Homogeneous nucleation of diamond powder is reported. The experiments were performed in a low-pressure microwave-plasma reactor. The deposits were collected downstream of the reaction zone and subjected to wet oxidation to remove nondiamond carbons. The residues were analyzed by optical and electron microscopy, electron diffraction, and Raman spectroscopy. A variety of hydrocarbons diluted in argon, hydrogen, or oxygen gas mixtures were tested. In most cases only nondiamond materials, like graphite and carbyne, were obtained. Homogeneous nucleation of diamond was clearly observed in dichloromethane- and trichloroethylene-oxygen mixtures. The particles formed had crystalline shapes, mostly hexagonal. The largest particles were about 0.2 μm, although most of the particles were on the order of 50 nm in diameter. The powder was identified to be a mixture of polytypes of diamond.

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Vaporization thermodynamics in the titanium-silicon binary system has been studied by a Knudsen effusion-mass spectrometer technique. The equilibrium vapor pressure of silicon gas over two-phase mixtures of TiSi 2 + TiSi, TiSi + Ti 5 Si 4 , and Ti 5 Si 4 + Ti 5 Si 3 has been measured over a temperature range of ca. 1700-2050 K, and the vapor pressure of titanium gas was measured over a two-phase mixture of Ti 5 Si 4 + Ti 5 Si 3 . Standard enthalpy and entropy changes for the appropriate dissociation reactions, within the temperature ranges of the experiments, were determined from the temperature dependence of the measured vapor pressures. Estimated thermal functions were combined with the vapor pressure data and used in a third-law evaluation of the standard enthalpies of formation at 298 K of the intermediate compounds. The results obtained are 1 / 3 TiSi 2 , -53.5 ( 5 kJ; 1 / 2 TiSi, -71.5 ( 5 kJ; 1 / 9 Ti 5 Si 4 , -75.9 ( 5 kJ; 1 / 8 Ti 5 Si 3 , -78.1 ( 5 kJ. The results obtained in this work are compared with previous estimates and measurements of the thermodynamics of phase formation of the titanium silicides.

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