Adjacent Carbon Nanotubes Research Articles

Removal of hydrogen sulfide from natural gas by the graphene-nanotube hybrid structure: A molecular simulation

The molecular simulations are adopted to investigate the separation of H2S/CH4 mixture by the graphene-nanotube hybrid structure (GNHS). The effects of pressure, temperature and H2S mole fraction on the adsorption and separation behaviors are examined. The results show that the separation performance of the GNHS can be enhanced by reducing the adjacent carbon nanotubes (CNTs) distance. The self-diffusivity of H2S molecules in the GNHS is 1.83×10−8m2s−1 which is less than that of CH4. Therefore, enlarging the diffusivity discrepancy of the two components can improve the separation performance of the GNHS.

Improved field emission properties from carbon nanotubes grown onto micron-sized arrayed silicon pillars with pyramidal bases

Carbon nanotubes (CNTs) were synthesized onto micron-sized arrayed silicon (Si) pillars with pyramidal bases in order to mitigate the electric-field-screening due to densely grown CNTs. Compared with the CNT emitters grown on a flat Si substrate, CNT emitters grown on arrayed Si pillars showed improved field emission properties with a low turn-on field of 2.16V/μm at 10μA/cm2 and current density of 550μA/cm2 at 3.46V/μm. Field enhancement factor was calculated according to the Fowler–Nordheim theory. The resultant field enhancement factor of CNT emitters on arrayed Si pillars was higher than that of CNT emitters on a flat substrate, which revealed that the formation of arrayed Si pillars could be an effective method to suppress electric-field-screening among adjacent CNT emitters.

Piezoresistive effect of a carbon nanotube silicone-matrix composite

A carbon nanotube silicone-matrix composite is fabricated into a specimen with two current-electrodes and two voltage-electrodes. The specimen is sealed transversely and the four-wire-method is used to measure the piezoresistivity of the composite during compression. The electrical resistance and the resistivity of the composite increase with the increase of the absolute value of the strain during compression. The absolute value of the gage factor decreases with the increase of the carbon nanotube content. By analyzing the changes in the effective conductive path (ECP), a piezoresistive mathematical model is established. The resistance of a tunneling film (i.e. the silicone film between the adjacent carbon nanotubes where the tunneling effect occurs) attenuates with the increase of the absolute value of the strain during compression. The attenuating rate decreases with the increase of the carbon nanotube content. The ratio of the number of the tunneling films in each ECP to the number of ECPs in the composite decreases with the increase of the carbon nanotube content and increases with the increase of the absolute value of the strain during compression.

Cross-Linked Carbon Nanotube Heat Spreader

ABSTRACTAmong the exceptional properties of isolated individual carbon nanotubes (CNTs), exceptional thermal conductivity along their axis has been demonstrated, However they have also shown poor thermal transfer between adjacent CNTs. Thick bundles of aligned CNTs have been used as heat pipes, but the thermal input and output power densities are the same, providing no heat spreading effect. We demonstrate the use of energetic argon ion beams to join overlapping CNTs in a thin film to form an interpenetrating network with an isotropic thermal conductivity of 2150 W/m K. Such thin films may be used as heat spreaders to enlarge the thermal footprint of laser diodes and CPU chips, for example, for enhanced cooling. At higher ion energies and fluence, the CNTs appear to collapse and reform, aligned parallel to the ion beam axis, and form dense high aspect ratio tapered structures. The high surface area of these structures lends themselves to applications in energy storage, for example. We consider the mechanisms of energetic ion interaction with CNTs and junction formation of two overlapping CNTs during the subsequent self-healing process, as well as the formation of high aspect ratio structures under more extreme conditions

Sensing Mechanism of Self-Monitoring CNT Cementitious Composite

Abstract Owing to its outstanding sensing properties, mechanical properties, and durability, self-monitoring carbon nanotube (CNT) cementitious composite can serve as an intrinsic smart sensor for structural health monitoring and traffic detection. However, it is not clear at present what the origin of the sensing property of the self-monitoring CNT cementitious composite is. This paper discusses the nature of the sensing property of the composite measured with dc and ac circuits based on a commonly used conductive model of CNT cementitious composite and reveals the origin of the sensing property of the composite through experiments. Experimental results indicate that the sensing property of the composite mainly results from the change in contact resistance between adjacent CNTs under external force or deformation.

Atomistic simulations, mesoscopic modeling, and theoretical analysis of thermal conductivity of bundles composed of carbon nanotubes

The propensity of carbon nanotubes (CNTs) to self-organize into continuous networks of bundles has direct implications for thermal transport properties of CNT network materials and defines the importance of clear understanding of the mechanisms and scaling laws governing the heat transfer within the primary building blocks of the network structures—close-packed bundles of CNTs. A comprehensive study of the thermal conductivity of CNT bundles is performed with a combination of non-equilibrium molecular dynamics (MD) simulations of heat transfer between adjacent CNTs and the intrinsic conductivity of CNTs in a bundle with a theoretical analysis that reveals the connections between the structure and thermal transport properties of CNT bundles. The results of MD simulations of heat transfer in CNT bundles consisting of up to 7 CNTs suggest that, contrary to the widespread notion of strongly reduced conductivity of CNTs in bundles, van der Waals interactions between defect-free well-aligned CNTs in a bundle have negligible effect on the intrinsic conductivity of the CNTs. The simulations of inter-tube heat conduction performed for partially overlapping parallel CNTs indicate that the conductance through the overlap region is proportional to the length of the overlap for CNTs and CNT-CNT overlaps longer than several tens of nm. Based on the predictions of the MD simulations, a mesoscopic-level model is developed and applied for theoretical analysis and numerical modeling of heat transfer in bundles consisting of CNTs with infinitely large and finite intrinsic thermal conductivities. The general scaling laws predicting the quadratic dependence of the bundle conductivity on the length of individual CNTs in the case when the thermal transport is controlled by the inter-tube conductance and the independence of the CNT length in another limiting case when the intrinsic conductivity of CNTs plays the dominant role are derived. An application of the scaling laws to bundles of single-walled (10,10) CNTs reveals that the transition from inter-tube-conductance-dominated to intrinsic-conductivity-dominated thermal transport in CNT bundles occurs in a practically important range of CNT length from ∼20 nm to ∼4 μm.

Modeling electrical conductivity of nanocomposites by considering carbon nanotube deformation at nanotube junctions

This paper investigates the effect of carbon nanotube (CNT) deformation on the electrical conductivity of CNT polymer composites at crossed nanotube junctions using a revised 3-dimensional CNT percolating network model. Two aspects of the work are considered. The first is concerned with the effect of CNT deformation on its intrinsic and contact resistances at CNT-CNT junctions. An analytical model based on electron ballistic tunneling theory and Landauer-Büttiker formula is proposed to describe the variation of CNT-CNT contact resistance at the CNT-CNT junction in terms of local deformation of CNT walls and CNT-CNT distance. In addition, a model exclusively based on experimental data to describe the change of CNT intrinsic resistance in terms of its cross-section deformation is adopted. The second is concerned with the relationship among the CNT-CNT distance, the angle between two adjacent CNTs, and the dimensions of local deformation of CNT walls and its impact on the corresponding intrinsic and contact resistance of CNTs near and at a CNT-CNT junction. Finally, Monte Carlo simulations are conducted to evaluate these effects on the electrical conductivity of nanocomposites for different CNT weight fractions. Our results reveal that the local deformation of CNT walls plays a significant role in the evaluation of electrical conductivity of CNT polymer composites. The intrinsic resistance in the deformed part of CNTs near a CNT-CNT junction increases much faster than the decrease of CNT-CNT contact resistance at the same junction when two CNTs are getting closer, resulting in a net increase of resistance at the junction. Numerical results show that the current model agrees with existing experimental data better than existing models without considering the effect of CNT deformation, which tends to overestimate the electrical conductivity of CNT polymer composites containing homogeneously dispersed percolating CNT network.

Piezoresistive response to changes in contributive tunneling film network of carbon nanotube/silicone rubber composite under multi-load/unload

The piezoresistive effect of carbon nanotube filled silicone rubber composite under the multi-load/unload are studied. For the composites with the mass ratio of carbon nanotube to silicone rubber ranging from 0.04 to 0.10, the piezoresistivities are monotonically increasing within the pressure ranging from 0 to 1MPa. The hysteresis (repeatability) error decreases with the increase of the number of the load/unload cycles. There exist the increasing and decreasing tendencies in the electrical resistance of the composite under the multi-load/unload, which are caused by the changes in the resistivity and the geometric size of the composite. The resistivity increases with the increase of the pressure. The absolute value of the deviation between the resistivities under the same pressure in the adjacent load (unload) cycles decreases with the increase of the number of the load (unload) cycles. The variations in the resistivity under the multi-load/unload are explained by analyzing the changes in the three-dimensional network composed of the contributive tunneling films (CTFs) between the adjacent carbon nanotubes.

Tunneling resistance and its effect on the electrical conductivity of carbon nanotube nanocomposites

In this paper, we examined the effect of electron tunneling upon the electrical conductivity of carbon nanotube (CNT) polymer nanocomposites. A CNT percolating network model was developed to account for the random distribution of the CNT network using Monte Carlo simulations, where the tunneling resistance between CNTs was established based on the electron transport theory. Our work shows several novel features that result from this tunneling resistance: (i) direct contact resistance is the result of one-dimensional electron ballistic tunneling between two adjacent CNTs, (ii) the nanoscale CNT-CNT contact resistance should be represented by the Landauer-Büttiker (L-B) formula, which accounts for both tunneling and direct contact resistances, and (iii) the difference in contact resistance between single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) can be modeled by the channel number in the L-B model. The model predictions reveal that the contact resistance due to electron tunneling effects in nanocomposites with dispersed SWCNTs plays a more dominant role than those with MWCNTs. These results compare favorably with existing experimental data and demonstrate that the proposed model can properly estimate the electrical conductivity of nanocomposites containing homogeneously dispersed percolating CNT network.

Experimental-Computational Study of Shear Interactions within Double-Walled Carbon Nanotube Bundles

The mechanical behavior of carbon nanotube (CNT)-based fibers and nanocomposites depends intimately on the shear interactions between adjacent tubes. We have applied an experimental-computational approach to investigate the shear interactions between adjacent CNTs within individual double-walled nanotube (DWNT) bundles. The force required to pull out an inner bundle of DWNTs from an outer shell of DWNTs was measured using in situ scanning electron microscopy methods. The normalized force per CNT-CNT interaction (1.7 ± 1.0 nN) was found to be considerably higher than molecular mechanics (MM)-based predictions for bare CNTs (0.3 nN). This MM result is similar to the force that results from exposure of newly formed CNT surfaces, indicating that the observed pullout force arises from factors beyond what arise from potential energy effects associated with bare CNTs. Through further theoretical considerations we show that the experimentally measured pullout force may include small contributions from carbonyl functional groups terminating the free ends of the CNTs, corrugation of the CNT-CNT interactions, and polygonization of the nanotubes due to their mutual interactions. In addition, surface functional groups, such as hydroxyl groups, that may exist between the nanotubes are found to play an unimportant role. All of these potential energy effects account for less than half of the ~1.7 nN force. However, partially pulled-out inner bundles are found not to pull back into the outer shell after the outer shell is broken, suggesting that dissipation is responsible for more than half of the pullout force. The sum of force contributions from potential energy and dissipation effects are found to agree with the experimental pullout force within the experimental error.

Solid‐state Bis‐hexahapto‐metal complexation of single‐walled carbon nanotubes

We report the effect of transition metal deposition on the conductivity of single‐walled carbon nanotube (SWNT) networks. The slow evaporation of small amounts of highly mobile transition metal atoms significantly reduces the resistance of SWNT thin films; we suggest that Cr, Fe, and Ti atoms form bis‐hexahapto complexes at the SWNT sidewalls, thereby bridging adjacent carbon nanotubes and reducing the inter‐nanotube junction resistance. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of potassium doping on electrical properties of carbon nanotube fibers

We investigate the effect of potassium (K) doping on the transport properties of aligned single-walled carbon nanotube fibers. The temperature dependence of the electrical resistance, the current-voltage characteristics, and the magnetoresistance vs external magnetic field of the fibers consistently show that doping enhances the metallic character of the fibers and that the response of the samples can be quantitatively explained in two thermal regimes separated by a characteristic temperature ${T}^{*}$. At temperatures higher than ${T}^{*}$, the data are interpreted in the framework of variable range hopping theory, suggesting that the increased conductance with potassium doping is due to the increase of the density of states, which enhances carriers hopping. For temperatures below ${T}^{*}$, experimental evidence of fluctuation temperature-induced tunneling mechanism suggests that the doping by K atoms affects the potential barriers established between adjacent carbon nanotubes, enhancing the metallic properties of the fibers.

Multi-scale modeling of carbon nanotube reinforced composites with a fiber break

Present paper deals with the study of stress distribution in the vicinity of a broken carbon nanotube (CNT) in CNT-based fiber reinforced composites. Three dimensional finite elements analysis (FEA) using multi-scale modeling has been done considering a square representative volume element (RVE) and the effect of a broken CNT on the adjacent CNTs and on the matrix has been studied for CNT/Epoxy and CNT/Titanium composites. The distributions of stresses in the broken CNT, at the interface, and in the adjacent CNTs have been analyzed for different volume fractions. It is observed that the ineffective length of the broken CNT is dependent on the volume fraction as well as on the type of matrix materials. The magnitude of the interfacial shear stress as well as the stress concentration in the vicinity of the broken CNT is also influenced by the fiber volume fraction and the type of matrix materials.

A Model for the Strength of Yarn-like Carbon Nanotube Fibers

A model for the strength of pure carbon nanotube (CNT) fibers is derived and parametrized using experimental data and computational simulations. The model points to the parameters of the subunits that must be optimized in order to produce improvements in the strength of the macroscopic CNT fiber, primarily nanotube length and shear strength between CNTs. Fractography analysis of the CNT fibers reveals a fibrous fracture surface and indicates that fiber strength originates from resistance to nanotube pull-out and is thus proportional to the nanotube-nanotube interface contact area and shear strength. The contact area between adjacent nanotubes is determined by their degree of polygonization or collapse, which in turn depends on their diameter and number of layers. We show that larger diameter tubes with fewer walls have a greater degree of contact, as determined by continuum elasticity theory, molecular mechanics, and image analysis of transmission electron micrographs. According to our model, the axial stress in the CNTs is built up by stress transfer between adjacent CNTs through shear and is thus proportional to CNT length, as supported by data in the literature for CNT fibers produced by different methods and research groups. Our CNT fibers have a yarn-like structure in that rather than being solid, they are made of a network of filament subunits. Indeed, the model is consistent with those developed for conventional yarn-like fibers.

Dielectric Spectroscopy and Tunability of Multi-Walled Carbon Nanotube / Epoxy Resin Composites

The dielectric response of oxidized multi-walled carbon nanotube / epoxy resin composites, is investigated with respect to filler content concentration, over a wide temperature and frequency range. Specimens, below the percolation threshold, exhibit similar behaviour to that of the neat epoxy. Two relaxation modes are observed in the low temperature region, attributed to the re-arrangement of small parts of the polymer chain (γ-mode) and the reorientation of polar side groups (β-mode) respectively, where in the high temperature region the evolution of the α-mode is present. Direct current (DC) conduction follows the Vogel – Tamann - Fulcher equation as expected. The dielectric response of specimens, above the percolation threshold, follows the “Random Free Energy Barrier Model”. DC conductivity exhibits Arrhenius temperature dependence with two distinct regions. The activation energies of both regions were evaluated. The contact resistance between two adjacent carbon nanotubes was also calculated. The effective dielectric constant as well as the capacitance of the specimens, above the percolation threshold, can be modulated by means of an applied DC bias voltage.

Effect of concentration on low-frequency noise of multiwall carbon nanotubes in high-density polyethylene matrix

Transport and noise measurements of multiwall carbon nanotubes in high-density polyethylene matrix are reported. In these composites current transport occurs through a random tunnel junctions network, formed by adjacent carbon nanotubes. Low-frequency noise investigations reveal a 1/f behavior induced by resistance fluctuations. An unusual temperature dependence in samples with different nanotube concentration is found. This can be explained by a transition from a fluctuation-induced tunneling mechanism to a thermally activated regime, occurring at increasing nanotube concentration and resulting in a decrease in the overall noise.

Advanced elastomer nano-composites based on CNT-hybrid filler systems

Different techniques to disperse multiwalled carbon nanotubes (CNT) in elastomers using an internal mixer are applied and physical properties of the composites are evaluated: stress–strain behavior, dynamic-mechanical, thermal diffusivity, dielectric and fracture mechanical properties. The electrical percolation threshold is found to decrease by using ethanol as dispersion agent, compared to “dry” mixing, correlating with improved optical dispersion. The effect of nanoscopic gaps between adjacent CNTs on the electrical and thermal conductivity of the composites and the missing percolation behavior of the thermal conductivity are discussed. We have found some technically promising synergetic effects of the hybrid filler systems. For all systems one observes significantly steeper stress–strain curves by addition of 1.6 vol.% CNT to the systems with conventional fillers. In natural rubber the fatigue crack propagation resistance, tensile strength and electrical conductivity is found to be improved also for dry mixed CNT-silica hybrid systems.

Field Emission Stability of Anodic Aluminum Oxide Carbon Nanotube Field Emitter in the Triode Structure

In this work, fabricated field emission (FE) triode arrays with the anodic aluminum oxide (AAO) template carbon nanotube (CNTs) as the field emitters are numerically analyzed. To obtain physically sound self-consistent solution, a set of Maxwell's equations coupling with Lorentz equation are solved simultaneously using a finite difference time domain particle-in-cell method. The FE current is then computed with the Fowler-Nordheim equation. To validate the simulation model, we firstly calibrate the collected electron current density between the measured AAO-CNTs and calculated result. The FE current is dominated by, such as density, height, diameter, and tilt angle of CNTs, and applied bias, respectively. A high density of CNTs will result in strong screening effect among adjacent CNTs and reduce the magnitude of electric field. Consequently, it significantly affects the emitted and collected electron current densities. However, the structure with much higher density of CNTs obviously emits more stable current than that of a low density of CNTs. There is an optimal setting on the height and diameter of the CNTs within the explored structure which exhibits the highest current density. When we vary the density of CNTs, the structure with high density (say the number of CNTs is greater than 30) shows the most stable and smallest fluctuation on the current density against the randomly generated samples of the height and tilt angle of CNTs.

Maximum nanotube volume fraction and its effect on overall elastic properties of nanotube-reinforced composites

The potential capability of improving overall elastic modulus of nanotube-reinforced composites is a fundamental concern in nanotechnology applications. Based on geometric analysis and micromechanics estimation, this study reports that the ratio of surface-to-surface distance of adjacent carbon nanotubes (CNTs) to the CNT diameter plays a key role in improving the overall elastic modulus of the CNT-reinforced composites when the tubes are perfectly aligned, completely separated from other tubes, and ideally bonded with the composite matrix. With the decrease of this ratio, that is, decrease of the surface-to-surface distance of adjacent CNTs and/or increase CNT diameter, the improvement capability increases. However, theoretical and experimental results show that an increase of the CNT diameters degrades the elastic moduli of CNTs. This paper discusses the criterion of choosing CNTs with larger diameter and addresses the factors influencing the surface-to-surface distance of adjacent CNTs.

Dispersing Carbon Nanotubes in Aqueous Solutions by a Starlike Block Copolymer

The ability of dispersing carbon nanotubes (CNTs) in aqueous solutions by a starlike amphiphilic block copolymer with PPO−PEO segments (AP432) was investigated in detail. For comparison, two commercially available linear amphiphilic block copolymers, Pluronics L64 and F127, were also selected. It was found that AP432 and F127 can get good CNT dispersions, while L64 was proved to be unable to disperse CNTs. AP432 with five branches could disperse CNTs efficiently at much lower concentrations compared with the linear F127, although it has a smaller molecular weight and shorter terminal EO groups. This indicated clearly that, once branched, copolymers would get a much better ability to disperse CNTs. Increasing concentration of AP432 or F127 would disperse more CNTs, but at high copolymer concentrations the aggregation of dispersed CNTs was observed, which may be related to the free micelles formed by AP432 or F127 around CNTs. Other influencing factors such as the mass ratio of CNTs to copolymers and sonication time and strength were also discussed. From the molecular dynamics simulation results, it can be found that copolymers with five branches can gain better steric repulsions between adjacent CNTs, which is consistent with the experimental results.