Figure 1. High resolution transmission electron micrographs of the
catalytically-grown multi-walled carbon nanotube material.
The scanning electron micrographs are illustrated in Figure 2 for the
catalytically-grown multi-walled carbon nanotube material used as a
filler. Multi-walled carbon nanotubes include concentric nanotubes with
an inter-wall spacing of about 0.34 nm and outside diameters on the
order of about 10 to about 100 nm. Multi-walled carbon nanotubes have
gained increasing interest in various industrial and technological
applications because of the development of techniques to produce bulk
quantities of high-quality carbon nanotubes with uniform diameters,
number of walls, and atomic structure. A key technological challenge is
growing high-quality single-walled carbon nanotubes in large quantities.
Although chemical vapor deposition provides the highest quality
single-walled carbon nanotubes, large quantity forest growth is hampered
by the presence of double and multi-walled carbon nanotubes. The ratio
of single-walled to multi-walled carbon nanotubes can vary greatly from
one process to another and is difficult to control. Several methods of
synthesizing fullerenes have developed from the condensation of
vaporized carbon at high temperature. Fullerenes may be prepared by
carbon arc methods using vaporized carbon at high temperature. Carbon
nanotubes have also been produced as one of the deposits on the cathode
in carbon arc processes. An important way to synthesize carbon nanotubes
is by catalytic decomposition of a carbon-containing gas by
nanometer-scale metal particles supported on a substrate. The carbon
feedstock molecules dissociate on the metal particle surface and the
resulting carbon atoms combine to form carbon nanotubes. The method
typically produces imperfect multi-walled carbon nanotubes. One example
of this method involves the disproportionation of carbon monoxide to
form single-walled carbon nanotubes and carbon dioxide catalyzed by
transition metal catalyst particles comprising molybdenum, iron, nickel,
cobalt, or mixtures thereof residing on a support, such as alumina.
Although the method can use inexpensive feedstocks and moderate
temperatures, the yield of single-walled carbon nanotubes can be low,
with large amounts of other forms of carbon, such as amorphous carbon
and multi-walled carbon nanotubes present in the product. The method
often results in tangled carbon nanotubes and also requires the removal
of the support material for many applications.