Investigation of the Multiscale Constitutive Behavior of Ferroelectric Materials Using Advanced Diffraction Techniques

Citation

Rogan, Robert Cashman (2004) Investigation of the Multiscale Constitutive Behavior of Ferroelectric Materials Using Advanced Diffraction Techniques. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BT3T-F608. https://resolver.caltech.edu/CaltechETD:etd-05282004-105848

Abstract

Ferroelectric ceramics are widely used in a diverse set of devices including sensors, actuators, and transducers. The technological importance of ferroelectrics originates from their large electromechanical coupling. Ferroelectric materials exhibit a complicated behavior in response to both electrical and mechanical loads which produce large internal stresses that eventually lead to failure. Efforts to model and predict the behavior of ferroelectrics have been hindered by the lack of suitable constitutive relations that accurately describe the electromechanical response of these materials. While many measurements have been conducted on the macroscopic response of single-crystals or polycrystals, multiaxial (and multiscale) data about the in situ internal strain and texture response of these materials is lacking; this information is critical to the development of accurate models, and diffraction techniques which directly measure internal crystal strains and material texture are aptly suited to supply it.

A neutron diffraction technique was employed which allowed for the simultaneous measurement of material texture and lattice strains in directions parallel and transverse to an applied mechanical load. By comparing the behaviors of single-phase tetragonal, single-phase rhombohedral, and dual-phase morphotropic compositions, information concerning mechanics of average macroscopic behavior was inferred. In an effort to probe more of the multiaxial constitutive behavior, a high-energy X-ray diffraction technique was employed. Using transmission geometry and a 2-D image plate detector, 36 different directions of sample behavior were measured simultaneously. Polychromatic scanning X-ray microdiffraction was used to investigate the microscale three-dimensional strain tensor in single-crystals. One investigation yielded the first ever direct measurement of the tri-axial strain fields associated with single domain walls in ferroelectrics. The second investigation recorded the domain switching mechanisms activated to accommodate indentation-induced fracture stresses. Finally, 3-D XRD was used to probe the mesoscale constitutive behavior of single, embedded grains of BaTiO3 within a polycrystalline matrix.

The experimental methods described in this thesis provide access to two-dimensional and three-dimensional multiaxial constitutive strain behavior in ferroelectrics for each of the microscopic, mesoscopic, and macroscopic length scales. Results from each of these length scales will provide critical data for models attempting to accurately describe the behavior of ferroelectric materials.

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