MATÉRIAUX CARBONES OBTENUS PAR DÉPÔT CHIMIQUE EN PHASE VAPEUR ASSISTÉ PAR UN PLASMA RÉACTIF

Nous avons fabriqué des matériaux carbonés par la technique du dépôt chimique en phase vapeur assisté par plasma. On obtient ainsi des matrices de carbones métastables aux propriétés physiques différentes, comparées à celles des phases graphitiques habituelles. Dans ce but, nous avons construit un dispositif expérimental comportant un plasma radiofréquence inductif ajouté à un four CVD basse pression standard. Il est possible de préparer une matrice contenant un mélange de carbone et d'hydrogène pour des paramètres bien définis et ajustables, le carbone étant essentiellement en hybridation sp2. Les cinétiques de densification de différents substrats poreux ont été observés ainsi que le gradient d'infiltration sur les échantillons composites finaux. Ce dernier point montre la compétition entre la réaction chimique et la diffusion thermique qui doit être reconsidérée pour de nouvelles conditions expérimentales plus contraignantes.We have used a plasma enhanced chemical vapour deposition technique for making carbon materials. The purpose of this technique is to prepare new metastable carbon matrices with different physical properties, compared to the classical graphitic phases. We have therefore built an experimental set-up with an inductively coupled plasma combined with a standard low pressure CVD furnace. Under adjustable and defined constraints we are able to prepare a matrix which is a mixture of carbon and hydrogen, where most of the carbons are under the sp2 hybridization. The kinetics of densification of different porous substrates have been examined and the infiltration gradient is observed on the final composite samples. This last point emphasizes the competition between the chemical reaction and the thermal diffusion which has to be reconsidered under these new more drastic experimental conditions

Multiwalled-carbon-nanotube-based carbon foams

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Matériaux composites carbone carbone conditions de fabrication par dépôt chimique en phase vapeur microstructures et propriétés physiques

Les conditions expérimentales du dépôt chimique en phase vapeur de Carbone sur un substrat fibreux, conduisant à un composite Carbone/Carbone, sont systématiquement étudiées en fonction des contraintes : température de dépôt, débit et taux de méthane, taux initial de fibres du substrat. Les différentes microstructures de pyrocarbone (laminaire rugueux, laminaire lisse, isotrope) sont caractérisées par microscopie optique et microscopie électronique en transmission, densité de grain et mesures magnétiques (susceptibilité et anisotropie). En choisissant des couples de contraintes, on a établi plusieurs coupes du diagramme d'existence de chaque microstructure. Enfin, une évaluation des caractéristiques mécaniques en flexion “trois points” conduit à une tentative de corrélation entre la résistance mécanique, la microstructure de la matrice de pyrocarbone et les conditions de fabrication du matériau composite

A comparison between Raman spectroscopy and surface characterizations of multiwall carbon nanotubes

The distribution of graphene units with an axial symmetry gives rise to different types of carbon filaments: nanotubes, nanofilaments and classical fibers. In this work the surfaces of different multiwalled nanotubes are characteirzed by two complementary techniques: chemical ones based on Total Surface Area and Active Surface Area measurements, associated with a physical approach the Raman scattering spectroscopy. From analysis of Raman data we deduce the values of the in-plane coherence lengths, identified as L1 the planar projection of gaphene sheets, and we propose an analysis for the observed line-width bahavior related to the graphitation step. From coherence length L1 for all types of MWNT. This analysis allows us to show the influence of both, the structural organization and the different treatments on the interfacial characteristics of these nanocarbons

Relationship Between the Structural Organization and the Physical-properties of Pecvd Nitrogenated Carbons

By a Plasma Enhanced Chemical Vapor Deposition process (PECVD), we are able to prepare nitrogenated amorphous carbon materials around room temperature from methane and nitrogen gas as precursors. We have also used chlorine gas as an additive to reduce the hydrogen content of our samples. Starting from the ''as-deposited'' materials, we have investigated their thermal stability by successive heat treatments up to 1400-degrees-C. These compounds suffer a weight loss mostly due to the hydrogen departure. They become nonfusible and it turns out that nitrogen, chemically bound to sp2 hybridized carbons, induces some changes in the physical properties. In order to understand the relationship between the local structural organization and the physical characteristics, we have investigated different spectroscopic techniques such as Nuclear Magnetic Resonance, IR Absorption, and X-ray Photoelectron Spectroscopy. We have also investigated several transport properties: (i) The dc electrical conductivity shows a kind of metal/insulator transition around 700-degrees-C. The temperature dependence for the conductive samples gives evidence for a pseudogap associated with the presence of localized states. (ii) The thermal conductivity exhibits, for the as-deposited compound, a very low value varying slowly with temperature; its magnitude as well as its temperature dependence, characteristic of noncrystalline materials, are modified by the annealing process. Finally, an electronic band model is proposed, explaining the structural evolution through a kind of Mott-Anderson pseudotransition