Judith Monnier

Solid-state hydrogen storage in intermetallic hydrides represents a promising alternative to compressed and liquefied hydrogen, combining higher volumetric density with improved safety. Among ABn-type compounds (A = rare earth, B = transition metal, 2 ≤ n ≤ 5), AB3 and A2B7 phases are of particular interest owing to their complementary hydrogenation properties. Their performance is largely dictated by the structural compatibility between the [AB5] and [A2B4] subunits, especially the volumetric mismatch between them.This work investigates the pseudo-binary La-Y-Ni-Mn system, with the objective of understanding the influence of Mn substitution on the structural, microstructural, and hydrogenation properties of ABn-type compounds. Samples were synthesised by arc melting, followed by annealing to improve homogeneity, and characterised using laboratory X-ray diffraction, neutron powder diffraction, electron microscopy, and pressure-composition isotherm (PCI) measurements.All samples contain both AB3 and A2B7 phases, Mn substitution favourites formation of AB3 Phase. In the LaY2.3Ni11.5-xMnx system, La and Y occupy A-sites and Mn occupies exclusively B-type sites, resulting in anisotropic lattice expansion for AB3 phase. With increasing Mn content (x ≥ 1.8), microstructural analyses by EPMA and TEM revealed the formation of nanoscale AB- and AB5-type precipitates, which indicates a limit of solubility of Mn in the ABn. PCI measurements indicated good hydrogen storage capacity, low pressures plateau and reversibility.

La compréhension des mécanismes de la corrosion atmosphérique sous abri à très long terme des alliages ferreux concerne plusieurs applications. D'une part, l'emploi massif du fer dans l'architecture médiévale pose, notamment, la question de l'évolution à long terme de ce matériau dans ces conditions. D'autre part, la période d'entreposage, lors du processus de traitement des déchets nucléaires, pointe le besoin d'une modélisation sur plusieurs centaines d'années du comportement des aciers doux, matériau envisagé pour les sur-containers. Une approche commune a été développée pour ces deux problématiques et appliquée sur le chaînage de renfort métallique de la cathédrale d'Amiens (XVe siècle). La corrosion atmosphérique sous abri à long terme est contrôlée par un cycle d'humidification à séchage, durant lequel la couche oxydée joue un rôle. Le premier axe de travail a donc consisté à caractériser finement le système de corrosion, à l'aide d'un croisement de techniques, depuis l'échelle macroscopique jusqu'à l'échelle nanométrique. L'accent a été en particulier mis sur les techniques d'analyse structurale micro-focalisées (μ-Raman, μ-DRX et μ-XAS), qui permettent de déterminer la nature des phases présentes, leur localisation et leurs proportions. Le système de corrosion est composé du milieu environnant, de la couche de produits de corrosion et du substrat métallique. Les couches de produits de corrosion sont constituées d'une matrice de goethite nanocristallisée, contenant de faibles quantités de lépidocrocite et d'akaganéite, ces deux phases étant principalement situées en couche externe. De plus, des marbrures plus claires sont observées au sein de la matrice. Parfois composées de maghémite, ces marbrures sont principalement constituées de ferrihydrite/feroxyhite et peuvent être connectées, ou non, au substrat métallique. Dans le second axe de travail, des expériences spécifiques ont été conduites pour tester certaines hypothèses de mécanismes liées au cycle humidification-séchage. En ce sens, les sites de réduction de l'oxygène ont été localisés en milieu insaturé et plusieurs cas ont été mis en évidence, fonction de la morphologie de la couche corrodée. Par ailleurs, des expériences en cellule électrochimique couplée avec des techniques d'analyse structurale ont permis de suivre in situ la réduction de composés de référence modèles. Ce couplage a montré l'influence du mode de réduction et du pH du milieu sur la nature de la ou des phases formées. L'ensemble de ces résultats a conduit à proposer un ensemble de mécanismes pour la corrosion atmosphérique sous abri du fer à très long terme, incluant la morphologie des couches de produits de corrosion et les propriétés des phases en présence. Ces différentes hypothèses ont été intégrées dans une proposition de méthode de diagnostic de la stabilité des systèmes ferreux anciens, mais elles permettent également de discuter les modélisations existantes de la corrosion atmosphérique.

Materials capable of withstanding extreme environments open promising opportunities for nuclear fusion reactors. In this study, equiatomic CrFeTaVW and CrFeTiVW high-entropy alloys are investigated as interlayer materials between W and CuCrZr. Monte Carlo and Molecular Dynamics simulations predicted a bcc-type structure for both systems. Additionally, the Monte Carlo simulation predicts lower potential energy and a more stable structure for both systems than Molecular Dynamics. For CrFeTaVW, the chemical segregation values are lower in MC than in the MD simulation, whereas for CrFeTiVW, the opposite trend is observed, with MC indicating stronger segregation values. After simulation, the high-entropy alloys were prepared by planetary ball milling, consolidated by spark plasma sintering, and analyzed using X-ray diffraction, scanning electron microscopy, and thermal diffusivity. The experimental results for the milled powders confirmed the formation of a bcc structure in both alloys. The consolidated material revealed a bcc-type structure and an Fe₂Ta Laves phase for the CrFeTaVW HEA, while the CrFeTiVW HEA exhibits two different bcc-type structures. The values of CrFeTaVW and CrFeTiVW thermal diffusivity are between 3.5 and 7 mm²/s, which is consistent with the expected values for high-entropy alloys. Overall, the findings indicate that these HEAs have promising properties that can be used in extreme environments.

Intermetallic compounds are key materials for energy transition as they form reversible hydrides that can be used for solid state hydrogen storage or as anodes in batteries. AB_y compounds (A = Rare Earth (RE); B = transition metal; 2 < y < 5) are good candidates to fulfill the required properties for practical applications. They can be described as stacking of [AB₅] and [AB₂] sub-units along the c crystallographic axis. The latter sub-unit brings a larger capacity, while the former one provides a better cycling stability. However, AB_y binaries do not show good enough properties for applications. Upon hydrogenation, they exhibit multiplateau behavior and poor reversibility, attributed to H-induced amorphization. These drawbacks can be overcome by chemical substitutions on the A and/or the B sites leading to stabilized reversible hydrides. The present work focuses on the pseudo-binary Sm₂Mn_xNi_7−x system (0 ≤ x < 0.5). The structural, thermodynamic and corrosion properties are analyzed and interpreted by means of X-ray diffraction, chemical analysis, scanning electron microscopy, thermogravimetric analysis and magnetic measurements. Unexpected cell parameter variations are reported and interpreted regarding possible formation of structural defects and uneven Mn distribution within the Ni sublattice. Reversible capacity is improved for x > 0.3 leading to larger and flatter isotherm curves, allowing for reversible capacity >1.4 wt %. Regarding corrosion, the binary compound corrodes in alkaline medium to form rare earth hydroxide and nanoporous nickel. As for the Mn-substituted compounds, a new corrosion product is formed in addition to those above mentioned, as manganese initiates a sacrificial anode mechanism taking place at the early corrosion stage.

For the first time, the nickel layer formed during the corrosion of intermetallic alloys with a stacked structure has been isolated. This nickel layer has been produced by chemical dealloying of Y₂Ni₇ through a corrosion process in alkaline medium. This anisotropic and nanoporous phase has been fully characterized regarding its morphology, chemical and structural properties, as well as its electrochemical behavior. The metallic phase is made of interconnected nanocrystalline cubic nickel, showing preferential orientation and nanopores. A cavity microelectrode has been used in addition to classical methods using bulk electrodes for electrochemical investigations of the layer. This full characterization is of primary importance to better understand the behavior of metallic anodes in Ni‐MH batteries upon cycling and to ensure a longer lifetime.