Mechanisms of Ferroelasticity and Ferroelastic Transformations in Ceramics

9972114 KrivenThe项目的目的是研究铁弹性陶瓷的铁弹性相变的背景下产生的域和它们的相互安排在所得的微观结构的机制。当前一代的致动器是基于通过各种电刺激、热刺激、压电刺激或光学刺激在铁电材料中诱导机械响应。 这些刺激在通常为正交钙钛矿的结构中引发相变。 通常，伴随的体积和/或晶胞形状变化非常小（远小于1%），使得为致动递送的应变为10 E-4的量级。 这种小的应变通常很少被关注，并且在循环非常快的传感器-致动器系统中通常是有益的。 然而，存在需要能够在环境温度和高温下传递大的机械力的致动器的应用。 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。 域重排机制及其局限性的理解，将使我们能够解决的可行性机械极化的铁弹域在非钙钛矿，从而产生纹理或晶体取向的厚膜的大的力产生致动器材料。 使用非钙钛矿晶体结构伴随着显著的体积和晶胞形状变化是产生大的力的关键。 在这个项目中，单晶以及多晶样品将被制备和检查。 确定了铁弹相变序列中各相的晶体结构、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，在高达2,000摄氏度的温度下，使用X射线衍射和同步辐射进行原位分析，并进行一些合作中子衍射研究。 一旦配备了基本的晶体学数据，调查将是定量的铁弹畴重排和变形孪晶的研究机制，通过一系列的电子显微镜技术（TEM，CBED，HREM和SEM，EBSP）。 实验观察结果将与文献中提出的理论机制和PI进行比较。 在选择材料时，主要考虑氧化物材料，因为它们的多产相变，这可能是由于混合的离子-共价型键合包括它们的晶体结构。 最初，一些已知的或怀疑的铁弹性变换将被检查作为模型系统，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最前沿知识，并从具有潜在技术适用性的已知置换变换中进行选择。计划使用各种互补技术对铁弹性和铁弹性转换机制进行全面调查。这种有前途的方法可能会导致大的力，环境和高温致动用于先进的技术应用。 PI将与各个国家实验室以及其他美国和国际机构的研究人员合作开展该项目，该项目也将涉及本科生。

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.***