1,720,979 research outputs found
Optimisation of metallic interconnects for hydrogen production by high temperature water vapour electrolysis (HTVE)
No full textLa technologie de l’électrolyse de la vapeur d’eau à haute température (EVHT) est unesolution alternative à la production d’hydrogène. Le principe est inversé à celui d’une pile àcombustible de type SOFC : on utilise la vapeur d’eau et de l’électricité afin de produire del’hydrogène. Une difficulté technique majeure repose sur la mise au point d’interconnecteursfonctionnant efficacement sur le long terme. Sur le plan électrique, l’interconnecteur doitprésenter une valeur de résistance de contact aux électrodes la plus faible possible, car elleaffecte directement le rendement de conversion électrochimique (eau en hydrogène) et peutpénaliser le procédé. Il ne doit donc pas présenter une cinétique d’oxydation élevée ni formerdes oxydes isolants électriquement. Sur le plan chimique, l’interconnecteur doit être résistantà l’oxydation sous atmosphère riche en oxygène côté anode et riche en vapeur d’eau côtécathode. De plus, le problème de la volatilisation des oxydes de chrome, qui peuvent diffuseret empoisonner les électrodes, déterminant ainsi une réduction de l’activité électrochimique etdes performances du « stack » sur des longues durées de fonctionnement, doit être réduit. Latempérature de fonctionnement comprise entre 700 et 900°C permet l’utilisationd’interconnecteurs métalliques, qui présentent l’avantage d’une mise en oeuvre plus facile etd’un coût plus faible par rapport aux interconnecteurs céramiques.Dans cette étude, deux matériaux ont été testés en tant qu’interconnecteurs pour lessystèmes EVHT : un acier ferritique chromino-formeur K41X et un alliage Fe-Ni-Co necontenant pas de chrome. Le comportement envers la corrosion à haute température et laconductivité électrique des deux alliages ont été évalués à 800°C sous un mélange 95%O2-5%H2O, pour le côté anodique, et 10%H2-90%H2O, pour le côté cathodique. Pour l’alliageK41X, l’effet de l’état initial de la surface des échantillons sur la nature des oxydes formés àhaute température sous mélange H2-H2O a été pris en compte, à travers une comparaison desalliages bruts de laminage avec des surfaces polies miroir. L’effet d’une pré-oxydation decourte durée à 800°C sur le comportement à haute température de l’alliage K41X brut deréception sous atmosphère H2-H2O a également été évalué. Mais, le travail le plus original decette étude a consisté à effectuer des essais de marquage à l’or et des marquages isotopiquessous mélange H216O-H218O, H2-D2O et D2-H2O. Ces tests ont permis d’étudier lesmécanismes responsables de la croissance de la couche de corrosion de l’alliage K41X brut deréception et poli miroir à 800°C sous atmosphère H2-H2O et d’évaluer le rôle de la vapeurd’eau et de l’hydrogène dans le mécanisme d’oxydationThe high temperature water vapour electrolysis offers a promising method for highlyefficient hydrogen production. It works as an inverse solid oxide fuel cell, using water vapourand electricity in order to produce hydrogen. A major technical difficulty related to hightemperature water vapour electrolysis (HTVE) is the development of interconnects workingefficiently on a long period. From the electrical point of view, the interconnect must have alow contact resistance with the electrodes. Indeed, it directly affects the electrochemicalconversion efficiency (water into hydrogen) and it can penalize the process. The interconnectmust present a slow oxidation kinetics and form as less as possible electrical insulatingoxides. From the chemical point of view, the interconnect has to be resistant against oxidationin an oxygen rich atmosphere (anode side) and water vapour rich atmosphere (cathode side).Moreover, the problem of the volatility of chromium oxide species, which might migrate andpoison the electrodes, leading to a decrease in their electrochemical activity and degradationof stack performance, over long-term operation, needs to be reduced. The operatingtemperature between 700°C and 900°C allows the use of metallic interconnects, which havehigher electrical and thermal conductivities, easier shaping and lower cost, with respect to theceramic materials.In this study, two materials were tested as interconnects for the HTVE systems: a ferriticchromia-forming alloy, the K41X, and a Fe-Ni-Co alloy, which does not contain chromium.High temperature corrosion behaviour and electrical conductivity were tested in both anode(95%O2-5%H2O) and cathode (10%H2-90%H2O) atmospheres at 800°C. Moreover, for theK41X alloy, the effect of the initial surface state of the samples on the chemical nature of theoxides formed at 800°C in H2-H2O atmosphere was evaluated, by comparing as received andmirror polished surfaces. The effect of a short-term air preoxidation at 800°C on the hightemperature behaviour of the K41X as received sample in H2-H2O atmosphere was tested.The most original part of this study consisted in the investigation of the oxidation mechanismsof both as received and mirror polished K41X samples at 800°C in H2-H2O atmosphere bymeans of marking experiments using Au and isotopes (H216O-H218O mixture). Moreover,marking tests using H2-D2O and D2-H2O were carried out, in order to further investigate therole of hydrogen and water vapour in the oxidation mechanis
High chromium-carbon’s steel microstructural and properties evolutions after Hot Isostatic Pressing (HIP) process : From powder to final product.
No full textL'alliage Norem02 (Fe - 25% Cr - 4% Ni - 4% Mn - 4% Si - 2% Mo – 1,3% C) est utilisé comme revêtements de revêtement dur pour les vannes nucléaires dans les réacteurs à eau sous pression (REP), en raison de sa bonne résistance mécanique et face à la corrosion. Cette alliage est présenté comme étant un acier duplex inoxydable malgré sa forte teneur en carbone. Les revêtements dur de Norem02 sont généralement déposés par un procédé de soudage à l'arc transféré au plasma (PTAW). Une microstructure grossière et hétérogène découle de ce procédé, présentant des propriétés de résistance à l'usure insatisfaisantes, principalement à haute température (T > 180°C). Dans la présente étude, le Norem02 est élaboré par Compression Isostatique à Chaud (CIC), une technique de frittage assisté par pression où une poudre est placée dans un conteneur en acier inoxydable plastiquement déformable dans les conditions de frittage. Ce qui aboutit à l'élaboration de matériaux denses avec une microstructure à grain fin et des propriétés de durabilité améliorées. La CIC peut être utilisée comme technique alternative d’élaboration de revêtement dur si le substrat à revêtir constitue une partie voire l’entièreté du conteneur.La poudre de Norem02 a été frittée pendant 3 heures à 1100°C et 1200 bars dans un conteneur en 304L. La microstructure se compose principalement d'austénite où sont dispersés de fin carbures de chrome de type M23C6 (M = Cr, Mn, Mo) ainsi qu’une faible quantité de ferrite. Malgré une densité mesurée de 100% caractéristique d’un frittage complet, la microstructure de la poudre initiale est toujours visible après le procédé CIC. Cependant, une microstructure présentant plus de défaut et des carbures globulaires s’est formée entre les anciennes particules de poudre.Une zone de transition a été révélée à l'intérieur du Norem02, à proximité de l'interface avec le 304L. Dans cette zone, la microstructure est formée d'une matrice duplex (austénitique / ferritique) et principalement de carbures globulaires de type M23C6.Afin d'étudier les transformations de phase et les mécanismes de densification de la poudre de Norem02, des cycles HIP interrompus à différentes températures (de 800 à 1100°C) ont été réalisés. Les évolutions microstructurales du cœur de l’échantillon et de la zone d'interface avec le conteneur ont été étudiées par des observations optiques, des analyses MEB et EBSD couplées à des caractérisations par diffraction des rayons X. Un test de frittage sous vide sans aucune pression appliquée a également été réalisé, afin de comprendre l’influence de l’application de la pression sur les transformations de phases.L'ensemble des résultats a permis d’améliore sensiblement la compréhension des phénomènes de densification ainsi que l’influence des différents paramètres de frittage sur la microstructure final du matériau fritté. Des résultats préliminaires ont été obtenus concernant les performances tribologiques et mécaniques du matériau. Ces derniers indiquant une amélioration des performances par rapport au procédé d’élaboration initial sur les tests effectués.Fe-Cr-C alloys are frequently used as hard coating coatings for nuclear valves in pressurized water reactors (PWRs), due to their good mechanical resistance and corrosion. The Norem02 alloy (Fe - 25% Cr - 4% Ni - 4% Mn - 4% Si - 2% Mo) is one of the most promising candidates. Hardfacing coatings are typically deposited by a plasma transferred arc welding (PTAW) process, which results in a coarse and heterogeneous microstructure, exhibiting unsatisfactory wear resistance properties, mainly at high temperature. In the present study, Norem02 is developed by Hot Isostatic Compression (CIC), a pressure assisted sintering technique where a powder is placed in a plastically deformable container under sintering conditions. This results in the development of dense materials with a fine-grained microstructure and improved durability properties. CIC can be used as an alternative hard coating technique if the substrate to be coated forms part of the container.The Norem02 powder was sintered for 3 hours at 1100 ° C and 1200 bar in a 304L container. The microstructure consists mainly of austenite in which chromium carbides of the M23C6 type (M = Cr, Mn, Mo) are dispersed as well as a small amount of ferrite. Despite a measured density of 100% characteristic of complete sintering, the microstructure of the initial powder is still visible after the CIC process. However, a more defective microstructure and globular carbides formed between the old powder particles.A transition zone was revealed inside the Norem02, near the interface with the 304L. In this zone, the microstructure is formed of a duplex matrix (austenitic / ferritic) and mainly of globular carbides of the M23C6 type.In order to study the phase transformations and the densification mechanisms of the Norem02 powder, interrupted HIP cycles at different temperatures (from 800 to 1100 ° C) were carried out. The microstructural evolutions of the volume and of the interface zone were studied by optical observations, SEM and EBSD analyzes coupled with characterizations by X-ray diffraction. A vacuum sintering test without any applied pressure was also carried out, in order to understand the influence of the application of pressure on phase transformations.All the results made it possible to begin understanding the phenomena of densification as well as the influence of the various sintering parameters on the final microstructure of the material. Preliminary results were obtained concerning the tribological and mechanical performances of the material. These indicate an improvement in performance over the initial development process
316L Stainless-Steel Carburizing Close to Eutectic Transformation Using the Spark Plasma Sintering Process
Full text linkThis work focuses on the 316L austenitic stainless-steel case-hardening microstructure, after the SPS process near the solid/liquid state transition temperature. This process, faster than conventional carburizing techniques, is equivalent to weld cladding, allowing the achievement of high surface carbon contents with large-size carbide grains in the case of partial melting. Three distinct zones were formed: internal carburizing, carburizing with melting, and carburizing with melting and chromium depletion; all three composed of mixed carbides (Cr0.4Fe0.6)7C3 distributed in an austenitic matrix. The internal carburizing layer growths following a parabolic kinetic law with kp 1027 cm2/s, while the advancement of the melting
front is very fast and follows a linear law with kl = 1.0 3 1024 cm2/s at 1100 °C. The Cr-depleted fusion zone microstructure is similar to a composite material with a metallic matrix, which includes graphite particles, Mo-rich intermetallic phases, and core-shell eutectic carbides. The partial melting zone without Cr depletion shows the formation of a dense carbide layer with diameters exceeding 10 µm, constituting 60% of the volume, and achieving a hardness of 850 HV5. Its wear rate is about 100 times lower than the 316L steel, indicating a significant improvement in the alloy's wear behavior
Elaboration of 316L/CU composite alloy using a hybrid pvd/sps process
No full textInternational audiencePowder metallurgy is an ideal field for developing advanced materials with complex geometries and compositions. Iron-based alloys produced through sintering often exhibit significant porosity
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
316L Stainless-Steel Carburizing Close to Eutectic Transformation Using the Spark Plasma Sintering Process
No full textInternational audienceThis work focuses on the 316L austenitic stainless-steel case-hardening microstructure, after the SPS process near the solid/liquid state transition temperature. This process, faster than conventional carburizing techniques, is equivalent to weld cladding, allowing the achievement of high surface carbon contents with large-size carbide grains in the case of partial melting. Three distinct zones were formed: internal carburizing, carburizing with melting, and carburizing with melting and chromium depletion; all three composed of mixed carbides (CrFe)C distributed in an austenitic matrix. The internal carburizing layer growths following a parabolic kinetic law with k ≈ 10 cm/s, while the advancement of the melting front is very fast and follows a linear law with k = 1.0 x 10 cm/s at 1100 °C. The Cr-depleted fusion zone microstructure is similar to a composite material with a metallic matrix, which includes graphite particles, Mo-rich intermetallic phases, and core-shell eutectic carbides. The partial melting zone without Cr depletion shows the formation of a dense carbide layer with diameters exceeding 10 µm, constituting 60% of the volume, and achieving a hardness of 850 HV. Its wear rate is about 100 times lower than the 316L steel, indicating a significant improvement in the alloy's wear behavior
Elaboration of 316L/CU composite alloy using a hybrid pvd/sps process
No full textInternational audiencePowder metallurgy is an ideal field for developing advanced materials with complex geometries and compositions. Iron-based alloys produced through sintering often exhibit significant porosity
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
