Mechanisms of Ferroelasticity and Ferroelastic Transformations in Ceramics

这个项目的目标是在铁弹相变的背景下研究陶瓷中的铁弹性机制，产生磁区和它们在所产生的微结构中的相互排列。当前一代的执行器是基于通过各种电、热释电、压电或光刺激在铁电材料中诱导机械响应的。这些刺激在通常为正交钙钛矿结构的结构中引发相变。正常情况下，伴随的体积和/或单位晶胞形状变化非常小(远小于1%)，因此用于驱动的应变为10e-4量级。这种小应变通常不太重要，在循环极快的传感器-执行器系统中通常是有益的。然而，在某些应用中，需要能够在环境和高温下提供大机械力的执行器。实现这一目标的一种方法是通过铁弹性和铁弹性变换现象。对磁区重排机制及其局限性的了解将使我们能够解决非钙钛矿中铁弹性磁区机械极化的可行性，从而制备出具有较大作用力的织构或晶体取向的厚膜。非钙钛矿晶体结构的使用伴随着显著的体积和晶胞形状变化，是产生大作用力的关键。在这个项目中，将制备和检验单晶和多晶样品。确定了铁弹相变序列中相的晶体结构、晶格参数和热膨胀系数。在可能的情况下，高温晶体结构将在空气中、温度高达2,000摄氏度的情况下，使用X射线衍射和同步辐射进行现场分析，并进行一些合作的中子衍射研究。一旦掌握了基本的结晶学数据，这项研究将定量地研究铁弹磁畴重排和形变孪晶的机制，如一系列电子显微镜技术(TEM、CBED、HREM和SEM、EBSP)所示。实验观察结果将与文献和PI提出的理论机制进行比较。在选择材料时，主要考虑氧化物材料，因为它们的大量相变，这可能是由于组成其晶体结构的混合离子-共价键。首先，将一些已知的或可疑的铁弹性变换作为模型系统来研究，以了解铁弹性和铁弹性变换的现象。稍后，搜索将扩展到该领域的前沿知识，并将从具有潜在技术适用性的已知置换变换中进行选择。%计划使用各种补充技术对铁弹性和铁弹性转变机制进行全面调查。这一有希望的方法可能导致在先进技术应用中使用大的力、环境和高温驱动。PI将与不同国家实验室以及其他美国和国际机构的研究人员在这个也将涉及本科生的项目中进行合作。

9972114KrivenThe objective of this project is to investigate mechanisms of ferroelasticity in ceramics in the context of ferroelastic phase transformations producing domains and their mutual arrangements in resulting microstructures. The current generation of actuators are based on inducing a mechanical response in ferroelectric materials by a variety of electrical, pyro-, piezo- or optical stimuli. These stimuli initiate phase transformations in structures that are usually orthorhombic perovskites. Normally the accompanying volume and/or unit cell shape changes are extremely small (much less than 1%) so that the strains delivered for actuation are of the order of 10E-4. This small strain is usually of little concern and often beneficial in sensor-actuator systems that are cycled extremely rapidly. However, there are applications where there is a need for actuators that are capable of delivering large mechanical forces, both at ambient and at high temperatures. One approach to achieving this goal is through the phenomena of ferroelasticity and ferroelastic transformations. An understanding of the domain rearrangement mechanisms and their limitations will enable us to address the feasibility of mechanical poling of ferroelastic domains in non-perovskites, so as to produce textured or crystallographically aligned thick films of large force-generating actuator materials. The use of non--perovskite crystal structures that are accompanied by significant volume and unit cell shape changes is the key to the large forces generated. In this project, single crystals as well as polycrystalline specimens will be prepared and examined. The crystal structures, lattice parameters and thermal expansion coefficients of phases in a ferroelastic transformation sequence will be determined. Where possible, the high temperature crystal structures will be analyzed in situ, in air, at temperatures up to 2,000 deg C, using X-ray diffraction and synchrotron radiation, with some collaborative neutron diffraction studies. Once armed with the basic crystallographic data, the investigation will be quantitative in studying mechanisms of ferroelastic domain rearrangements and deformation twinning as seen by a range of electron microscopy techniques (TEM, CBED, HREM and SEM, EBSP). The experimental observations will be compared with the theoretical mechanisms proposed in the literature and by the PI. In choosing materials, primarily oxides materials are considered because of their prolific phase transformations, that are presumably due to the mixed ionic-covalent type bonding comprising their crystal structures. Initially some known or suspected ferroelastic transformations will be examined as model systems to understand the phenomena of ferroelasticity and ferroelastic transformations. Later the search will be expanded to the forefront of knowledge in this field, and a choice will be made from known displacive transformations that have potential technological applicability. %%%A comprehensive investigation of ferroelasticity and ferroelastic transformation mechanisms, using a variety of complementary techniques is planned. This promising approach may lead to large force, ambient and high temperature actuation for use in advanced technological applications. The PI will be collaborating with researchers at various national laboratories and at other US and international institutions in this project that will also involve undergraduate students.***