电流变液的力学性能研究及其微观结构研究

发布时间：2019-04-10 11:09

【摘要】：作为一种典型的智能材料,电流变液的力学性能在外加电场下能够发生显著的变化,而且这种变化是快速可逆的。鉴于这种特殊性质,电流变液在力电耦合装置中具有广阔的应用前景。因此,电流变液力学性能变化的机理分析一直以来就是电流变液研究领域的重点和难点。在外加电场作用下,电流变颗粒在极板间形成链状结构使电流变液的剪切应力和表观粘度显著增加。电流变液工作状态下的微观结构与力学性能的关系是解释电流变机理的关键所在。由于实验条件的限制,电流变颗粒的微观结构很难直接被观测到,因此模拟计算成为了研究电流变液机理的重要方法。通过实验现象验证计算模型的正确性,用模拟计算的结果解释实验中观测的现象,是目前电流变液机理研究常用而有效的手段。本文采用实验与计算模拟相结合的方法全面研究了电流变液在压缩和剪切时的力学性能变化,并探究了其中不同影响因素的作用机理,具体工作内容如下:1.通过实验结合模拟计算的方法研究了电流变液在压缩模式下法向应力的变化情况。电流变液在压缩时表现出较高的法向应力,测试了压缩速度对电流变液法向应力的影响。同等条件下压缩速度越小,电流变液的法向应力越大。这是在外加电场作用下电流变液的颗粒与基液分离的现象导致的,压缩速度越小,颗粒与基液分离的现象越显著,电流变颗粒的链状结构更稳固,可以承载更大的应力。然后基于偶极子模型,提出了电流变液的压缩模型,将模拟结果和实验结果进行了比较,验证了压缩计算模型的可靠性。模拟研究了外加电场强度、压缩应变、剪切速率对电流变液法向应力的影响。剪切速率较小时,剪切作用对电流变液法向应力的影响很小;随着剪切速率的增大,电流变液的法向应力在逐渐减小。在剪切作用下电流变液的法向应力有振荡变化的现象。通过对微观结构的计算发现较大的剪切速率下电流变颗粒的微观结构不断地破坏与重组是法向应力振荡的原因。2.通过实验和模拟计算研究了剪切场中介电损耗对电流变液力学性能的影响。采用锶离子掺杂的方法改变二氧化钛颗粒的介电损耗性能,测试了改性后颗粒的介电损耗频谱图和电流变液的剪切流变曲线,发现了介电损耗对电流变液流变性能的影响。锶离子掺杂比例的提高降低了电流变颗粒的弛豫频率,介电损耗增大,电流变液的电流变效率也在逐渐降低。当颗粒的弛豫频率低于100Hz时,电流变液在一定的剪切速率范围内失去电流变效应,有效工作范围减小。模拟研究了介电损耗过程中的弛豫时间对电流变液力学性能的影响。当弛豫时间超过0.01 s后,临界剪切速率降低,电流变有效工作范围减小,与实验中的结论一致。给出了介电损耗对电流变液影响的机理解释,弛豫频率过大时颗粒偶极矩方向和颗粒链方向不一致,使得颗粒链方向颗粒间的相互作用力减弱,甚至由吸引力变为排斥力,颗粒链状结构的强度降低。3.模拟研究了剪切作用下电流变液力学性能的变化,结合实验研究了不同状态下的剪切速率对电流变液剪切应力变化的影响。利用基于偶极子极化理论的计算模型,研究了体积分数、电场强度和剪切速率对电流变液力学性能的影响。然后进行了稳态剪切下的二维模拟和微观结构的计算,以此来解释剪切速率对电流变液力学性能的影响。通过实验发现了剪切速率对电流变液剪切应力影响的三种不同状态,验证了模拟结果。通过计算的微观结构演化解释了不同状态下的剪切速率对电流变液力学性能影响的机理:低剪切速率下,颗粒链结构向剪切方向倾斜,剪切应力随着剪切速率的增加而增加;中等剪切速率下,颗粒链结构处于破坏与重组的动态平衡状态,剪切应力随时间振荡变化;高剪切速率下,电流变液达到剪切屈服状态,链结构被完全破坏,电场对剪切应力的影响很弱,电流变液的剪切应力由液体的粘性力主导,表现出宾汉流体的性质。
[Abstract]:As a typical intelligent material, the mechanical properties of the electrorheological fluid can change significantly under the applied electric field, and the change is fast and reversible. In view of this special property, the electrorheological fluid has a wide application prospect in the force-electric coupling device. Therefore, the mechanism analysis of the change of the mechanical property of the electrorheological fluid has always been the focus and difficulty in the field of current fluid-changing research. Under the effect of the applied electric field, the current-changing particles form a chain-like structure between the plates, so that the shear stress and the apparent viscosity of the electrorheological fluid are obviously increased. The relationship between the microstructure and the mechanical property in the working state of the electrorheological fluid is the key to explain the mechanism of the current transformation. Due to the limitation of the experimental conditions, the microstructure of the electrorheological particles is very difficult to be observed directly, so the simulation is an important method to study the mechanism of the rheological fluid of the current. The correctness of the calculation model is verified by the experimental phenomenon, and the phenomenon of the observation in the experiment is explained by the result of the simulation. In this paper, the mechanical properties of the electrorheological fluid at the time of compression and shearing are studied by the method of combination of experiment and calculation, and the action mechanism of the different influencing factors is explored. The specific work is as follows:1. In this paper, the variation of the normal stress in the current-changing liquid under the compression mode is studied by means of the simulation calculation of the experiment. The effect of the compression rate on the stress of the electrorheological fluid is tested by the high normal stress at the time of compression. Under the same conditions, the smaller the compression speed, the greater the normal stress of the electrorheological fluid. This is caused by the phenomenon that the particle of the electrorheological fluid is separated from the base liquid under the action of the applied electric field, the smaller the compression speed, the more obvious the phenomenon of the separation of the particles from the base liquid, and the chain structure of the current-changing particles is more stable and can bear more stress. Then, based on the dipole model, the compression model of the electrorheological fluid is put forward, and the simulation results and the experimental results are compared, and the reliability of the compression calculation model is verified. The effects of the applied electric field strength, compressive strain and shear rate on the stress of the electrorheological fluid are simulated. The shear rate is small, and the effect of shear on the stress of the electrorheological fluid is very small; with the increase of the shear rate, the normal stress of the electrorheological fluid is gradually reduced. The normal stress of the electrorheological fluid under the shearing action is the phenomenon of the oscillation change. Through the calculation of the micro-structure, it is found that the micro-structure of the current-variable particles is destroyed and the recombination is the reason of the normal stress oscillation. The effect of the dielectric loss on the mechanical properties of the rheological fluid in the shear field is studied by means of experiment and simulation. The dielectric loss performance of the titanium dioxide particles was changed by the method of ion doping. The dielectric loss spectrum of modified particles and the shear rheological curve of the electrorheological fluid were tested, and the effect of the dielectric loss on the rheological property of the rheological fluid was found. The increase of the ion doping ratio reduces the relaxation frequency and the dielectric loss of the current-variable particles, and the current-changing efficiency of the current-changing liquid is also gradually reduced. When the relaxation frequency of the particles is lower than 100 Hz, the current transformer loses the current variable effect in a certain shear rate range, and the effective working range is reduced. The effect of relaxation time in the dielectric loss on the mechanical properties of the electrorheological fluid is simulated. When the relaxation time is more than 0.01 s, the critical shear rate is reduced, and the effective working range of the current transformer is reduced, which is consistent with the conclusion in the experiment. The mechanism of the effect of the dielectric loss on the electrorheological fluid is given. The direction of the particle dipole moment and the direction of the particle chain are not uniform when the relaxation frequency is too large, so that the interaction force between the particles in the particle chain direction is weakened, and the strength of the particle chain structure is reduced even by the attractive force becoming a repulsive force. The changes of the mechanical properties of the rheological fluid under shear are simulated, and the effect of shear rate on the shear stress of the rheological fluid under different conditions is studied in combination with the experiment. The effect of volume fraction, electric field strength and shear rate on the mechanical properties of the electrorheological fluid is studied by using the calculation model based on the dipole-polarization theory. The effect of the shear rate on the mechanical properties of the electrorheological fluid is explained by the calculation of the two-dimensional simulation and the microstructure under the steady-state shear. In this paper, three different states of shear rate on the shear stress of current-varying fluid are found, and the simulation results are verified. The mechanism of the effect of shear rate on the mechanical properties of the electrorheological fluid under different conditions is explained by the calculated microstructure evolution: under the low shear rate, the particle chain structure is inclined to the shearing direction, and the shear stress is increased with the increase of the shear rate; at the moderate shear rate, the structure of the particle chain is in a dynamic equilibrium state of destruction and recombination, and the shear stress changes with time; under the high shear rate, the current variable reaches the shear yield state, the chain structure is completely destroyed, the influence of the electric field on the shear stress is weak, The shear stress of the electrorheological fluid is dominated by the force of the liquid, showing the properties of the Bingham fluid.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TB381

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