Oxidation of ultrafine (Si-) SiC powders

Abstract

The increasing usage of ultrafine ceramic powders in the fabrication of highly reliable ceramics results in a growing interest in appropriate processing conditions for these powders. During processing the extremely high surface areas might lead to significant absorbtion of oxygen even at low temperatures. But especially in this temperature regime, oxidation data of powders are rarely available; as far as the authors know, no investigations have been published in the case of ultrafine powders with particle sizes below 100 nm. In this study the oxidation kinetics of ultrafine (Si-) SiC powders (∼ 20 nm) in the temperature range between room temperature and 1000 °C in air were investigated. Thermobalance experiments showed that at least three different oxidation mechanisms are operating. At temperature above 650 °C the fraction of completion R is proportional to the square root of time, indicating a diffusion-controlled mechanism (activation energy ∼-1.8 eV). At lower temperatures the best data fit is obtained by a Cabrera-Mott-like equation. At room temperature and for thin silica-layer thicknesses a third oxidation mechanism was determined. The formation of the first monolayer of silicon oxide obeys the kinetics of a first-order reaction, namely an exponential one with a time constant of 1.25× 10⁻⁴ s⁻¹. An investigation of the influence of oxygen pressure on the oxidation of ultrafine Si-SiC powders revealed a low pressure influence at 500 °C. An approximately linear relation between pressure and oxidation rate constant is observed between 30 and 1000 mbar air pressure at 800°C. The kinetic data were used to construct an “oxidation map” for ultrafine SiC powders, as a help to determine appropriate processing conditions.