Jp. Issi

QUANTUM TRANSPORT IN A MULTIWALLED CARBON NANOTUBE

We report on electrical resistance measurements of an individual carbon nanotube down to a temperature T = 20 mK. The conductance exhibits a lnT dependence and saturates at low temperature. A magnetic field applied perpendicular to the tube axis increases the conductance and produces aperiodic fluctuations. The data find a global and coherent interpretation in terms of two-dimensional weak localization and universal conductance fluctuations in mesoscopic conductors. The dimensionality of the electronic system is discussed in terms of the peculiar structure of carbon nanotubes.

Electrical-resistance of a Carbon Nanotube Bundle

The first direct electrical resistance measurements performed on a single carbon nanotube bundle from room temperature down to 0.3 K and in magnetic fields up to 14 T are reported. From the temperature dependence of the resistance above 2 K, it is shown that some nanotubes exhibit a semimetallic behavior akin to rolled graphene sheets with a similar band structure, except that the band overlap, DELTA almost-equal-to 3.7 meV, is about 10 times smaller than for crystalline graphite. In contrast to graphite which shows a constant low-temperature resistivity, the nanotubes exhibit a striking increase of the resistance followed by a broad maximum at very low temperatures. A magnetic field applied perpendicular to the sample axis decreases the resistance. Above 1 K, this behavior is consistent with the formation of Landau levels. At lower temperatures, the resistance shows an unexpected drop at a critical temperature which increases linearly with magnetic field. These striking features could be related to the unique quasi-one-dimensional structure of the carbon nanotubes.

Electronic-properties of Carbon Nanotubes - Experimental Results

Band structure calculations show that carbon nanotubes exist as either metals or semiconductors, depending on diameter and degree of helicity. When the diameters of the nanotubes become comparable to the electron wavelength, the band structure becomes noticeably one-dimensional. Scanning tunneling microscopy and spectroscopy data on nanotubes with outer diameters from 2 to 10 nm show evidence of one-dimensional behavior: the current-voltage characteristics are consistent with the functional energy dependence of the density-of-states in 1D systems. The measured energy gap values vary linearly with the inverse nanotube diameter. Electrical resistivity acid magnetoresistance measurements have been reported for larger bundles, and the temperature dependence of the electrical resistance of a single micro-bundle was found to be similar to that of graphite and its magnetoresistance was consistent with the formation of Landau levels. Magnetic susceptibility data taken on bundles of similar tubes reveal a mostly diamagnetic behavior. The susceptibility al fields above the value at which the magnetic length equals the tube diameter has a graphite-like dependence on temperature and field. At low fields, where electrons sample the effect of the finite tube diameter, the susceptibility has a much more pronounced temperature dependence.

Properties and Characterization of Codeposited Boron-nitride and Carbon Materials

The physical properties of carbon‐boron nitride (C‐BN) prepared from mixtures of BCl3, NH3, and selected hydrocarbons by codeposition methods have been investigated using x‐ray diffraction, electron diffraction, transmission electron spectroscopy, x‐ray photoelectron spectroscopy, Raman scattering, optical reflectivity, thermal conductivity, thermopower, and electrical resistivity. Taken collectively, the results of these experiments indicate that the compression‐annealed C‐BN materials studied here consist of separated domains of pyrolytic boronated graphite and pyrolytic boron nitride, while as‐deposited samples may possibly be a single‐phase mixture of C, B, and N. As‐deposited materials containing more than 20% carbon were found to be more highly oriented than unannealed pyrolytic graphite, and the crystallinity of these materials was greatly enhanced by uniaxial compression annealing. Results of the thermal conductivity, thermopower, and electrical resistivity measurements are consistent with a netwo...

Differential scanning calorimetry and infra-red crystallinity determinations of poly(aryl ether ether ketone)

The differential scanning calorimetry (d.s.c.) heating thermograms of 12 poly(aryl ether ether ketone) (PEEK) samples of varying degrees of crystallinity have been recorded. The relation found between the degree of crystallinity as determined by specific gravity measurements, and the melting enthalpy of the polymer, shows that recrystallization is occurring during a heating scan rate of 10-degrees-C min-1. This implies that d.s.c. is not a convenient technique to assess PEEK crystallinity. The infra-red absorbance spectra of the same samples have also been examined in the range from 1030 to 880 cm-1. The 965 cm-1 band, up to now considered as indicative of the PEEK crystallinity, is shown to be practically independent of the degree of crystallinity above 15%. However, there is evidence to support the existence of a true i.r. crystalline band located at 947 cm-1. It is also suggested that the 965 cm-1 band is due to a normal vibration mode of a short segmental conformation, whose presence is favoured in the crystalline phase, but also in the amorphous zones nearest to the crystallite surface.

Diamond formation by thermal activation of graphite

Synthetic diamond is used in applications ranging from abrasives, tool coatings, bearing surfaces, microelectronics and optics to corrosion protection. The first artificial synthesis used high-pressure techniques to produce diamond as the thermodynamically stable form, but it can also be grown at low pressures as a metastable carbon phase. Here we report the production of high-purity cubic diamond microparticles (10–100 µm), which form in a highly concentrated carbon-vapour phase, followed by deposition of the crystals on the substrate. The carbon-vapour phase is generated by thermal activation of graphite, and the fast initial growth-rates of diamond, in the range 100–500 µm s-1, are at least two orders of magnitude higher than previously reported. We expect that tuning of experimental parameters to optimize the density of the carbon-vapour phase will allow us to grow larger diamond crystals, thereby opening a wider range of potential practical applications.

The temperature variation of the thermal conductivity of benzene-derived carbon fibers

The temperature variation of the in-plane thermal conductivity of benzene-derived carbon fibers (BDF) measured from 5 to 300K is reported and discussed. Very high thermal conductivity values-comparable to that of the best HOPG heat treated at the same temperature-are found. The data confirms the high structural order previously reported for BDF.

Surface treatments of vapor-grown carbon fibers produced on a substrate

Vapor-grown carbon fibers (VGCF) were grown from a methane-hydrogen mixture on a reconstituted graphite support using different catalyst precursors, the [Fe-3(CO)(12)] complex being found to be the most efficient for the production of VGCF. The fibers thus produced were characterized and submitted to different oxidative treatments, namely nitric acid, plasma, air and carbon dioxide. From analysis performed by scanning electron microscopy (SEM), nitrogen adsorption (BET) and X-ray photon spectroscopy (XPS) it appears that the air and carbon dioxide treatments do not lead to significant increase either of the surface area, or of the quantity of surface oxygen containing groups, despite the considerable weight loss attained (50%). This peculiar observation has been interpreted by considering the presence of traces of iron at the fibers surface, which can catalyze the gasification of carbon. The presence of iron on the VGCF has been evidenced for the first time by the time-of-flight secondary ion mass spectrometry technique. The cleansing of the fibers surface with concentrated hydrochloric acid results in the removal of the iron and leads, after CO2 activation, to an improvement of the BET surface area. The use of nitric acid or plasma as oxidation agents does not affect significantly the surface morphology of the fibers, but greatly increases the number of surface oxygen functions

Strong electron-phonon coupling from thermal conductivity measurements in a YBa2Cu3O7-type superconducting compound

Abstract Measurements on the temperature variation of the thermal conductivity, thermopower and electrical resistivity of a superconducting Y-Ba-Cu-O compound are reported. The results, which are qualitatively interpreted in terms of the ordinary electronic and phonon transport theory applied to superconducting metallic alloys, suggest that the strong electron-phonon coupling should be considered in the interpretation of superconductivity in these materials.

Apparatus for thermal conductivity measurements on thin fibres

A sample holder for the measurement of the low-temperature thermal conductivity of very thin samples is described. The method is based on a potentiometric principle. Some constructional details and typical results are also given.