Nonoxide ceramic powder synthesis

SiC was synthesized by injecting mixtures of carrier gas plus a silicon compound (Si(CH3)4, SiH4) 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 H2 at synthesis temperature. However, CO2 addition to the synthesis zone causes silicon oxidation to SiO2 with little SiC formation. Flame stoichiometry shows the same influence: mixtures around a molar C/O = 1, where the burned gas is CO and H2, favor ceramic synthesis. Richer flames are cooler and produce excessive soot while leaner flames produce H2O and CO2 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 SiO2 forms it remains a solid product. Thus silicon species do not equilibrate with the gas species. (2) SiC forms from reaction of SiH4 + C2H4, 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.