Une large investigation sur l’influence de plusieurs défauts métallurgiques sur les processus de diffusion et de piégeage de l’hydrogène a été conduite sur le nickel. Ce travail a été réalisé selon deux orientations scientifiques. Une première approche a consisté à évaluer l’impact des défauts intrinsèques et plus particulièrement les joints de grains et les dislocations géométriquement nécessaires sur les modes de transport et de ségrégation de l’hydrogène. Le couplage de caractérisations microstructurales avec les essais de perméation électrochimiques et de thermo-désorption a permis d’établir que les joints de grains présentant une structure ordonnée appelés « spéciaux » représentent des zones privilégiées à la ségrégation de l’hydrogène. Une seconde catégorie de joints de grains dits « généraux » ou « random » présentant un excès de volume important constituent des promoteurs à la diffusion de l’hydrogène. Ces derniers sont la principale source des phénomènes de courts-circuits de diffusion relatés dans les matériaux cubiques à faces centrées. La seconde approche de cette étude a consisté en l’étude de l’interaction de l’hydrogène avec les hétérogénéités de déformation plastique. Les essais de perméation électrochimique réalisés sur des microstructures obtenues par déformation ont montré qu’en traction monotone, les cellules équiaxes et les murs de dislocations représentent des pièges pour l’hydrogène. Celles-ci ralentissent son transport. Ce dernier est essentiellement assuré par le mécanisme de diffusion interstitielle. Par ailleurs, pour la microstructure de déformation résultant de l’essai en fatigue, une accélération de la diffusivité de l’hydrogène a été enregistrée ce qui suggère qu’un phénomène comparable au court-circuit de diffusion intervient dans le transport de l’hydrogène. Concernant les deux approches, les résultats obtenus suggèrent une contribution de l’hydrogène dans la formation de lacunes. / A thorough investigation on the influence of several metallurgical defects on the hydrogen diffusion and trapping was conducted on nickel. This work was conducted towards two scientific orientations. A first approach was to assess the impact of intrinsic defects, especially grain boundaries and geometrically necessary dislocations on the hydrogen transport and segregation mechanisms. Combining microstructural characterizations with electrochemical permeation tests and thermal desorption spectroscopy, it has established that the grain boundaries with ordered structure called "special grain boundaries" are preferential areas for hydrogen segregation. On the other hand, a second category of grain boundaries called "general" or "random" with high free volume and disordered structure are promoters for hydrogen diffusion, and they represent the main sources of the phenomena short-circuit diffusion reported in the face-centered cubic materials. The second approach of this work consisted in the study of the interaction of hydrogen with the plastic deformation heterogeneities. The electrochemical permeation tests performed on microstructures obtained by deformation showed that for the traction monotonous, the equiaxed cells and walls of dislocations are the potential traps for hydrogen and they slow its transport, this latter is mainly provided by the interstitial diffusion mechanism. In addition, for fatigue microstructure, rapid diffusivity of hydrogen was recorded, and suggesting that a phenomenon similar to short-circuit diffusion is involved in the transport of hydrogen. On two approaches, the results suggest a contribution of hydrogen in the formation of vacancies

Ceroxid (CeO2) wurde in den letzten Jahren als Katalysator für die Hydrierung von Alkinen zu Alkenen entdeckt und hat als solcher großes wissenschaftliches Interesse geweckt. Um weitere Einblicke in die Funktion von CeO2 in der Reaktion zu gewinnen, beschäftigt sich diese Arbeit mit der Adsorption von H2, CO2 und Propin, sowie mit der Interaktion von Hydroxylgruppen und Propin auf CeO2(111)-Oberflächen. Ein besonderer Fokus liegt dabei auf der Rolle von Sauerstoffleerstellen. / In recent years, ceria (CeO2) has attracted much scientific interest due to its activity as a catalyst in the selective hydrogenation of alkynes to alkenes. To gain further insights into the role of CeO2 in propyne hydrogenation, this thesis explores the fundamental processes of H2, CO2, and propyne adsorption, as well as the interaction of hydroxyls and propyne on well-defined CeO2(111) surfaces. A special emphasis thereby lies on the role of oxygen (O) vacancies in these processes.

This thesis explores multiscale approaches to describe structural and chemical defects in metals. Particular emphasis is placed on investigating processes involving grain boundaries (GBs) in combination with impurity and vacancy defects. The defects and their interactions are calculated to very high accuracy using density functional theory (DFT) and connected to the macroscopic behavior within the two multiscale formalisms presented here. We begin with a sequential approach to address chemical embrittlement of nickel by sulfur impurities. Effects at both a \(\Sigma 5 (012)\) symmetric tilt GB and in the bulk are studied by considering competing mechanisms for ductile and brittle behavior. For the bulk, this takes the form of Rice’s theory, where the ratio of the surface and unstable stacking energy is used as a measure of ductility. This is generalized to the GB by considering GB sliding (GBS) and intergranular decohesion. Clear evidence that chemical embrittlement of nickel by sulfur is a GB driven effect is found. Next, a concurrent multiscale approach is described. A small region, containing the defects, is treated with Kohn-Sham DFT and coupled to the bulk, described with the embedded atom method. We apply this novel method to elucidate the chemical embrittlement of a copper \(\Sigma 5 (012)\) symmetric tilt GB. Intergranular decohesion for three substitutional impurities, bismuth, lead and silver, is investigated by considering the work of separation \((W_s)\) and the tensile strength \((\sigma_t)\). Bismuth and lead show a significant decrease in \(W_s\) and \(\sigma_t\), consistent with embrittlement, whilst silver has only a minor effect. Then, the concurrent multiscale method is applied to the process of GBS in copper. It is found that the resistance against sliding increases significantly for bismuth, lead and silver impurities. The underlying mechanisms for this increase are found to be dominated by size effects for bismuth and lead. For silver, chemical effects are of greater importance. Similar results are found for the underlying mechanisms of intergranular decohesion. The effect of a mono-vacancy on GBS is studied for copper. The multiscale approach enables improved decoupling of the mono-vacancy. It is found that the monovacancy enhances GBS by 22%. / Engineering and Applied Sciences

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En spintronique, l’étude des hétérostructures multicouches composées d'une électrode ferromagnétique et d'une couche isolante mince, c'est-à-dire des jonctions tunnel magnétiques (JTM), est particulièrement importante. Le système canonique est le Fe/MgO/Fe où les hautes valeurs du rapport de la magnétoresistance tunnel (TMR) ont été mesurées. Le facteur crucial définissant la performance de la jonction est l’imperfection structurelle dans un dispositif réel. Dans notre travail, nous nous sommes concentrés sur des lacunes d'oxygène dans MgO. Au moyen de la théorie de la fonctionnelle de densité, nous avons étudié les propriétés électroniques de l'état fondamental des lacunes d'oxygène simples et doubles dans MgO massif, appelées respectivement centres F et M. Nous avons ensuite étudié l'impact de ces lacunes sur le transport balistique dans les jonctions magnétiques. Nous avons démontré le rôle supérieur joué par les centres M et nous avons prouvé qu'un transport cohérent, préservant le spin et la symétrie des électrons, est possible en présence de centres M. / In sprintronics, the study of multilayer heterostructures composed of a ferromagnetic electrodes and a thin insulating layer, i.e. magnetic tunnel junctions (MTJs), is of special importance. The canonical systems are MTJs made of Fe/MgO/Fe where hight tunneling mangetoresistance ratio (TMR) values were measured. The crucial factor defining the junction performance is the structural imperfection appearing in a real devices. In our work we focused in particular on oxygen vacancies in MgO. By means of density functional theory we studied ground state electronic properties of single and double oxygen vacancies, referred as F and M centers, respectively, in bulk MgO. We then switched to full junctions where we investigated the impact of vacancies on the ballistic transport. We demonstrated that M centers played a superior role and proved that coherent transport, preserving electrons spin and symmetry, is possible in presence of paired vacancies.

The purpose of this thesis is to investigate the possibilities of predicting vacancies in the real estate market by using machine learning models in terms of classification. These models were mainly based on data from contracts between a Swedish real estate company and their tenants. Attributes such as annual renting cost and rental area for each contract were supplemented with additional data regarding financial and geographical information about the tenants. The data was stored in three different formats with the first having binary classes which aim is to predict if the tenant is moving out within a year or more. The format of the second and third version were both multi classification problems that aims to classify if the tenants might terminate their contract within a specific interval with the length of three and six months. Based on the results from Microsoft Azure Machine Learning Studio, it is discovered that the multi classification problems perform rather poorly due to the classes being unbalanced. Regarding the  performance of the binary model, a more satisfying result was obtained but not to the extend to say that the model can be used to determine a vacancy with high accuracy. It should rather be used as a risk analysis tool to detect if a tenant is showing tendencies that could result in a future vacancy. A major pitfall of this thesis was the lack of data and the financial information not being specific enough. The performance of the models will likely increase with a larger dataset and more accurate financial information.

Orientadores: Peter Alexander Bleinroth Schulz, Ana Luiza Cardoso Pereira / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-17T03:19:21Z (GMT). No. of bitstreams: 1 Munoz_WilliamArmando_M.pdf: 8434738 bytes, checksum: d59e43facc73d2ca570f8291de7f22cc (MD5) Previous issue date: 2010 / Resumo: Nós estudamos os efeitos da quebra de simetria e de desordem no espectro de energia de um elétron numa bicamada de grafeno. O problema foi abordado através de simulações numéricas considerando um Hamiltoniano tight-binding para uma rede retangular topologicamente equivalente à rede hexagonal do grafeno que nos permite incluir de forma simples os efeitos do campo magnético através da substituição de Peierls. O problema de quebra de simetria foi observado quando consideramos a aplicação de uma diferença de potencial entre as camadas. Neste contexto, é observada uma quebra completa na degenerescência do nível de Landau de energia zero devida à aplicação desta diferença de potencial entre as camadas. Acompanhamos este mecanismo de quebra de degenerescência para uma situação onde a diferença de potencial entre camadas é mantida fixa enquanto o campo magnético é aumentado. Isto mostra a possibilidade de controlar a abertura de gaps através da variação do campo magnético na bicamada de grafeno e está de acordo com o que foi recentemente observado experimentalmente. Observamos também um comportamento para a separação em energia desta quebra de degenerescência do tipo vB para valores do campo magnético B<60 Teslas, enquanto que para campos maiores é obtido um comportamento assintótico para energias próximas da separação (Vg) do desdobramento do nível de Landau central. Um mapeamento das funções de onda dos estados envolvidos nesta quebra de degenerescência, mostra um esquema diferente para regimes diferentes de campo magnético. Conseguimos verificar claramente como a quebra da degenerescência do nível de Landau central está relacionada com uma quebra da degenerêscencia entre as duas camadas da bicamada e também entre as duas sub-redes que formam o grafeno. O segundo problema abordado neste trabalho é relacionado à introdução de duas vacâncias acopladas na bicamada de grafeno. Em presença de um campo magnético perpendicular ao plano das camadas mostramos como estados localizados ao redor do defeito com energias entre os níveis de Landau são introduzidos pela presença das vacâncias acopladas. Estes estados para baixas energias formam uma molécula de vacâncias que pode ser facilmente polarizável pela aplicação de um potencial entre as camadas / Abstract: We studied the symmetry breaking and disorder effects on energy spectrum of an electron in a bilayer graphene. We used numerical calculation considering a 2D tight-binding model for a rectangular lattice which not changes the hexagonal lattice topology and where the magnetic field effects are easily calculated. The breaking symmetry problem was considered through the application of a potential difference between both layers in the bilayer graphene. In this case, we observe a complete degeneracy breaking of the energy-zero Landau level due to the application of this potential difference between the two layers. We followed this degeneracy breaking considering a potential difference constant while the magnetic field was increasing. That shows a possibility to control the opening of the gap by means of the magnetic fields in the bilayer graphene which is in agreement with recent experimental results. We also shown that this gap increases with a root of B for values of B<60T, while in the high magnetic field regime (B>60T) the energy dependency of the gap shows a asymptotic behavior with B, which tends to energy values close to Vg. We mapped the wave functions amplitudes of states related to the splitting of the zero-energy Landau level and we found that this electronic charge distribution is different depending on magnetic field regime. We verified clearly as this splitting is related to a layers degeneracy breaking as well as a sublattice (valley) degeneracy breaking. The second problem considered in this thesis is related to the introduction of two coupled vacancies in the bilayer graphene. In presence of a perpendicular magnetic field, we show that two coupled vacancies in the bilayer graphene introduce states with a charge-density distribution localized close to the defect and energies between consecutives Landau levels. These states for lowest energies form a vacancy molecule which can be easily polarized by applying of a potential between the two layers in bilayer graphene / Mestrado / Física da Matéria Condensada / Mestre em Física

L'objectif de ce travail est l'étude des interactions entre des nanoparticules de platine et la zircone yttriée (YSZ pour Yttria-Stabilized Zirconia), oxyde conducteur ionique. Il s'agissait de transposer les effets de promotion électrochimique de la catalyse mis en évidence sur des films polarisés de platine de faible dispersion déposés sur des membranes denses de YSZ à des systèmes catalytiques conventionnels à base de nanoparticules métalliques dispersées sur des poudres de YSZ. La migration des ions oxydes promoteurs n'est plus contrôlée par une polarisation électrique mais induite thermiquement. Ces travaux ont permis de mettre au point une méthode de mesure de la dispersion du Pt déposé sur la zircone yttriée. Les interactions Pt/YSZ et notamment celles entre les lacunes d'oxygène de YSZ et les nanoparticules de Pt ont été étudiées par réduction en température programmée et spectroscopie infrarouge. L'importance des lacunes d'oxygène du support YSZ sur les propriétés de chimisorption du Pt et sur son activité catalytique pour l'oxydation du propane a été clairement montrée. La migration thermique des ions oxydes a été étudiée par échange isotopique 18O/16O. Un mécanisme de la réaction de combustion du propane a été proposé incluant le rôle prépondérant des oxydes de réseau de YSZ contrairement aux supports conventionnels en silice et en zircone non substituée. Finalement, les paramètres pouvant influencer les interactions Pt/YSZ comme la surface spécifique de YSZ, le taux d'oxyde d'yttrium, la méthode de préparation de YSZ ainsi que la teneur et la taille des nanoparticules de Pt ont été évalués. Les résultats ont mis en évidence la migration thermique des ions oxydes du support vers le Pt dès 100 °C. D'autre part, l'échange entre les oxygènes du réseau et ceux de la phase gaz est extrêmement rapide dès 100°C. L'activité catalytique du Pt semble promue par la mobilité des oxygènes du support / The aim of this work is to investigate the interactions between Pt nanoparticles and Yttria-Stabilized Zirconia (YSZ), an ionically conducting support. The idea was to overcome the effects of electrochemical promotion of catalysis (EPOC) observed on Pt/YSZ electrochemical catalysts which present low metal dispersion to conventional catalytic systems based on metallic nanoparticles finely dispersed on YSZ powered support. In that configuration, the migration of the oxygen ions from YSZ toward the Pt surface is not electrically controlled but thermally induced without any polarisation. First, we have established a new procedure to measure the Pt dispersion over YSZ. The metal support interactions between Pt and YSZ were characterized by Temperature Programmed Reduction and Infrared Spectroscopy. The importance of the YSZ oxygen vacancies on the chemisorptive behaviours of Pt as well as its catalytic for the propane oxidation was clearly demonstrated. The thermal migration of oxygen ions was validated by using the Isotopic Exchange procedure 18O/16O. The impact of these vacancies was evaluated and a mechanism of the propane deep oxidation on Pt/YSZ was proposed including the important role of bulk YSZ oxygen species in opposition with conventional supports such as silica and non-substituted zirconia. Finally, the key parameters that can influence the Pt/YSZ interactions such as the YSZ specific surface area, the yttria content, the YSZ preparation route as well as the loading and size of Pt nanoparticles were investigated. Our results point out that the thermal migration of oxygen ions from YSZ toward Pt surface occurs from 100 °C. In addition, the exchange between oxygen species from YSZ bulk and those from the gas phase is extremely fast starting from 100 °C. The Pt catalytic activity for the propane deep oxidation seems to be promoted by the mobility of the bulk YSZ oxygen species

Dans de nombreuses applications technologiques des alliages métalliques structurés, la fragilisation par l'hydrogène (HE) est une préoccupation majeure car elle peut pénétrer dans la plupart des métaux, dégrader leurs propriétés et conduire à des défaillances prématurées. Malgré de nombreux efforts durant ces dernières décennies, au cours desquelles de nombreux mécanismes microscopiques ont été proposées, une compréhension claire des mécanismes de fragilisation H n'a pas encore été atteint. Depuis qu'il processus se produisent sur une échelle atomique, les mécanismes exacts conduisant à HE ne sont pas facilement identifiés expérimentalement. Une amélioration possible serait d'utiliser des simulations à l'échelle atomique pour essayer de capturer les détails des processus de déformation et de rupture au niveau atomique, permettant l'enquête du mécanisme microscopique pertinente. Dans un tel contexte, l'objectif de ce travail de thèse est de comprendre et de quantifier les interactions H avec des défauts comme postes vacants, les luxations et les fissures dans les métaux cfc par la modélisation multi-échelle. L'étude est organisé en quatre parties principales. Dans la première partie, nous avons utilisé des premiers calculs principe (basée sur la théorie fonctionnelle de la densité) pour décrire H interaction avec une vacance de Nickel. Plus précisément, les énergies de ségrégation de plusieurs atomes d'hydrogène en un seul et di-postes vacants ont été calculés. Deux énergies caractéristiques qui ont été trouvés à clarifier les pics expérimentaux observés chez Spectra désorption thermique dans la littérature. Les concentrations de groupes H-inoccupation équilibre a ensuite été évalué, dans des conditions pertinentes à SE et de corrosion sous contrainte (CSC) des alliages à base de Ni (industrie nucléaire), par des simulations de Monte Carlo et un modèle thermodynamique développé à partir de nos données DFT. Dans la deuxième partie, nous avons quantifié l'effet de piégeage de postes vacants sur H diffusion dans Nickel. Avec DFT barrières de saut calculé, liés à H piégeage et dépiégeage dans les postes vacants, nous avons utilisé accéléré Kinetic Monte Carlo (KMC) simulations pour évaluer le coefficient H de diffusion en fonction de la concentration de vacance et de la température. Dans la troisième partie, nous avons étudié la diffusion des grappes H-vacance de Ni, basée sur la combinaison de DFT et une méthode statistique. Calculs DFT de barrières de saut d'inoccupation ont été effectuées pour les clusters contenant de un à six H à l'intérieur du poste vacant. Avec ces barrières calculés et les concentrations calculées précédentes de grappes H-inoccupation, un modèle simple stochastique similaire à la procédure KMC a été développé pour estimer le coefficient de diffusion de grappes H-inoccupation en fonction de la concentration et de la température M. Dans la dernière partie, nous avons étudié l'interaction de l'hydrogène avec une pointe de fissure émoussée en aluminium par EAM combinée (potentiel interatomique semi-empirique) et calculs DFT. Méthode atome embarqué (EAM) simulations potentiels ont été réalisées pour évaluer l'effet de H sur la dislocation émission d'une pointe de fissure émoussée en mode mixte chargement. Ce phénomène peut être comprise par le changement induit H de l'empilement instable faute de l'énergie (γus) dans le modèle de Rice. Par conséquent, DFT et EAM calculs de γus ont été effectuées, y compris les effets de H et des charges en mode mixte. Il est montré que l'effet de la charge perpendiculaire au plan de glissement est très forte, contrairement à l'effet de la surface sous-H, ce qui est négligeable. / In many technological applications of structured metallic alloys, hydrogen embrittlement (HE) is a major concern as it can penetrate in most metals, degrade their properties and lead to premature failures. Despite numerous efforts in the past decades during which many microscopic mechanisms were proposed, a clear understanding of H embrittlement mechanisms has not been achieved yet. Since HE processes occur on an atomic-scale, the exact mechanisms leading to HE are not easily identified experimentally. One possible improvement would be to use atomic-scale simulations to try to capture details of deformation and fracture processes at the atomic level, enabling the investigation of relevant microscopic mechanism. In such context, the goal of this PhD work is to understand and quantify H interactions with defects like vacancies, dislocations and cracks in fcc metals through multi-scale modeling. The study is organized in four main parts. In the first part, we employed first principle calculations (based on density functional theory) to describe H interaction with a vacancy in Nickel. More specifically, the segregation energies of multiple H atoms in a single and di-vacancies were computed. Two characteristic energies were found which clarify the experimental peaks observed in Thermal Desorption Spectra in the literature. The equilibrium concentrations of H-vacancy clusters was then evaluated, under conditions relevant to HE and stress corrosion cracking (SCC) of Ni based alloys (nuclear industry),by Monte Carlo simulations and a thermodynamic model developed from our DFT data. In the second part, we quantified the trapping effect of vacancies on H diffusion in Nickel. With DFT computed jump barriers, related to H trapping and detrapping in vacancies, we employed accelerated Kinetic Monte Carlo (KMC) simulations to evaluate the H diffusion coefficient as a function of vacancy concentration and temperature. In the third part, we studied the diffusion of H-vacancy clusters in Ni, based on the combination of DFT and a statistical method. DFT calculations of vacancy jump barriers were performed for clusters containing from one to six H inside the vacancy. With these computed barriers and previous calculated concentrations of H-vacancy clusters, a simple stochastic model similar to the KMC procedure was developed to estimate the diffusion coefficient of H-vacancy clusters as a function of H concentration and temperature. In the last part, we studied the interaction of hydrogen with a blunted crack tip in Aluminum by combined EAM (semi-empirical interatomic potential) and DFT calculations. Embedded atom method (EAM) potential simulations were performed to evaluate the H effect on dislocation emission from a blunted crack tip under mixed mode loading. This phenomenon can be understood by the H induced change of the unstable stacking fault energy (γus ) in Rice’s model. Therefore, DFT and EAM calculations of γus were performed including the effects of H and of the mixed mode loads. It is shown that the effect of the load perpendicular to the glide plane is very strong, contrary to the effect of sub-surface H, which is negligible