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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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(0,0)Ch = (10,0)
a1a2
x
y
ZIG-ZAG
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(0,0)
Ch = (10,10)
a1a2
x
y
ARM CHAIR
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(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