Study On The Flexoelectriclike Effect Based On The Surface Layer Effect

The flexoelectric effect describes the interaction between dielectric polarization and strain gradient.It is a common electromechanical coupling effect in solid dielectrics that is not limited by the symmetry of the material crystal structure.Therefore,the flexoelectric effect has wide application prospects in the fields of sensors,actuators and transducers.The flexoelectric coefficient of simple low-permittivity dielectrics is on the order of 10^-11-10-¹⁰ C/m.Until the early 2000s,people discovered that ferroelectric ceramics have large flexoelectric coefficients,which are several orders of magnitude larger than the theoretical values.In recent years,the large deviation between the measured value and the theoretical value of the flexoelectric response of ferroelectric oxide ceramics has attracted the attention of many researchers.Recent studies by our group show that the abnormally large flexoelectric coefficients of ferroelectric ceramics mainly come from the spontaneously polarized surface layer,which is a stress-related phenomenon,but the source of the stress remains to be explored.Sintered ferroelectric ceramics are often abraded to obtain a specific thickness before being subjected to various physical measurements.Mechanical abrasion will introduce additional residual stress and affect the structure and properties of ceramic surfaces.We further investigated the effect of mechanical abrasion stress on the flexoelectric coefficient of BaTiO₃ ceramics and found that the stress caused by abrasion has only a limited effect on the measured flexoelectric coefficient,and is not the main reason for the formation of polarized surface layers.In addition,the effect of grain size on the flexoelectric response of BaTiO₃ ceramics was systematically studied.Ceramic samples with grain sizes of 0.59～8.90μm were prepared by the two-step sintering method.It was found that the flexoelectric response was dependent on grain size.The flexoelectric coefficient of BaTiO₃ ceramics can be increased from less than 20 μC/m to more than 300 μC/m.At the same time,we found that the grain size effect of BaTiO₃ ceramics can be explained by the polarized surface layer effect.It is helpful to further explore the cause of the large deviation between the measured value and the theoretical value of the flexoelectric coefficient of ferroelectric ceramics.Finally,we investigate the effect of reduction on the flexoelectric response of SrTiO₃ ceramics.There is another surface phenomenon in SrTiO₃ ceramics sintered in a reducing atmosphere,and the existence of surface layers greatly increases the flexoelectric coefficient.The flexoelectric coefficient of SrTiO₃ ceramics sintered in a H2 atmosphere can reach 1300 μC/m,which is mainly due to the formation of a large number of defect dipoles in the ceramics in the reducing atmosphere,and the reduced sample has a relatively insulated surface layer of～20 μm thickness.In addition,the contribution of the Schottky barrier layer formed by the Au electrode and ceramics to the flexoelectric response cannot be ignored.