Effect of silica on in-situ synthesis of nano-SiC whiskers in porous Al2O3-SiC composite ceramics for solar thermal storage by aluminium-assisted carbothermal reduction

Abstract

For promoting in-situ synthesis of nano-SiC whiskers in porous Al2O3-SiC composite ceramics for solar sensible thermal storage by aluminium-assisted carbothermal reduction of coal series kaolin (CSK), silica (SiO2) was added as an extra silicon source to the basic formula composed of 68 wt% CSK and 32 wt% aluminium. Effect of silica on the in-situ synthesis and morphology of nano-SiC whiskers was studied and the influence of nano-SiC whiskers on the properties (including physical and thermophysical properties) of the composites was investigated. Results revealed that silica could facilitate the production of nano-SiC whiskers significantly by increasing the partial pressure of SiO gas, and the relative content of SiC reached the highest as 18.4% by adding 12 wt% silica when sintering at 1650 °C. Silica exhibited great effect on increasing the porosity of the composites through carbothermal reduction, which was essential for sufficient heat transfer. Optimum properties were obtained for sample CS3 with 12 wt% silica addition sintered at 1650 °C, whose porosity, bending strength, coefficient of thermal expansion, thermal conductivity (room temperature) and heat capacity (600 °C) were 44.4%, 33 MPa, 7.27 × 10−6·°C−1, 8.38 W·(m·K)−1, 1.1 J·(g·K)−1, respectively. The high content of nano-SiC whiskers with a high aspect ratio contributed to improve the thermal conductivity and lower the coefficient of thermal expansion of the porous composite ceramics.