SUPERSONIC DC-ARCJET SYNTHESIS OF DIAMOND

A supersonic, direct-current arcjet, designed for expansion into low pressures (<< 100 Pa) and operating entirely on hydrogen, is employed to produce diamond films on molybdenum substrates. This technique features strongly reduced boundary layer effects due to the near rarefied flow conditions, and short residence times for post-plasma injected reactants, thereby providing better control of the gas-phase chemistry. The results from a parametric study of diamond growth for methane as a carbon source gas are presented. Despite the low pressures, relatively high diamond growth rates (up to 10 mum h-1) are obtained. A ''saturation'' is observed in the variation of the diamond film thickness with methane to hydrogen flow ratio. Non-diamond phases become more pronounced at higher methane flow ratios, where the film thickness appears to increase linearly with methane concentration. The results are interpreted within the framework of a simplified five-step heterogeneous mechanism that accounts for the competition between the growth of diamond and non-diamond carbon.