Combustion Synthesis of Nanosized SiC x N y Powders
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*presentaddress,SciTec, Princeton,NJ 161 Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society
EXPERIMENTAL Combustion experiments were performed in both a closed vertical tube and a constant volume (9.3L) spherical vessel. Known mixtures of silane (purity: 99.95%), ammonia (99.99%), and acetylene (analysis indicates less than 0.5% nitrogen/air as the major contaminant after passing through activated carbon to remove acetone) were prepared by partial pressure to ± 0.2 kPa accuracy and ignited by an electric spark. We report here on experiments performed using a spherical vessel in which the discharge of a high voltage capacitor (0.03VF, •30 kVDC) provided spark energies exceeding 1 J. Quantitative product gas analysis was performed using an HP-5830 Gas Chromatograph with calibrated thermal conductivity detection. Temperature programming allowed clean separation and quantification of hydrogen, nitrogen, methane, all C2 hydrocarbons, silane and ammonia on a 3.6 meter 80/100 mesh Porapak N column. The moles of each element in the product gases were calculated based on the vessel volume, final gas pressure and temperature, and product distribution from the gas chromatography analysis. An inferred solid product elemental composition was then
calculated from the difference between the moles of each element in the (known) reactant gases and in the product gases. Ceramic powders resulting from ignition of various reactant mixtures were characterized both physically and chemically. The inferred elemental compositions based on gas chromatography were supplemented by elemental analysis of the silicon, carbon, nitrogen and hydrogen contents of selected powders. IR spectroscopy, TEM, and ESCA provided additional chemical insight. Powders were also characterized as to surface area (BET), particle density (He pycnometry), and particle size (TEM). X-ray diffraction of powders was measured before and after high temperature annealing in argon. RESULTS High voltage sparks initiated a combustion wave (flame) in a wide range of nonstoichiometric silane/acetylene/ammonia mixtures in the constant volume bomb, as evidenced by rapid pressure increases. Hydrogen was identified by gas chromatography as the major gas phase product (see Reaction 1). Fig. 1 is a map of the mixtures which were ignited in the 9.3L vessel. The reactant mixtures are defined by the two partial pressure ratios, P(C 2 H2 )/P(SiH 4) and P(NH 3)/P(SiH 4 ). The stoichiometry line defines mixtures which satisfy Reaction 1. Below this line, assuming all carbon and nitrogen are incorporated into SiC and Si3N4, excess silicon is available. Above the line, excess carbon and/or excess nitrogen are available. More of the stoichiometric region was tested in flametube experiments. We do not report on those experiments here, but do note that in the flametube, no ignitions were observed on or below the stoichiometric line at P(NH 3)/P(SiH 4) greater than 0.35. The 9.3L bomb ignitions in Fig. 1 are all above the stoichiometric line where all the available silicon c
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