THE PRODUCTION AND STRUCTURE OF PYROLYTIC CARBON NANOTUBES (PCNTS)

The structures of pyrolytic carbon nanotubes (PCNTs), produced by thermal decomposition of hydrocarbon vapor (benzene at ca 1100-degrees-C) have been studied. Electron microscope images of the PCNTs ''as-formed'' and after various degrees of subsequent heat treatment are presented and discussed on the basis of elongated giant fullerene structures and a possible primary growth mechanism. The images indicate that some PCNTs show evidence of thickening due to secondary pyrolytic deposition whereas others appear to be essentially exposed nanotubes (without further carbon deposition) or have sections with negligible secondary deposition. The nanotubes appear initially to grow as ultra-thin tubes with a central hollow core (diameters ca 2 nm or mom). No catalytic particles am observed at the tips of the tubes shown by high resolution transmission electron microscope (HRTEM) studies to be composed of concentric graphite cylinders with consecutive radii increasing by a factor close to that of the standard graphite interlayer spacing. The further pyrolytic carbon thickening occurs at roughly the same time as longitudinal nanotube growth. This phenomenon can cause the diameters of the tubes to increase to micron sizes. Heat treatment of the ''as-formed'' PCNTs results in encapsulation of the hollow cores by polyhedral hemispherical fullerene-like caps. Single or multi-layered nodes are also generated, yielding ''bamboo-like'' structures. The PCNTs are similar to those obtained by d.c. arc techniques using graphite electrodes.