Study On The Relationship Between Domain Configurations And Properties Of PMN-PT Based Relaxor Ceramics

Compared with relaxor ferroelectric crystals,Pb(Mg_1/3Nb_2/3)O₃-Pb Ti O₃（PMN-PT）ceramics have some special advantages,such as more convenient and cost-effective production processes,the homogeneity of properties and so on.Unfortunately,the piezoelectric and ferroelectric properties of PMN-PT ceramics are not comparable to that of corresponding crystals due to the difference in domain configurations.The design and modulation of domain configurations provide the potential possibility to enhance the properties of PMN-PT-based ceramics.As a result of the existence of defects,like grain boundaries and pores,as well as the random orientation of grains in ceramics,however,the domain configurations of ceramics are much more complex than that of single crystals,which make it difficult to study the domain structures of PMN-PT-based ceramics and to optimize electrical properties by modulating the domain configurations of the ceramics.In view of this,it is necessary to further study the domain configurations of PMN-PT-based ceramics and their evolution processes under the external stimulation of the electric field or temperature.A reasonable explanation from the perspective of domain structures for the changes in electrical properties of PMN-PT-based ceramics will be beneficial to guiding the design and modulation of domain structure,as well as enhancing the piezo/ferroelectric properties of PMN-PT-based ceramics.The domain switching process of ferroelectrics exists dynamic hysteresis behavor under a cycle of a time-varying electric field.As a consequence,every type of ferroelectric has its own characteristic ferroelectric hysteresis loop,which is capable of providing an insight into the domain evolutions under the electric field.Therefore,the first part of the present work was focus on the domain switching mechanism under the electric field by studying the scaling behavior of dynamic hysteresis of PMN-x PT ceramics with different phase structures near the morphotropic phase boundary（MPB）.Our results displayed that the evolution of scaling behavior of PMN-x PT ceramics with different compositions,that is,the relationship between the logarithm of the hysteresis loop area（ln<A>）and the logarithm of the field amplitude（ln E₀）,can all be divided into three stages,which is independent of the crystal structure of ceramics and the frequency of the external electric field.Specifically,the relationship between the hysteresis loop area<A>and the field amplitude E₀ followed the power law<A>∝f^αE ₀^βin both the low and high E₀ regions,where the reorientation of 180°and non-180°domain are dominant mechanisms,respectively.However,<A>did not obey the power law in the intermediate E₀ regions due to the interaction of different domain switching mechanisms.Furthermore,the hysteresis loop area<A>decreased with the increase in the frequency of a certain electric field,indicating that the domain switching under the external electric field is a time-depedent process.Due to the extrinsic contribution of domain wall movements to the electric field-induced strain,the electro-strain behavior of piezoelectric ceramics usually exhibits a large hysteresis,which restrict practical applications of piezoelectric ceramics for high-performance piezoelectrical actuators.In view of this,the second part of the work reported a critical state in relaxor ferroelectric ceramics that is capable of enhancing the electro-strain and reducing the strain hysteresis simultaneously by studying the relation between electro-strain behaviors and phase/domain structures.A room temperature ergodic relaxor state dominated by nanodomains was obtained by introducing Bi(Zn_1/2Ti_1/2)Ti O₃ component into 0.73Pb(Mg_1/3Nb_2/3)O₃-0.27Pb Ti O₃ ceramic matrix.Furthermore,the ergodic relaxor state exhibited the phase structural characteristic that is similar to MPB in normal ferroelectric,i.e.possessing the coexistence of two different local structural symmetries but exhibiting an average pseudocubic symmetry.This kind of MPB-like structure can promote the transition from the ergodic relaxor state to the ferroelectric under the external field due to the ease of polarization rotation,thereby leading to the enhancement of electro-stains.In addition,the field-induced ferroelectric can spontaneously reverse back to the initial ergodic relaxor state during unloading the external electric field,which is conductive to reducing the strain hysteresis.As a result,a giant electro-strain（0.24%at 50 k V/cm）with a low hysteresis（15.4%）was available in PMN-PT-BZT ceramics at room temperature.This work not only provided a ceramic system with good electro-strain performances but also afforded a guidance for manipulating the electro-strain behavior by modulating the phase/domain structures of piezoelectric ceramics.The third part of the work was to study the thermal depolarization behavior of PMN-PT-5BZT ceramic and to search for the cause of thermal depolarization from the view of domain evolutions.There was no other phase transition in PMN-PT-5BZT ceramic besides a structural transformation from MPB structure at room temperature to cubic phase with increasing temperature.However,the ceramic undergone a successive thransition from the ferroelectric to the nonergodic relaxor state and then to the ergodic relaxor state from the perspective of domain evolutions with temperature.The result of temperature dependent ferroelectric properties indicated the depolarization temperature（T_d）of PMN-PT-5BZT ceramic correspond to the transformation temperature from the ferroelectric to the nonergodic relaxor state(T_F-NR),where an obvious dielectric anomaly can also be observed.The thermally induced depolarization behavior was attributed to the decrease in the domain size and the transition from macrodomains to nanodomains with increasing temperature.In addition,the change in electro-strain performances with temperature showed that there is a critical electric field during the field-induced transition from the nonergodic/ergodic relaxor state to the ferroelectric.Finaly,the effect of grain size on domain configurations and properties of PMN-PT-5BZT ceramics was systemically investigated.The results showed that two-step sintering method can not only reduce the grain size but also enhance the density of PMN-PT-5BZT ceramics.The piezoelectric coefficient d₃₃ of PMN-PT-5BZT ceramics can dramatically increase from 550 p C/N to 768 p C/N when the average grain size reduced from 15.8μm to 7.7μm,which is related to the increase in the density and the fraction of monoclinic phase in favor of polazization rotations.More importantly,the domain size decreased significantly with the reduction of grain size,accompanied by the increase in nanodomain contents,which is supposed to promote the domain switching process under the electric field.However,a further reduction in grain size can result in the decrease of the ceramic density and the fraction of monoclinic phase.Meanwhile,the density of grain boundaries also increased with the further reduction in grain size and the clamping effect of grain boundaries in polycrystals can limit the movement of domain walls.As a result,the piezoelectric performance of PMN-PT-5BZT ceramics started to dercrease.Research results in this part are useful for optimizing the properties of piezoelectric ceramics by controlling the grain size without changing the chemical composition of the ceramics.