Claire Hérold

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.

KC4, A New Graphite Intercalation Compound

Abstract New stage 1 graphite-potassium intercalation compounds have been synthesized. They are very rich in metal, as each intercalated sheet consists of two superimposed potassium planes. The chemical formula is close to KC4. Five different phases were observed. They were studied by X-Ray diffraction. Electrical measurements were carried out on two phases.

A new graphite intercalation compound containing sodium associated with oxygen

Abstract A new second stage blue phase of donor-type with an interplanar distance of 745 pm has been synthesized by reaction of graphite with partly oxidized sodium. It contains oxygen in form of peroxide ions.

On the great difficulty of intercalating lithium with a second element into graphite

Abstract Lithium is able to intercalate into graphite leading to various binary graphite intercalation compounds, that are well defined by their stage. Concerning the ternaries, there is little literature on the subject. Thermodynamical and structural data, that differ largely from those of the other alkali metals, lead one to foresee some serious difficulties in synthesising such ternary compounds. Many experiments have attempted to synthesise ternary graphite intercalation compounds with lithium, using successively very electronegative elements, then fairly electronegative species and lastly electropositive metals. Numerous results, that are wholly negative, are described in this paper. The calcium–lithium system only allows one to prepare a novel intercalation compound, that is a first stage ternary phase exhibiting a large interplanar distance. This latter suggests that the intercalated sheets consist of several superimposed atomic layers. The synthesis of this ternary is not easy, because it needs reagents of very high purity. It possesses the brightness of metals and its strong hardness is very unusual among graphite intercalation compounds. On the other hand, the charge transfer between the graphene planes and the intercalated sheets, that just allows the intercalation, is especially high, and much higher than the LiC 6 compound.

Selective removal of metal impurities from single walled carbon nanotube samples

Large scale production of high quality CNT samples is still challenging. The presence of structural defects and metallic particles in pristine single walled carbon nanotubes (SWNTs) is responsible for the alteration of both their chemical stability and their magnetic and electrical properties. The commonly used purification procedures are based on multi-step treatments that are often too aggressive towards the CNTs, leading to disappointing yields. Here, we propose an alternative process that allows preparing high-quality and high-purity SWNT samples. The proposed process merely consists of heating up SWNT powder under high chlorine partial pressure and high temperature. These thermodynamic conditions favor high chlorine diffusion to metal impurities embedded in carbon shells thus inducing an avalanche process of metal chloride formation and sublimation. The purified samples have been characterized by transmission electron microscopy, thermogravimetric analysis, magnetic measurements and Raman spectroscopy. We show that the developed process combines selective elimination of catalytic impurities and high yields. More importantly, we show that this process preserves the quality of the resulting purified nanotubes.

Why mono- or poly-layered intercalated sheets in graphite-electron donors systems ?

Abstract Intercalation into graphite of electron donors is discussed, regarding the nature of the chemical species to be intercalated. In the binary compounds, the intercalated sheets are always mono-layered, while, in the ternaries, they are poly-layered, as soon as the electronegativities of both intercalated elements exhibit a sizeable difference. In the case of the ternary compounds, the intercalated sheets can be regarded as 2D slices of the corresponding bulk binary compounds. These slices are subjected to various distortions, that are caused by the presence of the adjacent graphene planes

Processing Carbon Nanotubes

Due to their combined superior chemical and physical properties, carbon nanotubes (CNTs) are recognized to have a huge potential in many fields of applications (Ajayan, 1999; Rao et al., 2001; Dai, 2002; Van Noorden, 2011). These molecular-scale tubes of graphitic carbon are one of the stiffest and strongest fibers known. Besides, they have remarkable electronic, optical, thermal and chemical properties. For these reasons their interest in both academic and industrial areas is unique. Nevertheless, the as-produced material is extremely difficult to process. Development of CNT-based devices or composites of interest for new applications has been consequently hindered. CNTs are hydrophobic and incompatible with a majority of solvents, including monomers and polymers; they indeed have a high tendency to agglomerate. Moreover, CNTs and especially single-walled carbon nanotubes (SWNTs) are assembled in bundles of generally several tens of tubes. Development of efficient processes and chemical treatments that are able to control the quality of the CNT samples and to induce both their dispersion and partial or complete debundling remains highly challenging. CNTs can be produced using different methods that basically consist in heating carboncontaining solid or gas. On the contrary to the preparation of multi-walled carbon nanotubes (MWNTs), SWNT growing requires a metal catalyst. The characteristics of the samples depend on the control and the choice of the experimental parameters used for the synthesis. A better understanding of the growth mechanisms has permitted the development of mass production processes (Grobert, 2007). Nevertheless, their uniformity (length, diameter, chirality), the quality of their walls (number of defects) and also their purity are still partially controlled. The quality of the samples has to be improved in order to benefit of the exceptional properties of CNTs in new materials. Depending on the type and the synthesis method, the CNTs can differently behave through the applied chemical treatments. Whatever the synthesis method, CNT samples persistently contain several kinds of heterogeneities: (i) carbonaceous species like fullerenes, amorphous carbon, graphitic and carbon particles, ...; (ii) impurities such as residual metallic catalyst often protected by more or less graphitized carbon shells or polyhedra; (iii) defects at the CNT surface or oxygenated grafted functions, (iv) dispersion in diameter, chirality and morphology (aspect ratio) and (v) aggregation into bundles. These heterogeneities represent a major obstacle for both the establishment of universal behaviors and the development of efficient processing methods. Nano-scaled particles exhibit an enormous surface area being of several orders of magnitude larger than that of conventional fibers. This surface area can potentially act as a

Toward the control of graphenic foams

Abstract Graphene-based materials are extensively studied, due to their excellent properties and their wide range of possible applications. Attention has recently been paid to three-dimensional-like graphenic structures, such as crumpled graphene sheets and graphenic foams: these kinds of materials can combine the properties of graphene associating high surface area and porosity, what is particularly interesting for energy or catalysis applications. Most of the synthesis methods leading to such structures are based on graphite oxide exfoliation and re-assembly, but in this work we focus on the preparation of graphenic foams by a solvothermal-based process. We performed a solvothermal reaction between ethanol and sodium at 220°C, during 72 h, under 200 bar, followed by a pyrolysis under nitrogen flow. An extended study of the influence of the temperature (800°C–900°C) of pyrolysis evidences an unexpected strong effect of this parameter on the characteristics of the materials. The optimal conditions provide multi-layer graphene (10 layers) foam with a surface area of 2000 m2·g−1. This work is an important step for the understanding of the mechanisms of the thermal treatment. Post-treatments in different experimental conditions are performed in order to modulate the structure and properties of the graphenic foams.

On the role of dihydrogen in the co-intercalation reactions into graphite of potassium and chalcogen

Abstract The influence of dihydrogen gas on the reactions between graphite and liquid potassium containing a very small amount of a chalcogen (O, S, Se, Te) was studied. The reactions were carried out under a pure argon atmosphere in a stainless steel reactor, between 400 and 600°C. Controlled amounts of dihydrogen gas can be added in this reactor. When dihydrogen is strictly absent, the co-intercalation of potassium and chalcogen does not take place at 400°C: only potassium intercalates, leading to the KC8 binary compound. The same experiments carried out with controlled amounts of dihydrogen at the same temperature lead to various ternary compounds with oxygen, sulphur, selenium and tellurium. However, at 600°C, and strictly without dihydrogen, co-intercalation occurs, but only for S, Se and Te, allowing the preparation of new well-defined ternary graphite intercalation compounds. The co-intercalation of potassium and oxygen is possible only in the presence of dihydrogen, at any temperature.

Momentum density of electrons in CsRb2C60, versus temperature

Abstract The electronic momentum density in C60 and CsRb2C60 are measured as a function of temperature from below Tc to room temperatue. The effect of the superconducting transition on the momentum distribution was found to be minimal and the effect of intercation appears to agree with a rigid band picture.