Pyrolytic carbon （PyC） was infiltrated into silicon nitride （Si3N4） ceramics by precursor infiltration and pyrolysis （PIP） of phenolic resin, and Ni nanoparticles were added into the phenolic resin to change the electric conductivity of Si3N4-PyC composite ceramics. Dielectric permittivity, electromagnetic interference （EMI） shielding and absorption properties of Si3N4-PyC composite ceramics were studied as a function of Ni content at 8.2-12.4 GHz （X-band）. When Ni nanoparticles were added into phenolic resin, the electric conductivity of the prepared composite ceramics decreased with increasing Ni content, which was attributed to the decrease of graphitization degree of PyC. The decrease in electric conductivity led to the decrease in both permittivity and EMI shielding effectiveness. Since too high permittivity is harmful to the impendence match and results in the strong reflection, the electromagnetic wave absorption property of Si3N4-PyC composite ceramics increases with increasing Ni content. When the content of Ni nanoparticles added into phenolic resin was 2 wt%, the composite ceramics possessed the lowest electric conductivity and displayed the most excellent absorption property with a minimum reflection loss as low as -28.9 dB.
The dielectric and electromagnetic properties of two types of SiC fibres with different compositions were investigated. The permittivity and electromagnetic shielding effectiveness （SE） of SiC fibre bundles were measured in 8.2-12.4 GHz by waveguide method. The reflection coefficient （RC） of unidirectional SiC fibre laminates was determined in 8-18 GHz using naval research laboratory （NRL）-arc method. Results showed that the electromagnetic wave （EMW） absorbing properties of SiC fibres were correlated with their composition, microstructure and instinct performance of electrical resistance. SiC fibres with higher content and greater size of nano-scale H-SiC showed higher permittivity, conductivity, SE and lower RC, which resulted in their better EMW absorbing ability, Le. the lower reflection to EMW.
