Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting

Abstract C f –Si 3 N 4 sandwich composites were prepared by gelcasting using α -Si 3 N 4 powder, SiC-coated carbon fibers and sintering additives as starting materials. The microstructure and composition, dielectric properties of C f –Si 3 N 4 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 Si 3 N 4 ceramics, both the real ( e ׳ ) and imaginary ( e ׳ ׳ ) permittivity of C f –Si 3 N 4 sandwich composites show strong frequency dispersion characteristics at X-band. Microwave absorption ability of the C f –Si 3 N 4 sandwich composites are significantly enhanced compared with pure Si 3 N 4 ceramic, and the reflection loss gradually decreases from −3.5 dB to −14.4 dB with the increase of frequency, while the pure Si 3 N 4 ceramic keeps at −0.1 dB. Particularly, the relationship between permittivity of C f –Si 3 N 4 sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. Results showed that both e ׳ and ω e ׳ ׳ are inversely proportional to the frequency square ω 2 , and the predicted results agree quite well with the measured data.