Thermal oxidation of carbothermal β′-sialon powder: reaction sequence and kinetics

Abstract

The oxidation of carbothermally synthesised β′-sialon powder (z=2.45) was found by X-ray powder diffraction and solid-state ²⁹ Si and ²⁷ Al MAS NMR spectroscopies to result in the initial formation of amorphous SiO ₂ and a non-crystalline mullite-like aluminosilicate which becomes crystalline at higher temperatures. The evolution of crystalline mullite is accompanied by a decrease in both the sialon z value and in the proportion of tetrahedral Al. The ratio of ²⁹ Si in the SiO ₂ to ²⁹ Si in the mullite oxidation products is about 4, consistent with the formation of Al ₂ O ₃ as an additional oxidation product. The oxidation kinetics at 1100–1300 °C are described by a parabolic rate law with an activation enthalpy of 161 kJ mol ^-1 , suggesting the rate-determining step to be the permeation of oxygen through the oxidised layer coating the sialon grains.