CNTs Class Presentation 2012

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CARBON NANOTUBES

Dr. A.Subramania

Centre for Nanoscience and Technology,

Pondicherry University

Puducherry-605 014, India

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OVERVIEW

INTRODUCTION TYPES OF CNTs

VECTOR NOTATION FOR CNT STRUCTURES

SYNTHESIS OF CNTs

PURIFICATION of CNTs CHARACTERIZATION OF CNTs

PHYSICAL PROPERTIES OF CNTs

FUNCTIONALIZATION OF CNTs

CNTs REINFORCED METAL MATRIX COMPOSITES CNTs REINFORCED POLYMER COMPOSITES

APPLICATIONS OF CNTs

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INTRODUCTION

CNTs are allotrope of carbon.

They are nanometers in diameter and

several micrometers in length.

CNTs discovered in 1991 by the Japanese

electron microscopist, Sumio Iijima, while

studying the arc - evaporation synthesis of

fullerenes.

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TYPES OF CNTs

There are two types of nanotubes;

1) Single Walled Carbon Nanotubes (SWCNTs)

2) Multi Walled Carbon Nanotubes (MWCNTs)

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1. SWCNT

Most SWCNTs have a diameter of 2nm with a tube

length of 100m, making it effectively a 1-D

structure called nanowires

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2. MWCNT

MWCNTs consist of multiple layers of graphene

rolled in themselves to form tube shape

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VECTOR NOTATION FOR CNT

STRUCTURES

The way to find out how the carbon atoms

are arranged in a molecule can be done by

pair of indices (n,m), called chiral vector.

By this way, it can be identified whether

the carbon atoms are arranged in a zig zag,arm chair or in a helical shape (chiral).

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1. ZIG-ZAG

If m=0, the nanotubes are called Zig zag.

In which the hexagon points lies along thelong axis of the tube.

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2. ARM CHAIR

If n=m, the nanotubes are called Arm

chair.

In which the flat side of the hexagon lies

along the long axis of the tube.

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3. CHIRAL

In which the configurations lies between

the two extremes.

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13

(0,0)Ch = (10,0)

a1a2

x

y

ZIG-ZAG

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14

(0,0)

Ch = (10,10)

a1a2

x

y

ARM CHAIR

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15

(0,0)

Ch = (10,5)

a1a2

x

y

CHIRAL

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SYNTHESIS OF CNTs

Carbon nanotubes are generally produced bythe following techniques

1. Arc discharge method

2. Laser ablation method3. Chemical vapour deposition method

4. Solar beam Evaporation method

5. Solvothermal synthesis

6.Electrochemical Method

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1. Arc discharge method

In this method, an electric arc is generated inbetween two closely spaced graphite electrodes (3000C) between the twoelectrodes.

At the plasma region, the carbon electrodessublime and condense rapidly to form CNTs andother carbonaceous by products.

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Arc discharge method


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2. Laser Ablation method

A laser source is used to generate high

temperature on a carbon target.

The vapourized carbon rapidly cools ina helium gas stream and forms CNTs and

other carbonaceous by products.

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3. Chemical vapour deposition method (CVD)

In this method, by putting a carbon source in the gas phase and

using an energy source such as plasma to get a gaseous carbon

molecule.

Commonly used carbon sources are methane, carbon monoxide,

acetylene etc.

CVD carbon nanotube synthesis is essentially a two step process

consisting of a catalyst preparation step followed by the actual

synthesis of nanotubes.

The temperature used for the synthesis of nanotubes by CVD is

generally in the range of 650-900C.

Typical yields for CVD are approximately 30%.

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4. Solar Beam Evaporation method

The graphite crucible is filled with a mixture ofgraphite powder

and metallic catalysts and placed in a graphite pipe heated at its top

by the sun light.

The evaporated material is drawn immediately through the

graphite pipe, which acts as a thermal screening by reducingradioactive loses.

On its wall either MWCNT or MWCNT

and SWCNT together or only SWCNT

will be formed.

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5. Solvothermal method

CNTs can be synthesized solvothermally using a planarhexagonal configuration such as hexachlorobenzene as acarbon precursor.

In a typical reaction, reduction of hexachlorobenzene iscarried out by metallic potassium in benzene as well as in thepresence of Co/Ni catalyst at 350C, resulting in the formationof MWCNTs with an average diameter of 40nm.

When catalysts are not used, the reaction usually givesnanostructures, such as carbon spheres, hollow spheres andhallow cones.

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6.Electrochemical method

A simple electrochemical process is used to synthesis CNTs atroom temperature, without the presence of any metal catalysts and

can be directly deposited onto a suitable substrate.

In this electrochemical approach, 1 vol. % of acetonitrile (CH3CN)

in deionised water is used as the electrolyte, the deposition will take

place at room temperature under the lower voltage of 16-20 Vbetween the substrate (tin-oxide coated glass) and counter electrode

(graphite).

The growth of the film deposition on the substrate was monitored

by scanning electron microscopy at an interval of 1hr till the

formation of carbon nanotubes, after a deposition period of 4 hrs.The carbon film obtained at intermediate duration was found to be

in its amorphous phase, which subsequently converted into carbon

nanotubes.


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Mechanism

CH3+

radicals generated from acetonitrile in the electrochemicalprocess giving rice to the formation of amorphous carbon in the

form of very small clusters with unsaturated bonds.

The dehydrogenation of carbon from CH3+ radicals can be

predicted thro the following reaction;


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SEM microstructures of the films on tin-oxide coated glass

deposited for (a) t=1 h (b) t=2 h (c) t=3 h and (d) t=4 h


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PURIFICATION of CNTs

CNTs prepared by various methods contain impurities suchas amorphous carbon, smaller fullerenes and metal catalyst

etc.

These impurities will interfere with most of the desired

properties of the CNTs.

Hence, purification process is required for the synthesized

nanotubes.

The purification process generally consists of six steps

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PURIFICATION of CNTs


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1.Thermal Oxidation

Oxidation treatment is a good way to

remove carbonaceous impurities or to clearthe metal surface at 300C in air

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3. Ultrasonication

In this, particles are separated due toultrasonic vibrations.

Agglomerates of different nanoparticles willbe forced to vibrate and will become moredispersed is highly dependable on thesurfactant, solvent and reagent used.

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4. Magnetic purification

In this method, ferromagnetic particles areremoved from their graphitic shells.

The SWCNTs suspension containinginorganic nanoparticles, mainly ZrO2. Theseparticles are then trapped with permanentmagnetic poles. After, a subsequent chemical

treatment, a high purity SWCNTs will beobtained.

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5. Microfiltration

Microfiltration is based on size of the particles

SWCNTs are trapped in a filter, and the other

nanoparticles such as metal catalyst, fullerene

etc. are passing through the filter.

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6. Annealing

Due to high temperatures (600-1000C) in vacuum,most of the defects created in the earlier steps can beremoved in the CNTs.

When using high temperature vacuum treatment(1600C), the metal will be melted and can also beremoved.

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CHARACTERIZATION OF CNTs

There are several techniques to characterize

the CNTs.

Among them, XRD, SEM/TEM and Raman

Spectroscopy studies are important.

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SEM

SEM study has been extensively used to study CNTsand its alignment.

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PHYSICAL PROPERTIES OF CNTs

CNTs have some incredible physical properties and they

are given below;

Mechanical properties

Electrical properties

Thermal properties

Optical properties

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1.Mechanical properties

CNTs are the strongest and stiffest materials due to its C-C

bond strength.

Youngs modulus of individual SWCNT and MWCNT are

measured as 1054 GPa (1.054 TPa) and 1200 GPa (1.200 TPa),

respectively from the amplitude of thermally driven vibrationsobserved in the TEM.

AFM can also be used to measure the mechanical properties

of individual CNTs

The tensile strength of CNT is about 150 GPa.

High strength steel alloys break at about 0.4 GPa.

Thus carbon nanotubes are about 30-40times stronger than

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A) AFM image of a SWNT bundle adhered on the alumina ultrafiltration membrane, leading

to a clamped beam configuration for mechanical testing.

(B) Schematic representation of the measurement technique. The AFM applies a load, F, to

the portion of nanotube with a suspended length ofL and the maximum deflection dat the

center of the beam is directly measured from the topographic image, along with L and the

diameter of the tube.

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Mechanical properties


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2. Electrical properties

CNTs have the most interesting property that they are metallic or

semiconducting, depending on the diameter and chirality of the tube.

Chirality refers to how the tubes are rolled with respect to the direction

of its (n,m) vector in the graphene plane, where n, m are two integers.

A metallic NT is obtained when the difference n-m is a multiple of 3.

If the difference is not a multiple of 3, a semiconducting NT is

obtained.

In theory, metallic NTs can have an electric current density more than

1000 times greater than metals such as silver and copper.

They have extremely low electrical resistance. It is an excellent

material for high current applications.

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3. Thermal properties

Along the tube axes, CNTs have excellent thermal

conductivity to the tune of ~6000 Wm-1K-1 at room

temperature.

On the other hand , copper has an excellent conductor of heatis valued at 385 W m-1K-1.

However, along the diameter of the tube, CNTs are insulating.

The thermal stability of CNTs is very high (Ca. 3100 K in

vacuum. But, in presence of oxygen, CNTs are easily oxidized

at ~ 900K.

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4. Optical properties

Metallic tubes of any other metals have no

band gas (0.0 eV).

But semiconductor CNTs have a band gap

that is a function of the diameter and its band

gap energy ranges from 0.4 to 0.7 eV.

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FUNCTIONALIZATION OF CNTs

CNTs have to be functionalized in order to acquireadditional physical-chemical properties.

Its unique properties make it desirable for differentapplications.

Functionalization of CNTs changing some of the graphiteproperties to make CNTs soluble in different media orattaching different groups or even inorganic particles forfurther utilization of modified nanotubes. Functionalization ofCNTs can be done as follows;

Exohedral (outside) Functionalization

Endohedral (inside) Functionalization

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1. Exohedral (outside) Functionalization

Meaning that the attachment of different

groups or compounds to the sidewall of the

nanotube.

This can be achieved by covalent attachment.

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First is the functionalization via end and

defect-side chemistry.

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The second is functionalization through side wall by

treating nanotubes in an oxidizing environment For example, in a mixture of concentrated HNO3 and

H2SO4, the oxygen containing groups such as COOH, -C=O

and OH introduced at the ends and side walls of the

tubes.These groups can serve as starting points for further

functionalization of the nanotubes.

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Other popular chemical functionalization of CNTs are;

Fluorine functionalized multiwalled nanotubes, which

achieves a high degree of functionalization by replacingfluorine with other functional groups.

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Third class of functionalization of nanotubes is the non-covalent exohedral functionalization.

For example, wrapping nanotubes in polymers, piptidesor surfactants.

The advantage of this method is that it does not destroythe electronic structure of the nanotubes.

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Solvent free covalent functionalization of CNTs.

In this, nanotubes are mixed with various anilines andisoamyl nitrite or sodium nitrite to produce nanotubes withcovalently attached different chemical groups.

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2. Endohedral (inside) Functionalization

Meaning that nanotubes are functionalized by filling them

with different nanoparticles.

This can be achieved by Filling of CNTs with colloidal

suspensions followed by evaporation of the carrier liquid.CNTs are filled with some compounds, which react under

particular thermal or chemical conditions and produce

nanoparticles, which are trapped in the nanotubes.

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CNT REINFORCED METAL


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CNTs REINFORCED METAL

MATRIX COMPOSITES

CNTs reinforced metal matrix composites are prepared thro avariety of processing techniques such as plasma spraying, ionimplantation, thermal vapour deposition, mechanical alloying andsintering (or) hot pressing etc., are very tedious and expensive.

Hence, electrodeposition techniques can be used to prepare CNTreinforced metal matrix composites.

CNT reinforced metal-matrix composites can be obtained byelectrodeposition and Electroless deposition processes.

The objective of adding fibrous reinforcement such as CNT is twofold;

1. To increase the tensile strength and

2. To increase the elastic modulus of the composite.

Due to these effects, the CNTs have a higher stiffness and strengthcompared to the metal matrix.

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1. Mechanical properties

The critical issues in mechanical properties MM-CNT compositesare the homogeneous distribution of CNTs in the metal matrix, andthe interfacial reaction and booking with the matrix to work as aneffective reinforcement.

For example, in the case of Ni/Ni alloy CNTs composite coatings,the maximum improvement in the hardness was 44% by the additionof 2 vol% of CNT in the composite coating deposited by electrolessmethod. In the case of Ti-Ni- shape memory alloy with 4.5 wt% ofCNT, improves the hardness 200%.

Elastic modulusImprovement in the elastic modulus of the composite is a result of thelarge tensile modulus of 350 970 GPa of CNTs.

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2. Wear and friction properties

Wear properties are more critical for coatings.

For example, in the case of Ni CNTs, Cu CNTs electrodepositedcoatings, a decrease in the coefficient of friction (COF) and increase inthe wear resistance was observed.

The decrease in COF is due to the lubricating nature of CNTs.

The improved wear resistance is due to preventing the surfaceroughness of the matrix by CNTs .

Deng et al have reported a maximum of 83% decrease in the wearand 60% decrease in COF for electroless Ni-P-CNTs coatingcontaining 2 vol. % of CNTs.

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3. Corrosion properties

Electrodeposited coatings are more prone to corrosion

due to the presence of pores and voids.

All the composite coatings with CNTs addition have

shown an increase in the corrosion resistance.Chen et al have measured the corrosion rate of the

composites to be 5 times lower than the Ni-coatings.

This is due to the chemical inertness of the CNTs that

helps in forming a passive layer on the coating surface.And also CNTs help to filling up voids and pores of

electrodeposited coatings leaving no place for the initiation

of localized corrosion.

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CNTs REINFORCED POLYMER


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CNTs REINFORCED POLYMER

COMPOSITES

By the addition of CNTs onto the polymer, we canimprove the strength, elasticity, toughness and

durability of the CNT reinforced polymer

composites and also improve the mechanical,

thermal, electrical and optical properties of the

composite materials.

CNTs reinforced polymer composites can be

prepared by direct mixing, in-situ chemicalpolymerization (or) electrochemical polymerization

processes.

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GENERAL APPLICATIONS OF CNTs

Depending on the size and length of the carbonnanotubes, its applications vary a lot.

When the diameter is large, they can be used

in energy devices such as Li- ion batteries,electric double layer capacitors, Fuel cells etc.

On the other hand, if the CNTs has larger in

length can be used in composites as a fillermaterial to reinforcing the composite.

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Applications

The incorporation of CNTs in polymer matrices are suitablefor variety of applications such as

1. Electrically conductive composites

2. Mechanically reinforced composites

3. Electrochemical capacitors

4. Solar cells

5. Light emitting diodes

6. Sensors and actuators7. Fuel cells

8. Electromagnetic absorbers etc.

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Thank you

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CNTs Class Presentation 2012