Research On Barium Strontium Titanate Ceramics For Tunable Microwave Application

Research works on the novel tunable ferroelectric materials are extremely significant to modern radar and microwave communication technology. The related research works presently focus on how to decrease the dielectric loss of the material and maintain a relatively high tunability. In this thesis, we have completed following research works for this purpose: Ba_0.6Sr_0.4TiO₃ was doped with low-loss, perovskite microwave ceramics including La(Mg0.5Ti0.5)O3, s:s: and Sr(Ga0.5Ta0.5)O3. The results showed that they formed single-phase solid solution with Ba_0.6Sr_0.4TiO₃ and decreased the dielectric loss and dielectric constant; Dielectric loss and dielectric constant decreased with the increase of the doping content; However, the tunability of Ba_0.6Sr_0.4TiO₃ doped with La(Mg_0.5Ti_0.5)O₃ or La(Zn_0.5Ti_0.5)O₃ decreased severely, a doping content more than 10mol% resulted in a tunability less than 2%. Higher tunability was found in Sr(Ga_0.5Ta_0.5)O₃ doped Ba_0.6Sr_0.4TiO₃. Although the tunability of the doped Ba_0.6Sr_0.4TiO₃ ceramics still needed to be improved, we believe that a tradeoff of permittivity, dielectric loss and tunability can be achieved by adjusting the doped content and Sr/Ba ratio. To improve the dielectric performance, La(Mg_0.5Ti_0.5)O₃ and La(Zn_0.5Ti_0.5)O₃ were synthesized through sol-gel process which may produce more homogeneous powders than solid state reaction. La(Zn_0.5Ti_0.5)O₃ powders were synthesized via citric acid precursor, all precursors produced a single La(Zn_0.5Ti_0.5)O₃ phase at 1000oC for 2h, though the molar ratio of citric acid to total metal cations and the pH of the starting solution were found to have an effect on the crystallization behavior; La(Zn_0.5Ti_0.5)O₃ powders were also prepared via EDTA precursor, the gel formed just when the pH of the starting solution was less than 0.5 and single-phase La(Zn_0.5Ti_0.5)O₃ was obtained at 900℃for 2h; La(Mg_0.5Ti_0.5)O3 powders were synthesized through EDTA gel combustion route. The auto-ignition of the EDTA gel directly resulted in single-phase La(Mg_0.5Ti_0.5)O₃ powder. The pH of the starting solution was found to have a significant influence on the gel formation.