Formation of the core-shell microstructure in lead-free Bi_1/2Na_1/2TiO₃-SrTiO₃ piezoceramics and its influence on the electromechanical properties

The Bi_1/2Na_1/2TiO₃-based materials exhibit the largest electric-field-induced strains among lead-free piezoceramics and are considered as promising candidates for actuation applications. A typical representative of this group is (1-x)Bi_1/2Na_1/2TiO₃-xSrTiO₃, where its excellent electromechanical properties were recently related to the existence of a core-shell microstructure. Although the latter was also reported in other Bi_1/2Na_1/2TiO₃-based ceramics, the formation mechanism remains unknown. In the present work we therefore first investigated the solid-state reaction occurring during calcination using simultaneous thermogravimetric analysis, X-ray diffraction, scanning and transmission electron microscopy. The reaction occurred in two steps, whereby the cores and shells had different formation reaction temperatures, which resulted in a metastable heterogeneous microstructure. Furthermore, a series of sintered samples with different relative densities, grain sizes, and core densities was prepared. Modifications of these microstructural parameters resulted in variation of the maximal strain by 17% and in the electric-field required to trigger the phase transitions by 38%.