THERMAL-ANALYSIS OF FORMATION OF SIALONS BY CARBOTHERMAL REDUCTION OF CLAYS

The formation at high temperatures of phases such as beta', X, and O'-sialon by carbothermal reduction of clays has been studied using simultaneous thermal analysis (STA) and thermomechanical analysis (TMA). Two reducing agents, silicon carbide and carbon, were used, with either kaolinite or bentonite. The progress of reaction was followed by interrupted and continuous thermal analysis procedures using STA and TMA and the changes correlated with the phases observed by X-ray diffraction at different stages. The conversion of kaolinite to beta'-sialon is characterised by the dehydration and transformation of kaolinite independent of the nitriding atmosphere below 1400 degrees C. At higher temperatures nitriding commences. Similar behaviour is observed with bentonite transforming to lower substitution sialon phases. The weight changes are correlated with oxidation/reduction reactions and mechanisms are suggested to explain the observed phenomena. The data obtained will permit the optimization of thermal cycles for batch production of sialon powders from natural minerals.