The structure of nanotubes fabricated by carbon evaporation at high gas pressure

Nanotubes were fabricated by carbon evaporation at high gas pressure (gas-nitrogen or nitrogen-argon, pressure up to 1300 atm). Three main types of tubes were observed and are characterised as follows: (1) Multilayered surface modulated micro- and nanotubes (SMMTs and SMNTs, respectively) with fluctuating diameters caused by periodically interrupted movement of catalytic particles during tube formation. All tubes of this type have a common principle of structural organization: the main part of the walls is formed by continuous graphene layers to which the side layers of inner caps are connected. An extreme form of SMNTs was observed in which the carbon nanotube walls were spheroidal. (2) Stacked conical layer carbon nanotubes (CLNTs). These nanotubes have open edges and were found to readily absorb argon. (3) Multiple walled carbon nanotubes (MWNTs) consisting of concentric cylindrical layers. The inner channels of these nanotubes were sometimes observed to be filed with Fe carbides. The growth of (1) and (2) type nanotubes is promoted by Fe catalytic particles. At reduced catalyst concentration and at high temperature, most of the observed nanotubes are type (3). In addition the presence of N-2 in the gaseous mixture was found to enhance the rate of graphite evaporation and increased the nanotube yield. (C) 2000 Elsevier Science Ltd. All rights reserved.