多畴低维铁电材料电热性能的调控研究
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摘要
基于电热效应的铁电固体制冷器具有高效、环保、应用范围广等优点被认为是最具潜力的固体制冷器之一。目前，铁电材料只有在居里温度附近的较小温度范围内才具有巨电热效应，限制了铁电材料电热制冷的商业化应用。因此，如何调控低维铁电材料的电热性能实现在较宽的温度范围内具有巨电热效应，是实现铁电材料制冷商业化应用的关键。本论文结合电畴结构模拟的相场方法和电热性能研究的热力学理论，建立了多畴低维铁电材料电热性能计算的理论模型，研究了多畴低维铁电材料的电热性能，获得了电热性能与畴变的关联机理和电热的调控机理。该研究结果为制备具有宽温域巨电热效应的铁电固体制冷器提供了理论依据。
本论文的具体工作和相应结果如下：
1、建立了PbZr_(0.4)Ti_(0.6)O_3/PbTiO_3铁电超晶格电热性能计算的热力学理论模型。研究了超晶格的层间作用、电极功函数差和厚度比等因素对电热性能的调控机理。获得了铁电超晶格电热性能的调控机理。
在PbZr_(0.4)Ti_(0.6)O_3/PbTiO_3铁电超晶格中，PbTiO_3层的厚度比(PTO)对其电热性能有较大的影响。当从0.69增大到0.7时，PbZr_(0.4)Ti_(0.6)O_3/PbTiO_3铁电超晶格中PbZr_(0.4)Ti_(0.6)O_3层从铁电c-相(P_1=P_2=0, P_3≠0)转变为顺电相(P_1=P_2=0, P_3=0)。与此同时，铁电超晶格的绝热温度变化出现峰值，这是由于铁电超晶格的层间失配应变使PbZr_(0.4)Ti_(0.6)O_3层从铁电相转变为顺电相所引起的。铁电超晶格中上下电极功函数不同形成的电极功函数差会使超晶格的绝热温度变化减小，这是由于电极功函数差形成的内建电场使外电场减小所导致的。
2、结合铁电材料90°畴结构模拟的相场方法和电热性能分析的热力学理论，建立了90°畴结构铁电材料电热计算的理论模型。研究了90°畴结构与电热性能的关联性。获得了畴壁密度、畴壁宽度及应变等因素对电热性能的调控机理。
(a)室温下，90°畴结构的BaTiO_3(BTO)绝热温度变化(ΔT)为2.94K(28nm)，大于室温下单畴BTO铁电薄膜的电热性能(ΔT=0.8K)，这是由于90°畴壁对电热性能有贡献所引起的。BTO的绝热温度变化随畴壁宽度和畴壁密度的增大而增大。当尺寸减小到临界尺寸(20nm)时，BTO从90°畴结构转变为单畴结构。与此同时，绝热温度变化从2.89K骤降到0.96K，这是由于单畴结构中缺少90°畴壁对电热贡献所引起的。
(b)室温下，90°畴结构的PbZr0.8Ti0.2O_3(PZT(80/20))铁电薄膜有巨电热效应(ΔT=8.53K，32nm)。PZT(80/20)薄膜绝热温度变化的大小随外加交变电场振幅的增大而增大。PZT(80/20)薄膜与基底的失配应变为压应变时(um=-0.012)，PZT(80/20)薄膜的绝热温度变化ΔT=9.8K，大于无失配应变时绝热温度变化；当失配应变为拉应变(um=+0.012)，ΔT=7.1K，小于无失配应变时绝热温度变化。由于失配应变改变了薄膜的晶格常数，引起极化大小改变，导致了PZT(80/20)薄膜绝热温度变化改变。
3、结合180°畴结构模拟的相场方法和电热性能分析的热力学理论，建立了180°畴结构PbTiO_3(PTO)电热性能计算的理论模型。研究了180°畴结构与电热性能的关联机理和电热的调控机理。并提出一种新型固体制冷器设计思想。
计算结果表明，室温下PTO铁电薄膜的180°畴壁两端发现正/负电热效应共存的现象，这是由外电场方向与极化方向不同所导致的。通过计算PTO薄膜与外电场的入射角、外电场的大小和薄膜与基底的失配应变的关系，180°畴结构PTO铁电薄膜电热性能的调控机理。最后根据PTO铁电薄膜180°畴壁两端正/负电热效应共存现象，提出了一种基于铁电薄膜正/负电热效应的新型固体制冷器设计方案。
4、结合涡旋畴结构模拟的相场方法和电热性能分析的热力学理论，建立了涡旋畴结构铁电材料电热计算的理论模型。研究了涡旋畴结构与电热性能的关联性。获得了外电场、屏蔽因子和表面张力等对铁电材料电热性能的调控机理。
(a) Bi4Ti3O12(BIT)铁电纳米片中，为减小退极化场作用会形成涡旋畴结构并形成新的序参量—涡旋矩。外加环形电场Q作用下，BIT涡旋矩(G)变化会产生巨电热效应。环形电场变化ΔQ=0.15mV/2时，BIT铁电纳米片绝热温度变化峰值出现在610oC附近(ΔT=16.6K)，且ΔT随ΔQ的增大而增大。退极化场屏蔽因子k减小到临界尺寸(0.7)以下，BIT从涡旋畴结构转变为单畴结构。在涡旋畴结构中，BIT纳米片的ΔT随k增大略有小幅增大。
(b)考虑表面张力，PTO铁电纳米片为减小退极化场作用会形成涡旋畴结构，表面张力不会影响PTO铁电纳米片的电畴结构。该结果证实在铁电纳米结构的涡旋畴结构模拟中可不考虑表面张力。PTO铁电纳米片的电热性能随表面张力增大而减小。这是由于表面张力改变PTO的晶格常数，导致极化变化改变最终而影响电热性能。PTO纳米片的绝热温度变化随其尺寸的变化而改变。
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_(0.4)Ti_(0.6)O_3/PbTiO_3ferroelectric 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_(0.4)Ti_(0.6)O_3/PbTiO_3ferroelectric superlattices. The ECE of the superlatticeconsisting of ferroelectric layers PbTiO_3(PTO) and PbZr_(0.4)Ti_(0.6)O_3(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 BaTiO_3(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 PbZr0.8Ti0.2O_3thin 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 PbZr0.8Ti0.2O_3thin 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 PbZr0.8Ti0.2O_3thin 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 thePbTiO_3thin 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.
本论文的具体工作和相应结果如下：
1、建立了PbZr_(0.4)Ti_(0.6)O_3/PbTiO_3铁电超晶格电热性能计算的热力学理论模型。研究了超晶格的层间作用、电极功函数差和厚度比等因素对电热性能的调控机理。获得了铁电超晶格电热性能的调控机理。
在PbZr_(0.4)Ti_(0.6)O_3/PbTiO_3铁电超晶格中，PbTiO_3层的厚度比(PTO)对其电热性能有较大的影响。当从0.69增大到0.7时，PbZr_(0.4)Ti_(0.6)O_3/PbTiO_3铁电超晶格中PbZr_(0.4)Ti_(0.6)O_3层从铁电c-相(P_1=P_2=0, P_3≠0)转变为顺电相(P_1=P_2=0, P_3=0)。与此同时，铁电超晶格的绝热温度变化出现峰值，这是由于铁电超晶格的层间失配应变使PbZr_(0.4)Ti_(0.6)O_3层从铁电相转变为顺电相所引起的。铁电超晶格中上下电极功函数不同形成的电极功函数差会使超晶格的绝热温度变化减小，这是由于电极功函数差形成的内建电场使外电场减小所导致的。
2、结合铁电材料90°畴结构模拟的相场方法和电热性能分析的热力学理论，建立了90°畴结构铁电材料电热计算的理论模型。研究了90°畴结构与电热性能的关联性。获得了畴壁密度、畴壁宽度及应变等因素对电热性能的调控机理。
(a)室温下，90°畴结构的BaTiO_3(BTO)绝热温度变化(ΔT)为2.94K(28nm)，大于室温下单畴BTO铁电薄膜的电热性能(ΔT=0.8K)，这是由于90°畴壁对电热性能有贡献所引起的。BTO的绝热温度变化随畴壁宽度和畴壁密度的增大而增大。当尺寸减小到临界尺寸(20nm)时，BTO从90°畴结构转变为单畴结构。与此同时，绝热温度变化从2.89K骤降到0.96K，这是由于单畴结构中缺少90°畴壁对电热贡献所引起的。
(b)室温下，90°畴结构的PbZr0.8Ti0.2O_3(PZT(80/20))铁电薄膜有巨电热效应(ΔT=8.53K，32nm)。PZT(80/20)薄膜绝热温度变化的大小随外加交变电场振幅的增大而增大。PZT(80/20)薄膜与基底的失配应变为压应变时(um=-0.012)，PZT(80/20)薄膜的绝热温度变化ΔT=9.8K，大于无失配应变时绝热温度变化；当失配应变为拉应变(um=+0.012)，ΔT=7.1K，小于无失配应变时绝热温度变化。由于失配应变改变了薄膜的晶格常数，引起极化大小改变，导致了PZT(80/20)薄膜绝热温度变化改变。
3、结合180°畴结构模拟的相场方法和电热性能分析的热力学理论，建立了180°畴结构PbTiO_3(PTO)电热性能计算的理论模型。研究了180°畴结构与电热性能的关联机理和电热的调控机理。并提出一种新型固体制冷器设计思想。
计算结果表明，室温下PTO铁电薄膜的180°畴壁两端发现正/负电热效应共存的现象，这是由外电场方向与极化方向不同所导致的。通过计算PTO薄膜与外电场的入射角、外电场的大小和薄膜与基底的失配应变的关系，180°畴结构PTO铁电薄膜电热性能的调控机理。最后根据PTO铁电薄膜180°畴壁两端正/负电热效应共存现象，提出了一种基于铁电薄膜正/负电热效应的新型固体制冷器设计方案。
4、结合涡旋畴结构模拟的相场方法和电热性能分析的热力学理论，建立了涡旋畴结构铁电材料电热计算的理论模型。研究了涡旋畴结构与电热性能的关联性。获得了外电场、屏蔽因子和表面张力等对铁电材料电热性能的调控机理。
(a) Bi4Ti3O12(BIT)铁电纳米片中，为减小退极化场作用会形成涡旋畴结构并形成新的序参量—涡旋矩。外加环形电场Q作用下，BIT涡旋矩(G)变化会产生巨电热效应。环形电场变化ΔQ=0.15mV/2时，BIT铁电纳米片绝热温度变化峰值出现在610oC附近(ΔT=16.6K)，且ΔT随ΔQ的增大而增大。退极化场屏蔽因子k减小到临界尺寸(0.7)以下，BIT从涡旋畴结构转变为单畴结构。在涡旋畴结构中，BIT纳米片的ΔT随k增大略有小幅增大。
(b)考虑表面张力，PTO铁电纳米片为减小退极化场作用会形成涡旋畴结构，表面张力不会影响PTO铁电纳米片的电畴结构。该结果证实在铁电纳米结构的涡旋畴结构模拟中可不考虑表面张力。PTO铁电纳米片的电热性能随表面张力增大而减小。这是由于表面张力改变PTO的晶格常数，导致极化变化改变最终而影响电热性能。PTO纳米片的绝热温度变化随其尺寸的变化而改变。
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_(0.4)Ti_(0.6)O_3/PbTiO_3ferroelectric 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_(0.4)Ti_(0.6)O_3/PbTiO_3ferroelectric superlattices. The ECE of the superlatticeconsisting of ferroelectric layers PbTiO_3(PTO) and PbZr_(0.4)Ti_(0.6)O_3(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 BaTiO_3(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 PbZr0.8Ti0.2O_3thin 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 PbZr0.8Ti0.2O_3thin 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 PbZr0.8Ti0.2O_3thin 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 thePbTiO_3thin 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.
引文
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