercritical ethanol conditions at 100bar (Pc=63bar) and 255 C (Tc=235 C). The

film morphology is modified by varying the aging time of the gel films that remains under

saturated atmosphere conditions. Figure VI.6 shows some of the experiments when using a

sol-gel process with a very short gelation time (few minutes). The reactive ratios are

TMOS/MetOH/H2O/NH4OH=1/12/4/6,5.10-4. Several types of morphologies were

produced by changing the aging times under saturated solvent (1 or 2 minutes).


Figure VI.6 shows four SEM images obtained in two different aging times (one and two

minutes) and in different saturated conditions, left side for non-saturated and right side for

methanol saturated atmosphere.

Figure VI. 6 Dip coating silica aerogels with reactives ratios of

TMOS/MetOH/H2O/NH4OH=1/12/4/6,5.10-4

. Figure VI.a) 1 min of aging

at room conditions. Figure VI.b) 1 min of aging under methanol saturated

atmosphere Figure VI.c) 2 min aging under room conditions Figure VI.d)

2 min aging under methanol saturated atmosphere.

It can be observed that in the two experiments without saturated atmosphere (a and c) small

particles have been formed on the film surface. The presence of particles increases when

increasing the aging time. On the other side, in presence of saturated atmosphere no particles

are found (b and d). The films are more homogeneous and without cracks.

Figures VI.7 and 8 show a detail of the porous microstructure of the aerogel coating under

different aging time, one and two minutes, respectively.

a)

b)

c)

d)


Figure VI. 7 Dip-coated substrate 1 min of aging time in methanol

Figure VI. 8 Dip-coated substrate 2 min of aging time in methanol

When the aging time is increased, the structure becomes more colloidal and less polymeric.

The film morphology seems to be more porous when decreasing aging time.

3.2 SPIN COATING

This technique has also been used in order to optimize the sol-gel procedure for film

formation and to use the obtained films as evaluation of the quality of the „in situ‟

experiments. A wide study has been performed using different catalysts and different ratio of

TEOS/ethanol. The film thickness obtained by this process is usually lower than 2 m. The


drying procedure has been performed at solvent supercritical conditions (section 3.2.a) and

with CO2 liquid exchange (section 3.2.b).

a) Spin coating with High-Temperature drying

In order to optimize the synthesis parameters of the aerogels films several spin-coating

experiments have been tested. The sol-gel spin coatings following a high temperature route

were dried under supercritical ethanol conditions at 100bar (Pc=63bar) and 255 C

(Tc=235 C). The reactive ratios in the methanol gel spin coating are

TMOS/MetOH/H2O/NH4OH=1/12/4/6,5.10-4. The film morphology and coating

thickness is modified by varying the aging time of the coated substrates that remains under

saturated atmosphere conditions. Figure VI.9 shows four SEM images for one of the

experiments when using a sol-gel process with a very short gelation time (few minutes). The

reactive ratios are TMOS/MetOH/H2O/NH4OH=1/12/4/6,5.10-4.


a) b)

c) d)

Figure VI. 9 a) An homogeneous film is observed with particles

deposited over the coating. Figure VI.b) shows a closer up of those

particles. In Figure VI.c) shows a cross -section of the film. It is possible

to observe that the thickness of the spin coating is of the order of 2 m.

The film shows some cracks as observed in Figure VI.d ).

Since the films are so thin, the gelification speed is accelerated because evaporation rate is

high. Then special attention must be taken to avoid the premature drying of the films at

ambient conditions. In view of the fact that in this experiment it was not possible to work

under a completed saturated atmosphere the coating presented some cracks (Figure VI.9b).

The microstructure of the spin-coating films was similar to that obtained by „one-pot‟

method.


4. PROPOSED NEW METHODS: ‘IN SITU’ PREPARATION AT HIGH

PRESSURE AND INJECTION AT SUPERCRITICAL CONDITIONS

A new technique based on supercritical fluid technology for the production and processing

of aerogel film has been developed in our group. Two alternative methods are shown. „In

situ‟ preparation will be described in section 4.1 and „spray in supercritical CO2 method‟ in

section 4.2.

4.1 ‘IN SITU’ PREPARATION AT HIGH PRESSURE

A substrate was placed in the autoclave directly over the recipient containing a diluted sol,

which was previously prepared out of the reactor. As a second option, the substrate was

placed over two independent vessels containing the sol-gel solutions: the precursor solution

(precursor dissolved by the solvent) and the water solution (water and in some cases a

catalyst).

The method consisted of four main steps:

i) The substrates -previously treated to enhance adherence-, the precursor, the

solvent, and some water were introduced in the autoclave before sol gellifies. See

Figure VI.10.

ii) The reactor was driven above solvent supercritical values. At supercritical

conditions, the sol components (the metal-organic precursor, the solvent and water)

were dragged by the supercritical fluid and the components were distributed over the

entire reactor.

iii) The sol gellifies and then, temperature is increased above the supercritical value.

iv) Finally, the solvent is extracted under supercritical conditions of the solvent.

Figure VI. 10 ‘In situ’ preparation at h igh pressure: The substrate is

placed over the recipient containing the sol that is dragged by the

supercritical flu id and deposited on the substrate surface.

sol

sol in process of

gelification

solvent

supercritical conditions

substrate


Some examples will be more extensively explained, in section 4.1.1 when using ethanol in

supercritical conditions and section 4.1.2 for supercritical CO 2.

4.1.1 ‘In situ’ high temperature

Some experiments were performed placing two vessels inside the autoclave one with the

dissolved precursor in ethanol solution and the other one with hydrolyzing solution, and then

the autoclave was driven until supercritical ethanol conditions following the above described

method. In all the attempted high temperature tests a white powder was found spread over

the inside walls of the autoclave vessel (see Chapter V: Silica aerogel particles). For that

reason, further studies were needed in order to improve the „in situ‟ method. To get a better

route two changes were proposed. The first change was to vary the sol-gel conditions by

varying the TEOS/EtOH/H2O ratios with the intention to increase the contact of the neck

surfaces between particles leading to film morphology. The second change is to cool the

substrate whiles remaining at supercritical conditions (during step iii) allowing the

condensation of droplets on the surface.

4.1.2 ‘In Situ’ low temperature coating method

Aerogels films were obtained at low temperature (without needing to use water) by using

CO2 as solvent in the synthesis of the gels (see more details of the synthesis on section 2.1.b).

As an example, Figure VI.11 illustrates one scheme of these experiments.


Figure VI. 11 ‘In situ’ Low-T experiment. (A) The precursor and the

hydrolysis solution were placed inside the autoclave as independent

solutions placing two vessels. The first vessel contains the precursor

solution (TEOS dissolved in ethanol) and the second vessel contains the

hydrolysis solution (HCOOH dissolved in ethanol). (B) Then, the two

solutions were dragged at low temperature by supercritical CO2. (C) After

a while, the sol gellify and supercritical CO2 extraction was processed.

The autoclave was leaded to supercritical CO2 (200Bar, 80 C) during 24 hours. The

experiment failed, since it was observed that the amount of the liquid in both vessels

diminished but no coating was obtained. Instead, few particles were found on the substrate

surface (described in Chapter V: Silica aerogel particles). In order to optimize the film

formation a series of „in situ‟ low temperature experiments were tried changing the following

parameters:

-Ratio of HCOOH/precursor.

-Pressure and temperature of the supercritical fluid.

-Time of permanence in supercritical conditions.

A B

C

Silica aerogel


One of the experiments performed trying to improve the results, was decreasing the TEOS

concentration in ethanol (EtOH/TEOS =12) and the substrates were treated by an alkaline

solution (NaOH) and dipped in a solution with water and acid catalyst (1M HF). The idea is

to avoid the formation of particles by using the immiscibility of water in CO2. In this case, a

thick porous film was obtained on the substrate.

4.2 SPRAY COATING BY DIRECT INJECTION IN SUPERCRITICAL CO2

AT LOW TEMPERATURE

A new initiative was seek to obtain silica aerogel films in a „one-pot‟ process. The idea was to

spray the sol-gel reactives directly in the autoclave at supercritical conditions.

This method was based on the injection (by using a nozzle) of the precursor (TEOS or

TMOS dissolved in EtOH) at supercritical CO2 conditions. After that, the injected precursor

reacts with the HCOOH that covers the substrate, in these reactions the supercritical CO 2

acts as the solvent of the sol. Subsequently, after the gelification of the sol, the supercritical

extraction of the solvent from the gel film was performed on supercritical CO 2 conditions

(temperature and pressure of 31 C and 71 bar, respectively). As a final result, a silica aerogel

film was achieved at low-temperature without the presence of water. Figure VI.12 shows the

scheme of the experiments for the injection of TEOS (at supercritical CO 2) over a substrate

covered with HCOOH. The scheme exemplifies two different routes marked as 1) and 2)

corresponding to two different ratios of the injected solutions.


Figure VI. 12 Injection of TEOS dissolved in EtOH (at supercritical

CO2 conditions) over a HCOOH covered substrate. For process 1), the

molar rat io was HCOOH/TEOS=6 and the volume ratio was

vEtOH/vTEOS=10. For p rocess 2), the molar rat io was HCOOH/TEOS=12

and the volume ratio was vEtOH/vTEOS=1.

For process 1), the molar ratio was HCOOH/TEOS=6 and the volume ratio was

vEtOH/vTEOS=10. A porous but cracked film was obtained, and no microparticles were found

on the substrate and inside the autoclave. Next Figure shows the SEM image of the film

under the above-described conditions.

Figure VI. 13 SEM image of a film obtained by spraying (in

supercritical CO2) TEOS+EtOH on the HCOOH wet substrate.


It was observed that this film was porous and reasonably homogeneous. Figure VI.14 shows

a closer up of the film morphology, its microstructure was very similar to that of a bulk

aerogel.

Figure VI. 14 Micrograph of a detail of the film microstructure.

For the process number 2), see Figure VI.8, the substrate was initially covered just by

HCOOH (not dissolved in ethanol) and the ratios were increased to HCOOH/TEOS =12,

and vEtOH/vTEOS =1 in order to accelerate the gelification process. A porous and non-

homogeneous 100 m thick film was obtained. Some microparticles were on the substrate

but no powder was found inside the autoclave.

In order to better understanding the process that occurs inside the reactor a scheme is shown

in Figure VI.15.

CO2

TEOSHCOOH

On the substrate

EtOH

Figure VI. 15 Cartoon picture of what may happen inside the

autoclave when TEOS (marked in violet) dissolved in EtOH (marked in

green) is injected on the reactor while the substrates are dipped in a

HCOOH solution (marked in yellow).


This cartoon gives an idea about what may happen inside the autoclave: When the autoclave

is full of CO2 molecules in supercritical conditions (marked in blue), the TEOS (marked in

violet) dissolved in EtOH (marked in green) is injected on the reactor while the substrates are

dipped in a HCOOH solution (marked in yellow). Then, supercritical CO 2 (marked in blue)

acts as the reactive media of the sol and contributes that the sol-gel process occurs on the

surface of the substrate.

5. CONCLUSIONS

1) The main objective of this chapter has been to synthesize aerogels as homogeneous

films and with good adherence to the substrate. The coated substrates were Pyrex

glass slides and silicon wafers.

2) Four methods were tried: dip and spin coating were used to evaluate the results of the

two „one-pot‟ proposed methods. Spin coating processed films were more

homogeneous and thinner than dip coated films. By varying the aging time and the

viscosity of the coating gel, we were able to obtain a variety of film porosities. The

processed films by spin coating were useful to evaluate the quality of the „in situ‟ and

spray coating experiments.

3) The „In situ‟ methods (at low or high temperature) give a wide variety of

morphologies. When the aging time was not long enough microparticles were

obtained. The presence of catalyst on the hydrolysis solution accelerates the sol-gel

process and then allows a better control of the homogeneity of the films. Another

important factor is the ratio between the sol-gel reactive a too large concentration of

hydrolysis solution results in particulate powder.

4) Spraying the sol-gel components directly in supercritical conditions seems to be the

most appropriate and simple method to obtain aerogel films by „one-pot‟ processing.

The substrate was covered by HCOOH and the precursor was sprayed through a

nozzle, the CO2 at supercritical conditions acts as the reactive medium. When the

aging time was too short, the sol-gel process was not finished and then microparticles

were found spread over the reactor. Under the optimized conditions, homogeneous

and porous films were manufactured. An important factor to control the porosity and

morphology of the coatings was the precursor/HCOOH molar ratio.

5) Further essays may be performed in order to find a technique to measure the porosity

of the films in order to ensure the presence of silica aerogel structure. One possible

technique is by elipsometry, which may allow measuring the density of the films.


6) Further characterization of the films should be carried out (density, thickness,

adherence to different substrates, etc.).

7) Accurate theories of in situ and spray directly in supercritical conditions would permit

better control of these processes and would allow the design of specific coating

protocols for specific applications.

6. REFERENCES

1. M. Bunzendahl, "Development of a

Low-Dielectric Constant Thin-Film

Composite Material." M.S. thesis,

Department of Mechanical,

Aerospace, and Nuclear Engineering

University of Virginia, August 1998

2. “Lightweight solar cell” Patent

US5221364, issued1993-06-22,

Hotaling Steven P

3. “Silica aerogel film and its

production, optical fiber“ Tsubaki

Kenji; Yokoyama Masaru; Yokogawa

Hiroshi; Sonoda Kenji, JP10300995,

issued 1998-11-13

4. “Developed sol-gel spray coatings for

solar energy collectors”. Brinker C

Jeffrey; Fraval Hanafi R, U.S

Patent:WO0010044, issued 2000-02-

24.

5. “Low mass optical coating for thin

film detectors”, Cho Chih-Chen;

Beratan Howard R US Patent

US5929441, issued 1999-07-27.

6. “Coated film with aerogel layer with

low thermal conductivity and good

mechanical stability” Patent

DE19606114, issued 1997-08-21

Frank Dierk; Schwertfeger Fritz;

Zimmermann Andreas.

7. “Coated film used for thermal

insulation, electronic applications,

noise absorption or membranes”

Patent DE19537821, issued 1997-04-

17 Frank Dierk, Schwertfeger Fritz,

Zimmermann Andreas

8. “Acoustic waveguide properties of a

thin film of nanoporous silica on

silicon”. Rogers, John A.; Case,

Carlye, Appl. Phys. Lett., 75(6), 865-

867, 1999

9. “Acoustic properties and potential

applications of silica aerogels”. Gibiat,

V.; Lefeuvre, O.; Woignier, T.;

Pelous, J.; Phalippou, J. Non-Cryst.

Solids, 186, 244-55 1995

10. “Applications for silica-based aerogel

products on an industrial scale” M.

Scmidt, F. Schwertfeger. Mat. Res.

Soc. Symp. Proc. Vol 521 1998

11. “Evaluation of the acoustic properties

of silica aerogels” Conroy, John F. T.;

Hosticka, Bouvard; Davis, Scott C.;

Norris, Pamela M. 82 Porous, Cellular

and Microcellular Materials, 25-33

1998

12. “Localization of acoustic vibrations in

aerogels” Vacher, R.; Courtens, E.


Ultrason. Symp. Proc., (2), 1237-9

(English) 1989.

13. “Modified silica aerogels as acoustic

impedance matching layers in

ultrasonic devices” Gerlach, R.;

Kraus, O.; Fricke, J.; Eccardt, P. C.;

Kroemer, N.; Magori, V. J. Non-

Cryst. Solids, 145(1-3), 227-32 1992.

14. “Flexible aerogel composite for

mechanical stability and process of

fabrication” Patent WO 9938610,

issued 1999-08-05 Coronado, Paul R;

Poco, John F.

15. „Aerogel mesh getter„, Patent

US5470612 , issued 1995-11-28

Hotaling Steven P, Dykeman Deidra

A .

16. “Rapid aging technique for aerogel

thin films” US5753305, issued 1998-

05-19, Jeng Shin-Puu; Smith Douglas

M; Ackerman William C

17. .D.E Bornside et al. J. Appl. Phys. 66

(1989) 5185.

18. "Preparation of High Porosity

Xerogels by Chemical Surface

Modification", Brinker, C.J.,

Desphande, R.,Smith, D.M., U.S.

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Preparing Aerogel Thin Films",

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1999

CONCLUSIONS

This section provides an overview of the conclusions of each of the chapters presented in

this thesis.

The preparation of silica aerogels deals with the combination of the silica solid matrix

(formed by interlinked silica nanoparticles) and nanometer-sized pores (filled with air). Such

combination results in the unique optical, thermal, acoustic, and mechanical properties of

aerogels. In this work it has been shown that there is a direct connection between the

chemistry of the sol-gel process and the structure of the gels and consequently, on the

microstructure of the aerogels. The choice of the precursors and the chemical reaction

parameters determines the physical properties of the final silica aerogels. It has been shown

that it is possible to control porosity and transparency by adjusting the so-called sol-gel

parameters, which include the type and concentration of alkoxide precursor, acid or base

catalyst, and water content.

Tetramethoxysilane as metal alkoxide precursor (TMOS):

To obtain transparent aerogels the best synthesis is obtained by using methanol as solvent

with the presence of base catalyst at low concentrations. This synthesis has been labeled as

M-series.

Varying the TMOS concentration in acetone gels proves to be an easy way for controlling

density and porosity of the resulting aerogels. The study of the influence of the

TMOS/acetone concentration of the sol on the density of the aerogels was performed by

using the so-called A-series. Methanol gels are very transparent while acetone aerogels have a

white shading and their opacity decreases with increasing TMOS content. For all the samples

of A-series, monolithic structures without cracks were obtained. The A1 aerogels were

especially fragile. Cracked gels were produced when ethanol was used as a solvent.

It was found that the gel time, tg , widely expands from few minutes to several days,

depending on the type of solvent and catalyst combinations, with shortest t g being for

methanol solvent and sodium hydroxide catalyst. This effect is a result of the shortest chain

length and branching of the methanol compared to the other solvents.

Conclusions 234

Tetraethoxysilane as metal alkoxide precursor (TEOS):

Aside from TMOS, other esters of orthosilicic acid like TEOS were used to obtain silica

aerogels. TEOS is not only less toxic when compared to TMOS but it is cheaper too. Hence,

TEOS is a more suitable precursor for the commercial production of silica aerogels.

The best quality TEOS aerogels, in terms of monolithicity and transparency, without much

shrinkage were obtained by using weak base or acid catalyst, TEOS molar concentration in

alcohol: between 5 and 7, and excess of stoichiometric amount of water with values between

5 and 8. Strong acidic catalyst gave transparent but cracked aerogels, whereas weak acids

yielded monolithic and transparent aerogels. The conditions considered optimal

concentration of reactive were for TEOS/EtOH/H2O =1/5/7, 1/7/5, 1/7/6, and 1/7/8.

The reaction was more easily controlled with the presence of low concentration of acid citric

catalyst (0.01<c<0.03). On the contrary, taking the m precursor concentration (m=

TEOS/EtOH) lower than 5 (m< 5) and higher than 9 (m> 9) values resulted in opaque, high

density as well as cracked aerogels. The effect of ethanol content over the TEOS gels allowed

aerogel density to decrease. In addition, it can be concluded, that an increase in solvent

content reduces the probability of mutual collisions of hydrolyzed alkoxides molecules (Si -

OH), resulting in a decrease in the rate of polymerization reaction.

One can state that to produced a good quality aerogels, the most favorable condition of the

preparation were when the ‘two-step method’ was followed, a first step in acidic conditions

and a second one in basic conditions. The two-steps aerogel density increases as the

concentration of catalyst increases, due to the fact that at too large catalyst concentrations,

the colloidal particles and pores are smaller and therefore the gels tend to shrink and become

denser. In all two-steps obtained aerogels to enhance the aging process, the gels were soaked

in an alcohol/water/catalyst mixture of equal proportions to the original sol. The gels were

maintained in this solution up to 24 hours. After that, water was removed by soaking the gels

in an ethanol solution. In conclusion, although a slightly more laboriously synthesis, the two-

step process appears to be the best synthesis method to obtain low-density (from 0.03 to

0.1g/cm3), non-cracked, and very transparent aerogels with a high surface area (800-

1000m2/g).

For all the metal alkoxide precursors, it was observed a decrease of gel time by either

increasing the amount of water or decreasing the concentration of the precursor. It should be

pointed out that for the same synthesis and drying conditions, gels washed in ethanol

Conclusions 235

exhibited a lower bulk density and the incidence of cracking or fracture of aerogels was

significantly lower. This difference in bulk density is attributed to the presence of water

during drying process that caused a larger shrinkage giving a denser bulk structure.

Physical characterization of silica aerogels has been widely investigated. The aerogels were

characterized by BET, IR, UV-VIS spectroscopy, light scattering, SEM, and TEM techniques.

Aerogels from diverse sets presented clear differences in shrinkage, transparency and porosity.

Initial parameters, such as solvent, catalyst and water content, have a pronounced influence in

the pore structure and optical transparency of the final aerogel. Different microstructures,

from macroporosity to mesoporosity, have been obtained by varying the initial sol parameters.

The surface area and porosity of the resulting aerogels has been discussed using the BET

technique, M and A-series showed similar surface areas (from 400 to 600 m2/g). The

translucence of the aerogel samples indicates the presence of macropores, although they are

not accounted by BET. In order to evaluate the effect of the macropores on the transparency,

light transmission experiments have been performed. The optical transmission of the aerogels

was measured at a wavelength of 900 nm in order to compare quantitatively the degree of

transparency of the samples at the visible range. The lowest transmission was obtained for

more diluted sol conditions (A1), the percentage of transmission was of 23% for a sample

with a thickness of 1 cm and the highest transmission for denser aerogel (A4), the percentage

of transmission increased up to 40%. Aerogel transparency increases with the TMOS

concentration due to lower macropores content. The percentage of light absorbance

depending on the intensity of the Rayleigh scattering has been related to the diameter size of

the scatter centers, < pore>Rayleigh, larger porous leads to lower transmission.

It is important to point out that pore size distribution has been described by a proposed

model using BET, optical transparency and density measurements. From that model, it has

been concluded that:

The opacity in A1 is explained by the large value of V, where V= Vpore -VBET, which

corresponds approximately to the macropores not measured by BET (~90% for A1). UV-

VIS measurements allowed to confirm this hypothesis giving a A1 pore diameter,

< pore>Rayleigh, of 150 nm. A2 and A3 have a pore size distribution with an important number

of micropores and/or macropores, but the mean pore diameter obtained is within the

Conclusions 236

mesopore range. A4 presented a mesopores distribution, which is totally accounted by the

BET technique, and agrees with the mean pore value obtained by optical measurements. M

have a pore size distribution that is mostly accounted by BET (~80%). Its mean value is

similar for both techniques, BET and optical measurements.

The nephelometer measured the angular and polarization dependence of light scattered at

visible regions. It has been proved that intensity measurements at fixed angle provide

information about the size of scattering centers. Angular measurements at different

wavelength have demonstrated the necessity of performing scattering measurements to

obtain information outside of the Rayleigh regime. The presence of the correlation function

(r) in the scattered intensity equations has permitted to use light scattering measurements to

determine density-density correlation in the aerogels medium, and to extract information

about the inhomogeneities of the aerogel microstructure. It was assumed that the correlation

function consists of a short-range exponential part, 1(r), and a long-range gaussian part,

2(r). The model proposed was able to fit the experimental data by the variation of the a1, a2

and w parameters. The model was fitted with four different wavelengths.

Further studies should be undertaken in order to improve the fitting of the data with the aim

of extracting structural information from the scattered intensity and to normalize the

intensity for each laser beam. New correlation functions are proposed to fit the experimental

data.

Since bulk material properties are a function of aerogel microstructure, then efforts to direct

imaging aerogels at molecular level have been taken by using SEM and TEM. The

morphological features, such as particle shapes and particle arrangements, of the series of

acetone silica aerogel A-series, and the base catalyzed aerogel with methanol as solvent (M)

has been examined by Scanning and Electron microscopy (SEM and TEM). An estimation of

the particle size has also been evaluated. SEM images show that the aerogels have a granular

appearance composed by spherical particles. A1 sample is built by smaller interconnected

particles than the denser A4 sample. A4 showed the smallest pores, although particles were

larger than those of A1. M aerogel shows an interconnected structure with smaller particle

size. TEM technique has provided molecular information about the aerogels and to

distinguish the different parts of the structure of the aerogel. Individual chains and

Conclusions 237

crosslinking junctions have been visualized. TEM microscopy was used to examine the

surface replicas in stereo with a tilt series at 20k-80k magnifications. The stereoscopic images

have made possible a three-dimensional visualization of the aerogel structure.

The microindentation technique has proved to be a non-destructive, suitable dynamical

method to assess the parameters that characterize the mechanical behavior of extremely

porous materials such as aerogels, despite their brittleness and softness. Silica aerogels of

different mechanical responses have been obtained by varying the initial parameters in the

TMOS sol-gel polymerization process, such as alkoxide concentration, solvent, drying process,

as well as the carbon addition. Young’s modulus, hardness and the elastic parameter, that

measures the percentage of elasticity versus plasticity, have been evaluated for these aerogel

samples. It has been shown that the evolution of the parameters describing the mechanical

behavior as a function of the bulk density follows power-scaling laws. A relation of the type,

E with ~2.9 was found for the A-series. The evaluated exponents are 2.0 and -3.2 for

hardness, and elastic parameter, respectively. As a function of aerogel density, two different

regimes of mechanical behavior are observed. The lowest density aerogels are elastic but the

denser aerogels are elasto-plastic materials. The large dependency of Young’s modulus on the

density as well as the change from elastic to plastic behavior has its origin on the aerogel

microstructure. Further structural and modelisation efforts are needed for a deeper

understanding of these dependencies.

The effects of the solvent, being methanol or acetone, on the silica aerogels have also been

studied. It was found that for samples with similar density values, the ones obtained using

acetone have higher hardness and Young’s modulus values than those obtained from

methanol, but with less elastic recovery. Moreover, the effect of the drying process has been

studied. It has been shown that the process with CO 2-acetone exchange causes a slight

improvement in hardness and a relevant increase in the elastic modulus, mostly due to larger

shrinking effects.

An improvement of the material’s elasticity is needed for some applications. Carbon-

reinforced aerogels present a more elastic response to indentation compared to silica

aerogels. The addition of small amounts of powdered carbon as filler in silica aerogels

increases the elasticity of the composite and keeps the hardness similar to silica matrix values.

Conclusions 238

It has been found that including small amounts of activated carbon inside the fragile silica

network dramatically increases its elastic indentation recovery. This is reflected in the shape

of the indentation curves as well as in the increase of the elastic parameter value. Young’s

modulus values obtained for carbon-reinforced aerogels show a similar variation to the

carbon mass fraction as that predicted by a commonly used model for composite materials.

The measured hardness values corresponding to the total elastoplastic deformation do not

show such a prominent dependency on the carbon mass fraction as the elastic parameter and

Young’s modulus do and values of hardness are similar to those measured for the pure silica

aerogel.

A simple and versatile method to obtain silica aerogel particles based on the hydrolysis and

subsequent condensation of silicon alkoxides (TMOS/TEOS) in several supercritical f luids

has been proposed. The sol-gel route at supercritical conditions reduces the number of steps

of the traditional microparticle sol-gel processing. To obtain aerogel silica powders at low

temperature the synthesis has been performed using supercritical carbon dioxide as solvent.

Following the ‘in situ’ high-temperature method, spherical and fiber silica particles have been

produced by ‘one-pot’ method using the sol-gel process with supercritical acetone as a

solvent. The spherical particles showed a very narrow size distribution. The particle size is

controlled by varying the relative amounts of alkoxysilane, water and acetone. Other

important parameters in order to control the particle size are the relative time, t/tgelification,

and the way of introducing the reactants in the autoclave. Silica aerogel microparticles have

also been produced at low-temperature by ‘one-pot’ process by injecting the reactive under

CO2 supercritical conditions. In this case, less spherical in shape particles were obtained

because the existence of necking between particles. The most porous aerogel microparticles

have been produced when using two-steps synthesis and introducing a liquid CO2 exchange.

Major focus was given on the understanding of the particle formation mechanisms and on

the conditions to tailor the particle morphology, size and porosity. The morphology of the

particles has been characterized by electron microscopy (SEM and TEM) and Atomic Force

Microscopy (AFM). The microstructure observed on the silica aerogel surface was similar to

that one of bulk silica aerogels. Coulter technique has also been used to account for the

particle size distribution. In addition, BET surface area, pore volume and mean pore diameter

of the aerogel microparticles have been obtained from nitrogen sorption isotherms.

Conclusions 239

Aerogel films have a wide range of applications. The main objective of film chapter has been

to synthesize aerogels as homogeneous films. Two ‘one-pot’ methods have been described. A

recently set up ‘laboratory scale plant’ has provided us with the possibility to better rationalize

the number of free parameters in the experiments and simplify the realization of such. We

have also performed some dip coating and spin coating experiments to compare the results

of those with our methods. When dealing with dip and spin coating methods to produce

aerogel films, the solvent evaporation rate should be slowed down since there is a large

contact area of the forming gel with the atmosphere. Thus, the coating is usually done in a

saturated solvent vapor chamber. A second step is required to supercritically dry the coating.

Further characterization of the films should be done (density, thickness, adherence to

different substrates, etc.).

A N N E X I

SUPERCRITICAL FLUIDS

A supercritical fluid is defined as a fluid above its critical temperature (TC) and its critical

pressure (PC). The critical point represents the highest temperature and pressure at which the

substance can exist as a vapor and liquid in equilibrium.

A ‘physical’ way to explain what is happening in a supercritical fluid is that when two

molecules approach each other in a fluid, at a temperature where their relative speed is likely

to be low, their mutually attractive forces will bring a temporary association between them. If

there is a sufficient density of molecules, there is the possibility of condensation to a liquid.

On the other hand, if the temperature and the probable relative speeds are high, the attractive

force will be too weak to have more than a slight effect on the molecular velocities, and

condensation cannot occur however high the molecular density. It is therefore reasonable to

expect, on the basis of molecular behavior, that for every substance there is a temperature

below, which condensation to a liquid (and evaporation to a gas) is possible, but above which

these processes cannot occur. That there is a critical temperature above, which a single

substance can only exist as a fluid and not as either a liquid or gas.

The phenomenon can be easily explained using the phase diagram for pure carbon dioxide

(Figure AI.1).

Annex I. Supercritical f luids 244

Figure AI. 1 Phase diagram of a single substance, carbon dioxide.

The phase-diagram is schematic, and the pressure axis is non-linear. It shows the areas where

carbon dioxide exists as a single gas, liquid, solid phase or as a supercritical fluid. The curves

represent the temperatures and pressures where two phases coexist in equilibrium (at the

triple point, the three phases coexist). Moving along the gas-liquid curve, increasing both

temperature and pressure, then the liquid becomes less dense due to thermal expansion and

the gas becomes denser as the pressure rises. Eventually, the densities of the two phases

converge and become identical, the distinction between gas and liquid disappears, and the

end of the coexistence curve is defined as the critical point. The substance is now described

as a fluid. The critical point has pressure and temperature coordinates on the phase diagram,

which are referred to as the critical temperature, Tc, and the critical pressure, pc, and which

have particular values for particular substances, some examples are shown in the table below.


Table AI. 1 Substances useful as supercritical fluids, with critical

parameters

Solvent

Critical pressure Pc (atm)

Critical temperature Tc (°C)

H20 216 374

NH3 110 132

CO2 70 31

CH3CH2OH 62 243

CH3OH 77 240

CH3CN 47 275

Hexane 32 235

It was 170 years ago that Baron Charles Cagniard de la Tour showed experimentally the

disappearance of the distinction between the liquid and gas phases. In the experiment, it was

observed through a view cell how the meniscus between a liquid and a gas disappeared at the

critical temperature.

Figure AI. 2 Disappearance of the meniscus at the critical point. The

meniscus separating a liquid (bottom) from its vapor (top) disappears at

the critical point. The liquid state does not exist above the critical

temperature, regardless of the pressure that might be applied to the

substance.


Supercritical fluids exhibit important characteristics such as compressibility, homogeneity,

and a continuous change from gas-like to liquid-like properties. These properties are

characteristic of conditions inside supercritical fluid region.

The more fundamental interest in supercritical fluids arises because they can have properties

intermediate between those of typical gases and liquids. Compared with liquids, they have

lower densities and viscosities and greater diffusivities. The conditions may be optimum for a

particular process or experiment. Furthermore, properties are controllable by both pressure

and temperature and this characteristic compared with a liquid, leads to that more than one

property can be optimized. The main disadvantages are their cost and inconvenience of the

higher pressures needed. Consequently, supercritical fluids are exploited only in particular

areas.

The critical point for carbon dioxide occurs at a pressure of 73.8 bar and a temperature of

31.1°C. These parameters make equipment design relatively simple. Carbon dioxide is

available as a convenient supercritical fluid substance. Carbon dioxide has so far been the

most widely used, because of its convenient critical temperature, low price, chemical stability,

non-flammability, stability in radioactive applications and non-toxicity. Large amounts of

CO2 released accidentally could constitute a working hazard, but hazard detectors are

available. It is an environmentally friendly substitute for other organic solvents. Its polar

character as a solvent is intermediate between a truly non-polar solvent and weak polar

solvents.

BIBLIOGRAPHY

This annex is adapted from the introduction to Fundamentals of Supercritical Fluids by T.

Clifford, published by the Oxford University Press in 1998.

A N N E X I I

TECHNICAL DESCRIPTION OF THE JOIN ICMAB-CM HIGH PRESSURE-HIGH

TEMPERATURE LABORATORY*

The laboratory is composed by two main equipments (a laboratory scale plant and pilot plant).

The high pressure-high temperature lab was created by ICMAB and Carburos Metalicos in

1995 to investigate develop and promote new supercritical fluid applications. The lab was

designed to allow the maximum possible types of experiments, proposed by research groups or

industries.

The high pressure-high temperature facility is able to work up to 400 ºC and 500 bars with

CO2, as a supercritical fluid. However, others supercritical fluid can be used, if necessary. The

flexibility is one of the more important aspects of the ICMAB pilot plant. Temperature,

pressure and flow rate can be easily tuned to work at different conditions. This flexibility

allows planning different operations and to control pressurization, depressurization and

heating rates with great accuracy. The collected pressures have an uncertainty of 0.7%.

CO2 was chosen as main supercritical fluid due to the advantages that offer (Table AI.1).

Although its safety, some preventive measures must be taken when CO 2 is used. It may

produce immediately hazardous and asphyxiating atmospheres. Fast liquid CO 2

depressurization produce solid CO2 and it can block pipes. CO2 can be recycled as much as

needed and product solubility can be tuned by controlling pressure and temperature

*This section was taken from Supercritical Training Course. J. Torres and R. Solanas

Table A.I. 1 Advantages of carbon dioxide as supercritical fluid

Advantages

Non-toxic Highly selective Non-flammable Low viscosity

Cheap High diffusivity Readily available

Annex II. Technical description of the join ICMAB-CM High pressure-high temperature Laboratory 248

1. DESCRIPTION OF THE HIGH PRESSURE-HIGH TEMPERATURE

PLANT

The experimental apparatus used for drying is shown in Figure A.I.1.

Figure A.I.1 Pilot plant and its three unites: 1) CO2 supplier and

liquefier (blue square), 2) reactors – pressure, temperature and flow rate

control (violet square), 3) and conditioning before releasing (green

square).

Table AI.2 gathers the technical description of the pilot plant.

Table A.I. 2 Pilot plant technical data

Maximum pressure: 500 bar Maximum temperature: 400 ºC Number of reactors: 3 Reactors volume: 1-2 liter Heating Power: 14.5 kW (5 heaters)

CO2 maximum flow rate: 40 Kg/h Cosolvent maximum flow rate: 1,8 l/h Computer controlled plant

The chamber pressures given have an uncertainty of ±0.7percentage.

Additional features:

Work with liquid and solid samples

Recycling CO2

Feed cosolvent


Spray liquids inside reactors

Data recording for temperature, pressure and CO2 flow rate

Different working conditions at each reactor

Adsorption tower before venting

Alternative gases (nitrogen,) can be fed

SCF sampling

2-litre agitated reactor with quartz viewer

Different types of filters are available

As it is shown in figure 1 the plant is divided in three unites: 1) CO 2 cylinders and its liquefier,

2) 3 reactors of 2 liters– pressure, temperature and flow rate control, and 3) the control

releasing facilities (a safety vessel of 51 liters). The first and third units are located outdoors

and the second inside the lab.

Unit 1: CO2 supplying and liquefier

Composed by:

- Two CO2 cylinders set each one with four cylinders

- 85 liters liquefier with its temperature and level control

- Two chillers

- Glycol cooling closed loops (with a pump included) to maintain the CO 2 in liquid phase

- Five pressure transmitters

- Six relief valves

- Three thermocouples, to control temperature of the cylinders

- Seven on/off valves, to control feeding of the CO2

- Several manual valves, to control the feeding in case of electronic failure


Figure A.I.2 Outdoors view: high-pressure pump, CO2 liquefier,

chillers, and cylinders.

Unit 2: Reactors with its corresponding pressure, temperature and flow rate control

Composed by:

High pressure pump (to feed the CO2) outdoor

Five control valves for pressure and flow rate control

Seven on/off valves to control CO2 feeding

Three reactors (pressure maximum: 500 bar temperature maximum: 400 ºC): two 2-liter

capacity reactor and one 1-liter capacity extractors.

Five heaters (one for CO2 preheating, one for each reactor and piping)

Two mass flow meters

Sampling loop

Two heat exchangers for CO2

Four pressure transmitters

Four-safety pack composed by one relief valve and one pressure switch.


Fourteen k thermocouples to control the temperature in different parts of the plant and

turn off the heating in case of safety

Several manual valves in case of automatic valves failure

Figure A.I.3 High pressure - high temperature vessel

Unit 3: System of releasing

Figure A.I.4 High pressure - high temperature pilot plant

Composed by:

Safety vessel (51 liters), designed to collect toxic or dangerous supercritical solutions,

avoiding releasing them to the atmosphere, during overpressures and emergency

shutdowns.


Adsorption tower filled with activated carbon

Two pressure transmitter

Two-safety pack composed by one relief valve and one pressure switch

One control valve

One on/off valve

Several manual valves

Safety system (safeguards)

The most dangerous hazard is a rupture of the reactor, vessels or pipes under pressure, due to

an overpressure or high temperature.

A safety 10 mm polycarbonate screen protects the plant.

Overpressure safeguard: At each reactor, safeguards are redundant in four levels: i) Software

does not allow pressure set point higher than the maximum pressure. ii) Presence of a pressure

transmitter control– if measured pressure is higher than safety set point, the plant stops. iii)

Pressure switch, and iv) level is controlled by relief, then, the overpressure is released.

High temperature safeguard: At each reactor, safeguards are redundant in two levels: i)

Software does not allow temperature set point higher than the maximum temperature of the

plant. ii) Thermocouple – if measured temperature is higher than safety set point, heater is

turned off.

CO2 high concentration safeguard: In the laboratory, there are two O2 sensors and a fan to

renew air.

Personal protection safeguards: high temperature gloves, safety glasses, small masks for

organic solvent and powder.

Wrong operation safeguard: special software was designed to avoid wrong operation.

Switches and position detectors avoid wrong operation, too.

Safety vessel.

Water hammer appears after suddenly opening or closing an On/off valve, when the

pressure difference between valve inlet and outlet is high. It could be dangerous and it must be

avoided or diminished.

1.1.1 Description of the laboratory scale plant

Lab scale plant has an agitated 300 ml vessel, able to work up to 414 bars at 20 ºC or 227 bars

at 454 ºC. Samples can be liquids and/or solids and co-solvents can be fed.


Figure A.I.5 Description of the laboratory scale plant and its two

unites: 1) CO2 supplier and liquefier (A square), 2) reactor– pressure,

temperature and flow rate control (B square),

esquema planta petita.dwgEsquema

Planta Petita

Laboratori Supercritic

Esquema Planta Petita

Laboratori Supercritic

B

A


Figure A.I.6 Picture of lab scale plant. It is composed by: (1) reactor

(red line), (2) chiller (g reen line), (4) pressure valves (black lines), (5)

nitrogen and carbon dioxide gas connection (white line), (6) CO2

liquid pump (yellow line), and (9) pressure, temperature, and flow rate

control system.

A ‘zoom’ up of the reactor allows distinguishing the pressure valves and the realizing system

and the connections for the cooling/heating system.

5

9

1

6

2


Figure A.I.7 Detail of the s mall of lab scale p lant

Analogical

manometer

CO2

Gas pipe

Magnetic

stirring drive

Pressure

valves

Rupture

disk

Control

panel

Thermocouple

Pre-heater

Valve

control

A n n e x I I I

INTRODUCTION TO ADSORPTION ANALYSIS AND TO THE BET

MODEL

1. INTRODUCTION TO ADSORPTION ANALYSIS

When a gas or vapour is brought into contact with a solid, the solid takes up part of it. The molecules

that disappear from the gas either enter the inside of the solid, or remain on the outside attached to

the surface. The former phenomenon is termed absorption (or dissolution) and the latter

adsorption. When the phenomena occur simultaneously, the process is termed sorption. The

phenomenon of adsorption was discovered over two centuries ago.

The solid that takes up the gas is called the adsorbent, and the gas or vapour taken up on the surface

is called the adsorbate. It is not always easy to tell whether the gas is inside the solid or merely at the

surface because most practical absorbents are very porous bodies with large internal surfaces. It is not

possible to determine the surface areas of such materials by optical or electron microscopy because of

the size and complexity of the pores and channels of the material. The gas adsorption itself, however,

can be used to determine the accessible surface area of most absorbents.

In this section the adsorption of N2 at cryogenic temperatures on silica aerogels and silica

microparticles was investigated

1.1 THEORY OF ADSORPTION

Molecules and atoms can attach themselves onto surfaces in two ways. In physisorption (physical

adsorption), there is a weak van der Waals attraction of the adsorbate to the surface. The attraction to

the surface is weak but long ranged and the energy released upon accommodation to the surface is of

the same order of magnitude as an enthalpy of condensation. During the process of physisorption,

the chemical identity of the adsorbate remains intact, i.e. no breakage of the covalent structure of the

adsorbate takes place. In chemisorption (chemical adsorption), the adsorbate sticks to the solid by the

formation of a chemical bond with the surface. This interaction is much stronger than physisorption,

and, in general, chemisorption has more stringent requirements for the compatibility of adsorbate and

surface site than physisorption. The physisorption classification for the pore size is given

by:Macropores: when pore diameter ( 0) is 50nm (0.05 m), Mesopores: when 2nm 0 50nm,

Micropores: when 0 2nm.

Annex III.Introduction to adsorption analysis and to the BET model 258

This definition is not exact because the filling of the pore depends on the shape of them and is

influenced by the adsorbate properties and for the adsorbent/adsorbate interactions.

The total accessible volume present in the micropores is considered as adsorption space, this process

is called micropores filling; different at the superficial coverage occurred on the open macropores -

mesopores walls. The mesopores physisorption occurs in two steps: monolayer adsorption and

capillary adsorption.

In the monolayer adsorption, all adsorbate molecules are in contact with the adsorbent surface. In the

multilayer adsorption not all the molecules adsorbate are in direct contact with the adsorbent surface

because the adsorption space is covered for multilayer molecules. In the capillary adsorption the

residual space of the pore that is empty after the multilayer adsorption is filled by a condensed,

separate of gaseous phase by meniscus. The capillary condensation is frequently followed by hysteresis.

This process does not exist in the micropores filling. In physisorption, the monolayer capacity, nmono,

is defined as the adsorbate quantity needed to cover all the absorbent surface with a complete

molecule monolayer and Vm as the volume of the maximum gas absorbed usually taken to be a

monolayer. The energy of adsorption depends on the extent to which the available surface is covered

with adsorbate molecules. This is because the adsorbate can interact with each other when they lie

upon the surface (in general they would be expected to repel each other). The fractional coverage of a

surface is defined by the quantity :

sitespossibleofnumberTotal

sitesadsorptionoccupiedofNumber (1)

At any temperature, the adsorbate and the surface come to a dynamic equilibrium, that is, the

chemical potentials of the free adsorbate and the surface bound adsorbate are equal. The chemical

potential of the free adsorbate depends on the pressure of the gas, and the chemical potential of the

bound adsorbate depends on the coverage, . Thus, the coverage at a given temperature is a function

of the applied adsorbate pressure. The variation of with P at a given T is called an adsorption

isotherm. Adsorption hysteresis is obtained when adsorption and desorption curves are different. The

specific surface area adsorbent (As) is calculated by:

mnNA monoAs

Where: NA is Avogadro's number, is the molecular area occupied per absorbate N2: 16.2Å², m is

the mass of the sample

1.2 THE LANGMUIR MODEL

Langmuir suggested the earliest model of gas adsorption. The model is limited to monolayer

adsorption. If one assumes that all adsorption sites are equivalent then the adsorption and desorption

rate is independent of the population of neighbouring sites. Then one can derive a simple formula for

an adsorption isotherm.


Consider the equilibrium:

A + S A.S (2)

Where: A is the free adsorbate, S is the free surface, A.S is the substrate bound to the surface.

The rate of adsorption will be proportional to the pressure of the gas and the number of vacant sites

for adsorption. If the total number of sites on the surface is N, then the rate of change of the surface

coverage due to adsorption is:

)(1NPkdt

da

(3)

The rate of change of the coverage due to the adsorbate leaving the surface (desorption) is

proportional to the number of adsorbed species:

NPkdt

dd (4)

In these equations, ka and kd are the rate constants for adsorption and desorption respectively and p

is the pressure of the adsorbate gas. At equilibrium, the coverage is independent of time and thus the

adsorption and desorption rates are equal. The solution to this condition gives us a relation for :

mV

V

Pk

Pk

1 (5)

where :k = ka / kd, .V= volume of gas absorbed at pressure P

Note that because k is equilibrium constant, the value of k at various temperatures determined from

the Langmuir isotherm allows for the evaluation of the enthalpy of adsorption, ads, through the

Van't Hoff equation:

2

ln

RT

H

dT

kd ads (6)

As the strength of the interaction between the adsorbent and the adsorbate increases the value of p

increases and the surface coverage increases faster as the pressure is increased.

To use the Langmuir model adsorption data are plotted in the form:

mm V

P

kVV

P 1 (7)

Vm is calculated plotting P/V against P and find Vm from 1/slope of the line.

In practice, it has been found that the Langmuir model is rarely a useful model to fit gas adsorption

data and hence calculate the surface area. The Langmuir model is only applicable when adsorption at

low coverage. The Langmuir isotherm is found to be useful only at very small coverage (sub-

monolayer) but is generally applied to all cases involving chemisorption.


2. THE BET MODEL

Brunauer, Emmett and Teller developed several models of gas adsorption on solids, which have

become the effective standard for surface area measurements.

The models were generalisations of Langmuir theory monolayer adsorption to multilayer adsorption.

The BET isotherm is useful in cases where multilayer adsorption must be considered.

Nitrogen is the most commonly used BET adsorption gas because of its inertness to chemical

interaction with most materials and the ready availability of liquid nitrogen to control the temperature

of the adsorption process. The standard 2 parameter BET isotherm gives the amount of gas adsorbed

as a function of the relative pressure of the adsorbing gas:

xcx

xc

PPcPP

PPc

V

V

n

n

mmono )1(1)1()/()1(1)/1(

)/(

00

0 (8)

where

V = Volume of gas adsorbed at pressure P, Vm = Volume of gas covering the surface with a

monomolecular layer, n/nmono is the ratio of the moles adsorbed to the moles adsorbed in a single

monolayer.Po = Saturation pressure of the gas (vapour pressure), i.e. the pressure of the gas in

equilibrium with bulk liquid at the temperature of the measurement., x = P/Po = Relative pressure.

c = a constant for the gas/solid combination.

The isotherm can be converted to a linear form for ease of extracting the values of Vm and c. The

constant c represents the relative strengths of adsorption to the surface and condensation of the pure

adsorbate. Simple theory predicts an approximate value of this constant as:

RTH

RTH

vap

ads

e

ec

/

/

(9)

The constant c is related to the difference between the heat of adsorption of the first layer (H1) and

the heat of liquefaction (HL), Where H1-HL= H, is also known as the net heat of adsorption. R = gas

constant (8.31447 J K-1 mol-1) T = temperature (K)

The BET model is used to measure the surface areas of several porous materials. The BET isotherm

is found to describe adequately the physisorption at intermediate coverage ( = 0.8 - 2.0) but fails to

represent observations at low or high coverage. The BET isotherm is reasonably valid around =1.0,

however, and this is useful in characterising the area of the absorbent. If one can determine

experimentally the number of moles of adsorbate required to give , =1.0 (i.e. a monolayer), one can

determine the specific surface area of the absorbent.

gabsorbentofmass

mabsorbentofareasurfaceA

2

(11)

Practically, one measures the number of moles adsorbed as a function of equilibrium pressure, i.e. one

does not directly measure . Algebraic rearrangement of the BET isotherm to produce a linear


equation is usually applied to experimental data. For surface area measurements the BET equation is

used in the form:

xcn

c

cnxn

x

monomono

)1(1

)1(

Over the range where the BET isotherm is valid (in the range P/Po = 0.05-0.30), a plot of x/n(1-x) vs

x will be linear. The slope and intercept of this line will allow the determination of nmono (or Vm) and c.

erceptYSlopeVm

int

1 from the best fit straight line.

1int erceptY

Slopec from the best fit straight line.

Finally, the BET surface area is then calculated from:

mnNA monoAgm //2 (13)

Where: is the molecular area occupied per absorbate N2: 15.8 (16.2)Å² NA is Avogadro's number m

is the mass of the sample Vm is in cm³ at STP/g.

The adsorption process is generally taken as completely reversible, but, under some conditions the

isotherm may exhibit different shapes upon desorption as compared to absorption. This is called

hysteresis. Sometimes hysteresis data can be used to determine the structure and size of pores in the

absorbent. We will therefore need to generate an isotherm for both absorption and desorption.

2.1 EXPERIMENTAL PROCEDURE

We will perform the adsorption measurements in a commercial vacuum manifold called the ASAP

2000, manufactured by Micromeritics Instrument Corporation. The ASAP 2ooo system consists of

one analyser and a multi-function control module. The analyser it is designed for completely

automatic operation. Two separate internal vacuum systems are included-one for sample analysis and

one for sample preparation. It contains one sample analysis port and two sample preparation ports.

The sample P0 (saturation pressure) tube is located next to the sample analysis port. Since the analysis

results are expressed in units of surface area per gram of sample, the true weight of the sample must

be known, is necessary to determine the weight of the sample before degassed: Must solid materials

absorb moisture and other contaminants when exposed to the atmosphere. The sample must be clean

when analysis is performed. The sample is heated and placed under vacuum to remove moisture and

other contaminants. This process is referred to as degassing the sample. The approach to equilibrium

is perceptibly slow, especially at high coverage. It would in principle take an infinite time for

equilibrium to be exactly established. The nature of the isotherm and the required precision of the

measurement suggest that equilibrium pressures need be known only to a few percent. Recordding of

the equilibrium pressure generates the data needed to determine the number of moles adsorbed on

the solid as a function of (equilibrium) pressure and to generate the adsorption isotherm.


The desorption measurement is performed to see if there is any hysteresis or non-equilibrium effects

in the adsorption/desorption cycle. This is basically performed in reverse of the procedure above. The

value of the constant c in the BET equation affects the shape of the isotherm mainly at low relative

pressures (P/Po) as is shown below.

2.2 BJH PORE VOLUME AND AREA DISTRIBUTION CALCULATION

For adsorption data, the relative pressure and volume adsorbed data points pairs collected during an

analysis must be arranged in reverse order from which the points were collected during analysis. All

calculations are performed based on a desorption model, regardless of whether adsorption or

desorption data is being used. The data used in these calculations must be in order of strictly

decreasing numerical value. The data set is composed of relative pressure (Pr), volume adsorbed (Va)

pairs from (Pr1, Va1) to (PrN, VaN) where (PrN=0, VaN=0) is assumed as a final point. Generally, the

desorption branch of an isotherm is used to relate the amount of adsorbate lost in a desorption step

to the average size of pores emptied in the step. A pore loses its condensed liquid adsorbate, known

as the core of the pore, at a particular relative pressure related to the core radius by the Kelvin

equation (16). After the core has evaporated, a layer of adsorbate remains on the wall of the pore. The

thickness of this layer is calculated for a particular relative pressure from the thickness equation. This

layer becomes thinner with the successive decreases in pressure, so that the measured quantity of gas

desorbed in a step is composed of a quantity equivalent to the liquid cores evaporated in that step plus

the quantity desorbed from the pore walls of pores whose cores have been evaporated in that and

previous steps. Barret, Joyner, and Halenda developed the method (known as BJH method).

A pore filled with condensed N2 liquid has three zones:

The core: evaporates all at once when the critical pressure for the radius is reached; the relationship

between the core radius and the critical pressure is defined by the Kelvin equation.(16)

The adsorbed layer: composed of adsorbed gas that is stripped off a bit at a time with each pressure

step; the relationship between the thickness of the layer and the relative pressure is defined by the

thickness equation.

The walls of the cylindrical pore itself: the diameter of the empty pore is required to determine the

pore volume and the pore area. End area is neglected.

The total pore volume, Vp, is often derived from the amount of vapour adsorbed at a relative

pressure close to unity by assuming that the pores are then filled with condensed adsorptive in the

normal liquid state.

The pore size distribution is the distribution of pore volume respect to pore size. The computation

of the pore size distribution involves a number of assumptions (pore shape, mechanism of pore

filling, validity of Kelvin equation, etc.).

Application of the Kelvin equation

Mesopore size calculation are usually made with the aid of the Kelvin equation in the form


og p

pIn

v

RT

rr 11

21

11

Which relates the principal radii, r1 and r2, of curvature of the liquid meniscus in the pore to the

relative pressure, p/po, at which condensation occurs; here is the surface tension of the liquid

condensate and v1 is its molar volume. It is generally assumed that this equation can be applied locally

to each element of liquid surface.In using this approach to obtain the pore radius or pore width, it is

necessary to assume: (i) a model for the pore shape and (ii) that the curvature of the meniscus is

directly related to the pore width.

The use of the physisorption method for the determination of mesopore size distribution is subject to

a number of uncertainties arising from the assumptions made and the complexities of most real pore

structures. It should be recognised that derived pore size distribution curves may often give a

misleading picture of the pore structure. On the other hand, there are certain features of physisorption

isotherms (and hence of the derived pore distribution curves) which are highly characteristic of

particular types of pore structures. Physisorption is one of the few non-destructive methods available

for investigating mesoporosity, and it is to be hoped that future work will lead to refinements in the

application of the method - especially through the study of model pore system and the application of

modern computer techniques.

3. REFERENCES

S. Brunauer, "Physical Adsorption" (Princeton

University Press, Princeton, N. J., 1945)

S. Brunauer, P. H. Emmett and E. Teller, J.

Amer. Chem. Soc., 60, 309-319 (1938)

I.Langmuir, J. Am. Chem Soc., 38,

2219,(1916); 40, 1368, (1918)

Langmuir, J. Amer. Chem. Soc., 40, 1361

(1918);

Langmuir, J. Amer. Chem. Soc., 54, 2798

(1932);

Langmuir, Nobel Lecture, 1932].

P. Atkins, "Physical Chemistry" 5th ed

(Freeman, New York, 1994)

G. A. Somorjai, "Principles of Surface

Chemistry (Prentice-Hall, Englewood Cliffs,

N. J. 1972)

Drake ,J.M.; Nitrogen Adsorption on Porous

Silica: Model-Dependent Analysis; Journal of

Physical Chemistry; v.98, 1994, p. 380-2

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