The Synthesis of Ultrafine Silicon Carbide in a Hybrid Plasma

Ultrafine silicon carbide powder was synthesized from SiCl4 and CH4 through thermal plasma CVD using a “hybrid plasma”, characterized by the superimposition of a DC arc jet on a radio-frequency plasma. For a better understanding of the process, thermodynamic equilibrium compositions for the Ar-H-C-Si-Cl system were calculated up to 6000 K.The reactants of SiCl4 and CH4 mixed with H2 were injected into an arc jet and a tail flame of the hybrid plasma through a narrow ring-slit, respectively. By this method, the productive reaction zone of SiC is very limited to the space at the slit level, leading to a homogeneous reaction history of each particle. The ratio of [C]/[Si] in the product depended upon the molar ratio of [CH4]/[SiCl4] and the total flow rate of H2 used as a quenching gas. Under appropriate conditions, nearly stoichiometric whitish gray β-SiC powder about 30 nm in size was obtained. Moreover, it was found that the particle size could be controlled by regulating the quenching process.According to equilibrium calculations, SiCl4 is completely dissociated in the plasma, and the main reaction for the formation of β-SiC is believed to be the reaction: Si(g)+CHx. Moreover, there was no possibility for the condensed phase except β-SiC to appear during the cooling process from the plasma. Accordingly, the synthesis of very pure SiC powder would not be so difficult if complete reaction occurred in the plasma tail flame. These thermodynamic considerations were consistent with the experimental results.