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PHASE-FIELD STUDY ON FERROELECTRIC OXIDE COMPOSITES AND HETEROSTRUCTURES
Restricted (Penn State Only)
Materials Science and Engineering
Doctor of Philosophy
Date of Defense:
December 13, 2017
Long-Qing Chen, Dissertation Advisor
Long-Qing Chen, Committee Chair
Venkatraman Gopalan, Committee Member
Sulin Zhang, Committee Member
Michael T Lanagan, Outside Member
Ferroelectric materials are a class of materials possessing a spontaneous electric polarization that is switchable by an external field. A composite approach can either greatly enhance the performance of ferroelectric materials or induce emerging properties and phenomena which expands the range of its applicability. Through microstructure design, performances of ferroelectric composites and heterostructures can be optimized for multiple applications. This dissertation focuses on theoretical understandings of various properties of ferroelectric composites and heterostructures by employing a phase-field model. Magnetoelectric coupling is a phenomenon in which a material exhibits a polarization response to an applied magnetic field. We formulate a phase-field model coupled with constitutive equations to investigate the magnetoelectric cross-coupling in magnetic/ferroelectric composites. The model allows us to obtain static piezoelectric, piezomagnetic, dielectric, and magnetoelectric properties under a given magnetic or electric field, from the local distributions of polarization, magnetization, and strain in the composites. As an example, effective magnetoelectric coupling coefficient, i.e., magnetic-field-induced voltage output (or changes in polarization), of the CoFe2O4-BaTiO3 composites is numerically calculated. Influences of the phase connectivity and the phase fraction of the composites on the magnetoelectric coupling coefficient are discussed. We further develop a phase-field model to study the local elastic coupling between magnetic and ferroelectric domains that show one-to-one pattern match. A multiferroic layered heterostructure of Co0.4Fe0.6/BaTiO3 is considered. Kinetics of the local elastic coupling is investigated by simulating the time-dependent electric-field-driven changes in local magnetization/polarization/strain distributions and by comparing the associated velocities of the magnetic and ferroelectric domain walls. It is found that the electric-field-driven magnetic domain evolution manifests itself as an alternating occurrence of local magnetization rotation and coupled motion of magnetic and ferroelectric domain walls with almost identical velocities. The electrocaloric effect is a phenomenon in which a dielectric material absorbs or releases heat in response to an applied electric field. An anomalous negative electrocaloric effect in ferroelectric/relaxor composites showing cooling upon applying an electric-field pulse without subsequent heating, is studied by applying a phase-field model. Evolution of domain structure and changes in dipole ordering upon applying the electric field pulse is simulated. It is revealed that coexistence of the normal ferroelectric phase and the ferroelectric relaxor phase in the composite enables stability of two distinct remnant states with ordered and disordered dipoles, respectively. Application of an electric field switches the composite between ordered and disordered states and induces the anomalous electrocaloric effect. The dependence of the electrocaloric cooling temperature and overshooting phenomena on operating temperature is simulated and discussed. Applying an ultrafast stimulus to a ferroelectric material allows one to explore possible new transient phenomena or new metastable domain patterns that may emerge during the relaxation from its excited state to its original or a new equilibrium state. In this work, we develop a phase-field model for understanding and predicting the dynamical responses of both ferroelectric and ferroelastic domain patterns under ultrafast electrical, thermal, mechanical, and optical stimuli, with advanced numerical algorithms for solving the governing dynamical equations. As an example, the nanoscale and mesoscale domain dynamics of BaTiO3 crystals under certain types of ultrafast stimuli were investigated. We demonstrate domain dynamics under an external local heat pulse with a combined characteristic of thermal conduction and polarization dynamics under pyroelectric effect. We predict possible deterministic 180° ferroelectric domain switching through the application of ultrashort mechanical stimuli. We further reproduce experimentally observed laser-pulse-induced domain dynamics with distinct responses determined by both the orientation and the location of the domains. Mesoscale mechanisms of ferroelectric domain and domain wall responses, as well as intrinsic lattice vibrations, are revealed. The theoretical insights on ultrafast domain dynamics will provide useful guidance for manipulating dynamic functionalities of ferroelectric materials.
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