Electromagnetic properties of porous Si3N4 ceramics with gradient distributions of SiC and pores fabricated by directional in–situ nitridation reaction

Abstract Porous Si 3 N 4 ceramics with gradient distributions of SiC and pores are fabricated by directional nitridation sintering diatomite preforms with phenolic resin as a carbon source and pore forming agent. The nitrogen flow rate during sintering and the phenolic resin content in the preforms greatly impact the distribution of SiC and pores on the ceramics. Trace SiC and high porosity make the ceramics show weak surface impedance mismatch at their upper surface, so electromagnetic waves can enter the ceramics from their upper surfaces with little reflection. The enhancement of interfacial polarization with increasing SiC in the downward direction is the reason for the excellent electromagnetic wave absorption by the ceramics. The ceramics are excellent electromagnetic wave absorbing materials with a mean reflectivity far lower than −10 dB in the frequency range of 8–18 GHz.