界面接枝羟基对碳纳米管运动和摩擦行为影响的分子动力学模拟

本文采用分子动力学模拟研究了界面间接枝羟基对碳纳米管在石墨基底上运动和摩擦行为的影响.结果表明:界面接枝羟基后碳纳米管所受的侧向力明显改变;仅石墨接枝羟基时碳纳米管侧向力波动增大;同时由于垂直碳纳米管与基底间接触面积小,碳纳米管所受的摩擦力随羟基含量的增加而增大;碳纳米管与石墨上均接枝羟基后体系中引入了氢键和库仑力作用,显著增加了界面间的摩擦,体系的滑移界面从碳纳米管与石墨间迅速转变为石墨层间,并且导致碳纳米管在垂直初始运动方向上也出现了滑移.     

单壁碳纳米管受压屈曲行为的数值模拟

采用以Tersoff Brenner势函数来描述碳纳米管中碳原子间的相互作用的分子动力学方法,模拟了单壁 碳纳米管(SWCNTs)的受压屈曲行为.计算结果表明,单壁碳纳米管的杨氏模量随着管径的增大而减小;碳纳米 管屈曲的临界应力和临界应变与碳纳米管细长比有关,不同的细长比决定了碳纳米管结构不同的屈曲模态;碳纳 米管的受压屈曲机理和连续介质力学中柱体壳的受压屈曲理论随细长比的不同而存在一些异同.     

碳纳米管包裹的硅纳米线复合结构的热稳定性研究

采用经典分子动力学方法模拟一定直径[111]晶向的硅纳米线填充不同扶手椅型单壁碳纳米管复合结构的加热过程, 通过可视化和能量分析的方法判断复合结构中硅纳米线和碳纳米管的热稳定性. 通过讨论碳纳米管的空间限制作用和分子间相互作用力的关系, 对碳纳米管和硅纳米线的热稳定性变化进行初步解释. 研究发现碳纳米管中硅纳米线的热稳定性和碳纳米管的直径关系密切: 当管径较小时, 硅纳米线的热稳定性有所提高, 当管径增大到一定大小时, 硅纳米线的热稳定性会突然显著地下降, 直到硅纳米线与管壁不存在分子间相互作用力, 硅纳米线的热稳定性才会恢复. 而硅纳米线填充到碳纳米管中对碳纳米管的热稳定性有着明显的降低作用.     

热力耦合作用下双层碳纳米管的扭转屈曲

考虑碳纳米管周边弹性介质和层间范德瓦耳斯力的作用,利用连续介质力学的壳体理论,建立了热力耦合作用下碳纳米管屈曲问题的控制方程,给出了相应的临界屈曲扭矩的解析解.数值模拟结果表明,在低温和室温环境下,碳纳米管的临界屈曲载荷随着温度变化量的增加而提高;在高温环境下,碳纳米管的临界屈曲载荷随着温度变化量的增加而降低. 关键词： 碳纳米管 屈曲 热力耦合     

纳米孔隙内混合气体流动特性的分子动力学模拟

纳米孔隙内气体流动的理论预测对气体微流控器件的设计和制造具有重要的理论指导作用,文章采用分子动力学方法研究了氮气、氧气和二氧化碳混合气体在平行壁纳米孔隙内的剪切流动特性和边界滑移特性.研究结果表明:随着加入二氧化碳比例的不断增加,混合气体滑移速度不断增大,并且当二氧化碳的比例低于20%时,混合气体流动速度沿孔隙宽度方向呈线性分布;而当比例达到40%后,其速度轮廓将呈现非线性趋势.当二氧化碳所占比例为20%时,随着孔隙宽度的增加,混合气体的整体边界滑移随之减小.探究了混合气体密度和气-固耦合强度对混合气体流动及边界滑移的影响机理.发现随着混合气体密度的减小,气流边界滑移增大;随着气-固界面耦合强度的增强,边界气体分子易被吸附而出现黏滑运动,气体分子在边界处的积聚现象增强,剪切应变率增大,边界滑移减小.      

相互作用势对模拟计算单壁碳纳米管物理吸附储氢的影响

采用巨正则Monte Carlo方法模拟氢分子在单壁碳纳米管中的储存与分布,重点研究了Lennard-Jones势、Crowell-Brown势和Silvera- Goldman势对模拟计算单壁碳纳米管物理吸附储氢的影响.计算结果显示,碳纳米管与氢分子间的相互作用宜采用Lennard-Jones势描述;氢分子与氢分子间相互作用的描述则与碳纳米管的管径有关,管径较小时选Lennard-Jones势较佳,管径偏大时取七参数Silvera-Goldman 势更为合理,而三参数Silvera-Goldman势则不宜采用;并给出了相应的理论解释.     

单壁碳纳米管受限空间内水的分布

碳纳米管管腔作为分子物质的纳米通道,其储存或输送水的能力具有重要研究价值.为了研究碳纳米管管腔受限空间对水分子团簇结构和分布的影响,本文采用分子动力学方法探究了管径、手性和温度对单壁碳纳米管管腔内水的结构和分布的影响.结果表明:在常温下,管径尺寸范围为1.018—1.253 nm的单壁碳纳米管管内易形成有序的多元环水结构,此范围以外碳纳米管管内难以形成水的有序结构;且随着管径尺寸增大,多元环水呈现由三元环至六元环的结构变化;范德瓦耳斯势分布分析表明,在上述管径范围内,水分子趋向于贴近碳纳米管管壁分布而形成水的有序结构.对比管径尺寸差别较小的碳纳米管,其手性对多元环水结构影响不大.多元环水结构的稳定性表现出温度依赖性,管径较大的碳纳米管内的多元环水的有序结构更易随温度升高而消失.     

单壁碳纳米管储氢的第一原理分子动力学模拟

本文用第一性原理平面波赝势方法模拟研究了手性单壁碳纳米管与氢分子的相互作用,考察了碳纳米管直径对储氢性能的影响.对单壁碳纳米管储氢的模拟结果表明:(1)物理吸附时,H2可以吸附在空腔内,也可以吸附在管与管之间的空隙中,纳米管内部的氢吸附力均高于管外,而“完好无损”的H2分子不能够穿过管壁而进入管内.(2)化学吸附时,碳纳米管对氢的吸附首先出现在管的边缘附近,碳纳米管局部会发生形变,SWCNTs的张力会随C-H键的增加而增大,系统不稳定.(3)随着直径的增加,纳米管内、外的氢吸附力差异减小.     

结构和变形对单壁碳纳米管力学性能的影响

使用分子动力学方法模拟了单壁碳纳米管的拉伸变形行为和泊松比,并从单壁碳纳米管晶胞单元的结构特征角度,系统分析了管径、螺旋性和应变对力学性能的影响．模拟结果显示,单臂性碳纳米管(8,8)-(22,22)和锯齿性碳纳米管(9,0)-(29,0)的拉伸弹性变形可以分别达到35％-38％和20％-27％,拉伸条件下这些碳纳米管的弹性模量随管径的增大从960 GPa下降到750 GPa,并且锯齿性碳纳米管的弹性模量比单臂性碳纳米管的弹性模量要高．通过对三根具有相同直径和不同螺旋性的碳纳米管(9,9),(12,6)和(16,0)分别在拉伸和压缩条件下的模拟发现,随着变形的增大,碳纳米管的泊松比将减小;在相同的拉伸应变下,碳纳米管的泊松比随其螺旋角的减小而减小,而在相同的压缩应变下,碳纳米管的泊松比随其螺旋角的减小而增大．     

界面羟基对碳纳米管摩擦行为和能量耗散的影响

本文采用分子动力学模拟研究了羟基对碳纳米管摩擦和能量耗散方式的影响.研究结果表明:由于界面间氢键的形成,碳纳米管所受的平均摩擦力明显增大;随着羟基比例的改变,界面间氢键的数量与摩擦力的变化趋势一致;碳纳米管的手性角对摩擦力有一定的影响,扶手椅型碳纳米管所受的摩擦力比其他类型的碳纳米管的大;直径对摩擦力的影响较大,直径越大界面间的摩擦力越大,其原因是大直径的碳纳米管底部变平导致界面接触面积增大;界面接枝羟基后,体系的声子态密度中出现羟基的振动峰;随羟基比例的增加,羟基的振动在能量耗散中起到更为重要的作用,当碳纳米管和硅基底的羟基比例为10%/20%时,体系能量耗散的主要途径由碳纳米管和硅基底的振动转变为羟基的振动.     

石墨烯/柔性基底复合结构双向界面切应力传递问题的理论研究

界面力学性能是影响石墨烯/柔性基底复合结构整体力学性能的关键因素,因此对该结构界面切应力传递机理的研究十分必要.考虑了石墨烯和基底泊松效应的影响,本文提出了二维非线性剪滞模型.对于基底泊松比相比石墨烯较大的情况,利用该模型理论研究了受单轴拉伸石墨烯/柔性基底结构的双向界面切应力传递问题.在弹性粘结阶段,导出了石墨烯双向正应变和双向界面切应力的半解析表达式,分析了不同位置处石墨烯正应变和界面切应力的分布规律.导出了石墨烯/柔性基底结构发生界面滑移的临界应变,结果表明该临界应变低于利用经典一维非线性剪滞模型得到的滑移临界应变,并且明显受到石墨烯宽度尺寸以及基底泊松比大小的影响.基于二维非线性剪滞模型建立有限元模型(FEM),研究了界面滑移阶段石墨烯双向正应变和双向界面切应力的分布规律.与一维非线性剪滞模型的结果对比表明,当石墨烯宽度较大时,二维模型和一维模型对石墨烯正应变、界面切应力以及滑移临界应变的计算结果均存在较大差别,但石墨烯宽度很小时,二维模型可近似被一维模型代替.最后,通过与拉曼实验结果的对比,验证了二维非线性剪滞模型的可靠性,并得到了石墨烯/聚对苯二甲酸乙二醇酯(PET)基底结构的界面刚度(100 TPa/m)和界面剪切强度(0.295 MPa).     

Analysis of single-fiber pull-out from composites by using stress birefringence

During a fiber pull-out test, it is desirable to analyze the stress profiles along the embedded fiber directly within the same time scale as the normal pull-out tests. In the present study, the axial tensile stress profiles of the fiber in a model composite are measured during the single-fiber pull-out tests by using stress birefringence of the fiber. It is concluded from the analysis of the measured stress profiles that an effective radius of matrix, i.e. a radius defining the region of the matrix where the major deformation takes place, is not constant but is an increasing function of the interfacial shear stress. By incorporating the variable values of the effective radius of matrix into the shear-lag model, the axial tensile and the interfacial shear stress profiles are calculated. To accurately estimate the interfacial shear strength, the stress distribution along the embedded fiber and the variability of the effective radius of matrix should be taken into account instead of calculating the interfacial shear strength simply from the pull-out stress and the embedded length.     

A cohesive law for multi-wall carbon nanotubes

We have established the cohesive law for carbon nanotube (CNT) walls in multi-wall CNTs. The interactions between CNT walls are characterized by the van der Waals force. The tensile cohesive strength and cohesive energy are given in terms of the area density of CNT and parameters in the van der Waals force. For an infinitely long CNT, the shear cohesive stress between CNT walls vanishes, and the tensile cohesive stress depends only on the opening displacement. For a finite CNT, the tensile cohesive stress remains the same, but the shear cohesive stress depends on both opening and sliding displacements, i.e. the tension/shear coupling. The simple, analytical expressions of the cohesive law are useful to study the interaction between walls in multi-wall CNTs.     

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack are dealt with in this paper. Utilizing the conformal mapping technique, the closed-form solutions are derived for complex potentials and stress fields due to a screw dislocation located near the interfacial blunt crack. The stress intensity factor on the crack tips and the critical stress intensity factor for dislocation emission are also calculated. The influence of the orientation of the dislocation and the morphology of the blunt crack as well as the material elastic dissimilarity on the shielding effect and the emission criterion is discussed in detail. The results show that positive screw dislocations can reduce the stress intensity factor of the interfacial blunt crack tip (shielding effect). The shielding effect increases with the increase of the shear modulus of the lower half-plane, but it decreases with the increase of the dislocation azimuth angle. The critical loads at infinity for dislocation emission increases with the increase of emission angle and curvature radius of blunt crack tip, and the most probable angle for screw dislocation emission is zero. The present solutions contain previous results as special cases.     

聚合物分子与官能化纳米管相互作用及扩散特性的分子动力学模拟

用全原子分子动力学方法研究典型聚合物分子（PE，PEO和PP）与碳纳米管（CNT）及官能化碳纳米管（FCNT）界面的相互作用及扩散特性.动力学模拟显示：—CH₃官能团具有减弱CNT与PE和PP的相互作用，但是，—CH₃官能化后的CNT与PEO之间确有增强作用.分析含氧官能团（—OH和—COOH）官能化的CNT与PE，PEO和PP的相互作用，可知含氧官能团的确具有增强表面相互作用的功能，而且含氧原子越多，相互作用就越强.此外，—CH₃，—OH，—COOH官能化后的CNT与PE，PP和PEO体系的总能量均减少，而且能量满足—COOH < —OH < —CH₃.分析非键相互作用势（库仑能和范德瓦尔斯能），可知库伦相互作用是增强界面相互作用的主要作用能.官能化后的CNT/PE，CNT/PEO，CNT/PP体系的扩散系数都明显减小，且扩散系数大小满足—COOH < —OH < —CH₃.     

Enhanced interfacial strength of carbon nanotube/copper nanocomposites via Ni-coating: Molecular-dynamics insights

The molecular bridging between carbon nanotube (CNT) within the meta matrix is hopeful for enhancing nanocomposite's mechanical performance. One of the main problems for nanocomposites is the inadequate bonding between nonstructural reinforcement and meta matrix. Ni-coating on CNT is an effective method to overcome the drawback of the inadequate strength, but the enhancing mechanism has not well interpreted yet. In this paper, the enhancing mechanism will be interpreted from the molecular-dynamics insights. The pullout process of CNT and Ni-coated CNT against copper matrix is investigated. The effects of geometric parameters, including CNT length and diameter, are taken into considerations and discussed. Results show that the interfacial strength is significantly improved after the Ni-coated CNT, which shows a good agreement with the experimental results available in the open literature. Besides, the sliding mechanism of Ni-coated CNTs against copper matrix is much more like a kind of friction sliding and directly related to the embedded zone. However, the pullout force of the CNT without Ni-coating is nearly proportional to its diameter, but independent of embedded length.     

Numerical simulation of micro-scratch tests for coating/substrate composites

In this paper the micro-scratch test is simulated by ANSYS finite element code for thin hard coating on substrate composite material system. Coulomb friction between indenter and material surface is considered. The material elastic-plastic properties are taken into account. Contact elements are used to simulate the frictional contact between indenter and material surfaces, as well as the frictional contact after the detachment of coating/substrate interfaces has taken place. In the case of coating/substrate interfaces being perfectly bonded, the distributions of interfacial normal stress and shear stress are obtained for the material system subjected to normal and tangential loading. In the case of considering the detachment of interfaces, the length of interfacial detachment and the redistribution of stresses because of interfacial detachments are obtained. The influences of different frictional coefficients and different indenter moving distances on the distributions of stresses and displacements are studied. In the simulation, the interfacial adhesion shear strength is considered as a main adhesion parameter of coating/substrate interfaces. The critical normal loading from scratch tests are directly related to interfacial adhesion shear strengths. Using the critical normal loading known from experiments, the interfacial adhesion shear strength is obtained from the calculation. When the interfacial adhesion shear strength is known, the critical normal loading is obtained for different coating thicknesses. The numerical results are compared with the experimental values for composite materials of thin TiN coating on stainless steel substrate.     

Interface end theory and re-evaluation in interfacial strength test methods

The experimental results of fragmentation, micro-indentation, pull-out and microdebond tests often exhibit large discrepancies. Since all specimens of the four test methods all have interface ends, the singularity theory of the interface end should be used to evaluate the exactness of the test methods. The eigenvalues of the specimens for the micro-indentation test, pull-out test and microdebond test are calculated and investigated. The results show that the stress singularity of the interface end depends on the Dundurs' parameters and the wedge angles. The interfacial shear strength (IFSS) obtained from the tests loses its rationality if the stress is singular at the interface end. In further analysis, for a carbon fiber-epoxy resin composite, it is found that the microdebond test gives the most reliable IFSS results, if the wedge angle of the resin droplet is less than 40°; the results from the pull-out test are dubious, due to the stress singularity at the interface end. In the micro-indentation test, there is a critical matrix stiffness value for a given fiber, above which the stress at the interface end will be non-singular. The fragmentation test assumes the interfacial shear stress on the fiber fragment of critical length is the IFSS. If debonding does not occur at the interface end, then apparently, the interfacial shear stress on the fiber fragment of critical length is less than the true value of IFSS.     

单层单晶石墨烯与柔性基底界面性能的实验研究

单晶石墨烯具有更优异的力学及电学性能,有望成为新一代柔性电子器件的核心材料.因此,有必要从实验的角度精细分析化学气相沉积法制得的大尺度单晶石墨烯与柔性基底复合结构的界面力学行为.本文通过显微拉曼光谱实验方法测量了不同长度的单层单晶石墨烯/PET(聚对苯二甲酸乙二醇酯)基底的界面力学性能参数及其在长度方向上界面边缘的尺度效应.实验给出了石墨烯在PET基底加载过程中与基底间黏附、滑移、脱黏三个界面状态的演化过程与应力分布规律.实验发现,单晶石墨烯与柔性基底间由范德瓦耳斯力控制的界面应变传递过程存在明显的边缘效应,并且与石墨烯的长度有关.界面的切应力具有尺度效应,其值随石墨烯长度的增加而减小,而石墨烯界面传递最大应变以及界面脱黏极限则不受试件尺度的影响.     

Surface adhesion between hexagonal boron nitride nanotubes and silicon based on lateral force microscopy

This study presents the surface adhesion between hexagonal boron nitride nanotube (BNNT) and silicon based on lateral manipulation in an atomic force microscope (AFM). The BNNT was mechanically manipulated by the lateral force of an AFM pyramidal silicon probe using the scan mechanism in the imaging mode. With a controlled normal force of the AFM probe and the lateral motion, the lateral force applied to the BNNT could overcome the surface adhesion between BNNT and silicon surface. The individual BNNT is forced to slide and rotate on the silicon surface. Based on the recorded force curve, the calculated shear stress due to surface adhesion is 0.5 GPa. And the specific sliding energy loss is 0.2 J/m². Comparing BNNTs and carbon nanotube (CNT), the shear stress and specific sliding energy loss of BNNT are an order of magnitude larger than that of CNT. Therefore, the results show that the surface adhesion between BNNT and silicon surface is higher than that of CNT.