Atomistic and Continuum Study of flexoelectricity in ferroelectric materials
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In this dissertation, we try to address some of the questions which arise while studying flexoelectricity in ferroelectric materials. 1. Most technologically-relevant ferroelectrics typically lose piezoelectricity above the Curie temperature. This limits their use to relatively low temperatures. In this dissertation, exploiting a combination of flexoelectricity and simple functional grading, we propose a strategy for high-temperature electromechanical coupling in a standard thin film configuration. We use continuum modeling to quantitatively demonstrate the possibility of achieving apparent piezoelectric materials with large and temperature-stable electromechanical coupling across a wide temperature range that extends significantly above the Curie temperature. With Barium and Strontium Titanate as example materials, a significant electromechanical coupling that is potentially temperature-stable up to 900 C is possible. 2. Piezoelectricity is a property of non-centrosymmetric crystals. In most typically used ferroelectrics, this property is lost as the temperature is increased beyond the Curie point thus strongly reducing the availability of efficient materials that can be used for high temperature energy harvesting. Flexoelectricity, as can be shown from simple symmetry arguments, is a universal and linear electromechanical coupling that dictates the development of polarization upon application of inhomogeneous strains. The implications of this phenomenon become amplified at the nanoscale. In this dissertation, we develop a molecular dynamics approach predicated on a specially tailored interatomic force-field to extract the temperature dependence of flexoelectricity. Surprisingly, we find that it, at least for Barium Titanate and Strontium Titanate nano structures, increases with temperature. Apart from cataloging this interesting observation for future use in high temperature energy harvesting, we also examine the physical mechanisms that lead to the observed temperature dependence. 3. A new theory for 180 domain wall in ferroelectric perovskite material is presented in this work. The effect of flexoelectric coupling on the domain structure is analyzed. We show that the 180 domain wall has a mixed character of Ising and Bloch type wall and that the polarization perpendicular to the domain wall is non zero though it is very small compared to the spontaneous polarization in the case of tetragonal Barium Titanate. Finally, we present the effect of the new finding on the domain wall interaction with defects in the material. 4. Pyroelectric materials generate electricity in response to change in temperature. These materials are commonly used to build temperature sensors, radiation detectors and alarm systems, among others. There are few materials that possess this property. In this work, we develop a nonlinear theoretical framework for pyroelectricity in soft materials. Using the concept of soft electrets materials, we illustrate a nonlinear relation between the Maxwell stress effect and pyroelectricity, and propose the design of a pyroelectric material whose constituents are intrinsically non-pyroelectric.