多壁碳纳米管弯曲导致的扭转

文章介绍了多壁碳纳米管弯曲的分子动力学模拟,从原子尺度上解释了由于屈曲失稳而导致的起皱现象,并发现多壁碳纳米管弯曲时会呈现出非线性的力学响应。同时,在模拟过程中,观察到了弯曲诱发扭转的现象,并揭示出扭转变形的内在起因是曲率诱导的晶格错配。     

VIBRATION ANALYSIS OF TWISTED CANTILEVERED CONICAL COMPOSITE SHELLS

The finite element method based on the Hellinger-Reissner principle with independent strain is applied to the vibration problem of cantilevered twisted plates and cylindrical, conical laminated shells. With a small number of elements, the present assumed strain finite element method is validated by convergence tests and numerical tests, comparing with the previous published vibration results for cantilevered conical shell. Computational effort and virtual storage reduce significantly due to good convergence. This study presents the twisting angle effect on vibration characteristics of conical laminated shells. Parameter studies with varying shallowness of cylindrical and conical shells are carried out. As the curvature increases, the fundamental mode shape changes from twisting mode to bending mode. For shells with a large curvature, the fundamental frequency, which is always characterized to bending mode, is almost constant independent of twisting angle. The twisting angle affects greatly twisting frequency and mode shape.     

Twisting carbon nanotubes: A molecular dynamics study

We simulate the twist of carbon nanotubes using atomic molecular dynamic simulations. The ultimate twist angle per unit length and the deformation energy are calculated for nanotubes of different geometries. It is found that the thick tube is harder to be twisted while the thin tube exhibits higher ultimate twisting ratio. For multi-walled nanotubes, the zigzag tube is found to be able to stand more deformation than the armchair one. We observed the surface transformation during twisting. Formation of structural defects is observed prior to fracture.     

Measuring the uniaxial strain of individual single-wall carbon nanotubes: resonance Raman spectra of atomic-force-microscope modified single-wall nanotubes

Raman spectroscopy is used to measure the strain in individual single-wall carbon nanotubes, strained by manipulation with an atomic-force-microscope tip. Under strains varying from 0.06%-1.65%, the in-plane vibrational mode frequencies are lowered by as much as 1.5% (40 cm(-1)), while the radial breathing mode (RBM) remains unchanged. The RBM Stokes/anti-Stokes intensity ratio remains unchanged under strain. The elasticity of these strain deformations is demonstrated as the down-shifted Raman modes resume their prestrain frequencies after a nanotube is broken under excessive strain.     

Experimental study of yttrium barium copper oxide superconducting tape’s critical current under twisting moment

Critical current (I _c) characteristics of 2G YBCO superconducting tape under the influence of twisting moment was experimentally investigated at varying current ramp rates in the self-field. Under a uniform twist, the degradation in the current-carrying capacity of YBCO tape up to 30% was observed at 77 K. The degradation is largely attributed to the shear stress and torsional shear strain resulting from the twisting. The superconductor to resistive transition index, n, is also found to behave in an identical manner with increase in the twisting. Finite element analysis (FEA) of the tape in the experimental configuration with twisting moment being applied on to it has been carried out in COMSOL. The torsional strain calculated analytically as per the experimental configuration matches closely with that of FEA results, which shows that the critical current degradation is a function of strain.     

Torsional buckling behavior of boron-nitride nanotubes using molecular dynamics simulations

Based on molecular dynamics simulations, the mechanical properties and buckling behavior of boron-nitride nanotubes under the action of torsional load are investigated. According to the results, the torsional properties of a boron-nitride nanotube are higher than those of its carbon counterpart. The effect of geometrical parameters on these parameters is also investigated. It is observed that by increasing the radius of nanotubes of the same length, unlike the critical shear strain, the critical torque considerably increases. The effect of chirality is also found to be negligible in the cases of critical shear strain and buckling mode, unlike the critical torque.     

Synthesis and structural determination of twisted MoS<Subscript>2</Subscript> nanotubes

In the present work we report the synthesis of MoS₂ nanotubes with diameters greater than 10 nm using a template method. The length and properties of these nanotubes are a direct result of the preparation method. High-resolution transmission electron microscopy is used to study the structure of these highly curved entities. Molecular dynamics simulations of MoS₂ nanotubes reveal that one of the stable forms of the nanotubes is a twisted one. The twisting of the nanotubes produces a characteristic contrast in the images, which is also studied using simulation methods. The analysis of the local contrast close to the perpendicular orientation shows geometrical arrays of dots in domain-like structures, which are demonstrated to be a product of the atomic overlapping of irregular curvatures in the nanotubes. The configuration of some of the experimentally obtained nanotubes is demonstrated to be twisted with a behavior suggesting partial plasticity. PACS 61.16.Bg; 79.60.Jv; 61.46.+w; 61.50.Ah     

Atomistic Failure Mechanism of Single Wall Carbon Nanotubes with Small Diameters

Single wall carbon nanotubes with small diameters （〈 5.0 A） subjected to bending deformation are simulated by orthogonal tight-binding molecular dynamics approach. Based on the calculations of C-C bond stretching and breaking in the bending nanotubes, we elucidate the atomistic failure mechanisms of nanotube with small diameters. In the folding zone of bending nanotube, a large elongation of C-C bonds occurs, accounting for the superelastic behaviour. The C-C bonds parallel to the axis direction of nanotube are broken firstly due to the sustained longitudinal stretching strain, giving rising to forming one-notch or two-notch bond-breaking mode depending on nanotube chirMities. The direct bond-breaking mechanism is responsible for the brittle fracture behaviour of nanotubes with small diameters.     

Nonlinear nonuniform torsional vibrations of bars by the boundary element method

In this paper a boundary element method is developed for the nonuniform torsional vibration problem of bars of arbitrary doubly symmetric constant cross-section taking into account the effect of geometrical nonlinearity. The bar is subjected to arbitrarily distributed or concentrated conservative dynamic twisting and warping moments along its length, while its edges are supported by the most general torsional boundary conditions. The transverse displacement components are expressed so as to be valid for large twisting rotations (finite displacement-small strain theory), thus the arising governing differential equations and boundary conditions are in general nonlinear. The resulting coupling effect between twisting and axial displacement components is considered and torsional vibration analysis is performed in both the torsional pre- or post-buckled state. A distributed mass model system is employed, taking into account the warping, rotatory and axial inertia, leading to the formulation of a coupled nonlinear initial boundary value problem with respect to the variable along the bar angle of twist and to an “average” axial displacement of the cross-section of the bar. The numerical solution of the aforementioned initial boundary value problem is performed using the analog equation method, a BEM based method, leading to a system of nonlinear differential-algebraic equations (DAE), which is solved using an efficient time discretization scheme. Additionally, for the free vibrations case, a nonlinear generalized eigenvalue problem is formulated with respect to the fundamental mode shape at the points of reversal of motion after ignoring the axial inertia to verify the accuracy of the proposed method. The problem is solved using the direct iteration technique (DIT), with a geometrically linear fundamental mode shape as a starting vector. The validity of negligible axial inertia assumption is examined for the problem at hand.     

THE DECISIVE ROLE OF m= 1 AND 2 PERTURBATIONS IN DENSITY LIMIT DISRUPTIONS ON TEXTOR

The behavior of magnetohydrodynamic phenomena associated with disruptions of ohmically heated density limit discharges (q_a=3.9) in the TEXTOR tokamak has been investigated in detail. A minor disruption is demonstrated to be only due to an tn = 2 mode instability, possibly due to the magnetic reconnection of an m=2 island. When the m = 2 mode oscillations cease, the internal m = 1 kink mode instability is drastically enhanced at once, twisting the plasma core considerably. If the twisting is too strong, the twisted plasma core is inevitably displaced to the outside and brings cold plasma to the central region, leading to the energy quench at a major disruption.     

Orientational melting in carbon nanotube ropes

Using Monte Carlo simulations, we investigate the possibility of an orientational melting transition within a "rope" of carbon nanotubes. When twisting nanotubes bundle up during the synthesis, orientational dislocations or twistons arise from the competition between the anisotropic intertube interactions, which tend to align neighboring tubes, and the torsion rigidity that tends to keep individual tubes straight. We map the energetics of a rope containing twistons onto a lattice gas model and find that the onset of a free "diffusion" of twistons, corresponding to orientational melting, occurs at T(OM) greater, similar160 K.     

Electronic structures of deformed B2C nanotubes under tensile strain

The vibrational properties and the band gaps of new B₂C nanotubes have been studied by the first principles calculations. It is found that (1) there is a typical Raman-active radial-breathing vibrational mode (RBM), which is similar to that of carbon nanotubes. The RBM frequency decreases in a linear proportion to the inverse diameter, whose variation slope depends on the types of B₂C nanotubes. (2) Under an applied tensile strain, the band gap of B₂C tubes is found to change greatly. For example, their band gaps can decrease to zero with increasing tensile strain for the (n, 0) B₂C tubes with odd n, showing clearly a metal–insulator transition, which cannot happen for the (n, 0) B₂C tubes with even n and the (0, n) B₂C tubes.     

Electronic properties of boron nanotubes with axial strain

The electronic properties of boron nanotubes with axial strain are investigated by first principle calculations. The band gaps of the (3, 3) and (5, 0) boron nanotubes are found to be modified by axial strain significantly. We find that the semiconductor-metal transition occurs for the (3, 3) boron nanotubes with both compressive and tensile strain. While for the (5, 0) boron nanotubes, only the tensile strain induces the semiconductor-metal transition. These boron nanotubes have the largest gaps under compressive strain.      

Strain effect on the visible emission in PbTiO3 nanotubes: Template and wall-thickness dependence

We investigated the strain effect on temperature-dependent photoluminescence property in the clamped (with template) and free-standing (without template) PbTiO₃ (PTO) nanotubes. The wall-thickness of nanotubes was varied from 25 to 80 nm with the outer diameter fixed to 420 nm. While all nanotubes show sizable green/yellow emission, the temperature dependent shift of the emission energy is significantly suppressed in the clamped PTO nanotubes, which is attributed to the lattice strain driven by the template clamping. This clamping effect is more significant for thinner nanotubes. Even in the free-standing PTO nanotubes the temperature-dependence of emission is affected by the wall-thickness. Our finding is the clear manifestation of the template and geometrical shape effect on the optical property of the nanotubes.     

Raman and SERS studies of carbon nanotube systems

In this paper, we report on Raman studies carried out on different carbon nanotube systems, namely single-walled and multi-walled carbon nanotubes and polymer/nanotube composites. We focus on different types of interactions which can take place in these materials. In single-walled nanotubes, the introduction of van der Waals interactions between tubes when arranged in bundles leads to an upshift of the radial breathing mode (RBM) ranging from 11 to 16 cm⁻¹ depending on the size of the bundle. In multi-walled carbon nanotubes, similar interactions between concentric tubes permit to interpret the low frequency Raman modes. In composites, PMMA/nanotubes, an upshift of the RBM is also observed, explained by the dynamical strain applied by the polymer on the bundles, in response to the breathing vibration. In addition, surface enhanced Raman scattering experiments have demonstrated the occurrence of interfacial reactions between the nanotubes and the metallic support. This is put in evidence by the degradation of tubes, especially metallic ones, and reconstruction of C₆₀-like molecules are in some cases observed.     

熔锥型光纤耦合器的扭转响应

对熔锥型光纤耦合器进行了扭转响应研究.通过放置在耦合器一端的旋转装置对未封装的熔锥型光纤耦合器耦合区施加扭转作用.实验表明:耦合器的耦合比不但对扭转作用敏感,而且变化呈单调性;同时耦合器的附加损耗和工作波长不受扭转作用影响,并且这种扭转操作具有可重复性.通过扭转操作能够对生产出来偏离预定耦合比的耦合器进行调整,使之符合产品要求,提高生产效率.     

Uniaxial deformation of nanotwinned nanotubes in body-centered cubic tungsten

We perform large-scale molecular dynamics simulations to delve into tensile and compressive loading of nanotubes containing {112} nanoscale twins in body-centered cubic tungsten, as a function of wall thickness, twin boundary spacing, and strain rate. Solid nanopillars without the interior hollow and/or nanotubes without the nanoscale twins are also investigated as references. Our findings demonstrate that both stress-strain response and deformation behavior of nanotwinned nanotubes and nanopillars exhibit a strong tension-compression asymmetry. The yielding of the nanotwinned nanotubes with thick walls is governed by dislocation nucleation from the twin boundary/surface intersections. With a small wall thickness, however, the failure of the nanotwinned nanotubes is dominated by crack formation and buckling under tensile and compressive loading, respectively. In addition, the strain rate effect, which is more pronounced in compressive loading than in tensile loading, increases with a decreasing twin boundary spacing.     

The effects of fibre orientation on free vibrations of composite beams

The torsion-flexure coupling effect of generally orthotropic beams is dependent on reinforcing fibre orientation and mode order. At higher ranks of vibration, this coupling effect is principally contributed by the twisting moment induced by bending. The influence of fibre orientation on normal mode shapes is more significant for small values of fibre orientation. The mode shapes can change suddenly with a small increase in fibre orientation.     

单壁纳米管的弹性性质

基于第一性原理计算了一系列单壁碳纳米管(椅型、锯齿型)和氮化硼锯齿型纳米管的杨氏模量.用实际的数值结果显示了杨氏模量与纳米管管径之间的依赖关系.还讨论了各类纳米管卷曲形变能以及平衡基矢长度a随管型的变化. 关键词： 单壁纳米管 平衡基矢长度 杨氏模量 形变能     

Nonlinear mechanical response and rippling of thick multiwalled carbon nanotubes

The measured drop of the effective bending stiffness of multiwalled carbon nanotubes (MWCNTs) with increasing diameter is investigated by a generalized local quasicontinuum method. The previous hypothesis that this reduction is due to a rippling mode is confirmed by the calculations. The observed ripples result from a complex three-dimensional deformation similar to the Yoshimura buckling pattern. It is found that thick MWCNTs exhibit a well-defined nonlinear moment-curvature relation, even for small deformations, governed by the interplay of strain energy relaxation and intertube interactions. Rippling deformations are also predicted for MWCNTs subject to torsion, resulting in an effective torsional modulus much smaller than that predicted by linear elasticity.