Carbothermal synthesis of low-z β′-sialon from silica or elemental silicon in the presence and absence of Y2O3: an XRD and MAS NMR perspective

Abstract

Carbothermal formation of low-z β′-sialon (z=0.5) from halloysite clay has been carried out using two different forms of the necessary additional silicon, namely, finely divided SiO ₂ (quartz) and elemental Si. XRD and solid-state ²⁹ Si and ²⁷ Al MAS NMR studies of the early stages of the reaction at 1000–1400 °C show that the SiO ₂ forms both Si ₃ N ₄ , via a series of oxynitrides, and SiC. These then react with the mullite produced by thermal decomposition of the clay, to form β′-sialon. The addition of 3 mass% Y ₂ O ₃ to the reaction mix does not change the reaction sequence, but facilitates the formation of both mullite and β′-sialon, in which it also promotes the formation of Al–N units. Using elemental Si, some SiC is formed, which reacts with the mullite and remaining Si to form O′-sialon, which may then react further with mullite to produce β′-sialon. The addition of Y ₂ O ₃ forms transient Y ₂ Si ₂ O ₇ at lower temperatures, but in the later stages facilitates the formation of β′-sialon with respect to O′-sialon.