Nanomechanics of imperfectly straight single walled carbon nanotubes under axial compression by using molecular dynamics simulation

The buckling characteristics of several curved forms of single walled carbon nanotubes (SWCNTs) were studied in this work via molecular dynamics simulation method. The structural morphology of the CNT was modified to induce curvature along the tube axis. The nature of mechanical properties of these classes of CNTs under compression deviates from the ideal ‘perfectly straight’ CNTs. We found that the inclusion of curvature along the tube axis can significantly impact the performance of the carbon nanotube under compression. These curvilinear CNTs, when combined with other CNTs to form a bundle, will have a greater weakening effect on the mechanical performance of the CNT bundle.     

Molecular dynamics simulation of polarizable carbon nanotubes

This paper is aimed to develop a modified force field for molecular dynamics (MD) simulations of polarizable carbon nanotubes (CNTs). The effects of electrical polarization and the associated electronic degrees of freedom are represented by a network of negative charged shell particles which move relative to the surrounding positively charged carbon atoms in response to an applied electric field. In this setting, the negative and positive charges are exactly balanced so that the total system remains electrically neutral, and the motion of the shell particles relative to their equilibrium positions leads to polarization within the nanotube. Potential applications of the proposed model include simulations of controlled translocation of ions, water and polymers through solid-state CNT membranes.     

Bond-order potential for transition metal carbide cluster for the growth simulation of a single-walled carbon nanotube

A classical many-body potential for transition metal carbide cluster is developed in the form of the bond-order type potential function. The parameter sets between carbon atoms and several transition metal atoms (Fe, Co and Ni) are constructed by fitting binding energies from Density Functional Theory (DFT) calculations. Using the potential function, clustering process of carbon atoms to a small metal cluster is studied by classical molecular dynamics (MD) simulation. The number of hexagonal rings in the Co cluster increases about twice as fast as in the Fe cluster. This implies that the graphitic lattice interacts more strongly with Co atoms than with Fe atoms. A Co cluster has a crystal structure where metal atoms are regularly allocated and embedded in the hexagonal carbon network in the simulation. In contrast, carbon atoms cover the entire surface in case of the Fe cluster. Additionally, the potential energy surface that a carbon atom feels from a 2D closed-packed facet is examined using a hypothetical FCC(1 1 1) facet of several transition metals. The potential energy minima are distributed on the hexagonal network showing the 2D closed-packed facet can be a template where a graphene is formed.     

Buckling of defective single-walled and double-walled carbon nanotubes under axial compression by molecular dynamics simulation

Buckling of defective single-walled and double-walled carbon nanotubes (SWCNTs and DWCNTs, respectively) due to axial compressive loads has been studied by molecular dynamics simulations, and results compared with those of the perfect structures. It is found that single vacancy defect greatly weakens the carrying capacity of SWCNTs and DWCNTs, though it does slight harm to the effective elastic modulus of the tubes. The influence of defects on the buckling properties of nanotubes is related to the density of the defects, and the relative position of defects also plays an important role in buckling of DWCNTs. The van der Waals force among atoms in the inner and the outer tubes of short defective DWCNTs makes the critical buckling strain of DWCNTs greater than that of the inner tube.     

碳纳米管/聚乙烯复合物分子动力学模拟研究

采用分子动力学方法模拟了碳纳米管/聚乙烯复合物的结构、热力学和力学特性,分析其随模拟温度和碳纳米管填充率的变化。模拟结果表明,碳纳米管/聚乙烯复合物为各向同性的无定形结构,聚乙烯和碳纳米管通过较强的范德华作用结合在一起,在聚乙烯基体作用下,碳纳米管壁上的碳原子排列的周期性下降,出现弯曲和褶皱。从能量上看,填充率较高的复合物更加稳定。碳纳米管/聚乙烯复合物具有比聚乙烯体系更高的等容热容和与聚乙烯体系相反的负值热压力系数,热容随碳纳米管填充率的变化较小,但随温度的升高而明显减小,具有显著的温度效应;热压力系数随温度的变化较小,温度稳定性比聚乙烯更好,但随填充率增加而减小。碳纳米管/聚乙烯复合物的力学特性表现出各向同性材料的弹性常数张量,弹性模量和泊松比比纯聚乙烯体系高得多,并且都随温度的升高和碳纳米管含量的降低而减小,说明加入碳纳米管可显著改善聚乙烯的力学性质。     

Load transfer of graphene/carbon nanotube/polyethylene hybrid nanocomposite by molecular dynamics simulation

Load transfer of the graphene/carbon nanotube (CNT)/polyethylene hybrid nanocomposite is studied here from molecular dynamics (MD) simulations. Simulations of this composite material under uniaxial tension were conducted by varying CNT’s position and diameter in the polymer matrix. The obtained results show that: (1) The peak strength of stress and strain evolution in the polymer matrix is lower than the peak strength of the graphene/graphene and graphene/polymer interfaces. Hence, the damage zone is always located in the polymer matrix. (2) Agglomerated two-layer graphenes do not possess an increased value in the peak strength compared with single-layer graphene-reinforced polymer nanocomposite (PNC), while two separate layers of graphene show slightly higher peak strength. (3) The largest peak strength is observed before CNT moves to the center of the polymer matrix. The damage location moves from the upper to the lower part of CNT when the CNT is located at the centre of polymer matrix. (4) The influence of the CNT diameter on the peak strength is not obvious, while the damage location and shape in the polymer matrix changes with respect to varying CNT diameters. In addition, the damage zone always falls outside the interphase zone.     

A molecular dynamics study on the thickness and post-critical strength of carbon nanotubes

This paper presents a molecular dynamics (MDs) study on the linear, buckling and post-buckling behaviour of carbon nanotubes (CNTs) under pure shortening and pure twisting. Its objectives are (i) to clarify the issue about the most correct thickness value to adopt in the simulation of CNTs using shell models and (ii) to evaluate their post-critical strength. Three CNTs with similar length-to-diameter ratio but different atomic structures (zig-zag, armchair and chiral) are selected for this study. Then, MD simulations are performed to investigate the pre-critical, critical buckling and post-critical behaviour of CNTs under pure shortening and pure twisting. Using available analytical formulae derived from shell models, the influence of CNT thickness on their critical strain and critical angle of twist is investigated. Some conclusions are drawn regarding (i) the most appropriate choice of the thickness value to use in shell models and (ii) the effectiveness of post-critical stiffness and strength of CNTs.     

Mechanical properties of graphene oxide: A molecular dynamics study

In this paper, the mechanical properties of graphene oxide are obtained using the molecular dynamics analysis, including the ultimate stress, Young modulus, shear modulus and elastic constants, and the results are compared with those of pristine graphene. It is observed that the increase of oxide agents (–O) and (–OH) leads to the increase of C–C bond length at each hexagonal lattice and as a result, alter the mechanical properties of the graphene sheet. It is shown that the elasticity modulus and ultimate tensile strength of graphene oxides (–O) and (–OH) decrease significantly causing the failure behavior of graphene sheet changes from the brittle to ductile. The results of shear loading tests illustrate that the increase of oxide agents (–O/–OH) results in the decrease of ultimate shear stress and shear module of the graphene sheet. It is shown that the increase of oxide agents in the graphene sheet leads to decrease of the elastic constants, in which the reduction of elastic properties in the armchair direction is more significant than the zigzag direction. Moreover, the graphene sheet with oxide agents (–O) and (–O/–OH) presents an anisotropic behavior.     

Tunable field emission properties of carbon nanotube arrays by engineering Fe catalysts

High-quality carbon nanotube (CNT) arrays composed of nanotubes with different diameters and wall numbers were synthesized by water-assisted chemical vapor deposition (CVD) from engineered Fe catalysts. Interestingly, the distribution of nanotube diameter and wall number broadened over 2.5 times as the catalytic Fe thickness increased. The mean diameter and wall number of nanotubes increased monotonically with the Fe thickness, while the calculated CNT area density within an array dropped about 32 times. Field emission measurements revealed that the turn-on voltage for CNT arrays decreased from 3.5 to 2.5 V/µm with the increased catalytic Fe thickness. It was believed that the screening effect from the proximity of neighboring nanotubes has a dominant influence than the CNT diameter on the resulting turn-on voltage.     

Molecular dynamics simulations of carbon nanotube dispersions in water: Effects of nanotube length, diameter, chirality and surfactant structures

Molecular dynamics simulations are used to compute the potential of mean force (PMF) governing the interactions between carbon nanotubes (CNTs) in water/surfactant systems. The effects of CNT length, diameter, chirality (armchair and zigzag) and surfactant structures on CNT interaction and dispersion in water/surfactant systems are investigated for (5, 5), (5, 0), and (10, 10) single walled CNTs with two commonly used surfactants [viz., sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS)] at room conditions. An adaptive biasing force method was used to speed up the calculations. Simulations revealed that CNT length and diameter as well as optimum amount of surfactant addition and its structures can significantly affect CNT interactions (i.e., PMFs vary significantly). Surfactant molecules were found to adsorb at the CNT surface and reduced interaction strength between CNTs. SDBS surfactant contributed weaker interactions between CNTs as compared with that of SDS surfactant by a factor of about 10 indicating that SDBS is better than SDS for dispersing CNTs in an aqueous suspension. This phenomenon agrees qualitatively with the experimental results reported in the literature. The understanding of detailed atomic arrangements and atomic interactions between CNTs and surrounding molecules reported in this study is significantly helpful to computationally screening different surfactants and improving the CNT dispersion in aqueous solution. The method will also facilitate the reduction of time and cost required to produce CNT reinforced nanocomposite materials as well as homogeneous CNT dispersed solutions for many biological applications.     

Synthesis and properties of novel materials at high pressure and temperature—molecular dynamics simulation studies

Molecular dynamics simulations, in combination with lattice dynamics studies, based on semiempirical interatomic potentials, have been very useful in the study of properties of complex novel materials at high temperature and pressure. Various properties such as the equation of state, elastic and thermodynamic properties, phase transitions and melting have been studied. These studies help in understanding the synthesis of important new and novel materials, especially the amorphous materials, compounds with unusually coordinated atoms, (e.g. with five-coordinated silicon atoms), materials with controlled thermal expansion, etc. A few examples will be discussed from our recent studies.     

High temperature electrical and thermal properties of the bulk carbon nanotube prepared by SPS

Bulk multi-walled carbon nanotube was prepared by spark plasma sintering at 1700 °C under a pressure of 50 MPa in vacuum. The density of the bulk sample reaches 72% of the theoretical density of the carbon nanotube, 2 g/cm³. The high temperature thermal conductivity and electrical conductivity of the bulk material were measured in the directions perpendicular and parallel to the pressure direction. Both the thermal conductivity and electrical conductivity show apparent anisotropy. The thermoelectric power has close values in the two different directions and takes positive values in the whole measured temperature range (360–840 K).     

碳纳米管阵列研究进展

在介绍CNT阵列性能的基础上,对国内外直接合成CNT阵列的方法进行了评述,重点阐述了各种方法的基本特点及CNT阵列的生长机理、结构控制和批量制备问题。进而探讨了CNT原生阵列、抽丝形成的CNT丝、以及CNT阵列分散后得到的CNTs在复合材料、力学增强、功能器件等方面的初步应用,展望了CNT阵列的发展趋势,指出低成本、大批量可控制备CNT阵列仍然是未来一段时间内国际研究热点。     

Lattice dynamics and molecular dynamics simulation of complex materials

In this article we briefly review the lattice dynamics and molecular dynamics simulation techniques, as used for complex ionic and molecular solids, and demonstrate a number of applications through examples of our work. These computational studies, along with experiments, have provided microscopic insight into the structure and dynamics, phase transitions and thermodynamical properties of a variety of materials including fullerene, high temperature superconducting oxides and geological minerals as a function of pressure and temperature. The computational techniques also allow the study of the structures and dynamics associated with disorder, defects, surfaces, interfaces etc.     

Mechanical properties of multi-walled carbon nanotube/epoxy composites

Untreated and acid-treated multi-walled carbon nanotubes (MWNT) were used to fabricate MWNT/epoxy composite samples by sonication technique. The effect of MWNT addition and their surface modification on the mechanical properties were investigated. Modified Halpin–Tasi equation was used to evaluate the Young’s modulus and tensile strength of the MWNT/epoxy composite samples by the incorporation of an orientation as well as an exponential shape factor in the equation. There was a good correlation between the experimentally obtained Young’s modulus and tensile strength values and the modified Halpin–Tsai theory. The fracture surfaces of MWNT/epoxy composite samples were analyzed by scanning electron microscope.     

High-aligned carbon nanotube film with netlike bulges

High-aligned carbon nanotubes film with netlike bulges made of catalyst particles has been synthesized on a silica wafer by pyrolyzing ferrocene/melamine mixtures. The structure and composition of carbon nanotubes are investigated by scanning electron microscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electron energy-loss spectroscopy (EELS). It is found that these nanotubes have uniform outer diameters of about 25 nm and lengths of about 40 μm. High-resolution TEM images show that each carbon nanotube is composed of graphite-like layers arranged in a stacked-cup-like structure. XPS spectrum shows that the crust covering the tops of the aligned carbon nanotube film consists of carbon, iron and ferric oxide. The EELS spectrum shows that these nanotubes are pure-carbon tubes. The formation mechanism of the netlike bulges has been provided.     

Movable hash algorithm for search of the neighbor atoms in molecular dynamics simulation

In this paper, we suggested an algorithm called as movable hash, by which the neighbor atoms of a given atom in the process of molecular dynamics simulation can be rapidly determined. The main idea of this algorithm is to divide the simulation box into cells with a size smaller than the diameter of the atom to ensure only one atom in a cell at any time. Each cell is represented by an element of a three-dimensional array, the value of which equals either to the address where the message of the atom in this cell is stored or to nil. Then in one cycle loop of the sub-indexes of the array, we can easily operate to all neighbor atoms of a special atom. The speed to find all neighbor atoms of a given atom by movable hash method is only one third of that by the cell index method when the cell size equals to half of the diameter of the atom.     

Nanoinfiltration behavior of carbon nanotube based nanocomposites with enhanced mechanical and electrical properties

In this work,carbon nanotube (CNT) based nanocomposites with high mass fraction are proposed by in-situ bridging carbon matrix into CNT paper through optimized chemical vapor infiltration (CVI).Nanoinfiltration behavior of CNTs is basically investigated under the CVI process.The contact between each CNT can be strengthened and the conductive pathways can be established,resulting in the better mechanical and electrical properties.Compared with the pristine CNT paper,the CNT/C composite after pyrolysis process confirms a remarkable advance in tensile strength (up to 310 ± 13 MPa) and Young's modulus (up to 2.4 ± 0.1 GPa).Besides,a notable feature of electrical conductivity also shows an improvement up to 8.5 S/cm,which can be attributed to the mass fraction of CNT (41 wt％) breaking the limits of percolation thresholds and the efficient densification of this sample to establish the conductive pathways.This study has a broad application in the development of the multi-functional electrical and engineering materials.     

Computer simulation of carbon nanotube pull-out from polymer by the molecular dynamics method

The influence of the polymer matrix density, chemical cross-links in the interface, and geometrical defect in the carbon nanotubes (CNTs) on the CNT pull-out from polymer has been analyzed by the molecular dynamics simulation. The interfacial shear strength (ISS) has also been estimated with the change of total potential energy. In the simulation, the crystalline polyethylene matrix is set up in a hexagonal array with the polymer chains parallel to the CNT axis. First, we investigate the effect of the polymer matrix density on the ISS by changing the distance between the chains. Simulated results show that the ISS increases with the increase of matrix density. Next, we examine the cross-link effect on the ISS by adding polyethylene cross-links in the interface. Here, an energy based switching criterion addressing cross-links traveling on the CNT has been proposed. It is found that the presence of cross-links and the cross-link positions affect the ISS. Finally, pentagon–heptagon defect, which reduces the tensile strength of the CNT and has been experimentally observed by Hashimoto et al. [Hashimoto A, Suenaga K, Gloter A, Urita K, Iijima S. Direct evidence for atomic defects in graphene layers. Nature 2004;430:870–3], has been addressed as a geometrical defect in the CNT. When cross-links are present between the CNT and the polymer, this defect reduces the ISS due to the improper connections of cross-links around this defective region.     

Improvement of emission reliability of carbon nanotube emitters by electrical conditioning

We have investigated the effect of electrical aging to improve emission reliability of carbon nanotube (CNT) emitter. The CNT emitters were prepared by the screen-printing of the CNT paste. The electrical aging treatment was carried out in a high vacuum chamber by applying the pulsed dc voltage. The field emission measurements and lifetime tests were performed on the CNT emitters depending on the electrical aging time and current density. After the electrical aging treatment, emission properties of CNT emitter were suppressed but the half lifetime was dramatically improved.