Improved electromagnetic absorbing properties of Si₃N₄-SiC/SiO₂ composite ceramics with multi-shell microstructure

Porous Si₃N₄-SiC composite ceramic was fabricated by infiltrating SiC coating with nano-scale crystals into porous β-Si₃N₄ ceramic via chemical vapor infiltration (CVI). Silica (SiO₂) film was formed on the surface of rod-like Si₃N₄-SiC grains during oxidation at 1100°C in air. The as-received Si₃N₄-SiC/SiO₂ composite ceramic attains a multi-shell microstructure, and exhibits reduced impedance mismatch, leading to excellent electromagnetic (EM) absorbing properties. The Si₃N₄-SiC/SiO₂ fabricated by oxidation of Si₃N₄-SiC for 10h in air can achieve a reflection loss of -30dB (>99.9% absorption) at 8.7GHz when the sample thickness is 3.8mm. When the sample thickness is 3.5mm, reflection loss of Si₃N₄-SiC/SiO₂ is lower than -10dB (>90% absorption) in the frequency range 8.3-12.4GHz, the effective absorption bandwidth is 4.1GHz.