STUDY ON FRACTURE BEHAVIOR OF FERROELECTRIC CERAMICS UNDER COMBINED ELECTROMECHANICAL LOADING BY USING A MOIR(?) INTERFEROMETRY TECHNIQUE

This paper discusses an in situ observation of fracture behavior around a crack tip in ferroelectric ceramics under combined electromechanical loading by use of a moiré interferometry technique.The deformation field induced by the electric field and the stress concentration near the crack tip in three-points bending experiments was measured.By analysis of the moiré images it is found that under a constant mechanical load,the electric field almost has no effect on the crack extension in the case that the directions of the poling,electric field and crack extension are perpendicular to each other.When the poling direction is parallel to the crack extension direction and perpendicular to the electric field,the strain decreases faster than that calculated by FEM with and without electrical loading as one goes away from the crack tip.In addition,as the electric field intensity increases,the strain near the crack tip increases,and the strain concentration becomes more significant.

Dielectric Characteristics in BiFeO3-Modified SrTiO3 Incipient Ferroelectric Ceramics

We investigate the dielectric and ferroelectric properties of Sr1-xBixTi1-xFex03 solid solutions （x =0, 0.05, 0.1, 0.15 and 0.2） together with their structures. Through the analysis of Rietveld refinement of powder x-ray diffraction, a cubic structure in space group Pm3m is determined for all the compositions. An obvious dielectric relaxation peak differing from SrTiO3 is observed in the present ceramics. The peak temperature Tm increases with increasing x, and it approaches room temperature at x =0.2. The Vogel Fulcher law and Curie Weiss law fittings further confirm the relaxor ferroelectricity in the present ceramics.

Effect of electric boundary conditions on crack propagation in ferroelectric ceramics

In this paper, the effect of electric boundary conditions on Mode I crack propagation in ferroelectric ceramics is studied by using both linear and nonlinear piezoelectric fracture mechanics. In linear analysis, impermeable cracks under open circuit and short circuit are analyzed using the Stroh formalism and a rescaling method. It is shown that the energy release rate in short circuit is larger than that in open circuit. In nonlinear analysis, permeable crack conditions are used and the nonlinear effect of domain switching near a crack tip is considered using an energy-based switching criterion proposed by Hwang et al.（Acta Metal. Mater.,1995）. In open circuit, a large depolarization field induced by domain switching makes switching much more diffcult than that in short circuit. Analysis shows that the energy release rate in short circuit is still larger than that in open circuit, and is also larger than the linear result. Consequently,whether using linear or nonlinear fracture analysis, a crack is found easier to propagate in short circuit than in open circuit, which is consistent with the experimental observations of Kounga Njiwa et al.（Eng. Fract. Mech., 2006）.

Study of Dielectric and Mechanical Spectra on Point Defects and Phase Transition in Ferroelectric Ceramics

Ferroelectric materials have enormous potential applications in advanced techniques. However, there are still many problems in its practical application. Dielectric and mechanical (internal friction) measurements are very sensitive to phase transitions, relaxation process of point defects, domain walls and their mobility, which have severe effect on ferroelectric properties. These make them become very good means to investigate substantial information on structural features and to explore the fundamental principles in ferroelectric materials and their applications. In this paper, the dielectric and internal friction measurement were used to investigate the behaviors for point defects and phase transition in ferroelectric ceramics such as Bi_ 4-x La_ x Ti_ 3 O_ 12 , Bi_ 4 Ti_ 3-y Nb_ y O_ 12 , SrBi_ 2 Ti_ 2 O_ 9 , PbZr_ x Ti_ 1-x O_ 3 ,_ PMN-PT. They were used to clarify the mechanism for some ferroelectric behaviors.

Energy principle of ferroelectric ceramics and single domain mechanical model

Many physical experiments have shown that the domain switching in a ferroelectric material is a complicated evolution process of the domain wall with the variation of stress and electric field. According to this mechanism, the volume fraction of the domain switching is introduced in the constitutive law of ferroelectric ceramic and used to study the nonlinear constitutive behavior of ferroelectric body in this paper. The principle of stationary total energy is put forward in which the basic unknown quantities are the displacement ui, electric displacement Di and volume fraction pI of the domain switching for the variant I. Mechanical field equation and a new domain switching criterion are obtained from the principle of stationary total energy. The domain switching criterion proposed in this paper is an expansion and development of the energy criterion. On the basis of the domain switching criterion, a set of linear algebraic equations for the volume fraction PI of domain switching is obtained, in which the coefficients of the linear algebraic equations only contain the unknown strain and electric fields. Then a single domain mechanical model is proposed in this paper. The poled ferroelectric specimen is considered as a transversely isotropic single domain. By using the partial experimental results, the hardening relation between the driving force of domain switching and the volume fraction of domain switching can be calibrated. Then the electromechanical response can be calculated on the basis of the calibrated hardening relation. The results involve the electric butterfly shaped curves of axial strain versus axial electric field, the hysteresis loops of electric displacement versus electric filed and the evo- lution process of the domain switching in the ferroelectric specimens under uniaxial coupled stress and electric field loading. The present theoretic prediction agrees reasonably with the experimental results given by Lynch.

Energy storage optimization of ferroelectric ceramics during phase-transition process of amorphous/nanocrystalline and polycrystalline by using a phaseffeld model for dielectric breakdown

Ferroelectric ceramics have the potential to be widely applied in the modern industry and military power systems due to their ultrafast charging/discharging speed and high energy density.Considering the structural design and electrical properties of ferroelectric capacitor,it is still a challenge to ffnd out the optimal energy storage of ferroelectric ceramics during the phase-transition process of amorphous/nanocrystalline and polycrystalline.In this work,a ffnite element model suitable for the multiphase ceramic system is constructed based on the phase ffeld breakdown theory.The nonlinear coupling relationship of multiple physical ffelds between multiphase ceramics was taken into account in this model.The basic structures of multiphase ceramics are generated by using the Voronoi diagram construction method.The speciffed structure of multiphase ceramics in the phase-transition process of amorphous/nanocrystalline and polycrystalline was further obtained through the grain boundary diffusion equation.The simulation results show that the multiphase ceramics have an optimal energy storage in the process of amorphous polycrystalline transformation,and the energy storage density reaches the maximum when the crystallinity is 13.96%and the volume fraction of grain is 2.08%.It provides a research plan and idea for revealing the correlation between microstructure and breakdown characteristics of multiphase ceramics.This simulation model realizes the nonlinear coupling of the multiphase ceramic mesoscopic structure and the phase ffeld breakdown.It provides a reference scheme for the structural design and performance optimization of ferroelectric ceramics.

Microscopic origin and relevant grain size effect of discontinuous grain growth in BaTiO_(3)-based ferroelectric ceramics

Barium titanate[BaTiO_(3)(BT)]-based ceramics are typical ferroelectric materials.Here,the discontinuous grain growth(DGG)and relevant grain size effect are deeply studied.An obvious DGG phenomenon is observed in a paradigmatic Zr^(4+)-doped BT-based ceramic,with grains growing from∼2.2–6.6 to∼121.8–198.4μm discontinuously near 1320℃.It is found that fine grains can get together and grow into large ones with liquid phase surrounding them above eutectic temperature.Then the grain boundary density(D g)is quantitatively studied and shows a first-order reciprocal relationship with grain size,and the grain size effect is dependent on D g.Fine grains lead to high D g,and then cause fine domains and pseudocubic-like phase structure because of the interrupted long-range ferroelectric orders by grain boundary.High D g also causes the diffusion phase transition and low Curie dielectric peak due to the distribution of phase transition temperature induced by internal stress.Local domain switching experiments reveal that the polarization orientation is more difficult near the grain boundary,implying that the grain boundary inhibition dominates the process of polarization orientation in fine-grain ceramics,which leads to low polarization but a high coercive field.However,large-grain ceramics exhibit easy domain switching and high&similar ferroelectricity.This work reveals that the grain boundary effect dominates the grain size effect in fine-grain ceramics,and expands current knowledge on DGG and grain size effect in polycrystalline materials.

Distinct microstructure and property evolution of 0.76(Bi_(0.5)Na_(0.5))TiO_(3)-0.24SrTiO_(3) ferroelectric ceramics synthesized with different TiO_(2) reactants

Single phase 0.76(Bi_(0.5)Na_(0.5))TiO_(3)–0.24SrTiO_(3) ferroelectric ceramics have been synthesized with homogenous anatase and hierarchical rutile TiO_(2) raw reactants(BNST-A and BNST-R).Either calcined powder persists the microstructure characteristics of raw reactants.As the result,when the sintering temperature increases from 1000 to 1200℃,the average grain size and density of BNST-A increase from 0.49 to 1.48μm and 5.02 to 5.61 g/cm^(3),while those of BNST-R from 0.86 to 1.44μm and 5.37 to 5.61 g/cm^(3).BNST-A illustrates a predominant ergodic relaxor state,and BNST-R prefers a non-ergodic relaxor state,as evidenced by the distinct polarization-electric field loops and current-electric field curves.Especially,such a distinct ferroelectric state is independent of sintering temperature.It is believed that the special hierarchical microstructure of rutile TiO_(2) reactant is beneficial to form denser ceramics with larger grains,and thus suppresses the contributions of polar nanoregions and defect-induced built-in field to ferroelectric property,leading to non-ergodic relaxor state.This work clearly demonstrates the nonnegligible effects of TiO_(2) reactants on the microstructure and properties of BNST ferroelectric ceramics.

Structure, ferroelectric and dielectric properties of Bi_2 WO_6 with different bismuth content

This paper reports that the Bi2WO6 ferroelectric ceramics with excess Bi2O3 of 0.0, 2.0, 3.5 and 5.0wt.% of the stoichiometric composition are prepared by the conventional solid-state reaction method. Their microstructure, ferroelectric properties, the concentration and mobility of the defects have been analysed systematically. With increasing Bi content, the remnant polarization decreases, and the broken-down voltage increases. The optimum Bi excess, 3.5, lowers the oxygen vacancy concentration, while further Bi-addition brings about more defects. The activation energies fitted from cole-cole plots are 0.97 eV, 1.07 eV, 1.18 eV, and 1.33 eV, respectively. This suggests that the mobility of the defects is weakened by Bi-addition, which may be due to the increase of the ratio of the number of Bi2O2 layers to that of the oerovskite blocks.

Dielectric and Pyroelectric Characteristics of the Infrared Sensitive (Pb_(1-x)Sr_x)TiO_3 Ceramics

PST ferroelectric ceramics were fabricated successfully by a two-step method, i e first, PbTiO3 and SrTiO3 were sintered respectively, then mixed and sintered together. The process and characteristics of PST ferroelectric ceramics were introduced and researched. Their dielectric and pyroelectric characteristics are as follows dielectric constant 10^4 order, low dielectric loss （lower than 3.0% in the working frequency range of 1-1 000 kHz）, saturation polarization intensity 10^-1 C/m^2 order, and pyroelectric coefficient 10^-3 C/m^2·K order.

Explosive Ferroelectric Power Supply of Flux Compression Generator

Useful electrical pulses of a few hundred kilowatts lasting for several microseconds can be obtained by the depolarization process of PZT95/5 ferroelectric ceramics. In this paper, taking account of the dielectric relaxation, and finite resistance, a new mathematical model of PZT95/5 ferroelectric ceramics subjected to normal-mode shock wave is suggested. Explosive shock wave techniques have also been used to investigate the response of PZT95/5 ferroelectric ceramics with inductive loads in experiments. The predictions from the model have a good agreement with observed results. In addition, an explosive ferroelectric generator composed of explosive shock wave generators, electric units, and additional capacitors is design to power small-size helical flux compression generators. The test results with the maximal output energy of up to 80 J are given and experimental results are also considered.

Composite materials ferropiezoelectric ceramics-polymer for hydroacoustic receivers

The purpose of this work is to study the processes of interfacial interactions,their influence on the piezoelectric properties in highly elastic composite structures,to establish the patterns that determine the volume-sensitive piezoelectric characteristics of composites and to create effective piezoelectric materials for hydroacoustic receiving devices.In the process of this work,the development of a method for obtaining anisotropic ferropiezoelectric ceramics,the development of methods for the preparation of perfect ceramic powders and composite materials based on thermoplastic fluoropolymers and the study of composite materials based on lead-calcium titanate were carried out.As a result of the study,the dependences of the hydrostatic parameters of the composites on the degree of filling,particle size distribution,piezoelectric anisotropy of the active phase and the elastic properties of the polymer matrix were established.Composite materials were based on ceramics of the PT-CT system and thermoplastic fluoropolymers F-2ME,F-62,F-2N with a degree of filling with a ceramic phase from 30 to 60%vol.The dependences of the hydrostatic piezoelectric moduli gh and dh,and the quality factor gh·dh on the degree of filling of the composite for polymers F-62 and F-2ME with different average sizes of ceramic particles are plotted.It has been established that the maximum values gh=119.1·10^(−3)V·m/N and the quality factor g_(h)·d_(h)=6074·10^(−15)m^(2)/N are achieved for a polymer with a higher elastic compliance(F-62)at a degree of filling of 60%vol.and the use of granular filler.

PHASE FIELD SIMULATION OF DOMAIN SWITCHING IN FERROELECTRIC SINGLE CRYSTAL WITH ELECTRICALLY PERMEABLE AND IMPERMEABLE CRACKS

Domain switching around electrically permeable and impermeable cracks in ferro-electric single crystals subjected to a mechanical load is investigated by using a phase field model.It is found that the electrical boundary conditions have little effect on the polarization distribution without any external load when the initial polarization is parallel to the crack,which is totally different from previous studies where the initial polarization is perpendicular to the crack.How-ever,the simulation results show that the electrical boundary conditions have great influence on the polarization distribution as well as the domain switching behavior when a mechanical load is applied.The critical mechanical load of domain switching with a permeable crack is much smaller than that in the case of an impermeable crack.

Eu-doped K_(0.5)Na_(0.5)NbO_(3)-(Ba,Sr)TiO_(3):A novel lead-free luminescent ferroelectric transparent ceramic with reversible photochromism

Photochromic(PC)luminescent ferroelectric materials have aroused great interest because of their potential applications in optical information memories,optical switches and bio-imaging.However,those materials are basically opaque.In this work,we prepared a PC luminescent K_(0.5)Na_(0.5)NbO_(3)-(Ba,Sr)TiO_(3)-based ferroelectric transparent ceramic by modifying with Eu^(3+),Ba^(2+,)Ti^(4+)and Sr^(2+),which not only have good optical transmittances(-60%at 900 nm),moderate ferroelectric properties and down-conversion photo-luminescence(PL)properties,but also exhibit reversible PC behavior.After the illumination by xenon lamp,the colors of the ceramics change from pale green to gray,and then reversibly recover to their initial state via thermal stimulus(200℃for 10 min).Interestingly,both the optical transmittances and PL intensities can be effectively tailored by controlling the PC reaction process.The results suggest that the ferroelectric transparent ceramics are promising for the modulation of photoenergy and the design of optoelectronic devices.

DIELECTRIC RESPONSE AND IMPEDANCE SPECTROSCOPY OF(Na_(0.5)Bi_(0.5))_(0.95)Ba0:05Zr_(0.04)Ti_(0.96)O_(3)FERROELECTRIC CERAMIC

A new lead-free ferroelectric relaxor ceramic was prepared by conventional solid-state synthesis by modifying A-site and B-site in(Na_(0.5)Bi_(0.5))TiO_(3)system,i.e.,(Na_(0.5)Bi_(0.5))_(0.95)Ba_(0.05)Zr_(0.04)Ti_(0.96)O_(3)(0.05BNBZT).X-ray diiffraction studies reveal a single phase rhombohedral structure.Crystallite size and strain analysis has been done by Debye-Scherrer and Williamson-Hall technique.The tolerance factor is 0.81,indicating a stable Perovskite structure of the material.Scanning electron micrograph of the material shows a distribution of grains,average grain size is 1.41μm.Dielectric response of(Na_(0.5)Bi_(0.5))_(0.95)Ba_(0.05)Zr_(0.04)Ti_(0.96)O_(3),ferroelectric ceramic has been studied as a function of frequency over a wide range of temperatures.The studied ceramic exhibited maximum frequency dispersion in both real and imaginary part of dielectric susceptibility at and around the dielectric transition temperature(T_(m)).The frequency dependence of transition temperature,T_(m)(temperature of the maximum of dielectric constant)was studied in terms of Vogel-Fulcher relation.The dielectric relaxation of(Na_(0.5)Bi_(0.5))_(0.95)Ba_(0.05)Zr_(0.04)Ti_(0.96)O_(3)ceramic was studied at different temperatures using the complex impedance(Z*)and electrical modulus(M*)formalism.Impedance measurements were made on over a wide range of temperatures(300-600℃)and frequencies(45 Hz-5 MHz)which show the presence of both grain and grain boundary effects in the material.The Impedance spectroscopy is shown to be an effcient method capable of detecting the contributions of the resistances of grain boundaries(at higher temperature),in addition to granular contribution(at all temperatures),which influences the device properties of a material.The electric modulus(M*)formalism used in the analysis enabled us to distinguish and separate the relaxation processes.Conductivity studies in the material obey the Jonscher's power law in frequency(45 Hz-5 MHz)and temperature(30-600℃).These results give evidence that the lead-free ferroelectric ceramics is extensively may be used for device and electronic applications,when compared with lead-based materials.

LARGE ELECTROCALORIC EFFECT IN RELAXOR FERROELECTRICS

Organic and inorganic relaxor ferroelectrics used for electrocaloric effect(ECE)applications areintroduced.Relaxor ferroelectrics offer several advantages for ECE devices,e.g.,infinite stateswithout applying electric field,field-induced large polarization,no-hysteresis ofheating and cooling,small-hysteresis polarization loss,room temperature phase transition,and broad temperaturerange.The ECE in relaxor ferroelectrics under a high electric field can be described using a theorysimilar to that for first-order phase transition materials.Large ECE was observed directly inhigh-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene)(P(VDF-TrFE)68/32 mol%copolymers,P(VDF-TrFE-CFE)(CFE-chlorofluoroethylene)59.2/33.6/7.2 mol%terpolymers,P(vDF-TrFE-CFE)-P(VDF-CTFE)(CTFE-chlorotrifluoroethylene)95/5 wt%terpolymer blended films,and(PbLa)(ZrTi)O_(3)(PLZT)ceramic thin films.ECE reported inPb(Sc_(1/2)Ta_(1/2))O_(3)(PST),Pb(Mg_(1/3)Nb_(2/3))O_(3)-PbTiO_(3)(PMN-PT)thin films is also summarized.Finally,the perspective of ECE devices is llustrated.

Ultralow frequency dielectric dispersion in PZT+PFS ferroelectric ceramic

The depolarization decaying currents were measured for the PZT compound with morphotropic composition modified by the Fe 1=3 Sb 2=3 cations,i.e.,Pb[Fe_(1/3)Sb_(2/3)_(x)Ti_(y)Zr_(z)]o_(3) with x+y+z=1,x=0.1,y=0.44(morphotropic boundary)and y=0.47.The measurements were performed at room temperature and a few temperatures above(up to 473K),as well as at a few lower temperatures(down to 77K).The samples were poling at fields 0.02kV/cm and 0.2kV/cm at higher and lower temperatures,respectively.In the high temperature range,the time dependence depolarization current follows the fractional power law with two different exponents(n<1 and m>1),while in the low temperature range with a single exponent(n).The appropriate procedure has been used to transform the results obtained in the time domain into the frequency domain in order to calculate real and imaginary parts of the dielectric permittivity in the frequency range 5×10^(-5)-0.1Hz.The possible dielectric relaxation mechanisms have been discussed.

Evaluation of dielectric and piezoelectric behavior of unpoled and poled barium titanate polycrystals with oxygen vacancies using phase field method

This article studies the dielectric and piezoelectric behavior of unpoled and poled barium titanate(BaTiO_(3))polycrystals with oxygen vacancies.A phase field model is employed for BaTiO_(3) polycrystals,coupled with the time-dependent Ginzburg–Landau theory and the oxygen vacancies diffusion,to demonstrate the interaction between oxygen vacancies and domain evolutions.To generate grain structures,the phase field model for grain growth is also used.The hysteresis loop and butterfly curve are predicted at room and high temperatures.The permittivity,and longitudinal and transverse piezoelectric constants of the BaTiO_(3) polycrystals are then examined for various grain sizes and oxygen vacancy densities.