Research On The Influences Of Phase Transitions On Electrocaloric And Piezoelectric Effect In Relaxor Ferroelectric Single Crystals

At the morphotropic phase boundary(MPB),the phase structures in ferroelectric materials are metastable,so they can produce excellent electrocaloric effects(ECEs)and electromechanical performance under the external stimulations of stress,temperature and electric-field.The Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMN-xPT)relaxor ferroelectric single crystal has been studied extensively due to the excellent ECEs,ultrahigh electrostrain and giant piezoelectric constant(d33>2000 pC/N).The PMN-xPT system forms a MPB conformation when the PT content covers a range of 30<x<35,wherein the rhmohedral(R)phase,the monoclinic(M)phase,the tetragonal(T)phase,the R-M and M-T phase boundaries coexist.Numerous physical factors that affect each other make it extremely difficult to study the inherent physical mechanism of the excellent performance of PMN-xPT single crystal.Moreover,the aforementioned microscopic processes are intertwined and highly dependent on the multi-field effects of electric-field and temperature in actual application environment,making the microscopic mechanism more complicated.There is still controversy in the academic community as to which physical mechanism plays a leading role in ECEs and piezoelectric performance.Therefore,it is very important for the theoretical research and optimization design of ferroelectric materials to clarify the contribution of various physical mechanisms to ECEs and piezoelectricity under multi-fields of electric-field and temperature.In this paper,we studied the complex evolution processes of phase structure and polarization switching as well as the corresponding responses of ECEs,electrostrain and piezoelectric constant in the<001>,<011>and<111>oriented PMN-30PT single crystals under multi-fields of electric-field and temperature..The intrinsic relations between the microscopic physical mechanisms and the macroscopic properties of ECEs,electrostrain and piezoelectric constant are established.The contribution and influence laws of various microscopic physical processes on the macroscopic properties are clarified.This paper mainly obtains the following conclusions and results:(1)The phase structure evolution processes of the<001>,<011>,and<111>oriented PMN-30PT single crystals under the multi-fields of electric-field and temperature was systematically studied.The detailed electric-field-temperature phase diagrams of single crystals with different orientations are drawn,and the pseudo-MPB region induced by the electric-field-temperature multi-fields is accurately depicted and further subdivided.According to the different determinants,the various phase transitions are divided into electric-field-dominated phase transitions and temperature-dominated phase transitions.The research results provide the foundation for establishing the intrinsic relation between the microscopic physical mechanisms and the macroscopic performance under different external fields.(2)The ECEs of the<001>,<011>and<111>oriented PMN-30PT single crystal were systematically characterized under the multi-fields of electric-field and temperature.Positive and negative ECEs coexist in the<001>and<011>oriented single crystals,while the<111>single crystal only produces positive ECEs.The positive ECEs peaks depend on the temperature-dominated phase transition.When E=4 kV/cm,the R-T or O-T phase transitions induce a positive ECEs peak with △Tmax=0.4 K.Near Tm,the positive ECEs induced by the ferroelectric-paraelectric phase transition reaches a maximum value with△Tmax=0.61 K,0.63 K and 0.68 K in the<001>,<011>and<111>oriented crystals,respectively.The negative ECEs originate from the generation of high-symmetric ferroelectric phases induced by non-collinear electric fields.The experimental results show that in the electric-field-induced pseudo-MPB region,only the MA-T phase transition in<001>single crystal and MB-O phase transitions in<011>single crystal with high energy barriers can produce a negative ECEs.The higher the energy barrier is,the larger the negative ECEs is.(3)Both positive and negative ECEs coexist in the<001>oriented PMN-30PT single crystal under different electric-fields at a certain temperature.We have proposed an electric-field drive scheme that effectively combines the positive and negative ECEs to achieve dual refrigeration cycles,thereby greatly improving the cooling capacity of ferroelectric materials(～150%,85℃).In addition,there are negative ECEs induced by the depolarization under the reversal electric-field in<001>crystal.Using this negative ECEs,the dual cooling cycle increases the cooling efficiency by 67%at 30℃.(4)The respective contributions of the physical mechanisms in the<001>oriented PMN-30PT single crystal to electrostrain and piezoelectric constants are quantitatively characterized.The experimental results prove that the electrostrain and piezoelectric constant are determined by different microscopic physical processes.For the electrostrain,with the increase of temperature,the polarization elongation in the R phase(～53%of total strain at 35℃)and the MA phase in pseudo-MPB(-43%at 55℃)phases as well as the MA-T phase transition region with high energy barrier in pseudo-MPB(-60%at 80℃),successively plays the dominant role in the electrostrain.For the piezoelectric constant,only the R-MA phase transition region with a low energy barrier in the pseudo-MPB can produces an ultrahigh piezoelectric constant peak with the d33>2100 pC/N.(5)The electrostrain and piezoelectric constants behaviors of the<001>,<011>and<111>oriented PMN-30PT single crystals were systematically studied.Both the electric-field-dominated phase transition and the temperature-dominated phase transition can produce electrostrain peaks,in which the electric-field-induced MA-T phase transition in the<001>oriented single crystal and the R-O phase transition in the<011>oriented single crystal are conducive to producing maximum electriostrain.The<001>single crystal and<011>single crystal produces maximum electrostrains of 0.52%and 0.22%,respectively.Different from the electrostrain,among the phase transitions dominated by the electric-field,only R-MA and R-O phase transitions with low energy barriers can generate piezoelectric constant peaks(R-MA:d33=2460 pC/N;R-O:d33=1500 pC/N),while the MA-T phase transition with a high energy barrier cannot produce a piezoelectric constant peaks.In addition,the temperature-dominated phase transition between ferroelectric phases,such as O-T phase transition in the<011>crystals and the R-T phase transition in the<111>crystal can also generate piezoelectric constant peaks of 1900 pC/N and 850 pC/N,respectively.