Electric-field-recoverable large shape memory in BNT-based lead-free ceramics

Abstract

Shape-memory ceramics exhibit several unique advantages such as high efficiency, quick response and low energy consumption, making them irreplaceable in some actuators, especially for lead-free ceramics with low electric-field-recoverable large strains. In this work, a critical non-ergodic relaxor ferroelectric state, in which the electric field long-range ordered ferroelectric state in the poled sample can be switched back to the initial non-ergodic relaxor state by applying a low reverse electric field, was found interestingly in (Bi_0.5Na_0.5)TiO₃-based lead-free ceramics in terms of in situ high-energy synchrotron XRD. The results indicate that the polarization reorientation process during the loading of reverse electric fields occurs through polarization contraction/extension instead of polarization rotation found in normal ferroelectrics, leading to the totally recoverable remanent strain. As a result, a large strain S_M of ∼0.25%, which can be restored by a relatively low electric field of ∼2.3 kV mm⁻¹ or heat treatment above 50 °C, was achieved. The polarization contraction/extension during polarization reorientation using a medium non-ergodic relaxor ferroelectric state could provide guidance for the design other lead-free ceramics with excellent shape-memory properties.