Effects of ZnO additive on crystalline phase and microwave dielectric properties of 0.90Al₂O₃–0.10TiO₂ ceramics

0.90Al₂O₃–0.10TiO₂ doped with x mol% ZnO (x = 0.2, 0.3, 0.4, 0.5) microwave dielectric ceramics, have been prepared by the conventional solid-state method with various sintering temperatures. A relationship between phase composition and microwave dielectric properties was systematically discussed for millimeter-wave applications. The formation of Al₂TiO₅ impurity phase was inhibited by ZnO addition without annealing treatment. The existence of Al₂TiO₅ impurity phase deteriorated seriously the microwave dielectric properties, especially quality factor (Q × f) and temperature coefficient (τ_f). Due to the liquid sintering effect, the addition of ZnO could reduce the sintering temperature to 1300 °C. To a large extend, ZnO additive improved the density of 0.90 Al₂O₃–0.10 TiO₂ ceramics, and ultimately enhanced the microwave dielectric properties. The ZnAl₂O₄ impurity phase could also enhance the Q × f value and permittivity (ε_r) of 0.90Al₂O₃–0.10TiO₂ ceramics. Excellent microwave dielectric properties was achieved for 0.4 mol% ZnO doped 0.90Al₂O₃–0.10TiO₂ ceramics sintered at 1300 °C for 4 h: ε_r = 12.4, Q × f = 159,000 GHz, τ_f = −0.2 ppm/°C.