Study On The Flexoelectricity In Ferroelectric Ceramics And Its Application

Compared with piezoelectricity,flexoelectricity is not limited by the crystal symmetry and flexoelectricity can still exists in the crystal with a central symmetric structure,which gives the flexoelectricity more widely application prospects at high temperature.Using flexoelectricity,one can design materials with high-temperature-stable and effective piezoelectric response,but the practical applications of these materials have rarely been explored.In this paper,the Na_0.5Bi_0.5TiO₃-based ferroelectric ceramic wafers were asymmetrically reduced using a mixture of 4%H₂ and 96%N₂ as reducing agent.We show that after the reduction,the flexoelectric?or flexoelectric-like?response of Na_0.5Bi_0.5TiO₃-based ferroelectric ceramics can be enhanced by 100 times,and the materials exhibit a high apparent piezoelectric response even at a temperature above the depolarization temperature?Td?of the materials.The performance of the reduced ceramics as an energy harvester was evaluated,especially at a temperature above Td.We found that the chemical reduction enhanced the output power of the ceramics above Td by more than 100 times.Although the performance of the flexoelectric energy harvester is inferior to that of conventional piezoelectric harvester below Td,flexoelectric harvester based on the reduced Na_0.5Bi_0.5TiO₃-based ferroelectric ceramics provides an approach to harvest the mechanical energy above Td.Inverse flexoelectricity is generally defined as the linear coupling phenomenon between the applied inhomogeneous electric field and the induced structure deformation stress in solid dielectric,which is a kinds of electromechanical coupling.Few researches have been published in inverse flexoelectric field and fewer papers were related to the mechanism since inverse flexoelectricity was put forward.xBaTiO₃-?1-x?SrTiO₃ ceramic materials were machined into rectangular specimen for the inverse flexoelectricity testing.In the testing the specimens were clamped in a fixture with one end fixed and the other end free.At room temperature,we found that the relationship between electric field and electric-field-induced displacement of BaTiO₃ is linear,while it is quadratic for SrTiO₃.The displacement vary along the length of the beam and with the distance from the fixed end under the same electric field and the maximum displacement appears on the free end.In the experiment,we explored the contribution of the sample's surface to the displacement by polishing the sample's surface.The displacement of the sample decreases after polishing the surface,which means the surface of the sample has a certain contribution to displacement of the sample.The charge injection is inevitable in the electric-field-induced displacement testing.By coating a polymer charge barrier layer on the surface of the sample,one can change the direction of the bending by changing the charge injection electrode,which means the injected charge has a significant influence on the bending.