Sergey Zhukov

Polarization hysteresis and piezoelectricity in open-porous fluoropolymer sandwiches

Open-porous polymers form an attractive class of piezoelectric materials. Up to now it has been demonstrated that fibrous polyterafluoroethylene films after proper polarizing are responsible for the high piezoelectric response if confined between two electrically blocking polyfluoroethylene propylene layers. The sandwich structures reveal large quasistatic piezoelectric coefficients of up to 1500 pC/N. Here a theoretical model is proposed for a three-layer sandwich which quantitatively explains the experimentally obtainable polarization and its hysteresis behavior for different poling voltages. It will be reported that each sandwich structure exhibits a limit for the remanent polarization induced by polarization backswitching due to insufficient charge compensation. The correlation of the remanent polarization to the device geometry and the dielectric constants will be highlighted and suggestions for optimization will be given. It will be reported that the limitation of the remanent polarization also limi...

Experimental and theoretical investigation on polarization reversal in unfatigued lead-zirconate-titanate ceramic

The dynamics of polarization switching in a soft lead-zirconate-titanate ceramic has been studied over a broad time window ranging from 10−6–106 for applied fields between 0.5 and 2.5 kV/mm. The classical Kolmogorov–Avrami–Ishibashi model of the polarization reversal was not able to satisfactory explain the obtained results. Therefore, a new concept for the polarization dynamics of ferroelectric ceramics has been suggested, which is based on two principal assumptions, (1) a strong dependence of the polarization switching time on the local electric field and (2) a random distribution of the local switching times caused by an intrinsic randomness in the field distribution within the system. Thereby the switching volume is composed as an ensemble of many regions with independent dynamics governed by local field exclusively. Such random field distribution could be well adjusted by a Gaussian distribution around the mean value of the field applied. A total polarization dependence on time and applied field was ...

Breakdown-induced light emission and poling dynamics of porous fluoropolymers

To date the charging mechanism of porous electret materials is not completely understood. The present study is focused on the investigation of light emission during corona poling of individual films of expanded polytetrafluoroethylene (ePTFE) with nominal porosity of 91% and of the same film sandwiched between two solid poly-fluroethylene propylene copolymer (FEP) films. One observes that as soon as the applied electric field exceeds a certain threshold value (Vth), the individual film and sandwiched structure start to emit light. For the sandwich structure the light intensity decays with time to zero, whereas for an individual ePTFE film a continuous light emission at V>Vth is observed. The emitted light is a result of electrical breakdown events in air developing in the pores of the media. For the sandwich sample the positive and negative charges generated during breakdown are trapped at opposite ePTFE/solid film interfaces, thus resulting in a macroscopic dipole formation (polarization). In the case of...

Polarization dynamics across the morphotropic phase boundary in Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ferroelectrics

Analysis of polarization switching dynamics by means of the inhomogeneous field mechanism model allows insight into the microscopic mechanism of reversed polarization domain nucleation. For all chemical compositions studied, two distinct field regions of nucleation are established. In the high-field region, the activation energy barrier is found to be inversely proportional to the local field according to the Merz law. In contrast, the barriers in the low-field region exhibit a linear field dependence with a minimum in the compositional region of phase instability, which can explain the corresponding peak ferroelectric properties.

Polarization switching dynamics by inhomogeneous field mechanism in ferroelectric polymers

The understanding of polarisation switching dynamics of ferroelectrics is of great importance for practical applications and is being steadily advanced for ferroelectric ceramics and polymers for more than half a century. The temporal behaviour of polarisation reversal in ferroelectric polymers like polyvinylidenedifluoride (PVDF) or its copolymer with trifluoroethylene P(VDF-TrFE) cannot be satisfactory explained by simple models such as the classical Kolmogorov-Avrami-Ishibashi nucleation and growth theory [1–3] or by models considering stretched exponential laws.

Effect of bipolar electric fatigue on polarization switching in lead-zirconate-titanate ceramics

From comparison of experimental results on polarization switching in fresh and electrically fatigued lead-zirconate-titanate (PZT) over a wide range of applied fields and switching times it is concluded that fatigue alters the local field distribution inside the sample due to the generation of discrete defects, such as voids and cracks. Such defects have a strong influence on the overall electric field distribution by their shape and dielectric permittivity. On this hypothesis, a new phenomenological model of polarization switching in fatigued PZT is proposed. The model assumes that the fatigued sample can be composed of different local regions which exhibit different field strengths but otherwise can be considered as unfatigued. Consequently the temporal response of a fatigued sample is assumed to be the superposition of the field-dependent temporal responses of unfatigued samples weighted by their respective volume fraction. A certain part of the volume is excluded from the overall switching process due to the domain pinning even at earlier stages of fatigue, which can be recovered by annealing. Suitability of the proposed model is demonstrated by a good correlation between experimental and calculated data for differently fatigued samples. Plausible cause of the formation of such regions is the generation of defects such as microcracks and the change in electrical properties at imperfections such as pores or voids.

Barrier heights, polarization switching, and electrical fatigue in Pb(Zr,Ti)O3 ceramics with different electrodes

The influence of Pt, tin-doped In2O3, and RuO2 electrodes on the electrical fatigue of bulk ceramic Pb(Zr,Ti)O3 (PZT) has been studied. Schottky barrier heights at the ferroelectric/electrode interfaces vary by more than one electronvolt for different electrode materials and do not depend on crystallographic orientation of the interface. Despite different barrier heights, hysteresis loops of polarization, strain, permittivity, and piezoelectric constant and the switching kinetics are identical for all electrodes. A 20% reduction in polarization after 106 bipolar cycles is observed for all the samples. In contrast to PZT thin films, the loss of remanent polarization with bipolar switching cycles does not significantly depend on the electrode material.

Piezoelectrets from sandwiched porous polytetrafluoroethylene (ePTFE) films: influence of porosity and geometry on charging properties

Sandwiched structures of open-porous and solid polymer dielectrics reveal a strong piezoelectric response after proper charging by high electric fields. Here, the electrical properties of corona-poled three-layer FEP/ePTFE/FEP sandwiches are studied theoretically and experimentally with the objective of its possible optimization for the highest piezoelectric activity. Modelling of the charging properties is performed for different sandwich geometries. It is shown that the maximum value of the remanent interface charge density accumulated during poling depends mainly on the ratio of the solid and porous layers thicknesses and on parameter EB, which characterizes the electric breakdown strength of air in the porous layer. EB exhibits specific dependences on porosity and thickness of the porous layer. For a given porous layer EB is independent of the solid film thickness. The obtained results can be utilized to further optimize the sandwich structure as an electromechanically active device.

Importance of geometry and breakdown field on the piezoelectric d 33 coefficient of corona charged ferroelectret sandwiches

Electrical and mechanical properties of corona poled three layer FEP/ePTFE/FEP sandwiches were studied theoretically and experimentally. Modeling of the properties has been performed, as well as their experimental verification. The piezoelectric d33 coefficient has been studied with the objective of its possible optimization. It has been shown that the maximum value of the d33 piezocoefficient depends on interrelation between solid and porous layers thicknesses, the intrinsic Young modulus of the porous layer and dependence of the breakdown field on the thickness of the porous layer. The best geometry of the sample can be designed considering the obtained results.

Effect of texturing on polarization switching dynamics in ferroelectric ceramics

Highly (100),(001)-oriented (Ba0.85Ca0.15)TiO3 (BCT) lead-free piezoelectricceramics were fabricated by the reactive templated grain growth method using a mixture of plate-like CaTiO3 and BaTiO3 particles. Piezoelectric properties of the ceramics with a high degree of texture were found to be considerably enhanced compared with the BCT ceramics with a low degree of texture. With increasing the Lotgering factor from 26% up to 94%, the piezoelectric properties develop towards the properties of a single crystal. The dynamics of polarization switching was studied over a broad time domain of 8 orders of magnitude and was found to strongly depend on the degree of orientation of the ceramics. Samples with a high degree of texture exhibited 2–3 orders of magnitude faster polarization switching, as compared with the ones with a low degree of texture. This was rationalized by means of the Inhomogeneous Field Mechanism model as a result of the narrower statistical distribution of the local electric field values in textured media, which promotes a more coherent switching process. The extracted microscopic parameters of switching revealed a decrease of the critical nucleus energy in systems with a high degree of texture providing more favorable switching conditions related to the enhanced ferroelectric properties of the textured material.