Grain size dependent physical properties in lead-free multifunctional piezoceramics: a case study of NBT-xST system

Grain size effect is one of the most important issues to develop next-generation functional devices. In this work, we firstly provide a systematic investigation on the grain size dependent physical properties based on a flexible (Na0.5Bi0.5)TiO3-xSrTiO3 (NBT-xST) system with multifunctionality. The NBT-20ST, −26ST and −35ST with multiple phase boundaries/structures were chosen as the studied compositions. The densified ceramics with a series of grain sizes were successfully fabricated by normal and two-step sintering method. For NBT-20ST and −26ST compositions, the coarse grain size is more favorable for improving the direct (small-signal d33) and converse (large-signal d33∗) piezoelectricity. The critical grain size of NBT-20ST and −26ST compositions for improving d33 and d33∗ is both around 1 μm. Rayleigh analysis and local PFM mapping indicate that the high d33 in coarse-grained NBT-20ST samples originates from the increased extrinsic contribution and easier domain wall motion, while the large d33∗ in coarse-grained NBT-26ST samples stems from the polarization enhancement through a linear electrostrictive effect. For NBT-35ST composition, an improved energy storage performance with high recoverable energy density over 1 J/cm3 was achieved in a 0.56 μm-sized sample owing to a large fraction of low-polarizability grain boundary layer. This study opens up a new way for designing novel lead-free multifunctional piezoceramics with superior electromechanical properties.