Crystallization Mechanism of CVD Si₃N₄–SiCN Composite Ceramics Annealed in N₂ Atmosphere and Their Excellent EMW Absorption Properties

To tailor a new electromagnetic wave (EMW) absorbing material with lower reflection coefficient (RC) and larger operating frequency band, the CVD Si₃N₄–SiCN composite ceramics were prepared from SiCl₄–NH₃–C₃H₆–H₂–Ar system and then annealed at the temperatures of 1400–1700°C in N₂ atmosphere. Effect of the annealing temperatures on the microstructure, phase composition, permittivity, and microwave-absorbing properties of the ceramic were investigated. Results showed that the CVD Si₃N₄–SiCN ceramics gradually crystallized into nanosized SiC grains, Si₃N₄ grains and graphite (T ≤ 1600°C), and then the grains grew up at T = 1700°C. The permittivity, dielectric loss, and electrical conductivity of as-annealed CVD Si₃N₄–SiCN ceramics (T ≤ 1600°C) increased firstly due to the formation of conductivity and polarity network and the increase in nanograin boundary, and then decreased at 1700°C because of the growth of nanograins and the disappearance of nanograin boundary. The minimal RC and effective absorption bandwidth of the as-annealed CVD Si₃N₄–SiCN ceramic at 1600°C was −41.67 dB at the thickness of 2.55 mm and 3.95 GHz at the thickness of 3.05 mm, respectively, demonstrating that the totally crystallized CVD Si₃N₄–SiCN ceramic (T = 1600°C) had the superior microwave-absorbing ability.