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.