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 (Na(0.5)Bio(0.5))TiO3-xSrTiO(3) (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 d(33)) and converse (large-signal d(33)*) piezoelectricity. The critical grain size of NBT-20ST and -26ST compositions for improving d(33) and d(33)* is both around 1 mu m. Rayleigh analysis and local PFM mapping indicate that the high d(33) in coarse-grained NBT-20ST samples originates from the increased extrinsic contribution and easier domain wall motion, while the large d(33)* 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/cm(3) was achieved in a 0.56 mu m-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. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.