The Study Of Electrocaloric Effect In Low Dimensional Ferroelectrics With Multi-domain Structures

Recently, there has been growing interest in study of the electrocaloric effect（ECE） which is a change in the temperature of a material upon the application orwithdrawal of an electric field under adiabatic conditions. The ECE can provide analternative cooling technology for reducing greenhouse gases that are used heavily indomestic and industrial refrigeration. However, it is noticed that the giant ECEreported in previous works mainly appears at the phase transform temperature fromthe ferroelectric phase into the paraelectric phase, which limits the commercialapplication of ECE. In fact, the giant ECE over a broad temperature range is veryimportant for exploiting the commercial refrigeration. In this research project, basedon the domain switching tuned by the strain in ferroelectric thin films, the mechanismof ECE tuned by the strain will be studied. A phase field method based on thetime-dependent Ginburg-Landau equation will be developed to study the domainswitching properties of ferroelectric thin films in electromechanical coupling. Amodel will be established to analyze the relationship between the asymmetric domainswitching and the ECE. And the effect of the domain switching with theelectromechanical coupling on the ECE is calculated and predicted by using thismodel. The main results have been presented as follow.1、ECE of the PbZr_0.4Ti_0.6O₃/PbTiO₃ferroelectric superlattices under the dcelectric field. The influence of the misfit strain and the electrode work functionon ECE has been studied.A thermodynamic model is developed to investigate the ECE of thePbZr_0.4Ti_0.6O₃/PbTiO₃ferroelectric superlattices. The ECE of the superlatticeconsisting of ferroelectric layers PbTiO₃（PTO） and PbZr_0.4Ti_0.6O₃（PZT） withthickness hAand hBare calculated as a function of the volume fraction βPTO=hPTO/（hPTO+hPZT）. An ECE anomaly could be found at the specific composition. Inparticular, we found that a maximum adiabatic temperature change （ΔT=5.3K）appears for the critical volume of PTO. This large ECE is attributed to the largechange in spontaneous polarization associated with the phase transform, caused by themisfit strain between the ferroelectric sublayers.2、ECE of the ferroelectric thin film with90°domain structures under the ac electric field. The influence of the domain wall density, the width of the domainwall and the misfit strain on the ECE has been studied.（a） The domain wall contribution to the ECE in the BaTiO₃（BTO） at roomtemperature is investigated by using a phase field model combined withthermodynamics analysis. The results show that the domain structure which relates tothe size has a significant effect on the ECE in the BTO. When the size of the BTOdecreases, the domain structure undergoes a transition from a multidomain structureto a monodomain structure. In the multidomain BTO, the adiabatic temperaturechange increases with increasing of the domain wall density and domain wall width.In the monodomain BTO, the adiabatic temperature change almost retains constantwith the value of0.95K which is smaller than that in the multidomain structure. Thevariation of the adiabatic temperature change at different domain structure isattributed to the domain wall contribution to ECE.（b） A phase field model based on the time-dependent Ginburg-Landau equation（TDGL） is developed to investigate the ECE of PbZr0.8Ti0.2O₃thin film with90°multi-domain structure. The rhombohedral domain morphology is obtained throughnumerically solving the TDGL equation with periodic boundary conditions at roomtemperature. Then the ECE of PbZr0.8Ti0.2O₃thin film is investigated by thethermodynamics analysis. It is shown that a great ECE exists with an applied acelectric field at room temperature. The magnitude of ECE greatly depends on theexternal applied electric field and becomes a periodic time-dependent quantity. Thetheoretical calculations also reveal that both compressive and tensile misfit strainscaused by the mismatch between the film and the substrate can largely affect theelectrocaloric properties. It reveals that the compressive substrate strain enhances theECE of PbZr0.8Ti0.2O₃thin film while the tensile substrate strain suppresses it.3、ECE of the ferroelectric thin film with180°domain structures under the dcelectric field. We provide the conceptual design of the ferroelectric thin film with180°domain structures as solid-state refrigerators.The coexistence of the negative and positive ECE is firstly found in theferroelectric thin film with180°domain structures using the phase field basedsimulation. The calculation results reveal that a negative adiabatic temperature change（ΔT=-3.4K） and positive adiabatic temperature change （ΔT=3.1K） coexist in thePbTiO₃thin film with180°domain structures under the dimensional electric field ΔE=0.1. The coexistence of the negative and positive ECE is caused by the directionof the applied electric field different from the dipole direction of180°domainstructures. And the influence of electric field and misfit strain on ECE are studied.The coexistence of negative and positive ECE in the thin film with180°domainstructures can provide a new way to design solid-state refrigerators.4、Electrocaloric effects of the ferroelectric nanoparticle with vortex domainstructures under the curled electric field. The domain wall contribution to theadiabatic temperature change in ferroelectric nanoparticle is investigated inmuch detail.（a） ECE of the Bi4Ti3O12（BIT） ferroelectric nanoparticle with vortex domainstructures under the curled electric field is investigated by using a phase field method.A large adiabatic temperature change （T=16.6K） is found in the BIT nanoparticlewith the vorticity vector of the curled field change Q=0.15mV-2at620oC. Thislarge T is attributed to the large change of toroidal moment which associated withvortex domain structures in the nanoparticle under the curled electric field. Theseresults indicate that the ferroelectric nanostructure with vortex domain structures canbe exploited for using as solid state refrigeration.（b） The influence of intrinsic surface tension on the ECE in the ferroelectricnanomaterial with vortex domain structures is studied by using the phase field method.The calculation results show that that a giant adiabatic temperature change （T=5.8K）related to the toroidal moment change appears in the PTO ferroelectric nanoparticlewith the surface tension coefficient=5N/m under the vorticity vector of curledelectric field （Q1=0mV/2, Q1=0.9mV/2） at room temperature. And themagnitude of the adiabatic temperature change decreases with the increase of surfacetension. In additional, the decrease of size is found to enhance the ECE of PTOnanoparticle with vortex domain structures.