Carbon nanotubes are cylindrical structures of carbon with diameters in the nanometer range. They have extremely high tensile strength and unique electrical properties depending on their structure. Carbon nanotubes can be single-walled or multi-walled and are synthesized using arc discharge, laser ablation, or chemical vapor deposition with transition metal catalysts. They have a variety of potential applications in electronics, optics, and other fields due to their novel properties.
2. CARBON NANOTUBES
(CNTs) Allotropes of carbon with cylindrical sructure
having diameter in nanometer range
Constructed with length -to – diameter ratio
(aspect ratio) upto (132 × 106 ) : 1 , significantly
larger than any other material
Have novel properties ,making them potentially
useful in many applications in
nanotechnology,electronics ,optics and other
fields of material science
3. BONDING IN CNT
Applied quantum chemistry , specifically, orbital hybridization
best describes chemical bonding in nanotubes
The chemical bonding of CNTs is composed of sp2 bonds
( as in graphite) which are stronger than sp3 bonds found in
alkanes ,provide their unique strength
NTs naturally align themselves into “ropes” held together
by van der Waals forces
7. SWNT
Most SWNT have a diameter close to 1 nm, with a tube length that can be
many million times longer
The structure of a SWNT can be conceptualized by wrapping a one –
atom-thick layer of graphite called graphene into a seamless cylinder
The way graphene sheet is wrapped is represented by a pair of indices ( n,
m) called the chiral vector
The intigers n,m denotes the number of unit vectors along the two directions in
the honeycomb crystal lattice of graphene
If m=0 , the nanotubes are called zig-zag
If n=m, the nanotubes are called armchair
Otherwise , the nanotubes are called chiral
14. MWNT
There are two models to describe the structure of MWNTs
MWNTs consist of multiple rolled layers (concentric
tubes) of graphite
15. SYNTHESIS AND PURIFICATION
OF CNTs
CNTs were first noticed in the graphitic soot deposited on the negatively charged
electrode used in the arc discharge synthesis of fullerenes
MODIFIED ARC DISCHARGE PROCESS
A smaller diameter anode evaporates on the face of a larger diameter cathode in a
direct current arc discharge apparatus
The bowl that grows on the cathode contains MWNTs
The bowl can be broken and ground and the nanotubes may be suspended in a suitable
solvent and deposited on theTEM grid for examination
The incorporation of transition metals in catalytic amounts into the anode results in the
formation of SWNTs
The most common metals used are Fe and Ni , but it is better to use bimetallic systems
such as Co-Ni, Co-Pt and Ni-Y
Optimized synyhesis utilizes an Ni-Y catalyst in the atomic ratio 4:1
16. SYNTHESIS AND PURIFICATION OF
CNTs
SWNT can be synthesized by heating a mixture of graphite
with Fe and Ni catalysts at a temperature of 12000C and
irradiating with laser
Nanotubes thus synthesised are found to form ropes in
which individual tube organize into hexagonal assembly,
showing the homogenity of the tubes synthesized
LASER
Fe Ni 12000C
(50-70 % )
17. SYNTHESIS AND PURIFICATION OF
CNTs
CHEMICAL VAPOUR DEPOSITION
SWNTs and MWNTs can be synthesized
An organometallic precursor is mixed with a carbon
containig feed gas, it is pyrolyzed in a quartz tube and the
nanotubes are collected from the cooler end of the
reaction vessel
Nanotubes are also grown solid catalytic substrates such
as silica, quartz, alumina etc .,which contain transition
metal precursors (important for making supported
MWNT assemblies for specific applications)
18. SYNTHESIS AND PURIFICATION OF
CNTs
Both MWNTs and SWNTs are formed with significant quantities of
carbonaceous material
One way of separating the tubes from the carbon mass is to heat- treat
the product
All carbon forms react with oxygen ,but they do so at different rates
All amorphous carbon materials can be burnt off by heating the soot at
7500C for half an hour (amorphous carbon reacts at high rate than nanotubes
due to the existence of large number of deffects in it)
At the end of the process the product (only less than 1% of the original
material is left ) obtained is essentially a mixture of nanotubes
Acid –based cleanig procedures can also be used
19. PROPERTIES OF CNTs
STRENGTH
CNTs are the strongest and stiffest materials yet
discovered interms of tensile strength and elastic
modulous respectively
This strength results from the covalent bonds
formed between individual carbon atoms
MWNT was tested to have a tensile strength of 63
GPa (in 2000)
20. PROPERTIES OF CNTs
HARDNESS
Standard SWCNTs can withstand a pressure upto
24 Gpa without deformation
They can undergo a transformation to superhard
phase nanotubes
The bulk modulus of superhard phase nanotubes is
462-546 Gpa, even higher than that of diamond
(420 Gpa for single diamond crystal)
21. PROPERTIES OF CNTs
KINETIC PROPERTY
MWNTs are multiple concentric nanotubes precisely nested
within one another - These exhibit a striking telescoping
property whereby an inner nanotube core may slide , almost
without friction ,within its outer nanotube shell,thus creating an
automatically perfect rotational bearing
This property has been utilized to create the world’s smallest
rotational motor
Applicatios such as gigahertz mechanical oscillator are also
envisaged
22. PROPERTIES OF CNTs
ELECTRICAL PROPERTIES
Because of the symmetry and unique electronic structure of
graphene, the structure of nanotube strongly affect its electrical
properties
For a given (n,m) nanotube
If n=m the nanotube is metallic
If n-m is a multiple of 3, the nanotube is semiconducting with a
very small band gap
Otherwise the nanotube is moderate semiconductor
But this rule has exceptions because of curvature effects in small
diameter CNTs
23. PROPERTIES OF CNTs
OPTICAL PROPERTIES
LEDs and photodetectors based on a single
nanotube have been produced
Their unique feature is not the efficiency , which
is relatively low, but the narrow selectivity in the
wavelength of emission and detection of light
and the possibility of its fine tuning through the
nanotube structure
24. PROPERTIES OF CNTs
THERMAL PROPERTIES
All nanotubes are expected to be very good
thermal conductors along the tube,exhibiting a
property known as ballistic conduction ,but good
insulators laterally to the tube axis
26. APPLICATIONS OF CNTs
A nanotube based single molecule field effect transistor has been built
Possible to construct a heterojunction by having a junction between
nanotubes of different helicities-this approach facilitates the creation of a
device with one molecule
In CNT based field emission displays-CNTs act as electron emitters
For hydrogen storage both in between and inside the NTs - fuel cell
applications for automobiles
Used in flow sensor
Nanotube tips used as nanoprobes in AFM and STM
In tissue engineering-CNTs act as scaffolding for bone growth