Carbon Nano Tubes, its types, synthesis process and important properties

1. CARBON NANOTUBES
BY- PROBAL CHOWDHURY

2. INTRODUCTION
Carbon nanotubes, long thin cylinders of carbon, were discovered in
1991 by Iijima. Carbon nanotubes (CNTs) are allotropes of carbon
which are members of the fullerene structural family, which also
includes the spherical buckyballs. These are large macromolecules
which are unique for there size, shape and remarkable physical
properties.
The nature of the bonding of a nanotube is described by applied
quantum chemistry, specifically, orbital hybridization. The chemical
bonding of nanotubes is composed entirely of sp2 bonds, similar to
those of graphite. This bonding structure, which is stronger than the
sp3 bonds found in diamond, provides the molecules with their unique
strength. Nanotubes naturally align themselves into “ropes” held
together by Van der Waals forces. Under high pressure, nanotubes can
merge together, trading some sp² bonds for sp³ bonds, giving the
possibility of producing strong, unlimited-length wires through high-
pressure nanotube linking.

3. STRUCTURE
The bonding in carbon nanotubes is sp², with each atom joined to three
neighbours, as in graphite. The tubes can therefore be considered as
rolled-up graphene sheets (graphene is an individual graphite layer).
There are three distinct ways in which a graphene sheet can be rolled
into a tube.
The first two of these, known as armchair and
zig-zag have a high degree of symmetry. The
terms “armchair” and “zig-zag” refer to the
arrangement of hexagons around the
circumference. The third class of tube, which
in practice is the most common, is known as
chiral, meaning that it can exist in two mirror-
related forms.

4. TYPES OF CARBON NANOTUBES
a) SINGLE-WALLED CNTs
Most single-walled nanotubes(SWNT) have a diameter close to 1nm, with a tube
length that can be many thousands of times longer. SWNTs are very important
carbon nanotube because they exhibit important electric properties that are not
shared by the multi-walled carbon nanotubes(MWNT) varients.
SWNTs can be excellent conductors and the most building block of SWNT system is
the electic wires. One useful application of SWNTs is in the development of the first
intramolecular field effect transistors(FETs).
b) MULTI-WALLED CNTs:
Multi-walled nanotubes (MWNT) consist of multiple rolled in on themselves to form a
tube shape. There are two models which can be used to describe the structures of
multi-walled nanotubes. In the Russian Doll model, sheets of graphite are arranged in
concentric cylinders. In the Parchment model, a single sheet of graphite is rolled in
around itself, resembling a scroll of parchment or a rolled up newspaper. The
interlayer distance in multi-walled nanotubes is close to the distance between
graphene layers in graphite, approximately 0.33 nm.
c) FULLERITE:
Fullerites are the solid-state manifestation of fullerences and related compounds and
materials. Being highly incompressible nanotube forms, polymerized single-walled
nanotubes (P-SWNT) are a class of fullerites and are comparable to diamond in
terms of hardness.

5. SYNTHESIS
1. ARC DISCHARGE METHOD
• The first macroscopic production of
carbon nanotubes was made in 1992 by two
researchers at NEC’s Fundamental Research
Laboratory at France.
• During this process, the carbon contained in
the negative electrode sublimates because
of the high temperatures caused by the
discharge. Because nanotubes were initially
discovered using this technique, it has been the most widely used method
of nanotube synthesis.
• The yield for this method is up to 30 percent by weight and it produces
both
single- and multi-walled nanotubes with lengths of up to 50 micrometres.

6. 2. LASER ABLATION PROCESS
• In the laser ablation process, a pulsed laser vaporizes a graphite
target in a high temperature reactor while an inert gas is bled into the
chamber. The nanotubes develop on the cooler surfaces of the
reactor, as the vaporized carbon condenses. A water-cooled surface
may be included in the system to collect the nanotubes.
• It was invented by Richard Smalley and co-workers at Rice University,
who at the time of the discovery of carbon nanotubes, were blasting
metals with the laser to produce various metal molecules. When they
heard of the discovery they substituted the metals with graphite to
create multi-walled carbon nanotubes. Later that year the team used
a composite of graphite and metal catalyst particles to synthesise
single-walled carbon nanotubes.
• This method has a yield of around 70% and produces primarily single-
walled carbon nanotubes with a controllable diameter determined by
the reaction temperature.

7. 3. CHEMICAL VAPOUR DEPOSITION
• In CVD, a substrate is prepared with a layer of
metal catalyst particles, most commonly nickel,
cobalt, iron, or a combination. The diameters of
the nanotubes that are to be grown are related to
the size of the metal particles. This can be
Controlled by patterned deposition of the metal,
annealing, or by plasma etching of a metal layer.
The substrate is heated to approximately 700°C.
To initiate the growth of nanotubes, two gases are
bled into the reactor: a process gas (such as
ammonia, nitrogen, hydrogen, etc.) and a carbon-
containing gas (such as acetylene, ethylene, ethanol, methane, etc.).
Nanotubes
grow at the sites of the metal catalyst; the carbon-containing gas is broken
apart at
the surface of the catalyst particle, and the carbon is transported to the edges
of
the particle, where it forms the nanotubes.

8. PROPERTIES
The most important properties of CNTs are
a) Strength: CNTs are the strongest and stiffest materials on earth, in
terms of tensile strength and elastic modulus respectively. This strength
results from the covalent sp² bonds formed between individual carbon
atoms.
CNTs are not nearly as strong under compression. Because of their
hollow structure and high aspect ratio, they tend to undergo buckling
when placed under compressive, torsional or bending stress.
b) Thermal: 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. The
temperature stability of carbon nanotubes is established to be up to
2800 degrees Celsius in vacuum and about 750 degrees Celsius in air.
c) Chemical Reactivity: The chemical reactivity of a CNT is, compared
with a graphite-sheet, enhanced as a direct result of the curvature of
the CNT surface.
d) Electrical Conductivity: Depending on their chiral vector, carbon
nanotubes with a small diameter are either semi-conducting or metallic.

9. APPLICATIONS
• Concrete: In concrete, they increase the tensile strength, and halt
crack propagation.
• Polyethylene: Researchers have found that adding them to
polyethylene increases the polymer’s elastic modulus by 30%.
• Sports equipment: CNTs are used in different sports equipments
such as tennis rackets, bike parts (racing bikes), golf balls etc.
• Space Elevators: This will be possible only if tensile strengths of
more than about 70 GPa can be achieved. Monoatomic oxygen in the
Earth’s upper atmosphere would erode carbon nanotubes at some
altitudes, so a space elevator constructed of nanotubes would need to
be protected (by some kind of coating). Carbon nanotubes in other
applications would generally not need such surface protection.
• Others: Bridges, clothes, combat jackets, ultrahigh-speed flywheels
etc.

10. CONCLUSION
• Rise in demand and production, and ease of accessibility of carbon
nanotubes would lead to the extensive use of carbon nanotubes in a
wide variety of applications. The use of nanotechnology for human
will become common need in 21st century. As world is suffering from
serious pollution problems, hydrogen will becoming need of 21st
century & carbon nanotubes provide better solution for hydrogen
storage.
• Nanotubes market, which was growing at a moderate rate till 2008-
2009, is expected to rise at a skyrocketing pace in the coming years.
Hence we can conclude that most of the demands of human, in this
and fore coming generation will be fulfilled by carbon nanotubes.

11. REFERENCES
• www.nanotechnology.com
• www.multidisciplinairprojects.com
• www.nanotechnology.com
• www.cnt.com