Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting

Cf–Si3N4 sandwich composites were prepared by gelcasting using α-Si3N4 powder, SiC-coated carbon fibers and sintering additives as starting materials. The microstructure and composition, dielectric properties of Cf–Si3N4 sandwich composites were investigated. SEM and EDS analysis results reveal that the SiC interphase could effectively overcome incompatibility between carbon fiber and silicon nitride matrix under the condition of pressure-less sintering at 1700 °C. The investigation of microwave absorbing property reveals that, compared with the Si3N4 ceramics, both the real (ε׳ε׳) and imaginary (ε׳׳ε׳׳) permittivity of Cf–Si3N4 sandwich composites show strong frequency dispersion characteristics at X-band. Microwave absorption ability of the Cf–Si3N4 sandwich composites are significantly enhanced compared with pure Si3N4 ceramic, and the reflection loss gradually decreases from −3.5 dB to −14.4 dB with the increase of frequency, while the pure Si3N4 ceramic keeps at −0.1 dB. Particularly, the relationship between permittivity of Cf–Si3N4 sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. Results showed that both ε׳ε׳ and ωε׳׳ωε׳׳ are inversely proportional to the frequency square ω2ω2, and the predicted results agree quite well with the measured data.