Fabrication and properties of in-situ mullite-bonded Si3N4/SiC composites for solar heat absorber

Mullite-bonded Si3N4/SiC composite ceramics were fabricated via different pressureless sintering methods (in air and in graphite powders) using Si3N4, SiC, andalusite, α-Al2O3 and calcined bauxite as starting materials. Phase composition, mechanical properties, microstructure, thermal shock resistance, thermophysical properties and oxidation resistance were investigated. Results show that buried sintering method is more feasible which can protect Si3N4 and SiC free from oxidation effectively. After sintered at 1460 °C in graphite powders, B3 sample exhibits the best mechanical properties, which also has high thermal conductivity of 8.84 W/(m K). The sample has no damage after 30 thermal shock cycles (wind cooling from 1100 °C to 25 °C), and the bending strength remains 87.86 MPa with a growth rate of 65.15%. Simulation results indicate that the average outlet air temperature of the volumetric absorber reaches 850 °C with the efficiency above 65%. It is believed that mullite-bonded Si3N4/SiC composite ceramic is suitable for the use as heat absorbing material in concentrating solar power generation.