High unipolar strain in samarium-doped potassium-sodium niobate lead-free ceramics

In this work, a high unipolar strain has been developed in the 0.9675(K_0.48Na_0.52)(Nb_0.915Sb_0.035Ta_0.05)O₃-0.0325(Bi_1-xSm_x)_0.5(Na_0.82K_0.18)_0.5ZrO₃ (KNNST-B_1-xS_xNKZ) ceramics by introducing Sm, and the composition dependence of their phase structures and electrical properties is also discussed. The addition of Sm³⁺ can change the phase structure of the ceramics by simultaneously shifting T_R-O and T_O-T to a higher temperature, and we obtained a rhombohedral-orthorhombic-tetragonal coexistence phase (R-O-T) with x = 0, a coexistence phase having orthorhombic and tetragonal (O-T) phases with 0.05 ≤ x ≤ 0.60 and an orthorhombic phase with 0.8 ≤ x ≤ 1.0. In addition, the doping with Sm³⁺ can greatly enhance the unipolar strain of the ceramics without significantly sacrificing its T_C, and a high unipolar strain (∼0.28%) was observed in the ceramics with x = 0.20. More importantly, a large S_max/E_max of ∼833 pm V^-1 was also observed in the ceramics with x = 0.20 under a low applied electric field of 1.8 kV cm^-1. We believe that such a high unipolar strain can benefit the practical applications of actuators.