Electromechanical Performance Optimization of Ni-Doped Bi(Sc, Zr)O₃-PbTiO₃ Ceramics and Microstructure Analysis

BiScO₃-PbTiO₃-based ceramics show great potential in high-temperature piezoelectric applications. However, their high dielectric loss tan δ and low mechanical quality factor Q_m have to be optimized. In this paper, a ceramic system of (1-y)Bi(Sc_0.975Zr_0.025)O₃-yPb(Ti_1-xNi_x)O₃ (BSZ-yPT-xNi, x = 0, 0.015, 0.025, and 0.035 and y = 0.62, 0.63, 0.64, and 0.65) was systematically investigated. Increase in x or y values leads to the enhancement of the tetragonal phase and tetragonal lattice distortion. The rhombohedral/tetragonal morphotropic phase boundary (MPB) locates in the vicinity of y = 0.64 for the x = 0 and 0.015 samples, and y = 0.63 for the x = 0.025 and 0.035 samples. For these MPB samples, the substitution of Ni²⁺ for Ti⁴⁺ causes domain refinement, evolving from the submicrometer lamellar domains to hierarchical domains, and finally to the high-density stripe-like nanodomains, which benefits the domain wall motion and makes the coercive field reduced. However, the alignment of defect dipoles (Ni_Ti^''-V_O^••) after the sufficient poling and aging treatment induces the noticeable internal bias field, which increases with the addition of Ni²⁺. Apparent piezoelectric "hardening" occurred, evidenced by the increase in Q_m and the reduction in tan δ. Among the MPB samples, the x = 0.025/y = 0.63 ceramic shows the superior comprehensive electromechanical performance with the d₃₃ of 380 pC/N, k_t of 0.51, Q_m of 112, and tan δ of 0.010. Besides, excellent temperature stability was achieved with the d₃₃ of 367-380 pC/N, Q_m of 106-112, and tan δ ≤ 0.010 in the temperature range of 25-250 °C.