Quenching effects on depolarization temperature and ¹⁸O tracer diffusion in (Bi_0.5Na_0.5)TiO₃ ceramics with acceptor and donor additives

Lead-free ferroelectric and piezoelectric ceramics, (Bi0.5Na0.5)TiO3 (BNT), with Mg and Nb additives were prepared and the concentration of oxygen vacancies as the 18O tracer diffusion of these ceramics were clarified by secondary ion mass spectroscopy (SIMS). Moreover, the samples were quenched after sintering to examine the depolarization temperature Td, and then their electrical properties were investigated. As a result, the obtained values of volume diffusion coefficient D of BNT doped with 0.4 wt % Mg and 0.4 wt % Nb were determined to be 9.2 × 10−11 and 1.1 × 10−13 cm2/s, respectively. The oxygen vacancy concentration increased with the amount of Mg added and decreased with that of Nb added. Moreover, quenching increased Td by ∼50 °C in BNT with the Mg additive (Mg x) and ΔTd also increased. Thus, the effect of quenching was promoted by Mg x. On the other hand, the Td of BNT with the Nb additive (Nb y) also increased despite the low concentration of oxygen vacancies. This is because the D of pure BNT (no additives) was revealed found to be 2.5 × 10−11 cm2/s, that is, pure BNT originally contained a high concentration of oxygen vacancies. The concentration was maintained even after quenching, then D showed 1.8 × 10−11 cm2/s. In addition, the electromechanical coupling factor k33 of the quenched ceramics remained similar to that of the ordinarily fired samples of BNT with the Mg and Nb additives.