Study On The Flexoelectric Effect In Ferroelectric Ceramics

The flexoelectric effect refers to an electromechanical coupling response between the polarization and strain gradient.Compared with the piezoelectric effect,the flexoelectric effect has the following advantages:1.Flexoelectric effect has no requirement for the symmetry of the material,and it is widely existed in various dielectric materials.No external electric field is required for the polarization;2.Flexoelectric response can be maintained to a high temperature which is higher than the Curie temperature,and it has good temperature stability;3.The flexoelectric effect has a broad research space at the nanometer scale due to the size effect it has.Since the twenty-first century,the rapid development of the microelectronics industry has made the demand for the miniaturization of electronic components increase urgently.The development of the automotive and aerospace industries has also made high temperature resistant electromechanical coupling materials much more popular.Therefore,more and more attention has been paid to the flexoelectric effect.Thus the research on the theory and application of flexoelectric effect has also made great progress,but there are still some remaining unresolved and disturbing more researchment.Flexoelectric coefficient is an important parameter to characterize the electrical response of flexoelectricity.According to the theoretical research carried by Tagantsev,the magnitude of flexoelectric coefficient should be 10-10C/m.However,the magnitude of the value measured in many experiment is about 10-6C/m,which is much larger than its theoretical predicted value.In addition,as another important parameter of the flexoelectric effect,the measured value of the flexocoupling voltage is also greater than its theoretical predicted value by more than two orders of magnitude.Such differences have aroused people's enthusiasm for the research of flexoelectricity.Clarifying the mechanism of the differences between the measured and theoretical predicted value is of great significance for exploring the source of the flexoelectricity and the enhancement of the flexoelectric response.Although some researchers have put forward corresponding theories to explain this phenomenon,the mechanism of the huge difference between the measured and theoretical values of the flexoelectric response is still inconclusive,and further investigation is needed.Based on the issues above,we selected(1-x)Na1/2Bi1/2TiO3-xBaTiO3(NBBT)and(1-x)BaTiO3-xSrTiO3(BST)ceramics for the research.of flexoelectricity,in order to find out the reason of the enhanced flexoelectricity.Chapter 1 is mainly about the research background of this article.The history and current situation of ferroelectric materials and flexoelectric response was introduced,including the testing method and application.The problems remains unresolved was also mentioned in order to point out the motivation of this study.Chapter 2 introduces the sample preparation process of the ceramic,including the raw materials,preparation method and the equipment used in the process.In addition,it briefly set forth the characterization of the flexoelectric,dielectric and ferroelectric properties of the sample.The sodium bismuth titanate-barium titanate sample was selected in chapter 3 to explore the mechanism of the enhancement in flexoelectricity by carrying out different treatment on the sample.Comparation was made between the abraded and slowly-cooled samples.The results show that a spontaneously polarized surface with a thickness of 10-15?m plays an important role in the enhancing of the flexoelectric coefficient.This surface can be removed by grinding and then formed by annealing at a temperature above Tc.The temperature dependence of the flexoelectric coefficient of NBBT8 shows that the enhancement begins to show a downward trend above the Curie temperature,evev though the flexoelectric response of the sample is still larger than theoretical predicated value..In chapter 4,the(1-x)BaTiO3-xSrTiO3(BST)with a low Curie temperature and simple lattice structure was chose to study the contribution of spontaneously polarized surfaces to the flexoelectric response above the Curie temperature and to measure the intrinsic flexeelectric response.The temperature dependence of flexoelectric response was measured and it shows that when the temperature is much higher than its Curie temperature,the contribution of the spontaneously polarized surface completely disappeared.The measured value of the flexocoupling voltage at this temperature was equivalent to the theoretical value.