Electric field-induced strain in Sr(Hf0.5Zr0.5)O3-modified Bi0.5(Na0.8K0.2)0.5TiO3 piezoelectric ceramics

Abstract

Lead-free Sr(Hf_0.5Zr_0.5)O₃-modified Bi_0.5(Na_0.8K_0.2)_0.5TiO₃ (SHZ-BNKT) ceramics were synthesized using a conventional solid-state, mixed-oxide method. In complex solid solutions such as BNKT, the role of a ternary additive is important because it can break up the long-range dipole order, which destabilizes the ferroelectric phase leading to relaxor behavior. In this system, as verified by x-ray diffraction, SHZ was incorporated into the BNKT perovskite lattice throughout the studied compositional range. The coexistence of tetragonal and rhombohedral phase was observed for x = 0.00 and with the addition of SHZ, a transition to pseudocubic symmetry was observed, which is indicative of the onset of relaxor behavior. To characterize the relaxor properties of the material, the polarization–electric field hysteresis, dielectric, and electric-field-induced strain behaviors were studied as a function of composition. The temperature-dependent dielectric spectra showed frequency dependence for all the SHZ-modified BNKT ceramics, which is a typical characteristic of relaxor ferroelectrics. Furthermore, constriction in polarization loops and an absence of negative strain in the bipolar strain measurement for SHZ-modified BNKT compositions confirms that the addition of SHZ significantly disrupts the ferroelectric order. The addition of 2 mol. % SHZ in BNKT markedly enhanced the electric field-induced strain from 0.10% (for pure BNKT) to 0.33% (for 2% SHZ-BNKT). The corresponding normalized strain coefficient $(d_{33}^{\ast }=S_{max}/E_{max})$ increased from 196 pm/V to 663 pm/V at a moderate electric field of 50 kV/cm. These results indicate that BNKT–SHZ ceramics can be designed for an improved strain response via a cation substitution-induced relaxor state for electromechanical actuator applications.