Ce que le jour doit à la nuit

National audiencePour beaucoup de peintres, l'opposition du noir et du blanc suffit à dire l'essentiel. En observant le carré noir sur fond blanc de Malevitch, nous pouvons comprendre cette œuvre comme une image sacrée, une représentation iconique détachée de la terre (un pur espace). Magritte donne une autre lecture de l'opposition Noir Blanc dans son tableau l'empire des lumières qui présente une variation sur le jour et la nuit. Ce tableau est fait d'azur et de lumière noire sans aucun mélange. Ce n'est pas du clair obscur mais la juxtaposition de la clarté et de la noirceur. Malgré il reste une image crédible, habitable. En littérature, Pierre Corneille dans Le Cid en 1636 fait déjà un clin d'œil à cette opposition Noir Blanc : " Cette obscure clarté qui tombe des étoiles ". On voit à travers les interrogations que suscite l 'un des plus célèbre poème de Baudelaire la difficulté de décrire par une expression concrète une couleur

Oxidation of stainless steel powder

International audienceTo understand the corrosion behavior of a model 304L(p)-ZrO2(s) composite, a 304L stainless steel powder was studied under oxygen at high temperature. Oxidation tests were performed with thermogravimetry. The so-called jumps method, which involves a sudden change of the temperature, was also applied to propose a kinetic model. Two periods with different rate-determining steps could be distinguished for short (<12 h) and long time experiments (12-20 h). SEM observations of oxidized particles revealed an oxide layer structure similar to that of alloy plates of same composition: during the first ten hours period, the external scale surrounding stainless steel particles was found to be chromium oxide; for the second oxidation period, the outer oxide layer was enriched in iron. Considering the relatively short-term oxidation period, a kinetic model based on an outward growth of chromia from oxidation of Cr in solution in the spherical alloy particles was successfully compared to the experimental mass gain curve. The kp value deduced from this modeling was found to be in agreement with the literature data. The diffusion of interstitial chromium ions is the rate-determining step in agreement with the absence of influence of the oxygen partial pressure

Couplage analyse thermogravimétrique et émission acoustique pour l'étude de la corrosion haute température

National audienceAfin d'améliorer les connaissances du comportement à haute température des matériaux, le couplage de plusieurs techniques d'analyse physique in situ est une voie prometteuse. Dans ce but, des thermobalances ont été équipées d'un appareil spécifi que de mesure des signaux acoustiques émis par les échantillons dont la variation de masse est continûment mesurée sous diverses atmosphères représentatives de conditions industrielles. Les corrosions de surface à haute température sont à l'origine d'émission d'ondes acoustiques générées lors de la croissance ou de la fi ssuration de couches formées. Des mesures simultanées in situ des variations de masse et des signaux acoustiques apportent donc des informations sur ces mécanismes de corrosion haute température. L'application de cette méthodologie aux mécanismes d'oxydation, réduction ou attaque par les atmosphères carbonées (cokage) est présentée dans ce document. Une perspective de ces études est de pouvoir établir des référentiels pour une éventuelle utilisation de l'émission acoustique pour l'étude de la corrosion des équipements fonctionnant à haute température. // In order to improve the knowledge on the high temperature behaviour of materials, the coupling of in situ physical analysis is a promising way. For this purpose, thermobalances have been equipped with specific devices to measure the acoustic signals emitted by samples for which the mass variation is continuously recorded under simulated industrial atmospheres. The high temperature corrosion of materials surfaces generates acoustic emission signals associated to the growth or the cracking of the formed layers. In situ simultaneous measurements of the mass variation and the acoustic signals give information on the high temperature corrosion mechanisms involved. In this document, this methodology has been applied to the study of oxidation, reduction or attacks under carbon-rich atmosphere (coke deposition). These studies could lead to reference data for a possible application of acoustic emission for the corrosion monitoring of high temperature equipments

Zirconia matrix composite dispersed with stainless steel particles: Processing and oxidation behavior

International audienceMaterials with non uniform properties are being developed to optimize several functions of industrial components in severe atmospheres at high temperature. These composites called M(p)-CMC(s): "ceramic matrix composites dispersed with metal particles" are candidates for high-temperature structure materials as functionally graded materials (FGMs) such as intermediate components between electrolyte and interconnecting components in SOFC. Preparation of a model composite M(p)-CMC(s) is described. Powder metallurgy process is used to obtain a dense composite (98% of the theoretical density) based on yttrium stabilized zirconia for the ceramic part and on 304L stainless steel for the metallic part. The characteristics of this material as well as its behavior under oxidation at high temperature are reported

Extrapolation of DEM simulations to large time scale. Application to the mixing of powder in a conical screw mixer

International audienceThe paper proposes an original algorithm which allows a long time scale extrapolation of DEM results at a very low computational cost. This algorithm can be adapted to any periodic processes. In this study, it is applied to the mixing process of powders within a conical screw mixer. The results are then compared with long time DEM simulations. It appears that this method is able to predict the DEM results with a very good accuracy

Oxidation behaviour of unirradiated sintered UO2 pellets and powder at different oxygen partial pressures, above 350°C

International audienceThe oxidation of sintered UO2 pellets and powder into U3O8 has been studied by thermogravimetry at 370°C, under controlled oxygen partial pressures (PO2 ranging from 2-40 kPa). Sigmoidal curves of oxidation weight gain were measured for both pellet and powder test samples. The rate of oxidation increased as the oxygen partial pressure increased. It has been shown, by simultaneous TG-DSC, that the reaction proceeds in a pseudo steady state. An experimental methodology based on temperature or PO2 jumps has shown that the assumption of a rate-limiting step is validated, and a mean value of activation energy for the formation of U3O8 of 103 kJ.mol-1 was estimated

Study of zircaloy-4 cladding air degradation at high temperature

Prix de la Meilleure Communication EuropéenneInternational audienceZircaloy cladding, providing the first containment of UO2 fuel in Pressurised Water Reactors, can be exposed to air during accidental situations. This might occur during reactor operation (in case of a core meltdown accident with subsequent reactor pressure vessel breaching), under shutdown conditions with the upper head of the vessel removed, in spent fuel storage pools after accidental loss of cooling or during degraded transport situations. The fuel assemblies inadequately cooled, heat up and as a result, corrosion of Zircaloy claddings takes place. This paper is devoted to the kinetic analysis of Zy4 corroded at 850°C in 20% oxygen - 80% nitrogen partial pressure atmosphere to support the comprehension of the degradation mechanisms involved during the post-transition stage

Thermogravimetric experiments coupled with acoustic emission analysis dedicated to high-temperature corrosion studies on metallic alloys

International audienceHigh temperature corrosion of metallic alloys in industrial equipments, such as refinery and petrochemical equipments concerns several phenomena: oxidation, carburization... These phenomena can create stresses in the materials, the relaxation of which mostly produces transient elastic waves. Several methods enable the recording and analyzing of these transient elastic waves. Piezoelectric sensors fixed directly on the sample can record elastic waves with a low decrease in energy and frequency at ambient temperature. In case of high temperature environments, a waveguide can also be used to transmit waves from sample to sensors. For this purpose, alumina or platinum are mainly used as waveguide materials because these materials conserve the waveform. The goal of this study is to assign the elastic waves to the corrosion phenomena. This data base will then be useful for the monitoring of industrial equipment using acoustic emission methods. For this purpose, thermogravimetric analysis (TGA) has been coupled with acoustic emission (AE) devices. Simultaneous measurements of the mass variation and of the acoustic signals emitted during the corrosion of samples of the Zirconium based alloy Zircaloy 4 at high temperatures in the range of 400 °C to 900 °C can provide complementary information to increase the level of understanding of high temperature corrosion mechanisms. Our work focuses on a specific waveguide (WG) conception and on the transmission of elastic waves (acoustic signals) through the waveguide at high temperature. Results on experiments concerning the corrosion of zirconium alloy plates under oxygen atmosphere are presented

Modifications of oxidized Zircaloy-4 surface in contact with radiolysed wet air

International audienceIn the framework of radioactive waste geological disposal, the long term evolution of the nuclear wastes packages and the release of the radionuclides from the wastes have to be studied. Regarding compacted wastes (cladding tubes) coming from reprocessing of spent fuel, the Zircaloy-4 (zirconium alloy) cladding tubes have been activated and oxidized in reactors. In the disposal, the radioactive waste is exposed to humid air in a first phase and to water after the resaturation phase. In order to better assess the degradation process of these nuclear waste package, the influence of wet air proton radiolysis on the behavior of surface oxidized Zircaloy-4 has been investigated. Radiolysis experiments were performed using an irradiation cell which is associated to an extracted beam. Samples are exposed to wet air, under and without radiolysis, during 12 and 24 h. The water partial pressure has been fixed at 6 and 50 mbar in order to have, respectively, localized adsorbed water molecules and a thin film of adsorbed water. Before and after each treatment, sample surfaces were characterized by X-ray Photoelectron Spectroscopy (XPS) in order to identify the elements at the topmost surface of the solid. The wet air radiolysis causes changes at the surface of oxidized Zircaloy-4 and influences the corrosion phenomenon. Indeed, an enrichment of tin and the presence of nitrogen species were observed. It could be due to the formation of tritin(II) tetrahydroxide dinitrate and a Zr4+ tetramer on the topmost oxide surface