Investigation On Dielectric Response And Fatigue Of Ferroelectric And Piezoelectric Devices Under Combined Mechanical And Electric Loads

With the multilayer ceramic capacitors (MLCCs) and piezoelectric stack actuators as representative devices, their dielectric responses and field-induced fatigue were systematically investigated under combined mechanical and electric loads.Firstly, the residual stress in multilayer ceramic capacitor (MLCC) has been evaluated by two-dimensional finite element simulation in combination with sin 2ψ?Xmeasurement. Subsequently, the influences of the residual stress existing in MLCCs on the dielectric properties of BaTiO3-based ferroelectrics were investigated under DC bias field by both dielectric measurement method and a phenomenological thermodynamic model. Finally, the relationships among the number of dielectric layers, residual stress and dielectric permittivity were established. The results show that a compressive in-plane stress exists in the active layers of the MLCC, which increases with increasing of the number of dielectric layers when both dielectric layer thickness and electrode thickness are kept constant. The compressive in-plane stress existing in the MLCC leads to an increase in the dielectric permittivity, but the suppression of the dielectric permittivity induced by DC bias field is strengthened due to the residual stress. Both the residual stress existing in MLCCs and DC bias field result in up shift of the Curie temperature of BaTiO3-based ferroelectrics. The residual stress strengthens the frequency depression while the dc bias field weakens the frequency depression. DC bias field causes a significant decrease in the dielectric loss at lower temperature, but it has little effect at higher temperature.When temperature is above 423K, the dielectric loss almost keeps invariable. In addition, the dielectric loss is not influenced by the residual stress existing in MLLCs.The changes in the dielectric properties and ferroelectric phase transition of BaTiO3-based ferroelectrics in MLCCs were investigated under the combined external uniaxial compressive stress and dc bias field by both dielectric measurement and a phenomenological thermodynamics. It is found that the application of external uniaixal compressive stress along a direction parallel to the internal electrode layer in the MLCCs can cause an increase in the dielectric permittivity, while the application of external uniaxial compressive stress along a direction perpendicular to the internal electrode layer in the MLCC can strengthen the suppression of the dielectric permittivity due to DC bias field. Additionally, the first order paraelectric-ferroelectric phase transition of the modified BaTiO3 ceramic becomes stronger with increasing DC bias when a uniaxial stress is also applied, meanwhile, the Curie temperature shifts up. As the DC bias increased up to 5.6MV/m, the Curie peak becomes narrower and higher, and two new dielectric peaks at lower temperature of the dielectric permittivity vs temperature curve were induced. These results are related to the transition from the polar microregions (PMRs) to polar macroregions in the shell part, nduced by DC bias field.The dielectric nonlinearity and electric-induced fatigue of PZT-based piezoceramics were investigated. It is observed that as external uniaxial compressive stress increases, the dielectric nonlinearity in the intermediate electric field region (Rayleigh region) initially increases, and subsequently declines, while it decreases continuously and disappears completely in the high electric field. In addition, a marked improvement of fatigue resistances can be achieved by the application of the external uniaxial compressive stress along a direction parallel to the internal electrode layer in the investigated piezoelectric stack actuator, which can be attributed to both non 180o domain switching and suppression of microcracking in the fatigue process, triggered by the external mechanical loads.