Nonoxide ceramic powder synthesis

Abstract

SiC was synthesized by injecting mixtures of carrier gas plus a silicon compound (Si(CH 3 ) 4 , SiH 4 ) into hot burned gas downstream of a fuel rich hydrocarbon-oxygen flame. The gas temperature during ceramic synthesis was measured at 1500°–1650°C. Reaction stoichiometry was investigated by changing the carrier gas injected with the silicon compound. Both experimental data and equilibrium calculations show that SiC is stable in mixtures of CO and H 2 at synthesis temperature. However, CO 2 addition to the synthesis zone causes silicon oxidation to SiO 2 with little SiC formation. Flame stoichiometry shows the same influence: mixtures around a molar C O = 1 , where the burned gas is CO and H 2 , favor ceramic synthesis. Richer flames are cooler and produce excessive soot while leaner flames produce H 2 O and CO 2 which oxidize silicon species. Two key results give guidance about the ceramic formation mechanism: (1) In these experiments silicon species reactions are path dependent; once SiO 2 forms it remains a solid product. Thus silicon species do not equilibrate with the gas species. (2) SiC forms from reaction of SiH 4 + C 2 H 4 , which shows silicon species both breaking and forming chemical bonds during the reaction process. This result opens the possibility of a gas phase kinetic mechanism for ceramic powder formation. In general, these experiments present a new combustion driven material synthesis route that may apply to a variety of compounds.