The formation and effect of defect dipoles in lead-free piezoelectric ceramics: A review

As one of the most crucial intrinsic contributions, defect dipoles show unique and prominent influence on the responsive behavior of piezoelectric ceramics. Here, we present the formation of the defect dipoles and the state-of-the-art effect on lead-free piezoelectric ceramics in detail. First, the formation of defect dipoles in ceramics are demonstrated. The coupling effect between defect dipole polarization and normal ferroelectric spontaneous polarization, which results in the unusual ferroelectric properties (e.g., “pinched” or asymmetric polarization hysteresis loops) and high electric field induced-strain performance in piezoelectric ceramics, is elaborated. Thereafter, we systematically introduce the switching and decoupling process of defect dipoles under different external conditions. Finally, the researches about the effect of defect dipoles on various lead-free piezoelectric ceramics are presented herein, with special attention on the three systems: the barium titanate, sodium-bismuth titanate and potassium sodium niobate. The remaining challenges, however, such as weak temperature/cycling stability, still need further effort for the extensive applications of these materials.