介电高弹聚合物力电行为研究与器件设计

相对于人们通常熟悉的金属、陶瓷、玻璃等“硬材料”,“软材料”是自然界、生命体系、日常生活和工程应用中广泛存在的物质体系,例如动植物组织结构、人造聚合物、液晶、胶体、泡沫、颗粒物质等.许多软材料在受外界激励后会发生变形或引发相应的功能响应,这种材料被称为功能软材料(soft active material,SAM).介电高弹聚合物属于典型的功能软材料,通过电压作用,介电高弹聚合物可产生超过100％的应变,并具有轻质量、快反应和高能量密度等优点,在智能仿生、航空航天、机械、新能源等领域有巨大的应用潜力.本学位论文主要研究介电高弹聚合物这一类功能软材料,研究了介电高弹聚合物的失拉、断裂、电击穿、粘弹性、失稳等力电失效和力电耗散现象,通过分析这些现象的产生机理和特性,得到克服或利用介电高弹聚合物特有的力电性能,实现换能器极大电致变形(1692％电致面积变形)、高效能量收集、主动调频振动等性能的理论方案.通过仿真模拟,进行换能器结构与介电高弹聚合物材料的优化设计,在试验上完成介电高弹聚合物换能器在不同工作模式下的高性能实现.     

人工肌肉作动器的参数优化设计

由于电活性聚合物材料在电场作用下所表现出的许多优异的力学性能,如大应变、响应快、能量转换率高等特点,使得这种高分子智能材料引起了广泛关注,有望被加工成作动器、传感器及俘能器等能量转换器,在工程应用中发挥巨大作用。 但是,由于描述电活性聚合物材料变形的状态方程的非线性性,及其在力电载荷作用下多种失效模式的存在,使得设计电活性聚合物能量转换器面临诸多的困难。本文针对美国人工肌肉公司(Artificial Muscle Inc.)开发的一款电活性聚合物薄膜作动器的优化设计开展研究,主要研究了不同初始预拉伸对薄膜厚度、拉伸变形、应力及电场强度等的影响效应,结果表明,在某一预拉伸下,薄膜中的电场分布将趋于均匀。本文所提供的研究方法,可为此类作动器的优化设计提供基本的分析模式。     

一种新型介电弹性体仿生可调焦透镜的变焦分析

介电弹性体 (dielectric elastomer) 是电活性聚合物智能材料的一种,在外加电场作用下,可产生多种形式的响应.在驱动柔性透镜的变焦方面,相对于传统的机械操控变焦方法 显示出独特的优势.针对一款在电压激励下可高效调节焦距的介电弹性体仿人眼变焦透镜,该透镜由上下两层介电弹性薄膜和固定框架构成,并在封闭腔内充入盐水,上层薄膜涂覆环形柔性电极.在电压激励下,上层膜发生变形,由于盐水的体积保持恒定,引 起下层膜随之变形,使得透镜的焦距发生改变.采用 neo-Hookean 模型,利用变分原理导出了该透镜的控制方程、边界条件和连 续条件.利用打靶法求解了该非线性问题并高效地处理了非线性问题的界面连续条件. 理论分析结果与实验结果相吻合. 利用此模型开展了广泛的参数分析,研究表明,透镜的几何形状、初始焦距、介电弹性体薄膜的预拉伸率、涂覆的电极面积、材料的剪切模量等对透镜焦距的调节性能都有重要的影响.所建立的理论分析模型可为柔性仿生透镜的设计和参数优化提供有效的分析方法.      

界面黏接对软电介质薄膜临界击穿场强影响的试验研究

软电介质薄膜因模量低、可大变形、适应性强等特点,被广泛应用于电气和电子系统中。但由于材料的介电常数较低,通常需要在较高的电场环境下服役。在高电场环境下,软电介质薄膜易产生失稳和电击穿破坏,影响着材料的应用范围和稳定性。本研究针对电极对软电介质薄膜有无约束两种情况对此类器件的稳定性展开了研究：首先定量描述了电极对电介质无约束时,电压加卸载过程中失稳形貌的演化规律;其次讨论了例如氯化锂水凝胶等软电极对薄膜力电失稳的抑制,并利用化学黏接的方式增强水凝胶电极对电介质薄膜的约束,提升了电介质薄膜的临界击穿场强。     

THEORETICAL NONLINEAR ANALYSIS OF A BIOMIMETIC TUNABLE LENS DRIVEN BY DIELECTRIC ELASTOMER 1)

介电弹性体 (dielectric elastomer) 是电活性聚合物智能材料的一种,在外加电场作用下,可产生多种形式的响应.在驱动柔性透镜的变焦方面,相对于传统的机械操控变焦方法 显示出独特的优势.针对一款在电压激励下可高效调节焦距的介电弹性体仿人眼变焦透镜,该透镜由上下两层介电弹性薄膜和固定框架构成,并在封闭腔内充入盐水,上层薄膜涂覆环形柔性电极.在电压激励下,上层膜发生变形,由于盐水的体积保持恒定,引 起下层膜随之变形,使得透镜的焦距发生改变.采用 neo-Hookean 模型,利用变分原理导出了该透镜的控制方程、边界条件和连 续条件.利用打靶法求解了该非线性问题并高效地处理了非线性问题的界面连续条件. 理论分析结果与实验结果相吻合. 利用此模型开展了广泛的参数分析,研究表明,透镜的几何形状、初始焦距、介电弹性体薄膜的预拉伸率、涂覆的电极面积、材料的剪切模量等对透镜焦距的调节性能都有重要的影响.所建立的理论分析模型可为柔性仿生透镜的设计和参数优化提供有效的分析方法.     

离子导体驱动的介电弹性体软机器研究进展

基于介电弹性体的软机器在柔性机器人、能量收集、柔性电子等领域有着广泛的应用。传统的介电弹性器件由不透明的流体碳膏电极或其他硬材料制作的电子导体来驱动。可拉伸透明离子导体作为电极驱动介电弹性体时,两种软材料互相结合,为软机器提供了一种独特的实现方式。本文综述了离子导体驱动的介电弹性体软机器的近期研究进展,展望了未来的研究方向和存在的问题。     

塑封硅器件中介电薄膜的温湿效应分析

随着铜互连以及 low-k电介质在超大规模集成电路中地广泛使用,low-k电介质的机械完整性及其对互连可靠性变得更加重要. 影响介电膜的机械完整性和互连可靠性的因素包括介电膜的工艺制程, 芯片与封装材料的相互影响, 以及环境温度和湿度的影响.本文研究集中于了解环境温度和湿度对塑封硅器件中介电薄膜的可靠性影响. 采用快速温度和湿度实验条件,对塑封硅器件中介电薄膜受水分和温度损伤的敏感性进行了分析. 运用商业有限元(FEA)分析软件, 对水分在塑封材料和硅器件中的扩散过程进行了建模及仔细分析. 并对硅器件周边密封圈的防水分扩散效力进行了研究. 通过这一系列实验与分析,对塑封硅器件中介电薄膜的温湿效应有了完整地了解,并提出和建立了相关的物理模型和经验公式.运用这物理模型和经验公式可对在各种使用环境温度和湿度条件下,塑封硅器件中介电薄膜的可靠性进行评估及分析.     

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。     

塑封硅器件中介电薄膜的温湿效应分析(英文)

随着铜互连以及low-k电介质在超大规模集成电路中地广泛使用,low-k电介质的机械完整性及其对互连可靠性变得更加重要。影响介电膜的机械完整性和互连可靠性的因素包括介电膜的工艺制程,芯片与封装材料的相互影响,以及环境温度和湿度的影响。本文研究集中于了解环境温度和湿度对塑封硅器件中介电薄膜的可靠性影响。采用快速温度和湿度实验条件,对塑封硅器件中介电薄膜受水分和温度损伤的敏感性进行了分析。运用商业有限元(FEA)分析软件,对水分在塑封材料和硅器件中的扩散过程进行了建模及仔细分析。并对硅器件周边密封圈的防水分扩散效力进行了研究。通过这一系列实验与分析,对塑封硅器件中介电薄膜的温湿效应有了完整地了解,并提出和建立了相关的物理模型和经验公式。运用这物理模型和经验公式可对在各种使用环境温度和湿度条件下,塑封硅器件中介电薄膜的可靠性进行评估及分析。     

双层薄膜与弹性梯度基底三层结构表面失稳分析

硬薄膜/软基底结构的表面失稳问题一直是柔性电子器件的难题,基于此,本文考虑了双层结构与弹性梯度基底间的界面剪切力,建立了双层薄膜/弹性梯度基底模型;利用位移协调条件,理论推导得到了双层薄膜/弹性梯度基底结构的临界应变和失稳波长的表达式并通过有限元仿真,验证了本研究解析解的有效性。在此基础上,应用此解析解进一步研究了弹性梯度基底的材料、双层薄膜结构厚度比等参数对临界应变和波长的影响。结果表明：减小器件层的厚度或者增加封装层的厚度,可以提高双层膜/弹性梯度基底结构的稳定性;当弹性梯度材料基底表面“较软”或器件层“较硬”时,器件层与基底界面的剪切力的影响较大,可以提升三层膜/基结构抵抗界面破坏的能力。本研究成果将为硬薄膜/弹性梯度基底结构的柔性电子器件的制备提供理论支撑。     

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.     

Voltage-induced beating vibration of a dielectric elastomer membrane

Nonlinear Dynamics - An AC voltage induces a nonlinear vibration of dielectric elastomers (DEs), which enables DE to be served as soft dynamical devices and robots. As is known, a special beating...     

Temporal evolution and instability in a viscoelastic dielectric elastomer

Dielectric elastomer transducers are being developed for applications in stretchable electronics, tunable optics, biomedical devices, and soft machines. These transducers exhibit highly nonlinear electromechanical behavior: a dielectric membrane under voltage can form wrinkles, undergo snap-through instability, and suffer electrical breakdown. We investigate temporal evolution and instability by conducting a large set of experiments under various prestretches and loading rates, and by developing a model that allows viscoelastic instability. We use the model to classify types of instability, and map the experimental observations according to prestretches and loading rates. The model describes the entire set of experimental observations. A new type of instability is discovered, which we call wrinkle-to-wrinkle transition. A flat membrane at a critical voltage forms wrinkles and then, at a second critical voltage, snaps into another state of winkles of a shorter wavelength. This study demonstrates that viscoelasticity is essential to the understanding of temporal evolution and instability of dielectric elastomers.     

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.     

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.     

A mathematical model for cylindrical,fiber reinforced electro-pneumatic actuators

In recent years, dielectric elastomers have received increasing attention due to their unparalleled large strain actuation response (>100%). The force output, however, has remained a major limiting factor for many applications. To address this limitation, a model for a fiber reinforced dielectric elastomer actuator based on the deformation mechanism of McKibben actuators is presented. In this novel configuration, the outer cylindrical surface of a dielectric elastomer is enclosed by a network of helical fibers that are thin, flexible and inextensible. This configuration yields an axially contractile actuator, in contrast to unreinforced actuators which extend. The role of the fiber network is twofold: (i) to serve as reinforcement to improve the load-bearing capability of dielectric elastomers, and (ii) to render the actuator inextensible in the axial direction such that the only free deformation path is simultaneous radial expansion and axial contraction. In this paper, a mathematical model of the electromechanical response of fiber reinforced dielectric elastomers is derived. The model is developed within a continuum mechanics framework for large deformations. The cylindrical electro-pneumatic actuator is modeled by adapting Green and Adkins’ theory of reinforced cylinders to account for the applied electric field. Using this approach, numerical solutions are obtained assuming a Mooney–Rivlin material model. The results indicate that the relationship between the contractile force and axial shortening is bilinear within the voltage range considered. The characteristic response as a function of various system parameters such as the fiber angle, inflation pressure, and the applied voltage are reported. In this paper, the elastic portion of the modeling approach is validated using experimental data for McKibben actuators.     

Band-gaps in electrostatically controlled dielectric laminates subjected to incremental shear motions

The thickness vibrations of a finitely deformed infinite periodic laminate made out of two layers of dielectric elastomers is studied. The laminate is pre-stretched by inducing a bias electric field perpendicular to the layers. Incremental time-harmonic fields superimposed on the initial finite deformation are considered next. Utilizing the Bloch-Floquet theorem along with the transfer matrix method we determine the dispersion relation which relates the incremental fields frequency and the phase velocity.Ranges of frequencies at which waves cannot propagate are identified whenever the Bloch-parameter is complex. These band-gaps depend on the phases properties, their volume fraction, and most importantly on the electric bias field. Our analysis reveals how these band-gaps can be shifted and their width can be modified by changing the bias electric field. This implies that by controlling the electrostatic bias field desired frequencies can be filtered out. Representative examples of laminates with different combinations of commercially available dielectric elastomers are examined.     

Constitutive modeling of isotropic hyperelastic materials in an exponential framework using a self-contained approach

In this paper, an exponential framework for strain energy density functions of elastomers and soft biological tissues is proposed. Based on this framework and using a self-contained approach that is different from a guesswork or combination viewpoint, a set strain energy density functions in terms of the first and second strain invariants is rebuilt. Among the constructed options for strain energy density, a new exponential and mathematically justified model is examined. This model benefits from the existence of second strain invariant, simplicity, stability of parameters, and the state of being accurate. This model can capture strain softening, strain hardening and is able to differentiate between various deformation-state dependent responses of elastomers and soft tissues undergoing finite deformation. The model has two material parameters and the mathematical formulation is simple to render the possibility of numerical implementations. In order to investigate the appropriateness of the proposed model in comparison to other hyperelastic models, several experimental data for incompressible isotropic materials (elastomers) such as VHB 4905 (polyacrylate rubber), two various silicone rubbers, synthetic rubber neoprene, two different natural rubbers, b186 rubber (a carbon black-filled rubber), Yeoh vulcanizate rubber, and finally porcine liver tissue (a very soft biological tissue) are examined. The results demonstrate that the proposed model provides an acceptable prediction of the behavior of elastomers and soft tissues under large deformation for different applied loading states.     

Modeling and simulation of the coupled mechanical-electrical response of soft solids

Dielectric elastomer (DE) is one type of electro-active polymers (EAP) that responds to electrical stimulation with a significant shape and size change. As EAPs, dielectric elastomers are lightweight, inexpensive, pliable and can be fabricated into various shapes, all of which are attractive properties to justify the intense research in the field. This paper presents a nonlinear, electrical and mechanical coupled, large deformation finite element formulation for DEAs. Maxwell’s equations for the electroquasistatic fields were solved simultaneously with equation of linear momentum. The hyperelastic Ogden model and total Maxwell stress method were combined to describe the material. The formulation was based on the weak forms of Maxwell’s equation and linear momentum expressed in the reference configuration. The closed form consistent tangent moduli for dielectric elastomers were derived. The results of the simulation compared with the experiments have demonstrated the validity of the method from the computational aspect.