Che5700 陶瓷粉末處理 Synthesis of ionic compounds Majority liquid phase method involves ionic compounds, calculation of supersaturation requires ionic products

Che5700 陶瓷粉末處理

Synthesis of ionic compounds Majority liquid phase method involves ionic

compounds, calculation of supersaturation requires ionic products Ksp;

Another characteristics: charge neutrality Ions in solution: often hydrated!, dehydration process

is always involved; ions with slow dehydration – often form hydrated compounds.

In general: hydration number, for cations: 4-6; for anions: 0-2;

Interaction between ions and water molecule: electrostatic forces, thus electronic structure or charge density – main factor to determine this force

pH always an important factor: affects ion-pairing; complex ion formation etc.;

Characteristics of ionic solutions


Solvent molecule: may adsorb on particle surface, thus shows effect, different solvents (or small additions, co-solvent) will affect nucleation and growth behavior;

Other factors: e.g. ultrasonic, electrical field, magnetic field, etc. may also show effects

Ion pairs formation change distribution of species CA = [A] + [AB] + 2 [A2B] + [AB2] + ….

driving force in ionic solution: (I.P) 1/2 – Ksp 1/2 (IP = ionic product; Ksp solubility product)

Right figure: 5,6,7 region: suspended solution, 5,6 with charged, stable suspension

Different pH: often obtain different ionic compounds, in theory, it is the one with the lowest solubility (thermodynamic reasons)

Yet different ionic compounds involves complex chemistry, difficult to predict (kinetic reasons)

Nielsen found: solubility of ionic compound related to its surface energy: /kT = 2.82 – 0.272 ln(Cs) Cs: mol/m3

Metal Ions in Solution


Ionic radius Na+ < Ag+ < K+; number of salts containing water of crystallization 19 for Na+; 14 for K+; 4 for Ag+

Metal ions: in addition to ion pair formation, may also form polynuclear complex (sort of precursor to nucleus)

All metal ions in solution are hydrated; when z/d increase, easier to release H+, or first hydrolysis constant

Bronsted acid: provide proton (proton donor) Lewis acid: to provide empty electron orbital to

accept electrons

Classification of metal ions


======================

Another classification


Metal ion can associate with OH in water, to release H+, act like an acid

Stability Sequence


To the left: A type metal cations B type

Stepwise Hydrolysis Reaction for Al

==

6 Al 8 Al

Distribution of Fe Species

Calculated based on stability constant

From Fe(OH)3 to Fe2O3

• source: 李崧助 MS thesis (NTHU, 2008)•Two transformation routes from hydroxide to oxide under boiling condition: (a) dissolution – re-crystallization; (b) in-situ solid state transformation• Case 1: adding Fe+2 as catalyst to accelerate dissolution and re-crystallization process; • Case 2: adding urea or NaHCO3 to accelerate solid state transformation process (possible via action of CO2 bubbles – need to be proven)

time (min)

0 10 20 30 40 50 60 70

Fe3

+ C

once

ntra

tion

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

Fe2

+ C

once

ntra

tion

0.062

0.064

0.066

0.068

0.070

0.072

0.074

0.076

0.078

0.080

0.082

Fe3+

Fe2+

•Fe+2 concentration dropped at first, then gradually increase back to original concentration; • Fe+3 concentration increase with time, while pH of solution decrease with time• FeOOH formed immediately when Fe+2 was added;

Some Possible Reactions

• Fe(OH)3 + Fe+2 + H2O FeOOH + Fe(OH)2 + H+

• Fe(OH)3 + H+ + e- Fe+2 + H2O (reductive dissolution)• e- may come from: • Fe+2 Fe+3 + e- or• Fe(OH)2 + OH- Fe(OH)3 + e-• hydrolysis of Fe+3 and subsequent transformation into oxide• Fe+3 + 2H2O Fe(OH)2

+ + 2 H+

• 2 Fe(OH)2+ Fe2O3 + H2O + H+

2*theta

20 30 40 50 60 70 80

Inte

nsity

0

50

100

150

200

250

220

311

400

422

511

440

2*theta

20 30 40 50 60 70 80

Inte

nsity

0

50

100

150

200

250

220

311

400

422

511

440

Initial pH = 7; t= 20, 50 & 90 minutes; (final pH = 5.4 (20 min; 3.1 (90 min))

Initial pH = 9, t = 4 hours, still pure Fe3O4 phase; (final pH = 7.3)

Possible Mechanism for Solid State Transformation

Dirk Walter (Thermochimica Acta, 445 (2006)

195)

含水氧化鐵

O2-

OH- continue to move toward interface

Urea and sodium bicarbonate decompose to produce CO2, possibly exhibiting similar effects

H2O

H+ and O2- combine together

First Hydrolysis Constant

}/{}}{{ )1(1

zz MHMOHK* In figure d=M-O bond distance; four types of metal ions

Solubility Product & Hydrolysis Constant

zzz OHMMzzMOH )()1()1(

)1()1( }/{}{ zzzz MMOHK

}/{}}{{ )1(1

zz MHMOHK Overall reaction

• K = (*K1)z Kso • As shown in previous figure, according to its

charge, four types can be found;• Often when the first water molecule left, the

precipitation occurs;

Solubility and supersaturation

Due to changes in species, at different pH, solubility and corresponding supersaturation also changes


Hydroxo, Oxo complexes from metal ions

Start with simple hydration lost one H+, to become hydroxo lost another H+ to become oxo complex

Effects of anions


Mostly: chloride, nitrate, carbonate, sulfate, perchlorate, phosphate, etc; [counter-ions]

They will affect particle size, shape, composition, etc. Anions entering particle: may not be easy to eliminate

by calcination, may require higher temperature to lower to ppm level;

Different anions – different morphology, e.g. Fe(OH)+2 + 2 FeSO4+ + 6 H2O hexagonal

Fe3(OH)5(SO4)2 2H2O + 4 H+

3 Fe(OH)+2 + Fe(SO4)+ + 4 H2O monoclinic Fe4(SO4)(OH)10 + 4 H+

Mixing


Cation and anions from two different sources, always require mixing, though very fast, still take several seconds, its effect may not be negligible.

Method of addition: direct strike, reverse strike (add alkaline to metal ion solution or reverse), it will affect reaction rate and thus rate of supersaturation.

Sequence of events: mixing reaction supersaturation nucleation/growth/agglomeration aging final particle size distribution, shape, crystallinity;

Co-precipitation


Classification: Isomorphous mixed crystal formation: structure similar or formation of compounds

Anomalous mixed crystal formation: one of which should not precipitate, yet was removed from solution

Adsorption Occlusion (inclusion): trapped during growth Post-precipitation: after precipitation of the first

compound, either independent or in coating form distribution coefficient (B/A) in whole crystal = D (B/A)sol (B/A) on surface = (B/A)sol D: homogeneous distribution coefficient; : logarithmic distribution coefficient (more often)

Chemical Reactions

• Examples

NiSO4 + 4 FeSO4 + 5 (NH4)2C2O4. H2O (at 60oC) 5 Ni 0.2 Fe 0.8 C2O4. 2H2O + 10 NH4

+ + 5 SO4

-2

Ba(OC3H7)2 + Ti(OC5H11)4 in alcohol solution + H2O BaTiO3 + 2 C3H7OH + 4 C5H11OH

Monodispersed Colloids


example: micelles, latex, micro-emulsion, photographic emulsion, ceramics, inorganic chemicals (pigment, pharmacy, catalyst, etc); stress micron or sub-micron size; now: nanosize

Applications: model system for fundamental studies; may have better packing and sintering results;

Formation: (a) separate nucleation and growth steps: best strong dependency of nucleation on supersaturation and low growth rate; (b) resist agglomeration (based on electrostatic force, surfactant, polymers, gel network etc.); (c) choice of growth modes: diffusion or surface reaction controlled

Typical Monodispersed Systems

• Homogeneous systems: Redox reaction Hydrolysis: such as alkoxide in alcohol; high

temperature forced hydrolysis in acid; etc. Decomposition of compounds: Cd ion + TAA CdS Reaction of chelates: M-EDTA complex

• Heterogeneous systems: Emulsion polymerization Reaction in microemulsions Reaction in aerosols Recrystallization: involves dissolution

Homogeneous system: depend on control of reaction rate, in a clean system, rapid nucleation to consume supersaturation, so particle grow to the same size in a short period

Core may have time to become crystallized; material in diffusion – may be nanosized colloids

ZnS colloids: use TAA, TAA = thioacetamide (S=C-(CH3)(NH2); it will slowly hydrolyze to release S-2, to do the precipitation reaction, slower than use of Na2S, with more uniform particle size但粒徑均勻 . COOHCHNHSHOHCSNHCH 332223 2

Reaction or Diffusion Control


/)(/)4( 2MsbM VCCDJVdtdrr

MesM VCCkJVdtdrr )(/)4( 2 •Diffusion control or surface reaction control of growth rate;•Results show: diffusion control – size distribution become narrower with size; similar for reaction control (different scale)

質傳控制定值,~/)( 2 dtrd2)/)(/(/ rrrrrr ooo

表面反應控制定值,~/ dtdr

)/)(/(/ rrrrrr ooo

Diameter (nm)

50 100 200 400 600 800

Re

lativ

e In

ten

sity

0

20

40

60

80

100

120

S G1 G2 G3 G4 G5

Growth size distribution

Results from semi-batch Stober process SiO2 (from 陳貞志 )

0 1 2 3 4 510

15

20

25

30

50

100

150

200

250

300

350

400

450

experiment

Re

lativ

e s

tan

da

rd d

evi

atio

n (

%)

Growth times

Ave

rag

e d

iam

ete

r (n

m)

Hydrothermal Method


Higher reaction temperature (higher pressure), need to use autoclave, thus higher reaction rate.

To some strong acid solid, we can obtain oxide directly, or at least oxo-hydroxides (e.g. Zr, Zn…)

Use hetereoaggregation to prepare core-shell particles

example: latex particle + polyallylamine hydrochloride to make it positive charge, easy to adsorb negative charged CdTe colloids(from adv. Mater. 13(22), 1685, 2001

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Che5700 陶瓷粉末處理 Synthesis of ionic compounds Majority liquid phase method involves ionic compounds, calculation of supersaturation requires ionic products