Enhanced microwave dielectric properties of Al2O3 ceramics via a cold sintering assisted two-step sintering route

The effects of cold sintering assisted two-step sintering on the structural evolution and microwave dielectric properties of Al2O3 ceramics are systematically investigated in this work. XRD analysis reveals that the crystal structure remains stable throughout the cold sintering and following high-temperature sintering processes. Rietveld refinement confirms the trigonal Al2O3 configuration and the absence of impurities. Increasing cold sintering pressure results in improved relative density, reaching over 94% after post-sintering at 1575 degrees C. Additionally, the sintering temperature of Al2O3 ceramics via cold-sintering assisted two-step sintering route is 100 degrees C lower than that via direct conventional sintering. The microwave dielectric properties, including epsilon(r) and Qf, exhibit enhancements with increasing cold sintering pressure and post-sintering temperature. The measured epsilon(r) closely aligns with the theoretical value, indicating a successful densification. Qf achieves its peak of 100,220 GHz at 1575 degrees C under 375 MPa, nearly 2 times higher than the value obtained at 125 MPa (53,280 GHz). The optimal microwave dielectric properties (epsilon(r) = 9.67, Qf = 100,220 GHz, and tau(f) = - 54.5 ppm/degrees C) are achieved at a cold sintering pressure of 375 MPa and post sintering temperature of 1575 degrees C.