Synthesis of β-SiAlON from clays: effect of starting materials

Abstract

β-SiAlON powder was synthesized by simultaneous reduction and nitridation of different clays in a nitrogen atmosphere, using graphite and carbon black as reducing agents, the reaction being complete in 10 h at 1400 °C or 5 h at 1475 °C. The process takes place in three basic stages: (i) thermal decomposition of kaolin into mullite and vitreous silica; (ii) reduction of silica and formation of silicon carbide; (iii) reaction of carbon with silicon carbide and mullite (in a nitrogen flow) to give β-SiAlON. The first two stages are independent of the atmosphere, but the reaction rate of the second stage is a function of the carbon used. The impurities of the starting materials affect the intermediate products. Thus, if K is present, the X-phase is formed, whereas the presence of Ca leads to the formation of the O′-phase. The carbon/clay ratio plays an important role since an excess of carbon produces SiC, this being the main phase when the reaction takes place at 1475 °C.