Thermal characteristic difference between α-Al2O3 and cristobalite powders during mullite synthesis induced by size reduction

The thermal behavior resulting from size reduction of α-Al2O3 and cristobalite powders during mullite formation were examined to understand the effects of the individual size reduction on the initiation and completion temperatures and the reaction rate of mullite formation. Both powders with particle sizes (D50) of approximately 200, 300, and 400 nm were mixed at a stoichiometric composition of 3Al2O3·2SiO2 and thermally treated up to 1400–1600 °C. The results indicated that the cristobalite amorphized in advance and then reacted with the α-Al2O3 particles. The size of α-Al2O3 particles determined that of the mullite particles formed. Decreasing α-Al2O3particles reduced the temperature of the amorphization and resulted in lowering and determining the temperature at which mullite formation initiated. While reducing the cristobalite sizes narrowed the temperature ranges of amorphization. It accelerated the rate of mullite formation, allowing the formation reaction to be completed at lower temperatures.