Thermal depolarization regulation by oxides selection in lead-free BNT/oxides piezoelectric composites

For the bismuth sodium titanate (BNT)-based materials, the thermal depolarization temperature (T_d) is always an obstacle for practical applications. Recently, BNT/ZnO composite has provided one method to resist the thermal depolarization, and however, T_d values just increase to a small extent and the conclusions derive from ZnO merely. A universal selection principle for the oxides will be helpful for us to choose the suitable oxides to effectively resist the thermal depolarization, which is desperately demanded but still lacks in BNT/oxide composites. Here, we report that the deferred thermal depolarization can be also obtained in piezoelectric Bi_0.5(Na_0.8K_0.2)_0.5TiO₃: Al₂O₃(BNKT:Al₂O₃) composites. T_d is deferred to the higher temperatures (from 116 °C to 227 °C) with increasing Al₂O₃ contents, as evidenced by the temperature dependence of dielectric, ferroelectric and piezoelectric properties. In addition, the piezoelectricity of BNKT:0.15Al₂O₃ remains stable at a high temperature (∼210 °C). And, the thermal deviatoric stress from the coefficients of thermal expansion (CTE) discrepancies between Al₂O₃ and BNKT matrix provides a stronger stabilization force than the ions diffusion-induced destabilization force, resulting in the ultimate deferred thermal depolarization and the significantly increased T_d values. In particular, according to the results from the representative BNT/oxides (i.e., ZnO, Al₂O₃, ZrO₂, HfO₂) composite, the oxide selection principle (regulating several competing factors) is given to form the appropriate thermal resistant BNT/oxide composite, which may further open the door for piezoelectric BNT-based materials from the research and application scope.