Wednesday, June 07, 2006

US Patent 7056479 - CVD Nanotube Fabrication On Porous Carbon Substrate

There are three basic strategies commonly employed to fabricate nanotubes-arc discharge, laser vaporization, and chemical vapor deposition (CVD). Arc discharge usually employs graphite electrodes in a low pressure, helium environment and electric breakdown between the electrodes result in a buildup of nanotube containing deposits on the negatively biased electrode. Laser vaporization usually employs laser evaporation of a graphitic sample in an argon environment resulting in nanotube containing byproducts. Both arc discharge and laser vaporization result in a mixture of nanotubes, carbon soot, and other byproducts and, while refinements of these techniques as well as purification methods have allowed for increased bulk production of nanotubes, they are not suited for producing a well ordered array of pure nanotubes on a substrate such as required for many electronics and sensor applications.

CVD, however, is capable of producing well-ordered nanotube arrays on a substrate (such as a silicon wafer or aluminum oxide template) by using catalytic particles (e.g. Ni, Co, Fe) and a flow of hydrocarbon gas at a high temperature. Unfortunately, this technique suffers from low yield and contamination of amorphous carbon. The inventors of this patent propose the use of porous carbon as the substrate on which the catalytic CVD growth of nanotubes can occur. This apparently reduces heating requirements, increases yield, and reduces contamination. Claim 1 reads:

1. A process for producing carbon nanotubes comprising the steps of: (a) depositing a metallic catalyst on a porous carbon substrate; (b) passing a feedstock gas containing a source of carbon through the substrate; and (c) applying an electrical current to said substrate to heat the substrate sufficiently to generate a reaction between said catalyst and said gas, resulting in the formation of carbon nanotubes.