Chemical vapor deposition of C on SiO2 and subsequent carbothermal reduction for the synthesis of nanocrystalline SiC particles/whiskers

Abstract

This study investigates chemical vapor deposition of C from CH4 on particulate SiO2 and subsequent carbothermal conversion of the resultant composite particles to SiC powders. Mass measurements, HR-TEM, SEM and XRD were used to characterize the products at various stages of the processes. It was found that oxide particles gained mass rapidly at 1300 K under CH4 atmosphere owing to enhanced C uptake. Pyrolytic carbon layers 5-8 nm thick were deposited on SiO2 particles. The coated powders with high C loadings (40-42.6 wt.% C) were converted to SiC under Ar flow in a temperature range of 1700-1800 K. Almost pure SiC powders containing a mixture of particles and whiskers of similar to 100 nm were synthesized at 1750K for 45 min and at 1800K for 30 min using the starting powder with 40 wt.%C. Whisker diameter increased with the C content of the coated powder. It was proposed that SiC whisker was grown by a vapor-solid mechanism. Equilibrium thermodynamic analysis by the method of minimization of Gibbs' free energy predicted the reaction pathways to SiC and to the product species in the Si-O-C-Ar system. This study demonstrated that either C shell-SiO2 core powders or SiC powders could be synthesized rapidly in the same reactor. (C) 2011 Elsevier Ltd. All rights reserved.