Papers by Keyword: Domain Switching

Abstract: Effects of the electric inclusion in ferroelectrics on the stress and electric field concentration and polarization switching are investigated based on a phase field approach containing the time-dependent Ginzburg-Landau equation. To capture a clear physical picture with the simulation, the crack medium in the electric inclusion is taken into account explicitly as a crack fluid medium as water, oil or air in region. The simulations exhibit a macroscopic electric field concentration in the electric inclusion filled with air, and a significant influence on the domain evolution from the micro perspective, while the electric inclusion filled with water have little influence. The numerical calculations indicated that, when the dielectric constant inside the flaw are much smaller than the dielectric constant of the ferroelectric matrix, the electric field inside the crack medium are enhanced much higher than the applied electric loading. The result implies the domain evolution takes place with the minimization of total free energy, which involves the high electric field energy derived from the electric inclusion. Therefore, the crack fluid medium in ferroelectric plays in importation role in the effect on the non-uniform distribution of the stress and electric field.

Abstract: A lanthanum doped lead zirconate titanate (PLZT) ceramic specimen was prepared by the conventional solid state reaction method. The crystal phase and morphology of the PLZT specimen were characterized by XRD and SEM techniques, with the hysteresis loop by RT6000HVS system. The compressive stress was applied to the PLZT specimen through the microtest mechanical loading device. In-situ Raman spectra focused on a fixed grain under various compressive stresses were recorded for different polarization directions of the scattered light. The effects of stresses on the Raman spectra and the intensity ratio between the E+B1 and E(2TO) modes are discussed.

Abstract: By employing a dipole defect model, two-dimensional phase field simulations of domain switching in the crack tip vicinity of a crack embedded in a relaxor ferroelectric single crystal, which was subjected to mechanical loading and electric field, have been carried out. The interaction between the dipole defects and crack, the influence of the dipole defect concentration density on the switching process, and the coupling effect of mechanical stress and electric field on domain switching in the vicinity of the crack tip have been studied. Comparing the results obtained from relaxor ferroelectrics with those of normal ferroelectrics, the former showed that, due to the interaction between the dipole defects and crack, polarization switching in the vicinity of the crack tip was suppressed. Moreover, the coupling between applied mechanical stress and electric field can either promote or suppress domain switching in the vicinity of a crack.

Abstract: In-situ observation of the fatigue crack growth and 90o domain switching was carried out for BaTiO3 ferroelectric single crystals under alternating electric field. It is shown that during the electric cycling, the crack propagates continuously. Parallel lines of 90o domain boundaries can be seen and they flip at each reversal of the alternating electric field. The width of the 90o domain switching zone grows with the number of cycles and its frontal always lies ahead of the crack tip. It is suggested that the cyclic stress field induced by the repeated 90o domain switching at the crack tip, as well as the stress field caused by the electrically activated material between the electrode and the material under the electrodes contribute to the observed fatigue crack growth.

Abstract: In the present paper, lead zirconate titanate (PZT) and lead titanate (PT) piezoelectric ceramics were subjected to both high electric field (which is higher than the coercive electric field) with low frequency and low electric field with high frequency (which is the resonance frequency). After applying certain electric field systematically, resonance and anti-resonance frequencies and an electrostatic capacity were measured by means of an impedance analyzer, and an electromechanical coupling coefficient, a dielectric constant, an elastic coefficient and a piezoelectric constant were evaluated from the frequencies and capacity measured. Then variation of the material properties in process of time was investigated experimentally, and the dependence of the variation of the properties due to mainly domain switching on conditions of applied electric field was elucidated.

Abstract: In this paper, a phenomenological, nonlinear constitutive relation of ferroelectric ceramics is established by considering the fact that domain switching happens gradually with the external loading. A simplified gradual switching model is suggested and implemented into the constitutive law of ferroelectric materials. The volume fraction of domain switching is used as the internal variable in the model, and its evolution equation with mechanical/electrical loading is given. Comparison with experimental results shows that this simple constitutive model can predict the nonlinear hysteresis responses of ferroelectric materials.

Abstract: Fatigue damage behavior under repeated electric loading was investigated on two kinds of PZT ceramics with discontinuous electrodes. Intergranular cracking was observed at the electrode edge in soft PZT under electric fields greater than ±400 V/mm. However, under the same loading conditions, no damage was observed in hard PZT. When cracking occurred, permittivity of specimens decreased with the number of cycles corresponding to the amount of mechanical damage. FEM analyses of the electroelastic field of the specimens showed that cracking due to cyclic electric loading was related to 180˚ domain switching caused by concentrated electroelastic field.

Abstract: Motivated by predicting the performance and reliability of piezoelectric devices, considerable research efforts have been made in recent years to mathematically describe the largesignal nonlinear behaviour of ferroelectric piezoceramics. The success of such modelling approach depends strongly on our knowledge of the loading history dependence of the material properties. In this experimental work, the nonlinear ferroelastic behaviour was investigated in detail for a commercially available soft lead zirconate titanate (PZT) material under low-frequency compressive stress loading. It was found that the strain response was significantly loading rate dependent. The evolution of the elastic moduli was determined using fast small-stress partial unloading method, which was specially designed to minimize the time-dependent effect as much as possible. The results were used to separate the reversible elastic deformation from the irreversible plastic strain due to domain switching.

Abstract: Applications of above-coercive electric fields lead to domain switching of a large or global scale. Large scale switching model is proposed to deal with load-induced domains witching in experiment. Both a discussion of crack initiation via the stress intensity factor and a discussion of crack path stability via T-stress are presented. The theoretical predictions and the experimental data roughly coincide for crack initiation, propagation and stability phenomena. Attention is also extended to consider the effect of non-uniform ferro-elastic domain switching in the vicinity of a crack. The domain switching zone is divided into a saturated inner core and an active surrounding annulus. Toughening for ferroelectrics with different poling states is estimated via Reuss type approximation. Solutions obtained according to spherical and cylindrical inclusions cover the range of experimental data.

Abstract: LiTaO3 (5--25vol%) dispersed Al2O3 matrix composite (LTA) with high relative density were fabricated by hot pressing at 1300°C. The microstructure and mechanical properties of the composite were studied. The main results are as follows: Al2O3 was found to be phase compatible with piezoelectric LiTaO3 during high temperature sintering process. The relative density of the LTA composites was greatly improved by the addition of LiTaO3 particles. Domains are clearly discerned in LiTaO3 grains in the LTA composite, confirming that LiTaO3 particles remain ferroelectrics. 90 degree domains were observed in LiTaO3 grains constrained strongly by Al2O3 matrix, which has never been found in LiTaO3 single crystal before. Comparing with those of pure Al2O3, sintered at same temperature (1300°C), both the flexural strength and the fracture toughness of the composites are all enhanced. Domain switching is suggested as a new toughening mechanism in the ferroelectric particles dispersed structural ceramics.