Carbon nanotubes are novel materials with unique electrical and mechanical properties. MOst of the natures of carbon nanotubes depend on their diameters and chiralities. However, it is very difficult to control the diameter and chirality of carbon nanotubes at high temperatures, and thus limits prospects of the nanotubes. For instance, one of the most promising applications of carbon nanotubes in the future is flat panel display, but the carbon nanotubes synthesized at high temperatures is not useable for this purpose because the strain point of glass is very low. For these reasons, synthesis of carbon nanotubes at low temperatures has become a challenge and received much attention. Among all the procedures for synthesizing nanotubes, chemical vapor deposition, especially plasma chemical vapor deposition (PCVD), has great potential in synthesizing carbon nanotubes at low temperatures. In this paper, cobalt nanoparticles supported by SiO_2 was prepared by sol-gel process and plasma reducing process. Using methane as carbon resources, hydrogen as carrier, carbon nanotubes were synthesized by microwave plasma chemical vapor deposition (MWPCVD) at temperatures above 460 ℃ but below 500 ℃ under the catalytic effect of cobalt supported by SiO_2. During the process of nanotubes growth, the total pressure in the chamber was kept at 2 kPa. The microwave plasma input power was 600 W, with the flow rates of H_2 and CH_2 being 80 and 5～20 sccm, respectively. Though some other carbonaceous particles were also deposited and attached on the carbon nanotubes when the flow rate of CH_2 was 20 sccm, the amount of carbonaceous particles decreased with the decrease of the flow rate of CH_4. These results demonstrate that MWPCVD is a very efficient process for the synthesis of carbon nanotubes at low temperatures.

Key words: NANOPHASE MATERIALS, CARBON, PLASMA, CHEMICAL VAPOUR DEPOSITION