Kinetics of carbon deposition on diamond powder

The rate of carbon deposition on 0–1-μm diamond seed crystals was investigated in the temperature range 1140–1475 °C, methane partial pressures form 10−3 to 44.4 Torr and hydrogen partial pressures from 0 to 16.4 Torr. Two distinct kinetic regimes were found. A relatively rapid initial rate of diamond growth decayed exponentially to a final slow constant rate as graphitic carbon was codeposited and covered the surface. A maximum initial diamond growth rate of 1.06×10−7 g cm−2 min−1, which corresponded to a nominal linear growth rate of only 0.45 μm per day, was observed. Dilution of methane with hydrogen decreased the nucleation rate of graphitic carbon relative to that of diamond resulting in a increase in the diamond yield. The activation energy for diamond deposition from pure methane was 55 kcal mole−1. The dependence of the initial rate data on methane partial pressure could be described by a Langmuir-type equation. The deposits were identified by chemical etching, chemical analysis, density measurements, x-ray and electron diffraction, transmission electron microscopy, and optical measurements. The crystalline quality of the diamond was not determined and could be highly defective.