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

Abstract A new method is described for the preparation of essentially monophase X-phase sialon, Si 12 Al 18 O 39 N 8 , by silicothermal reduction of mixtures of kaolinite, γ-Al 2 O 3 and elemental Si in purified nitrogen at 1450–1500 °C. Powder X-ray diffraction and solid state 29 Si and 27 Al 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 Si 3 N 4 , 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 Y 2 O 3 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 Y 2 O 3 retains a high proportion of SiO 2 N 2 groups, by comparison with X-phase formed without Y 2 O 3 , in which SiO 3 N 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.