Effect of pre-oxidation on the microstructure, mechanical and dielectric properties of highly porous silicon nitride ceramics

Abstract

Highly porous Si3N4 ceramics have been fabricated via freeze casting and sintering. The as-sintered samples were pre-oxidized at 1200–1400°C for 15min. The effect of pre-oxidation temperature on the microstructure, flexural strength, and dielectric properties of porous Si3N4 ceramics were investigated. As the pre-oxidation temperature increased from 1200°C to 1400°C, firstly, the flexural strength of the pre-oxidized specimens remained almost constant at 1200°C, and then decreased to 14.2MPa at 1300°C, but finally increased to 25.6MPa at 1400°C, while the dielectric constant decreased gradually over the frequencies ranging from 8.2GHz to 12.4GHz. This simple process allows porous Si3N4 ceramics to have ultra-low dielectric constant and moderate strength, which will be feasible in broadband radome applications at high temperatures.