Study Of The Domain Structures And Physical Properties Of Ferroelectric Materials Using Landau Theory

Ferroelectric materials are widely used functional materials,which have good piezoelectric,dielectric and pyroelectric properties and so on.They are widely used in electronic devices and instruments.The properties of ferroelectric materials are not only dependent on their quality and chemical composition,but also determined by their domain structures.For example,the lead-based ferroelectric solid solutions Pb(Zr_1-x-x Ti_x)O₃?PZT?and Pb(Mg_1/3Nb_2/3)O₃-x PbTiO₃?PMN-xPT?reveal very high piezoelectric performance at the compositions close to morphotropic phase boundary?MPB?,which is related to the formation of monoclinic?M?phase domains.The physical properties can be enhanced by changing the domain patterns,domain sizes and domain structures of ferroelectrics,which can benefit to the applications of ferroelectric materials.However,the formation mechanism of some special domain structures are still unclear,and the studies on how to improve the properties of ferroelectric materials by engineered domain structures are still insufficient.Therefore,it is very meaningful to study the formation mechanism of the domain structures and their special characteristic under external loading.In this dissertation,by combing the Landau phenomenological theory with phase field simulation,the influences of the domain structures on the properties of ferroelectric materials are studied in the following aspects.Based on the finite difference algorithm,we establish the discretized phase field model for the simulation of domain structures.We present the treatments of the dimensionless of parameters and periodic boundary conditions.By using phase field simulation,the influence of the elastic energy and gradient energy on the formation of the domain structures is studied.The phase field simulation model provides us an effective method on investigating the physical mechanism of the domain structures and properties in this work.Using Landau theory and phase field simulation,the domain structures of different compositions in PZT ferroelectric solid solution are studied.We focus on the formation mechanism of the M phase at the MPB composition.For bulk PZT,large strain can be induced due to the lattice misfit of the tetragonal?T?and rohmbohedral?R?phases at the MPB compositions.Using the three dimensional phase field simulation,we show that the misfit strains between T and R phases can lead to an adaptive M structure in the MPB region,similar to the effects of misfit strains between a crystal and substrate in epitaxial ferroelectric thin films.The results indicate that the existence of the M phase in the MPB region can lead to a high piezoelectric coefficient.Then,the formation of the vortex structure in ferroelectric nanowire-polymer nanocomposite and its dielectric energy storage property have been investigated.Using the three-dimensional phase field simulation,we show that multivortex structures can exist in ferroelectric nanowires without charge defects and free charges on the interface between the filler and matrix.The switching behavior of the topological structure under external electric field is calculated in nanocylinderwires.The results indicate that the small remnant polarization and very narrow hysteresis loop due to the vortex structure in the nanocomposites can lead to a large enhancement of energy storage density and efficiency.Furthermore,the influence of the domain size and grain size on the electrocaloric properties of ferroelectric systems is studied.The calculated results indicate that the electrocaloric property improves with the decreasing of domain size at room temperature.In order to further enhance the adiabatic temperature change at room temperature,we have studied how the phase transformation temperature and electrocaloric property can be tuned by changing the grain size close to the critical size.Our results show that the electrocaloric coefficient at room temperature can be greatly enhanced by controlling the grain size.The results provide an approach for improving the electrocaloric properties at room temperature in experiments by engineering the grain size.