Defects evolution and temperature stability of low-loss magnesium titanate stannate - calcium strontium titanate microwave dielectric ceramics

Magnesium titanate stannate - calcium strontium titanate (MgTi_0.95Sn_0.05O₃-xCa_0.8Sr_0.2TiO₃; abbreviated as MTS-xCST, x = 0.02, 0.04, 0.06, 0.08) microwave dielectric ceramics were synthesized via the conventional solid-state reaction. The optimum densities corresponded to higher sintering temperatures than the optimum quality factor, so the ceramics were classified as MTS-xCST-D and MTS-xCST-Q. All compositions exhibited a composite phase consisting of the rhombohedral structure MTS and the orthorhombic structure CST. As the sintering temperature rose, the relative densities of the ceramics increased significantly, while the formation of oxygen vacancies and titanium reduction were intensified. Finally, the x = 0.06 compositions sintered at 1425 °C (MTS-0.06CST-D) and 1375 °C (MTS-0.06CST-Q) had excellent temperature stability and low loss. At low frequency, the change of dielectric permittivity at 25–150 °C did not exceed 0.1, and the loss tangent was less than 0.003. In addition, microwave dielectric properties of MTS-0.06CST-D (ε_r = 20.4, Qf = 63,770 GHz, τ_f = −1.8 ppm/°C) and MTS-0.06CST-Q (ε_r = 18.4, Qf = 74,740 GHz, τ_f = −4.3 ppm/°C) compositions were obtained. These results suggest that MST-0.6CST ceramics could be a candidate material for 5 G microwave communication devices.