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2006-05-09 22:15:28
论文题目：电流变机理及应用研究
作者简介：田煜 ，男,1975年05月出生，1998年09月师从于清华大学温诗铸教授，于2002年01月获博士学位。
摘 要
智能材料系统与结构（Intelligent Material Systems & Structures）是机械科学技术与材料科学技术交叉的前沿领域。当今人类社会已经高度机械化、自动化和信息化，智能材料的应用往往会孕育出一个新的产业，或者引起相关传统产业的巨大变革，因此智能材料系统与结构作为新兴的科学技术在近几十年得到了广泛的研究和应用。如智能压电材料不但被广泛用于照相机的光学自动调焦，而且更是诸如原子力显微镜等尖端科学仪器的关键工作部件，从而极大地促进了人类在原子尺度上对自然界的认知和改造。电流变液是20世纪中叶由美国人Winslow发明的性能独特的智能流变材料，在外电场作用下它可由液态向类固态转化，此转化可逆，且响应速度很快。其优异的机电控制特性使它在航空航天、武器控制、机器人工程、噪声防治、汽车工程、船舶工程、液压工程、体育健身器械等领域具有广阔的应用前景。在近十几年来，电流变技术吸引了物理、化学等基础学科和材料、机械、航空和土木结构等工程学科的大批研究力量，并取得了显著进展，但在材料、机理、性能和应用等方面仍有一些关键问题有待突破。由于电流变技术对国民经济发展的重要性，国家自然科学基金委于1999年专门立项资助了电流变机理和电流变材料两个重点基金项目。本论文工作是“电流变液的机理研究”重点项目（19834020）的一部分，以电流变液的两个最重要的力学和电学参数：剪切屈服应力和电流密度为核心，对电流变效应的物理模型、高性能电流变液的制备与性能表征，以及在机械传动和健身车方面的应用进行了系统研究，得到了一些重要结果。论文的主要研究工作和创新成果如下：
1．发展了电流变效应的电导模型，并据此配制了沸石/硅油型的高强度电流变液。从两个荷电表面间库仑力作用的基本物理模型出发，根据电导模型从颗粒间相互作用力推导了电流变液的剪切屈服应力，并与基于界面极化模型和双电层极化模型进行了比较，认为用电导模型来描述直流电场作用下的电流变效应更加合理。根据电导模型提出将吸附在固体颗粒表面的添加剂看作颗粒表面的薄膜，该薄膜的电导率远大于基础液的电导率，薄膜在电场作用下沿着电场方向产生拉伸变形，减小颗粒间实际间隙，增强颗粒间局部电场强度，从而获得强电流变效应。根据该模型描述，从各种沸石材料中筛选出一种NaY型沸石，使用添加剂对该沸石表面改性，使其具有高的电导率（10 –4s/m量级）和较高的相对介电常数（小于10 kHz时，大于100），与普通绝缘硅油混合后配制的电流变液在5 kV/mm直流电场作用下具有20 kPa以上的剪切屈服应力。该值远高于国内外其他报道中电流变液的剪切强度，并突破了传统极化理论通常预测的几个kPa的极限值。提出沸石颗粒和基础液间的高电导率失配和高介电常数失配是得到具有强电流变效应的关键，用改进的电导模型能够合理解释这种强电流变效应。另外研究了颗粒体积比率和温度对电流变液的剪切屈服应力的影响，发现颗粒体积比率越高，同样外电场强度作用下电流变液的剪切屈服应力越高，该类电流变液的剪切屈服应力随温度升高变化不大，而零电场粘度明显降低，同时电流密度急剧升高。用电导模型对这些实验现象进行了合理解释。
2．用电流变液的宏观力学性能验证了产生剪切屈服应力的微观物理模型，系统研究了电流变液在剪切、拉伸和压缩作用下的力学性能。由于电流变液的拉伸屈服应力代表了外电场作用下被极化的颗粒间相互作用力沿外加电场方向的统计力学特性，而剪切屈服应力则代表了颗粒间作用力沿着剪切方向（垂直于电场方向）的统计力学特性，这两个参数之间可通过剪切屈服应变角联系起来：。在本研究中＝15°～18.5°，与Conrad等人实验测试剪切屈服应变得到的16.7°吻合很好，从而直接验证了基于颗粒链剪切与外电场形成一定剪切屈服角而产生剪切屈服应力的微观物理模型。另外在实验中还发现，在同样的电场强度作用下，电流变液在两平行极板间产生的压应力要远大于剪切屈服应力和拉伸屈服应力，电流变液在压应变约为0.1时具有最小的压缩弹性模量。
3．提出电流变液中基础液电导率在与固体颗粒混合后可发生改变，并据此提出了改进电流变液性能的热处理方法。根据电导模型中颗粒间饱和局部电场、电流密度和颗粒间相互作用力之间的关系分析发现，饱和电场区域半径越大，要得到一定大小的颗粒间相互作用力，所需要的电流密度越小。这样可使用立方体颗粒模型来计算某一电流变液剪切屈服应力所对应的最小电流密度。据已有关于高剪切强度电流变液的实验报道中给出的电流密度都比根据基础液电导参数及以上方法计算得到的最小电流密度还要小，表明电流变液基础液的电导在与固体颗粒混合后已发生改变。据此结论，用电导模型可对以往无法解释的电流变液的含水量对电流变效应的非单调影响进行合理解释。另外还给出了减小电流变液中基础液电导率的一种热处理方法，并提出使用具有强吸附能力的颗粒分散相可有效减小基础液电导率，提高颗粒与基础液的电导比率，增强电流变液剪切屈服应力，降低电流变液电流密度。
4．系统研究了电流变液的动态响应特性。研制了基于同心圆筒剪切和平行平板剪切的两套电流变液动态响应测试装置，对电流变液在不同剪切速率、不同电场强度作用下的动态响应进行了实验研究。实验结果表明剪切速率越高，剪切应力的上升就越快，上升响应时间常数在毫秒量级。去掉电场后，剪切应力回复过程远快于上升过程，均在2 ms以内。在实验中还发现，剪切应力变化对外加电场施加有短时间延迟（约0.2-0.3 ms），与电流变液的充电时间一致。由此提出电流变液首先被电场极化，然后在剪切作用下链结构与外电场方向形成一定角度产生剪切抗力的物理过程，即极化－结构－力的电流变液动态响应过程。
5．综合考虑电流变液的力学和电学性能进行了电流变器件的优化设计及应用基础研究。研究了同心圆筒式电流变液联轴器在不同电场强度作用下的力矩和速度传递特性，预测值与实验值吻合较好。通过摩擦加载，施加方波高压直流电场实现了输出轴的间歇旋转进给运动。另外，采用同心圆筒剪切，以所需达到的力学指标为约束条件，以消耗电源功率最小为优化目标对阻尼部件进行了优化设计，研制了一台基于电流变液的健身车样机，并进行了力学和温升特性测试。实验结果表明，在2250 V/mm的电场作用下，健身车产生阻力矩约12.2 Nm，而电流变液的电流密度只有0.26 mA/cm2(室温25 °C)，电极面积523 cm2，消耗电源功率仅为0.3 W。在1500 V/mm电场作用下进行40分钟锻炼。测得电流变液的电流密度和温度随着锻炼时间的增长而升高，最后温度上升到37.5 °C，电流变液的电流密度也只有0.31 mA/cm2，此时健身车消耗的电源功率仍非常小，仅为0.24 W。这种综合材料力电性能进行优化设计的电流变阻尼器具有较宽的阻尼力调节范围和极小的电源功率消耗，很好展示了电流变液的工作特性和乐观的应用前景。
本论文工作大幅度提高了电流变液的剪切屈服强度，可使电流变器件具有更强的力电耦合输出能力；直流电场作用下电流变液机理的研究结果对配制其他体系的高性能电流变具有指导作用；电流变液动态响应的研究结果对优化电流变器件的控制参数和模式具有重要的参考价值；电流变液的拉伸和压缩性能的研究结果将拓宽电流变液应用的工作模式和电流变器件的设计思路，从而促进电流变技术的发展。
关键词：电流变液、电导率、介电常数、电场强度、剪切屈服应力、电流密度
Abstract
Intelligent Material Systems & Structures (IMSS) is one of the frontier interdisciplinary areas of mechanical engineering and materials science. Application of intelligent materials usually generates a new industry, or leads to a great revolution of the involved traditional industries in today’s mechanization, automation and information society. Therefore, as a rising area, IMSS has been widely explored during the past several decades. Electrorheological (ER) fluid is a kind of intelligent rheological material invented by Winslow in the middle of the 20th century. It is a suspension composed of particles and insulating oils, and is notable for abrupt and reversible change of fluid viscosity under the application of an external electric field. Its excellent mechatronic controllability exhibits a great potential in applications to aeronautics and aviation, armament control, robotic engineering, noise control, automobile engineering, watercraft engineering, hydraulic engineering, and physical exercising apparatus etc. The ER fluid has attracted much attention of researchers in fundamental science areas, such as physics and chemistry, and in engineering areas, such as materials science and engineering, mechanical engineering, aeronautic engineering and civil engineering. Although great developments in ER materials have been achieved during recent years, there are still some key issues unresolved in mechanism, property, and applications of ER fluids. Considering the importance of the ER technology to the national economy, the National Natural Science Foundation of China sponsored two projects of key program in ER mechanism and ER materials in 1999. This dissertation is a part of the project of key program of ER mechanism (19834020). The physical model of ER effect, the development and characterization of ER fluids, and ER applications to mechanical transmission and human exercising bicycle were systematically investigated. And special attention was paid to the most important mechanical and electrical parameters of ER fluids, shear yield stress and current density. The main work and innovative contributions of this dissertation are as follows:
1. The conduction model of ER effect was improved, and a kind of zeolite/silicone oil ER fluid with high strength was developed in line with the improved model. Based on the physical model of Coulomb force between two charged surfaces and the conduction model, the shear yield stress of ER fluids was derived from the two-particle interaction system. Compared with the interfacial polarization model and the double layer model, the conduction model was more suitable for the description of strong ER effect under DC electric fields. According to the conduction model, we proposed that the additives added to particles could be treated as a coated liquid layer outside the particle surface. The conductivity of the liquid layer is much higher than that of the insulating oil. The layer can be extended along the direction of the applied electric field. It can reduce the real gap distance, enhance the local field strength between particles, and finally lead to high shear yield strength and strong ER effect. According to the description, we selected a NaY type zeolite as the dispersion particle in the common insulating silicone oil. The surface of the zeolite was modified by additives to obtain high conductivity (in the order of 10－4 s/m) and high relative dielectric constant (<10 kHz, <100). Then, ER fluids with shear yield stress of higher than 20 kPa under a DC field of 5 kV/mm were achieved. The obtained shear yield stress is much higher than those reported by others. And it breaks through the prediction by the traditional polarization model of several kPa. The high mismatches of conductivity and dielectric constant between the zeolite particles and the base oil were considered to be responsible for the strong ER effect. In addition, the particulate volume fraction effect and the temperature effect on the ER effect of this kind of ER fluids were studied. The experiments showed that a higher particulate volume fraction corresponded to a higher shear yield stress of ER fluids under the same electric field strength. There was little change in shear yield stress in the temperature range of 10-90 °C, while the zero field viscosity of the suspension decreases quickly and the current density increases quickly with the increase of temperature. The reasonable explanation of the phenomena by the conduction model was given.
2. The mechanical properties of ER fluids under shearing, tension, and compression have been systematically investigated. The tensile yield stress of ER fluid represents the effect of the interaction force between the polarized particles along the direction of the applied electric field. Shear yield stress represents the effect of the interaction force along the shear direction (perpendicular to the field direction). Experiments showed that the two parameters could be related by a shear yield angle by , which is 15 °～18.5 ° in this investigation. It agreed well with the experimental value of 16.7 ° obtained by Conrad et al. The results directly verified the physical model of the origin of shear yield stress. Experiments also showed that, under the same electric field strength, the compressive stress of the ER fluid was much higher than its shear yield stress and tensile yield stress. The compressive modulus is minimum when the compressive strain is about 0.1.
3. The change of the conductivity of base oil after mixed with the particles was verified, and a thermal treatment of ER fluids to improve the ER property was proposed. According to the analyses of the local saturation field, current density, and the interaction force between particles based on the conduction model, the larger the radius of the saturation field region is, the smaller the current density needed to attain a certain interaction force between particles will be. Thus cubic particle model can predict the minimum current density of a certain ER fluids needed to obtain a shear yield stress. However, almost all ER fluids with high shear strength reported in literatures gave current densities lower than the minimum values predicted by the above model if taking the conductivity of base oil as the value measured before mixed with particles. The reason for this conflict is that the conductivity of base oil was reduced after it was mixed with particle phase. By taking this effect into account, the conduction model can explain the non-monotony effect of water content on the ER effect very well. In addition, a thermal treatment was proposed to decrease the conductivity of base oil in ER fluids. Employing particles with high absorbability can effectively reduce the conductivity of base oil, increase the conductivity ratio between particles and base oil, and consequently increase shear yield stress, decrease current density.
4. The dynamic response of ER fluids under shearing was experimentally determined. Two sets of apparatus based on shearing between two concentric cylinders and between two parallel plates were established. The dynamic response of ER fluids under different shear rates, and different applied electric fields was experimentally determined. Experiments showed that, shear stress rose more quickly at a higher shear rate under the same stepwise electric field. The rising time was in ms scale. The results verified the fact that when the ER fluid reached its shear yield strain, shear stress attained shear yield stress. After the retraction of the applied electric field, the shear stress decayed at a rate much quicker than rising, which was less than 2 ms. Also, experiments showed that there was a short delay time of about 0.3-0.3 ms, which accords with the charging and discharging time of the ER fluids. So we proposed the following process for the dynamic response. Firstly, the ER fluids are polarized by the applied electric field, then under the mechanical shearing, particle chains form angles to the applied field and show the shear stress, i.e., following the sequence of polarization-structure-stress.
5. Optimal design and fabrication of ER application devices were carried out with consideration of both the mechanical and electrical aspects of ER fluids. The torque and speed transmission characteristics of a newly designed ER coupling under different electric field strengths were investigated. The experimental results agreed well with the predictions. By applying a frictional resistant torque and high electric field pulse, step rotation of the output axle was realized. A prototype of exercising bicycle was designed based on two concentric cylinders. Taking the mechanical performance of the cylinders as the constraint condition, the minimization of the electric power consumption as the aim of the optimization, the dimensions of the cylinders were optimized. The mechanical and thermal performances of the prototype were experimentally determined. Experiments showed that, under an applied field of 2250 V/mm, the induced resistant torque was approximately 12.2 Nm, the current density of the ER fluid was 0.26 mA/cm2 (room temperature 25 °C), the consumption of electric power was only 0.3 W with an electrode area of 523 cm2. An exercising test under 1500 V/mm for 40 minutes showed that the current density of the ER fluid increased slowly from the room temperature to 37.5 °C, with a final current density of 0.31 mA/cm2 and an electric power consumption of 0.24 W. The resistance of this shearing type damper can be conveniently adjusted with very small electric power consumption. It clearly showed the technical characteristics of ER fluids and the optimal potential of using ER fluids in real applications.
The work in this dissertation greatly improved the shear yield strength of ER fluids, which can increase the mechanical output ability of ER actuators. The conclusions on ER mechanism under DC field are useful for preparing other kinds of ER fluids with high performance. The measured dynamic response property of ER fluids under shearing is important for the optimization of control parameters and control modes of ER actuators. The study on tensile and compressive behaviors of ER fluids has broadened the working type of ER fluids in applications, and can expedite the development of ER technology.
Key words: electrorheological fluids, conductivity, dielectric constant, electric field strength, shear yield stress, current density
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论文题目：电流变机理及应用研究
作者简介：田煜 ，男,1975年05月出生，1998年09月师从于清华大学温诗铸教授，于2002年01月获博士学位。
摘 要
智能材料系统与结构（Intelligent Material Systems & Structures）是机械科学技术与材料科学技术交叉的前沿领域。当今人类社会已经高度机械化、自动化和信息化，智能材料的应用往往会孕育出一个新的产业，或者引起相关传统产业的巨大变革，因此智能材料系统与结构作为新兴的科学技术在近几十年得到了广泛的研究和应用。如智能压电材料不但被广泛用于照相机的光学自动调焦，而且更是诸如原子力显微镜等尖端科学仪器的关键工作部件，从而极大地促进了人类在原子尺度上对自然界的认知和改造。电流变液是20世纪中叶由美国人Winslow发明的性能独特的智能流变材料，在外电场作用下它可由液态向类固态转化，此转化可逆，且响应速度很快。其优异的机电控制特性使它在航空航天、武器控制、机器人工程、噪声防治、汽车工程、船舶工程、液压工程、体育健身器械等领域具有广阔的应用前景。在近十几年来，电流变技术吸引了物理、化学等基础学科和材料、机械、航空和土木结构等工程学科的大批研究力量，并取得了显著进展，但在材料、机理、性能和应用等方面仍有一些关键问题有待突破。由于电流变技术对国民经济发展的重要性，国家自然科学基金委于1999年专门立项资助了电流变机理和电流变材料两个重点基金项目。本论文工作是“电流变液的机理研究”重点项目（19834020）的一部分，以电流变液的两个最重要的力学和电学参数：剪切屈服应力和电流密度为核心，对电流变效应的物理模型、高性能电流变液的制备与性能表征，以及在机械传动和健身车方面的应用进行了系统研究，得到了一些重要结果。论文的主要研究工作和创新成果如下：
1．发展了电流变效应的电导模型，并据此配制了沸石/硅油型的高强度电流变液。从两个荷电表面间库仑力作用的基本物理模型出发，根据电导模型从颗粒间相互作用力推导了电流变液的剪切屈服应力，并与基于界面极化模型和双电层极化模型进行了比较，认为用电导模型来描述直流电场作用下的电流变效应更加合理。根据电导模型提出将吸附在固体颗粒表面的添加剂看作颗粒表面的薄膜，该薄膜的电导率远大于基础液的电导率，薄膜在电场作用下沿着电场方向产生拉伸变形，减小颗粒间实际间隙，增强颗粒间局部电场强度，从而获得强电流变效应。根据该模型描述，从各种沸石材料中筛选出一种NaY型沸石，使用添加剂对该沸石表面改性，使其具有高的电导率（10 –4s/m量级）和较高的相对介电常数（小于10 kHz时，大于100），与普通绝缘硅油混合后配制的电流变液在5 kV/mm直流电场作用下具有20 kPa以上的剪切屈服应力。该值远高于国内外其他报道中电流变液的剪切强度，并突破了传统极化理论通常预测的几个kPa的极限值。提出沸石颗粒和基础液间的高电导率失配和高介电常数失配是得到具有强电流变效应的关键，用改进的电导模型能够合理解释这种强电流变效应。另外研究了颗粒体积比率和温度对电流变液的剪切屈服应力的影响，发现颗粒体积比率越高，同样外电场强度作用下电流变液的剪切屈服应力越高，该类电流变液的剪切屈服应力随温度升高变化不大，而零电场粘度明显降低，同时电流密度急剧升高。用电导模型对这些实验现象进行了合理解释。
2．用电流变液的宏观力学性能验证了产生剪切屈服应力的微观物理模型，系统研究了电流变液在剪切、拉伸和压缩作用下的力学性能。由于电流变液的拉伸屈服应力代表了外电场作用下被极化的颗粒间相互作用力沿外加电场方向的统计力学特性，而剪切屈服应力则代表了颗粒间作用力沿着剪切方向（垂直于电场方向）的统计力学特性，这两个参数之间可通过剪切屈服应变角联系起来：。在本研究中＝15°～18.5°，与Conrad等人实验测试剪切屈服应变得到的16.7°吻合很好，从而直接验证了基于颗粒链剪切与外电场形成一定剪切屈服角而产生剪切屈服应力的微观物理模型。另外在实验中还发现，在同样的电场强度作用下，电流变液在两平行极板间产生的压应力要远大于剪切屈服应力和拉伸屈服应力，电流变液在压应变约为0.1时具有最小的压缩弹性模量。
3．提出电流变液中基础液电导率在与固体颗粒混合后可发生改变，并据此提出了改进电流变液性能的热处理方法。根据电导模型中颗粒间饱和局部电场、电流密度和颗粒间相互作用力之间的关系分析发现，饱和电场区域半径越大，要得到一定大小的颗粒间相互作用力，所需要的电流密度越小。这样可使用立方体颗粒模型来计算某一电流变液剪切屈服应力所对应的最小电流密度。据已有关于高剪切强度电流变液的实验报道中给出的电流密度都比根据基础液电导参数及以上方法计算得到的最小电流密度还要小，表明电流变液基础液的电导在与固体颗粒混合后已发生改变。据此结论，用电导模型可对以往无法解释的电流变液的含水量对电流变效应的非单调影响进行合理解释。另外还给出了减小电流变液中基础液电导率的一种热处理方法，并提出使用具有强吸附能力的颗粒分散相可有效减小基础液电导率，提高颗粒与基础液的电导比率，增强电流变液剪切屈服应力，降低电流变液电流密度。
4．系统研究了电流变液的动态响应特性。研制了基于同心圆筒剪切和平行平板剪切的两套电流变液动态响应测试装置，对电流变液在不同剪切速率、不同电场强度作用下的动态响应进行了实验研究。实验结果表明剪切速率越高，剪切应力的上升就越快，上升响应时间常数在毫秒量级。去掉电场后，剪切应力回复过程远快于上升过程，均在2 ms以内。在实验中还发现，剪切应力变化对外加电场施加有短时间延迟（约0.2-0.3 ms），与电流变液的充电时间一致。由此提出电流变液首先被电场极化，然后在剪切作用下链结构与外电场方向形成一定角度产生剪切抗力的物理过程，即极化－结构－力的电流变液动态响应过程。
5．综合考虑电流变液的力学和电学性能进行了电流变器件的优化设计及应用基础研究。研究了同心圆筒式电流变液联轴器在不同电场强度作用下的力矩和速度传递特性，预测值与实验值吻合较好。通过摩擦加载，施加方波高压直流电场实现了输出轴的间歇旋转进给运动。另外，采用同心圆筒剪切，以所需达到的力学指标为约束条件，以消耗电源功率最小为优化目标对阻尼部件进行了优化设计，研制了一台基于电流变液的健身车样机，并进行了力学和温升特性测试。实验结果表明，在2250 V/mm的电场作用下，健身车产生阻力矩约12.2 Nm，而电流变液的电流密度只有0.26 mA/cm2(室温25 °C)，电极面积523 cm2，消耗电源功率仅为0.3 W。在1500 V/mm电场作用下进行40分钟锻炼。测得电流变液的电流密度和温度随着锻炼时间的增长而升高，最后温度上升到37.5 °C，电流变液的电流密度也只有0.31 mA/cm2，此时健身车消耗的电源功率仍非常小，仅为0.24 W。这种综合材料力电性能进行优化设计的电流变阻尼器具有较宽的阻尼力调节范围和极小的电源功率消耗，很好展示了电流变液的工作特性和乐观的应用前景。
本论文工作大幅度提高了电流变液的剪切屈服强度，可使电流变器件具有更强的力电耦合输出能力；直流电场作用下电流变液机理的研究结果对配制其他体系的高性能电流变具有指导作用；电流变液动态响应的研究结果对优化电流变器件的控制参数和模式具有重要的参考价值；电流变液的拉伸和压缩性能的研究结果将拓宽电流变液应用的工作模式和电流变器件的设计思路，从而促进电流变技术的发展。
关键词：电流变液、电导率、介电常数、电场强度、剪切屈服应力、电流密度
Abstract
Intelligent Material Systems & Structures (IMSS) is one of the frontier interdisciplinary areas of mechanical engineering and materials science. Application of intelligent materials usually generates a new industry, or leads to a great revolution of the involved traditional industries in today’s mechanization, automation and information society. Therefore, as a rising area, IMSS has been widely explored during the past several decades. Electrorheological (ER) fluid is a kind of intelligent rheological material invented by Winslow in the middle of the 20th century. It is a suspension composed of particles and insulating oils, and is notable for abrupt and reversible change of fluid viscosity under the application of an external electric field. Its excellent mechatronic controllability exhibits a great potential in applications to aeronautics and aviation, armament control, robotic engineering, noise control, automobile engineering, watercraft engineering, hydraulic engineering, and physical exercising apparatus etc. The ER fluid has attracted much attention of researchers in fundamental science areas, such as physics and chemistry, and in engineering areas, such as materials science and engineering, mechanical engineering, aeronautic engineering and civil engineering. Although great developments in ER materials have been achieved during recent years, there are still some key issues unresolved in mechanism, property, and applications of ER fluids. Considering the importance of the ER technology to the national economy, the National Natural Science Foundation of China sponsored two projects of key program in ER mechanism and ER materials in 1999. This dissertation is a part of the project of key program of ER mechanism (19834020). The physical model of ER effect, the development and characterization of ER fluids, and ER applications to mechanical transmission and human exercising bicycle were systematically investigated. And special attention was paid to the most important mechanical and electrical parameters of ER fluids, shear yield stress and current density. The main work and innovative contributions of this dissertation are as follows:
1. The conduction model of ER effect was improved, and a kind of zeolite/silicone oil ER fluid with high strength was developed in line with the improved model. Based on the physical model of Coulomb force between two charged surfaces and the conduction model, the shear yield stress of ER fluids was derived from the two-particle interaction system. Compared with the interfacial polarization model and the double layer model, the conduction model was more suitable for the description of strong ER effect under DC electric fields. According to the conduction model, we proposed that the additives added to particles could be treated as a coated liquid layer outside the particle surface. The conductivity of the liquid layer is much higher than that of the insulating oil. The layer can be extended along the direction of the applied electric field. It can reduce the real gap distance, enhance the local field strength between particles, and finally lead to high shear yield strength and strong ER effect. According to the description, we selected a NaY type zeolite as the dispersion particle in the common insulating silicone oil. The surface of the zeolite was modified by additives to obtain high conductivity (in the order of 10－4 s/m) and high relative dielectric constant (<10 kHz, <100). Then, ER fluids with shear yield stress of higher than 20 kPa under a DC field of 5 kV/mm were achieved. The obtained shear yield stress is much higher than those reported by others. And it breaks through the prediction by the traditional polarization model of several kPa. The high mismatches of conductivity and dielectric constant between the zeolite particles and the base oil were considered to be responsible for the strong ER effect. In addition, the particulate volume fraction effect and the temperature effect on the ER effect of this kind of ER fluids were studied. The experiments showed that a higher particulate volume fraction corresponded to a higher shear yield stress of ER fluids under the same electric field strength. There was little change in shear yield stress in the temperature range of 10-90 °C, while the zero field viscosity of the suspension decreases quickly and the current density increases quickly with the increase of temperature. The reasonable explanation of the phenomena by the conduction model was given.
2. The mechanical properties of ER fluids under shearing, tension, and compression have been systematically investigated. The tensile yield stress of ER fluid represents the effect of the interaction force between the polarized particles along the direction of the applied electric field. Shear yield stress represents the effect of the interaction force along the shear direction (perpendicular to the field direction). Experiments showed that the two parameters could be related by a shear yield angle by , which is 15 °～18.5 ° in this investigation. It agreed well with the experimental value of 16.7 ° obtained by Conrad et al. The results directly verified the physical model of the origin of shear yield stress. Experiments also showed that, under the same electric field strength, the compressive stress of the ER fluid was much higher than its shear yield stress and tensile yield stress. The compressive modulus is minimum when the compressive strain is about 0.1.
3. The change of the conductivity of base oil after mixed with the particles was verified, and a thermal treatment of ER fluids to improve the ER property was proposed. According to the analyses of the local saturation field, current density, and the interaction force between particles based on the conduction model, the larger the radius of the saturation field region is, the smaller the current density needed to attain a certain interaction force between particles will be. Thus cubic particle model can predict the minimum current density of a certain ER fluids needed to obtain a shear yield stress. However, almost all ER fluids with high shear strength reported in literatures gave current densities lower than the minimum values predicted by the above model if taking the conductivity of base oil as the value measured before mixed with particles. The reason for this conflict is that the conductivity of base oil was reduced after it was mixed with particle phase. By taking this effect into account, the conduction model can explain the non-monotony effect of water content on the ER effect very well. In addition, a thermal treatment was proposed to decrease the conductivity of base oil in ER fluids. Employing particles with high absorbability can effectively reduce the conductivity of base oil, increase the conductivity ratio between particles and base oil, and consequently increase shear yield stress, decrease current density.
4. The dynamic response of ER fluids under shearing was experimentally determined. Two sets of apparatus based on shearing between two concentric cylinders and between two parallel plates were established. The dynamic response of ER fluids under different shear rates, and different applied electric fields was experimentally determined. Experiments showed that, shear stress rose more quickly at a higher shear rate under the same stepwise electric field. The rising time was in ms scale. The results verified the fact that when the ER fluid reached its shear yield strain, shear stress attained shear yield stress. After the retraction of the applied electric field, the shear stress decayed at a rate much quicker than rising, which was less than 2 ms. Also, experiments showed that there was a short delay time of about 0.3-0.3 ms, which accords with the charging and discharging time of the ER fluids. So we proposed the following process for the dynamic response. Firstly, the ER fluids are polarized by the applied electric field, then under the mechanical shearing, particle chains form angles to the applied field and show the shear stress, i.e., following the sequence of polarization-structure-stress.
5. Optimal design and fabrication of ER application devices were carried out with consideration of both the mechanical and electrical aspects of ER fluids. The torque and speed transmission characteristics of a newly designed ER coupling under different electric field strengths were investigated. The experimental results agreed well with the predictions. By applying a frictional resistant torque and high electric field pulse, step rotation of the output axle was realized. A prototype of exercising bicycle was designed based on two concentric cylinders. Taking the mechanical performance of the cylinders as the constraint condition, the minimization of the electric power consumption as the aim of the optimization, the dimensions of the cylinders were optimized. The mechanical and thermal performances of the prototype were experimentally determined. Experiments showed that, under an applied field of 2250 V/mm, the induced resistant torque was approximately 12.2 Nm, the current density of the ER fluid was 0.26 mA/cm2 (room temperature 25 °C), the consumption of electric power was only 0.3 W with an electrode area of 523 cm2. An exercising test under 1500 V/mm for 40 minutes showed that the current density of the ER fluid increased slowly from the room temperature to 37.5 °C, with a final current density of 0.31 mA/cm2 and an electric power consumption of 0.24 W. The resistance of this shearing type damper can be conveniently adjusted with very small electric power consumption. It clearly showed the technical characteristics of ER fluids and the optimal potential of using ER fluids in real applications.
The work in this dissertation greatly improved the shear yield strength of ER fluids, which can increase the mechanical output ability of ER actuators. The conclusions on ER mechanism under DC field are useful for preparing other kinds of ER fluids with high performance. The measured dynamic response property of ER fluids under shearing is important for the optimization of control parameters and control modes of ER actuators. The study on tensile and compressive behaviors of ER fluids has broadened the working type of ER fluids in applications, and can expedite the development of ER technology.
Key words: electrorheological fluids, conductivity, dielectric constant, electric field strength, shear yield stress, current density
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