Powder Preparation And Dielectric Property Of Low-sintering BST Ceramics

Barium Strontium Titanate (Ba1-xSrxTiO₃,BST) ferroelectric materials have excellent electrical properties. Pure BST and BST-based materials are commonly sintered at >1350°C, which is too high for using silver electrodes and co-firing with LTCC multilayer modules (850-900°C). In this paper, Ba0.6Sr0.4TiO₃ powders with high activity by modified oxalate co-precipitation method were chosen as a basic ferroelectric BST ceramic. The different combination of Bi2O3-ZnO-Nb2O5, B₂O₃-Li₂CO₃ and B₂O₃-SiO₂–Li₂CO₃ were tested as sintering aids. The stabilization mechanism of [H2TiO(C2O4)2] (HTO) was studied. In order to obtain clear HTO solution, the disturbance of pH value must be avoided and proper quantities of H2O must be added to inhibit the polymerization of [TiO(C2O4)2]2-. Through adding quantitative ammonia into a precursor solution containing stoichiometric quantities of Ba and Sr ions before the co-precipitation procedure, a simple oxalate co-precipitation method based one-step cation-exchange reaction between the stoichiometric solutions of oxalotitanic acid (H2TiO(C2O4)2:HTO) and barium+strontium nitrate is investigated successfully for the quantitative precipitation of barium–strontium titanyl oxalate (Ba0.6Sr0.4TiO(C2O4)2·4H2O:BSTO) precursor powders. The pyrolysis of BSTO produced the homogeneous barium–strontium titanate (Ba0.6Sr0.4TiO₃: BST) powders. The BST powders were micron-sized with regular near-spherical aggregate structure. The BST powders possess high fluidity as well as high specific surface area of 18.52g/m2 and thus can be sintered at rather lower sintering temperature and broader temperature range. The BST-BZN samples can be sintered at 1050℃. Also the Curie peak was suppressed and broadened withαTin of 2.93% andαTde of 3.78% in the range of -40-80℃. However, the tunability was only 0.15% when a DC field of 1kV/mm was installed (measured at 1MHz). It can be explained considering the substitution of Bi³⁺ to Ba²⁺/Sr²⁺, the decrease of BST grain size and the increase of grain boundary. B₂O₃-Li₂CO₃ effectively decreased the sintering temperature of BST. The BST with 2.32wt%Li₂CO₃-0.68wt%B₂O₃ can be sintered at as low as 850°C. Some B³⁺ and Li+ enter the BST crystal lattice. The tunability of BST-BL samples was over 15% when a DC field of 1kV/mm was installed (measured at 1MHz). BST-BSL can be sintered at relatively higher temperature and broader temperature range. And thus the grain size of BST can be adjusted easily by altering the sintering temperature. The change of liquid property during the sintering process was due to the complex chemical reaction between B₂O₃-SiO₂–Li₂CO₃ and BST. The permittivity-temperature character of BST-BSL was improved while the tunability was decreased. The tunability of BST-BSL samples was over 5% when a DC field of 1kV/mm was installed (measured at 1MHz).