The 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. ***
