What Are Carbon Nanotubes? Essay

620 words - 3 pages

Carbon nanotubes are long cylindrical structures of small nano-scale diameters formed only by
carbon atoms. These structures can be understood as rolled sheets of graphene into a tube. The
interactions between carbon atoms are mostly defined as Van der Waals forces.

If we categorize CNTs by their structure, the following division can be obtained:

- single-walled nanotubes (SWNTs)

- multi-walled nanotubes (MWNTs)

The above structures differ in the arrangement of graphene cylindrical structures . SWNTs have
only one single layer of graphene tubes, whereas MWNTs have many. There are three geometrical
types of nanotubes: armchair, zig-zag and chiral. Between these types there is a difference of how
the graphene is formed into a tube. The zig-zag (n, 0) has carbon-carbon bonds (C-C bonds)
parallel to the axis of the nanotube, while the armchair (n,n) has bonds perpendicular to the axis.
A chiral (n,m) nanotube has a spiral symmetry ...view middle of the document...

The most important features of CNTs in medical applications are, however, the possibility of
encapsulation with ferromagnetic element and attachment of proteins or other molecules to the
inside or outside of the nanotubes. They have potential to, for instance, carry drugs or target and
destroy cells that may be cancerous or infected by a virus. It is important for such applications to
have purely synthesized carbon nanotube material.
There are many different techniques used to synthesize carbon nanotubes (CNTs) such as arc
discharge, laser ablation and chemical vapor deposition (CVD). The latter is the most popular one
as it has many advantages from large area deposition to low costs. What is more, it enables
aligned CNTs growth at moderate temperatures. There are various configurations of CVD reactor
used in the synthesis of CNTs such as horizontal, vertical or barrel. The vertical one is commonly
used for large-scale CNTs production in which catalyst is fluidized with high carrier gas flow

The other method in CNTs growth uses a horizontal quartz reactor. It relies on the following
carbon sources: gaseous hydrocarbons, e.g. methane or acetylene, or liquid ones such as alcohol,
benzene or toluene. The most widely used carrier gases are argon and nitrogen. To prevent the
oxidation of catalyst particles and production of various carbon impurities, in the process of CNTs
catalytic synthesis hydrogen is used with the carrier gas [4]. In this method a non-uniform growth
is observed as a result of the position dependent chemical reaction kinetics in the reactor. The
growth rate, hybrid structure, diameter and number density of CNTs varies greatly, depending on
the temperature and hydrogen ratio of the process.

On the other hand spray pyrolysis CVD enables CNTs synthesis without the presence of hydrogen
and at low carrier gas flow rates. Here the growth rate and other significant features of CNTs is
also highly dependent on the parameter of the reaction, i.e. temperature, injection speed,
precursor volume and catalyst concentration. This method is a relatively safe one in comparison
with others as it does not employ the usage of hydrogen and happens at low temperatures (around
750 ℃.

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