Effect of silicon on properties of Al2O3-SiCw composite ceramics in-situ synthesized by aluminium-assisted carbothermal reduction of coal series kaolin for solar thermal storage

Abstract

For further improving the yield of SiC whiskers (SiCw), the heat capacity and the thermal conductivity of Al2O3-SiCw composite ceramics in-situ synthesized by aluminium-assisted carbothermal reduction of coal series kaolin (CSK) for solar thermal storage, silicon (Si) was added to consume the silica residual in CSK and formulae of series CA with 0.8–8 wt% Si additive were designed on the basis of the previous formula (CSK 68 wt%, Al 32 wt%). Effects of Si additive on the yield of SiCw as well as the morphology and the properties of Al2O3-SiCw composites were investigated in detail. Results indicated that Si additive was superior in terms of improving the SiCw yield and the properties of the composites. Samples CA3 (with 5.6 wt% Si additive) fired at 1600–1650 °C achieved the highest relative content of SiCw as 21.4–21.6%, which was much higher than that of the basic formula with increasing rates of 44.6–45.9%. The added Si was also highly in favor of densifying the composites and samples CA3 fired at 1600 °C obtained the optimal properties: 2.45 g cm−3 for bulk density, 63.5 MPa for bending strength, 7.38 × 10−6 °C−1 for coefficient of thermal expansion, 9.36 W (m K)−1 for thermal conductivity (room temperature), 0.85 J (g K)−1 for heat capacity (room temperature). The great effect of Si additive on the consumption of silica and the improvement of SiC yield could increase the heat capacity and the thermal conductivity of the composites effectively.