Zero temperature coefficient microwave alumina ceramics via TiO₂ doping and post sinter annealing

Alumina (Al₂O₃) is a promising candidate for microwave dielectric ceramics due to its exceptional quality factor (Qf ~ 333,000 GHz). This study investigates the influence of additions of titania (TiO₂) and annealing treatments on the dielectric properties of polycrystalline Al₂O₃. The dielectric properties of Al₂O₃-x wt% TiO₂ (x = 2.5, 5, 7.5, 10) were evaluated after various heat treatment conditions. The optimal composition, Al₂O₃-7.5 wt% TiO₂, achieved notable properties when sintered at 1400 °C for 8 h: a relative permittivity (εr) of 10.73 and a quality factor-frequency product (Qf) of 126,347 GHz at 13.814 GHz. Specifically, subsequent annealing at 1000 °C for 5 h adjusted the temperature coefficient of frequency ( $$\:{\tau}_{f}$$ ) to + 1.25 ppm/°C, near zero. The study reveals that the thermal treatment induces the decomposition of Al₂TiO₅ into Al₂O₃ and TiO₂, accompanied by titanium redistribution. Characterization using X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, and dielectric measurements established correlations between the microstructure and properties. This work provides a scalable framework for designing ultra-stable millimeter-wave ceramics, particularly for 5G/6G resonators and satellite communication systems, where minimal signal delay and thermal drift are critical.