Effects Of The Grain Boundary And Orientation On The Electromechanical Response Of Ferro-electric Polycrystals

A real-space phase field model which based on time-dependent Ginzburg-Landau equation is employed to simulate the nonlinear electromechanical behaviors of ferroelectric polycrystals under electric and/or stress field. The effects of grain boundaries and grain orientation on the domain switching of ferroelectric polycrystals are investigated. The grain boundaries in the phase field model are regarded as dielectrics or semiconductor in which the ferroelectric properties are degraded completely. As the grain boundaries are dielectrics, the phase field simulations show that the presence of dielectric grain boundaries results in a large build-in depolarizing field in grains. The depolarizing field has a significant influence on the coercive field, the switching behaviour of ferroelectric domain under an electric field or stress, and the piezoelectric and dielectric properties of the ferroelectric polycrystal. It is found that both coercive field and remnant polarization decrease with the increase of the thickness of dielectric grain boundary. However, the piezoelectric coefficient and permittivity of the ferroelectric polycrystal become larger when the thickness of dielectric grain boundary increases. It is found that the grain orientation plays an important role in the domain switching of ferroelectric polyscrystals. For the ferroelectric polyscrystals with the random grain orientation, the domain switching takes place from the grains with the large crystallographic angles. When the grain orientations are uniform, new domains nucleate from the cross-sections between the grain boundary and material surface. Due to the effect of grain orientation, the coercive field, remanent polarization and strain of ferroelectric polycrystals with the uniform grain orientation are larger than those with random grain orientations. As the grain boundaries are semiconductors, each grain almost becomes an equipotential body, which makes the grains are difficult to be poled to a single domain but domain switching becomes easier to take place. Therefore, the coercive field and residual polarization of the polycrystal are smaller, while the dielectric property keeps unchanged in a certain electric field range.