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 °C. Additionally, the sintering temperature of Al₂O₃ ceramics via cold-sintering assisted two-step sintering route is 100 °C lower than that via direct conventional sintering. The microwave dielectric properties, including ε_r and Qf, exhibit enhancements with increasing cold sintering pressure and post-sintering temperature. The measured ε_r closely aligns with the theoretical value, indicating a successful densification. Qf achieves its peak of 100,220 GHz at 1575 °C under 375 MPa, nearly 2 times higher than the value obtained at 125 MPa (53,280 GHz). The optimal microwave dielectric properties (ε_r = 9.67, Qf = 100,220 GHz, and τ_f = − 54.5 ppm/°C) are achieved at a cold sintering pressure of 375 MPa and post sintering temperature of 1575 °C.