Carbonization behavior in microwave heating of silicon powder

Silicon carbide (SiC) powders have been synthesized by microwave-heating of mixtures of silicon (Si) and graphite (C) powders embedded in graphite powder (susceptor). The influence of the mixing composition of these powders on the SiC formation has been investigated. For 1-min heating, the products qualitatively followed the rules of the Si-C phase diagram, depending on the Si/C mixing ratio. For heating periods of 10 min or longer, however, almost single-phase β-SiC powders have been obtained irrespective of the Si/C mixing ratio, not being in accordance with the phase diagram. This result suggests that the initial mixing ratio was not retained during the carbonization for long heating periods. A comparative experiment using an electrical furnace, providing thermally equilibrium heating, also confirmed the rapid carbonization of solid Si at 1673 K in a carbon monoxide (CO) atmosphere. These results suggest that the carbonization occurs mainly via solid-state reaction among the neighboring Si/C particles for an early stage of the heating period, followed by the gas-solid reaction between CO gas and solid silicon for the subsequent heating period. The latter mechanism contributes to the carbonization independent of the Si/C mixing ratio and the long-distance mass transfer of extra carbon from the susceptor. © 2021 Japan Society of Powder and Powder Metallurgy