纳米碳管与刚性壁的正碰撞

用分子动力学方法模拟纳米碳管与刚性壁的正碰撞过程,并与弹性动力学方法的结果进行对比．在分子动力学模拟中,采用Tersoff-Brenner势描述碳管的原子间相互作用,用6～12形式的Lennard-Jones,势描述碳管与刚性壁间相互作用．结果表明,两种方法所得到的应力波传播速度吻合较好．与弹性动力学分析结果不同的是,在发生屈曲以前,纳米碳管与刚性壁的接触时间不仅与纳米碳管的长度近似成线性关系,还与管径及碰撞初速度有关．碰撞过程中,纳米碳管端部应力并非定值,但其平均值与弹性动力学计算结果相差不大．     

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。     

基于高阶Cauchy-Born准则的单壁碳纳米管本构模型

提出了一种基于高阶Cauchy—Born准则建立单壁碳纳米管本构模型的方法。通过引入高阶变形梯度,合理地修正了传统Cauchy—Born准则在描述纳米管变形几何关系时所存在的缺陷。利用原子间相互作用势以及能量等效原理,得到了基于广义连续介质模型的单壁碳纳米管的本构关系。由此得到的本构参数不仅与变形梯度张量F,而且与其梯度F相关,因此是一种广义连续介质模型。利用这样的本构模型,本文还对单壁碳纳米管的杨氏模量进行了预测,并与采用其他方法得到的结果进行了对比,从而证实了所提出方法的有效性。     

纳米碳管/铝基复合材料的制备及摩擦磨损性能研究

利用粉末冶金法制备纳米碳管/铝基复合材料,研究不同纳米碳管含量对复合材料硬度和稳态摩擦磨损行为的影响,采用扫描电子显微镜观察复合材料的磨损表面形貌,并对其磨损机制进行探讨.结果表明:随着纳米碳管质量分数的增加,复合材料的硬度呈现先增大而后减小的趋势,含质量分数为2%的纳米碳管复合材料硬度比铝增加约80%;复合材料的摩擦系数逐渐降低,磨损率先减小而后增大;含质量分数为1%的纳米碳管复合材料磨损机制为磨粒磨损和粘着磨损,而含质量分数为2%的纳米碳管复合材料以剥层磨损和疲劳磨损为主.     

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分析了圆柱、柱锥形及回形纳米碳管弹簧在轴向载荷作用下的变形,给出了它们轴向刚 度$K_{\rm p}$的统一表达式. 为验证$K_{\rm p}$公式的有效性,还将圆柱形碳管纳米弹簧$K_{\rm p}$ 的理论解和相应的试验结果、有限元结果进行了比较. 结果发现,碳管纳米弹簧的$K_{\rm p}$ 理论解和试验结果、有限元结果均较为接近.     

Ni—P—纳米碳管化学复合镀层的摩擦磨损特性

用化学镀方法制备了 Ni- P-纳米碳管复合镀层 ,研究了热处理对复合镀层微观结构及摩擦学性能的影响 .结果表明 :Ni- P-纳米碳管复合镀层比 Ni- P- Si C和 Ni- P-石墨镀层具有更好的摩擦磨损性能 ;在 6 73K条件下热处理 2 h后 ,复合镀层的耐磨性能显著改善 ;除 Ni- P-纳米碳管复合镀层的摩擦系数基本不变以外 ,其余复合镀层的摩擦系数均降低 .     

纳米碳管增强铜基复合材料的滑动磨损特性研究

以纳米碳管作为增强体制备了铜基复合材料,采用ＭＭ－２２０型环－块摩擦磨损试验机考察了该复合材料的滑动磨损行为,并观察分析了复合材料的组织结构、磨损表面形貌及磨屑组成．结果表明,其磨损过程存在跑合和稳态磨损２个阶段,在稳态磨损阶段主要发生氧化磨损,同时也存在磨粒磨损．工作环境影响复合材料的耐磨性．纳米碳管体积分数在１２％～１５％时,可以较好地发挥其润滑和阻止基体氧化的作用．     

有限转动张量的保角参量及在多柔体系统中的应用

采用保角转动参数描述了多体系统中的大转动张量．该方法消除了传统的欧拉参数描述所必需的约束方程，并且适于大变形部件的建模需要．利用以上结果建立了含大变形梁状部件的多体系统的力学模型．     

分子动力学模拟纳米镍单晶的表面效应

对单晶镍纳米丝、纳米薄膜零温准静态拉伸破坏过程进行了分子动力学模拟．模拟表明表面效应对单晶纳米材料的原子运动及整体力学行为有显著影响．自由表面增加纳米材料的塑性、降低其强度,影响纳米材料的变形机制．受表面效应的作用,纳米镍丝强度与弹性模量均低于纳米镍薄膜．纳米薄膜的断裂接近脆性断裂,断裂强度符合Griffith理想晶体脆断理论;纳米镍丝在断裂过程中表现出微弱塑性．     

单壁碳纳米管屈曲的原子/连续介质混合模型

用数学和力学研究所，上海 200072)//力学学报.--2004,36(6).--744～748 提供了一种运用原子/连续介质混合(hybrid atomic/continuum,HAC)方法解决纳米力学问题的思路. 通过在连续介质力学模型中引入利用分子力学方法获得物性参数，建立了预测单壁碳纳米管临界屈曲参数的HAC模型. 结果表明, HAC模型具有与连续介质力学模型可比拟的简洁性, 同时可表征纳米管微观结构特征对屈曲参数的影响. 计算结果表明，Zigzag纳米管的抗屈曲性能优于Armchair纳米管. 基于Tersoff-Brenner作用势的分子动力学结果证实了这一结论.     

Parametric variational principle and quadratic programming method for van der Waals force simulation of parallel and cross nanotubes

A parametric variational principle for van der Waals force simulation between any two adjacent nonbonded atoms and the corresponding improved quadratic programming method for numerical simulation of mechanical behaviors of carbon nanotubes are developed. Carbon nanotubes are modeled and computed based on molecular structural mechanics model. van der Waals force is simulated by the network of bars (called bar network) with a special nonlinear mechanical constitutive law (called generalized parametric constitutive law) in the finite element analysis. Compared with conventional numerical methods, the proposed method does not depend on displacement and stress iteration, but on the base exchanges in the solution of a standard quadratic programming problem. Thus, the model and method developed present very good convergence behavior in computation and provide accurate predictions of the mechanical behaviors and displacement distributions in the nanotubes. Numerical results demonstrate the validity and the efficiency of the proposed method.     

典型纳尺度结构的热振动

热振动是一定温度下纳尺度结构的固有运动,对其动力学行为有着重要的影响.当空间进入纳米尺度,结构呈现离散性,量子效应、边界效应、范德华力等变得不可忽略,纳尺度结构在热噪声随机激励下的动力学行为表现出众多异乎寻常的特性.以碳纳米管和石墨烯为代表的纳尺度碳材料具有优良的力学、电学和化学性质.在此介绍多种针对纳尺度结构热振动问题的研究方法、及碳纳米管和石墨烯的低温热振动、碳纳米管的非线性热振动研究进展.     

双层碳纳米管在扭矩作用下的屈曲

考虑双层碳纳米管层间范德华力的作用,利用连续介质力学的壳体理论,建立了扭矩作用下碳纳米管屈曲问题的双层弹性壳体模型,给出了相应的临界屈曲扭矩,分析了双层碳纳米管层间范德华力对临界屈曲扭矩的影响。     

Nonlocal beam theory for nonlinear vibrations of embedded multiwalled carbon nanotubes in thermal environment

A nonlocal elastic beam model is developed to investigate the small scale effects on the large-amplitude vibration analysis of embedded multiwalled carbon nanotubes (MWCNTs) at an elevated temperature. The nested slender nanotubes are coupled with each other through the van der Waals (vdW) interlayer interaction. The curvature-dependent vdW force employed incorporates not only pairwise nearest-neighbor but also nonneighbor interactions between nested nanotubes. The incremental harmonic balance method is adopted to analytically solve the nonlinear equations that are governed by the vibrations of nested nanotubes. The influences of small scale parameter, geometrical parameters, temperature rise, and the elastic medium are fully examined.     

Effects of van der Waals force on nonlinear vibration of electromechanical integrated electrostatic harmonic actuator

An electromechanical integrated electrostatic harmonic actuator is promising for the miniaturization of electromechanical devices. As the dimensions of the actuator decrease, the effects of the van der Waals force become obvious. In this study, by considering the nonlinearity of electrostatic and van der Waals forces, nonlinear vibration equations of the flexible ring of an electrostatic harmonic actuator are deduced. Using these equations, the nonlinear free vibration and nonlinear forced response of the actuator are investigated. The effects of the van der Waals force on the nonlinear vibration of the flexible ring are analyzed. Results show that these effects of the van der Waals force are relatively obvious under some conditions and should be considered.     

碳纳米管增强Nb-Si基复合材料界面应力传递的研究

基于均匀化理论,建立了碳纳米管增强Nb-Si基复合材料的代表体积元模型,并采用剪切滞后模型对碳纳米管增强Nb-Si基复合材料界面上的应力分布和传递机制进行了研究,探讨了分子间作用力、杨氏模量比β、长径比α、体积分数?等对其应力分布和传递机制的影响。结果表明,复合材料界面应力分布的变化主要集中在碳纳米管的两端,最大的应力都是分布在加载端或拔出端,然后向另一端传递;分子间作用力、杨氏模量比、长径比、体积分数等参数对界面应力的传递均有一定的影响,其中长径比和体积分数的影响最明显,体积分数为0.02时拔出端的界面剪切应力值相对于体积分数为0.0025时增大幅度达到近7倍,而长径比从200减小到50时,其应力传递距离增大了近8倍。     

Bifurcation and pull-in voltages of primary resonance of electrostatically actuated SWCNT cantilevers to include van der Waals effect

In this work the voltage response of primary resonance of electrostatically actuated single wall carbon nano tubes (SWCNT) cantilevers over a parallel ground plate is investigated. Three forces act on the SWCNT cantilever, namely electrostatic, van der Waals and damping. While the damping is linear, the electrostatic and van der Waals forces are nonlinear. Moreover, the electrostatic force is also parametric since it is given by AC voltage. Under these forces the dynamics of the SWCNT is nonlinear parametric. The van der Waals force is significant for values less than 50 nm of the gap between the SWCNT and the ground substrate. Reduced order model method (ROM) is used to investigate the system under soft excitation and weak nonlinearities. The voltage-amplitude response and influences of parameters are reported for primary resonance (AC near half natural frequency).     

Frequency response of primary resonance of electrostatically actuated CNT cantilevers

This paper deals with electrostatically actuated carbon nanotube (CNT) cantilever over a parallel ground plate. Three forces act on the CNTs cantilever, namely electrostatic, van der Waals, and damping. The van der Waals force is significant for values of 50 nm or less of the gap between the CNT and the ground plate. As both forces electrostatic and van der Waals are nonlinear, and the CNTs electrostatic actuation is given by AC voltage, the CNT undergoes nonlinear parametric dynamics. The methods of multiple scales and reduced order model (ROM) are used to investigate the system under soft AC near half natural frequency of the CNT and weak nonlinearities. The frequency–amplitude response and damping, voltage, and van der Waals effects on the response are reported. It is showed that only five terms ROM predicts and accurately predicts the pull-in instability and the saddle-node bifurcation, respectively.     

范德华力对磁头/硬盘系统气体润滑特性的影响

通过数值方法分析范德华力对磁头承载力的影响以及随飞行姿态的变化规律.结果表明,范德华力主要作用在磁头滑块尾部的凸台上,当飞行高度低于10 nm后,范德华力使得滑块的承载能力明显降低,从而导致飞行高度下降.范德华力主要与飞行高度和俯仰角有关,随着飞行高度和俯仰角减小,范德华力使承载能力和俯仰转矩明显减小.范德华力对侧翻角的影响不敏感,随着侧翻角的增加,范德华力使滑块的承载能力和侧翻转矩略微减小.在磁头设计时,通过增加俯仰角和减小尾部凸台面积可以减小范德华力的影响.     

A glance on the effects of temperature on axisymmetric dynamic behavior of multiwall carbon nanotubes

In this paper the effects of temperature on theradial breathing modes(RBMs) and radial wave propagation in multiwall carbon nanotubes(MWCNTs) are investigated using a continuum model of multiple elastic isotropicshells.The van der Waals forces between tubes are simulatedas a nonlinear function of interlayer spacing of MWCNTs.The governing equations are solved using a finite elementmethod.A wide range of innermost radius-to-thickness ratioof MWCNTs is considered to enhance the investigation.Thepresented solution is verified by comparing the results withthose reported in the literature.The effects of temperature onthe van der Waals interaction coefficient between layers ofMWCNTs are examined.It is found that the variation of thevan der Waals interaction coefficient at high temperature issensible.Subsequently,variations of RBM frequencies andradial wave propagation in MWCNTs with temperatures upto 1 600 K are illustrated.It is shown that the thick MWCNTs are more sensible to temperature than the thin ones.