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

Highly porous Si ₃N ₄ ceramics have been fabricated via freeze casting and sintering. The as-sintered samples were pre-oxidized at 1200-1400 °C for 15 min. The effect of pre-oxidation temperature on the microstructure, flexural strength, and dielectric properties of porous Si ₃N ₄ 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 14.2 MPa at 1300 °C, but finally increased to 25.6 MPa at 1400 °C, while the dielectric constant decreased gradually over the frequencies ranging from 8.2 GHz to 12.4 GHz. This simple process allows porous Si ₃N ₄ ceramics to have ultra-low dielectric constant and moderate strength, which will be feasible in broadband radome applications at high temperatures.