Temperature-Insensitive High Strain in Lead-Free Bi_0.5(Na_0.84K_0.16)_0.5TiO₃-0.04SrTiO₃ Ceramics for Actuator Applications

Lead-free piezoelectric ceramics, 0.96[{Bi_0.5 (Na_0.84K_0.16)_0.5}_1-xLi_x(Ti_1-yNb_y)O₃]-0.04SrTiO₃ (BNKLiTN-ST) with x, y = 0-0.030, were synthesized by solid-state reaction method. X-ray diffraction patterns indicated that Li and Nb successfully diffused into the BNKT-ST lattice and formed a pure perovskite structure with x, y ≤ 0.025. Increasing the Li and Nb contents (x, y = 0.020) induced a phase transformation from the coexistent rhombohedral-tetragonal phases for pure BNKT-ST ceramics to a pseudocubic phase, resulting in degradation of the remnant polarization and coercive field. However, the field-induced strain was markedly enhanced at x, y = 0.020, giving rise to a giant dynamic piezoelectric constant (d₃₃∗ = S_max/E_max = 800 pm/V). Furthermore, the temperature dependence of the field-induced strain response showed temperature-insensitivity up to 120°C. To explore its potential for device applications, a 10-layered stack-type multilayer actuator was fabricated from the optimal composition (x, y = 0.020). This actuator showed a large S_max/E_max of 600 pm/V at a relatively low driving field of 4.5 kV/mm suggesting highly promising results in lead-free BNT-based ceramics.