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

Highly porous Si3N4 ceramics have been fabricated via freeze casting and sintering. The as-sintered samples were pre-oxidized at 1200-1400 degrees C for 15 min. 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 degrees C to 1400 degrees C, firstly, the flexural strength of the pre-oxidized specimens remained almost constant at 1200 degrees C, and then decreased to 14.2 MPa at 1300 degrees C, but finally increased to 25.6 MPa at 1400 degrees C, while the dielectric constant decreased gradually over the frequencies ranging from 8.2 GHz to 12.4 GHz. 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. (c) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.