Molecular Dynamics Study Of Electrocaloric Effect Under The Combined Action Of Multi-Physics Fields

With the development of electronic equipment towards miniaturization and integration,thermal failure has become the key factor restricting the efficient operation of electronic equipment.There is an increasingly urgent need for efficient and micro-miniature refrigerators in the field of electronic technology.The highlyefficient solid-state refrigeration based on electrocaloric effect(ECE)has become the international research hotspot in recent years.The influencing factors of ECE can be divided into external field excitation and intrinsic factor.The former mainly includes temperature,electric field and stress field,and the latter mainly includes material composition and microstructure(defects,pores,grain boundaries,dislocations and internal bias field caused by defect dipoles,etc).When multiphysics fields including temperature,electric field,stress field and internal electric field exist at the same time,they affect the phase transition process and polarization response of ferroelectric materials,and further affect the ECE behaviors of adiabatic temperature change,working temperature region and reversibility.There are still difficulties in studying the ferroelectric behaviors and mechanism under multiphysics fields by means of experiment,and the influence and associated physical mechanism of multi-physics fields on the phase transition process and ECE are not clear.Moreover,how to effectively control the ECE by means of multi-physics fields is also an urgent problem needed to be settled.Computational materials science is an important tool for material research.Molecular dynamics simulation is an important means to study the physical mechanism of ferroelectrics,which can make up the deficiency of experimental research in microstructure characterization and mechanism explanation.In this paper,the molecular dynamics(MD)simulations based on effective Hamiltonian are used to study the ECE of ferroelectric materials under multiphysics fields.The direct method is used to characterize the ECE,where the microcanonical ensemble is adopted to directly simulate the temperature change under adiabatic conditions.BaTiO3,a typical perovskite ferroelectric,is used as the research object.With the decrease of temperature,BaTiO3 exhibits cubic(C),tetragonal(T),orthorhombic(O)and rhombohedral(R)phases.This paper systematically studied the phase transitions and ECEs of BaTiO3 single crystal under the effect of two or three physical fields of temperature,electric field,stress field and internal electric field,established multi-dimensional phase diagrams under multi-physics fields,and elucidated the influence and physical mechanism of multiphysics fields on the phase transition behaviors and ECEs.The main results of this thesis are as follows:(1)The phase transitions and ECEs of BaTiO3 single crystals under arbitrary directions of electric field were systematically studied.The electric field noncollinear with the spontaneous polarization direction of the ferroelectric phase induces complex polarization states and phase structures,and produces abundant ECE behaviors.The electric-field-induced phase transitions produce remarkable ECE peaks/valleys,whereas the polarization switching only produces small positive ECEs.The T-C phase transition produces positive ECEs regardless of the electric field direction,and the value and width of ECE peak both increase with the reduction of the angle between electric field and the spontaneous polarization direction of T phase.For the O-T and R-O phase transitions,the positive ECE gradually changes to negative ECE when the electric field direction rotates between crystallographic orientations of two ferroelectric phases.Moreover,when the electric field is applied along critical direction,a single phase transition induces the coexistence of positive and negative ECEs.The sign of the ECE is determined by the lattice symmetry of the phase structures before and after the electric-fieldinduced phase transition.(2)The phase transitions,ECEs and their reversibility of BaTiO3 single crystals under four typical thermal and electrical histories of zero-field heating,zero-field cooling,field heating and field cooling were systematically studied,and the physical mechanism of thermal hysteresis and electrical hysteresis of first-order phase transition on ECE was analyzed.BaTiO3 produces large ECEs in the hysteresis region of first-order phase transitions,and the temperature span of thermal hysteresis region shrinks with increasing electric field.The reversibility of ECE is analyzed combined with the hysteresis region in the equilibrium phase diagram:for a given thermal and electric history,the ECE is irreversible when one of the initial or final states of varying electric field is located inside the hysteresis region;while the ECE is reversible when two states fully cross the hysteresis region.For the irreversible ECE,the phase transitions produce large ECEs for the first change of electric field,but the polarization switching produces small ECEs in subsequent field cycles.For the reversible ECE,regardless of positive or negative ECEs,electrical hysteresis loss results in the gradual increase of the temperature baseline in continuous field cycles.(3)The phase transitions and ECEs of BaTiO3 single crystals under different uniaxial strains were systematically studied,and the detailed uniaxial straintemperature diagrams were drawn.The phase transition sequences under different strain conditions are the same as that without strain.However,the O phase and T phase exhibit multi-dimain states under specific strains.The R-O,O-T,T-C phase transition and domain switching of T phase from multi-domains to single domain phase produce positive ECE peaks.The phase transition induced by the change of polarization intensity along the electric field direction produces the largest ECE peak.When the uniaxial strain changes from compressive to tensile,if the electric field is parallel to the uniaxial strain direction,the ECE peak increases and moves to higher temperature;if the electric field is perpendicular to the uniaxial strain direction and in mechanically free boundary condition,the ECE peak decreases and moves to lower temperature;if the electric field is perpendicular to the uniaxial strain direction and in mechanical boundary condition,the ECE peak is basically unchanged.(4)The polarization responses,phase transitions and ECEs of BaTiO3 single crystals in the presence of defect dipoles perpendicular to the electric field direction were systematically studied.At a certain defect dipole concentration,the internal bias field caused by the defect dipoles and external electric field lead to the overshoot and rapid recovery of the polarization vector near the coercive field,resulting in the abnormal polarization with negative slope in the hysteresis loops for both simulation and experiment results.At high defect concentration,the coercive field becomes positive and a double hysteresis loop appears;at low defect concentration,the restoring force of defect dipoles decreases and a normal hysteresis loop appears.The relative magnitude of external electric field and internal electric field leads to two orientations of T phase,and the temperature region of T phase shows a waist shape with temperature in the electric field-defect dipole concentration-temperature phase diagram.With the increase of defect dipole concentration,the positive ECE peak produced by the T-C phase transition decreases,while the negative ECE produced by the T-O phase transition decreases and changes to be positive,and the positive ECE peak continues to increase and widen.