Large strain with low hysteresis in Sn-modified Bi_0.5(Na_0.75K_0.25)_0.5TiO₃ lead-free piezoceramics

For the sake of excellent strain performance for actuator application, Sn-modified Bi_0.5Na_0.5TiO₃–Bi_0.5K_0.5TiO₃ (BNT–BKT) ceramics were designed and prepared by solid-state reaction method. The crystal structures and microstructures of Bi_0.5(Na_0.75K_0.25)_0.5(Ti_1−xSn_x)O₃ (abbreviated as BNKT–xSn, x = 0, 0.02, 0.05, 0.08) were systematically investigated together with their strain performance. It is found that all the compositions possessed a single perovskite structure phase by X-ray diffraction patterns. With increasing Sn content, the ferroelectric phase with mainly tetragonal structure gradually transformed into the ergodic relaxor phase in pseudocubic structure with nanodomains further confirmed by high-resolution transmission electron microscope images. Both the P–E and I–E loops confirmed this structural change and the coexistence of ferroelectric phase and ergodic relaxor phase at x = 0.02, where a large strain of 0.37% was achieved with low hysteresis (21.8%). By analyzing the bipolar and unipolar strain curves and the origin of strain, we believe that the large strain is contributed to the phase transition from ferroelectric phase into ergodic relaxor phase, and the low hysteresis is beneficial from the existence of ergodic relaxor phase, which should pave a way for future developing high-performance actuators.