Properties of BST ceramics prepared by high temperature hydrothermal process

Barium strontium titanate (BST) was produced in a pressure vessel at 220 °C using the hydrothermal technique. Ba and Sr concentrations, temperature, reaction time and Ti concentration were varied to study the effects of processing on the formation of BST. The chemical composition and crystal structure were analysed by X-ray diffractometry. Peak fitting software was used to separate contributions from different BST phases and to calculate the relative volumes of the phases. From the obtained data Ba losses relative to initial Ba:Sr ratio were estimated. It was found that the amount of Ti was critical for BST phase formation. A small amount of Ti, less than 0.19 mol/l, in a Ba rich solution leads to a single-phase structure, as Sr greatly outperforms Ba. An increase of Ti above 0.225 mol/l decreases Ba losses and creates a solid solution of two phases: one is tetragonal, almost Sr-free, and the other is cubic BST. When the initial concentration of Sr is increased above 30% the obtained BST became triple-phased with different Ba:Sr ratios.