铁电纳米多晶动态电-力学特性外场频率相关性的微结构动力学机理研究

It has been experimentally demonstrated that there is apparent frequency dependence in the electromechanical characteristics of ferroelectric ceramics.Such dependence of the material properties on the field frequency is further intensified in nanocrystalline polycrystals, and can directly affect the material's engineering application. Therefore it is worth of the efforts to do microstructural dynamics studies on the mechanism behind the observed frequency-dependent phenomena by taking the view that frequency dependence is a result of direct competition between the speed of microstructure evolution and the speed of external loading. In this proposal we intend to develop phase field model to describe the microstructural evolution dynamics of nanocrystalline ferroelectric polycrystals by introducing the electric field frequency through the boundary condition values. In the meanwhile we consider the microstructural characteristics of the grain-grain interface which leads to grain-size effects. By phase field analysis and experimental verification, the objective of this work is to understand the microstructural dynamics in ferroelectric nano-polycrystals under applied alternating electric field, mimic the competition between the microstructural evolution speed and the applied field frequency, develop the microstructural dynamics mechanism for the observed frequency-dependent electromechanical characteristics of nanocrystalline ferroelectric polycrystals, and eventually provide a theoretical foundation for the improvement of the stability and efficiency of such functional materials.

实验观测证实铁电多晶在外加交变电场环境下存在电-力学特性上的外场频率相关性。对于晶粒尺寸更小的铁电纳米多晶，其材料特性对外场频率的依赖性更大，将直接影响其在工程中的应用。从晶体微结构相变动力学的角度来分析其宏观材料动态特性，将频率相关性视为材料微结构演化速度与外场加载速率竞争的结果，对其进行微结构动力学机理性的研究具有十分重要的科研价值。本项目拟建立用以描述铁电纳米多晶微结构动力学演化过程的相场计算模型，通过边界值条件引入外场频率的参与；同时考虑纳米多晶晶粒界面层的微结构特征及其衍生的尺寸效应。通过进行相场数值模拟分析和宏观辅助实验验证，掌握在动态电加载环境下晶体微结构动力学演化过程的时空规律；再现微结构演化与外场频率的竞争过程；揭示铁电纳米多晶宏观电-力学特性与电场频率相关性的内在微结构动力学机理；为提高该类功能元件的稳定性和工作效率提供理论依据。

本课题针对铁电纳米多晶在外加交变电场环境下存在电-力学特性上的外场频率相关性开展了分析研究。从晶体微结构相变动力学的角度来分析其宏观材料动态特性，将频率相关性视为材料微结构演化速度与外场加载速率竞争的结果，通过发展铁电体电-力学全场耦合的相场计算方法，对铁电纳米多晶材料动态电-力学响应的外场频率相关性进行分析。通过研究掌握了动态频率加载环境下晶体微结构动力学演化过程的规律以及电畴反转的产生和空间分布的特点；再现了晶体微结构演化与外加电场的竞争过程；厘清了这一竞争过程中纳米晶粒界面的本征效应和非本征效应在电场频率相关性问题上的贡献，提出了两者在不同频率水平的重要性不同的理论观点。同时本研究中还对连带的晶粒尺寸效应进行了深入探讨。在本项目的执行过程中，课题组成员解决了相场模型中电场加载频率的引入、动力学参数的确定、数值模型的选取、纳米晶粒界面层的几何与电学特性的表征等几个关键性问题。通过数值分析以及与实验观测的比较研究最终揭示铁电纳米多晶动态电-力学特性与电场加载频率相关性的微结构动力学机理。研究表明，在决定纳米多晶材料性能的频率依赖性问题上，晶粒间的纳米晶界层起到了十分重要的作用。由于纳米晶界层的存在导致了其本征和非本征效应在不同频率电场下的竞争关系，从而使材料表现出宏观的高、低频行为特征。同时，纳米晶界层还会导致材料性能的晶粒尺寸效应。