Robust and Tunable Ferroelectricity in Ba/Co Codoped (K_(0.5)Na_(0.5))NbO_(3) Ceramics

The 0.98(K_(0.5)Na_(0.5))NbO_(3)-0.02Ba(Nb_(0.5)Co_(0.5))O_(3-δ) ceramics with doped Ba^(2+) and Co^(2+) ions are fabricated,and the impacts of the thermal process are studied.Compared with the rapidly cooled (RC) sample,the slowly cooled (SC) sample possesses superior dielectric and ferroelectric properties,and an 11 K higher ferroelectricparaelectric phase transition temperature,which can be attributed to the structural characteristics such as the grain size and the degree of anisotropy.Heat treatment can reversibly modulate the content of the oxygen vacancies,and in turn the ferroelectric hysteresis loops of the samples.Finally,robust and tunable ferroelectric property is achieved in SC samples with good structural integrity.

Study on Ferroelectric and Dielectric Properties of La-Doped CaBi_4Ti_4O_(15)-Based Ceramics

Lanthanum doped bismuth layer structured ferroelectrics （BLSFs） Ca1 - x LaxBi4 （Ti0.9W0. 1 ）4O15 （ x = 0, 0.2, 0.3, 0.4, 0.6） ceramics were prepared by solid-state reaction method. X-ray diffraction （XRD） patterns showed that single phase was formed when x = 0 - 0.6. The effects of La^3＋ doping on dielectric, piezoelectric and ferroelectric properties of Ca1-xLaxBi4（Ti0.9W0.1）4O15 ceramics were studied. Ca0.7La0.3Bi4（Ti0.9W0.1）4O15 ceramic had optimal properties, its dielectric constant was 166.85, dielectric loss was 0.0063, piezoelectric strain constant was 11 pc·N^-1, remanent polarization was 18.1μC·cm^-2 and coercive field was 118 kV·cm^-1. SEM micrographs showed that the grains of CaBi4Ti4O15-based ceramics were plate-like. The results of energy spectrum analysis （EDS） showed that La^3＋ incorporation could increase Bi/Ca ratio.

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.

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.

Phase evolution and room temperature ferroelectric and magnetic properties of Fe-doped BaTiO_3 ceramics

To make the ferroelectric BaTiO3 possess ferromagnetism simultaneously,magnetic Fe was doped into BaTiO3 ceramics at doping levels up to 10%(molar fraction).Both tetragonal and hexagonal phases coexisted in the Fe-doped BaTiO3 ceramics except at 1% doping level.X-ray diffraction analysis indicated that higher doping level of Fe,higher sintering temperature and longer sintering time promoted the formation of hexagonal phases in Fe-doped BaTiO3 ceramics.Ferroelectricity was observed in all samples at room temperature,but it was greatly depressed by Fe doping.Except at doping level of 1%,room-temperature ferromagnetism was observed in the BaTiO3 ceramics.The dependence of the saturation magnetization and coercivities of the Fe-doped BaTiO3 ceramics on doping level was systematically studied.Both the saturation magnetization and magnetic coercivities were found to be dependent on the doping level as well as the fraction of the hexagonal phase in the ceramics.

MARTENSITE-LIKE PHASE TRANSITIONS DUE TO POLAR REGION STRUCTURE IN DISORDERED FERROELECTRIC CERAMICS

Clear experimental evidence for phase transitions was shown in titanium doped lead magnesium niobate compositional disordered ferroelectric ceramics. One is the diffused phase transition around the temperature of dielectric permittivity maxima, which is often assumed as the characteristics of relaxor ferroelectrics. Another is a first order transition from frequency dispersion relaxor ferroelectrics to normal ferroelectrics, corresponding to a zero field spontaneous polar micro macrodomain switching. According to the x ray diffraction, thermal analysis and transmission electron microscope results, it is pointed out that the relaxor state corresponds to a coexistence of cubic parent phase and nucleating rhombohedral ferroelectric microregion which is similar to a precursor martensite. After the spontaneous relaxor normal ferroelectrics transition, the lower symmetry phase is sure to be a long range rhombohedral phase. Thus a dynamic behavior of polar microregions is suggested to explain the phenomena, which is more similar to a stress induced martensitic transformations from cubical stabilized perovskite parent phase.

Ferroelectric behaviour of 30nm BaTiO3 ceramics prepared by high pressure assisted sintering

Dense nanocrystalline BaTiO3 ceramics with a homogeneous grain size of 30 nm was obtained by pressure assisted sintering. The ferroelectric behaviour of the ceramics was characterized by the dielectric peak at around 120 ℃, the P-E hysteresis loop and some ferroelectric domains. These experimental results indicate that the critical grain size for the disappearance of ferroelectricity in nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering is below 30 nm. The ferroelectric property decreasing with decreasing grain size can be explained by the lowered tetragonality and the ＇dilution＇ effect of grain boundaries.

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）.

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.

Impedance and ferroelectric properties of Sr^(2+) modified PZT-PMN ceramics

Sr^2＋ modified polycrystalline PZT-PMN ceramics were synthesized by a semi-wet route. Impedance spectroscopy studies indicate the bulk and grain boundary effects of PZT-PMN material along with the negative temperature coefficient of resistance. The bulk conductiv-ity exhibits an Arrhenius-type thermally activated hopping process which is supported by the AC conductivity behavior as a function of fre-quency and temperature. It is observed that the remnant polarization increases with an increase in the Sr2＋content in PZT-PMN.

Dielectric, piezoelectric, and ferroelectric properties of lanthanum-modified PZTFN ceramics

Specimens of Pb1-1.5xLax（Zr0.53 Ti0.47）1-y-zFeyNb2O3 （x = 0, 0.004, 0.008, 0.012, and 0.016, y = z = 0.01） （PZTFN） ceramics were synthesized by a semi-wet route. In the present study, the effect of La doping was investigated on the structural, microstructural, dielectric, piezoelectric, and ferroelectric properties of the ceramics. The results show that, the tetragonal （space group P4mm） and rhombohedral （space group R3c） phases are observed to coexist in the sample at x = 0.012. Microstructural investigations of all the samples reveal that La doping inhibits grain growth. Doping of La into PZTFN improves the dielectric, ferroelectric, and piezoelectric properties of the ceramics. The hys- teresis loops of all specimens exhibit nonlinear behavior. The dielectric, piezoelectric and ferroelectric properties show a maximum response atx 〉 0.012, which corresponds to the morphotropic phase boundary （MPB）.

Ferroelectric and dielectric properties of La/Mn co-doped Bi_4Ti_3O_12 ceramics

La/Mn co-doped Bi4Ti3O12 ceramics, Bi3.25La0.75Ti3-xMnxO12 （x = 0.02, 0.04, 0.06, 0.08）, were prepared by the solid-state reaction method. The influence of manganese substitution for the titanium part in Bi3.25La0.75Ti3O12 on the sintering behaviour, microstructure, Raman spectra and electrical properties was investigated. The experimental results show that the phase composition of all samples with and without manganese doping, sintered at 1000 ℃, consists of a single phase with a bismuth-layered structure belonging to the crystalline phase Bi4Ti3O12. There is no evidence of any impurity phase, but a small change in crystallographic orientation is observed. The Curie temperature of Bi3.25La0.75Ti3-xMnxO12 ceramics is steadily shifted to lower temperature with increasing Mn-doping content. Moreover, the remnant polarisation （Pr） of Bi3.25La0.75Ti3- xMnxO12 samples increases with Mn-doping content, and the Bi3.25La0.75Ti2.92Mn0.08O12 sample exhibits the largest Pr of 16.6 μC/cm^2.

Preparation, Characterization and Dielectric Properties of Sr_5RTi_3Ta_7O_(30) (R=Pr and Eu) Ferroelectric Ceramics

Polycrystalline samples of Sr5PrTi3Ta7O30 （SPTT） and Sr5EuTi3Ta7O30 （SETT） compounds were prepared by high-temperature solid-state reaction method and their formation, structure and dielectric properties were studied. They are found to be ferroelectric phase of filled tetragonal tungsten bronze （TB） structure at room temperature and undergoes diffuse type of ferroelectric-paraelectric phase transition around 34 ℃ and 31 ℃, respectively. At 1 MHz SPTT exhibits high dielectric constants of 177 and low dielectric losses of 3.5×10^-4 and SETT has high dielectric constants of 125 and low dielectric losses of 2.4×10^-3.

Effect of Pb Substitution on the Microstructure and Ferroelectric Properties of Ba_(0.7)Sr_(0.3)TiO_3 Ceramic

Ferroelectric Ba0.7Sr0.3TiO3（BST） and partially Pb^2＋ substituted for Ba^2＋ ceramics （Ba0.7-xPbx）Sr0.3TiO3 （x=0.1-0.4, BPST） were prepared by using conventional solid-reaction method. XRD analysis shows that the samples microstructure changes from cubic phase to tetragonal one with the Pb^2＋ content increasing. ESEM analysis shows that the Pb^2＋ substituted samples have a denser and more uniform surface morphology than that of pure BST. Measured electrical properties suggest that the Pb^2＋ substitution for Ba^2＋ in the BST system enhances the ferroelectric performance obviously when x=0.2. In addition, the substitution increases the samples Curie temperature （To） r （Ba0.5Pb0.2）Sr0.3TiO3 ceramic has good ferroelectric properties measured at a maximal electric field of 30 kV/cm under the condition of room temperature. The corresponding saturated polarization （Ps）, remnant polarization （Pr） and coercive field （Ec） is respectively 15.687 μC/cm^2, 8.100 μ C/cm^2 and 6.611 kV/cm. The measured Tc of （Ba0.5Pb0.2）Sr0.3TiO3 is 117 ℃.

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.

Modeling of PZT Ferroelectric Ceramic Depolarization Driven by Shock Stress

Shock-induced phase transition of ferroelectric ceramic PZT 95/5 causes elastic stiffening and depolarization,releasing stored electrostatic energy into the load circuit.We develop a model to describe the response of the PZT ferroelectric ceramic and implement it into simulation codes.The model is based on the phenomenological theory of phase transition dynamics and takes into account the effects of the self-generated intensive electrical field and stress.Connected with the discharge model and external circuit,the whole transient process of PZT ceramic depoling can be investigated.The results show the finite transition velocity of the ferroelectric phase and the double wave structure caused by phase transition.Simulated currents are compared with the results from experiments with shock pressures varying from 0.4 to 2.8 GPa.

PIEZOELECTRIC AND PYROELECTRIC PROPERTIES OF FERROELECTRIC GLASS-CERAMICS

In this paper we report the ferroelectric glass-ceramics possessing both piezoelectric and pyroelectric properties.The properties can be comparable to those of corresponding ceramics or single crystals.

THE APPLICATION OF FE1-FE2 PHASE TRANSITION IN Nb DOPED(95/5)FERROELECTRIC CERAMICS TO HEAT-ELECTRIC ENERGY CONVERSION

Formerly,direct conversion of heat to electricity was carried out by making use of ferroelectric to paraelectric(FE-P)transition.In this paper a new method utilizing pyroelectric effect between two ferroelectric phases(FEI-FE2)is suggested and the Nb doped PZT(95/5)ceramic materials are used as actuating media.The intrinsic efficiency as well as the output electric power in energy conversion is estimated in terms of the charge-voltage behaviour during phase transition.Two methods of the energy conversion concerned are compared.

Influences of Ho Doping on Structure,Ferroelectric,Energy Storage and Optical Properties of KNN-SYbN Transparent Ceramics

Ho doping 0.825K_(0.5)Na_(0.5)NbO_(3)-0.175Sr(Yb_(0.5)Nb_(0.5))_(O3)(KNN-SYbN-x%Ho)transparent ceramics were prepared by solid-state sintering method.The structure,ferroelectric,energy storage,and optical properties of KNN-SYbN-x%Ho were explored.With the addition of Ho,under the excitation of a 980 nm laser,the ceramics exhibit up-conversion luminescence properties with wavelengths of 550 nm and 670 nm,however,the ceramics change from pseudo-cubic phase to triphase-orthorhombic phase and the light transmittance decreases.The addition of Ho significantly enhances the ferroelectric properties and the energy storage performance of KNN-SYbN-x%Ho ceramics.When x=0.15,the residual polarization P_(r)=9.11μC/cm^(2),while x=0.20,the maximum energy storage density W_(rec) reaches 0.26 J/cm^(3),and the energy storage efficiencyηreaches 87.1%.

Dielectric and Ferroelectric Properties of Complex Perovskite Ceramics Under Compressive Stress

Dielectric and ferroelectric properties of complex perovskite PZT-PZN ceramic system were investigated under the influence of the compressive stress.The results showed that the dielectric properties,i.e.dielectric constant(ε_r)and dielectric loss(tan δ),and the ferroelectric characteristics,i.e.the area of the ferroelectric hysteresis loops,the saturation polarization(P_ sat),and the remnant polarization(P_r)changed significantly with increasing compressive stress.These changes depended strongly on the ceramic compositions.The experimental results on the dielectric properties could be explained by both intrinsic and extrinsic domain-related mechanisms involving domain wall motions,as well as the de-aging phenomenon.The stress-induced domain wall motion suppression and non-180° ferroelectric domain switching processes were responsible for the changes observed in the ferroelectric parameters.In addition,a significant decrease in those parameters after a cycle of stress was observed and attributed to the stress induced decrease in switchable part of spontaneous polarization.This study clearly show that the applied stress had significant influence on the electrical properties of complex perovskite ceramics.