【作者】 朱黎辉；

【导师】 朱喜林；

【作者基本信息】 吉林大学 ， 机械电子工程， 2011， 博士

【摘要】 介电弹性体(DE)是一种新型的智能高分子材料,是电场型电活性聚合物的一种。当其被置于直流电场中时,将沿电力线方向产生收缩,并在与电力线垂直正交的平面内扩展延伸。介电弹性体是制造主动驱动器最有潜力的电活性聚合物材料。它具有超大变形、超快响应速度、高弹性能密度、良好的负载匹配性、高机电转化效率、超强的环境适应性、高疲劳寿命以及好的仿生性能等特点,因此,可被广泛地应用于各种智能驱动器的设计。介电弹性体驱动器的潜在应用领域有小型侦察机器人、微泵、微阀、磁盘驱动、人机械面、弯曲辊、扩音器、微型飞行器、表面特性和智能表面、线性执行机构、麦克风、假肢、管路机器人、内诊镜等。本文结合国家自然科学基金项目(No.50777028),以电活性介电弹性体膜型材料为研究对象,对该材料的电致应变特性进行了相关的基础理论及实验研究。本文的主要研究工作可概括如下：1.介电弹性体驱动单元电致应变基本影响因素分析介电弹性体驱动单元是各类介电弹性体驱动器的基本功能单元,它由介电弹性体膜型材料与均匀覆盖其上下表面导电性良好的柔顺电极共同构成。当在柔顺电极上施以偏置电压,介电弹性体薄膜将在静电力作用下发生变形,即：在厚度方向产生收缩,在薄膜平面内扩展延伸,从而呈现一种“电致伸缩”特性。本文首先根据介电弹性体驱动单元的电致应变机理,定性地分析了其电致应变的主要影响因素,(如介电常数、弹性模量、材料的厚度、预加载荷等),以及这些因素对其电致应变特性的影响。2.介电弹性体材料力学特性的研究介电弹性体属于一种超弹性材料,其力学特性的本构关系具有明显的非线性。因此,对其力学特性进行精确建模是其电致应变特性研究的先研工作。本文介绍了基于连续介质力学的非线性超弹性材料的基本理论。根据介电弹性体材料大变形状态下的非线性、不可压缩以及具有超大可逆弹性变形等力学特点,采用超弹性应变能函数对其力学特性建模。在此基础上,推导了单向拉伸条件下,介电弹性体膜型材料基于Yeoh和Ogden应变能函数的名义应力—实际应变之间的关系,并与单轴拉向拉伸实验中获得的实验数据进行拟合从而获得了典型的介电弹性体膜型材料—丙烯酸VHB4910基于Yeoh和Ogden两种应变能函数的材料系数。3.介电弹性体驱动单元静电应力的分析本文在COMSOL Multiphysics的环境下,对介电弹性体驱动单元在电场激励状态下的静电应力分布情况进行有限元仿真。仿真结果表明,作用在驱动单元上实际应力,同时均匀分布于水平以及垂直两个方向。根据能量转换与守恒定律,基于介电弹性体驱动单元上所受的静电应力垂直分布假设,推导了其垂直静电等效应力。应用类似方法,推导了其水平静电等效应力。并采用垂直静电等效应力对DE驱动单元在电场中所受的静电应力进行建模。针对电活性介电弹性体膜型材料在大应变条件下相对介电常数变化的这一特点,提出了一种该膜型材料相对介电常数的测定方法,并据此获得了其相对介电常数随拉伸率变化的规律。通过在介电弹性体驱动单元的静电等效应力公式中引入变化的介电常数,使得静电等效应力的计算更为精确。这一研究为后续分析该材料电致应变特性奠定了理论基础。4.电活性介电弹性体材料电致应变特性的研究本文从介电弹性体驱动单元基本致动原理及材料的机械特性出发,根据材料在不同边界配置条件下的机电耦合关系,分别建立了其基于Yeoh和Ogden应变能函数的电致应变数学模型。该数学模型综合地考虑了材料在变形过程中力学特性的非线性、相对介电常数的变化以及柔顺电极间电场强度的变化等因素对其电致应变的影响。基于Ogden应变能函数的电致应变数学模型,对典型电活性介电弹性体材料—丙烯酸VHB4910在自由边界、单轴预拉伸以及等轴预拉伸配置下的单向以及面积电致应变特性进行了数值仿真研究。研究结果表明,该材料在等轴预拉伸配置下可以获得更大的单向以及面积电致应变。通过剖析电活性介电弹性体膜型材料正反馈的机电耦合过程中伴随的机电稳定性问题,分析了该材料发生机电失稳的原因,提出了划分材料在电致变形过程中的稳定工作区及失稳区的方法,并据此获得了该材料在不同等轴拉伸率下的极限失稳电压及极限失稳电致应变。数值仿真结果表明,通过提高等轴预拉伸率可以显著增加该介电弹性体膜型材料的稳定工作区间,并提高其极限失稳场强。分析了介电弹性体膜型材料的三种主要失效模式,即机械强度、介电强度以及机电失稳,并阐述其基本失效机理。在此基础上,通过经验公式、实验研究以及基于数学模型的数值仿真分别获得了该丙烯酸聚合物基于这三种失效模式的极限电致面积应变及安全工作区间。研究结果表明：丙烯酸VHB4910在其等轴预拉伸率λopt=3.17时,可以获得最大的电致面积应变。这一结论可用于指导基于丙烯酸聚合物的介电弹性体驱动器的设计。5.介电弹性体膜型材料电致应变特性的实验研究本文以丙烯酸VHB4910为基质材料,以石墨—硅胶混合物为柔顺电极的基本材料,制成一种电活性膜型功能材料,并将其应用于圆形介电弹性体驱动器的制作。选取不同边界配置条件、激励电压以及电极材料分别进行实验研究,获得了这些因素对介电弹性体材料电致应变特性的影响规律。实验结果表明,等轴预拉伸配置下,该介电弹性体膜型材料可以获得较大的电致面积应变,且其最大电致面积应变发生在等轴预拉伸率λp∈[2.5,3.2]情况下；提高激励电压可以提高材料在各种边界配置条件下的电致面积应变；柔顺电极材料对介电弹性体驱动器的电致面积应变特性及其失效模式的影响都比较显著。实验结果与基于电致应变数学模型的数值仿真结果基本一致,从而验证了所建模型的正确性。本文的研究成果可为介电弹性体驱动器的制作提供理论指导和应用参考。更多还原

【Abstract】 Dielectric elastomer (DE) as a new kind of intelligent polymer material belongs to electric-filed-type electroactive polymers. When the material is put into electric field, it will be compressed in the direction of electric field and meanwhile be stretched in the plane perpendicular to the field, according to Maxwell effect.Dielectric Elastomer shows most promising in the manufacturing of actuators. With excellent performance parameters such as large and reversible elastic deformation, short response time, high energy desity, good flexibility and matching of load, high efficiency, high reliability and fatigue life, strong environmental tolerance and high actuation presure, etc., it is widely used in the design of all types of intelligent actuators.The potential applications of the electroactive material includes small size detecting robots, micro-pumps, micro-values, disk driving system, face robots, bend rollers, loudhailer, micro-aerocrafts, unidirection linear actuators, artificial limb equipments, loud speaker, tube robots and rulable medicine pipes, etc.This work was supported by National Natural Science Fundation of China (grant No.50777028). Relevant theoretical and experimental research were carried out to figure out characteristics of electrostrictive strain in Electro-active membraniform dielectric elastomer material. Main research work of this thesis can be summed up as following:1. Basic Concernning Factors of Electrostrictive Strain on Dielectric Elastomer Actuating UnitDielectric elastomer actuating unit is a basic unit of dielectric elastomer actuators (DEAs) which is composed of DE film and two compliant electrodes adhering to its upper and bottom sides. Once a high voltage difference is placed on the both electrodes, the unit will change its shape according to Maxwell stress. On the basis of introduing basic configuration and working principle of dielectric elastomer unit, qualitative analysis of main factors that might influence characteristics of its electrostrictive strain were analysised, such as dielectric permitivity, elastic modulus, thickness of the film, preload, etc..2. Research on Mechanical Characteristics of Dielectric Elastomer MaterialStarting from mechanical characteristics of DE material such as nonlinear, volume incompressible and large and reversible elastic deformation etc., several hyper-elastic material models such as Mooney-Rivlin, Neo-Hookean, Yeoh and Ogden models were selected to describe mechanical property of it. Relationship between nominal stress and practical strain of this volume compressible hyper-elastic material were derived from the bounduary condition of uniaxial pre-stretch, on the basis of strain energy function of Yeoh and Ogden models. Also, method to determine material coefficients of strain energy function in these two hyper-elastic materials were given. Material coefficients of these two strain energy function about a kind of typical DE material, Acrylic elastomer VHB4910, were obtained by fitting experiment data from uniaxial tension. This work laid the foundation for modeling and numerical simulation of characteristics of electrostrictive strain in electro-active membraniform DE materials.3. Research on Static Stress of Dielectric Elastomer Actuating UnitSimulation program COMSOL Multiphysics 3.5a was used in simulating distribution of the static stress on dielectric elastomer actuating unit. Simulation result indicates that static stress caused by electric filed is homogenously distributing on the the internal suface area and lateral surface area of the unit. Base on the hypothesis of the distribution of static stress on dielectric elastomer actuating unit only existing along the direction of electric field, the vertical static equivalent stress on the unit was obtained from derivation, and also the horizontal one, according to the Energy Conversion and Conservation Law.A method to obtain the relative permittivity of the film under different stretch rate by fitting the data from experiment was adopted considering the fact that the relative permittivity of the material is changing during deformation. According to the method, the law of relative permittivity changing with stretch rate of acrylic elastomers VHB4910 was obtained through experiments and it was found out that the relative permittivity of acrylic elastomers VHB4910 keep linearly decreasing with the increasing of stretch rate. On that basis, the method to calculate the vertical static equivalent stress on the unit more accurately has been proposed with revised relative permittivity. The work is useful for furhter calculating and simulating of electrostrictive strain of membraniform dielectric elastomer material.4. Research on Characteristics of Electrostrictive Strain in Electro-active Membraniform Dielectric Elastomer MaterialsCharacteristics of electrostrictive strain in electro-active membraniform DE materials were studied under the boundary condtions of free boundary, uniaxial and equi-biaxial pre-stretch, respectively. Mathematical models about characteristics of electrostrictive strain in DE material were proposed, based on Yeoh and Ogden strain energy function, respectively.The model took into account the influence of non-linearity mechanical property of membraniform material, decreasing of relative permittivity and changing of electrostatic equivalent stress during deformation process on characteristics of electrostrictive strain of electroactive DE material, synthetically. Thus it can efficiently describe the relationship among pre-stretch rate, actuation voltage and electrostrictive strain of the material under actuating process.The thesis took acrylic elastomer VHB4910, a typical kind of DE material, as an example, demonstrated a research method for electrostrictive strain characteristics based on Ogden strain energy function, under the boundary condtions of free boundary, uniaxial and equi-biaxial pre-stretch, respectively. The results show that the best atrain on one axis strain and area electrostrictive strain of the material can be obtained under the boundary condiction of equi-biaxial pre-stretch. By properly selecting pre-stretch rate, the electroactive film can achieve higher electrostrictive strain as well as lower the possibility of pull-in failure.The reason of pull-in failure was analysised by explaining the deformation process of electroactive dielectric elastomer material as a positive feedback electromechanical couple process.By numerical simulation of electrostrictive area strain with the mathematic model, stable and instable zone of the unit can be devided and also the limit stable voltage and top area strain can be achieved.Three main failure modes of electroactive film were analysised, that is mechanical strength, dielectric strength and pull-in instability. Also, their corresponding failure mechanism were set forth. Limit electrostrictive area strain of each failure mode can be obtained by experience formula, mathematic model and experiment research, synthetically. According to numerical simulation, the unit built with acrylic elastomer VHB4910 can obtain its highest area strain under boundary condition of equi-biaxial pre-stretch rateλopt=3.17. The result can be used in the design of DEAs with substrate material of VHB4910.5. Experiment Research on Characteristics of Electrostrictive Strain in Electro-active Membraniform Dielectric Elastomer MaterialsA new type of electroactive membraniform functional material (E-ACE) was bulit with acylic VHB4910 and silicon-gel mixture. A method for pre-stretching dielectric elastomer was proposed and also all kinds of circular actuator with different pre-stretch rate was built with the funtional material.Different boundary conditions, actuation voltages and compliant electrodes were chosen in the experiments and relative experiments were carried out to find out the influence of them on electrostriction strain of DEAs.Experiment results indicated that electrostrictive area strain of the actuators under equi-biaxial pre-stretch condition was much larger than non-prestretch and uniaxial pre-stretch condition, and the largest area strain of the DEAs occured between 250%-320% under equibiaxial pre-stress condition. Electrostrictive area strain of the DEAs can be improved by increasing bias voltage for any boundary condition. The influence of compliant electrodes material on the characteristics of electrostrictive area strain of DEAs also can not be ignored.Experiment results coincide with numerical simulation results, which further validates the applicability of the mathematics model. The result of this paper can provide theoretical guidance and application reference for the design of DEAs.更多还原