Crystallization Mechanism of CVD Si 3 N 4 –Si CN Composite Ceramics Annealed in N 2 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 Si 3 N 4 –Si CN composite ceramics were prepared from SiCl 4 – NH 3 –C 3 H 6 –H 2 –Ar system and then annealed at the temperatures of 1400–1700°C in N 2 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 Si 3 N 4 –Si CN ceramics gradually crystallized into nanosized SiC grains, Si 3 N 4 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 Si 3 N 4 –Si CN 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 Si 3 N 4 –Si CN 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 Si 3 N 4 –Si CN ceramic ( T = 1600°C) had the superior microwave‐absorbing ability.