Solid-state NMR examination of the formation of β-sialon by carbothermal reduction and nitridation of halloysite clay

Abstract

β-Sialon (Si_{6 –z}Al_zO_zN_{8 –z}; 0 < z < 4.2] can be readily formed from halloysite clay [Al₂Si₂O₅(OH)₄·2H₂O] at 1400 °C by carbothermal reduction using brown coal and nitridation. A combination of X-ray diffraction, transmission electron microscopy and ²⁹Si and ²⁷Al solid-state NMR are used to establish the details of the intermediate stages of the reaction. NMR clearly shows a distinct sequence of reactions of silica and mullite formed from the thermal decomposition of halloysite with carbon. The carbon initially reacts with free silica and then removes silicon from the mullite, to form a β-SiC-like phase, leaving an alumina-rich mullite framework. SiC subsequently reacts with the remaining silicon from the mullite phase to form some Si₃N₄ which then combines with silicon from SiC, the alumina-rich mullite and nitrogen to form β-sialon. The ²⁹Si and ²⁷Al NMR spectra from sialon X-phase are reported.