The Domain Switching Theory For The Ferroelectric/piezoelectric Behaviors Of Ferroelectric Thin Film

Ferroelectric thin films are widely used as microactuators,accelerometers and pressure transducers due to their excellent dielectric, ferroelectric, and piezoelectric properties. Domains exist in ferroelectric thin film, and the domain switching results in the nonlinear behavior. At present, there are several domain switching criteria used to simulate the nonlinear behavior of bulk ferroelectric materials under external electric and mechanical fields. However, modeling of the nonlinear behavior for ferroelectric thin film is rather limited. So we focus on the domain switching under external electric and mechanical fields, and propose the proper model to investigate the nonlinear behavior for ferroelectric thin film. The research can be served as the basis for the development of advanced ferroelectric devices.The residual stress/strain is induced inevitably during the deposition of ferroelectric thin film, and it can be regard as the response strain of domain for ferroelectric thin film. The effect of electromechanical strain on domain switching has been taken into account. On the other hand, we study the dead layer forming at the interface between ferroelectric thin film and electrode. It is well known that the permittivity of a ferroelectric layer decreases with the decrease of thickness and the observation can be explained by assuming the existence of the dead layer. In this dissertation, at first we use an improved model to study the effect of residual/ electromechanical strains on domain switching of lead zirconate titanate thin film. Secondly, based on the improved model, the effect of dead layer on domain switching is discussed.1. The residual strain developed in the thin film during fabrication consists of phase transition, intrinsic and thermal mismatch strains. The residual strain regarded as the spontaneous strain is taken consider into the piezoelectric constitutive relation. At the same time, the electromechanical strain has been considered into the piezoelectric constitutive relation. The difference of Gibbs energy for domain before and after switching is defined as the driving force. If the driving force exceeds the corresponding threshold, the 90°or 180°domain switching will occur. The electric displacement and the strain of domain can be derived through the piezoelectric constitution, and the electric displacement and the strain of ferroelectric thin film can be gained by averaging all domains. The hysteresis loops and butterfly curves are simulated with the improved and traditional models and the simulated results are compared with the experimental results. The simulated results with the improved model are more close to the experimental results. From the hysteresis curves at different stresses, we can have the conclusions that with the increases of the compressive stress, the remanent polarization increases and coercive electric field decreases. With the increases tensile stress, the remanent polarization decreases and coercive electric field increases. From the butterfly curves at different stresses, we can have the conclusions that the remanent strain and field-induced strain increase with the increases of the compressive stress, decrease with the increase of the tensile stress.2. We consider the sandwich structure to describe the dead layer clearly. The domain switching behavior can be analyzed starting from the condition of continuity of the normal component of the electric displacement field and from the equation for the potential drop across the system. The piezoelectric constitutive relation considering the dead layer has been derived and the electric displacement and the strain of domain can be calculated through it. The electric displacement and the strain of ferroelectric thin film can also be gained by averaging all domains. The hysteresis loops and butterfly curves are simulated by the improved and traditional models considering the dead layer. The simulated results considering the dead layer are most close to the experimental results.