Phase-field Simulation Of Domain Evolution And Properties In Ferroelectric Materials

Nowadays,progresses in materials science and engineering have shown increasing reliance on the advantages of materials properties that originated from multi-field couplings.In ferroelectric materials,these properties include electrical-mechanical coupling(i.e.piezoelectricity and flexoelectricity),electrical-thermal coupling(i.e.pyroelectricity),magneto-electricity coupling,dielectricity and so forth.Strain controlled ferroelectric thin films,composites and mult-layer structures have also emerged as promising designs for materials applications.All above properties involve the dynamics of ferroelectric polarization as well as reactions to externally applied fields.As a powerful tool in materials researches,computational materials methods have been acting like a bridge connecting microscopic structures and macroscopic properties.In this work,thermodynamic calculation and phase field method were used to study properties of ferroelectric materials.Firstly,a phase-field model is developed to elucidate the process of polarization switching in BiFe03 thin film.The results demonstrated an energy-favorable mechanism for domain switching path and revealed possible ferroelectric domain switching modes.It is shown that 71° switching is dominant among the three possible switching paths,namely 71°,109°and 180°switching.This might provide significant references for the application of ferroelectric materials under electric fields.Secondly,thickness effects in electric-field induced domain switching in BiFe03 thin film is studied using the above model.Time evolutions of domain switching percentage for films with different thicknesses have been explored to reveal the primary switching path and its dependence on film thickness.In addition,hysteresis loop for these films are calculated to obtain their coercive fields.Results provide a nonlinear thickness dependence of coercive field for ultrathin films.A parametric study of the interactions between film thickness,coercive field,I-V response and polarization switching behavior has been discussed,which could provide physical insights into materials engineering.Thirdly,a modified phenomenological potential was constructed for BaTiO3-CaTiO3 solid solution single crystal based on the Landau-Devonshire theory.The Ba1-xCaxTiO3 solid solution phase diagram of temperature vs concentration(T-x)is obtained and shown to perfectly agree with experimental observations.Sustained Curie temperature can be obtained by the increase of Ca concentration x,while the transition temperatures from tetragonal to orthorhombic phase,and further to rhombohedral phase decrease with increasing Ca content.In this work,spontaneous polarization and dielectric constant dependence on concentration are studied and relationship between hydrostatic pressure and transition temperatures is established to pave the way for the generation of temperature-concentration-pressure phase diagram.At last,by utilizing the thermodynamic potential with ferroelectric phase-field model,microstructure evolutions of Ba1-xCaxTiO3 films at certain temperature were obtained.With different Ca concentration,these films would reach different equilibrium phases.Simulation results also showed ferroelectric domain formations in different phases.In summary,this work accomplished a comprehensive study about ferroelectric material.By using phase-field method,electric field induces domain evolution in ferroelectric thin film was obtained.Polarization switching path was analyzed based on these results.The effects of electric field and film thickness on multidomain switching behavior were further studied.By using thermodynamic theory,a phenomenological potential was developed for Ca/Sr doped BaTi03,and temperature-composition,pressure-composition,and 3D phase diagrams were obtained.The calculated results were consistent with experimental data,moreover,it could be used to predicted kinds of physical properties.The adoption of this thermodynamic potential in phase field model could help study microstructure evolution,domain wall motion,and multidomain switching in Ca/Sr doped barium titanate solid solutions.