Thermal oxidation of carbothermal beta'-sialon powder: Reaction sequence and kinetics

The oxidation of carbothermally synthesised beta'-sialon powder (z = 2.45) was found by X-ray powder diffraction and solid-state Si-29 and Al-27 MAS NMR spectroscopies to result in the initial formation of amorphous SiO2 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 Si-29 in the SiO2 to Si-29 in the mullite oxidation products is about 4, consistent with the formation of Al2O3 as an additional oxidation product. The oxidation kinetics at 1100-1300 degrees 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.