Si3N4/nano-carbon black (CB) composite ceramics with boron nitride (BN) as interphase were fabricated by gelcasting. The results of scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that the structures of BN interphase and BN coated CB were formed and were uniform distributed in the Si3N4 ceramic. The permittivity (ε′ε″) of BN-coated CB was obviously lower than that of the pure CB, and the impedance of the BN-coated CB particles was close to the Z0 (Z0 = 1, Z0 is impedance under vacuum) compared with that of the CB particles; therefore, electromagnetic wave can enter into the BN-coated CB particles and be dissipated due to the high conductivity of CB particles. When incorporating the BN(CB) into Si3N4 substrate, the electromagnetic wave can pass through Si3N4/nano-CB composite ceramics easily and be absorbed by CB, so the tgδ of the composite ceramic increased as the mass content of BN(CB) increased. The tgδ of the composite ceramic was about 0.43 when the content of BN(CB) increased to 15 wt%.

Gelcasting was employed to fabricate Si 3 N 4 /SiC whisker (SiCw) composite ceramics, and the effects of heattreatment temperature on the length-to-diameter ratio of the whiskers and SiCw content on microwave dielectric properties were studied. Compared with pure SiCw of spherical structure obtained at temperature of 1,750°C(Ar), pure SiCw treated at 1,600°C(Ar) showed rod-like structure, higher dielectric properties and more evenly distribution in Si 3 N 4 /SiCw composite ceramics. Both the real (e 0 ) and imaginary (e 00 ) permittivity of Si 3 N 4 /SiC whisker (SiCw) composite ceramics decreased with increasing frequency and increased as the whisker content raised owing to the interface and SiCw playing a role of dipole in the frequency range of 8. 2-12.4 GHz. In addition, comparing the ceramics with lower content of SiCw, the reflectivity of the composite ceramics moved to a lower frequency; the maximum absorption peak reached -22.4 dB at the whisker content of 15 wt%.