Dielectric And Tunable Properties Of Modified Barium Titanate Ceramics

Recently, field tunable dielectrics have been extensively studied due to their important applications in the field tunable devices such as voltage-controlled oscillators, tunable filters and phase shifters. In these applications, both high tunability (=[ (0)- (E)]/ (0)) and low dielectric loss are required. The currently most studied materials are perovskite SrTiO3 (ST) and (Ba,Sr)TiO3 (BST). In order to explore potential candidates for field tunable dielectric materials, the present work is focused on the investigation of the microstructure, dielectric and tunable properties of A- and B- site substituted barium titanate ceramics. The origin of the tunable dielectric characteristics is discussed.The partial substitution of Ca for Ba in barium titanate results in the (Ba1-xCax)TiO3 limited solid solution. The BaTiO3-based solid solution with tetragonal structure is obtained for x=0.1, and the CaTiO3-based solid solution with cubic structure appears for x 0.3 and becomes the major phase for x 0.7. This suggests that the tetragonal to cubic phase transition occurrs with increasing x in the present system. The dielectric constant decreases with increasing x, while the dielectric loss and the temperature dependence of dielectric constant is significantly suppressed. The Curie point keeps almost the same, but the Curie peak of dielectric constant is significantly depressed with increasing x and disappears for x=0.9. The field dependence of dielectric constant is observed, and it becomes weaker with increasing x. Considering the tunable microwave applications, the modification of the present system with incorporating SrTiO3 is worth investigating.Ba(Ti1-xSnx)O3 (BTS) forms the complete solid solutions within the selected Sn concentration range. With increasing Sn content, the Tmax of BTS solid solutions decreases almost linearly and BTS shows more DPT and relaxor behaviors. Obvious DC field dependence of dielectric properties is observed for BTS, especially at the vicinity of the phase transition temperature Tmax. The tunability increases with increasing external electric-field, while it drops when the temperature is above Tmax-With increasing Sn content, the tunability of BTS ceramics at the room temperature drops because of Tmax decreasing gradually. Ba(Sn0.15Ti0.85)03 ceramics exhibits the high tunability (~56%) and low dielectric loss (tan ~0.003) at relatively low electric-field, and BTS materials are the promising candidates for field tunable dielectrics working at the room temperature.The origin of dielectric tunability is the non-linear polarization under applied DC electric field. For the present A- and B-site substituted barium titanate materials, the origin of non-linear polarization is the spontaneous polarization of ferroelectric or ferroelectric micro-regions. The dielectric tunable behaviors of BTS are consistent with Devonshire phenomenological theory. With the high effective coefficient a(T), the higher tunability up to 84% is predicted for BTS ceramics with x=0.15 under the bias field of 40kV/cm.