Effect of Pre-Polarization Process on the Apparent Piezoelectric Response Measured by Point-Ring Method in Ferroelectric Perovskite Oxide Ceramics

Both flexoelectricity and piezoelectricity belong to the electromechanical coupling effect. While piezoelectricity only exists in materials whose crystal structure is noncentrosymmetric and a pre-polarization process is necessary for ferroelectric piezoelectric ceramics, flexoelectricity theoretically exists in all dielectric materials and does not require pre-polarization. However, this does not mean that flexoelectricity would not be affected by the pre-polarization process, considering that flexoelectricity is a polarization phenomenon. In this work, we prepared ferroelectric perovskite oxide ceramics Ba1−xCaxTiO3 and revealed a strong effect of the pre-polarization process on the flexoelectric response of the ceramics, characterized by the apparent piezoelectric response measured by the point-ring method. The effective piezoelectric coefficient was separated into the one contributed by the flexoelectric(-like) response and the piezoelectric(-like) response by quasi-static d33 measurement and a two-step point-ring testing method. The effective piezoelectric coefficient contributed by the flexoelectric(-like) response of the ceramics could be largely enhanced to be over 350 pC/N after a 900 V polarization, larger than the standard piezoelectric response. The pre-polarization process was suggested to alter the polarization state and defect distributions, which would further change the overall flexoelectric response (both intrinsic and extrinsic parts) of the samples. Our work indicates a facile method to enhance the apparent piezoelectric response of flexoelectric materials under a bending mode.