Improved depolarization behavior and electric properties in (Bi_0.5Na_0.5)TiO₃-based piezoelectric composites

(Bi_0.5Na_0.5)TiO₃ (BNT) based ceramics have been restricted by the drawback of the low depolarization temperature T_d and ferroelectric-to-relaxor transition temperature T_F-R. Many works have confirmed conventional improving strategies like doping and forming solid solution are not efficient to solve the problem. To overcome this obstacle, we introduced semiconductor ZnO into 0.93(Bi_0.5Na_0.5)TiO₃-0.07Ba(Ti_0.945 Zr_0.055)O₃:xZnO matrix to prepare 0-3 type ceramic composites (abbreviated as BNT-BZT:xZnO). Energy disperse spectroscopy (EDS) result reveals that there are two distributions of Zn. Most of Zn²⁺ ions gather around the boundaries of BNT-BZT grains to form ZnO enrichment regions, and the others diffuse into the lattice. In consideration of the fact that the distribution of Na also appears to be separated, a new model on the Na migration is proposed to explain experimental results. Both the substitution of lower valent Zn²⁺ ions at B site and the migration of Na are benefit for the formation of oxygen vacancies. As a consequence, the coercive field E_c and mechanical quality factor Q_m have been significantly promoted. In terms of depolarization behavior, the temperature-dependent P-E loops show a completely different polarization process after ZnO incorporation. A slower decay of the remnant polarization P_r indicates ferroelectric state is more stable in the composites. The influence of internal bias field on asymmetric P-E loop has been discussed. The experiments on thermally stimulated depolarization current (TSDC) and temperature-dependent dielectric permittivity demonstrate T_d and T_F-R increase about 17 °C and 19 °C, respectively. Our research confirms forming ZnO composites is an effective method to improve depolarization behavior, and BNT-BZT:xZnO composites should be an alternative for application in electromechanical devices.