Homogeneous nucleation of diamond powder by CO2-laser-driven gas-phase reactions

Diamond powders with grain diameters up to 0.3 μm were obtained by CO2 -laser-induced decomposition of C2H4 at low pressures and temperatures. C2H4 or mixtures of C2H4, H2, and SiH4 were irradiated in a gas-flow reactor with the 10P14 line (10.532 μm) of a grating-tunable CO2 laser with 50-W cw output power. Solid products were produced in a yellow-to-orange colored flame (500–550 °C) and collected in filters. The product contained polyaromatic species, high molecular polymers, graphite, amorphous carbon, and spherical diamond particles. Several diamond particle populations, with mean diameters of 6–120 nm, were observed by transmission electron microscopy. Both diamond modifications, cubic and hexagonal, were identified by electron diffraction. Diamond formation is dependent on the residence time in the reaction flame, but relatively independent of the reactant gas compositions. Hydrogen-free pyrolysis of pure ethylene produced diamond of high purity (no diffraction rings of graphite detectable).