碳纳米管纤维及其传感器力电性能实验研究

基于UTM T150微纳米力学测试系统搭建了纤维力电耦合测试平台,并对碳纳米管纤维进行了多组力电耦合作用下的拉伸性能实验研究。通过观察分析碳纳米管纤维的电阻-应变曲线,发现其电阻变化与应变大小密切相关,随着应变的不断变大,碳纳米管纤维的电阻也在逐渐增加。基于此,设计制作了碳纳米管纤维柔性应变传感器,并将其贴在标准试件上分别进行了弯曲变形测试、单次/循环加载性能测试以及标定实验。结果表明:当对试件进行加载时,碳纳米管纤维柔性应变传感器的电阻会随着载荷的增大而增大;当载荷逐渐消失时,其电阻会逐渐减小直至回归初始电阻值附近。这表明碳纳米管纤维具备优异的电学与力学特性,并且利用它所制作的柔性应变传感器也有着较好的灵敏度与稳定性。     

光纤传感器在相似材料模型测试中的相容性研究

为了找出光纤传感器弯曲损耗与岩层变形破坏过程中变形的关系 ,在相似材料模型铺设过程中将光纤传感器埋入在层合材料之间。研究了相似材料模拟实验中 ,光纤与模型岩层的相互作用机理 ,光纤与模型岩层的相容性 ,以及提高二者相容性的方法 ;并设计了一种基于光时域反射技术的新型微弯光纤传感器 ,用于相似材料模型的应变、位移检测中。实验研究表明 ,这种传感器结构不仅能实现对岩体变形的监测 ,而且在该情况下传感器不会失效 ,具有较好的相容性。构建了岩体光纤检测的理论基础     

碳纳米管增强硫化天然橡胶拉伸性能的分子动力学模拟

基于分子动力学(MD)模拟方法,建立了碳纳米管/硫化天然橡胶复合材料体系模型,采用ReaxFF势函数,模拟了不同碳纳米管(CNT)含量的复合材料的拉伸过程.通过计算复合材料体系的自由体积分数、均方位移及回转半径,分析了材料基本微观性质和碳纳米管团聚的机制,计算结果与实验相符.通过碳纳米管与硫化天然橡胶界面能的计算,发现在加载过程中系统总势能的变化主要由硫化天然橡胶基体引起,其中非键能起主导作用;碳纳米管由于其自身力学性能较好,且与天然橡胶分子链相互作用产生界面能,导致材料力学性能提升,材料的屈服应力随碳纳米管含量的增加而显著升高.     

碳纳米管在简单荷载作用下的变形屈曲行为研究

采用分子动力学方法研究了不同螺旋性的单壁碳纳米管在轴压、扭转和外压分别作用下的变形屈曲行为。针对每种加载形式，给出了宏观形式的内力一变形曲线来反映碳原子间相互作用力的变化与碳管变形量之间的关系。对碳管在这三种不同荷载作用下的失稳特性、微观结构和应变能的变化进行了详细的分析。结合经典弹性理论和本文的模拟结果，得出了单壁碳纳米管的等效壁厚、弹性模量和Poisson比。另外，通过对比不同螺旋性碳管的模拟结果，讨论了螺旋性对碳纳米管力学行为的影响。文中采用的势能函数为Brenner多体势。     

复杂加载下混凝土的弹塑性本构模型

混凝土材料在不同应力路径下或复杂加载条件下会表现出差异性显著的应力应变关系,在小幅循环加载条件下,其应力应变关系会表现出类似于弹性变形的滞回曲线.在不同应力水平下,混凝土的应力应变关系以及破坏特性都具有静水压力相关特点,即随着静水压力增大,各向异性强度特性弱化.此外,混凝土受压及受拉破坏机理不同,因而对应于混凝土硬化损伤亦有不同,即可分为受压硬化损伤,受拉硬化损伤及两者的混合硬化损伤类型.基于Hsieh模型,对该模型进行了三点改进.（1）针对小幅循环加载下混凝土无塑性变形的试验规律,而模型中在应力水平较低的循环加载条件下始终存在塑性变形的预测问题,采用在边界面模型框架下,设置了应力空间的弹性域,初始屈服面与后续临界状态屈服面几何相似的假定.（2）基于广义非线性强度准则将原模型采用变换应力方法将其推广为三维弹塑性本构模型,采用变换后模型可合理的考虑不同应力路径对于子午面以及偏平面上静水压力效应形成的影响,并避免了边界面应力点奇异问题.（3）分别对拉压两种加载损伤模式建议了相应的硬化参数表达式,可分别用于描述上述加载中产生的应变软化及强度退化行为.基于多种加载路径模拟表明:所建立的三维弹塑性本构模型可合理地用于描述混凝土的一般应力应变关系特性.      

碳纳米管的等价柔性节点空间框架模型

小变形情况下,碳纳米管C-C共价键间的相互作用可以用基于分子力学的宏观力学模型进行模拟.其中,基于分子结构力学的等价结构力学模型是最为有效的碳纳米管弹性参数的预测模型.现有的碳纳米管的等价结构力学模型是用具有刚性节点的空间框架结构模拟碳纳米管的原子晶格受力和变形的关系.根据碳纳米管的原子晶格的变形特点,本文首次提出了一个用柔性节点空间框架模拟碳纳米管原子晶格键角变化的分析模型,再通过应变能等价推导了柔性节点的等价抗弯刚度与分子力学中力常数的关系,从而给出了一个更精确的计算碳纳米管等价弹性参数的分子结构力学模型.文中用ANSYS计算了不同尺寸的锯齿型(zigzag)和扶手型(armchair)单壁碳纳米管的轴向杨氏模量、泊松比、剪切模量及径向杨氏模量,分析了碳纳米管的尺寸效应,并且与其它各种模型所得结果进行了比较.计算结果表明,本文所给碳纳米管的等价柔性节点空间框架模型不仅计算简单、高效,而且准确;并可以直接推广到多壁碳纳米管等价弹性模量的计算及碳纳米管的稳定和动力分析.     

埋入混凝土的光纤传感器包层力学特性研究

本文用解析解探索光纤传感器埋入混凝土中其包层的特性及重要性。针对外荷载平行与光纤方向施加于混凝土构件时，其包层的材料特性和厚度变化而引起其内部应力分布特性作了一些研究。从解的分析可以看出，在埋入光纤的附近横截面应力集中很大程度上取决于包层的厚度及材料性质。对于给定的混凝土材料存在包层材料和厚度相结合的优化问题，选择某一优化目标，可能减小或消除其应力集中，而这对于光纤传感器在整个运行期间是至关重要。     

单壁碳纳米管的扭转力学特性研究

利用基于高阶Cauchy-Born准则所建立的单壁碳纳米管本构模型,针对不同手性的单壁碳纳米管的扭转力学特性进行了研究.研究发现结构呈现对称性的锯齿型和扶手型单壁碳纳米管具有完全对称的扭转特性,而结构不对称的手性型单壁碳纳米管具有正反相异的扭转特性.同时,针对一系列手性不同的单壁碳纳米管的扭转力学特性展开了详细的研究.研究的部分结果与采用其他方法得到的结果进行了对比,证实了所提出方法以及预测结果的有效性和可行性.     

颗粒材料内部滑动变形局部化的离散元模拟

颗粒材料在加载过程中表现出非常复杂的变形性质.在微观尺度上,单个颗粒的运动、单个孔隙胞元内的变形都是非均匀的,但也不是完全随机、没有规律可循的,而是呈现一定结构性的分布,如微带、剪切带等.本文用基于孔隙胞元的离散元方法对颗粒体进行双轴加载数值试验,模拟了以滑动变形表征的变形局部化现象.数值结果发现,对应加载过程中的不同...     

基于Ottosen本构关系的混凝土三维损伤模型

提出基于混凝土已有本构模型建立混凝土三维损伤模型的方法，该方法无需再进行新的参数标定，计算损伤不必通过对损伤演化方程的积分获得。基于Ottosen本构模型建立了相应的损伤模型，就损伤参量和Ottosen非线性指标的不同选取方式，对典型比例加载情形的全过程进行损伤分析，发现基于刚度下降和基于变形能力损失的两种不同损伤描述具有互补性，非线性指标采用比例增大法计算较为简单合理。     

Thermal vibration and apparent thermal contraction of single-walled carbon nanotubes

It is of fundamental value to understand the thermo-mechanical properties of carbon nanotubes. In this paper, by using molecular dynamics simulation, a systematic numerical investigation is carried out to explore the natural thermal vibration behaviors of single-walled carbon nanotubes and their quantitative contributions to the apparent thermal contraction behaviors. It is found that the thermo-mechanical behavior of single-walled carbon nanotubes is exhibited through the competition between quasi-static thermal expansion and dynamic thermal vibration, while the vibration effect is more prominent and induces apparent contraction in both radial and axial directions. With increasing temperature, the anharmonic interatomic potential helps to increase the bond length, which leads to thermally induced expansion. On the other hand, the higher structural entropy and vibrational entropy of the system cause the carbon nanotube to vibrate, and the apparent length of nanotube decreases due to various vibration modes. Parallel analytical and finite element analyses are used to validate the vibration frequencies and provide helpful insights. The unified multi-scale study has successfully decoupled and systematically analyzed both thermal expansion and contraction behaviors of single-walled carbon nanotube from 100 to 800 K, and obtained detailed information on various vibration modes as well as their quantitative contributions to the coefficient of thermal expansion in axial and radial directions. The results of this paper may provide useful information on the thermo-mechanical integrity of single-walled carbon nanotubes, and become important in practical applications involving finite temperature.     

Model for measurement of thermal expansion coefficient of concrete by fiber optic sensor

The existence of a coating on an optical fiber results in a difference between the strain of the matrix concrete and that sensed by the embedded fiber optic sensor. This paper deals with the theoretical model for measurement of thermal expansion coefficient of concrete by embedded fiber optic sensor. The fiber core, the coating of the optical fiber, and the matrix concrete were all supposed to be elastic. And the bonding surfaces were supposed to be intact, that is to say, there is no relative slip deformation at the interface between the matrix concrete and the coating of the optical fiber, and that between the coating and the fiber core. Based on these assumptions, a theoretical model was developed for measurement of thermal expansion coefficient of concrete. Some experiments were done to verify the validation of the theoretical model. And the comparison indicates a good agreement between the experimental and theoretical results.     

The thermal effect on axially compressed buckling of a double-walled carbon nanotube

The thermal effect on axially compressed buckling of a double-walled carbon nanotube is studied in this paper. The effects of temperature change, surrounding elastic medium and van der Waals forces between the inner and outer nanotubes are taken into account. Using continuum mechanics, an elastic double-shell model with thermal effect is presented for axially compressed buckling of a double-walled carbon nanotube embedded in an elastic matrix under thermal environment. Based on the model, an explicit formula for the critical axial stress is derived in terms of the buckling modes of the shell and the parameters that indicate the effects of temperature change, surrounding elastic medium and the van der Waals forces. Based on that, some simplified analysis is carried out to estimate the critical axial stress for axially compressed buckling of the double-walled carbon nanotube. Numerical results for the general case are obtained for the thermal effect on axially compressed buckling of a double-walled carbon nanotube. It is shown that the axial buckling load of double-walled carbon nanotube under thermal loads is dependent on the wave number of axially buckling modes. And a conclusion is drawn that at low and room temperature the critical axial stress for infinitesimal buckling of a double-walled carbon nanotube increase as the value of temperature change increases, while at high temperature the critical axial stress for infinitesimal buckling of a double-walled carbon nanotube decrease as the value of temperature change increases.     

简谐载荷下嵌入式单壁碳纳米管动态响应的Magnus方法

基于两端固支的弹性梁模型,研究嵌入式单壁碳纳米管在横向简谐载荷作用下的非线性振动问题。利用Galerkin方法对运动微分方程进行近似处理,将原方程从非线性动力学系统转化到二阶动力学系统,对于二阶动力学方程采用Magnus级数方法进行求解。通过数值实验,分析了嵌入式单壁碳纳米管非线性振动幅频特性,根据非线性动力学理论分析了碳纳米管动态响应,结果表明倍周期分岔产生混沌。     

A carbon nanotube sensor for wall shear stress measurement

A novel carbon nanotube (CNT) sensor is being developed to measure the mean and fluctuating wall shear stress (WSS) in a turbulent boundary layer. The CNT WSS sensor is based on the thermal principle and featured by high spatial and temporal resolutions (in the order of nm and kHz, respectively), low power consumption (in the order of μW), and a compact fabrication process compared with traditional WSS measurement sensors. The CNT WSS-sensing element was characterized in detail before its calibration. The CNT sensor was operated under a constant temperature (CT) operation mode and an overheat ratio range of −0.15 to −0.19 and calibrated in a fully developed turbulent channel flow. It has been observed for the first time in a macroscopic flow that the sensor output power is approximately proportional to the 1/3 powered WSS, as expected for a thermal-principle-based WSS sensor, and the wall shear stress measurement is demonstrated for a low Reynolds number flow.     

有限空间爆炸静态压力的温度补偿方法

为改善压阻式压力传感器的温度漂移特性,构建了基于遗传算法和小波神经网络的压力传感器温度补偿模型。针对小波神经网络收敛速度慢且易陷入局部最优解的问题,采用遗传算法对小波神经网络的连接权值、伸缩参数和平移参数进行优化。基于压力传感器的标定数据,分别采用BP神经网络、小波神经网络和遗传小波神经网络对其进行温度补偿研究,结果表明:遗传小波神经网络兼容了小波分析的时频局部特性和神经网络的自学习能力,表现出良好的收敛速度和补偿精度,经补偿后传感器的输出值更接近于标定值,其最大误差由?17.44 kPa变至0.38 kPa,最大相对误差由?14.0%变至0.38%。将该模型应用于有限空间爆炸静态压力的温度补偿中,取得了较好的实际应用效果。     

面内随机堆叠石墨烯复合材料压阻传感机理与压阻性能

面内随机堆叠石墨烯复合材料(graphene composites, GC)是可穿戴柔性传感器的基础材料之一, 但是其压阻传感机理与压阻性能仍然有待深入研究. 本文基于GC的微观结构特征, 利用0 $\sim$ 1间均匀分布随机数获得石墨烯在复合材料中的位置和方向, 建立了二维GC压阻传感器模型. 根据GC均匀变形的特点和有限单元法发展了GC压阻性能的计算方法, 计算得到了相对电阻、灵敏度系数、石墨烯片的微观形态与电流密度云图. 研究结果表明, GC中的压阻效应是由于在变形过程中石墨烯形态的改变, 包括GC中石墨烯片的密度随应变变化、石墨烯片滑移、分离导致电子迁移路径和无效片数量的改变, 而GC中石墨烯片密度随应变的变化是影响压阻效应的主要因素. 石墨烯片间的相对滑移产生线性传感特征, 分离反之. 高面分比GC与大尺寸石墨烯的GC拥有较大的感知范围, 低面分比GC和小尺寸石墨烯的GC具有更高的灵敏度系数. 最后将接触面的面内电阻率设为应变的函数, 研究了石墨烯片的接触效应对GC压阻性能影响, 解释了GC压阻性能的接触效应和影响机理. 研究结论可为GC生产方法的改进与创新、以及GC压阻传感器件的制备提供理论依据和技术参考.      

PIEZORESISTIVE SENSING MECHANISM AND PIEZORESISTIVE PERFORMANCE OF IN-PLANE RANDOM STACKED GRAPHENE COMPOSITES 1)

面内随机堆叠石墨烯复合材料(graphene composites, GC)是可穿戴柔性传感器的基础材料之一, 但是其压阻传感机理与压阻性能仍然有待深入研究. 本文基于GC的微观结构特征, 利用0 $\sim$ 1间均匀分布随机数获得石墨烯在复合材料中的位置和方向, 建立了二维GC压阻传感器模型. 根据GC均匀变形的特点和有限单元法发展了GC压阻性能的计算方法, 计算得到了相对电阻、灵敏度系数、石墨烯片的微观形态与电流密度云图. 研究结果表明, GC中的压阻效应是由于在变形过程中石墨烯形态的改变, 包括GC中石墨烯片的密度随应变变化、石墨烯片滑移、分离导致电子迁移路径和无效片数量的改变, 而GC中石墨烯片密度随应变的变化是影响压阻效应的主要因素. 石墨烯片间的相对滑移产生线性传感特征, 分离反之. 高面分比GC与大尺寸石墨烯的GC拥有较大的感知范围, 低面分比GC和小尺寸石墨烯的GC具有更高的灵敏度系数. 最后将接触面的面内电阻率设为应变的函数, 研究了石墨烯片的接触效应对GC压阻性能影响, 解释了GC压阻性能的接触效应和影响机理. 研究结论可为GC生产方法的改进与创新、以及GC压阻传感器件的制备提供理论依据和技术参考.     

Application of thermal convection to stability analysis of lithograph

Thermal conduction and thermal convection are two major cooling methods. The experiment applies these two methods simultaneously to cool photo-masks. The combination of thermal conduction and thermal convection can solve the problems of thermal expansion of masks during long-term exposure. Highly thermal conductive material can conduct heat that accumulates on chromium to the diamond film. However, heat that is conducted on the diamond film is not easily discharged into the air. Therefore, a thermal convection method to cool a diamond film is designed. The method involves blowing N ₂ gas at 23°C onto the surface of masks.