Jean-François Marêché

Superconductivity of Bulk CaC 6

We have obtained bulk samples of the graphite intercalation compound, CaC6, by a novel method of synthesis from highly oriented pyrolytic graphite. The crystal structure has been completely determined showing that it is the only member of the MC6, metal-graphite compounds that has rhombohedral symmetry. We have clearly shown the occurrence of superconductivity in the bulk sample at 11.5 K, using magnetization measurements.

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 ...

Intercalation of sodium with its halides into graphite

Abstract Ternary GICs containing sodium associated with chloride, bromide and iodide ions have been synthesized by heating graphite with liquid sodium in the presence of NaCl, NaBr, and NaI in solid or liquid state. Similar ternary compounds could not be obtained with NaF, but quaternary phases G–Na–I–F and G–Na–Cl–F have been synthesized.

Intercalation into graphite of sulphur or selenium with potassium

New first stage ternary graphite-potassium-chalcogen compounds were synthesized. They are very rich in alkali metal : their chemical formulas are close to KC3AX (A = sulphur or selenium). We studie...

Superconductivity in Li3Ca2C6 intercalated graphite

In this paper, we report the discovery of superconductivity in Li{sub 3}Ca{sub 2}C{sub 6}. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors [T. Enoki, S. Masatsugu, E. Morinobu, Graphite Intercalation Compounds and Applications, Oxford University Press, Oxford, 2003]. It is probably not astonishing, since it is generally admitted that low dimensionality promotes high superconducting transition temperatures. Superconductivity is lacking in pristine graphite, but after charging the graphene planes by intercalation, its electronic properties change considerably and superconducting behaviour can appear. Li{sub 3}Ca{sub 2}C{sub 6} is a ternary GIC [S. Pruvost, C. Herold, A. Herold, P. Lagrange, Eur. J. Inorg. Chem. 8 (2004) 1661-1667], for which the intercalated sheets are very thick and poly layered (five lithium layers and two calcium ones). It contains a great amount of metal (five metallic atoms for six carbon ones). Its critical temperature of 11.15 K is very close to that of CaC{sub 6} GIC [T.E. Weller, M. Ellerby, S.S. Saxena, R.P. Smith, N.T. Skipper, Nat. Phys. 1 (2005) 39-41; N. Emery, C. Herold, M. dAstuto, V. Garcia, Ch. Bellin, J.F. Mareche, P. Lagrange, G. Loupias, Phys. Rev. Lett. 95 (2005) 087003] (11.5 K). Both CaC{sub 6} and Li{sub 3}Ca{submorexa0» 2}C{sub 6} GICs possess currently the highest transition temperatures among all the GICs.«xa0less

Effects of coal moisture content on carbon deposition in coke ovens

Abstract The formation of carbon deposits from coal pyrolysis in a two-stage reactor was studied at a constant cracking temperature of 900°C for several moisture contents of the coal charge ranging from 0.8 to 15 wt%. Whatever the coal moisture content, the formation of the deposits, the mass of which was continuously monitored during a pyrolysis run by a gravimetric balance system, was characterized by two distinct stages. The average deposition rate decreased progressively with increasing moisture content up to 8 wt%, then increased sharply beyond 8 wt%. Deposits observed under the polarized light microscope presented two different structures depending on the coal moisture content. For contents

Pyrolysis of coal tar pitch mixed in the presence of a graphite intercalation compound : a kinetic study

Abstract A kinetic study has been carried out to determine the influence of the addition of first stage FeCl 3 -graphite intercalation compound (GIC) to coal tar pitch (CTP) on the formation and the development of the mesophase. The mesophase formation was followed by the determination of the tetrahydrofuran insoluble fraction (THFI) of the solid residues, which were further characterized by polarized light microscopy. The presence of the Lewis acid (desorbed from the graphite layers) has a great influence during the mesophase spherule formation and development. The activation energy of the formation and the development of the mesophase (E a ) is equal to 159 and 204 kJ/mol in the case of the CTP-GIC 8% mixture and the CTP respectively. This corresponds to a decrease of 22% of E a when the CTP is mixed with the GIC. The authors want to highlight the difficulties and mainly the limitations of the method used to calculate this activation energy by THFI measurements. However, we assume that the mesophase formation and growth rates are accelerated in the presence of compounds such as GIC for two reasons: the absence of QI particles on the surface of the mesophase spherules and the catalytic activity of the Lewis acid.

Pyrolysis of coal tar pitch with graphite-MoCl5 intercalation compound

Coal tar pitch (CTP) was pyrolysed in the presence of graphite-MoCl5 intercalation compound (MoCl5-GIC). Only a third-stage GIC could be obtained using the classical two-temperature method. GIC concentrations in the starting mixtures were 2, 5 and 10 wt%. The samples were characterized by elemental analysis, X-ray diffraction and scanning electron microscopy. The distribution of GIC particles in the CTP matrix is homogeneous and their presence leads to a significant increase in green coke yield on pyrolysis, in agreement with many published results. The influence of heating rate on the green coke yield was also studied: the lower the heating rate, the higher the yield. After thermal treatment at 750°C, molybdenum is always present in the green coke at a content close to the initial one, whereas most of the chlorine has been outgassed. It appears that CTP-MoCl5-GIC mixtures have a behaviour similar to that of CTP-FeCl3-GIC mixtures on pyrolysis. The final aim of this work is to synthesize catalysts supported on carbonaceous materials.

On the relevance of certain transport-structure correlations in SbCl5-intercalated graphite to our overall understanding of GIC c axis conductivity

This paper presents some new data on the SbCl5-intercalated graphite family as concerns the relationships between c axis resistivity, ρc(T,p), and intercalate layer structure. Results on the influence of intercalate layer crystallization and the nature of the host graphite are discussed and compared with those on other GIC families. Certain data are examined in the light of available theories and such an analysis raises a number of questions.

Electromigration of Hydrogen and Deuterium in Titanium

The general principle of these experiments follows the earlier study of C. Wagner and G. Heller [1], but several improvements have been made: First, the horizontal capillary with its droplet of bromonaphthalene and the travelling microscope were changed for an oil manometer and a cathetometer. Secondly the sealing of the capillary on the wire is made in situ, this allowing the study of most reactive metals and good temperature control during the sealing. Two copper wires connected to the specimen permit the flow of a direct current, which causes movement of the dissolved hydrogen, and the differential pressure can be read on the manometer. In the past we have worked at high temperatures (i.e. 800°C), and we had serious problems due to contamination of titanium by the oxygen of the glasses [2], We have obtained better results working with 747 glass in the 550