| Ferroelectric thin films can generate temperature change when external electric field is applied in adiabatic condition. Such an electric field-induced temperature change is known as the electrocaloric effect(ECE). The electrocaloric refrigeration based on ECE has great potential applications in electronic packaging and micro electro mechanical system(MEMS) due to high efficiency and high reliability. In order to meet the needs in commercial application, large ECE in a wide temperature range is necessary for electrocaloric refrigeration. So, how to obtain giant ECE in a wide temperature range is the key issue. The ECE of ferroelectric thin films can be improved by controlling and adjusting the domain structure in ferroelectric thin films. In addition, researches show that the domains are influenced greatly by the crystal orientation of ferroelectric thin films. Therefore, the ECE of ferroelectric thin films can be controlled and adjusted by the crystal orientation of ferroelectric thin films. In this thesis, the ECE of ferroelectric thin films with different crystal orientations was investigated by using phase field method, and the main contents and results are summarized as follows:(1) A phase field model of ferroelectric single crystal thin films was developed, and the electrocaloric properties of PbTiO3 single crystal thin film with double-vortex domain structure was studied. Simulation results show that PbTiO3 single crystal thin film with double-vortex domain structure can generate giant ECE in a wide temperature range, and the minimum and maximum value of adiabatic temperature change(?T) is 7.2 K and 16.5 K, respectively. In addition, the influence of misfit strain on the ECE of PbTiO3 single crystal thin film with the double-vortex domain structure was investigated, and results show that the compress misfit strain can enhance the ECE, but the tensile misfit strain reduces the ECE.(2) A phase field model of ferroelectric single crystal thin films with different crystallographic orientations was developed, and the influence of crystallographic orientations on the ECE of PbTiO3 single crystal thin film was studied. Simulation results show that the electrocaloric properties of PbTiO3 thin film with vortex domain and 90o domain change continuously and discontinuously respectively when crystallographic orientation change. And the ?T of PbTiO3 single crystal thin film with vortex domain and 90o domain has the maximum value when the crystallographic orientation is 45o and 14o, respectively.(3) A phase field model of ferroelectric polycrystal thin films with different grain orientations was developed, and the influence of grain orientations on the ECE of PbTiO3 polycrystal thin film was investigated. Simulation results show that the ECE of PbTiO3 polycrystal thin film with preferred grain orientations is larger than the ECE of PbTiO3 polycrystal thin film with random grain orientations. What's more, the ECE of PbTiO3 polycrystal thin film changes periodically as the preferred grain orientations change, and ?T has the maximum value when the preferred grain orientations is 90o. |
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