The price fluctuation and depletion of crude oil have led to the fervent interest in finding alternatives that can satisfy our increasing need for energy. In the past decades, two primary approaches are seen as promising ways to remedy our dependence on crude oil: first, the use of natural gas, primarily methane, to produce high-value hydrocarbons, and second, the use of ammonia as a hydrogen carrier. In this dissertation, we used density functional theory (DFT) calculation and kinetic modeling to investigate methane activation and C-C coupling on WC as well as the ammonia decomposition over the CoNi alloy surface. From our methane conversion project, we investigated the reactivity of W-terminated WC(0001) and WC(112 ̅0) surface toward methane activation and conversion to produce C2 moieties using DFT. We first calculate the intermediates binding energies and activation and reaction energies of methane dissociation. We found that WC(112 ̅0) is better at dissociating the first C–H bond than WC(0001). Our results also indicate that the surface is likely populated by (CH)ads species. The mobility of (CH)ads species on both surfaces allows the possibility of C-C coupling, resulting in a precursor for higher hydrocarbon formation. Our results also demonstrate that the WC(0001) surface favors the production of the (C2H2)ads species, whereas the WC(112 ̅0) surface dissociates CHx completely, resulting in coke formation. Thus, methane dissociates readily on the WC surfaces whereas the formation of the C2 species is sensitive to the surface structure. The DFT study on ammonia decomposition has been performed in close collaboration with the experimental study. A highly active catalyst consisting of CoNi alloy nanoparticles well-dispersed on a MgO–CeO2–SrO mixed oxide support with potassium promotion exhibited a performance matching that of the Ru-based catalysts. Extensive characterization in combination with the DFT results revealed that the CoNi alloy surface and metal/oxide interfaces are the active sites for catalytic decomposition of ammonia. Moreover, the much improved catalytic activity stems mainly from the presence of interface where the recombinative desorption of nitrogen has been greatly enhanced. These have been demonstrated by examining the detailed elementary steps of ammonia decomposition on the Co, Ni, Co2Ni, CoNi2 (111) surfaces and at the CeO2/Co2Ni interface. We calculated the binding energies of intermediates and the activation energies of each elementary step in ammonia decomposition. We found that on the Co, Ni, Co2Ni, CoNi2 surfaces, N–N bond formation is the rate-determining step, with the CoNi alloy surfaces having a lower activation energy than the pure metal surfaces. Over the CeO2/Co2Ni interface, however, N–H bond dissociation becomes rate-determining. The high catalytic activity at the CeO2/Co2Ni interface originates from the localized charge polarization due to alloying and the presence of the oxide which drastically facilitates N2* formation. We then integrated the DFT-calculated adsorption and activation energies in the microkinetic modeling of ammonia decomposition on the Co, Co2Ni, CoNi2, and Ni surfaces, focusing on the alloying effect. Two cases were investigated: ammonia decomposition in the 1) absence and 2) presence of product re-adsorption. In both cases, we determined the turnover frequencies, the apparent activation energies, the steady-state coverages, the degree of rate control, and the reaction orders. Our results show that in both cases, the alloys have higher catalytic performance than the pure metals. We also found that as the temperature increases, ammonia decomposition switches from being limited by N–N (and N–NH) bond formation to N–H bond dissociation. This change of mechanism is predicted to occur at lower temperatures on the alloy surfaces. In the case of hydrogen re-absorption, the surface H* adatom retards the last N–H bond-breaking step, resulting in the high coverage of NH* species on the surfaces, making N–NH coupling an alternative pathway for N2 formation. Furthermore, our microkinetic results show that alloying Ni with Co reduces the effect of hydrogen inhibition at high hydrogen partial pressures. In summary, this dissertation provides information for the design of efficient catalysts toward methane conversion and ammonia decomposition.

No description available.

La creciente demanda energética por el crecimiento demográfico y el desarrollo de países antes no industrializados, ocurrido en las últimas décadas, ha derivado en la imperiosa necesidad de obtener combustibles nuevos, limpios y eficientes. En esta búsqueda ha surgido el hidrógeno como potencial vector energético para el futuro. Es conocida ya la alta eficiencia del hidrógeno como recurso energético, como así también la baja contaminación ambiental que el mismo produce. Sin embargo, su utilización como combustible todavía conlleva dificultades técnicas no resueltas. Por ello es conveniente pensar en un combustible alternativo, versátil, y de fácil aplicación en su uso cotidiano, como lo es el metanol, para así disminuir el consumo de combustibles derivado de los hidrocarburos. En esta tesis se estudian, por un lado, todos los pasos para la síntesis de metanol a partir del gas CO en superficies catalíticas de óxidos de Cerio, Galio y Zinc. Complementariamente, se desarrolla una metodología teórica que puede ser extendida al estudio y análisis de otras reacciones superficiales. En los primeros capítulos se estudia la adsorción y reactividad de las moléculas de CO, CO2 y H2 sobre las superficies de CeO2 con cortes (111) y (331), Ga2O3(100) y ZnO(0001), perfectas y con vacancias de oxígeno. Las mismas reacciones se estudian en las superficies soportadas ZnO/CeO2(111) y Ga2O3/CeO2(111), como modelado del crecimiento epitaxial de una monocapa de ZnO y Ga2O3, respectivamente. Luego se estudia la síntesis de metanol a partir de reacciones de hidrogenación progresiva de CO. Se calculan las barreras de activación en cada paso de la síntesis y se describen también las posibles direcciones de cada reacción. Se concluye que la metodología de estudio es óptima para reacciones similares en un conjunto de óxidos. / The growing demand for energy due to population growth and development of non-industrialized countries in recent decades has led to the urgent need for new, clean and efficient fuels. In this search hydrogen an energy vector potential for the future. It is already known the high efficiency of hydrogen as an energy source is widely known, as the well as low environmental pollution it produces. However, its use as a fuel has difficulties still unresolved. Therefore it is convenient to think of an alternative fuel, versatile, and easy to apply in daily use (such as methanol) to reduce the consumption of oil-derived fuels. In this thesis, studies- on the one hand, all the steps for the synthesis of methanol from CO gas in catalytic surfaces of oxides of cerium, gallium and zinc; and on the other hand, it develops a theoretical methodology that can be extended to the study and analysis of other surface reactions. The early chapters, study the adsorption and reactivity of molecules of CO, CO2 and H2 on surfaces CeO2 with cuts (111) and (331), Ga2O3(100) and ZnO(0001)-perfect and with oxygen vacancies. The same reactions are studied on surfaces supported ZnO / CeO2 (111) and Ga2O3 / CeO2 (111)as a modeling of epitaxial growth of a monolayer of ZnO and Ga2O3, respectively. Then, the methanol synthesis is studied from reactions of progressive hydrogenation of CO. Activation barriers at each step of the synthesis are calculated the possible directions of each reaction are describe. In conclusion, the methodology used in this thesis is appropriate for similar reactions in a set of oxides.

With ever shrinking geometries, growing metal density and increasing clock rate on chips, delay testing is becoming a necessity in industry to maintain test quality for speed-related failures. The purpose of delay testing is to verify that the circuit operates correctly at the rated speed. However, functional tests for delay defects are usually unacceptable for large scale designs due to the prohibitive cost of functional test patterns and the difficulty in achieving very high fault coverage. Scan-based delay testing, which could ensure a high delay fault coverage at reasonable development cost, provides a good alternative to the at-speed functional test. This dissertation addresses several key challenges in scan-based delay testing and develops efficient Automatic Test Pattern Generation (ATPG) and Design-for-testability (DFT) algorithms for delay testing. In the dissertation, two algorithms are first proposed for computing and applying transition test patterns using stuck-at test vectors, thus avoiding the need for a transition fault test generator. The experimental results show that we can improve both test data volume and test application time by 46.5% over a commercial transition ATPG tool. Secondly, we propose a hybrid scan-based delay testing technique for compact and high fault coverage test set, which combines the advantages of both the skewed-load and broadside test application methods. On an average, about 4.5% improvement in fault coverage is obtained by the hybrid approach over the broad-side approach, with very little hardware overhead. Thirdly, we propose and develop a constrained ATPG algorithm for scan-based delay testing, which addresses the overtesting problem due to the possible detection of functionally untestable faults in scan-based testing. The experimental results show that our method efficiently generates a test set for functionally testable transition faults and reduces the yield loss due to overtesting of functionally untestable transition faults. Finally, a new approach on identifying functionally untestable transition faults in non-scan sequential circuits is presented. We formulate a new dominance relationship for transition faults and use it to help identify more untestable transition faults on top of a fault-independent method based on static implications. The experimental results for ISCAS89 sequential benchmark circuits show that our approach can identify many more functionally untestable transition faults than previously reported. / Ph. D.

An internet-based, real-time, Global Positioning System (GPS) ---synchronized relative to the wide-area frequency-monitoring network (FNET) ---has been developed at Virginia Tech. In this FNET system, an algorithm that employs the relationship between phasor angles and deviated frequency [13] is used to calculate both frequency and its rate of change. Tests of the algorithm disclose that, for non-pure sinusoidal input (as compared to pure sinusoidal input), significant errors in the output frequency will result. Three approaches for increasing the accuracy of the output frequency were compared. The first---increasing the number of samples per cycle N---proved ineffective. The second---using the average of the first estimated frequencies rather than the instant first estimated frequency as the resampling frequency---produces a moderate increase in accuracy of the frequency estimation. The third---multiple resampling---significantly increased accuracy. But both the second and the third become ineffective to the extent the input is not pure sinusoidal. From a practical standpoint, attention needs to be paid toward eliminating noise in the input data from the power grid so as to make it more purely sinusoidal. Therefore, it will be worthwhile to test more sophisticated digital filters for processing the input data before feeding it to the algorithm. / Master of Science

Cette thèse comprend trois sujets différents, tous en rapport à des problèmes de structures électroniques. Ces trois sujets sont présentés dans trois parties indépendantes.Cette thèse commence par une introduction générale présentant les problématiques et les principaux résultats.La première partie traite de la théorie de la fonctionnelle de la densité lorsqu'elle est appliquée aux modèles d'électrons avec spins polarisés. Cette partie est divisée en deux chapitres. Dans le premier de ces chapitres, nous introduisons la notion de N-représentabilité, et nous caractérisons les ensembles de matrices de densité de spin représentables. Dans le second chapitre, nous montrons comment traiter mathématiquement le terme de Zeeman qui apparaît dans les modèles comprenant une polarisation de spin. Le résultat d'existence qui est démontré dans (Anantharaman, Cancès 2009) pour des systèmes de Kohn-Sham sans polarisation de spin est étendu au cas des systèmes avec polarisation de spin.Dans la seconde partie, nous étudions l'approximation GW. Dans un premier temps, nous donnons une définition mathématique de la fonction de Green à un corps, et nous expliquons comment les énergies d'excitation des molécules peuvent être obtenues à partir de cette fonction de Green. La fonction de Green peut être numériquement approchée par la résolution des équations GW. Nous discutons du caractère bien posé de ces équations, et nous démontrons que les équations GW0 sont bien posées dans un régime perturbatif. Ce travail a été effectué en collaboration avec Eric Cancès et Gabriel Stoltz.Dans le troisième et dernière partie, nous analysons des méthodes numériques pour calculer les diagrammes de bandes de structures cristallines. Cette partie est divisée en deux chapitres. Dans le premier, nous nous intéressons à l'approximation de Hartree-Fock réduite (voir (Cances, Deleurence, Lewin 2008)). Nous prouvons que si le cristal est un insolant ou un semi-conducteur, alors les calculs réalisés dans des supercellules convergent exponentiellement vite vers la solution exacte lorsque la taille de la supercellule tend vers l'infini. Ce travail a été réalisé en collaboration avec Salma Lahbabi. Dans le dernier chapitre, nous présentons une nouvelle méthode numérique pour le calcul des diagrammes de bandes de cristaux (qui peuvent être aussi bien isolants que conducteurs). Cette méthode utilise la technique des bases réduites, et accélère les méthodes traditionnelles. Ce travail a été fait en collaboration avec Eric Cancès, Virginie Ehrlacher et Damiano Lombardi / This thesis contains three different topics, all related to electronic structure problems. These three topics are presented in three independent parts.This thesis begins with a general introduction presenting the problematics and main results.The first part is concerned with Density Functional Theory (DFT), for spin-polarized models. This part is divided in two chapters. In the first of these chapters, the notion of N-representability is introduced and the characterizations of the N-representable sets of spin-density 2X2 matrices are given. In the second chapter, we show how to mathematically treat the Zeeman term in spin-polarized DFT models. The existence of minimizers that was proved in (Anantharaman, Cancès 2009) for spin-unpolarized Kohn-Sham models within the local density approximation is extended to spin-polarized models.The second part of this thesis focuses on the GW approximation. We first give a mathematical definition of the one-body Green's function, and explain why methods based on Green's functions can be used to calculate electronic-excited energies of molecules. One way to compute an approximation of the Green's function is through the self-consistent GW equations. The well-posedness of these equations is discussed, and proved in the GW0 case in a perturbative regime. This is joint work with Eric Cancès and Gabriel Stoltz.In the third and final part, numerical methods to compute band-diagrams of crystalline structure are analyzed. This part is divided in two chapters.In the first one, we consider a perfect crystal in the reduced Hartree-Fock approximation (see (Cances, Deleurence, Lewin 2008)). We prove that, if the crystal is an insulator or a semi-conductor, then supercell calculations converge to the exact solution with an exponential rate of convergence with respect to the size of the supercell. This is joint work with Salma Lahbabi. In the last chapter, we provide a new numerical method to calculate the band diagram of a crystal (which can be either an insulator or a conductor). This method, based on reduced basis techniques, speeds up traditional calculations. This is joint work with Eric Cancès, Virginie Ehrlacher, and Damiano Lombardi

L'industrie de la microélectronique est en perpétuelle évolution, surtout concernant la diminution des dimensions des composants. Ainsi, pour les nouvelles générations de mémoires non volatiles Flash, le poly-silicium de la grille flottante pourrait être remplacé par des nanoplots discrets de silicium. L'élaboration de ces nanoplots par LPCVD (Low Pressure Chemical Vapor Deposition) à partir de silane SiH4 sur un substrat amorphe SiO2 demeure l'une des voies privilégiées par l'industrie. Le fonctionnement de ce type de mémoires est fortement dépendant des conditions de synthèse des nanoplots de silicium. Ce travail de cette thèse visait donc à améliorer la compréhension des mécanismes de nucléation et de croissance en jeu. Nous avons étudié les premiers instants de la nucléation en chimie quantique, grâce à l'utilisation de la théorie DFT, en considérant l'oxyde de silicium comme surface de dépôt. Des lois cinétiques intrinsèques ont été tirées de ces résultats DFT et elles ont été implémentées dans un modèle de simulation à l'échelle du procédé industriel, sur la base du code de CFD Fluent. Pour la nucléation, il est apparu que seul le silylène, SiH2, peut se chimisorber à la surface du substrat. De plus, sa faible concentration et la première désorption de H2, qui est très lente, expliquent le temps d'incubation. Pour la croissance, le caractère auto-catalytique des dépôts a été expliqué par la contribution très forte du silane au dépôt dès la seconde chimisorption. L'étape limitant la croissance est clairement la désorption de H2. La réalisation d'essais expérimentaux et la comparaison avec le modèle multi-échelles issu de notre travail a permis d'expliquer pourquoi les cinétiques classiques de la littérature surestiment la vitesse de dépôt des nanoplots. Il est aussi apparu que la vitesse de dépôt du silicium sur des nanoplots en croissance est plus forte que celle d'un film de silicium continu « épais ». La prise en compte des sites de chimisorption lors des premiers instants et la description détaillée de la désorption de H2 sont des paramètres clés pour rendre compte du comportement des dépôts de nanoplots de silicium. / The need of high integrated systems of the everyday life involves a permanent evolution of the microelectronic industry. Integrated circuits involving non volatile Flash memories are good examples of these trends. In this technology, the poly-silicon floating gate could be replaced by a discrete trap floating gate in which discrete traps are made up of silicon nanodots. The synthesis of nanodots by LPCVD (Low Pressure Chemical Vapor Deposition) from silane SiH4 on SiO2 surfaces remains one of the most promising ways of industrial synthesis. Despite a huge experimental effort, fundamental understanding of the key mechanisms of nanodots nucleation and growth remains elusive. Here we find the main objectives of the thesis. For nucleation, our main results reveal that only silylene SiH2 is involved in the very first steps of nucleation. The incubation time experimentally observed can be explained by the low SiH2 concentration and the first slow H2 desorption process. For growth, silane is the main responsible for deposition, which explains the autocatalytic behaviour of silicon deposition. The growth limiting step is clearly the H2 desorption process. Comparisons between experimental and multiscale modelling allow to explain why classical kinetics of the literature overestimate nanodots deposition rate. We have found that the silicon deposition rate is higher on nanometer silicon dots than on a continuous silicon film. Key parameters to conveniently model nanodots deposition are good descriptions of the first chemisorption sites and of the H2 desorption process.

Lucie Ducháčková Bipyridine N,N'-dioxides are organocatalysts, which are used as a chiral Lewis bases in enantioselective catalysis. The aim of this diploma thesis was systematic investigation of the proton affinities of bipyridine N,N'-dioxide derivatives. Further, the complexation properties and chiral recognition in the gas phase of bipyridine N,N'-dioxide derivatives were examined. Mass spectrometry complemented with infrared multiphoton dissociation spectroscopy and quantum-chemistry calculations using the density functional theory (DFT) were used as the main experimental methods. Bipyridine N,N'-dioxides are a new class of oxygen superbases with proton affinities larger than 1030 kJ/mol. Complexation properties and reactivities of their metal complexes are comparable to 2,2'-bipyridine.

L’étude des défauts ponctuels (lacunes, impuretés) et des complexes (cavités, joints de grains, etc.) est un domaine important de la physique du solide. Les propriétés physiques et chimiques des matériaux (élasticité, plasticité, fragilité, etc.) sont très souvent corrélées à la présence de ces défauts. Les techniques expérimentales ne sont cependant pas toujours en mesure de fournir une compréhension suffisante de ceux-ci. Dans ce cas-là, les simulations numériques et la modélisation à l’échelle atomique sont des outils utiles pour interpréter les résultats expérimentaux. Mon travail de thèse se situe dans ce cadre. Le manuscrit comporte 4 principales parties. Dans la première et la deuxième partie, nous présentons les principes de la DFT et la méthodologie utilisée dans cette thèse. La troisième partie de ce manuscrit résume l’étude théorique par DFT de la formation, la migration et la diffusion de différentes configurations de multi-lacunes Vn(n=1-15) dans le fer cubique centré. Les configurations de Vn les plus stables y sont décrites en détail : les énergies de formation, de liaison et de piégeage. La migration de différentes multilacunes est discutée en détail d’un point de vue énergétique et configurationnelle. Dans la dernière partie, nous nous intéressons à l’absorption du soufre dans différents sites (interstitiel et substitutionnel) du massif de Ni et sa ségrégation vers les surfaces Ni (100) et Ni (111). L’étude du soufre en volume permet de clarifie sa position en solution solide. Nous discutons des interactions soufre-soufre et avec le métal de base. La ségrégation et l’absorption sur des surfaces libres du nickel est aussi abordée en détail. / The study of point defects (vacancies, impurity) and complex defects (cavities, grain boundary etc.) is a major challenge for solid state physics. The physical and chemical properties of materials (such elasticity, plasticity, embrittlement etc.) are correlated to the presence of these defects. The experimental study is not always able to bring sufficient information about them. In this way computational simulations and the modeling on the atomic level is efficient to interpret the experimental results and to obtain new informations. This PhD work consists also in a theoretical study of defects in metals. This manuscript is organized in four main parts. In the first and second part, we present the theoretical principles and the methodology used in this work. In the third part, we report a long discussion on the study of formation, migration and diffusion of multivacancies Vn in the bcc-iron system. The main objective of this study is to identify and to analyze most stable configurations of multivacancies for different sizes of Vn (n=1-15) in Fe. The migration is also investigated. In the last part, we present a study of S atoms in nickel. The different sites (interstitial and substitutional sites) in solid solution of nickel-fcc and the segregation of sulfur on and in the Ni(100) and Ni(111) surfaces are discussed. The study of sulfur in the bulk shows that the atoms S occupy the substitutional sites. The S-S and S-Ni interactions are also presented. We conclude by a discussion on the segregation and the adsorption on free surfaces.

Dans notre laboratoire rémois, le travail sur les sucres est à la base de toutes les recherches, que ce soit pour la synthèse de molécules d'intérêt biologique telles que des analogues du KRN 7000 (un glycolipide présentant une activité antitumorale), ou encore la mise en place de stratégies de synthèse (avec par exemple la stratégie NOE : addition nucléophile stéréosélective suivie d'une ouverture intramoléculaire régiosélective d'un époxyde). Mon trravail a consisté à étudier la flexibilité, et donc les conformations stables ou moins stables, de petites molécules que sont les C-furanosides, le but étant d'établir un certain nombre de règles permettant d'anticiper les questions concernant à la fois l'entrée et la pose de la structure dans un site actif. J'ai donc débuté avec une étude théorique dans le vide, puis j'ai considéré le milieu solvaté, l'objectif à long terme étant la prise en compte d'un site actif. Durant ces études, jai aussi eu la possibilité de réaliser un certain nombre de travaux en synthèse organique, allant jusqu'à l'établissement d'une voie de synthèse vers différents C-furanosides possédant un bras alcyne pouvant être personnalisé par notre partenaire lyonnais via une réaction de chimie click, pour la synthèse de molécules actives contre le diabète. / In our laboratory (in Reims), working on sugars is the base of all researches, whether for biologically interesting molecules synthesis as analogues of KRN 7000 (a glycolipid showing antitumor effects), or for the development of strategies for synthesis (for example the NEO stragegy : stereoselective nucleophilic addition followed by a regioselective intramolecular epoxide opening). My work was about studying the flexibility, and consequently stable and less stable conformations of small molecules : C-furanosides, the aim being the establishment of several rules anticipating the questions concerning the entry and the pose of the structure in an active site. So I have started with a theoretical study in vacuum, then I have considered solvation. The long term goal being the consideration of an active site. During these stdies, I have also realized several works in the field of organic chemistry, going to the establishment of a synthetic pathway to different C-furanosides with an alkyne arm, which could be functionalized by our partner (in Lyon) with a click reaction, for the synthesis of bioactive molecules against diabetes.