Study On Field-induced Strain Of SM-doped PMN-PT Ceramics

Relaxors exhibit excellent dielectric and piezoelectric properties and are widely used in ultrasonic transducers,sensors,filters,actuors and other electrical devices.（1-x）Pb(Mg_1/3Nb_2/3)O₃-x Pb Ti O₃（PMN-PT）is a typical relaxor ferroelectric,which is a hot-spot in the research field.In 2018,Li presented that Sm-doped PMN-PT ceramics show ultrahigh piezoelectricity,which also can produce giant strain under electric field and be used in piezoelectric actuators.However,research about field-induced strain in Sm-doped PMN-PT ceramics is not detailed at present,such as temperature stability of field-induced strain and its internal mechanism.Therefore,based on PMN-PT,a series of 2.5 mol%Sm-doped PMN-x PT（x=26-35 mol%）ceramics by two-step solid state reaction method were prepared.The structure and properties of ceramics were systematically studied.The field-induced strain and its internal mechanism were also investigated.The results are as follows:（1）The microstructure of samples was characterized.Through XRD phase structure analysis,it is found that all the samples formed perovskite phase structure.The introduction of Sm makes MPB move towards the low-PT side,and MPB is located at x=28-29 mol%.With increasing PT content,crystal structure of system transforms from pseudocubic to tetragonal phase.The SEM results show that all samples are sintered densified,and the average grain size of samples first decreases and then increases with incresaing PT content.When x=29 mol%,the average grain size reached the minimum,which is about 4.7μm.（2）The dielectric,piezoelectric and ferroelectric properties of samples were measured.Through measurements of dielectric and resonance frequency spectra of temperature,it turns out that:with increasing PT content,the system undergoes a transition from relaxor to normal ferroelectrics,and the maximum dielectric constant temperature（T_m）and depolarization temperature（T_d）increase linearly and gradually approached.At room temperature,the dielectric constant and dielectric loss decrease in unpoled and poled samples with increasing PT content.The introduction of Sm improves the dielectric property and piezoelectricity of system.When x=29 mol%,sample presents the best piezoelectricity,and the performance parameters are as follows:d₃₃=1281 p C/N,k_p=0.70,Q_m=28.02,ε_r=11550,tanδ=0.038.All the samples exhibit a rectangular hysteresis loop.With the increase of PT content,the hysteresis loop changes from"thin"to"fat"gradually,and the coercive field（E_c）increases monotonously.（3）The field-induced strain of samples was investigated at room temperature and in a certain temperature range,and the driving electric field was 0.8k V/mm.At room temperature,the maximum field-induced strain can be obtained when x=31 mol%.Its performance parameters are as follows:bipolar strain S_max=7.7×10^-4,unipolar strain S_uni=3.9×10^-4.In the temperature range from 17℃to 65℃,the temperature stability of field-induced strain is improved with increasing PT content,but large PT content will make field-induced strain decrease and hysteresis increase drastically.Therefore,the optimal overall performance is obtained at x=31 mol%,where the sample not only possesses giant strain,but also obtains good temperature stability of field-induced strain.The field-induced strain is closely related to domain structure:When the temperature is below frozen temperature（T_f）,the nanodomains in samples are"frozen"to form microdomains,so the field-induced strain is very small;When T_f<T<T_d,microdomains are cracked into nanodomains,and field-induced strain increases;When T>T_d,nanodomains are decoupled,and field-induced strain decreases.When the content of Sm is fixed,T_fis not related to PT content.When the content of Sm is 2.5mol%,T_f=-20℃remains unchanged.