Phase Transition Behaviors And Electrocaloric Effects In Lead-based Antiferroelectric Ceramics

Due to the advantages of high energy efficiency,zero emission,low cost and easy manipulation,ferroelectric refrigeration technology based on the electrocaloric effect（ECE）is regarded as a new promising solid-state refrigeration solution.Numerous studies have indicated that the ECE is closely correlated with the ferroelectric phase transition.The variation of temperature and/or electric field can produce a series of phase transition behaviors in lead-based antiferroelectric materials,therefore giant ECE adiabatic temperature changes and even anomalous negative ECE responses are expected to obtained.However,the research on ECE in antiferroelectrics with complex phase transition behaviors is extremely deficient.Moreover,missing the consideration of the special polarization behaviors in antiferroelectrics,the indirect ECE characterization based on Maxwell relation is still widely adopted,as the way in normal ferroelectrics.This leads to unclear and irregular ECE results.There are also academic controversies on some important physical issues,for example the origin of the negative ECEs in antiferroelectrics.Therefore,it is in urgent need to clarify the ECE evolution laws and relevant microphysical mechanisms in antiferroelectrics comprehensively and accurately.In this thesis,taking lead-based antiferroelectric ceramics as the research object,we systematically obtain the ECE evolution law in antiferroelectric materials by direct isothermal heat flow measurement based on modified differential scanning calorimetry.Combining microstructure analysis and macroscopic properties disccusion,the electric field-temperature phase diagrams of typical materials are established to elucidate the phase transition behaviors of antiferroelectric materials under influence of electric field together with temperature.Futhermore,we explore the correlation between phase transition and ECEs,and clarify the contribution of different physical mechanisms to the ECE in antiferroelectric materials.The main findings of this thesis are as follows:（1）Through investigating the phase transition behaviors in the ceramic system of Pb_0.99Nb_0.02[(Zr_0.6Sn_0.4)_1-xTi_x]_0.98O₃,a composition-temperature phase diagram is established covering the tetragonal antiferroelectric（AFET）,rhombohedral ferroelectric（FER）,multicell cubic paraelectric（PEMCC）and single-cell cubic paraelectric（PEscc）phases.The highest ECE peak of △T_max=2.44K（@40kV/cm）is obtained at the multiple critical point of x=0.12.The composition of x=0.08 exhibits the most complex phase transition behavior,based on which,the phase transition law of the material system as well as its relationship to the ECEs are clarified.Besides the positive ECE peak produced by electric-field-induced paraelectric-ferroelectric phase transition,the phase transition from AFET to lowtemperature FER with a positive-slope boundary in the electric-temperature phase diagram brings about giant positive ECEs,while the phase transition from AFET to high-temperature PEMCC with a negative-slope boundary generates significant negative ECEs.In addition,due to the different microstructure evolution under the unipolar poling process from that of ferroelectrics,it is no longer suitable to calculate the ECEs in antiferroelectrics indirectly by Maxwell relation,and the deviation of the direct and indirect measured results can be as high as 100%.Moreover,the hysteresis loss,evidented by the double hysteresis loop in antiferroelectrics,directly causes the phenomenon that the exothermal value is obviously greater than the endothermal value under unipolar electric field excitation.The phase transition behaviors of the antiferroelectrics are also influenced by polarization memory effect.The poling history favors the isotropic field-induced phase transition,but hinders the reverse field-induced phase transition,resulting in different ECE properties.Especially,near the FER-AFET phase transition temperature,the depoling process from field-induced ferroelectric to antiferroelectric phase is extremely slow,leading to a sluggish ECE thermal response.（2）Through investigating the phase transition behaviors in the ceramic system of Pb_0.97-xBa_xLa_0.02Zr_0.95Ti_0.05O₃,a morphotropic phase boundary between orthorhombic antiferroelectric（AFEO）and rhombohedral ferroelectric（FER）phases is constructed,and a coexistence of giant positive and negative ECEs was obtained in composition of x=0.04,i.e.the MPB adjacent to the AFEO end.The giant positive ECE response is up to 3.2K（@60kV/cm）near the Curie temperature（TC）,and a negative ECE response of-0.41K（@10kV/cm）is achieved at the temperature just slightly below Tc.The electric field-temperature phase diagram indicates that the positive and negative ECEs origin from the field-induced phase transition from cubic paraelectric（PEc）to low-temperature FER（with a positive slope of the phase boundary）and the field-induced phase transition from AFEO to high-temperature FER（with a negative slope of the phase boundary）,respectively.Moreover,the positive and negative ECEs are combined effectively by a welldesigned refrigeration cycle,so that the ECE cooling capacity is enhanced by～17%.（3）Through a series of structure and performance characterizations,we clarify the relaxor properties of the Pb_1-1.5xLa_xZr_0.7Ti_0.2O₃ antiferroelectric-like ceramic system,and construct a nonergodic-ergodic boundary at the composition of x=0.07,which also shows an optimal room-temperature ECE and good temperature stability.The electric field-temperature phase diagram for the composition of x=0.06 indicates that field-induced transition from short-range ordered polar nanomicroregions to long-range ordered macroscopic ferroelectric domains evokes more significant ECEs,compared to self-ordering change process of macroscopic ferroelectric domains or polar nano-microregions.（4）To resolve the irreconcilable contradiction between the low phase transition temperature and large phase transition enthalpy in simple ABO₃-type perovskite materials,PbMg_0.5W_0.5O₃ antiferroelectric ceramic with complex B-site structure is designed.It undergoes a first-order phase transition from the AFEo to the PEc phase near room temperature（36℃）,accompanied by a large enthalpy up to 3.92J/g.The electric field of 120kV/cm can induce a positive ECE up to 1.8K（@36℃）and negative ECE of -2K（@34℃）.The electric field-temperature phase diagram shows that the positive ECE is associated with the electric field-induced phase transition from paraelectric to low-temperature ferroelectric（positive slope of the phase boundary）,while the negative ECE corresponds to the electric field-induced phase transition from antiferroelectric to high-temperature ferroelectric（negative slope of the phase boundary）.If the enthalpy of the AFEo-PEc phase transition is fully released under higher electric fields,superior ECE performace can be expected in PbMg_0.5W_0.5O₃ antiferroelectric ceramics.