Giant Flexoelectric-Like Response via Macroscopic Symmetry Design

Flexoelectricity is enabled by symmetry in all materials. However, flexoelectric material application is limited by the normally low charge density produced in bulk materials. In this study, a universal strategy involving a macroscopic symmetry design is proposed to enhance the flexoelectricity. Through theoretical derivation, flexoelectricity can be improved by designing the macroscopic symmetry of the material parameter distribution (including the piezoelectric coefficients) and device structure. As a demonstration, typical piezoelectric bimorph cantilevers (PBCs; Ag/PZT-5H/Ag/PZT-5H/Ag) are constructed with the two PZT-5H layers arranged in "head-to-tail" polarization (mirror symmetry) and "tail-to-tail" polarization (centrosymmetry), to design the macroscopic symmetry and thus to tune the flexoelectricity. The theoretical predictions and experimental results show that the tail-to-tail PBC achieves a flexoelectric coefficient (1.47 × 10⁶ nC m^-1), 20 times higher than that of the head-to-tail PBC (7 × 10⁴ nC m^-1) and conventional piezoelectric cantilevers (Ag/PZT-5H/Ag). Furthermore, by introducing spaced-interdigitated electrodes, the macroscopic symmetry of the head-to-tail PBC can be transformed from mirror to centrosymmetry, yielding a giant flexoelectric coefficient of 2.53 × 10⁶ nC m^-1. This strategy offers a dimension beyond traditional approaches for understanding and enhancing flexoelectricity, paving the way for its practical application.