Intertube Van Der Waals Interactions Research Articles

Effects of ultrasonication on the microstructures and mechanical properties of carbon nanotube films and their based composites

Carbon nanotube (CNT) film has been considered as a promising preform of CNTs for developing high performance composites that possess high content of CNTs. However, due to the intertube van der Waals interactions, CNTs in the films are often tightly bundled together, making the impregnation of resin materials to CNTs very challenging. In this work, an eco-friendly ultrasonic-assisted resin infiltration process has been developed, and its effects on CNT bundles diameter, CNT crystallinity, CNT film stretchability, and the mechanical properties of CNT films and their based composites have been systematically investigated. It has been found that CNT bundles became thinner and the film stretchability became higher upon the ultrasonication, and thus the performance of CNT film-based composites were improved. However, extensive sonication would severely destroy the crystal structures of CNTs and thus degrade the mechanical properties of CNT films and their based composites. The underlying mechanisms for the evolution of CNT networks during the ultrasonic treatment were also discussed.

Interfacial characteristics of a carbon nanotube-polyimide nanocomposite by molecular dynamics simulation

Abstract With molecular dynamics simulations, nanocomposites were characterized that are comprised of a polyimide (PI) polymer and carbon nanotubes (CNTs) with the same outer diameter but with one, two, or three walls. The simulations indicate that the PI/CNT interaction is strong, regardless of the number of CNT walls, and that there is some degree of alignment of the PI chains near the CNT interface. As the number of CNT walls increased, the density of PI chains near the CNT interface also increased and the average radius of gyration of the PI chains decreased, and these observations were attributed to changes due to the intertube van der Waals interactions. From simulations of the constant force pullout process of the CNT from the PI matrix, the limiting pullout force was calculated to be higher for the triple-walled CNT than for the single-walled one. The interfacial shear strength of the nanocomposites was also calculated from the pullout energy, and the results indicate that increasing the number of walls is a critical factor for enhancing the interfacial stress transfer during tension.

Allotropes of Phosphorus with Remarkable Stability and Intrinsic Piezoelectricity

In this letter, we show that a new class of two-dimensional phosphorus allotropes can be constructed via assembling the previously proposed ultrathin metastable phosphorus nanotube into planar structures in different stacking orientations. Based on first-principles method, the structures, stabilities and fundamental electronic properties of these new two-dimensional phosphorus allotropes are systematically investigated. These two-dimensional phosphorus allotropes possess remarkable stabilities due to the strong inter-tube van der Waals interactions, which cause an energy release of about 30-70 meV/atom depending on their stacking manners. Our results show that most of these two-dimensional van der Waals phosphorene allotropes are energetically more favorable than the experimentally viable black alpha-P and blue beta-P. Three of them showing relatively higher probability to be synthesized in future are further confirmed to be dynamically stable semiconductors with strain-tunable band gaps and remarkable piezoelectricity, which may have potential applications in nano-sized sensors, piezotronics, and energy harvesting in portable electronic nano-devices.

Strain engineering for thermal conductivity of single-walled carbon nanotube forests

We perform classical molecular dynamics simulations to investigate the mechanical compression effect on the thermal conductivity of the single-walled carbon nanotube (SWCNT) forest, in which SWCNTs are closely aligned and parallel with each other. We find that the thermal conductivity can be linearly enhanced by increasing compression before the buckling of SWCNT forests, but the thermal conductivity decreases quickly with further increasing compression after the forest is buckled. Our phonon mode analysis reveals that, before buckling, the smoothness of the inter-tube interface is maintained during compression, and the inter-tube van der Waals interaction is strengthened by the compression. Consequently, the twisting-like mode (good heat carrier) is well preserved and its group velocity is increased by increasing compression, resulting in the enhancement of the thermal conductivity. The buckling phenomenon changes the circular cross section of the SWCNT into ellipse, which causes effective roughness at the inter-tube interface for the twisting motion. As a result, in ellipse SWCNTs, the radial breathing mode (poor heat carrier) becomes the most favorable motion instead of the twisting-like mode and the group velocity of the twisting-like mode drops considerably, both of which lead to the quick decrease of the thermal conductivity with further increasing compression after buckling.

Carbon nanotube dispersed conductive network for microbial fuel cells

Microbial fuel cells (MFCs) are promising devices for capturing biomass energy. Although they have recently attracted considerable attention, their power densities are too low for practical use. Increasing their electrode surface area is a key factor for improving the performance of MFC. Carbon nanotubes (CNTs), which have excellent electrical conductivity and extremely high specific surface area, are promising materials for electrodes. However, CNTs are insoluble in aqueous solution because of their strong intertube van der Waals interactions, which make practical use of CNTs difficult. In this study, we revealed that CNTs have a strong interaction with Saccharomyces cerevisiae cells. CNTs attach to the cells and are dispersed in a mixture of water and S. cerevisiae, forming a three-dimensional CNT conductive network. Compared with a conventional two-dimensional electrode, such as carbon paper, the three-dimensional conductive network has a much larger surface area. By applying this conductive network to MFCs as an anode electrode, power density is increased to 176 μW/cm2, which is approximately 25-fold higher than that in the case without CNTs addition. Maximum current density is also increased to approximately 8-fold higher. These results suggest that three-dimensional CNT conductive network contributes to improve the performance of MFC by increasing surface area.

Ultra‐broadband Polarisers Based on Metastable Free‐Standing Aligned Carbon Nanotube Membranes

A carbon nanotube free-standing linearly dichroic polariser is developed using solid-state extrusion. Membrane cohesion is experimentally and numerically demonstrated to derive from inter-tube van der Waals interactions in this family of planar metastable morphologies, controlled by the chemical vapour deposition conditions. Ultra-broadband polarisation (400 nm – 2.5 mm) is shown and corroborated by effective medium and full numerical simulations.

Thermal performance of vertically-aligned multi-walled carbon nanotube array grown on platinum film

Vertically-aligned carbon nanotube array is expected to inherit high thermal conductivity and mechanical compliance of individual carbon nanotube and serve as thermal interface material. In this paper, vertically-aligned multi-walled carbon nanotube arrays have been directly grown on Pt film and the thermal performance has been studied by using laser flash technique. The determined thermal diffusivity decreases from 0.187 to 0.135cm2s−1 and the thermal conductivity increases from 1.8 to 3.1Wm−1K−1 as temperature increases from 243.2 to 453.2K. The fracture surface of the array peeled off the Pt film was characterized by scanning electron microscopy. It has been illustrated that the tearing surface is not smooth but fluffy with torn carbon nanotubes, indicating strong interfacial bonding and consequent small interface resistance between carbon nanotube array and Pt film. According to Raman spectra and transmission electron microscopy image, the possible mechanisms responsible for the thermal transport degradation are low packing density, twist, and the presence of impurities, amorphous carbon, defects and flaws. The influence of intertube van der Waals interactions has been studied by comparing the phonon dispersion relations and is expected to be not significant.

Finite element and molecular dynamics models for predicting effective mechanical behaviors of carbon nanotube bundles

As an important type of potential interconnection nanostructures, metallic carbon nanotube bundles (CNBs) are expected to replace the copper interconnect lines as interconnects in the microelectronic packaging. By embedding discontinuous CNBs in a soft matrix for considering the intertube Van der Waals interaction, a finite element (FE) model is proposed to estimate the effective mechanical properties of CNBs. We also propose a molecular dynamics (MD) model to predict the intertube spacing and elastic parameters of both the carbon nanotubes and soft matrix by combining the analytical equations. The present FE numerical results show a good agreement with the analytical and MD results. Compared to the micromechanics-based analytical model, the proposed FE model can give a more exact description for the CNBs with a finite size while offering tremendous improvements in computational cost. The influences of the number of the tubes, the radius of the individual tube, and the length of the CNBs on the effective elastic properties, are especially examined. It is found that the geometric factors play an important role in characterizing the effective elastic properties.

FREE VIBRATION ANALYSIS OF DOUBLE-WALLED CARBON NANOTUBES USING THE SMOOTHED FINITE ELEMENT METHOD

This paper studies the resonant frequencies and the associated vibration modes of an individual double-walled carbon nanotube, using gradient smoothing technique. The study uses a double-elastic beam based on the Euler beam theory with the consideration of the intertube van der Waals interactions. A gradient smoothed formulation is deployed to deal with the fourth-order differential equation of the Euler beam problems for dynamic analysis of the double-walled carbon nanotubes. In the double-thin beam problems, a set of linear shape functions are adopted to approximate the displacement field. Smoothing domains are then formed for computing the smoothed curvature and bending moment field. Compared with analysis results of multi-beam theory, numerical examples indicate that very accurate results could be yielded when a reasonable number of nodes were used. The results also showed that non-coaxial intertube resonance would be excited at the higher resonant frequencies of double-walled carbon nanotubes. In addition, free vibration of a double-walled carbon nanotube with inner wall simple supported and outer wall one end fixed and the other end free is studied.

A three-dimensional model of electrical percolation thresholds in carbon nanotube-based composites

A three-dimensional modeling approach is developed to simulate the electrical percolation thresholds in carbon nanotube-based composites, wherein the effects of intertube Van der Waals interactions and electrical tunneling are taken into consideration. Numerical studies using the Monte Carlo method show that these two factors play significant roles when the aspect ratio of the constituent carbon nanotubes is low. For high aspect ratios, the effects of these two factors are negligible. The influence of carbon nanotube waviness on electrical percolation thresholds has also been studied.

A theoretical study of possible shape and phase changes of carbon nanotube crystals during contraction and expansion

We present a theoretical study of the phase transformations of crystals composed of single wall carbon nanotubes self-assembled by the intertube van der Waals interaction under contraction and expansion, and report a new hexagonal crystal structure caused by expansion of the crystal. We show that when the diameter of the nanotubes is larger than certain critical value, the nanotubes in the crystals will undergo three phase transformations: the closely-packed hexagonal phase I→assembled circular phase→new hexagonal phase II→isolated circular phase. These transformations are proved by both a hybrid atomic-continuum approach and the density functional theory calculations.

The effect of intertube van der Waals interaction on the stability of pristine andfunctionalized carbon nanotubes under compression

This paper investigates the effect of intertube van der Waals interaction on the stability ofpristine and covalently functionalized carbon nanotubes under axial compression, usingmolecular mechanics simulations. After regulating the number of inner layers of thearmchair four-walled (5, 5)@(10, 10)@(15, 15)@(20, 20) and zigzag four-walled (6, 0)@(15,0)@(24, 0)@(33, 0) carbon nanotubes, the critical buckling strains of the correspondingtubes are calculated. The results show that each of the three inner layers in thefunctionalized armchair nanotube noticeably contributes to the stability of the outermosttube, and together increase the critical strain amplitude by 155%. However, the threeinner layers in the corresponding pristine nanotube, taken together, increase thecritical strain of the outermost tube by only 23%. In addition, for both the pristineand functionalized zigzag nanotubes, only the (24, 0) layer, among the threeinner layers, contributes to the critical strain of the corresponding outermosttube, by 11% and 29%, respectively. The underlying mechanism of the enhancedstability related to nanotube chirality and functionalization is analyzed in detail.

Aggregate structure of hydroxyproline-rich glycoprotein (HRGP) and HRGP assisted dispersion of carbon nanotubes

Hydroxyproline-rich glycoproteins (HRGP) comprise a super-family of extracellular structural glycoproteins whose precise roles in plant cell wall assembly and functioning remain to be elucidated. However, their extended structure and repetitive block co-polymer character of HRGPs may mediate their self-assembly as wall scaffolds by like-with-like alignment of their hydrophobic peptide and hydrophilic glycopeptide modules. Intermolecular crosslinking further stabilizes the scaffold. Thus the design of HRGP-based scaffolds may have practical applications in bionanotechnology and medicine. As a first step, we have used single-molecule or single-aggregate atomic force microscopy (AFM) to visualize the structure of YK20, an amphiphilic HRGP comprised entirely of 20 tandem repeats of: Ser-Hyp4-Ser-Hyp-Ser-Hyp4-Tyr-Tyr-Tyr-Lys. YK20 formed tightly aggregated coils at low ionic strength, but networks of entangled chains with a porosity of ~0.5–3 μm at higher ionic strength. As a second step we have begun to design HRGP-carbon nanotube composites. Single-walled carbon nanotubes (SWNTs) can be considered as seamless cylinders rolled up from graphene sheets. These unique all-carbon structures have extraordinary aromatic and hydrophobic properties and form aggregated bundles due to strong inter-tube van der Waals interactions. Sonicating aggregated SWNT bundles with aqueous YK20 solubilized them presumably by interaction with the repetitive, hydrophobic, Tyr-rich peptide modules of YK20 with retention of the extended polyproline-II character. This may allow YK20 to form extended structures that could potentially be used as scaffolds for site-directed assembly of nanomaterials.

Self-Assembly of Single-Walled Carbon Nanotubes into Multiwalled Carbon Nanotubes in Water: Molecular Dynamics Simulations

We report discoveries from a series of molecular dynamics simulations that single-walled carbon nanotubes, with different diameters, lengths, and chiralities, can coaxially self-assemble into multiwalled carbon nanotubes in water via spontaneous insertion of smaller tubes into larger ones. The assembly process is tube-size-dependent, and the driving force is primarily the intertube van der Waals interactions. The simulations also suggest that a multiwalled carbon nanotube may be separated into single-walled carbon nanotubes under appropriate solvent conditions. This study suggests possible bottom-up self-assembly routes for the fabrication of novel nanodevices and systems.

Elastic axial buckling of carbon nanotubes via a molecular mechanics model

Based on a molecular mechanics model, analytical solutions for the critical buckling strain of single-walled achiral carbon nanotubes under axial compression are obtained. The results show that zigzag tubes are more stable than armchair tubes with the same diameter. Comparison with the results given by continuum mechanics model shows that the continuum mechanics model underestimates the critical buckling strain for smaller tubes if a Young’s modulus for larger tubes (or for graphene sheets) is adopted. The effect of intertube van der Waals interaction from the inner tube of multi-walled carbon nanotubes on the buckling of the outermost tube is also qualitatively discussed and it is found that the van der Waals interaction has little effect on the critical buckling strain for double-walled carbon nanotubes.

Ring formation of single-walled carbon nanotubes: Competition between conformation energy and entropy

We investigate ring formation of single-walled carbon nanotubes theoretically in this paper. The conformation energy of the nanotubes is identified as the curvature elastic energy of a curved graphite and the intertube van der Waals interaction. The entropy of ring nanotubes depends on the total number of carbon atoms. Assuming continuity, we find that the ring single-walled carbon nanotube is stable only when the radius of the tube is valued 6.958 \AA{}. The ring diameter ranges from 200 to 700 nm.

High Pressure Studies of the Raman-Active Phonons in Carbon Nanotubes

We report high pressure Raman studies on as-prepared, purified, and solubilized single-walled and aligned multi-walled carbon nanotubes. The pressure dependence of radial (R) and tangential (T) vibrational bands in these samples is measured and compared with the results from other studies. In single-walled nanotubes, an abrupt drop in the intensity of these bands is seen near 2 GPa, indicative of a phase transition. Experiments on single (unbundled) tubes reveal a ≈10 cm—1 upshift of the R band relative to its frequency in bundled tubes. This is opposite to the predictions of calculations that include the intertube van der Waals interaction only and is explained by the changes in the electronic band dispersion driven by tube–tube interactions. Surprisingly, the pressure dependence of the R and T bands in unbundled tubes is very similar to that seen in bundled tubes, which indicates that the changes in the electronic band structure might significantly influence the pressure dependence.