Origin of the large strain response in (K0.5Na0.5)NbO3-modified (Bi0.5Na0.5)TiO3–BaTiO3 lead-free piezoceramics

TLDR

Giant unipolar strain has been observed in the lead‑free piezoceramic 0.92(Bi0.5Na0.5)TiO3−0.06BaTiO3−0.02(K0.5Na0.5)NbO3. The study examined the validity of a previously proposed mechanism attributing the high strain to both a volume change during a field‑induced antiferroelectric‑to‑ferroelectric phase transition and domain contributions from the induced ferroelectric phase. To test this mechanism, the authors measured simultaneous longitudinal and transverse strains on disk‑shaped samples and performed temperature‑dependent hysteresis measurements on unpoled samples of a nearby composition. The measurements revealed that the phase transition involves no significant volume change, indicating that volume change contributes little to the total strain, and that the large unipolar strain originates from a nonpolar phase that restores the material to its unpoled state after the field is removed. The work references S.-T.

Abstract

The mechanism of the giant unipolar strain recently observed in a lead-free piezoceramic, 0.92(Bi0.5Na0.5)TiO3−0.06BaTiO3−0.02(K0.5Na0.5)NbO3 [S.-T. Zhang, A. B. Kounga, E. Aulbach, H. Ehrenberg, and J. Rödel, Appl. Phys. Lett. 91, 112906 (2007) was investigated. The validity of the previously proposed mechanism that the high strain comes both from a significant volume change during the field-induced phase transition, from an antiferroelectric to a ferroelectric phase and the domain contribution from the induced ferroelectric phase was examined. Monitoring the volume changes from the simultaneously measured longitudinal and transverse strains on disk-shaped samples showed that the phase transition in this specific material does not involve any notable volume change, which indicates that there is little contribution from a volume change due to the phase transition to the total strain response. Temperature dependent hysteresis measurements on unpoled samples of a nearby ferroelectric composition, 0.93(Bi0.5Na0.5)TiO3−0.06BaTiO3−0.01(K0.5Na0.5)NbO3 demonstrated that the origin of the large strain is due to the presence of a nonpolar phase that brings the system back to its unpoled state once the applied electric field is removed, which leads to a large unipolar strain.