Design and preparation of sandwich structured Si₃N₄ ceramics for broadband microwave transmission

To develop high-temperature structural materials with broadband microwave transmission ability used for high-speed aircraft radome, a sandwich-structured silicon nitride (Si₃N₄) ceramic consisting of porous core and dense shell is designed based on CST simulation, which helps to obtain the optimized structural and dielectric properties. After that, its near-net-shape preparation strategy is presented by a controllable combined process including gel-casting (GC), polymer infiltration and pyrolysis (PIP), and chemical vapor deposition (CVD). The mechanical and electromagnetic performances were studied. The results show that the optimally-structured sandwich Si₃N₄ processes a flexural strength of 216.5 MPa and a microwave transmissivity of 80 % in 2–13.4 GHz, which are obviously higher than that of traditional Si₃N₄ ceramics. Besides, this sandwich-structured Si₃N₄ ceramics also shows excellent dielectric and mechanical stabilities after a high-temperature oxidization at 1600 °C for 2 h in air atmosphere. Our prepared Si₃N₄ ceramics achieves excellent mechanical strength, high microwave transmissivity, and good high temperature stability, indicating a great application potential as high-speed aircraft radome materials.