Effect of cooling rate on microstructure and microwave dielectric properties of MgO doped (Sr,Ca)TiO₃-(Sm,Nd)AlO₃ ceramics

The effect of MgO additive on phase composition, microstructure, sintering behavior, and microwave dielectric properties of 0.7(Sr_0.01Ca_0.99)TiO₃-0.3(Sm_0.75Nd_0.25)AlO₃ (7SCT-3SNA) ceramics prepared via conventional solid-state route were systematically investigated. MgO as additive showed no obvious influence on the phase composition of 7SCT-3SNA ceramics and all the samples exhibited pure perovskite structures. The presence of MgO additive effectively reduced the sintering temperature of 7SCT-3SNA ceramics from 1500 to 1380 °C. Besides, it is found that 0.5 wt% MgO could improve the uniformity of the grain morphology significantly, thus Q × f value enhanced consequently. Results also showed that enhanced Q × f value could be obtained as cooling rate slowed down. Superior microwave dielectric properties with an ε _r of 44.96, a Q × f value of 45738 GHz (at 5.5 GHz), and τ _f value of −2.46 ppm/°C are obtained for 0.5 wt% MgO doped 7SCT-3SNA ceramics sintered at 1380 °C with a cooling rate of 60 °C/h.