Carbothermal production of β′-sialon from alumina, silica and carbon mixture

Abstract

Mixtures of pure nanometer-sized amorphous silica and γ-alumina with the atomic ratio Si∶Al=1 were reduced by a stoichiometric amount of carbon between 1100 and 1450 °C in flowing nitrogen in order to produce β′-sialon powder. Using aqueous suspensions of starting materials, compacts with different microstructures were prepared for reaction. Silica reduction to SiO occurred at a temperature as low as 1300 °C and part of it was removed with flowing nitrogen. Carbothermal reaction involving nitrogen stated at 1350 °C and Si₂N₂O was found as an intermediate together with SiC, resulting in β′-sialon formation. Loss of silica from the system led to AlN formation. Decomposition of β′-sialon into sialon polytypoids (15R, 12H) was observed as a result of sialon and AlN reaction at 1450 °C. The reaction rate of sialon formation was slowed down compared to the carbothermal reduction of kaolin because of the lack of impurities. The microstructure of the reacted pellets influenced the reaction products, and the narrow pore size distribution as well as good homogeneity enhanced β′-sialon formation.