Multi-Scale Mechanics Issues in Ferroelectric Ceramics

铁电陶瓷是一类重要的工程材料，在传感器、执行器、换能器、超声医学成像等方面有着广泛的应用。这类材料可以以多种尺度存在或制造：单晶、多晶、复合材料和薄膜，但它们的独特性质尚不清楚。为了充分挖掘它们的潜力，并为材料设计和选择提供指导，我们将开展一个研究项目来研究它们的性质，并为它们的非线性耦合机电行为开发独特的本构模型。在这一过程中，将考虑晶体结构从高温拟电态到低温铁电态的变化，并研究在电场和/或机械应力的作用下从一个极化态到另一个极化态的畴切换。这将通过考虑相变和畴切换过程中涉及的力学、物理和不可逆热力学来实现。特别是，由非均相体系的吉布斯自由能变化引起的热力学驱动力和与畴壁运动有关的阻力将被用来构建动力学方程。在单晶水平上，将在给定的机电场下识别所有潜在变体的层状结构域，而在多晶水平上，将考虑晶粒相互作用来解决组成晶粒的问题。对于由铁电/压电颗粒或棒或球形夹杂物驱动的铁电复合材料，将检查微观结构特征（如颗粒形状，体积浓度和分布）的影响。对于厚度方向通常包含具有某种优选织构的柱状晶粒的薄膜，将强调其与块体不同的特性。该理论将通过实验数据进行验证，其中一些来自公开文献，另一些来自两位合作者的工作。提出的研究结果将是一组基于物理的，实验验证的本构模型，揭示每个尺度机电行为的独特特征。提出的问题涉及几个不常同时遇到的新方面：非均质材料的微观力学、机电耦合、非线性响应、微观结构的演化、不可逆热力学、相变和畴切换物理以及尺度转变。因此，它将有助于这一重要领域的科学知识的基本进步。该研究项目将被整合到罗格斯大学的研究生和本科生教学中。在研究生阶段，将开始关注固体中的耦合现象。这将包括压电和铁电的机电耦合，以及电致伸缩、铁磁行为、磁致伸缩和形状记忆合金。在本科阶段，罗格斯大学的J. J.斯莱德学者项目将为对力学和电子材料都感兴趣的大四学生提供一个项目。该项目的参与者将有独特的机会学习这两种方法，并获得撰写这一跨学科主题的论文的经验。在这个过程中，将积极寻求少数民族和贫困学生的参与。＊＊＊

0114801WengFerroelectric ceramics are an important class of engineering materials that have wide applications as sensors, actuators, transducers, and ultrasonic medical imaging. This class of materials can exist or can be fabricated in several scales: single crystals, polycrystals, composites, and thin films, but their distinctive properties are not clearly known. In order to explore the full potential for each of them and to provide a guideline for material design and selection, a research project will be undertaken to study their properties, and to develop for each of them the unique constitutive models for their nonlinear, coupled electromechanical behaviors. In this process, the change of crystal structures from the high temperature paraelectric to the low temperature ferroelectric state will be considered, and domain switch from one poled state to another under application of an electric field and/or mechanical stress will be investigated. This will be carried out by consideration of the mechanics, physics, and irreversible thermodynamics involved during phase transformation and domain switch. In particular, the thermodynamics driving force arising from the change in Gibbs free energy of a heterogeneous system and the resistance force associated with the domain wall movement will be used to construct the kinetic equations. On the single crystal level, domains with lamellar structures for all potential variants will be identified under a given electromechanical field, while on the polycrystal level it will be addressed for the constituent grains considering grain interactions. For ferroelectric composites actuated with ferroelectric/piezoelectric particles or rods, or spheroidal inclusions in general, the effect of microstructural features, such as particle shape, volume concentration, and distribution, will be examined. For the thin films whose thickness direction usually contains columnar grains with some preferred texture, their distinctive characteristics from the bulks will be emphasized. The theory will be checked with experimental data, some from open literature and others from work of two collaborators. The outcome of the proposed study will be a set of physically based, experimentally verified constitutive models that reveal the unique features of the electromechanical behavior for each scale. The proposed problems touch upon several novel aspects not commonly encountered simultaneously: micromechanics of heterogeneous materials, electric-mechanical coupling, nonlinear response, evolution of microstructures, irreversible thermodynamics, physics of phase transformation and domain switch, and scale-transition. As such, it will contribute to the basic advancement of scientific knowledge in this important field. The research program will be integrated into both graduate and undergraduate teaching at Rutgers. At the graduate level a new focus on the coupled phenomena in solids will be initiated. This will include the electromechanical coupling in piezoelectricity and ferroelectricity as proposed, and electrostriction, ferromagnetic behavior, magnetostriction, and shape-memory alloys. At the undergraduate level a project for the seniors with an interest in both mechanics and electronic materials will be offered through Rutgers' J. J. Slade Scholars program. Participants of this project will have the unique opportunity to learn both, and gain experience to write a thesis on this interdisciplinary topic. In this process, minority and under-privileged students will be actively sought to participate. ***