In-situ synthesis of nitride whiskers-bonded SiAlON–Al2O3 ceramics for solar thermal storage by aluminothermic nitridation of coal-series kaolin

Abstract

Nitride whiskers-bonded SiAlON–Al2O3 ceramics used for solar thermal storage were in-situ synthesized by aluminothermic nitridation of coal-series kaolin, an abundant solid waste in China. Mechanisms on the aluminothermic nitridation and in-situ synthesis of Si3N4 and SiAlON whiskers were studied by TG-DSC, XRD, SEM, TEM, etc. The effect of sintering temperature on the phase evolution, microstructure and properties of the ceramics was investigated, and the thermophysical properties were compared to those of the available solar thermal storage materials. Results indicated that Si3N4 and SiAlON whiskers could be in-situ synthesized simultaneously at 1300 °C via Vapor–Solid mechanism. Increasing sintering temperature decreased the amount of Si3N4 whiskers, but increased that of SiAlON whiskers, which transformed to be columnar SiAlON grains at 1550–1600 °C. By increasing the sintering temperature, the enhancing densification and the formation of the columnar SiAlON grains were in favor of improving the strength of the ceramics, and the increasing SiAlON content decreased the thermal expansion coefficient of the ceramics to 5.73 × 10−6 °C−1. Compared with other thermal storage materials, the advantages of the SiAlON–Al2O3 ceramics are their relatively higher thermal conductivity (6.34 W/m/K, at room temperature), higher specific energy density (2941 kJ/m3/K, at 900 °C), and better high-temperature thermal stability, making them competent for high-temperature thermal storage uses.