Study On Flexoelectric Effect And Physical Mechanism Of Perovskite Compounds

The flexoelectric effect,also called flexoelectricity,which describes the coupling between strain gradient and polarization（positive effect）,as well as gradient of electric field and strain（inverse effect）.As a special coupling effect of gradient-related electromechanical,the flexoelectricity has two characteristics:（i）there is no requirements on the symmetry of the material,in principle,it can be widely discovered in all insulators and polymers;（ii）the strain gradient is usually inversely proportional to the size of the material,therefore,the flexoelectric effect at the micro-nano scale is particularly significant.However,due to the weak flexoelectric properties of the materials,the flexoelectric effect has considerable application potential,the development of its device applications is still limited.At present,the exploration of flexoelectric enhancement effect and the development of materials with high flexoelectric coefficients have become one of the important research topics.Based on the study of flexoelectric properties of ferroelectric perovskite metal oxides,the flexoelectric coefficients have been improved effictively by modifying the properties of the dielectric.And the flexoelectric effect in new organic-inorganic hybrid perovskite semiconductors has been developed.New progress has been made in the study of flexoelectric enhancement effect.The main research content and innovations of the article are:1.The relationship between the flexural electrical properties and the structural phase transition of ferroelectric perovskite were investigated.Taking PIN-PMN-PT single crystal as the research object,the flexoelectric coefficient in a wide temperature range was tested,and the change of the flexoelectric coefficient in different phases was compared.Through the comparison of the flexocoupling coefficient,the flexoelectric enhancement effect of ferroelectricity property on the was obtained.2.The flexoelectric enhancement effect in ferroelectric perovskite.By further modification of ferroelectric perovskite,the flexoelectric properties are enhanced and controllable,including:（1）By doping alumina into barium titanstannate ceramics and experimental tests,the effect of internal micro-strain on flexoelectric enhancement was verified.（2）Local structural heterogeneity was formed by doping Sm and Bi in PMN-PT ceramics.The flexoelectric coefficient with different proportions of PT was compared.The variation of the flexoelectric coefficient was well explained by the local structural heterogeneity,which confirmed the enhancement effect of the local structural heterogeneity on the flexoelectricity.At the same time,due to the smaller ion radius,the incorporation of Sm ions introduced greater local structural inhomogeneity,showing a stronger flexoelectric coefficient enhancement.（3）The flexoelectricity of lead-free ferroelectric perovskite（K,Na）Nb O₃（KNN）ceramics were enhanced by introducing oxygen vacancies.The aging characteristics of oxygen vacancies in ferroelectric materials were also studied.Through the advanced continuous exchange of bending vibration method,the dependencies between flexoelectricity and cyclic vibration（strain gradient）was studied.Due to the atomic defects（oxygen vacancy）,the flexoelectric aging of materials was related to the time dependence（aging）and cycle dependence（fatigue）of ferroelectric materials.By studying the cyclic dependence of flexoelectricity in materials,a concise relationship across microstructure and related flexoelectric properties was established.3.The flexoelectric nonlinear effect was derived by using the ion chain model.Flexoelectric devices with high performance require high flexoelectric coefficients.However,as a function of the strain gradient,it has not been effectively paid attention to whether the high flexoelectric coefficient can always be maintained as the strain gradient increases,that is,whether have nonlinear flexoelectric effect in the materials.Here,the suitable range of strain gradient for material with high flexoelectric coefficient is studied.It is found that in solid medias and PVDF,when the strain gradient is low level,the electro-mechanical coupling is a strictly linear.However,when the strain gradient is higher than the correlation threshold,there is a strong non-linearity between the polarization and the strain gradient.This nonlinear behavior can be explained by ion chain theory and fitted by nonlinear equations.The observed non-linear behavior of flexoelectric will help to understand the differences between the properties of bulk materials and their nanometer counterparts,and provide guidance for flexoelectric deviceds suitable for large strain gradients.4.Flexoelectric enhancement effects in semiconductors.The flexoelectricity of halide perovskite single crystals were studied and the novel flexoelectric enhancement effect of semiconductor was found.The principle of flexoelectric enhancement effect was proved by semiconductor interface theory.