Ultrahigh-Q and thermally stable (Sr_1−xCa_x)₂Ce_0.665Ti_0.335O₄ microwave dielectric ceramics with low permittivity

The Sr₂Ce_0.665Ti_0.335O₄–0.75 wt% LiF-x mol% CaCO₃ (SCT-xCa, x = 0, 0.25, 0.5, 1.0, 1.5, 2.5) ceramics were prepared by a conventional solid-state reaction technique. The effect of Ca²⁺ substitution for Sr²⁺ on the phase composition, microstructure and microwave dielectric properties was investigated systematically. The coexistence of Sr₂CeO₄ and Sr₂TiO₄ without any other phases indicates that Ca²⁺ can dissolve into the SCT lattice. An appropriate amount of Ca²⁺ can accelerate the grain growth and produce a compact and homogeneous microstructure, so an ultra-high Q × f value can be achieved. The deterioration in Q × f values with further increasing x is caused by the abnormal grain growth, as well as the increased internal strain, which tends to decrease the structural stability. Meanwhile, the τ_f values were found to slightly shift toward the positive direction with decreasing unit cell volume. Excellent microwave dielectric properties of ε_r = 20.9, Q × f = 168,580 GHz (8.14 GHz) and τ_f =  + 0.8 ppm/^oC can be obtained in the x = 0.25 sample sintered at 1150 °C for 4 h, showing great potentials in millimeter wave devices.