非饱和介电凝胶的力学性能及力电失稳行为研究

力-电耦合场作用下,介电凝胶的体积、形状会发生相应的变化,在智能传感器与促动器等领域具有广阔的应用前景．浸入溶液中,介电凝胶将吸收溶液而溶胀．当外界溶液的量足够多时,介电凝胶将吸收足量的溶液而达到饱和状态;但如果外界溶液的量较小,没有足够的溶液可被吸收时,介电凝胶将处于非饱和状态．基于介电凝胶大变形与极化理论,对非饱和介电凝胶在等双轴应力与电场耦合作用下的力学行为及力电失稳现象进行研究．结果表明,饱和度越低介电凝胶的刚性越大,材料越硬;预应力技术可以提高介电凝胶的力电稳定性,预应力越大,力电失稳出现时的临界电压值越高．     

Energy-momentum tensors in nonsimple elastic dielectrics

We use Noether's theorem to derive energy-momentum tensors for a simple elastic material, a nonsimple elastic material of grade two, a simple elastic dielectric and a nonsimple elastic dielectric of grade two. The technique is easily extendable to a nonsimple elastic dielectric of any grade.     

Mathematical analysis of the radiation from a magnetic line source on a cylindrically capped wedge covered by a plasma sheath

Summary Radial eigenfunctions appropriate for the dielectric (or plasma) layered medium and air are used to obtain an exact solution of the problem of radiation from a magnetic line source (or a slotted antenna) on a conducting infinite wedge with a cylindrical cap covered by a dielectric (or plasma) sheath. In order to satisfy the boundary conditions at the interfaces between the dielectric (or plasma) and the air, it is necessary to expand the radial eigenfunctions of one medium in terms of those of the other.This method can be used to solve any other two dimensional diffraction or radiation problem involving a cylindrically capped wedge surrounded by a dielectric (or plasma) sheath.     

The dielectric cavity resonator applied to electron spin resonance

Summary The design curves for TE _01n dielectric cavity resonators, the quality factor, and the effective volume are computed from the characteristic equation of a hollow dielectric cylinder.A method for measuring relative values of the effective volume of different cavities is described.In an X-band electron spin resonance spectrometer the dielectric cavity gives a good signal to noise ratio.Advantages of the dielectric cavity resonator are described.     

Experimental analysis of microwave heating of dielectric materials using a rectangular wave guide (MODE: TE10) (Case study: Water layer and saturated porous medium)

The heating process of dielectric materials by microwave with rectangular wave guide has been investigated experimentally. This experiment is operated in the TE₁₀-dominated mode at a frequency of 2.45 GHz. It was performed for two dielectric materials, water layer and saturated porous medium. In this work, the microwave powers level, a type of dielectric materials, dimensions and positions of dielectric material on the microwave power absorb and average temperature within dielectric materials were examined in details. The results show that the locations of sample have greater effects than the other parameters. The analyses from this research serve as essential fundamentals to development of mathematic models of heat and mass transfer phenomena.     

Calculations on the time domain reflection behaviour of a transmission line partly filled with a polar dielectric

In this paper the reflection behaviour of a transmission line, partly filled with a polar dielectric, is calculated in the time domain, applying an ideal voltage step (zero risetime) to a dielectric sample of infinite length (no multiple reflections).To extend and generalise former work, the polar dielectric is characterised by three relations of the complex permittivity belonging to three current, partly empirical, models. Low frequency conductivity is also included.It turns out to be impossible to determine the dielectric relaxation time immediately because the type of dielectric model involved cannot be recognised from the shape of the response curve. A Fourier transform of this response is then necessary.The high frequency permittivity can be determined directly but not for all dielectrics with the same accuracy.A determination of the low frequency permittivity can be disturbed by a slow approach of the response curve to its large time asymptote and by low frequency conductivity processes.To avoid problems it proves useful to determine the low frequency permittivity and low frequency conductivity seperately with the aid of known standard dielectric measurement techniques.     

Temperature effects on the dynamic response of viscoelastic dielectric elastomer

Viscoelasticity and temperature can significantly affect the performance of a dielectric elastomer. In the current study, we use a thermodynamic model to describe the effect of temperature and viscoelasticity on the electromechanical response undergoing a cyclic electric load by taking into account of the temperature dependent dielectric constant. Because of the significant viscoelasticity in the dielectric elastomer, the deformation and the nominal electric displacement can not keep in phase with the electric field at low frequencies. The results show that the magnitude of the cyclic electromechanical actuation strain increases with the decrease of the temperature and decreases with the increasing frequency, and viscoelasticity can result in significant hysteresis for dielectric elastomers under a relative low temperature and a low frequency.     

Investigation of pulsed discharges in a dielectric channel with ablating walls

The current-voltage characteristics of a pulsed discharge in a hollow cylindrical channel formed by dielectric walls are investigated. The erosion of the channel walls is measured, together with the mean velocity of the plasma flowing into a vacuum through an opening in one of the electrodes, and related to the channel geometry. Conclusions are drawn regarding the temperature of the plasma in the channel and the mechanism of heat transfer to the walls.Notation d channel diameter - l channel length - C capacitance of capacitor bank - U capacitor voltage; - M consumption of dielectric per discharge - m specific consumption - W discharge energy - r₀ resistance of discharge gap at instant of maximum current - R variable resistance of discharge channel - I_m maximum discharge current - W₁ chemical bond rupture energy of dielectric of mass M - W₂ ionization energy of dielectric of mass M - W₃ kinetic energy of dielectric of mass M - W₄ electrode heating energy per discharge - W₅ thermal energy of dielectric of mass M - T mean temperature of channel plasma - S energy of light flux on dielectric layer - h depth of dielectric layer heated to temperature T - density of dielectric - c specific heat of dielectric - m₀ mass of dielectric layer heated to temperature T - T temperature of heated layer - k₁, k₂ constants - u residual stress     

Model of electrothermal convection of an ideal dielectric in a horizontal capacitor

The behavior of a nonuniformly heated dielectric in a variable electric field of a horizontal capacitor is studied. The action of the dielectrophoretic mechanism of instability associated with the temperature dependence of the dielectric permeability is considered. The model of electroconvection of the ideal dielectric is obtained for the boundary conditions corresponding to the no-flow condition on the rigid surface. Nonlinear flow regimes are investigated in the case of microgravity. A map of the regimes is constructed.     

介电高弹聚合物理论

软材料受刺激会发生变形, 该变形会引起相应的功能, 这种材料称为活性软材料(soft active material, SAM). 本综述主要讨论介电高弹聚合物这一类活性软材料. 当介电高弹聚合物薄膜受到厚度方向的电压作用时, 薄膜厚度减小同时面积增大, 可导致超过100{\%}的应变. 介电高弹聚合物作为转换器被广泛应用, 包括柔性机器人、智能光学器件、盲文显示屏、发电机等. 本文综述了建立在连续介质力学和热力学框架内的、并且基于分子理论描述和经验观测的介电高弹聚合物理论. 该理论耦合了大变形和电势, 描述了非线性和非平衡行为, 如力电失稳和黏弹性. 采用该理论能够通过有限元方法模拟实际构型的转换器, 计算力电能量转换的效率, 给出电致大变形的可行途径. 该理论有助于材料和器件设计.     

具有线性介电常数的Ogden型介电弹性体的本构关系和机电稳定性

应用多材料常数的Ogden弹性应变能函数分析了介电弹性体的力学行为,研究了介电弹性体的机电稳定性.数值结果表明,通过对材料系数(如材料常数比和电致伸缩系数等)的恰当调节可以使得介电弹性体材料或介电弹性体结构更趋稳定.这些有益于深入理解介电弹性体的机电稳定性行为,进而设计恰当的介电弹性体器件.     

The optimization of dissipation in dispersive dielectric slabs

Consider an electromagnetic plane wave incident on a dispersive dielectric slab with susceptibility kernel varying as a function of depth in the slab. After dissipation in Lorentz and Debye dielectric media, it is shown in this paper that it is possible to choose from among all plane waves of fixed energy and maximum duration a signal which maximizes joule heating in any preselected portion of the dielectric slab. This optimal signal is shown to be a normalized eigenfunction corresponding to the eigenvalue of a compact, self-adjoint, integral operator. Numerical approximations of these optimal signals are presented for examples of Lorentz and Debye media.     

Study of a propagating finite crack in functionally graded piezoelectric materials considering dielectric medium effect

This paper provides a study of the problem of a propagating finite crack under in-plane loading in functionally graded piezoelectric materials (FGPMs). The analytical formulations are developed by Fourier transforms and the resulting singular integral equations are solved by using Chebyshev polynomials. By using a dielectric crack model with deformation-dependent electric boundary condition, numerical simulations are made to show the effects of the dielectric medium, the gradient of material properties and the speed of crack propagation on the fracture parameters, such as the stress, electric displacement and crack opening displacement intensity factors. A critical state for the electromechanical loading applied to the FGPMs is observed, which determines whether the traditionally impermeable (or permeable) crack model serves as the upper or lower bound for the dielectric model. The validity of this dielectric crack model is also examined by comparing the results of different existing crack models.     

Influence of stretch and temperature on the energy density of dielectric elastomer generators

Application of dielectric elastomers(DE) has remarkably increased in mechatronics because they are suitable candidates for energy harvesting due to their low cost,light weight, and high energy density. The dielectric elastomer generators(DEGs) exhibit high performance regardless of the applications scale. However, functioning as a generator, a DE may lose its efficiency due to several failure modes including material rupture, loss of tension(LT), electrical breakdown(EB), and electromechanical instability(EMI). The failure modes confine the area of allowable states for generation process.Dielectric constant and dielectric strength of such elastomers depend on the amount of applied deformation and also working temperature, which are often ignored in theoretical simulations. In this paper, variations of the above-mentioned parameters are considered in mechanical and electrical modellings to investigate their effects on energy density and efficiency of generators. Obtained results show that, ignoring the variations of material dielectric constant and dielectric strength leads to overestimation of the specific energy.Furthermore, it is shown that, for an acrylic-based generator, the specific energy sharply decreases with temperature rise.     

Dependence of electrical strength of pressed crystalline powders on filling factor and dielectric constant of crystals

Experimental results of determination of the dependence of the electrical strength of several pressed crystalline powders, differing considerably in dielectric constant, on the filling factor are given. The obtained data are attributed to the distribution of the electric field within a two-phase dielectric.     

Polymer motion as detected via dielectric spectra of 1,4-cis-polyisoprene under large amplitude oscillatory shear (LAOS)

In order to investigate the global polymer chain motion under large amplitude oscillatory shear (LAOS), the dielectric properties under LAOS are measured by a new rheo-dielectric combination. The design of the rheo-dielectric setup including a new fixture and modified oven is explained in detail. For 1,4-cis-polyisoprene, having type-A dipoles parallel to the backbone, the dielectric dipoles can detect the global polymer chain motion via the end-to-end vector. Thus rheological and dielectric (rheo-dielectric) properties reflect the dynamics of the polymer chain motion under LAOS. In this study, we investigate the rheo-dielectric properties under LAOS with 1,4-cis-polyisoprene as model component. As the strain amplitude was increased under LAOS, the relaxation strength from dielectric properties decreased for the whole spectra without changing the shape of the dielectric spectra. These results are analyzed on the basis of the molecular model of dynamic tube dilation (DTD) induced by the convective constraint release (CCR). It is found that the global chain motion under LAOS flow is affected by both rheological frequency and strain amplitude. It is also observed that segmental motion is affected via the oscillatory frequency under LAOS. This result differs from experiments under steady shear.     

介电弹性材料的机电耦合性能研究

作为一种新型的电活性聚合物,介电弹性材料可被用作柔性致动器。其中材料的介电性能和机械性能是影响其机电耦合致动性能的关键因素。通过实验方法研究了一种典型的介电弹性材料VHB4910在不同温度和频率下的介电常数和弹性模量,基于实验结果分析了该材料的机电耦合性能。结果表明:依赖于频率和温度的弹性模量是影响该介电弹性材料致动变形的主要因素,对致动性能的影响最大可达4个数量级,材料的介电常数对其致动性能的影响相对较小。     

THEORY OF DIELECTRIC ELASTOMERS

In response to a stimulus, a soft material deforms, and the deformation provides a function. We call such a material a soft active material （SAM）. This review focuses on one class of soft active materials： dielectric elastomers. When a membrane of a dielectric elastomer is subject to a voltage through its thickness, the membrane reduces thickness and expands area, possibly straining over 100%. The dielectric elastomers are being developed as transducers for broad applications, including soft robots, adaptive optics, Braille displays, and electric generators. This paper reviews the theory of dielectric elastomers, developed within continuum mechanics and thermodynamics, and motivated by molecular pictures and empirical observations. The theory couples large deformation and electric potential, and describes nonlinear and nonequilibrium behavior, such as electromechanical instability and viscoelasticity. The theory enables the finite element method to simulate transducers of realistic configurations, predicts the efficiency of electromechanical energy conversion, and suggests alternative routes to achieve giant voltage-induced deformation. It is hoped that the theory will aid in the creation of materials and devices.     

Elastic and Dielectric Properties of Active Ag/BaTiO<Subscript>3</Subscript> Composites

This study examines the elastic and dielectric properties of active composites consisting of barium titanate (BaTiO₃) and silver (Ag) constituents using experimental and numerical approaches. The elastic constants including Young’s modulus, shear modulus and Poisson’s ratio were measured by resonant ultrasound spectroscopy (RUS), a nondestructive dynamic technique, while a dielectric (impedance) spectroscopy was used to measure the relative permittivity and dielectric loss at different frequencies. The dielectric tests were also conducted at temperature ranges from ?50 to 200 °C where the two phase transformations of barium titanate at around 0 °C and 120 °C were examined. The experimental results in this study were compared to data available in the literature. In addition to the experimental work, a numerical method is also considered in order to study the effects of blending silver into barium titanate on the effective elastic and dielectric properties of the composite and the local field fluctuations. For this purpose, two micromechanics models describing the detailed composite microstructures were constructed. The first model is based on two dimensional (2D) images of realistic microstructures obtained by the scanning electronic microscopy (SEM), while the second model is based on randomly generated three-dimensional (3D) microstructures with spherical particles. The effects of loading direction, porosity, particle shape and dispersion were examined using the micromechanics models. Numerical predictions of the effective elastic and dielectric constants were compared to the experiment results.