Carbothermal reduction of sillimanite

Abstract

Preliminary studies on the carbothermal reduction of sillimanite (Al₂O₃SiO₂) powders are reported. The effects of various reaction parameters, e.g. reaction temperature (1550–1700 °C), C: Si molar ratio (4.5–9.0) in the reactant mixtures, the nature and source of carbon (carbon black or activated charcoal) and the reaction atmosphere (argon or nitrogen under a constant positive gas pressure of 0.15 MPa), on the carbothermal reduction have been investigated. It has been observed that Al₂O₃ and β-silicon carbide whiskers [β-SiC(w)] are the ultimate products formed in an argon atmosphere. Under a nitrogen atmosphere, β′-sialons are also formed. In all cases, mullite forms as the first intermediate phase at lower temperatures, which later converts progressively to the higher-temperature products.