铁电薄膜的宽温域巨电热效应及其与畴变的关联研究

Ferroelectric thin films with electrocaloric effect (ECE) have a potential application in chip refrigeration. However, it is noticed that the giant ECE reported in previous works mainly appears at the phase transform temperature from the ferroelectric phase into the paraelectric phase, which limits the commercial application of ECE. In fact, the giant ECE over a broad temperature range (BTECE)is very important for exploiting the commercial refrigeration. In this research project, based on the domain switching tuned by the strain in ferroelectric thin films, the mechanism of BTECE tuned by the strain will be studied. On the one hand, combined with the phase field method and the thermodynamic theory, the thermodynamic model based on the domain switching properties will be developed to calculate and emulate electrocaloric properties of ferroelectric thin films in electromechanical coupling. The domain switching behavor will be prognosticated when the BTECE appears. These resuts will help us how to use strain in the thin films to tune the ECE into the BTECE. On the other hand, the epitaxial ferroelectric thin film will be prepared by using pulsed laser molecular beam epitaxy deposition. The strain will be adjusted by the lattice mismatch which caused between the substrate and epitaxial thin films. The piezoresponse force microscopy(PFM) will be used in researching the effect of electromechanical coupling on the domain switching characteristic. Combined with the result of ECE measuring and analyzing, the relationship between the domain switching and the performance of ECE in ferroelectric thin films will be analyzed and summarized. Based on the theoretical and experimental studies, the physical mechanism of BTECE will be established and experimental technology tuning the temperature-dependent ECE into a BTECE will be developed. Finally, based on the analysis of the asymmetric domain switching in the thermodynamic refrigeration cycle, the failur behavors will be studied.

铁电体由于具有电热效应在芯片制冷等领域具有很好的应用前景，宽温域巨电热效应是其实现电热制冷商业化应用的前提。但是大部分铁电体仅在铁电-顺电相变温度附近具有巨电热效应，限制了其应用。本申请项目将从应变调控铁电薄膜畴变的角度探索宽温域巨电热效应的实现方法和产生机理。一方面，结合相场方法和热力学理论，建立基于畴变的热力学计算模型，计算和模拟铁电薄膜在力电耦合作用下的电热性能，预测宽温域巨电热效应下的畴变行为，为应变调控畴变进而调控宽温域巨电热效应提供指导。另一方面，通过控制基底和薄膜以及梯度薄膜之间的晶格失配和热力学失配来调节应变，研究力电耦合作用下的畴变行为，再结合电热性能分析，探讨畴变对宽温域巨电热效应影响的规律。结合实验和理论研究结果，研究铁电薄膜宽温域巨电热效应的产生机理。最后，通过研究电热制冷循环中绝热升温和降温两个过程中的非对称性畴变，探讨宽温域巨电热效应在制冷循环中的失效行为。

基于电热效应的铁电固体制冷器被认为是最具潜力的固体制冷器之一。铁电材料在居里温度附近小温度范围内才具有巨电热效应，限制其商业化应用。基于此，本申请项目从理论和实验两个方面研究了铁电材料畴变和电热性能之间的关联机理和调控机理。理论方面，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，建立了多畴铁电材料电热性能计算的理论模型，计算了不同畴结构、外电场和应力作用下铁电材料的电热性能，获取了电热性能与畴变的关联机理和电热的调控机理。实验方面：通过制备一系列的铁电薄膜，测试在失配应变、相变、微结构下铁电薄膜的电热性能，结合力电耦合作用下的畴变行为分析，掌握了畴变对宽温域巨电热效应影响的规律。最后，结合实验观测和理论模拟结果，掌握了铁电薄膜宽温域巨电热效应的产生和调控机理。