Showing papers by "François Béguin published in 1994"

Polyether-modified fullerenes

Abstract: Polyether-modified fullerenes have been prepared under mild conditions by reacting C60 or C70 in toluene with a precursor poly(oxyethylene) or poly(oxypropylene) polymer possessing amino end-groups. In the early stages of the interaction, soluble products are formed. Later, cross-linking proceeds between the polyfunctional fullerenes and the bifunctional polyethers. The non-crosslinked fullerene derivatives are soluble both in toluene and water; by evaporation of toluene, very adhesive films are formed.

Method of obtaining a sialon-based ceramic material by reducing an aluminosilicate material, and use thereof in forming a ceramic coating on a refractory substrate

Abstract: A ceramic material based on Sialon, in particular on β'-Sialon is obtained by reducing an aluminosilicate precursor by means of a gas phase comprising a mixture of hydrogen and nitrogen doped with a gaseous carbon compound. The gaseous carbon compound is a gas containing carbon in combined form, e.g. an alkane, and its concentration by volume in the gas is less than 3%. The method is particularly suitable for forming Sialon-based coatings on substrates that are solid or fibrous. For composite materials constituted by a fiber preform densified by means of a matrix, the method can be used to form an interphase coating on the fibers of the preform prior to densifying the preform, or the method can be used to form a matrix that is based on Sialon.

Influence of methane on the nitriding gas reduction of kaolinite

Abstract: A new synthesis of β′-SiAION via the reduction and nitridation of aluminosilicates in the presence of a carbon source by a hydrogen–nitrogen gas mixture is presented.The formation of β′-SiAION on surfaces (SiC, alumina) devoid of carbon was found to be impossible. Thermogravimetric analysis and mass spectrometry showed that, in a carbon crucible, methane was formed from the reaction of hydrogen with carbon. β′-SiAION was obtained in the absence of free carbon by adding a small amount of methane to the hydrogen-nitrogen gaseous mixture. The formation of β′-SiAION is thus the consequence of reduction by methane rather than by hydrogen. Parasitic reactions of hydrogen lead to silicon elimination as SiO in the gas phase, and to a Si/Al ratio lower than in the starting kaolinite.

Reduction of the second stage FeCl4- and FeCl3 graphite intercalation compounds

Abstract: FeCl4 − graphite intercalation compounds (GIC) were obtained by anodic oxidation of graphite in an aqueous solution and were subsequently submitted to galvanostatic reduction in a: Li / LiClO4 1M in propylene carbonate/GIC cell. The intensiostatic and structural data were compared to those obtained on partly reduced 2nd stage FeCl3 GIC. Whereas a “one step reduction” is observed with the FeCl3 GIC, the reduction of the 2nd stage FeCl4 GIC takes place with at least four well defined plateaus. The beginning of the last step corresponds to a stable 6 th stage structure. Increasing the charge passed through the samples leads to the exfoliation of the graphitic lattice and to the formation of metallic iron, exhibited by important ferromagnetic susceptibility.

A new synthesis of β’-SiAION using the vapor phase technique reduction of kaolin

Abstract: Silicon-aluminum oxynitrides and/or aluminum nitride were prepared by the reduction-nitridation of kaolin in graphite crucibles under hydrogen/nitrogen flow at temperatures in the range 1100 °C–1450 °C. Almost pure β'-SiAION was obtained in less than 24 h at 1200 °C. At high temperatures (e.g., 1450 °C) and for long reaction times (e.g., 10 h), β'-SiAION is fully reduced to AlN. In most preparations, β'-SiAION is formed together with small amounts of AlN. However, the formation of AlN can be limited by using short reaction times and/or by adjusting the reducing power of the atmosphere, i.e., the N 2 /H 2 ratio. Compared to the carboreduction of aluminosilicates which always yields mixtures, the present method leads to almost pure products under appropriate conditions. The temperature for the formation of β'-SiAION is at least 200 to 400 °C lower than the temperatures used for the carboreduction of aluminosilicates or sintering of powders, respectively. A gaseous species formed by the reaction of hydrogen with graphite is suspected to be responsible for the nitriding reduction of kaolinite.

Low Temperature Structures of Stage 2 Cesium Graphitides with Various Compositions

Abstract: Compounds with the compositions CsC 22 , CsC 24 , and CsC 2 appears as an almost pure phase, whereas CsC 22 and CsC 28 contain respectively a trace of 1 st stage CsC 8 , and a relatively large, amount of 3 rd stage phase. Fourier analysis of the 00l reflexions shows that the composition range for the 2nd stage compound seems to be very limited, and close to CsC 24 . From the 00l scans realized in the range 300 K-77 K, a first order transition was observed for the 2nd stage phase at about 155 K, giving two new 2nd stage phases, respectively with Ic=9.33 A (main phase), and Ic=9.58 A

Sulphur Contaminated Fullerenes

Abstract: We report on the presence of sulphur in a commercial C60/C70 mixture and in C60 separated by column chromatography on activated carbon. By heating under dynamic vacuum, ca. 2 wt% sulphur was eliminated from the mixture and ca. 4 wt% from C60.

Process for obtaining a sialon containing ceramic material by reduction of an aluminosilicate precursor and use thereof for applying a ceramic coating onto a refractory substrate.

Abstract: of EP0611180A ceramic material based on Sialon, especially ss'-Sialon, is obtained by reduction of an aluminosilicate precursor with a gaseous phase comprising a mixture of hydrogen and of nitrogen doped with a carbon compound. The carbon compound is a gas containing carbon in combined form, for example an alkane, and its proportion by volume in the gaseous phase is lower than 3 %. The process is especially suitable for forming Sialon-based coatings on bulk or fibrous substrates. In the case of composite materials consisting of a fibrous preform densified with a matrix, the process makes it possible to form an interphase coating on the fibres of the preform before the latter is densified, or else to form a Sialon-based matrix.

Transport-properties of a Graphitized Polyimide Film and its Stage-2 Fecl4- Intercalation Compound

Abstract: The temperature variation of the thermal conductivity, the electrical resistivity and the thermoelectric power of a graphitized polyimide film has been measured in the temperature range 2 < T < 300 K. The effect of electrochemical intercalation with FeCl4- ions has also been studied. The thermal conductivity results confirm the high degree of graphitization that may be obtained with polyimide films. It shows how intercalation increases the structural disorder and how the intercalate substantially contributes to the thermal conductivity at low temperatures. The electrical-resistivity and thermoelectric-power measurements reveal that the density of free carriers is about three times lower in stage-2 FeCl4- solvated intercalation compounds obtained by an electrochemical way than in stage-2 FeCl3 compounds synthetized by a classical method.