Oxidation of CVD diamond powders

Spherical diamond powders have been synthesised by a multi-cathode d.c. discharge in hydrogen–methane mixtures under conditions conducive to homogeneous nucleation. The polycrystalline powder can be deposited at rates of up to 100 mg/h at substrate temperatures below which thin film diamond growth is negligible. The diameters of the diamond spheres vary from 150 to 600 nm depending on the radial distance on the substrate. The diamond powders have been thermally oxidised in the range of 450–610 °C under dry oxygen. Depending on the degree of thermal contact between the powders and the heater block the rate of oxidation can be ‘explosive’ in which a rapid rise in temperature is observed caused by the exothermic nature of the oxidation of diamond.