1,721,204 research outputs found
Developping Numerical Tools for the Understanding and Quantification of Chemical Interactions of Adsorbates on Metallic Surfaces
No full textCette thèse se focalise sur le développement d’outils pour l’étude fondamentale des interactions latérales au travers de multiples projets : D’un point de vue quantique, les effets énergétiques des interactions chimiques sont décomposables rigoureusement en utilisant des orbitales moléculaires absolument localisées (ALMO). Au cours de cette thèse, une approximation de champ moyen a été développée et implémentée dans CP2K afin d’unifier le formalisme ALMO avec la théorie des états mixtes dans une description DFT, permettant d’étendre l’analyse en composantes énergétiques aux systèmes métalliques.D’un point de vue purement topologique, un nouvel algorithme de détermination de liaisons chimiques (ASANN) a été développé durant cette thèse. Au travers d’un simple terme correctif, ASANN étend l’algorithme de référence SANN aux systèmes présentant une anisotropie locale, sans introduire aucun paramètre. Les nombres de coordination ainsi produits sont particulièrement adaptés pour la descriptions d’interfaces. Ces points de vue complémentaires se combinent sous la forme d’un Hamiltonien effectif basé sur les interactions latérales. Un nouvel outil non-stochastique a été développé et implémenté durant cette thèse, utilisant des algorithmes d’apprentissage par renforcement pour l’entraînement automatique de tels modèles destinés à simuler un système à N corps sur surface réactive. Cet outil repose sur l’adaptation d’un UCT avec une pré-exploration guidée par le modèle en cours d’apprentissage. La mise à jour d’un tel modèle linéaire a été optimisée par la formulation d’un nouvel algorithme de résolution des moindres carrés récursifs exploitant les déficiences de rang.Chemical bonding can be described using the density functional theory (DFT) framework as electronic density reorganisation between chemical entities. Relying on Absolutely Localized Molecular Orbitals (ALMO), the density reorganization can be separated into polarization and charge transfer. However, the ALMO formalism is intractable when combined with mixed-states theory, which is required for describing metals. As shown herein, these two theories can be unified into a mean-field approximation called S-ALMO, allowing for a fundamental density-based description of most chemical bonds. In practice, bonds are most often defined by geometrical considerations. Based on an intuitive and powerful idea (an atom’s nearest neighbours must cover its field of view), SANN (Solid Angle based Nearest Neighbours) provides with locally adaptive parameter-free coordination numbers. A natural extension of this algorithm, called ASANN, is developed to tackle local anisotropy while remaining parameter-free. We demonstrate that this method effectively provides with a fundamental topology-based definition of coordination numbers applicable to close packed bulks, liquids and interfaces. Even though bonds are topologically defined, they contain chemically relevant information, providing descriptors for describing the associated energy. Lattice-based cluster expansion model Hamiltonians are particularly popular for determining the adsorption energy in heterogeneous catalysis. Fitting the corresponding model Hamiltonian is usually performed on a training set composed of hundreds of structures hand-picked by a chemist. For the model Hamiltonian to be relevant for describing a given reaction, the chemist uses his own expertise and chemical intuition to select a diverse set of chemically relevant structures. Treating the generation of a relevant input as a strategy-based game, a novel active learning scheme for model Hamiltonians is designed based on a UCT (Upper Confidence Tree). Updating the model Hamiltonian on-the-fly is achieved by a novel recursive least-squares algorithm, called rank-Greville, exploiting the rank deficiencies of the occurrence matrix to reduce the scaling of the update. Furthermore, a domain knowledge extension on top of the UCT framework allows to optimize the training set construction, leading to the Reinforcement Sampling scheme. This approach effectively replaces chemical intuition with reproducible reinforcement learning techniques
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
Atomistic modeling of hydrogen production on MoS2-derived electrocatalysts : the impact of water and electrochemical potential on their stability and activity
No full textDans une société dépendante des énergies fossiles, la production d'hydrogène à partir de l'eau par électro-catalyse (HER) est une alternative que les chimistes doivent explorer. Le problème majeur est le remplacement du catalyseur platine, le plus efficace, par un autre, moins cher. Le bisulfure de molybdène (MoS2) s’est présenté comme un candidat très prometteur. Cependant, la nature de ses sites actifs reste controversée et la densité des sites actifs est généralement limitée ce qui empêche une utilisation à large échelle. Contrairement aux travaux précédents, cette thèse théorique focalisant sur le MoS2 considère l’influence du solvant, du potentiel électrochimique ainsi que la désactivation par le dégagement de H2S sous conditions HER. Tout d’abord, en revisitant les sites actifs connus sur les bords du MoS2, nous avons mis en évidence que l’eau participe dans la production de H2 et change la structure des bords, menant à une désactivation partielle. Ensuite, nous avons exploré la possibilité de rendre la surface la plus exposée, le plan basal, active via la création des défauts et le dopage tout en considérant la stabilité et l’activité ainsi que l’étude de la variation des barrières d’activations d’HER en fonction du potentiel électrochimique. La dernière partie de cette thèse comprend une modification de la méthode de calcul pour incorporer le potentiel électrostatique généré par la dynamique moléculaire dans les calculs DFT périodiques au niveau de la mécanique moléculaire. Il s'agit de surmonter les limites de l'équation de Poisson-Boltzmann et d'améliorer la description des effets de solvant. Basé sur ces résultats théoriques qui traitent MoS2 de manière réaliste, notre collaborateur IFPEN pourra passer à la valorisation et l’industrialisation du catalyseur MoS2.In a society dependent on fossil fuels, the production of hydrogen from water by electrocatalysis (HER) is an alternative that chemists need to explore. One of the problems is to replacethe platinum catalyst, which is the most efficient, with another catalyst that is less expensive. Molybdenum disulfide ( MoS2) has emerged as a promising candidate. However, the nature ofits active sites remains controversial, and the density of the active sites is generally limited,preventing large-scale use. In contrast to previous work, this theoretical work considers theinfluence of the implicit solvent with some explicit water molecules, electrochemical potentialby grand canonical density functional theory (GC-DFT), and deactivation due to the releaseof H2S under HER conditions. First, by reexamining the known active sites on the edges ofMoS2, we showed that water is involved in the production of H2 and alters the structure of theedges, leading to partial deactivation. We then investigated the possibility of making the mostexposed surface, the basal plane, active by creating defects and doping, examining stabilityand activity as well as variation in activation barriers of HER as a function of electrochemicalpotential. The final part of this work involves a modification of the computational method.The implicit solvent used for the periodic DFT computations is replaced by the electrostaticpotential generated by molecular dynamics at the molecular mechanics level. This shouldovercome the limitations of the linearized Poisson-Boltzmann equation and thus improve thedescription of the potential-dependent solvent effects
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Modélisation moléculaire de l'interphase électrolyte solide dans les batteries au lithium
No full textLa performance des batteries lithium-ion a tendance à se détériorer avec le temps en raison de divers changements chimiques. Cette baisse de performance est régie par une couche appelée l'interface électrolytique solide (SEI). La SEI est une couche hétérogène ultrafine de l'ordre du nanomètre qui se forme à l'interface anode/électrolyte en raison de la réduction irréversible des composants de l'électrolyte par les électrons provenant de l'anode. Alors que la formation initiale de la SEI agit comme une couche passive pour protéger l'électrolyte contre une réduction continue, stabilisant ainsi la batterie, la SEI subit divers changements chimiques et poursuit de croître lentement avec le temps, consommant ainsi des électrons actifs et des espèces d'électrolyte, entraînant une perte de capacité et finalement la défaillance de la batterie. Dans cette thèse, nous éclairons la formation et la croissance de la couche SEI en utilisant des techniques de modélisation moléculaire. Étant donné que la formation et la croissance de la SEI et la perte de capacité correspondante se produisent à des échelles de temps différentes, diverses techniques de modélisation moléculaire sont appliquées pour une description atomistique : les principales voies de réaction de décomposition (initiale) sont calculées à l'aide de la DFT pour obtenir les données cinétiques et thermodynamiques, qui sont à leur tour injectées dans un modèle de Monte-Carlo cinétique pour étendre les échelles de temps et de longueur et simuler la croissance de la SEI et la perte de capacité. La structure multicouche de la SEI prédite et le comportement de perte de capacité au fil du temps concordent avec des études expérimentales et théoriques précédentes. Deuxièmement, des méthodes de structure électronique à faible coût sont étalonnées et ont été utilisées pour modéliser des réactions chimiques liées à la SEI.The performance of lithium-ion batteries tends to deteriorate over time due to various chemical changes. This performance decline is governed by a layer called the solid electrolyte interphase (SEI). The SEI is a nanometer thin heterogeneous layer that forms at the anode/electrolyte interface due to the irreversible reduction of electrolyte components by electrons from the anode. While the initial SEI formation acts as a passive layer to protect the electrolyte from further reduction, thereby stabilizing the battery, the SEI undergoes various chemical changes and continues to grow slowly over time, accordingly consuming active electrons and electrolyte species, leading to capacity loss and eventually the death of the battery. In this thesis, we shed light on the formation and growth of the SEI layer using molecular modeling techniques. Since the formation and growth of the SEI and corresponding capacity loss take place at different timescales, various molecular modeling techniques are applied for an atomistic description: the major (initial) decomposition reaction pathways are calculated using DFT to obtain the kinetical and thermodynamical data, which in turn are injected in a kinetic Monte Carlo model, to extend the time and length scales to simulate SEI growth and capacity loss. The predicted multi-layered SEI structure and capacity loss behavior over time agrees with previous experimental and theoretical studies. Secondly, low-cost electron structure methods are benchmarked and have been used to model additional SEI related chemical reactions
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