Relationship of Composition and Microstructure to Sr/Ba Ratio of BSTN Composite Ceramics

Abstract

In our previous work, the BSTN composite ceramics in which two phases of the perovskite (BST) and the tungsten bronze (SBN) coexist were successfully prepared in situ by controlling excess components in BaO-SrO-TiO2-Nb2O5 system. What the formation, microstruc-ture and properties of BST and SBN mainly depend on Sr/Ba ratio is well known. The Sr/Ba ratio may affect the structure and properties of the composite ceramics, too. In this paper, the effects of Sr/Ba ratio on the phase composition and microstructure of the composite ceramics were investigated in detail. X-ray diffractometry (XRD) and scanning electron microscope (SEM) were used to characterize the phase composition and microstructure of the composite ceramics. The results show that the Sr/Ba ratio increases in the perovskite phase and keeps almost constant in the tungsten bronze phase with increasing the content of strontium in the two-phase-coexistence composite BSTN system. The constant Sr/Ba ratio in the tungsten bronze phase is about 0.667. The crystal lattices of the perovskite phase in composite system are larger or smaller respectively than them in pure (1-x)BaO·xSrO·TiO2 system when the Sr/Ba ratio is respectively 0.667 or 0.667. The crystal lattices of the perovskite phase in both systems show the same crystal lattices when the Sr/Ba ratio is 0.667. Affected by the ratio of Sr/Ba required in the tungsten bronze phase in the composite system, the contents of the perovskite phase decreases while the tungsten bronze phase increases with increasing ratio of Sr/Ba. For the same reason, the average grain size of the perovskite phase and the density of the composite ceramics decrease as the ratio of Sr/Ba increases. If the substitution of ions can take place both in the two phases in a composite system with two phases, the substitution will take place first in the phase with higher solubility of ions while it will be restrained in the other phase.