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Carbon nanotubes are tubular forms of carbon that can be envisaged
as graphitic sheets rolled into cylindrical form. These nanotubes
have diameters in the range of few nanometers and their lengths
are up to several micrometers. Each nanotube is a single molecule
made up of a hexagonal network of covalently bonded carbon
atoms.
Carbon nanotubes are
of two types: single-walled and multi-walled. A single-walled
carbon nanotube (SWNT) consists of a single graphene cylinder,
whereas a multi-walled carbon nanotube (MWNT) comprises several
concentric graphene cylinders. A schematic representation of a
SWNT structure is shown in the figure below.
Strong covalent bonding,
unique one-dimensional structure and nanometer size, together
impart unusual properties to the nanotubes. These properties
include exceptionally high tensile strength, high resilience,
electronic properties ranging from metallic to semi-conducting,
the ability to sustain high current densities and high thermal
conductivity. Thus carbon nanotubes could be used as fillers in
super-strong composite materials, as wires and components in nano-electronic
devices, as tips of scanning probe microscopes and in flat panel
displays and gas sensors.
Iijima[1] reported the
preparation of MWNTs by the arc-discharge of graphite electrodes
in 1991. In 1993, Ijima and Ichihashi at NEC[2] and Bethune et al.
at IBM[3] independently reported the preparation of SWNTs. Today,
MWNTs are prepared in large quantities by the chemical vapor
deposition process. SWNTs can be prepared in reasonably high
yields by three techniques: arc-discharge of Ni-Y catalyzed
graphite electrodes,[4] laser ablation of Ni-Co catalyzed
graphite targets[5] and vapor phase pyrolysis of CO and Fe(CO)5 (HiPCO
process).[6] Carbon nanotube samples are always contaminated with
impurities including amorphous carbon, residual metal catalyst and
graphitic nanoparticles. Thus the purification and chemical
processing of carbon nanotubes remains as a key step in any
application.
next page:preparation of CNTs >>>>
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