Dependence of microwave dielectric properties on ceramic connectivity in ultralow-<i>ε</i>_r Al₂O₃ porous ceramics

Introducing pores is an effective approach for fabricating ultralow-ε r ceramics; however, it is still unclear how the microstructures affect the microwave dielectric properties. In the present work, Al2O3-A and Al2O3-B porous ceramics were prepared by incomplete sintering at 1,000-1,650 °C and sintering at 1,650 °C with a porogen, respectively, so that the effects of porosity and ceramic connectivity can be clarified. The introduction of pores led to a significant decrease in ε r, Qf , and |τf| , and Al2O3-B ceramic exhibits a slightly higher ε r, a larger |τf| , and a much higher Qf value compared to the Al2O3-A counterpart with a similar relative density. For Al2O3-A, increasing the sintering temperature and relative density notably enhanced ceramic connectivity, while Al2O3-B's ceramic connectivity remained relatively insensitive to density variations. The improved ceramic connectivity in Al2O3-B enhanced the contribution of the ceramic phase to ε r and τf , while also increased the Qf value by reducing ceramic-pore interfaces. Notably, Al2O3-B, with a low relative density of 48.31%, demonstrated a good combination of microwave dielectric properties, with ε r = 4.16, Qf = 38,400 GHz, and τf = -44.3 ppm/°C. These findings 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.