Dependence of microwave dielectric properties on ceramic connectivity in ultralow-ε_r Al₂O₃ porous ceramics

It is an effective approach to fabricate ultralow-εr ceramics by introducing pores, while it is still unclear how the microstructures affect the microwave dielectric properties. In the present work, Al₂O₃-A and Al₂O₃-B porous ceramics are prepared by incomplete sintering at 1000–1650 ^oC and sintering at 1650 ^oC with introducing porogen, respectively, so that the effects of porosity and ceramic connectivity can be clarified. The introducing of pores leads to the significant decrease in ε_r, Qf, and |τf|, and Al₂O₃–B ceramic exhibits a slightly higher εr, a larger |τf|, and a much higher Qf value than the Al₂O₃–A counterpart with the similar relative density. With increasing the sintering temperature and relative density for Al₂O₃–A, the ceramic connectivity is significantly improved, while it is insensitive to relative density for Al₂O₃–B. The improved ceramic connectivity in Al₂O₃–B porous ceramics enhances the contribution of ceramic phase to the ε_r and τf of porous ceramics, and increases the Qf value through decreasing the ceramic-pore interfaces. The good combination of microwave dielectric properties with ε_r = 4.16, Qf = 38,400 GHz, and τf = -44.3 ppm/°C is obtained in Al₂O₃–B ceramic with low relative density of 48.31%. Such results reveal the strong dependence of microwave dielectric properties on ceramic connectivity in porous ceramics, and also inspire the development of ultralow‐ε_r materials by regulating both the porosity and ceramic connectivity.