Influence of starting material composition and carbon content on the preparation of Mg-α SiAlON powders by carbothermal reduction-nitridation

This paper investigated the synthesis of Mg–α SiAlON from Talc (Mg3(Si2O5)2(OH)2) and halloysite clay (Al2Si2O5(OH)4). Talc, halloysite and carbon black as a reducing agent were used as the starting materials, and 3 wt.% α-Si3N4 was added as the seed in all cases. The mixtures were heated in flowing N2 (gas) to synthesize Mg–α SiAlON powders. The chosen molecular ratios of talc to halloysite were 1.0, 1.5, 2.0 and 2.5. The influences of various reaction parameters such as the carbon content, temperature and holding time at the top temperature were studied. The results showed that the synthesized powders were composed of α-SiAlON, β-SiAlON, β-SiC, 15R-AlN polytypoid and AlN phases; the phases and the particle morphology greatly depended on starting material composition and synthesis parameters. A larger amount of talc was needed to compensate for Mg evaporation loss in the starting composition. Higher carbon content seemed to retard the reaction rate, resulting in coarse particle size with an irregular grain shape. The highest content of Mg–α SiAlON, 90 wt.%, was achieved at 1480 °C for 4 h at talc to halloysite ratios of 1.5 and 2.0.