Edward McRae

RAMAN SPECTROSCOPIC STUDIES ON WELL-DEFINED CARBONACEOUS MATERIALS OF STRONG TWO-DIMENSIONAL CHARACTER

Using 514 nm radiation and a careful experimental protocol allowing us to make quantitative intensity analyses, we have examined the first- and second-order Raman spectra of five varieties of graphite. These include single-crystal graphite and highly oriented pyrolytic graphite as references, then two with a significant content of c-axis translation faults introduced through grinding and exfoliation, and a last sample which is purely turbostratic. We show that full c-axis stacking disorder results in a strong increase of the scattered E2g-mode intensity. We have further shown that even in those strongly disordered stacking sequences, but in which the AB sequence persists over only a few layers, the doublet around 2700 cm−1 is resolved. We argue against assigning the peak around 1355 cm−1 to “disorder,” as is so often done, but to the finite in-plane domain size as proposed a number of years ago; moreover, we suggest that this peak is a more sensitive probe of such small domains than is x-ray diffraction.

Composites based on micron-sized exfoliated graphite particles: Electrical conduction, critical exponents and anisotropy

Abstract Electrically conducting composites, made of epoxy resin and micron-sized graphite platelets with diameter-to-thickness ratios of the order of 100 are presented. Samples of various volume percentages of graphite particles are characterized by X-ray diffraction and electron microscopy: orientation of the filler and porosity of the polymer matrix are observed. DC conductivity measurements have been made both within and perpendicular to the plane of the film. Use of percolation theory shows that taking into account the swelling effects of the composite occurring during reticulation yields quasi-universal values for the conductivity critical exponent in both directions and a low critical volume concentration of Φ c = 1.3%.

Conduction mechanisms in some graphite - polymer composites: the effect of a direct-current electric field

This paper deals with the possible conduction mechanisms in highly anisotropic composites comprising 0 - 10 vol% graphite flakes within a polymer host. Conductivity measurements as a function of DC electric field have been made. In most cases, a non-linear behaviour of the current - voltage relationships is observed. A number of theoretical models are considered and we show that none of them is, by itself, able to explain our results. We further develop several arguments which lead us to consider the existence of a combined tunnelling effect and ionic conduction mechanism.

Conduction mechanisms in some graphite–polymer composites: Effects of temperature and hydrostatic pressure

This article is devoted to the conduction mechanisms involved in some highly electrically anisotropic resin–graphite particle composites. These materials are known to show a percolation phenomenon ...

Classification for double-walled carbon nanotubes

A classification method for double-walled carbon nanotubes is proposed taking account of the geometric constraints. The different possibilities of interlayer interactions are examined.

Synthesis and electrical properties of some new ternary graphite intercalation compounds

Abstract New ternary graphite intercalation compounds (GICs) of general formula MBi x C 4n (M=K, Rb, Cs; x∼-0.6 ; n=stage) have been synthesized. The electrical resistivity, both parallel (π a ) and perpendicular (π c ) to the basal planes has been studied from 1.4 to 295 K. All in-plane behaviour is metallic ; the c axis conductivity is thermally activated except for the richest compounds. The cesium compounds are superconducting.

OPENING OF SINGLE-WALLED CARBON NANOTUBES: EVIDENCE GIVEN BY KRYPTON AND XENON ADSORPTION

Adsorption isotherms of krypton between 77 and 93 K and xenon between 110 and 120 K on mechanically-opened single-walled carbon nanotubes (SWNTs) have been measured. The comparison of the results with those obtained under the same conditions on closed nanotubes is discussed. Evidence of adsorption inside the tubes is given through the appearance of an additional adsorbed amount at very low relative pressure on the isotherms. Different orderings are predicted for xenon and krypton, based on their respective adsorbed amounts inside the nanotubes, and on geometrical considerations. Finally, the fraction of opened nanotubes accessible to xenon and krypton was estimated, as well as the impurity content originating from the cutting process.

Anisotropic percolation in an epoxy - graphite disc composite

Abstract Composite comprising φ = 0.5 to 9 volume % of micron-sized graphite platelets with diameter to thickness ratios of the order of 100 have been made and deposited in the form of partially oriented films. Room temperature electrical conductivity measurements both parallel and perpendicular to the plane of deposition reveal a low threshold of φ ∗ = 0.8 ± 0.1 vol. % . The conductivity critical exponents are anisotropic with values around 7 and 10. While isotropic at the threshold, the composite system attains an anisotropy near 105 at the highest concentration limit.

Image analysis characterization of multi-walled carbon nanotubes

An original image analysis method is presented to characterize multi-walled carbon nanotubes from transmission electron microscopy images. The analysis is performed in three steps: (i) image preprocessing in order to isolate the nanotubes from the background, (ii) image segmentation, aiming at keeping only the measurable sections of nanotubes, and finally (iii) tube characteristics measurement. The measurement is based on a Lambert-like electron absorption law and is performed on the original gray level image itself. Two geometrical and one physical characteristics are determined for each tube, namely, its outer and inner radius and a linear electron absorption coefficient. The method is illustrated by comparing a pristine and an annealed carbon nanotube samples. The compaction of the tube walls during annealing is shown to result from a lowering of the external radius while the inner radius is left unchanged.

Insertion du chlore dans le graphite

Abstract The action of liquid chlorine on graphite has been studied by gravimetric, dilatometric and radiocrystallographic means. Around −20°C and according to the nature of the graphite, the compounds C12Cl and C16Cl corresponding to stages 3 and 4 respectively, were obtained. The interplanar distance is practically double that of pure graphite or 6.70 A. The intercalation involves a decrease in electrical resistivity in the direction parallel to the layers comparable to that observed for bromine. It was not possible to prepare compounds richer in chlorine by cooling to −40 and −78°C.