单壁碳纳米管杨氏模量的掺杂效应

用分子动力学方法研究了N,O,Si,P,S等5种杂质对扶手椅型(5，5)和锯齿型(9，0)单壁碳纳米管杨氏模量的影响.结果表明：直径为0.678和0.704 nm的扶手椅型(5，5)和锯齿型(9，0)碳纳米管在无掺杂时其杨氏模量分别为948和804 GPa.在掺杂浓度10％以下，碳纳米管的拉伸杨氏模量均随掺杂浓度增加近似呈线性下降规律，下降率以Si掺杂最大，N掺杂最小.对与C同周期的元素掺杂，随原子序数增加碳纳米管的杨氏模量下降率增大；与C不同周期的元素掺杂，碳纳米管的杨氏模量随掺杂浓度增加下降率更大，但 关键词： 碳纳米管 杂质 杨氏模量 分子动力学方法     

单壁碳纳米管弹性性质的羟基接枝效应

用分子动力学方法研究了端口接枝不同数量羟基对扶手椅型和锯齿型单壁碳纳米管弹性模量的影响.结果表明,未接枝的扶手椅型(5, 5),(10,10)管和锯齿型(9, 0),(18, 0)管杨氏模量分别为948,901和804,860GPa.在接枝2—8个羟基情况下,锯齿型单壁碳纳米管拉伸杨氏模量基本不随接枝数量增加发生变化,而扶手椅单壁碳纳米管则不同,接枝状态下的弹性模量比未接枝状态小很多,但接枝一定数量后,其杨氏模量又略增到某一稳定值.分别从接枝后碳纳米管变形电子密度等值线结构、C—C键长和系统结合能变化规律等方面,对单壁碳纳米管弹性模量的接枝效应进行了分析. 关键词： 碳纳米管 羟基 接枝效应 杨氏模量     

单壁碳纳米管在石墨基底上运动的分子动力学模拟

采用分子动力学模拟方法研究单壁碳纳米管在石墨基底上的运动.首先碳纳米管在基底弛豫至平衡状态，然后对其施加一固定外力，撤去外力后，碳纳米管在基底上逐渐减速至停止.为了研究管径、手性角对运动方式的影响，本文选择了C(10,10)，C(10,9)，C(10,8)，C(10,5)，C(10,0)，C(8,8)六种单壁碳纳米管进行模拟.结果表明，碳纳米管在石墨基底上的运动方式由手性角决定，与管径无关.手性角等于30°时，碳纳米管与石墨基底之间为公度结构，碳纳米管的运动出现周期性的滑动和翻滚现象；手性角大于28.3°小于30°时，碳纳米管一边向前滑动一边滚动；手性角小于26.3°时，碳纳米管在基底上滑动.碳纳米管的手性角决定了它与石墨基底接触界面的微观构型，从而决定了碳纳米管的运动方式. 关键词： 分子动力学模拟 碳纳米管 动能 结构公度性     

单壁碳纳米管拉伸变形的原子尺度模拟

采用分子动力学方法对单壁碳纳米管的拉伸变形行为进行了模拟,结果表明,碳纳米管具有较高的断裂应变.在结构产生缺陷之前,碳纳米管表现出弹性变形的特征.通过对能量变化的分析可以看出,能量分布的不均匀是导致结构失稳产生缺陷的主要因素.通过对含初始结构缺陷的碳纳米管在拉伸变形过程中的构型变化进行分析,发现在缺陷附近原来相邻的两个六边形蜂窝结构,随着拉伸变形的发展转变成5 7结构(Stone Wales转变),能量产生突变,应变能的释放使系统能量降低.分析也表明,较少数目的初始缺陷对碳纳米管的力学性质并不会有太大影响.     

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.     

拉伸条件下双壁碳纳米管弹性性能的原子模拟

利用分子动力学方法，对双壁碳纳米管在拉伸条件下的弹性性能进行模拟，研究了直径在1 nm以上的4根双壁碳纳米管，模拟了它们的应力-应变关系以及泊松比.计算结果表明，在弹性范围内，双壁碳纳米管的应力与应变呈非线性关系，切线弹性模量大致从720 GPa减小至570 GPa，出现软化现象；随着拉伸应变的增大，泊松比从0.3变化到0.17，但结构尺寸对泊松比的影响不大. 关键词： 双壁碳纳米管 分子动力学 弹性模量 泊松比     

乙醇水混合物在碳纳米管中的吸附与结构性质

用分子动力学模拟研究乙醇水混合物在碳纳米管中的结构与吸附．在(6,6)到(10,10)碳纳米管内,几乎总是充满乙醇分子,很少有水分子．在更粗的碳纳米管中有一些水分子,管内的乙醇质量分数远远高于体相值．对管内外的分子进行了径向、轴向、角向的密度和取向的分布以及氢键数目的分析．管外第一溶剂化层中分子的角向密度分布指出乙醇分子的甲基和碳壁有最强的作用,被钉扎在碳纳米管的六角形中心位置．基于对这些现象微观机制的理解,推测碳纳米管在甲醇和乙醇中更倾向吸附乙醇,通过对乙醇甲醇混合物与碳纳米管的分子动力学模拟验证了这个预测．     

硅纳米颗粒在碳纳米管表面生长的分子动力学模拟

采用分子动力学模拟方法研究了硅纳米颗粒在碳纳米管上的生长，并分析了这种复合材料的基本结构.研究表明，由于硅原子和碳纳米管之间的相互作用以及碳纳米管的巨大的表面曲率，硅原子在碳纳米管表面不是形成覆盖碳纳米管的二维薄膜，而是生成具有三维结构的硅纳米颗粒.小纳米颗粒的结构和无基底条件下生成的颗粒结构基本一致.对于大纳米颗粒，不同于无基底条件下形成的球状纳米晶体硅结构，硅纳米颗粒沿管轴方向伸长，其结构为类似于硅晶体的无定形网络结构. 关键词： 纳米颗粒 碳纳米管 硅 分子动力学模拟     

液体水银在碳纳米管中传输的压力控制模型

碳纳米管在纳米技术中有一个很重要的应用,就是它可以作为纳米管道传输液体. 本文用分子动力学模拟方法,结合一个液体压力控制模型,对液体水银由于外部压力作用在碳纳米管中传输的现象进行研究. 研究结果表明,当液体水银的内部压强超过一个临界值时,液体水银能够浸入到碳纳米管内部;不断增大液体内部的压强,碳纳米管可以连续地传输液体;而当对液体水银进行循环加压时,碳纳米管可以间断地传输液体水银. 关键词： 碳纳米管 流体传输 分子动力学     

液体水银在碳纳米管中传输的压力控制模型

碳纳米管在纳米技术中有一个很重要的应用，就是它可以作为纳米管道传输液体. 本文用分子动力学模拟方法，结合一个液体压力控制模型，对液体水银由于外部压力作用在碳纳米管中传输的现象进行研究. 研究结果表明，当液体水银的内部压强超过一个临界值时，液体水银能够浸入到碳纳米管内部；不断增大液体内部的压强，碳纳米管可以连续地传输液体；而当对液体水银进行循环加压时，碳纳米管可以间断地传输液体水银.     

Influence of the electric field on the alignment of carbon nanotubes during their growth and emission

The problems of the electric field action on carbon nanotubes (CNTs) during their growth and under the electron field emission conditions are considered. The relations determining the growth rate of an extended structure under the action of the electric field are established. The relation connecting the angle of orientation of a CNT inclined to the substrate surface and the applied electric field is used for computing current-voltage characteristics of the cathode consisting of inclined CNTs. The degree of deviation of these characteristics from the Fowler-Nordheim classic dependence is determined, on the one hand, by the parameters characterizing the CNT spread over the angles of inclination and, on the other hand, by the value of the Young modulus characterizing the bending stiffness of a nanotube. It is shown that in zero external electric field, a certain effect on the CNT orientation can be produced by the CNT potential relative to the substrate, which is due to the effect of the contact potential difference.     

On the buckling behavior of connected carbon nanotubes with parallel longitudinal axes

The application of hetero-junction carbon nanotubes (CNTs) is increasing continuously due to their outstanding properties in nano-mechanical systems. Several investigations have been conducted to study the behavior of CNTs. In this paper, straight hetero-junctions and their constituent CNTs (armchair and zigzag) were simulated by a commercial finite element package. Then, the buckling behavior of CNTs was evaluated by comparing the critical buckling load of each straight hetero-junction and its constituent CNTs. Both obtained, i.e. analytical calculations and computational, results were compared. The investigations showed that, first, the behavior of homogeneous CNTs under cantilevered boundary conditions follows the assumption of the classical Euler equation. Second, the analytical solutions are in good agreement with the finite element simulation results. In addition, it was shown that the first critical buckling load of hetero-junctions lies within the value of the fundamental homogeneous CNT range. It was also concluded that the buckling load of straight hetero-junctions and their fundamental CNTs increases by increasing the chiral number of both armchair and zigzag CNTs. The current study provides a better insight towards the prediction of straight hetero-junction CNTs behavior.     

A Comprehensive Numerical Investigation on the Mechanical Properties of Hetero-Junction Carbon Nanotubes

A set of forty-three hetero-junction CNTs, made of forty-four homogeneous carbon nanotubes of different chiralities and configurations with all possible hetero-connection types, were numerically simulated, based on the finite element method in a commercial finite element software and their Young's and shear moduli, and critical buckling loads were obtained and evaluated under the tensile, torsional and buckling loads with an assumption of linear elastic deformation and also compared with each other. The comparison of the linear elastic behavior of hetero-junction CNTs and their corresponding fundamental tubes revealed that the size, type of the connection, and the bending angle in the structure of hetero-junction CNTs considerably influences the mechanical properties of these hetero-structures. It was also discovered that the Stone-Wales defect leads to lower elastic and torsional strength of hetero-junction CNTs when compared to homogeneous CNTs. However, the buckling strength of the hetero-junction CNTs was found to lie in the range of the buckling strength of their corresponding fundamental tubes. It was also determined that the shear modulus of hetero-junction carbon nanotubes generally tends to be closer to the shear modulus of their wider fundamental tubes while critical buckling loads of these heterostructures seem to be closer to critical buckling loads of their thinner fundamental tubes. The evaluation of the elastic properties of hetero-junction carbon nanotubes showed that among the hetero-junction models, those with armchair-armchair and zigzag-zigzag kinks have the highest elastic modulus while the models with armchair-zigzag connections show the lowest elastic stiffness. The results from torsion tests also revealed the fact that zigzag-zigzag and armchair-zigzag hetero-junction carbon nanotubes have the highest and the lowest shear modulus, respectively. Finally, it was observed that the highest critical buckling loads belong to armchair-armchair hetero-junction carbon nanotubes and the lowest buckling strength was found with the hetero-junction models with armchair-zigzag connection.     

Synthesis of modified multi-walled carbon nanotube poly(benzimidazole-imide) composites: assessment of morphological and thermo-mechanical properties

A fully aromatic poly(benzimidazole-imide) (PBI) containing triazole side units and amine-modified multi-wall carbon nanotube (MWCNT)/PBI composites were fabricated via a polymerization process of monomer reactants and solution mixing with ultrasonication excitation. The polymer and composites were characterized by field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction. According to the microscopic characterizations, the MWCNTs homogeneously dispersed in the composites. The mechanical properties of the composite films were also measured by tensile test. The test results evidently indicated that the Young’s modulus increased by about 60.0% at 1 wt% CNT loading, and further modulus growth was observed at higher filler loading. The composite films hold preferable thermal stability the same as the pure PBI. The improvement of the mechanical and thermal properties was attributed to the incorporation of the surface modified CNTs. For CNT-reinforced polymer composites, strong interfacial adhesion and uniform dispersion of CNTs are more crucial factors for improving such properties.     

Radial buckling of multi-walled carbon nanotubes under hydrostatic pressure

Radial buckling stresses of carbon nanotubes (CNTs) need to be studied in high-pressure resonance Raman scattering spectrum. In this work, the closed-form expression of the critical buckling stress of multi-walled carbon nanotubes (MWCNTs) under hydrostatic pressure is derived that can be conveniently employed. Using the derived formulae, the critical buckling stresses of single-walled carbon nanotubes and double-walled carbon nanotubes with different diameters are calculated. The results are in good agreement with other reported literatures. In addition, the critical buckling stresses of each layer of a quintuple-walled CNT in different buckling modes are predicted, showing that the buckling instability can occur not only in the outermost rolled layer, but also in other rolled layer of MWCNTs by considering different diameters and buckling modes.     

Multi-scale bending,buckling and vibration analyses of carbon fiber/carbon nanotube-reinforced polymer nanocomposite plates with various shapes

Using a finite element-based multi-scale modeling approach, the bending, buckling and free vibration of hybrid polymer matrix composites reinforced by carbon fibers and carbon nanotubes (CF/CNT-RP) are analyzed herein. Thick composite plates with rectangular, circular, annular and elliptical shapes are considered. First, the equivalent material properties of CF/CNT-RP are calculated for different volume fractions of CF and CNT. To accomplish this aim, a two-step procedure is presented through which the coupled effects of nano- and micro-scale are taken into account. In the first step, modeling of dispersion of CNTs into the polymer matrix is done with considering interphase formed by their chemical interaction with the matrix, and the equivalent properties of resulting composite material are determined accordingly. CFs are then dispersed into CNT-RP which is considered a homogenous material in this step. Both distributions of CNTs and CFs are assumed to be random. After computing the equivalent properties of CF/CNT-RP for different volume fractions of its constituents, the bending, buckling and free vibration analyses of plates with different shapes are performed. It is shown that the reinforcement of the polymer matrix with both CF and CNT significantly affects the bending, buckling and free vibration characteristics of plates.     

A comparison between the mechanical and thermal properties of single-walled carbon nanotubes and boron nitride nanotubes

Carbon nanotubes (CNTs) are semimetallic while boron nitride nanotubes (BNNTs) are wide band gap insulators. Despite the discrepancy in their electrical properties, a comparison between the mechanical and thermal properties of CNTs and BNNTs has a significant research value for their potential applications. In this work, molecular dynamics simulations are performed to systematically investigate the mechanical and thermal properties of CNTs and BNNTs. The calculated Young’s modulus is about 1.1 TPa for CNTs and 0.72 TPa for BNNTs under axial compressions. The critical bucking strain and maximum stress are inversely proportional to both diameter and length-diameter ratio and CNTs are identified axially stiffer than BNNTs. Thermal conductivities of (10, 0) CNTs and (10, 0) BNNTs follow similar trends with respect to length and temperature and are lower than that of their two-dimensional counterparts, graphene nanoribbons (GNRs) and BN nanoribbons (BNNRs), respectively. As the temperature falls below 200 K (130 K) the thermal conductivity of BNNTs (BNNRs) is larger than that of CNTs (GNRs), while at higher temperature it is lower than the latter. In addition, thermal conductivities of a (10, 0) CNT and a (10, 0) BNNT are further studied and analyzed under various axial compressive strains. Low-frequency phonons which mainly come from flexure modes are believed to make dominant contribution to the thermal conductivity of CNTs and BNNTs.     

Large variation in Young's modulus of carbon nanotube yarns with different diameters

Twist-spun carbon nanotube (CNT) yarns are composed of numerous CNTs and their bundles with entangled and twisted structures. In this paper, we studied the mechanical properties of CNT yarns. The individual CNT, a component of yarn, is well known to have an extremely high mechanical strength. However, CNT yarns are very flexible and relatively free to transform their shapes, showing the potential for application in the design of wearable devices. Since CNT yarns have two opposing characteristics at the same time, a wide range of Young's modulus can be achieved by simply changing the geometrical structure while using the same fabrication process. We also suggest that CNT yarns can be utilized as the base material for several applications that require different stresses in a structure, such as bioimplants or foldable devices.     

In-plane vibration analysis of curved carbon nanotubes conveying fluid embedded in viscoelastic medium

The effect of the induced vibrations in the carbon nanotubes (CNTs) arising from the internal fluid flow is a critical issue in the design of CNT-based fluidic devices. In this study, in-plane vibration analysis of curved CNTs conveying fluid embedded in viscoelastic medium is investigated. The CNT is modeled as a linear elastic cylindrical tube where the internal moving fluid is characterized by steady flow velocity and mass density of fluid. A modified-inextensible theory is used in formulation and the steady-state initial forces due to the centrifugal and pressure forces of the internal fluid are also taken into account. The finite element method is used to discretize the equation of motion and the frequencies are obtained by solving a quadratic eigenvalue problem. The effects of CNT opening angle, the elastic modulus and the damping factor of the viscoelastic surrounded medium and fluid velocity on the resonance frequencies are elucidated. It is shown that curved CNTs are unconditionally stable even for a system with sufficiently high flow velocity. The most results presented in this investigation have been absent from the literature for fluid-induced vibration of curved CNTs embedded in viscoelastic foundations.     

Nonlinear structural mechanics based modeling of carbon nanotube deformation

A nonlinear structural mechanics based approach for modeling the structure and the deformation of single-wall and multiwall carbon nanotubes (CNTs) is presented. Individual tubes are modeled using shell finite elements, where a specific pairing of elastic properties and mechanical thickness of the tube wall is identified to enable successful modeling with shell theory. The effects of van der Waals forces are simulated with special interaction elements. This new CNT modeling approach is verified by comparison with molecular dynamics simulations and high-resolution micrographs available in the literature. The mechanics of wrinkling of multiwall CNTs are studied, demonstrating the role of the multiwalled shell structure and interwall van der Waals interactions in governing buckling and postbuckling behavior.