A new silicothermal route to the formation of X-phase sialon: The reaction sequence in the presence and absence of Y2O3

A new method is described for the preparation of essentially monophase X-phase sialon, Si12Al18O39N8, by silicothermal reduction of mixtures of kaolinite, γ-Al2O3 and elemental Si in purified nitrogen at 1450–1500 °C. Powder X-ray diffraction and solid state 29Si and 27Al MAS-NMR of reaction mixtures heated to 1200–1570 °C suggest that the reaction proceeds in several parallel steps, including the nitridation of the elemental Si to Si3N4, thermal decomposition of the kaolinite to mullite and amorphous silica, reaction between the amorphous silica and γ-alumina, and reaction between silicon nitride and the aluminosilicate phase. Two forms of O′-sialon also appear as minor phases, possibly formed in the early stages by nitridation of the silica or aluminosilicate. The addition of up to 3 wt% of Y2O3 to the reaction mixture increases the rate of all the reactions occurring above 1100 °C without changing the reaction sequence, but the X-phase product formed in the presence of Y2O3 retains a high proportion of SiO2N2 groups, by comparison with X-phase formed without Y2O3, in which SiO3N groups predominate. XRD and MAS-NMR show the fully reacted silicothermal product to be comparable with reputable X-phase sialon prepared by a two-stage carbothermal method.