Superconductivity in Li3Ca2C6 intercalated graphite

In this letter, we report the discovery of superconductivity in Li3Ca2C6. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors. 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. Li3Ca2C6 is a ternary GIC, 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 CaC6 GIC (11.5 K). Both CaC6 and Li3Ca2C6 GICs possess currently the highest transition temperatures among all the GICs.Comment: 5 pages, 3 figure

Nouvelles données sur l’électromigration de l’hydrogène dans le nickel

Les quelques expériences d'électromigration de l’hydrogène dans le nickel décrites dans la littérature ont donné des résultats très discordants. Nos mesures montrent que l’état du métal influe de façon considérable sur la valence efficace. De nouvelles études devront préciser l’influence des taux de défauts du métal sur l’électromigration

Étude des systèmes hydrogène-métaux: données expérimentales sur la solubilité et l’électromigration des isotopes de l’hydrogène dans les métaux de la colonne V

Cette étude apporte un certain nombre de résultats concernant la solubilité et l’électromigration de l’hydrogène dans les métaux 5A. Les valeurs de solubilités données par la littérature se trouvent confirmées et l’absence d'effet isotopique établi. Sous l'action d’un courant continu les atomes en insertion migrent en sens inverse des électrons. Cette migration a fait l’objet d'une étude quantitative dans tout l’intervalle accessible; le rôle de la contamination du métal par l’oxygène a été abordé

Étude des systèmes hydrogène-métal : potentiel induit par la diffusion de l’hydrogène dans le palladium

La mesure directe du potentiel associé au mouvement les atomes d’hydrogène diffusant sous l’effet d’un gradient le concentration dans le métal permet d'en évaluer la valence efficace. Sa valeur est comparée à celles obtenues à partir te mesures d’électromigration

Recherches sur les systèmes hydrogène-métal : données expérimentales sur la solubilité et l'électromigration de l'hydrogène et du deutérium dans les alliages palladium-argent

L'action des isotopes stables de l'hydrogène a été étudiée sur des alliages de palladium et d'argent de composition comprise entre 0 et 50 % d'argent, dans un large domaine de température et de pression limité par les conditions d'existence des phases α ou solutions solides. Les valeurs de la solubilité ont pu être établies ou précisées. Le calcul des enthalpies correspondantes met en évidence, pour beaucoup d'alliages, une transition vers 350 °C.Sous l'action d'un courant continu, les atomes d'hydrogène ou de deutérium en insertion se déplacent en sens inverse des électrons comme dans le palladium pur. La comparaison de leurs mobilités déduites des mesures d'électromigrations avec celles déduites des coefficients de diffusion thermique conduit pour leur charge apparente à des valeurs positives comprises entre 0,5 et 1 unité élémentaire

Analogies and differences between calcium-based and europium-based graphite intercalation compounds

International audienceAn original method using lithium-based liquid alloys has been developed, allowing studies on bulk graphite intercalation compounds with calcium and europium. We showed that binary and ternary compounds belonging to graphite-lithium-calcium and graphite-lithium-europium systems are synthesized in both cases for equivalent reaction conditions but amazingly with many different structural and physical properties. Concerning CaC6 and EuC6, even if their 2D unit cells are hexal, their c-axis stacking sequences lead to different symmetries. Regarding kinetical data, formation mechanisms of these graphite intercalation compounds appear comparable however different, with a first common step before differenciation in the intercalation mechanism. Obviously, their physical properties are strongly different due to the nature of the intercalated metallic element. So, the different ternary compounds from these systems also show very considerable differences concerning their electronic properties: complex magnetic ordering for Eu-based ternary GIC and superconducting behaviour for Ca-based ternary GIC. However, common points are highlighted

Reduction of MoCl5\bf MoCl_5 Graphite Intercalation Compounds by Heavy Alkali Metal Vapour

The reduction of $\rm MoCl_5$ graphite intercalation compounds by heavy alkali metal vapour (potassium, rubidium and cesium) leads to the formation of complex products. The analysis of these reduced compounds indeed shows that they contain alkali metal halide inclusions and also transition metal in several two-dimensional and three-dimensional phases depending on the reducing agent.
La réduction des composés d'insertion du graphite avec le chlorure de molybdène V par les métaux alcalins lourds (potassium, rubidium et césium) en phase vapeur conduit à la formation de produits complexes. L'étude de ces produits réduits par Microscopie Électronique à Transmission montre en effet qu'ils contiennent des inclusions de chlorures alcalins mais également des phases du métal de transition présentant plusieurs organisations bidimensionnelles et tridimensionnelles selon la nature du réducteur

Soft Oxidation of Single-Walled Carbon Nanotube Samples

International audienceThermal dry oxidation treatment is a straightforward and commonly used technique in carbon nanotube (CNT) processing. Among its advantages, it plays a key role in guaranteeing the efficiency of numerous purification procedures. It also enables a chemical modification of the nanotube surface by covalent bonding of oxygen-containing functions. Nevertheless, a careful optimization of the experimental conditions used for the treatment is a crucial step in order to obtain the desired effect and avoid a strong damaging of the CNTs. Soft oxidation conditions are thus in particular of great interest. We report on the oxidation of CNTs in a large scale of oxidation conditions. The treated samples were characterized using several techniques including transmission electron microscopy, thermal gravimetric analysis, dispersion tests, and Raman spectroscopy to investigate both morphological and surface properties. We were thus able to discriminate between the behavior of the CNTs from that of other carbonaceous species. The problem of the selectivity of the oxidation treatment is especially discussed

Gold nano-sheets intercalated between graphene planes

International audienceThe lamellar structure of graphite is known for allowing intercalation of numerous chemical species between its graphene planes. Considering the intercalation of the electron donors, some pure metals, several metallic alloys and even electronegative elements associated with electropositive metal lead to binary or ternary graphite intercalation compounds. In all cases, the presence of an alkali metal is essential in order to open the van der Waals's intervals, what is performed in this study dealing with the graphite-potassium-gold original system. Gold is very particular since it is a true metal but strongly electronegative too. Associated with potassium, it becomes able to intercalate easily into graphite, leading to a first stage graphite-potassium-gold compound. This study displays the best synthesis conditions to prepare this pure ternary compound. The study of its 001 reflections leads to determine the repeat distance (1311 pm) with a five-layered stacking sequence of the intercalated sheet along the c-axis. Two potassium mono-layers surround a three-layered gold nanosheet, so that the c-axis expansion reaches 237%. A chemical formula K1.30Au1.50C4, namely 0.7 metallic atom for one carbon atom, has been determined for this compound. A discussion concerning the charge transfer between graphene planes, potassium and gold atoms is also proposed