Effects of electric field and poling on the fatigue of cracked piezoelectric ceramics in cyclic three-point bending

This paper deals with the fatigue behavior of cracked piezoelectric ceramics in cyclic bending under electric fields both numerically and experimentally. Fatigue tests were carried out in three-point bending with the single-edge precracked-beam specimens. The crack was created normal to the poling direction. Number of cycles to failure was measured under different electric fields. A plane strain finite element analysis was also performed, and the effect of polarization switching on the energy release rate was discussed under a high negative electric field. In addition, possible mechanisms for crack growth were discussed by scanning electron microscope examination of the fracture surface of the piezoelectric ceramics.     

Electric field dependence of piezoelectric coefficient in ferroelectric thin films

A simple model is developed to explain the hysteretic electric field dependence of the piezoelectric coefficient in ferroelectric thin films. The nonlinear susceptibility and hysteretic electrical polarization behavior can explain the piezoelectric hysteresis characteristics. An empirical model introducing a weighting factor is utilized to represent the electric field dependence of the nonlinear susceptibility. Experimentally, the magnitude of the maximum of the piezoelectric coefficient measured in the backward direction of the piezoelectric hysteresis loop is usually larger than that of the maximum measured in the forward direction. The proposed model shows that the weighting factor in modeling the nonlinear susceptibility may account for this observed phenomenon. The nonlinear susceptibility may also explain the peaked shape observed in piezoelectric coeffieint-electric field hysteresis loops. The approach in this work does not require the “hard ferroelectric approximation” of the polarization characteristics, and provides a method of predicting the piezoresponse from measured capacitance and polarization properties of the ferroelectric thin film.     

Transform relationship between Kerr-effect optical phase shift andnonuniform electric field distributions

Electric field distributions measured using the Kerr effect cause a phase shift between light components polarized parallel and perpendicular to the electric field, proportional to the magnitude squared of the electric field components in the plane perpendicular to light propagation integrated over the light path length. One wishes to recover the electric field distribution from measurements of light phase shifts. For axisymmetric geometries where the electric field depends only on the radial coordinate and whose direction is constant along the light path, as is the case along a planar electrode, the total phase shift for light propagating at a constant distance from the center of symmetry and the electric field distribution are related by an Abel transform pair, a special case of Radon transforms typically used in image reconstructions with medical tomography and holography. The more general Radon transform relates the optical phase shift to non-axisymmetric electric field distributions but is restricted to cases where the applied electric field is perpendicular to the plane of light propagation. If the applied electric field direction changes along the light path, it becomes necessary to account for the change in direction of the light components parallel and perpendicular to the applied electric field and the light polarization equations are generalized     

500kV GIS盆式绝缘子的暂态电场分析

推导出了考虑介质极化损耗和位移电流情况下暂态电场计算的数学模型。采用有限元法对 5 0 0kVGIS盆式绝缘子在雷电冲击电压下的暂态电场进行了计算 ,雷电冲击电压波用De hamel积分来处理。给出了典型时刻的场域电位分布图 ,并与静态电场进行比较。找出了绝缘子表面的切向电场强度Et 及法向电场强度En 沿径向分布规律。讨论了松弛时间对切向电场强度Et 及法向电场强度En 的影响 ,给出了几个典型位置的电场强度随时间变化的曲线 ,为GIS的绝缘设计提供参考。     

Piezoelectric and elastic properties of layered materials of Cu(In,Cr)P2(S,Se)6 system

In this contribution, we present new results of ultrasonic and piezoelectric investigation in layered materials of Cu(In,Cr)P₂(S,Se)₆ system including the phase transitions points. It was shown, that in low temperature phase the electromechanical coupling parameters substantially increased after polarization of the material. The large anisotropy of linear and nonlinear elastic properties was observed. The largest critical ultrasonic attenuation anomalies appeared for ultrasonic wave propagating normal to layers: i.e. along polarization direction. From measurements of the amplitude of second longitudinal ultrasonic harmonic we showed that these layered crystals have extremely high elastic nonlinearity in the direction normal to layers. DC bias electric field induced piezoelectricity due to electrostiction was observed in the paraelectric phase of pure materials and their solid solutions. The piezoelectric memory was observed after samples were kept for long time in DC electric field along polar axis at fixed temperature in the paraelectric phase.     

Domain behavior in lead zirconate titanate (PZT) thin film capacitors

Abstract

Domain behavior based on surface energy and volumetric free energy was discussed in relation to imprint. Surface enery and volumetric free energy result in net free energy, which determines the critical size for domain growth. If a domain size is greater than the critical size, they will grow spontaneously under a constant electric field or even under zero electric field. Sometimes, they can grow to the extent that they dominate reverse domains. This phenomenon appears to be polarization reversal under zero voltage bias. Spontaneous domain growth either under zero electric field or a constant electric field was demonstratred. This phenomenon can be partly a source of asymmetric imprint mechanism.     

超声电机定子的简化复合梁模型裂纹扩展分析与实验

超声电机工作环境非常复杂,压电陶瓷极易发生断裂失效。为了分析研究电场、预压力及裂纹位置对压电陶瓷断裂失效的影响,将超声电机的定子沿周向简化为弹性复合梁模型,并在压电陶瓷上预制贯穿裂纹。首先用Ansys有限元软件对简化模型进行仿真,分析得到增大电场、预压力能增加裂纹尖端应力值,裂纹更加容易扩展。然后通过实验验证得到增大电场强度裂纹更加容易扩展。     

基于单谐振结构的金属裂纹CRFID传感器设计

为实现单结构谐振器对金属裂纹参数的无损检测,设计了一款同时检测裂纹宽度及方向的无芯片射频识别(CRFID)金属裂纹传感器。将圆形、切角矩形谐振器一体化设计,利用HFSS有限元分析软件对传感器进行结构优化与性能仿真。深层次探究了不同裂纹缺陷下传感器雷达散射截面(RCS)的响应特征。结果表明金属结构的裂纹方向和裂纹宽度可以通过传感器谐振频率的变化进行识别。双极化下RCS幅频特性的频偏方向对应裂纹的方向,而频偏量与裂纹宽度成正比。设计的CRFID传感器能够检测0°水平方向、90°竖直方向、45°或135°斜向等三类方向下的亚毫米宽度裂纹,其中裂纹宽度检测灵敏度最高可达43.5 MHz/0.1 mm。     

输电线路雷击概率及影响因素研究

雷击概率的研究对于有效的防雷、减小雷灾造成的危害有着非常重要的意义。首先分析了雷击的基本物理过程,根据雷击路径与场强最大方向一致的原理,假定场强最大方向为雷击概率较大的方向。在此基础上建立了雷击过程中先导放电阶段上下行先导电场分布的物理模型。分别研究了输电线路高度、所带电压极性、雷击角度几方面的因素对于雷击概率的影响,并得出以上几种因素对雷击概率影响的变化规律。对输电线路防雷工程有一定指导作用。     

光学电压互感器偏光干涉测量模式:(一)常见偏光干涉测量模式局限性分析

现有的光学电压互感器(OVT)大多基于电光晶体的Pockels效应,即电光晶体在外加电场的作用下其折射率发生变化,使得沿某一方向射入晶体的线偏振光产生电光相位延迟,延迟量与外加电场成正比。由于无法实现对晶体电光相位延迟的直接测量,通常采用偏光干涉测量模式,将相位延迟转变为光强调制,通过对光强的检测实现电压测量。这种测量模式反映了光功率的大小,但仅能近似线性地测量有限的电光相位延迟,其稳定性与可靠性受到温漂、晶体附加相位延迟、半波电压等问题的限制,无法满足电力系统的实用要求。文中分析了偏光干涉测量模式的局限性,并对双光路补偿法进行了讨论。     

直流电压下SF_6气体中电极覆膜对金属微粒启举的影响机理

为研究直流电场下SF_6气体中低压电极覆膜对金属微粒启举的影响机理,搭建了实验平台并使用高速摄像机记录运动轨迹。实验结果表明,随着SF_6气体压力的增大,微粒启举场强升高,且启举后到达高压电极的时间缩短。基于图像处理获得了微粒的瞬时位移,结合运动力学方程和最小二乘法提出了启举时电荷量的计算方法,微粒电荷量的计算分析表明启举时的电荷量减小。建立了覆膜后金属微粒周围电场分布的理论模型,电场分析表明金属微粒与薄膜间的电场明显增大及表面电荷密度分布的改变,使得金属微粒受到向下的极化作用力。研究认为:电荷量减小和极化作用力向下综合导致金属微粒的启举场强提高;覆膜后局部放电是金属微粒的带电机理;SF_6气体压力增大使得金属微粒发生局部放电的起始场强升高,导致极化作用力增大,需要更高场强发生启举。     

放电反应器中掺杂介质受电场力分析

借助有限元方法分析计算了气中水滴和水中气泡两类掺杂介质在电场作用下所受力,包括电场力在两相介质交界面上的分布规律、掺杂介质大小及位置改变时作用力的变化、作用力对掺杂介质形态和运动的影响等。分析结果表明,掺杂介质在电场中所受力作用在交界面上,方向由介电常数大的介质指向小的介质,二者介电常数相差很大时作用力与交界面法线方向相同,作用力在交界面上的分布与反应器电极结构尺寸、介质相态、掺杂介质形状尺寸及在反应器中的位置和电场分布局部畸变等有关。气中水滴受电场线方向的拉伸作用,水中气泡受等位线方向的压缩作用,越靠近电极受力越大,水滴受电场力合力指向线电极,气泡则相反,合力将影响其运动状态。     

Principles of electrocentrifugal separation

Theoretical aspects of electrocentrifugation of monodisperse and polydisperse particle systems are considered. It is shown tha if concentration is an unconstrained thermodynamic variable in an imposed uniform electric field, then electrocentrifugation depends on the balance between sedimentation and chemical and electric diffusion. This balance depends also on gradients of the imposed field when it is nonuniform. The effect of electric diffusion can, however, be significant also in cases involving nonuniform fields. Electrocentrifugal separation of polarizable particles can be achieved using differential diffusion in uniform fields and differential polarization and charge in nonuniform fields. Some design considerations regarding electrocentrifugal separation zones are outlined     

Magnetoelectric Effect in Composites of Magnetostrictive and Piezoelectric Materials

In the past few decades, extensive research has been conducted on the magnetoelectric (ME) effect in single phase and composite materials. Dielectric polarization of a material under a magnetic field or an induced magnetization under an electric field requires the simultaneous presence of long-range ordering of magnetic moments and electric dipoles. Single phase materials suffer from the drawback that the ME effect is considerably weak even at low temperatures, limiting their applicability in practical devices. Better alternatives are ME composites that have large magnitudes of the ME voltage coefficient. The composites exploit the product property of the materials. The ME effect can be realized using composites consisting of individual piezomagnetic and piezoelectric phases or individual magnetostrictive and piezoelectric phases. In the past few years, our group has done extensive research on ME materials for magnetic field sensing applications and current measurement probes for high-power electric transmission systems. In this review article, we mainly emphasize our investigations of ME particulate composites and laminate composites and summarize the important results. The data reported in the literature are also compared for clarity. Based on these results, we establish the fact that magnetoelectric laminate composites (MLCs) made from the giant magnetostrictive material, Terfenol-D, and relaxor-based piezocrystals are far superior to the other contenders. The large ME voltage coefficient in MLCs was obtained because of the high piezoelectric voltage coefficient of the piezocrystals and large elastic compliances. In addition, an optimized thickness ratio between the piezoelectric and magnetostrictive phases and the direction of the magnetostriction also influence the magnitude of the ME coefficient.     

一种计算任意形状孔缝平均电极化率密度的方法

孔缝耦合是电磁干扰最主要的途径,如何计算孔缝耦合对排查电磁干扰十分很重要。现有文献中解析计算任意形状孔缝平均电极化率密度的方法是基于超几何函数理论的。该方法虽然能够较为精确地计算出任意形状孔缝的平均电极化率密度,但由于超几何函数对于非数学专业的工程人员来说过于复杂,因此这种计算孔缝平均电极化率密度的方法很少被应用于电磁场工程实践中。针对这一问题,提出了一种新的解析公式,在不损失精度的前提下,大大简化了计算,可以显著提高孔缝耦合问题的解决效率。核心思想是将任意孔缝的平均电极化密度计算问题转化计算无数小圆孔平均电极化率密度的算术平均值,进而推导出一个简单函数沿孔缝边缘进行线积分的解析式,应用该解析式即可十分方便地计算出任意形状孔缝的平均电极化率密度。     

The effect of dielectric heterogeneity on the pyroelectric responseof PVDF

Understanding the dynamical response to the applied electric and heat field is crucial in pyroelectric applications of polymers, although their heterogeneity on molecular and crystalline level and the nonuniform distribution of electric charge make it complex. We studied the effect of dielectric heterogeneity induced during electric polarization in uniaxially and biaxially oriented PVDF films. Both pyroelectric and dielectric response of PVDF poled in low dc field and corona-charged was measured over a wide frequency range. We find that heterogeneity on the crystalline level is responsible for the piling up of the field-induced electric charges which causes an increase in the relaxation time ascribed to the relaxation process in the MHz range. It is also concluded that the dielectric heterogeneity in is important only in the case when the front electrode of the pyroelectric sensor is that which was grounded during dc poling     

Kerr electro-optic theory and measurements of electric fields withmagnitude and direction varying along the light path

Differential equations that govern light propagation in Kerr media are derived when the applied electric field direction and magnitude vary along the light path. Case studies predict Kerr electro-optic fringe patterns for the specific case of point/plane electrodes. We apply the characteristic directions theory of photoelasticity to understand these fringes. We also study birefringent media with small Kerr constant, in particular transformer oil. For this case we show that simplifications in the characteristic parameter theory is possible, resulting in simple integral relationships between the characteristic parameters and the applied electric field. We use these simple relationships to extend the ac modulation method to measure the characteristic parameters for small Kerr constant media. Measurements of the characteristic parameters using the ac modulation method are presented for point/plane electrodes in transformer oil. The measurements agree reasonably well with space charge free theory for infinite extent electrodes for which analytical expressions are available. We finally employ the `onion peeling' method to reconstruct the axisymmetric electric field magnitude and direction from the measured characteristic parameters and compare the results to the analytically obtained electric field     

Measurement of space-charge field in doped LiNbO3 single crystals

In order to measure the space-charge field in MgO-doped LiNbO₃, we examined the variation of second-harmonic intensity by an external DC electric field and a single beam pump. Intensities of illuminated beam were 3.9 and 7.8 W/cm². An external DC field was applied in the direction of spontaneous polarization (i.e. c-axis). Generated second-harmonic intensity as a function of DC electric field represented by the function of (cosine)². Space-charge field formed as a function of the exposure time of a laser beam had a exponential function. In 4 mol% MgO-doped LiNbO₃ of 6.8 kV/cm at 3.9 W/cm², the saturated space-charge field had a minimum value and the relaxation time had a maximum value.     

Effect of "Stagnation Plane" in the Electric Field during the Electrospinning Process

Recent experiments demonstrated that the essential electrospinning mechanism is a rapidly whipping dielectric liquid jet in an electric field. There are generally two main methods to control the jet, one is to control the solution and the other is to control the flight of the electrospinning jet through manipulation of the electric field. In this paper, a modified electric field distribution, providing a stagnation surface, is discussed in order to know the effect on the electrospinning jet. The results illustrate that if the bending starting point happened below the stagnation point, the jet movement could be random to any direction as no electric force on the jet at the stagnation point and if the bending starting point happened before the jet reached the stagnation point, the jet movement could be bending as the modified jet path. The results also show that the modified electric field distributions could change the jet path and control the fiber morphology.     

Study of partially-switched states of ferroelectrics in relation to the spatial inhomogeneity of their domain structure

In this paper we reexamine the possibility of using ferroelectric materials for adaptive learning artificial intelligence applications, by exploiting their capability to be set in electrically-controlled multivalue polarization states. Our experiments on a Pb(Zr,Ti)O₃ ceramic sample consisted in conveniently selecting the input pulse electric field and temperature of the sample during field application, in order to create partially-switched states. Employing strong/weak electric field pulses allows to control the analog polarization levels by a linear or logarithmic addition of pulses. The partially-switched states are mapped with enhanced resolution when domains with dissimilar evolution stages are present. Applying electric fields while heating the sample allows to reduce the switching time and shifts down the switching threshold. Thus, in addition to artificial intelligence applications, these results provide hints for energy-saving devices that exploit the intrinsic high mobility of small fluctuating domains.