Microstructure and dielectric properties of highly tunable Ba _0.6Sr _0.4TiO ₃/MgO/Al ₂O ₃/ZnO composite

The effect of co-doping ZnO, Al ₂O ₃ and MgO on the microstructures and dielectric properties of Ba _0.6Sr _0.4TiO ₃ ceramics prepared by the mixed-oxide route at 1400 °C was investigated. And a comparison criterion for the BST-based tunable ceramics was proposed. The results show that a cubic perovskite phase Ba _0.6Sr _0.4TiO ₃ and a face-centered-cubic spinel phase Mg(Zn)Al ₂O ₄ were observed in BST/MgO/Al ₂O ₃/ZnO (BMAZ) composite ceramics. When Zn ²⁺ content exceeds solid solubility limit in the BST, excess ZnO will react with Al ₂O ₃ to produce ZnAl ₂O ₄ phase. With the increase of ZnO, the paraelectric-ferroelectric phase transition points of BMAZ ceramics shift to the lower temperatures, and the maximum of dielectric constant increased first and then decreased. A higher tunability which amounts to 27.4% under 1.0 kV/mm biasing was obtained for BMAZ ceramics doped with 1.0 wt% ZnO.