Toughening due to domain switching in single crystal ferroelectric materials

In this paper Mode I steady state crack growth in single crystal ferroelectric materials is investigated. Specifically, the fracture toughness enhancement due to domain switching near a steadily growing crack tip is analyzed. For this purpose, an incremental phenomenological constitutive law for single crystal ferroelectric materials is implemented within a finite element model to calculate the stress and remanent strain fields around the crack tip. Also, the ratio of the far field applied energy release rate to the crack tip energy release rate, i.e. the toughening, is calculated. The numerical computations are carried out for single crystal ferroelectric materials of tetragonal or rhombohedral structure with different switching hardening and irreversible remanent strain levels. Toughening levels for crack growth along different crystallographic directions and planes are obtained and compared. Results from numerical computations for the toughening anisotropy for both tetragonal and rhombohedral crystals are presented and discussed.     

铁电陶瓷中电畴翻转研究进展

电畴为铁电陶瓷固有的独特微观组织特征之一,铁电陶瓷的许多性能均与其密切有关.综述了铁电陶瓷中的电畴结构,系统介绍了电场、机械作用引起的电畴翻转,概述了电畴翻转对铁电陶瓷断裂韧性的影响及其研究进展.     

Numerical analysis of ferroelectric/ferroelastic domain switching in ferroelectric ceramics

A numerical approach predicting the behavior of ferroelectric ceramics under electric field and mechanical loading is proposed in this paper. In the model, macroscopic properties of ferroelectric ceramics are determined by microscopic structures. Ferroelectric ceramics are seen to be composed of many domains with different orientations, and domain switching is the source of the nonlinear constitutive behavior of the ferroelectric ceramics. Numerical calculations based on the model were carried out, and the computational results are compared with the experimental results, which shows the two sets of results consist with each other. The calculation approach can provide a guidance for the ceramics component design.     

Variations in predicting domain switching of ferroelectric ceramics

In this article, a set of equivalent variational formulations for computing the driving forces for domain switching in ferroelectric materials is presented. It is proven that these formulations allow the free adoption of any couple of mechanical and electric fields as independent variables while obtaining consistent results. In addition, explicit expressions are provided for each formulation which allows for the study of the phase transformation process under different constraints.     

Nanoscale domain switching behaviour in polycrystalline ferroelectric thin films

We report on the nanoscale domain switching behaviour in polycrystalline tetragonal perovskite lead zirconate titanate (PZT) ferroelectric thin films investigated via piezoresponse force microscopy (PFM). Local domain structures were imaged as a function of varying biasing conditions and spatial location of the tip within 50-100?nm sized grains. Nanoscale piezoresponse images provided direct visual evidence of the complex interplay between electrical and mechanical fields in a polycrystalline system, which causes effects such as correlated switching between the grain of interest and neighbouring grains, ferroelastic domain switching, inhomogeneous piezostrain profiles and domain pinning on very minute length scales. Detailed investigations on mechanisms which induce such domain behaviour are presented.     

Development of ferroelectric oxides based resistive switching materials

Resistive random access memory (RRAM) is one of the most promising candidates that satisfies the requirements of new generation non-volatile memories, as a consequence of its high density, outstanding scalability, and low power consumption. The review is based on a summary of recent studies in ferroelectric oxides based resistive switching (RS) materials and devices. It highlights the various ferroelectric oxide materials with RS behaviour and the underlying mechanisms including filament-type and interface-type mechanism. In the end, the challenge in current RRAM for future high-density data storage applications is addressed.     

High-resolution studies of domain switching behavior in nanostructured ferroelectric polymers

We have demonstrated an effective electrical control of polarization in the individual crystalline nanomesas of the ferroelectric polymer, poly(vinylidene fluoride)-trifluoroethylene (PVDF-TrFE) and its relation to the polymer structure. The mechanism of polarization reversal has been investigated via sub-10 nm real space imaging of domain pattern evolution under an applied electric field. The domain switching behavior revealed in PVDF-TrFE nanomesas is drastically different from that observed in inorganic solid-state crystalline ferroelectrics. The nanoscale features of the switching process include remote domain nucleation and spatially nonuniform wall velocity. Local switching spectroscopy and domain dynamics studies relate the observed switching features to a random-bond type disorder associated with defects in conformation and molecular packing.     

On modeling of phase transformations in ferroelectric materials

Summary This paper represents an effort to model phase transformations in ferroelectric materials induced by the combined influence of electric fields and thermo-mechanical loading. The implications of thermodynamics principles for the stability of phases exhibited by a ferroelectric material and for the corresponding phase transformations are studied. Some of the theoretical predictions concerned with the dependence of the ferroelectric-paraelectric phase transformation temperature on the applied electric fields and stresses are in qualitative agreement with experimental observation. A ferroelectric material is capable of exhibiting more than one phase for given stresses, applied electric fields and temperature, a transformation among these phases can occur only if it does not result in an increase of the Gibbs energy of the system. In studying polarization reversal, it is found that the difference of the Gibbs energy between the electric twins, i.e. the two ferroelectric phases which share the same polar axis, coincides formally with thedriving traction introduced by Abeyaratne and Knowles [6] in modeling phase transformations induced by thermomechanical loading. An expression for the generalized driving traction, acting on a surface of discontinuity within a continuum in the presence of electromagnetic fields, is presented in this paper.     

Large electric-field-induced strain in ferroelectric crystals by point-defect-mediated reversible domain switching

Ferroelectric crystals are characterized by their asymmetric or polar structures. In an electric field, ions undergo asymmetric displacement and result in a small change in crystal dimension, which is proportional to the applied field. Such electric-field-induced strain (or piezoelectricity) has found extensive applications in actuators and sensors. However, the effect is generally very small and thus limits its usefulness. Here I show that with a different mechanism, an aged BaTiO(3) single crystal can generate a large recoverable nonlinear strain of 0.75% at a low field of 200 V mm(-1). At the same field this value is about 40 times higher than piezoelectric Pb(Zr, Ti)O(3) (PZT) ceramics and more than 10 times higher than the high-strain Pb(Zn(1/3)Nb(2/3))O(3)-PbTiO(3) (PZN-PT) single crystals. This large electro-strain stems from an unusual reversible domain switching (most importantly the switching of non-180 degrees domains) in which the restoring force is provided by a general symmetry-conforming property of point defects. This mechanism provides a general method to achieve large electro-strain effect in a wide range of ferroelectric systems and the effect may lead to novel applications in ultra-large stroke and nonlinear actuators.     

Phase field modeling of domain structures in ferroelectric thin films

Phase-field simulations were used to explore the effect of the characteristics of the Landau-Devonshire free energy and values of electrostatic and elastic interactions on the formation of different types of domain structures in ferroelectric thin films. Simulations were performed at different constant-applied electric fields and by using a cyclic continuously changing field. It is shown that the 180 degrees or 90 degrees domain structures can be produced depending on the relative strength of elastic interactions and the ratio of barrier heights that determine the energy of the 180 degrees and 90 degrees domain boundaries. It is shown that the applied field strength and the thickness of the dead layer can play a minor role in the transition between the 90 degrees and 180 degrees domain structures. It is also demonstrated that the poling history can affect the type of the domain structure.     

Effect of domain switching on kinking of a crack in a ferroelectric material

Summary Crack kinking induced by domain switching in a ferroelectric material under purely electric loading is investigated. Boundaries of domain switching zones for the asymptotic problem of a semi-infinite crack under the small scale conditions are determined based on the nonlinear electric theory. Stress intensity factors induced by the domain switching are numerically evaluated using the solution of the switching zone. Numerical results of the kink angle are obtained as a function of the ratio of the coercive electric field to the yield electric field for various polarization angles. Crack kinking in ferroelectric materials subjected to a cyclic electric field is also examined. The crack in the fully poled materials branches with different directions at application of the positive and negative electric fields, respectively. The electric fatigue crack is shown to have a forked crack pattern in the fully poled materials.     

Unexpectedly low barrier of ferroelectric switching in HfO2 via topological domain walls

Fluorite-structure ferroelectrics — in particular the orthorhombic phase of HfO₂ — are of paramount interest to academia and industry because they show unprecedented scalability down to 1-nm-thick size and are compatible with Si electronics. However, their polarization switching is believed to be limited by the intrinsically high energy barrier of ferroelectric domain wall (DW) motions. Here, by unveiling a new topological class of DWs, we establish an atomic-scale mechanism of polarization switching in orthorhombic HfO₂ that exhibits unexpectedly low energy barriers of DW motion (up to 35-fold lower than given by previous conjectures). These findings demonstrate that the nucleation-and-growth-based mechanism is feasible, challenging the commonly held view that the rapid growth of the oppositely polarized domain is impossible. Building on this insight, we describe a strategy to substantially reduce the coercive fields in HfO₂-based ferroelectric devices. Our work is a crucial step towards understanding the polarization switching of HfO₂, which could provide a means to solve the key problems associated with operation speed and endurance.     

Comment on "modeling of elastic nonlinearities in ferroelectric materials"

This letter discusses certain aspects in the modeling of nonlinear mechanical effects that were not correctly considered in a recent article in this journal     

Estimation of nonlinear interfacial capacitance from domain switching current of ferroelectric thin films

We introduce a methodology to estimate the nonlinear capacitance of interfacial “passive” layers of ferroelectric thin films from domain switching currents directly. The methodology has the advantage over the traditional extrapolation technique of a linear plot of the inversed capacitance against the film thickness which neglects the size effect on the ferroelectricity. Expectedly, this technique remains suitable in ultrathin films with the thickness scaling down into a few nanometers, where the size effect is important. From our measurements, we found that the interfacial capacitance increases nonlinearly with the reduction of the applied voltage in a tendency similar to the capacitance-voltage curve of ferroelectric thin films above the coercive voltage. Nevertheless, the capacitance at a high field drops down closely to a value derived from the traditional extrapolation technique. The pertinent physics is discussed in this work. Finally, we observed the reduction of the interfacial capacitance with the rising temperature, which suggests the thickening of interfacial layers at high temperatures.     

Driving forces on domain walls in ferroelectric materials and interaction with defects

The driving force on a domain wall in a piezoelectric material is identified by using a variational principle. In addition to the domain wall, point defects are considered. These defects are modeled as electrically charged centers of dilatation (or contraction). For representative numerical simulations, the finite element method (FEM) is used. The simulations allow the analysis of the interaction of point defects and domain walls. It is found that point defects have the ability to reduce the driving force on the domain wall. This can potentially lead to the hindering of the domain wall mobility which is suspected to be a mechanism in electric fatigue.     

The influence of fabrication processing on domain structure and polarization switching in PZN-based ferroelectric ceramics

Lead zinc niobate (Pb(Zn_1/3Nb_2/3)O₃, PZN) based ceramics are prepared by using conventional mixing oxide and complex phase reaction-sintering ceramic techniques. From the results of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), it is clear that these two fabrication processing routes produce different microstructures and ferroelectric domains in the same Pb(Zn_1/3Nb_2/3)O₃–BaTiO₃–Pb(Zr_0.4Ti_0.6)O₃ composition. Furthermore, different phase transitions are observed for the temperature dependence of the dielectric permittivity that can be confirmed by differential scanning calorimetry (DSC). Different polarization switching characteristics are also examined by using high field-induced strain and ferroelectric hysteresis loop. It is suggested that the distribution of the inner stress and domain configuration should be related with the fabrication processing of ferroelectric ceramics.     

Domain switching in polycrystalline ferroelectric ceramics

Ferroelectric ceramics are widely used as sensors and actuators for their electro-mechanical properties, and in electronic applications for their dielectric properties. Domain switching--the phenomenon wherein the ferroelectric material changes from one spontaneously polarized state to another under electrical or mechanical loads--is an important attribute of these materials. However, this is a complex collective process in commercially used polycrystalline ceramics that are agglomerations of a very large number of variously oriented grains. As the domains in one grain attempt to switch, they are constrained by the differently oriented neighbouring grains. Here we use a combined theoretical and experimental approach to establish a relation between crystallographic symmetry and the ability of a ferroelectric polycrystalline ceramic to switch. In particular, we show that equiaxed polycrystals of materials that are either tetragonal or rhombohedral cannot switch; yet polycrystals of materials where these two symmetries co-exist can in fact switch.     

Modeling of charge switching in ferroelectric capacitors

To simulate charge switching in ferroelectric capacitors, a pair of exponential growth and decay currents is mapped to the process of polarization reversal. This is based on the fact that these exponential currents [i.e., i = I(m) e(t/tau) (t < or = 0) and i = I(m) e(-t/tau) (t > or = 0)], are completely specified by two constants I(m) and tau and each accommodates an integral charge Q = I(m) x tau. Equating this charge to the remanent spontaneous polarization allows for the modeling of switching current. For practical circuit simulations for charge switching, this modeling of switching current is simplified to an exponential decay current whose integral charge is set equal to the total reversed spontaneous polarization. This is because an exponential decay current can be conveniently implemented by charging a series resistor and capacitor (RC) circuit with a pulse-voltage source. The voltage transitions of the pulse source are associated with the polarization reversal and can be controlled with a noninverting Schmitt trigger that toggles at the positive and negative coercive voltages of a ferroelectric capacitor. The final circuit model incorporates such electrical and geometrical parameters as capacitance, remanent spontaneous polarization, coercive field, electrode area, and film thickness of a ferroelectric, thin-film capacitor.