NUMERICAL ANALYSIS OF DIELECTRIC RESPONSE IN RELAXOR FERROELECTRICS

1.IntroductionDuringthepastseveraldecadesyears,muchattelltionhasbeenfocusedonpropertiesofrelaxorferroelectricsfromtheoreticalandapplicationresearcher,becausethecomplexperovsldterelaxorferroelectricsarepromisingmaterialsformultilayercapacitors,piezo-electrictransducersandactuatorsfortheirhighdielectricpermittivity,largepiezoelectriccoefficientsandexcellentelecLrostrictiveproperties[1].Mostoftheresultsindicatedthatthepolarmicroregionsarethekeytounderstandingtherelaxationpolarization,butthemechan…     

Cofiring behavior of NiCuZn ferrite／PMN ferroelectrics di-layer composites

The cofiring compatibility between ferrite and relaxor ferroelectrics materials is the key issue in the production of multilayer chip LC filters. The cofiring behavior, interracial microstructure and diffusion of di-layer composites of NiCuZn ferrite/PMN relaxor ferroelectrics are studied. In order to analyze the matching condition of thermodynamic properties between ferrite and relaxor ferroelectric ceramics, TMA is performed on PMN ferroelectrics and NiCuZn ferrite with certain percentage of Bi2O3, respectively. EDS results demonstrate that serious element diffusions exist at the interface, which is in accordance with the phase analysis based on XRD patterns.     

A distributed step-like switching model of the continuous field-driven phase transformations observed in PMN–xPT relaxor ferroelectric single crystals

The field-driven phase transformation behavior of relaxor ferroelectric single crystal PZN–xPT is discontinuous and displays well-defined forward and reverse coercive fields, whereas the same transformation in PMN–xPT is nearly continuous and occurs over a range of field levels. In analogy to the broad Curie range in relaxor ferroelectrics arising from property fluctuations at the nanometer length scale, the continuous field-driven phase transformations in PMN–xPT are modeled as a step-like series of discontinuous transformations associated with similar spatial property fluctuations. An increase in the applied field gradually increases the volume fraction of the new phase at the expense of the old phase, resulting in a continuous transition between phases. The model simulation produces excellent agreement with the measured material response of 〈0 1 1〉 cut PMN–0.32PT single crystals under conditions of cooperative stress and electric field loading.     

Engineered domain configuration and piezoelectric energy harvesting in 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals

The performance of cantilever piezoelectric energy harvesters using three types of piezoelectric materials, relaxor ferroelectric 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) single crystals oriented along the ??110?? and ??001?? directions, and a PZT-based ceramic, were investigated. The ??110?? and ??001?? oriented PMN-PT single crystals, which have a rhombohedral phase and spontaneous polarization along the ??111?? direction, presented electromechanical coupling factor k ₃₁??s of 0.78 and 0.42, respectively. The cantilever-type energy harvester operated by 31 resonance mode generated a larger output power of 1.07 mW for the ??110?? oriented PMN-PT single crystal compared to those of the other materials. The effective electromechanical coupling factor of the piezoelectric energy harvester with the ??110?? oriented crystal also reached 0.25, not achievable with the other piezoelectric materials. These results demonstrate that the domain engineering of the piezoelectric single crystals can provide higher design flexibility for a tiny energy harvester.     

Application of the FITSC method for characterization of PZT-type ceramics with the diffuse phase transition

In the present work the (Pb_0.84Ba_0.16)(Zr_0.54Ti_0.46)O₃ (PBZT 16/54/46) ceramics has been studied from the point of view of its electrical properties. Dielectric properties of PBZT were measured within the temperature range of the ferroelectric-paraelectric (FE-PE) phase transition region. The method of field-induced thermally stimulated currents (FITSC) was applied and thus the thermal relaxation effects were studied. The observed field dependence of thermally stimulated depolarization currents has revealed the existence of different frequency-dependent relaxation processes in the temperature range between 200 and 400 °C.Our experiment demonstrates that modification of ferroelectric materials with isovalent ions, but bigger radii than the origin atom, significantly affect its properties, particularly the PBZT real part of electrical permittivity shows the phase transition character change from ferroelectric to relaxor and finally, that electrical examination can be effectively used for drawing decisive applications conclusions considering polarization parameters distribution.     

Giant electrostrains accompanying the evolution of a relaxor behavior in Bi(Mg,Ti)O3–PbZrO3–PbTiO3 ferroelectric ceramics

Extremely enhanced electrostrains (up to 0.39%) were surprisingly observed in (0.67 ? x)Bi(Mg_0.5Ti_0.5)O₃–xPbZrO₃–0.33PbTiO₃ (BMT–xPZ–PT) ternary solid solutions, possibly resulting in BMT–xPZ–PT ceramics having great potential for large-displacement actuator applications. The generation of giant strains was found to be closely associated with the evolution of a weak relaxor behavior from diffuse-type BMT–PT binary ferroelectrics, during which the domain switching is actively facilitated owing to a change in the dynamics of the polar nanoregions from a static state to a dynamic state. It can be also attributed to a ferroelectric nature of the evolved relaxors in PZ substituted BMT–PT ceramics instead of a dipole glass freezing state. These judgements were reasonably supported by a couple of measurements, including strains vs. electric field, Raman scattering, dielectric spectroscopy and the time- and electric-field-dependent polarization. The present study can provide a general approach towards an appropriate compositional design for large electrostrains in BMT-based and related systems.     

Relaxor ferroelectric and dielectric properties of (1–x)Ba(Zr0.1Ti0.9)O3–xBa(Mg1/3Ta2/3)O3 ceramics

The environmentally-friendly (1–x)Ba(Zr_0.1Ti_0.9)O₃–xBa(Mg_1/3Ta_2/3)O₃ (x=0, 0.02, 0.04, 0.06, 0.08) relaxor ferroelectric ceramics were prepared by the conventional solid-state method and sintered in air at 1400 °C for 2 h. SEM and XRD analyses were utilized to study the surface morphologies and the crystalline structures, respectively. The effects of Ba(Mg_1/3Ta_2/3)O₃ on the phase transformation, dielectric and ferroelectric properties of Ba(Zr_0.1Ti_0.9)O₃ ceramics were also investigated. It is found that the average grain size of (1–x)Ba(Zr_0.1Ti_0.9)O₃–xBa(Mg_1/3Ta_2/3)O₃ (BZT–BMT) perovskite single-phase ceramics decreases as the content of Ba(Mg_1/3Ta_2/3)O₃ (BMT) increases. The relaxor ferroelectric behavior with diffuse phase transition and well-defined frequency dispersion of dielectric maximum temperature is found for the ceramic with increasing x values. 0.98BZT–0.02BMT ceramic shows very good dielectric properties with the relative permittivity and the dielectric loss, measured at 100 kHz as 6034 and 0.01399 respectively at room temperature. Both remnant polarization and coercive field decreased with increasing BMT content, indicating a transition from the ferroelectric phase to the paraelectric phase at room temperature.     

Effect of external stress on phase diagrams and dielectric properties of epitaxial ferroelectric thin films grown on orthorhombic substrates

1 Introduction Ferroelectric thin films have been widely investigated in recent years. Not only the ferroelectric, dielectric, and piezoelectric properties were found to be promising for microelectric and micromechanical applications[1], but also the phy…     

Ferroelectricity in synthetic metals: Reality and hypotheses

Ferroelectricity is one of the demanded effects in fundamental and applied solid state physics. Till now, the ferroelectrics were available mostly in the inorganic world. The breakthrough of 2000s was an unexpected discovery of the ferroelectricity related to the charge ordering in quasi-1D organic conductors (TMTTF)₂X, and in some layered compounds. The achieved understanding of underlying mechanisms allows to speculate on synthesis of a would-be ferroelectric polymer: it must possess a combination of dimerizations of sites and bonds, one of which must be build-in and another spontaneous. The theory of these “combined Peierls states” predicts an existence of solitons (dimerization kinks) with non-integer variable charges—they are the walls separating domains with opposite electric polarization. The physics of these exotic solitons will serve to describe transient processes in ferroelectric polymers, linking optical- and low-frequency properties. The promising example of such an (AB)_x conjugated polymer has already appeared but was not yet tested for ferroelectricity. A wide chemical search is necessary because of a problematic competition of an anti-ferroelectric phase, whose occurrence depends on complicated details of the interchain coupling.     

A constrained domain-switching model for polycrystalline ferroelectric ceramics. Part II: Combined switching and application to rhombohedral materials

A combined switching assumption (CSA) is incorporated into the constrained domain-switching model presented in part I to address the nonsymmetric deformations of ferroelectric ceramics under tension and compression. Using the CSA, the domain-switching process in rhombohedral ferroelectric/ferroelastic ceramics is analyzed in detail. The results show that in rhombohedral lead titanate zirconate (PZT) ceramics, most 109° switching can be accomplished with a minor fraction of 71° switching during electric poling, which is quite different from BaTiO₃ and tetragonal PZT ceramics in which only a few per cent of 90° switching can occur. Domain switching under combined uniaxial electric and mechanical tension/compression is also studied. Uniaxial tension is found to enhance electric poling while uniaxial compression inhibits it.     

(1-4x)NBT-3xKBT-xBT系无铅压电陶瓷性能研究

采用传统固相法制备了(1-4x)NBT-3xKBT-xBT(x=0.020～0.035)体系压电陶瓷.通过XRD分析,发现该体系陶瓷都能形成单一的钙钛矿型固溶体,并在0.025≤x≤0.032范围内具有三方和四方共存结构,为该体系的准同型相界.当x=0.028时,d_(33)=162 pC/N,Q_m=203.29,k_p=0.234.同时分析了该体系陶瓷材料在1, 10, 100 kHz下介电常数-温度曲线和介电损耗-温度曲线,发现该体系陶瓷样品的介电温谱都存在两个介电反常峰,且介电常数和介电损耗与频率存在很强的依赖性,表明该体系材料具有弛豫型铁电体性质.     

铁电材料湿敏性能的研究进展

对湿度的精密监测关系着一些潮解材料的保存,电子仪器的测量精准度等各个方面。高性能湿度传感器在现代工业、农业、医疗等领域均具有广泛的用途。湿度敏感材料包括介电材料、半导体材料、金属材料等。作为一种特殊的含有自发电极化的介电材料,铁电材料在湿敏传感器领域的应用越来越受到人们的关注。理论上,铁电材料的电极化对于表面的极性水分子具有强的吸附作用,同时,表面附着的极性水分子也可以反过来影响铁电材料的铁电极化、介电、电阻抗等性能。因此,铁电材料在高性能湿敏传感器件中具有重要的应用前景,铁电湿敏材料具有灵敏度高、响应快、稳定性好等优点。本文综述了铁电湿敏材料的发展历史和现状,详细总结了铁电材料湿度传感的物理机制。将铁电湿敏材料按类别、性质分为铁电纳米、铁电陶瓷、铁电薄膜、铁电单晶四大部分,分别综述了它们湿敏特性的研究进展及影响湿敏性能的各种因素,以期为未来新型铁电材料的湿敏研究提供一些科学参考。     

Critical properties of nanoscale asymmetric ferroelectric tunnel junctions or capacitors

Critical properties, such as the phase transition temperature, critical thickness and Curie-Weiss-type relation, of nanoscale asymmetric ferroelectric tunnel junctions or capacitors are investigated, considering the effects of size, surfaces, asymmetric interfaces and electrodes on the stability and magnitude of the two spontaneous polarization states. Using the modified thermodynamic model and taking into account contributions of the depolarization field, built-in electric field, interface and surface energies to the thermodynamic potential, explicit expressions of the critical properties are derived. For the asymmetric ferroelectric tunnel junction or capacitor, the results illustrate two important behaviors of vanishing critical thickness for the spontaneous polarization and smearing of the phase transitions, respectively. In addition, other critical properties are discussed as functions of the ambient temperature, misfit strain, surface coefficients, work function steps, dielectric constants and screening lengths of electrodes. Owing to the high-sensitivity of the critical properties to structures of asymmetric interfaces and electrodes, the results also suggest that the critical and other functional properties of nanoscale asymmetric ferroelectric tunnel junctions or capacitors can be completely controlled by adjusting the difference between asymmetric interfaces or electrodes.     

Effect of space charge on the polarization hysteresis characteristics of monolithic and compositionally graded ferroelectrics

A non-linear thermodynamic analysis of ferroelectric systems with localized space charges for monolithic and compositionally graded materials is described wherein the electrostatic interlayer interactions are specifically accounted for. The electrostatic coupling is established through the built-in polarization due to the space charges and the intrinsic polarization variations between the ferroelectric layers. The findings show that the polarization hysteresis response of monolithic stress-free barium strontium titanate (BST) ferroelectrics with asymmetrically distributed space charges result in a displacement of the hysteresis loop along the applied electric field axis. In compositionally graded BST multilayers, the hysteresis response is characterized by off-sets along both the polarization and the electric field axes, yet with magnitudes of displacement that are markedly larger than those for monolithic ferroelectrics.     

The effect of mechanical strains on the ferroelectric and dielectric properties of a model single crystal – Phase field simulation

The effect of applied mechanical strains on the ferroelectric and dielectric properties of a model single crystal is investigated using a phase field model based on the time-dependent Ginzburg–Landau equation, which takes both multiple-dipole–dipole-electric and -elastic interactions into account. The evolution of the ferroelectric domain structure is simulated at different temperatures and applied strains. The results show that the paraelectric/ferroelectric phase transition temperature linearly increases with the applied mechanical strain under mechanical clamping conditions. Analogous to the classical Ehrenfest equation, a thermodynamics equation is derived to describe the relationship between the transition temperature and the applied strain. The change in the domain structure with temperature under applied inequiaxial strains is different from that under applied equiaxial strains. The simulations also illustrate the changes in the coercive field, the remanent polarization and the nonlinear dielectric constant with the applied strain.     

铁电相SrBi_2Ta_2O_9的原子位移和价电子结构

根据晶体空间群理论,对SrBi_2Ta_2O_9进行了从正交相到四方相的晶体重构,计算出SrBi_2Ta_2O_9在铁电相变中的原子位移,确定了SrBi_2Ta_2O_9沿a轴方向的自发极化。在此基础上,采用固体与分子经验电子理论(EET)计算了SrBi_2Ta_2O_9的价电子结构,进而得到SrBi_2Ta_2O_9中各原子的有效价电子数,由原子位移和有效价电子数求得正交铁电相SrBi_2Ta_2O_9的自发极化强度为18.14μC/cm~2,与实验数据和其他理论方法的结果吻合较好。     

Effects of superimposed electric field and porosity on the hydrostatic pressure-induced rhombohedral to orthorhombic martensitic phase transformation in PZT 95/5 ceramics

The hydrostatic pressure-induced martensitic transformation from the ferroelectric rhombohedral to antiferroelectric orthorhombic phase in PZT 95/5 ceramics has been studied using neutron diffraction. The transition to the orthorhombic phase initiates at a pressure of 260 MPa and is almost complete at 290 MPa. This stress range is much narrower than that observed in uniaxial loading, which starts at 200 MPa and is incomplete even at 400 MPa. The narrower stress range observed under hydrostatic loading is attributed to a lack of internal stress developed during the transformation. By contrast, the work required to start the transformation is approximately the same under both types of loading. The transformation progresses more gradually with increasing pressure when a static electric field is applied to a specimen in a pre-poled state. Tests carried out on porous specimens, having a relative density of approximately 90%, demonstrated that the transformation occurred over a narrow pressure range but with a lower transformation pressure of approximately 220 MPa.     

Ultrasound studies of phase transitions in tungsten bronze ferroelectric materials

Attenuation and velocity measurements at ultrasonic frequencies were performed in two ferroelectric solid solutions with tungsten bronze structure type, SBN and PBN, as a function of temperature (from 200 to 380 K). Both materials, in polycrystalline form, presented sensitive changes in the temperature dependence curves of attenuation and velocity. One of these anomalies (at 350 K for SBN and 323 K for PBN) was associated with a second-order phase transition, confirming the ferroelectric orthorhombic–ferroelectric tetragonal phase transition suggested for both systems in this temperature range.     

Relationship between dielectric properties and structural long-range order in (x)Pb(In1/2Nb1/2)O3:(1 − x)Pb(Mg1/3Nb2/3)O3 relaxor ceramics

The dielectric and structural order–disorder properties of as-sintered complex perovskite (x)Pb(In_1/2Nb_1/2)O₃:(1 − x)Pb(Mg_1/3Nb_2/3)O₃ ceramics are highly influenced by the quantity of Pb(In_1/2Nb_1/2)O₃ (PIN). A high PIN quantity causes the relative permittivity maxima (ε_max) to decrease and the temperature (T_max) to increase. Also, strong frequency dispersion is dominant in the relative permittivity when plotted against the temperature. In ferroelectric hysteresis loop measurements, the maximum values of electric displacements (D_max) decrease with increasing PIN. The ceramics in the composition range x = 0.1–0.8 behave as ferroelectric relaxors and exhibit very slim hysteresis loops for all these compositions. Transmission electron microscopy (TEM) studies show that the size of the 1:1 structural ordered domains is influenced by the PIN quantity. The relationship between the dielectric properties and the long-range 1:1 order in these relaxors appear to be in conflict with the commonly accepted order–disorder behavior in complex perovskite ferroelectrics, in which large structural domains correspond with the tendency to depart from the relaxor state. TEM observations show that individual 1:1 ordered domains in (x)PIN:(1 − x)PMN ceramics are composed of numerous nano-sized ordered domains, separated by fine antiphase boundaries.     

Reorientation transitions in rhombohedral magnets

Reorientation transitions in a rhombohedral magnet have been studied taking into account invariants up to the sixth order inclusive. Unlike the hexagonal, tetragonal, and orthorhombic magnets, in rhombohedral magnets transitions into the high-symmetry phase are possible only through a first-order phase transition. It has been shown that critical, triple, and quadruple points can exist in the phase diagram.