Atomistic Simulation Of Domain Structures And Electromechanical Coupling Responses In Ferroelectric

In recent years, micro-electromechanical systems (MEMS) and micro/nano devices have been widely applied in industries and engineering fields. Ferroelectric materials are very important functional materials, and widely applied in nanoscale functional devices due to their distinguished mechanical, thermal, and electric coupling performances. With considerable decrease of the ferroelectric device characteristic size, the electromechanical coupling responses in the nano-scale ferroelectrics are obviously different from traditional large-scale cases, because the polar domain structure possibly exhibits some special configurations and evolution patterns in nano-scale. Complex domain configurations and untraditional evolution patterns in nanoscale pose a severe challenge to application and design of nano-scale ferroelectric devices. Nano-film is an important form of micro-and nano-scale devices, so understanding domain evolution and electromechanical coupling response in a mechanically loaded ferroelectric nano-film is an important issue for ferroelectric device design.In this dissertation, based on the shell model and the molecular dynamics method, an electromechanical coupling calculation framework is proposed, a calculation model for the dielectric constant of nano-scale ferroelectric is established, and ferroelectric domain evolution behaviors in nono-scale under displacement loading are simulated. After that, initiation, movement, collision and annihilation processes of vortex and anti-vortex pairs are observed, and the strain-rate effect is also investigated. Some creative achievements are summarized as follows.(1) The calculation method for the dielectric constant of nano-scaled perovskite is presented. Based on Lorenz’s effect electric field model, a calculation formula for dielectric constant of nano-crystal perovskite is given. After the structural parameters in the formula are obtained by using the molecular dynamics (MD) simulation, the dielectric constant of perovskite at nano-scale can be calculated. As an example, the dielectric constant of nano-crystal BaTiO3at various temperatures is calculated via the presented method. The numerical results are compared with the existing experimental results at all interval of phase transition temperature. It is shown that the numerical results are in a good agreement with the experimental results. The proposed method is demonstrated to be reasonable.(2) The single crystal BaTiO3nanofilm subjected to a monotonically increasing uniaxial compressive strain is simulated with the MD method combined with the Shell model. The3-stage evolution process of180°stripe domain to a flux closure vortex-like domain consisting of four90°stripe domains, then to a vortex-anti-vortex-vortex (VAV) array, and finally to a new180°stripe domain perpendicular to the initial stripe domain is observed when the strain varies from zero to2%. Both the stable condition and the vortex configuration in the compressed nano-film are discussed.(3) The mechanical and electromechanical coupling responses of a ferroelectric single crystal nano-film under displacement loading with different strain rates are simulated. While the linear stress-strain relation independent of the strain rate is given, strong strain rate dependence is exhibited in the electromechanical coupling response when the strain rate is between0and0.5ns"1. There is an approximate semi-logarithmic linear relationship between the polarization stability strain and the strain rate. With the increasing strain rate,180°domain switches take place sequentially from inside to outside in the stable domain structure evolution, and the number of domain walls increases. But after the strain rate exceeds0.5ns"1, it has almost no effect on the domain structure.(4) The initiation, movement, collision and annihilation process of the vortex and anti-vortex pairs in single crystal BaTiO3nano-film under strain loading is simulated, and their velocity and the balance time required for the annihilation are estimated. Based on the interaction relation between the ions inside the ferroelectric in atomic scale, the interaction relationship between the polar vectors is derived, and then the interactions between two vortexes with a common rotation direction, between two vortexes with different directions, between a vortex and an anti-vortex, and between an anti-vortex and an anti-vortex are also obtained. Based on them, the interaction inside the polarization structure and the evolution regularity can be roughly analyzed.