Materials World Network: Experimental Observation and Theoretical Modeling of Domain Evolution in Ferroelectrics

该项目是得克萨斯大学奥斯汀分校和英国牛津大学科学家之间的国际合作，这两个大学都拥有铁电材料方面的专业知识。铁电晶体是一种可以保持永久电荷状态的材料，这使它们在传感器和存储设备等应用中具有许多有用的特性。它们在这些应用中的行为强烈地受到晶体中缺陷的影响，例如磁区壁。目前，对这些缺陷的理解受到缺乏实验数据的阻碍，这些实验数据与之仔细匹配，因此可以直接评估当前模型的预测。例如，相场模型提供了在陷阱电荷附近演化的磁区结构的预测，但缺乏直接观察来验证这些预测。相反，扫描电子显微镜(SEM)和原子力显微镜(AFM)已经提供了铁电材料中关键的微结构特征的观察，但这些观察结果与模型的直接比较仍然难以捉摸。这种比较提出了几个挑战：在建模方面，直接与原子力显微镜或扫描电子显微镜的数据进行比较通常需要微米的长度尺度，这是目前通过相场模型等方法可以实现的计算上限。这就要求随着缺陷几何形状的演变，在模型中进行自适应网格细化，并限制三维模拟中可实现的模型深度。相反的挑战出现在这项研究的实验部分：一些关键的相互作用发生在亚微米尺度上。因此，能够为模型验证提供基本数据的测量结果的解释和材料配置的工程设计是具有挑战性的。美国和英国的研究人员为该项目带来了互补的专业知识：德克萨斯大学的团队开创了铁电及相关材料微观结构演变的连续介质模型和数值方法；它还开发了热力学一致的断裂力学框架，用于研究机电裂纹问题。英国小组开发了测试多轴铁电行为的实验方法，最近还启动了一个实验室，用于现场观察微结构的演变。在该项目中，将使用同步加速器X射线衍射法进行磁畴结构的三维绘制。材料配置将被选择来捕捉诸如磁区针形成以及电极或夹杂物附近的磁区形核等特征。这将提供对结构域结构演变的直接观察。然后，扩展到3维的现有相场模型将被用来探索观察到的组态。压电力显微镜和扫描电子显微镜将被用来评估表面的模型预测。该项目的成果将在基础水平上有助于理解铁电晶体中的磁畴结构演变、断裂和增韧。该项目还将为研究型学生提供参与国际合作的机会，从而使他们的科学培训多样化。

This project is an international collaboration between scientists at University of Texas, Austin, and University of Oxford, UK, each having expertise in ferroelectric materials. Ferroelectric crystals are materials that can hold a permanent state of charge, which gives them many useful properties for applications such as sensors and memory devices. Their behaviour in these applications is strongly governed by defects in the crystals such as domain walls. Understanding of these defects is at present held back by a lack of experimental data that are carefully matched to, and thus can directly evaluate the predictions of current models. For example, phase field models have provided predictions of the domain structure evolving near a trapped charge, but there is a lack of direct observation to verify these predictions. Conversely, scanning electron microscopy (SEM) and atomic force microscopy (AFM) have provided observations of key microstructural features in ferroelectrics, but a direct comparison of these observations with models remains elusive. The comparison raises several challenges: On the modelling side, the length scale of microns typically needed for a direct comparison with AFM or SEM data is at the upper computational limit of what is currently feasible through methods such phase-field models. This then demands adaptive mesh refinement in the models as the defect geometry evolves, and constrains the achievable model depth in 3-dimensional simulations. The opposite challenge presents itself in the experimental part of the study: Some of the key interactions occur at sub-micron length scales. Hence, the interpretation of measurements and the engineering of material configurations that can provide essential data for model validation are challenging. The US and UK investigators bring complementary expertise to the project: The group at University of Texas has pioneered continuum models and numerical methods for microstructural evolution in ferroelectric and related materials; it has also developed a thermodynamically consistent fracture mechanics framework for use in the study of electromechanical crack problems. The UK group developed experimental methods to test multi-axial ferroelectric behaviour, and has recently initiated a laboratory for in-situ observation of microstructure evolution. During the project, 3-dimensional mapping of domain structure using synchrotron X-ray diffraction will be carried out. Material configurations will be chosen to capture features such as domain needle formation, and domain nucleation near electrodes or inclusions. This will provide direct observations of the evolution of domain structure. Existing phase field models, extended to 3-dimensions will then be used to explore the observed configurations. Piezo-force microscopy and scanning electron microscopy will be used to evaluate model predictions at surfaces. The outcomes of the project will contribute at a fundamental level to the understanding of domain structure evolution, fracture, and toughening in ferroelectric crystals. The project will also give research students an opportunity to engage in international collaboration and thus to diversify their scientific training.