Carbon Nanotube Ropes Research Articles

Electrical properties of nanoceramics reinforced with ropes of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) were used to convert insulating nanoceramics to metallically conductive composites. Dense SWCNT/Al2O3 nanocomposites with CNT contents ranging from 5.7 to 15 vol % and with nanocrystalline alumina matrices have been fabricated by spark-plasma-sintering that retains the integrity of SWCNT in the matrix. The conductivity of these composites increases with increasing content of CNTs. The conductivity has been increased to 3345 S/m in the 15 vol % SWCNT/Al2O3 nanocomposite at room temperature. This is an increase of 13 orders of magnitude over pure alumina and of more than 735% over previously reported results in CNT–ceramic composites.

Individual single-walled nanotubes and hydrogels made by oxidative exfoliation of carbon nanotube ropes.

Single-walled carbon nanotubes were oxidized by a technique previously developed for the oxidation of graphite to graphite oxide (GO). This process involves treatment with concentrated H(2)SO(4) containing (NH(4))(2)S(2)O(8) and P(2)O(5), followed by H(2)SO(4) and KMnO(4). Oxidation results in complete exfoliation of nanotube ropes to yield individual oxidized tubes that are 40-500 nm long. The C:O:H atomic ratio of vacuum-dried oxidized nanotubes is approximately 2.7:1.0:1.2. XPS and IR spectra show evidence for surface O-H, C=O, and COOH groups. The oxidized nanotubes slowly form viscous hydrogels at unusually low concentration (>or=0.3 wt %), and this behavior is attributed to the formation of a hydrogen-bonded nanotube network. The oxidized tubes bind readily to amine-coated surfaces, on which they adsorb as smooth and dense monolayer films. Thin films of the oxidized nanotubes show ohmic current-voltage behavior, with resistivities in the range of 0.2-0.5 Omega-cm.

Load transfer mechanism in carbon nanotube ropes

We used molecular mechanics and molecular dynamics to study the nature of load transfer in a single walled carbon nanotube (SWCNT) bundle consisting of seven (10,10) SWCNTs: one core tube surrounded by six tubes on the perimeter. The surface tension and the inter-tube corrugation are identified as the two factors that contribute to load transfer. The surface tension effectively acts over a “line” (roughly over the circumference of each tube). The inter-tube corrugation scales linearly with respect to the contact surface area, and increases non-linearly as the inter-tube distance decreases. Relaxation in the nanotube cross-section leads to better inter-tube load transfer as a slight “faceting” develops; the tubes appear to be partially polygonized, rather than perfect cylinders. Compared with parallel bundles, twisting can significantly enhance the load transfer between neighboring tubes; this has been computed as a function of twist angle for this nanotube bundle system.

Deformations and Thermal Stability of Carbon Nanotube Ropes

Structural and thermal characteristics of crystalline ropes of single-wall carbon nanotubes (SWCNTs) are investigated. Novel crystalline ropes of polygonized SWCNTs produced by laser irradiation exhibit rounded-hexagonal cross sections in contrast to earlier observations of circular tubes. Extensive molecular dynamics (MD) simulations lead to several metastable structures of the lattice characterized by different tube cross sections, hexagonal, rounded-hexagonal and circular, and increasing cell volume. The competition between different tube shapes is analyzed and compared to experiments. On the other hand, bundles of SWCNTs coalesce forming multiwall carbon nanotubes (MWCNTs) under thermal treatment at high temperatures. Extensive MD simulations confirm the SW-to-MW transformation and suggest the physical patching--and--tearing mechanism underlying the concerted coalescence of the tubes.

Superconductivity in ropes of single-walled carbon nanotubes

Abstract We report measurements on ropes of single-walled carbon nanotubes (SWNT) in low-resistance contact to nonsuperconducting metallic pads, at low voltage and at temperatures down to 13 mK . Large resistance drops and strong non-linearities in the IV characteristics are observed below 0.4 K . These features, which disappear in magnetic field in the Tesla range, strongly suggest the existence of superconductivity in ropes of SWNT.

Superconductivity in carbon nanotube ropes

We investigate the conditions in which superconductivity may develop in ropes of carbon nanotubes. It is shown that the interaction among a large number of metallic nanotubes favors the appearance of a metallic phase in the ropes, intermediate between respective phases with spin-density-wave and superconducting correlations. These arise in samples with about 100 metallic nanotubes or more, where the long-range Coulomb interaction is very effectively reduced and it may be overcome by the attractive interaction from the exchange of optical phonons within each nanotube. We estimate that the probability for the tunneling of Cooper pairs between neighboring nanotubes is much higher than that for single electrons in a disordered rope. The effect of pair hopping is therefore what establishes the intertube coherence, and the tunneling amplitude of the Cooper pairs dictates the scale of the transition to the superconducting state.

Single walled 0.4 nm carbon nanotube bundles prepared by pyrolysis of n-hexane catalyzed by ferrocene

AbstractBundles of single walled 0.4 nm carbon nanotubes were prepared by chemical vapor deposition using n-hexane as carbon source catalyzed by ferrocene with additives of thiophene at 1373 K. 8 cm long rope of single walled carbon nanotubes with diameters of 0.8-1.18 nm was also synthesized. The products were characterized by scanning electron microscopy, high resolution transmission electron microscopy and Raman spectroscopy.

Fatigue Behaviour of Unidirectional Single-Walled Carbon Nanotube Reinforced Epoxy Composite under Tensile Load

Tension-tension fatigue behaviour of unidirectional, aligned single-walled carbon nanotube (SWNT) rope reinforced epoxy composites were studied. While the slope of the stress-life (S-N) curve of the SWNTs in SWNT/epoxy composites obtained is flat, similar to those of carbon fibre reinforced epoxy composites, the fatigue strength of the former is at least twice that of the latter. Morphology of the fatigue fracture surface of SWNT reinforced epoxy involves matrix plastic deformation and SWNT-bridged matrix cracks. Evidence of good adhesion between SWNTs and epoxy was seen, and pullout length of SWNTs from the matrix is about 30 μm. Results suggest that carbon nanotubes can be used for fatigue resistant, high fracture toughness composites.

Single-Walled Carbon Nanotube Rope for Gas Sensor Application

ABSTRACTA chemical gas sensor with a novel pad pattern applicable to NH3, NO2, and H2 gas was developed based on single-walled carbon nanotube ropes. In this pattern named BCE (Barricade confronting electrode), several electrodes were located at the circumference of a sample and each pair of electrode tips was designed to confront to each other with a narrow interval. Several single-walled carbon nanotubes were shifted to the interval by the centrifugal force generated through the rapid rotation of a sample equipped on the spinner. Our device showed improved performance for NO2, NH3, and H2 gases compared with others in references.

Consistent picture of strong electron correlation from magnetoresistance and tunneling conductance measurements in multiwall carbon nanotubes

Both the magnetoconductance and the nonlinear differential conductance of multiwall carbon nanotube ropes have been studied at low temperture. Suppression in the differential conductance was observed at low bias voltages and at low temperatures, indicating the formation of a Coulomb gap. The magnetoconductance was found to follow a scaling law at both high and low temperatures, but with different mechanisms. The analysis of the data provides a sign of a non-Fermi-liquid-like behavior through magnetotransport measurement.

Electronic properties of bromine-doped carbon nanotubes

Intercalation of bromine molecules (Br 2) into single-wall carbon nanotube (SWNT) ropes is studied using the ab initio pseudopotential density functional method. Electronic and vibrational properties of the SWNT and Br 2 are studied for various bromine concentrations. A drastic change in the charge transfer, bromine stretching-mode, and bromine bond-length is observed when the bromine–bromine distance decreases. Calculated electronic structures show that, at high bromine concentrations, the bromine ppσ ∗ level broadens due to the interbromine interaction. These states overlap with the electronic bands of the SWNT near the Fermi level which results in a substantial charge transfer from carbon to bromine.

Orientation dependence of magneto-resistance behaviour in a carbon nanotube rope

The orientation dependence of magneto-resistance behaviour for a single-walled carbon nanotube (SWCN) rope is reported. A clear delineation of behaviours is observed between applying a magnetic field perpendicular or parallel to the rope axis. For a perpendicular field, monotonic negative magneto-resistance is observed due to two-dimensional weak localisation within the rope. By contrast, for a parallel field, complicated oscillatory behaviour is observed due to the Altshuler–Aronov–Spivak effect around closed electron trajectories on discrete cylinders within the SWCN rope. A dominant oscillatory mode can be identified which corresponds to closed paths around the outer circumference of the SWCN rope. However, due to the composite filamentary nature of the rope, the overall oscillatory behaviour is rather complicated and can be classified as universal conductance fluctuations. With a backgate voltage applied to the sample, Coulomb blockade peaks are observed in the transport current with additional peak structure superimposed due to resonant tunnelling. We find an interesting effect whereby these peaks are suppressed in the presence of a magnetic field.

Very low shot noise in carbon nanotubes

We have performed noise measurements on suspended ropes of single wall carbon nanotubes (SWNT) between 1 and 300 K for different values of dc current through the ropes. We find that the shot noise is suppressed by more than a factor 100 compared to the full shot noise 2eI. We have also measured an individual SWNT and found a level of noise which is smaller than the minimum expected. Another finding is the very low level of 1/f noise, which is significantly lower than previous observations. We propose two possible interpretations for this strong shot noise reduction: i) Transport within a rope takes place through a few nearly ballistic tubes within a rope and possibly involves non integer effective charges. ii) A substantial fraction of the tubes conduct with a strong reduction of effective charge (by more than a factor 50).

Thermoelectric power of single-walled carbon nanotube films

We report measurements of the thermoelectric power (TEP) S and electrical resistance R of thin films of tangled ropes of single-walled carbon nanotubes. Experiments at 300 K under applied force normal to the plane of the film revealed a $\ensuremath{\sim}10--20 %$ decrease in the four-probe resistance, indicating that the contact resistance between ropes had improved. The TEP, however, was not affected by the applied force and the concomitant decrease in the contact resistance between ropes. A quasilinear, diffusion TEP is observed for temperatures $T\ensuremath{\gtrsim}120 \mathrm{K},$ indicating that the metallic tubes provide the dominant contribution. As the sample is heated in a vacuum (degassed), the TEP is observed to decrease with time and changes sign, retaining its metallic character. This behavior is explained as a consequence of the balance between charge ${\mathrm{O}}_{2}^{\ensuremath{-}\ensuremath{\delta}}$ species which can be removed by degassing and an unidentified donor state. In fact, by stopping the degassing process at the appropriate time, one can achieve a $S\ensuremath{\sim}0$ state over the range $4 \mathrm{K}lTl500 \mathrm{K}.$ Model calculations of the TEP are presented which show that the small TEP is identified with the near mirror symmetry bands in metallic tubes and that the Fermi energy of the rope is determined by the balance of acceptor and donor states on the semiconducting tubes. A broad low-$T$ feature in the TEP with maxima in the range $80 \mathrm{K}lTl100 \mathrm{K}$ is tentatively identified with phonon drag.

Low-energy electron point source microscope as a tool for transport measurements of free-standing nanometer-scale objects: Application to carbon nanotubes

We have developed a simple and reliable technique for two-terminal transport measurements of free-standing wire-like objects. The method is based on the low-energy electron point source microscope. The field emission tip of the microscope is used as a movable electrode to make a well-defined local electrical contact on a controlled place of a nanometer-size object. This allows transport measurements of the object to be conducted. The technique was applied to carbon nanotube ropes.

Experimental probes of the molecular hydrogen–carbon nanotube interaction

Electrical transport (resistance R and thermoelectric power S), Raman scattering, and hydrogen adsorption are used to study the interaction of hydrogen molecules with ropes of single-walled carbon nanotubes. The data are consistent with H 2 physisorption under the experimental conditions investigated (4 K< T<500 K; 0.1 atm< P<20 atm). The response of S, R to 1 atm hydrogen at 500 K is consistent with the introduction of a new scattering channel for electrons/holes in the metallic tubes. Raman scattering from the Q-branch of hydrogen molecules adsorbed on the surface is found shifted only by 1–2 cm −1 from their frequencies in the free molecule and indicates that two different adsorption sites can be detected. Finally, H 2 wt% storage in heavily processed ropes of SWNTs are found to exceed 6% at ∼1 atm and T=77 K and the isosteric heat of adsorption is found to be 120 meV.

Volumetric hydrogen storage in single-walled carbon nanotubes

Macroscopically long ropes of aligned single-walled carbon nanotubes (SWNTs), synthesized by a hydrogen and argon arc discharge method, were cold pressed into tablets without any binder for measurements of their volumetric hydrogen storage capacity. The typical apparent density of the tablets was measured to be around 1.7 g/cm3 with respect to a molding pressure of 0.75 Gpa. A volumetric and mass hydrogen storage capacity of 68 kg H2/m3 and 4.0 wt %, respectively, was achieved at room temperature under a pressure of 11 MPa for suitably pretreated SWNT tablets, and more than 70% of the hydrogen adsorbed can be released under ambient pressure at room temperature. Pore structure analysis indicated that the molding process diminished the mesopore volume of the SWNT ropes, but exerts little influence on their intrinsic pore textures.

Electrochemical Hydrogen Storage Behavior of Ropes of Aligned Single-Walled Carbon Nanotubes

Massive macroscopic ropes of well-aligned single-walled carbon nanotubes (SWNTs) with a larger mean diameter of about 1.72 nm, synthesized by a semicontinuous hydrogen arc discharge method, were employed for electrochemical hydrogen adsorption experiments. A high discharge capacity of 503 mAh/g, corresponding to 1.84 wt % hydrogen, was achieved reproducibly at 25 °C under normal atmosphere for about 200 mg SWNT sample. This result implies that the SWNTs are highly promising electrochemical hydrogen storage materials for rechargeable batteries.

Simple method to prepare individual suspended nanofibers

A simple and general technique for producing a suspended nanofiber has been developed using coordinate markers and a sacrificial layer of poly(methylmethacrylate). The simple procedure does not involve etching processes or chemical vapor deposition and makes it easier to investigate the physical properties of nanofibers in a suspended configuration. As a demonstration, a suspended carbon nanotube rope was fabricated and Young’s modulus was determined to be 0.4 TPa from the force calibration of an atomic force microscope.

Self-assembled, deterministic carbon nanotube wiring networks.

A “minimal-lithography” technique for chemically assembling small deterministic crossbars of single-walled carbon nanotube (SWNT) ropes: The formation of electrically conducting crossbar circuits with 8 to 14 junctions (see picture) from a suspension of SWNTs is described. The critical structural parameters of the circuits were controlled by chemically directed self-assembly, rather than lithographic patterning, and all steps were carried out under ambient conditions.