Double-walled carbon nanotubes (DWCNTs) have been proposed to be the leading candidates for high-speed nanobearings owing to superlubric characteristics between adjacent nanotubes. Performance of the DWCNT bearings is closely related to intertube friction, which is influenced by many factors, and in this work, we focus on the issue of flexibility of the nanotubes. Using molecular dynamics simulation, it has been found that considerable deformation of the nanotubes can emerge in the (5, 5)/(18, 0) DWCNT bearing with a length of similar to 80 angstrom if the angular speed of the shaft reaches 1.3 rev/ps. Such flexile deformation results in two distinct states with differing frictional characteristics. One of the two states, the slippery rotation, represents an interim period characterized by in-phase distortions of the inner and outer tubes, while the other state, the resistant rotation, is a steady state with the inner-tube curving lags behind that of the outer tube. Such a lag leads to a considerable increase of circular deflection of the outer tube and a sharp decrease of the minimal distance between tubes, therefore preventing the inner tube from slippery rotation. (c) 2013 AIP Publishing LLC.

That a commensurate contact usually leads to greater friction than an incommensurate one is a commonly held view in nanotribology. However, this perception seems paradoxical as commensurability is found to have negligible effect on the energy dissipation in double-walled carbon nanotube (DWNT) based oscillators. Using molecular dynamics simulations, we investigate such a paradox from the viewpoint of the atomic origin of friction. It is revealed that the commensurability cannot have a pronounced effect on friction unless the intertube interaction strength and the energy corrugation exceed their critical values. Both the commensurate and incommensurate oscillators constructed from natural DWNTs with an intertube distance of about 3.4 Å, may thus exhibit similar performance.

The mechanism of phase transition and evolution in graphite under uniform compression and spherical nanoindentation along the c-direction is investigated through systematical molecular dynamics simulations. Under both the loading conditions, the soft graphite phase can sustain pressure up to 16–20 GPa, beyond which it transforms into a new phase characterized by a much higher stiffness. More and more interlayer bonds will be created in the new hard phase with the increase of the pressure until an unstable state is reached. The critical pressure to produce the quenchable hard phase with a permanent sp3 bonding remaining after unloading is shown to be as high as ∼880 GPa under uniform compression, as opposed to only ∼75 GPa under nanoindentation. Therefore, application of non-uniform pressure is significantly more helpful for creating diamond-like sp3 structures in graphite by cold-compressive technique.

针对培养应用型人才的目标,结合自己的教学体会,对应用型高等院校的"理论力学"课程的教学内容与学时安排提出了个人的见解。为了相对完整地安排理论力学知识体系,需要64课时讲解理论力学课程。建议"静力学"28课时、"运动学"18课时、"动力学"12课时、期中考试与小结4课时、复习2课时。对各部分教学内容的安排作了探讨。

The behavior of nanobearings constructed from double-walled carbon nanotubes (DWCNTs) is investigated with molecular dynamics simulations. The results show that the (5, 5)/(10, 10) DWCNTs can work as stable and reliable nanobearings to a speed as high as ∼2.65 r ps(-1) with an inner tube as rotator. When the speed is lower than ∼0.75 r ps(-1), the nanobearings remain in an ultrasmooth state, beyond which the intertube friction increases and fluctuates sharply. The rotational friction is sensitive to many factors such as rotation speed, radial size, and flexibility of CNTs. Increase in rotation speed and the radial sizes of CNTs leads to increase of centrifugal force and decrease of intertube distance, thus, increases the intertube friction. As a result, both the critical speed for ultrasmooth rotation and the ultimate speed decrease with increasing radius of the inner tube with constant intertube distance. The centrifugal force and thermal motion of atoms will stimulate flexile deformation of CNTs, namely waving tube axis and distorting cross-section, which will lead to an increase in rotational friction. When the outer tube serves as the rotator, the DWCNT nanobearing becomes more easily damaged.

A new method for evaluating contact area and hardness is proposed for atomistic simulation of nanoindentation. The indenter is designed as a spherical virtual potential and atoms that enter the indenter are counted to evaluate the contact area. It is found that the nanohardness determined with the present method can be affected by the thermal activity of contact atoms, as well as the rigidity and dimensions of the virtual indenter. With the influencing factors carefully considered, molecular dynamics simulations employing the developed method are performed to investigate the indentation of carbon nanotubes (CNTs) along the radial direction and graphite sheets along the c-direction. It is found that the simulation results coincide well with available experimental findings, which demonstrates that the method is efficient for layer-configured nanomaterials such as CNTs and graphite crystal.

Temperature control in non-equilibrium nano-electromechanical systems is a vital issue to both scientific and engineering fields. The effect of temperature control in a specific dynamic system is studied using the concepts of thermodynamic temperature and kinetic temperature in a molecular dynamic model. It is demonstrated that isothermal simulations can be realized by constant thermodynamic temperature control of the system, but a constant kinetic temperature control scheme yields misleading results for the mechanical movements of the system. The isothermal simulations yield similar trends as an adiabatic approximation, at least at low temperature.

Molecular dynamics (MD) simulations of nanoindentation of multiwalled carbon nanotubes (MWCNTs) are carried out to study the deformation mechanism and the mechanical properties of MWCNTs in the radial direction. The MWCNT is found to be soft in its radial direction, with nanohardness rising slowly from about 6 to 15 GPa. The soft phase persists until all spaces between adjacent layers of the MWCNT are compressed beyond a critical value of about 1.9 angstrom. Beyond the critical stage the formation of new bonds between different layers starts to increase, producing a super-hard amorphous phase with a hardness of up to 94 GPa. Though locally compressed to a large radial strain of about 63%, the amorphous phase with a mixture of sp(3) and sp(2) bonds is completely reversible upon unloading, showing super-elasticity. Further indentation afterwards leads to permanent sp(3) and even sp rehybridization, the MWCNT is badly damaged and the hardness fluctuates with a maximum of about 124 GPa which is comparable to the microhardness of diamond.

纳米碳管振荡器外管对内管的作用相当于一个方向不断变化的交变恒力弹簧,在此交变恒力弹簧的回复作用下内管速度以线性规律变化而内管位置则按抛物线规律作周期性往复运动,其振荡频率可高达GHz甚至更高.随着温度的增加,振荡器内外管间摩擦力明显增大,因而能量衰减、振幅衰减都会明显加剧.

Nanoindentation-induced interlayer bond switching and phase transformation in carbon nanotubes (CNTs) and graphite are simulated by molecular dynamics. Both graphite and CNTs experience a soft-to-hard phase transformation at room temperature at compressive stresses of 12 and 16 GPa, respectively. Further penetration leads to the formation of interlayer sp(3) bonds, which are reversible upon unloading if the compressive stress is under about 70 GPa, beyond which permanent interlayer sp(3) bonds form. During nanoindentation, the maximum nanohardness of graphite can reach 109 GPa, and CNTs 120 GPa, which is comparable to that of diamond.

The uniaxial tensile deformation and shear deformation of nano bicrystal copper within (111) atomic layer are studied with molecular dynamics. Simulation shows that both tensile and shear deformation consists of a elastic and a plastic periods. When deformation is elastic the initial configuration of atomic system changes little, obvious change is observed in plastic period, such as break of metal bond,movement of atoms and pores,migration of grain boundary. The deformation mechanism directly determines the corresponding stress strain curve:the curve is linear in elastic period which coincides with the Huke Law; the curve fluctuates greatly in plastic period according to the microscopic deformation mechanism. It is also found that the two copper grains can be linked by just one atom at the end of plastic period, which manifests the excellent plasticity of nanocrystalline copper.

用分子动力学方法模拟了纳尺度铜单晶与铜双晶的(111)面原子的静拉伸力学行为.模拟结果表明纳米铜晶体的静拉伸变形过程中,原子系统的排列结构在弹性阶段没有变化;在塑性阶段铜单晶出现两个方向的交错滑移,铜双晶的塑性变形机制远比铜单晶复杂从而导致两者应力应变曲线的差异.模拟还发现纳米铜晶体具有良好的塑性变形自组织能力.

从压电弹性介质三维本构方程及平面问题的简化物理方程出发,求出了悬臂压电梁在没有外加电场情况下自由端受集中力时位移、电势的解析解,为探索压电层的感测机理提供了参考依据.