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

Abstract

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.

Multiscale designed SiCf/Si3N4 composite for low and high frequency cooperative electromagnetic absorption

“…By now, most of researches on dielectric CMCs focus on the target frequency range of X‐band (8.2‐12.4 GHz) and Ku‐band (12.4‐18 GHz). Current researches mostly focus on broadening the absorbing frequency range since the reflection loss (RL) of CMCs generally adopts a concave relationship with increasing frequency . Meanwhile, the absorbing properties of these materials would be impaired at high temperature due to permittivity changing with increasing temperature .…”

“…Current researches mostly focus on broadening the absorbing frequency range since the reflection loss (RL) of CMCs generally adopts a concave relationship with increasing frequency. [7][8][9][10][11][12] Meanwhile, the absorbing properties of these materials would be impaired at high temperature due to permittivity changing with increasing temperature. 7,8,10 Thus, the specific EM wave absorbing material adopting the convex RL function with increasing frequency, which would realize strong absorptions at both high and low frequency in [8][9][10][11][12][13][14][15][16][17][18] GHz in a relative large temperature range, is critical but not particularly designed and investigated by now.…”

Microstructure and EMW absorption properties of CVI Si3N4–SiCN ceramics with BN interface annealed in N2 atmosphere

“…The most widely used high-temperature electromagnetic wave (EMW) absorbers are nanocarbon materials, [5][6][7] SiC grains or fibers 8,9 and SiCN-based ceramics, [10][11][12] which have low density, high environmental stability, and excellent electromagnetic wave attenuation characteristics. 16,17 The amorphous SiCN ceramics prepared by polymer derived ceramics (PDC) route such as pyrolysis of siliconbased organometallic precursors had lower complex permittivity. At present, it is still an issue to design and fabricate dielectric ceramics with strong absorbing properties at broader frequency band.…”

“…At present, it is still an issue to design and fabricate dielectric ceramics with strong absorbing properties at broader frequency band. 18,19 The microstructure and EMW absorbing properties of CVI Si 3 N 4 -SiCN ceramics before and after annealed at the temperature of 1400°C-1700°C had been studied, 17 and the minimum RC and the maximum EAB of the composite ceramics with C-SiC-Si 3 N 4 phase annealed at 1600°C was À41.67 dB and 3 GHz, indicating the annealed CVI Si 3 N 4 -SiCN ceramics had excellent EMW absorbing properties. [13][14][15] The electromagnetic properties of SiCN-based ceramics depend on its phase composition and microstructure which is affected by the preparation process and crystallization degrees.…”

“…[13][14][15] The electromagnetic properties of SiCN-based ceramics depend on its phase composition and microstructure which is affected by the preparation process and crystallization degrees. 16,17 The amorphous SiCN ceramics prepared by polymer derived ceramics (PDC) route such as pyrolysis of siliconbased organometallic precursors had lower complex permittivity. 12,16 The phase composition of PDC SiCN ceramics varies with the increase in annealing temperatures, and the lowest reflection coefficient (RC) of À53 dB and the maximum effective absorbing bandwidth (EAB) of 3.0 GHz was obtained after annealing at 1500°C, but their mechanical properties were not reported.…”