Torsional buckling of carbon nanopeapods

Torsional buckling of carbon nanopeapods (carbon nanotubes filled with fullerenes) is studied using a continuum-based multi-layered shell model. The model takes into account non-bonded van der Waals interactions between nested fullerenes and the innermost layer of host nanotube. For nanopeapods with linearly arranged nested fullerenes, equivalent pressure distribution is proposed to model these interactions. Deriving explicit equations governing the torsional stability, it is concluded that the critical torsional load of a carbon nanopeapod is less than that of a carbon nanotube under otherwise identical geometric and mechanical conditions. Performing numerical calculations, it is also shown that increasing the number of layers of the host carbon nanotube decreases the weakening effect of encapsulated fullerenes on torsional stability of the nanopeapod.     

Torsional strain energy evolution of carbon nanotubes and their stability with encapsulated helical copper nanowires

Torsional strain energy evolution of a single-walled carbon nanotube (CNT) and torsional stability of a CNT encapsulating a helical copper nanowire are investigated by molecular dynamics simulations. The strain energy transfer of the pristine CNT reveals a unique wave-like mode along the torsional axis and afterwards isoenergetic strips form helical distributions in the vicinity of the critical point. Ripples appear together with structural buckling and strain energy release at the buckling point. Although the insertion of a copper nanowire impedes abrupt structural buckling and increases the stability of the CNT, the increase depends greatly on the number of the metal atoms. This kind of hybrid would increase the critical torsional angle and the maximum torque of the corresponding CNT in the future application of nano-mechanical devices.     

Transportation of hydrogen molecules using carbon nanotubes in torsion

The transportation of hydrogen molecules using carbon nanotubes subjected to torsion is studied with molecular dynamics. Molecular dynamics simulations reveal that the transportation in a (10, 0) carbon nanotube is a result of the van der Waals effect through the propagation of the kink initiated at the onset of the tube torsional buckling. In addition, the applied torsional loading rate has an obvious effect on the orientation of the molecular transportation. On the other hand, the motion of the molecules in a (10, 10) carbon nanotube is found to be less oriented. The mechanism of the transportation in the larger carbon nanotube is investigated through the transform of the collapsed wall of the tube in the dynamic process of the torsional buckling.     

Torsional buckling of double-walled carbon nanotubes

The mechanical instability of doubled-walled carbon nanotubes subject to torsion motion is investigated through molecular dynamics. A newly revealed buckling mode with one or three thin, local rims on the outer tube was discovered while the inner tube shows a helically aligned buckling mode in three dimensions. The distinct buckling modes of the two tubes imply the inapplicability of continuum mechanics modeling in which it is postulated that the buckling modes of the constituent tubes have the same shape. In view of this problem, a new concept of the equivalent thickness of double-walled carbon nanotubes is introduced, which enables the Kromm shell model to be applied to the derivation of the torsional buckling angle without the restraint of the two tubes having identical shapes.     

Dispersion of a bundle of carbon nanotubes by mechanical torsional energy

Dispersion of a bundle of carbon nanotubes by applying torsional energy is realized by molecular dynamics simulations. The torsional energy applied on the two ends of the bundle leads to a local buckling of the tube structures. The impulse between carbon nano-tubes owing to the local buckling of the tubes forms the driving force through a process of releasing the energy for a successful dispersion. The critical dispersion energy between two carbon nanotubes in a bundle in different solutions and the critical torsional energy for a successful dispersion are calculated by molecular dynamics simulations, and thus the dispersion efficiency is obtained. Effects of different parameters, such as the length and the diameter of the carbon nano-tubes and the temperature of the solution, on the dispersion efficiency are discussed. The dispersion of a bundle of five carbon nano-tubes in an aqueous solution is realized to further prove the feasibility of the proposed dispersion method by torsional energy. This paper provides an effective mechanical approach for dispersion of carbon nano-tubes from their bundle structure.     

Torsional elastic instability of double-walled carbon nanotubes

In this paper, a theoretical analysis of the torsional buckling instability of double-walled carbon nanotubes (DWCNTs) and the DWCNTs embedded in an elastic medium is presented based on the continuum elastic shell model and Winkler spring model. Using the proposed theoretical approach, the influences of the aspect ratio, the buckling modes and the surrounding medium on the torsional stability are examined in detail. The simulation results show that the torsional instability of DWCNTs can occur in different buckling modes according to the aspect ratio. The van der Waals (vdW) interaction force between nanotubes reinforces the stiffness of nanoshells. Thus, the DWCNTs possess higher buckling stability than the SWCNTs without considering vdW interaction force.     

Compressive buckling of carbon nanotubes containing polyethylene molecules

The instability of a carbon nanotube containing a polyethylene molecule subjected to compression is investigated using molecular dynamics. A decrease up to 35% in the buckling strain of the (6,6) and (10,10) carbon nanotube/polymer structures due to the attractive van der Waals interaction between the tube wall and the polymer molecule is reported. In particular, the decrease in the buckling strain of the (6,6) carbon nanotube/polymer structure is attributed to the initiation of two flattenings on the tube wall. Simulations show that the buckling strain of the structure is insensitive to the number of units of the polymer molecule.     

Thermal buckling of initially compressed single-walled carbon nanotubes by molecular dynamics simulation

Thermal buckling of initially compressed single-walled carbon nanotubes subjected to a uniform temperature rise is presented by using molecular dynamics simulations. Comprehensive numerical calculations are carried out for armchair and zigzag carbon nanotubes with various geometric dimensions. The results show that thermal buckling can occur beyond a critical value of temperature when the tube is initially compressed to a point prior to buckling. The critical buckling temperature increases as the compressive load ratio parameter decreases, and varies dramatically with nanotube helicity, radius and length. Owing to strong thermal oscillations of carbon atoms, a zigzag carbon nanotube with relatively small radius can buckle at a surprisingly lower temperature than the expected one.     

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Buckling of functionalized single-walled nanotubes under axial compression

Buckling characteristics of functionalized single-walled carbon nanotubes under axial compression are investigated by molecular mechanics simulation. The influences of the content, the distribution density and the location of the sp³-hybridized carbon atoms as well as the chirality on the critical buckling strains of functionalized single-walled carbon nanotubes are carefully studied. The results indicate that the chirality and the distribution density have dominant effect on the critical buckling strains. The critical buckling strains of present armchair (5, 5) and zigzag (10, 0) carbon nanotube are degraded by about 43% and 70%, respectively, due to the dense distribution of the sp³-hybridized carbon atoms. The reduction amplitude of the critical strain increases with increasing the tubule radius of an armchair or zigzag single-wall carbon nanotube. The dramatic reduction of the critical strain could cause a great loss of reinforcing role of carbon nanotubes in composites.     

A molecular dynamics simulation of the mechanical characteristics of a C60-filled carbon nanotube under nanoindentation using various carbon nanotube tips

The mechanical characteristics of a single-walled carbon nanotube (SWCNT) filled with C₆₀ fullerene subject to nanoindentation is studied using molecular dynamics (MD) simulations. The effects of temperature, indentation velocity, adhesion, and tip sizes were evaluated. The simulated results clearly show that the exerted load, Young’s modulus, elastic energy, and plastic energy decrease significantly with increasing temperature and decreasing indentation velocity and tip size. C₆₀ fullerenes can effectively increase the mechanical strength of a SWCNT because they act as a “barrier” to resist the radial deformation, as well as an inner wall in a double-walled carbon nanotube. With the same indentation depth, the ratio of elastic energy to plastic energy for a material gradually increases with the increase in the radius of the tip. This indicates that the elastic recovery of a material is better when the tip has a larger radius.     

Effect of Peierls transition in armchair carbon nanotube on dynamical behaviour of encapsulated fullerene

The changes of dynamical behaviour of a single fullerene molecule inside an armchair carbon nanotube caused by the structural Peierls transition in the nanotube are considered. The structures of the smallest C₂₀ and Fe@C₂₀ fullerenes are computed using the spin-polarized density functional theory. Significant changes of the barriers for motion along the nanotube axis and rotation of these fullerenes inside the (8,8) nanotube are found at the Peierls transition. It is shown that the coefficients of translational and rotational diffusions of these fullerenes inside the nanotube change by several orders of magnitude. The possibility of inverse orientational melting, i.e. with a decrease of temperature, for the systems under consideration is predicted.     

A dynamic continuum of nanostructured carbons in the combustion furnace

The presence of C60, carbon onions and higher fullerenes are confirmed in products from a carbon black combustion furnace. Acetylenic, C2, units participate in the formation and growth phases in the combustion flame. High-resolution TEM data clearly show fullerenes incorporated into the matrix of the carbon; MALDI mass spectral data show C60 and higher fullerenes in the soot itself. The toluene extracts of experimental carbons also contain C60.     

Deformation of gold-filled single-walled carbon nanotubes under axial compression

Molecular dynamics simulations were used to investigate the deformation behavior of gold-filled single-walled carbon nanotubes under axial compression. The simulation results show that the buckling strength of Au-filled carbon nanotubes is increased compared with that of a hollow tube, and is similar to the effect of filling with gases or fullerenes. The interactions between filling elements and the carbon wall help restrain the collapse of the tube. With Au-filling, the filled tube experiences an elastic-inelastic transition, somewhat like the behavior of metals, which is different from the cases when it is filled with gases or fullerenes, particularly for low filling density. Analysis of the transition using the potential energy map showed that several Au atoms began to slide before the strain reached the critical value. This is more obvious in the stress concentration zone, where the original Au bonds are first broken and then are re-formed with new neighbors.     

Torsional behavior of single-walled carbon nanotubes

Atomistic simulations were performed to investigate the deformation behavior of single-walled carbon nano-tubes (SWCNTs) under torsional loading. The evolutions of the potential energy and stresses were presented. Radial distribution functions (RDFs) were calculated to analyze structural evolution during torsional deformation. The results show that during torsion, the tensile stress component along the tube axis is most significant and other stress components are almost negligible. The tensile stress stretched the C–C bonds until they reached the bond length of 0.18 nm. The torsional strength of the SWCNTs is about 30% of the tensile strength. Buckling took place at a few degrees of torsional angle and propagated along the tube as the torsional angle increased, and collapse of the tube wall followed buckling. These structural evolutions can be well described with the RDFs. Two new peaks appeared at 0.21 nm and 0.18 nm in the RDFs, corresponding to the minimum spacing between the atoms in the collapsed layers, and the maximum bond length that can be reached in stretching before rupture.     

Computational investigation of the mechanical properties of nanomaterials

The mechanical responses of carbon nanotubes are examined using classical molecular dynamics simulations. Several different types of nanotubes are considered, including pristine single-walled tubes that are empty, filled with fullerenes to form peapods, filled with other nanotubes to form multi-walled tubes, or chemically functionalized. In addition, the responses of single-walled nanotubes with wall vacancies are considered. The results show how the bending force of filled nanotubes increases relative to the bending force of empty nanotubes and indicates how these increases come about. In addition, the simulations reveal the way in which the magnitude of these increases depend on the type of filling material and, in the case of multi-walled tubes, the number of inner tubes. These simulations further illustrate the way in which the inner nanotubes support higher external loads than the fullerenes in cases when the outer nanotubes are identical. The results also indicate that both the bending and buckling forces depend on temperature and the reasons for this dependence are discussed. Lastly, the simulations demonstrate the way in which the introduction of vacancy defects and covalently bound functional groups to the nanotube walls degrades the nanotubes' mechanical properties.     

Coal and carbon nanotube production

Strong expectations exist for future use of carbon nanotubes as composite materials in a large number of industries. Production cost and control of the purity and properties of such materials will influence the impacts nanotubes on the chemical, computer and construction industries. As a source material, coal is cheap and abundant, and has unique chemical structure, therefore, may be utilised in the nanotube synthesis. In the present paper, the synthesis of carbon nanotubes using coal as source material has been reviewed. Current nanotubes production largely followed the way of the production for fullerenes, most relying on plasma arcing methods. Non-arcing methods were also explored by a number of researchers. Catalytic synthesis is highlighted which has significant potential in the future nanotubes production directly from coal. Mechanism of the nanotube formation from coal is different from that using carbon graphite. Coal properties in this case are important. Weak bonds and mineral matter in the coal play an important role in the formation of the nanotubes.     

Electronic coupling in fullerene-doped semiconducting carbon nanotubes probed by Raman spectroscopy and electronic transport

We investigate the electronic properties of individual fullerene peapods by combining micro-Raman spectroscopy and (magneto)-transport measurements on the same devices. We bring evidence that the encapsulated C₆₀ molecules strongly modify the electronic band structure of semiconducting nanotubes in the vicinity of the charge neutrality point, including a rigid shift and a partial filling of the energy gap. Using a selective UV excitation of the fullerenes, we demonstrate that the electronic coupling between the contained C₆₀ molecules and the containing carbon nanotube is strongly modified by the partial coalescence of the C₆₀ and their distribution inside the tube. Our experimental results are supported by both numerical simulations of the Density of States and the measured conductance of carbon nanotubes with coalesced fullerenes inside.     

Effect of Topological Defects on Buckling Behavior of Single-walled Carbon Nanotube

Molecular dynamic simulation method has been employed to consider the critical buckling force, pressure, and strain of pristine and defected single-walled carbon nanotube (SWCNT) under axial compression. Effects of length, radius, chirality, Stone–Wales (SW) defect, and single vacancy (SV) defect on buckling behavior of SWCNTs have been studied. Obtained results indicate that axial stability of SWCNT reduces significantly due to topological defects. Critical buckling strain is more susceptible to defects than critical buckling force. Both SW and SV defects decrease the buckling mode of SWCNT. Comparative approach of this study leads to more reliable design of nanostructures.     

Stability analysis of double-walled carbon nanotubes as AFM probes based on a continuum model

The instability of single-walled carbon nanotubes (SWCNTs) under compressive loading has been observed experimentally by TEM and may limit their performance and structural integrity as atomic force microscopy (AFM) tips. Double-walled carbon nanotubes (DWCNTs) with inner and outer nanotubes of different lengths are proposed as AFM probes, and a theoretical approach based on a nanobeam model is developed for investigating the critical buckling stress of the DWCNTs under an axial compressive load. The influence of structural parameters on the buckling stress of DWCNT AFM probes are analyzed using this approach. The results show that the influence of the length mismatch between inner and outer nanotubes, as well as buckling modes on the buckling stress of DWCNTs was significant.