Le béryllium, utilisé comme matériau de première paroi dans la chambre à vide d'ITER, peut piéger une fraction significative de tritium provenant du plasma. Du point de vue de l'analyse de sûreté, il s'agit d'évaluer l'efficacité des dispositifs mis en place par l'exploitant pour maîtriser l'inventaire en tritium durant le fonctionnement nominal d'ITER et de prédire le comportement du tritium en situation accidentelle. L'objectif de ces travaux de thèse est de déterminer, par le biais de la Théorie de la Fonctionnelle de la Densité, le comportement du tritium dans le béryllium en termes de mécanismes de rétention et de désorption. La stabilité des défauts ponctuels dans le béryllium a d'abord été évaluée et analysée au regard des données expérimentales, mettant en évidence la prédominance du défaut lacunaire dans le béryllium. Ensuite, tous les sites d'occupation stables du tritium et les chemins de diffusion les plus favorables ont été déterminés. Cette analyse a été étendue au processus de multi-piégeage, mettant en évidence que cinq atomes de tritium peuvent être piégés dans une lacune. Enfin, les données obtenues ont été introduites dans un code Monte-Carlo Cinétique permettant d'évaluer le coefficient de diffusion du tritium et un modèle couplant mécanismes de réaction et de diffusion pour l'interprétation de spectres expérimentaux de thermo-désorption (TDS) obtenus à l'IEK-4 (Jülich -- Allemagne). Les spectres modélisés reproduisent de manière satisfaisante les spectres expérimentaux pour les faibles fluences. A forte fluence, la mise en évidence expérimentale d'un pic à basse température pourrait s'expliquer par la formation d'hydrure de béryllium. / Beryllium will be used as a plasma-facing material for the ITER vacuum vessel. Due to high plasma fluxes, significant amounts of hydrogen isotopes could be retained in the beryllium walls. From the safety point of view, it is important to assess the capability of devices which will be used to limit the tritium inventory in the tokamak in nominal conditions and secondly, to predict the behaviour of tritium in case of accident. The objective of this work is to evaluate within the framework of the Density Functional Theory the behaviour of tritium in beryllium in terms of retention and desorption mechanisms. Firstly, the stability of point defects in beryllium has been evaluated and compared to experiments. Vacancies are shown to be the dominant defect in beryllium. Then, the most stable interstitial sites for tritium atoms and the most favorable migration pathways have been determined. This study has been extended to multiple-trapping phenomenon in monovacancy, in which up to five atoms can be trapped. These data have been used in a kinetic Monte-Carlo code to calculate the diffusion coefficient of tritium and a reaction-diffusion based model, which provides a good agreement with experimental thermal desorption spectra made at IEK-4 (Jülich -- Germany). The emergence of desorption peak at low temperature under high fluence could be explained by the hydride formation.

Nanoligas bimetálicas têm atraído a atenção de pesquisadores nas últimas décadas devido a possibilidade de ajustar suas propriedades físico-químicas, tais como propriedades elétricas, ópticas, magnéticas e de reatividade, por meio da variação do número de átomos, da composição química e do formato geométrico. As nanoligas bimetálicas que combinam Pt com outros metais são especificamente interessantes na área de catálise heterogênea, devido a possibilidade de se obter materiais com propriedades distintas de seus respectivos sistemas unários no que se refere ao surgimento ou aumento da atividade catalítica, à seletividade e, muitas vezes, ao preço reduzido. Esse trabalho tem como objetivo investigar as propriedades estruturais, energéticas, eletrônicas e de estabilidade de nanoligas bimetálicas core-shell de 55 átomos que combinam Pt com metais de transição (MT) pertencentes às séries de transição 3d, 4d e 5d dos grupos de Fe a Zn. Esses sistemas foram estudados utilizando cálculos de primeiros princípios (ab initio) baseados na teoria do funcional da densidade, tal como implementada no código computacional VASP (Vienna Ab initio Simulation Package). As nanoligas putativas de mínimo global energético (pGMC) de composição Pt13MT42 e Pt42MT13 calculadas nesse estudo apresentaram características particulares quanto à geometria e ao arranjo das espécies metálicas na nanoliga. Ao que diz respeito aos arranjos core-shell, foram observados dois arranjos em que a fica Pt no caroço (Pt13Ag42 e Pt13 Au42) e sete arranjos em que a Pt fica na superfície (Pt42Fe13, Pt42Co13, Pt42Ni13, Pt42Cu13, Pt42Ru13, Pt42Rh13 e Pt42Os13). Os mecanismos que levam à formação destas e das demais nanoligas pGMC foram investigados com base em três fatores: raio atômico, energia de superfície e cargas de Bader. Verificou-se que raio e a energia de superfície competem como fator determinante pelas posições preferenciais de cada espécie metálica na nanoliga. Nos casos em que houve divergência, o raio apresentou-se como o fator de maior importância, entretanto, quando o raio das espécies são muitos próximos, a energia de superfície exerce um papel de maior importância. A partir da análise de cargas de Bader, observou-se ocorrência de transferência de carga da região do caroço para a região da superfície para a maioria das nanoligas. No mais, observou-se que as nanoligas core-shell contam com atração coulômbica de maior magnitude do que as demais nanoligas pGMC, como resultado de altas cargas de sinal oposto em cada uma das regiões. / Bimetallic nanoalloys have been attracting attention since the last decades due to the possibility of adjusting their physical-chemical properties, such as electrical, optical, magnetic and reactivity properties, by means of the variation of the number of atoms, chemical composition and geometry. Bimetallic nanoalloys that combine Pt with other metals are especially interesting for heterogeneous catalysis given the possibility of obtaining materials with properties that differ from their respective unary systems regarding the appearance or increase of catalytical activity, selectivity and, in many cases, reduced cost. The aim of this work is the evaluation of the stability and of structural, energetic and electronic properties of 55 atom core-shell bimetallic nanoalloys that combine Pt with transition metals (MT) from the 3d, 4d, and 5d transition periods from Fe to Zn groups. These systems were studied using first principle (ab initio) calculations based on density functional theory, as implemented in the VASP (Viena Ab initio Simulation Package) computer code. The nanoalloys with Pt13MT42 and Pt42MT13 compositions which were observed as putative global minimum configuration (pGMC) presented unique characteristics regarding their geommetry and the arrangement of the different metals within the nanoalloy. Considering the core-shell nanoalloys, two arrangements in which Pt is located in the core were observed (Pt13Ag42 e Pt13 Au42) as well as seven arrangements with Pt in the surface (Pt42Fe13, Pt42Co13, Pt42Ni13, Pt42Cu13, Pt42Ru13, Pt42Rh13 e Pt42Os13). The mechanisms that lead to the formation of these and of the remaining pGMC nanoalloys were investigated considering three factors: atomic radius, surface energy and Bader charges. It was verified that atomic radius and surface energy compete directly for the determination of preferential sites for the atoms in the nanoalloy. When these two factors diverge, the atomic radius is the most important factor. However, when the radii of the species involved are similar, the surface energy becomes the determining factor. In addition, Bader charges analysis showed that, for most nanoalloys, the core is positively charged and the shell accumulates negative charge, indicating that charge is transfered from the atoms in the core to the ones in the surface. Besides, the core-shell nanoalloys have a higher coulombic attraction in comparison with others pGMC, due to high quantities of charge with opposite sign in each region.

Dans cette étude, nous proposons des procédures basées sur des calculs informatiques et des modèles théoriques qui peuvent être utilisés pour prévoir le comportement de certains matériaux membranaires d'intérêt pour les applications de séparation de gaz. En particulier, nous nous sommes concentrés sur: i) des alliages VNiTi de structure cubique centrée, considérés comme de nouveaux matériaux pour les membranes denses sélectives à H2 et ii) sur des matériaux poreux cristallins qui sont des systèmes attractifs pour la séparation de gaz légers tels que H2, O2, CO, CO2, CH4 et N2. Les deux groupes de matériaux sont traités en utilisant une méthodologie différente, adaptée aux besoins des recherches associées à ces matériaux.Dans le cas des membranes métalliques denses, nous nous sommes intéressés à la controverse, connue de longue date, concernant l'occupation de l'hydrogène interstitiel dans les alliages à base de vanadium. Le système V-Ni-Ti est en effet particulièrement intéressant grâce à sa perméabilité élevée pour H2 et à ses propriétés mécaniques accrues par rapport au vanadium pur. Ce travail cible la compréhension de la structure ces alliages à l'échelle atomique, de façon à les optimiser et à activer la conception et le développement de ces matériaux comme nouvelles membranes pour la séparation de H2. Notre approche, basées sur les premiers principes, donne un aperçu des sites préférentiels de l'hydrogène et évalue le rôle des solutés de substitution Ti et Ni, sur l'affinité d'absorption de l'hydrogène. La méthode basée sur la théorie de la fonctionnelle de la densité (DFT) ne nécessite aucune donnée expérimentale autre que l'information sur la structure cristalline. En outre, cette méthode n’utilise aucun paramètre empirique ou d’ajustement, contrairement à d'autres techniques de calcul. Ainsi, cette approche est une voie alternative pour explorer de nouveaux alliages métalliques utilisables comme membranes de séparation de H2. La méthodologie appliquée pourra être utilisé ensuite dans des calculs à haut débit pour cribler diverses compositions d'alliage. Les résultats reportés ici seront utilisés comme guide pour adapter la formulation des solutions solides VNiTi et préparer des membranes en alliage denses à faible coût (par rapport aux membranes à base de palladium) dans le cadre d’autres projets (e.g. Projet Européen FP7- DEMCAMER).Dans le cadre de notre étude sur les membranes cristallines microporeuses, nous démontrons comment les données sur un composant unique peuvent être utilisées pour prédire la sélectivité idéale de ces membranes envers les gaz légers. Des modèles théoriques sont ainsi proposés pour décrire les propriétés de séparation de gaz de matériaux de type zéolithiques (« zeotypes ») en fonction de leurs paramètres structuraux et des conditions de fonctionnement. Les paramètres du modèle peuvent être obtenus expérimentalement ainsi que par le calcul. Afin d’analyser le degré de validité et les limites des modèles, les sélectivités idéales de deux membranes zéolithes classiquement étudiées (NaA, CaA) et d’une membrane MOF (ZIF-8) ont été évaluées. Les résultats démontrent que les expressions théoriques peuvent être utilisées pour le criblage de séries de matériaux microporeux cristallins sous réserve que des données fiables sur l'adsorption de gaz purs soient disponibles pour ces matériaux. Cependant, étant donné que les modèles n’intègrent pas tous les paramètres (notamment liés au design des membranes) et mécanismes impliqués dans le transport de gaz à travers ces membranes, les prédictions doivent être considérées comme celles correspondant à un cas idéal. / In this study, we propose procedures based on computational calculations and theoretical models that can be used to predict the behaviour of some of the membrane materials of interest for gas separation applications. In particular, we focus on: i) body-centred cubic VNiTi alloys as novel materials for H2-selective dense membranes and ii) crystalline porous materials that are attractive media for separation of light gases such as H2, O2, CO, CO2, CH4 and N2. These two types of materials are treated using different methodologies, adapted to the needs of our research objectives associated to each material.In the case of dense metal membranes, the long-standing controversy over occupancy of interstitial hydrogen in V-based alloys is addressed. The V-Ni-Ti system is of particular interest here, exhibiting high H2 permeability and improved mechanical properties relative to pure V. This work intends to gain understanding of hydrogen-metal interactions as function of alloy composition and thereby to optimize these new materials and advance their development as novel membranes for H2 separation. We use a first-principles approach that gives insights into the sites preference of hydrogen and assesses the role of Ti and Ni substitutional solutes for the hydrogen absorption affinity. The method based on Density Functional Theory requires no experimental input except crystal structure information. Furthermore, it uses no empirical or fitting parameters in contrast to other computational techniques. Hence this approach provides an alternative way to explore new metal alloys for H2 separation membranes. The applied methodology can be used further in high-throughput calculations to screen various alloy compositions. The hereto-reported results will be used as guidance for tailoring the formulation of VNiTi solid solutions and preparation of low cost dense alloy membranes in the frame of other projects (e.g. European DEMCAMER project).Further, we explore how single-component inputs can be used to forecast the ideal selectivity towards light gases of crystalline porous materials, used for membrane preparation. Theoretical models for describing gas separation properties of zeotype materials as function of structural characteristics and operation conditions are proposed. The model parameters can be obtained as experimentally as well as computationally. To analyse the extent of validity and limitations of the models, ideal selectivities of few crystalline porous materials are evaluated, including widely used zeolites (NaA, CaA) and a metal organic framework structure (ZIF-8). The results verified that the theoretical expressions could be used for screening series of zeotype materials when reliable single gas adsorption data are available. However, since the models don’t take into account all parameters (namely related to the membrane design) and mechanisms involved in gas transport through porous membranes, their predictions should be considered as values referring to an ideal case.

L'absorption d'hydrogène dans les sites interstitiels des métaux se situe au cœur de problématiques majeures comme la fragilisation des alliages ou le stockage de l'hydrogène à des fins énergétiques. En effet, ce phénomène modifie les propriétés physico-chimiques du métal hôte et peut conduire à la formation de composés ordonnés MHy appelés hydrures. Dans ce cadre, la méthode de modélisation Calphad (CALculation of PHAse Diagrams) constitue un outil pertinent pour comprendre et prédire le comportement des métaux et alliages en présence d'hydrogène. Toutefois, il n'existe pas de base de données Calphad centrée sur l'hydrogène, permettant de calculer des équilibres de phases dans des systèmes multi-constituants (ternaires, quaternaires…).Ainsi, la présente thèse est consacrée à l'étude de systèmes binaires métal-hydrogène (M-H), par une approche modélisatrice multi-échelle. Ces systèmes binaires représentent la première étape à la conception d'une telle base de données Calphad. Tout d’abord, des calculs DFT (Density Functional Theory) systématiques ont été réalisés pour 31 systèmes binaires M-H considérant 30 structures cristallographiques potentielles, soit 30 × 31 = 930 hydrures, stables ou métastables. Cette approche de criblage a permis notamment de déterminer les enthalpies de formation à 0 K, données d’entrée essentielles de la méthode Calphad. De nouveaux hydrures n'ayant jamais été observés expérimentalement ont été prédits à haute pression (TaH2, ZrH3…).Puis, des calculs de phonon dans l'approximation harmonique ont été réalisés sur les hydrures les plus stables. D’une part, ils permettent de corriger les enthalpies de formation issues de la DFT, en tenant compte de l'énergie et de l'entropie dues aux vibrations des atomes, non négligeables pour l'atome léger d'hydrogène. D’autre part, une étude à grande échelle a porté sur les modifications de l'enthalpie libre de formation résultant de la substitution de l'hydrogène par ses isotopes, dit « effet isotopique ». Des prédictions ont pu être réalisées sur la nature de cet effet en fonction de la température. De plus, pour étudier l'insertion aléatoire des atomes d'hydrogène en solution solide, nous avons utilisé des méthodes de thermodynamique statistique : CVM (Cluster Variation Method) et simulation de Monte-Carlo. Ces méthodes ont été implémentées dans des codes de calcul, appliqués aux métaux cubiques faces centrées (cfc) et cubiques centrés (cc). Les données d'entrée de ces codes sont les énergies associées aux interactions locales entre atomes plus proches voisins. Elles sont fournies par la CEM (Cluster Expansion Method) couplée à la DFT. Une étude comparée des systèmes Ni-H et Pd-H a mis en évidence les spécificités des comportements thermodynamiques des solutions solides interstitielles de chacun de ces systèmes. Par ailleurs, la pression en dihydrogène constitue un paramètre important car de nombreux hydrures ne se forment qu'à très haute pression. Afin d'améliorer la précision des modèles Calphad à pression élevée, le modèle de Lu et al. a été appliqué aux phases condensées des systèmes Ni-H, Rh-H et Mg-H. Ce modèle permet de déterminer la contribution à l'enthalpie libre du travail des forces de pression, en admettant en entrée aussi bien des calculs de phonon quasi-harmoniques que des données expérimentales. Enfin, la modélisation Calphad complète du système Ni-H a été réalisée en intégrant le modèle de Lu et al. L'enthalpie de formation calculée par DFT et l'enthalpie de mélange déterminée par CEM ont également été utilisées en données d'entrée, en complément des données expérimentales disponibles / Hydrogen absorption in the interstitial sites of metals is crucial for major issues such as alloy embrittlement or hydrogen storage for energy applications. This phenomenon modifies the physicochemical properties of the host metal and may lead to the formation of ordered MHy compounds called hydrides. Within this framework, the Calphad modeling method (CALculation of PHAse Diagrams) is a relevant tool for understanding and predicting the behavior of metals and alloys in the presence of hydrogen. However, there is no Calphad database centered on hydrogen for calculating phase equilibria in multi-constituent systems (ternary, quaternary…).The present thesis proposes to use a multi-scale modeling approach to study metal-hydrogen (M-H) binary systems, which are the first step in designing such a Calphad database. First, systematic DFT (Density Functional Theory) calculations were carried out for 31 binary M-H systems considering 30 potential crystal structures, resulting in 30 × 31 = 930 hydrides, stable or metastable. This high throughput approach allowed in particular to determine the enthalpies of formation at 0 K, which represent important input data for the Calphad method. New hydrides that have never been experimentally observed could be predicted at high pressure (TaH2, ZrH3 ...).Then, phonon calculations in the harmonic approximation were performed on the most stable hydrides. They allow, on the one hand, to correct the DFT calculated enthalpies of formation by considering the energy and entropy due to the atom vibrations, which are not negligible for the light hydrogen atom. On the other hand, a large-scale study focused on the modification of the free energy of formation due to hydrogen substitution by its isotopes, known as "isotopic effect". Predictions were made on the nature of this effect as function of temperature. Moreover, the random insertion of hydrogen atoms in solid solution was studied using statistical thermodynamic methods: CVM (Cluster Variation Method) and Monte-Carlo simulation. These methods have been implemented in calculation codes, applied to face centered cubic (fcc) and body-centered cubic (bcc) metals. The input data are the interactions energies between nearest neighbor atoms. They are provided by the CEM (Cluster Expansion Method) coupled with DFT calculations. A comparative study of the Ni-H and Pd-H systems revealed the specificities of the thermodynamic behaviors of both solid interstitial solutions. Furthermore, dihydrogen pressure is an important parameter because many hydrides form only at very high pressure. To improve the Calphad model accuracy at high pressure, the model of Lu et al. was applied to the condensed phases of the Ni-H, Rh-H and Mg-H systems. This model allows to determine the contribution to the free enthalpy due to the pressure force work. The input data may be both quasi-harmonic phonon calculation results and experimental data. Finally, a comprehensive Calphad model of the Ni-H system was carried out by integrating the model of Lu et al. The DFT enthalpy of formation and the mixing enthalpy determined by CEM were used as input data, to complement the available experimental data

Cette thèse vise à étudier la diffusion protonique, et dans une moindre mesure ionique, au sein d’un matériau électrolyte pour pile à combustible BaCeO3 dopé Gd, en adoptant une démarche multi-échelle. Tout d’abord, des calculs ab initio ont été réalisés afin de déterminer les positions stables des défauts protoniques OH_O et des lacunes d’oxygène VO dans le matériau. Puis, en utilisant toujours le formalisme de la théorie de la fonctionnelle de la densité, les barrières d’énergies pour les deux types de défauts entre deux positions stables ont été calculées. Enfin, ces barrières ont été utilisées dans un algorithme de Monte-Carlo cinétique afin de simuler des trajectoires de protons et de lacunes d’oxygène. Cette méthode permet d’accéder à des grandeurs macroscopiques, accessibles expérimentalement, telles que l’énergie d’activation, le coefficient de diffusion ou la mobilité, en se basant uniquement sur des données atomiques issues de simulations ab initio. Le gadolinium semble être un dopant intéressant pour le cérate de barium au vu de son faible pouvoir attractif sur le proton : il permet ainsi la création de nombreuses lacunes d’oxygène, qui pourront incorporer des molécules d’eau, sans toutefois piéger l’hydrogène. Ces deux conditions sont nécessaires pour obtenir un bon électrolyte pour les oxides solides conducteurs de protons. / This thesis deals with the study of protonic diffusion, and to a lesser extent ionic, inside a Gd-doped BaCeO3, a possible electrolyte for fuel cell, using a multi-scale approach. First of all, first principles calculations have been made to determine stable positions for protonic defects OH_O and oxygen vacancies V O in the material. Then, using the same formalism of density functional theory, energy barriers for both kinds of defects have been computed between two stable positions. Finally, these barrier heights have been used in a Kinetic Monte-Carlo algorithm to simulate trajectories of protons or oxygen vacancies. This method allows to access macroscopic values that can be found by experiments, such as activation energy, diffusion coefficient or mobility, using only atomic data coming from ab initio simulations. Gadolinium seems to be an interesting dopant in barium cerate considering its weak power of attraction on the proton: the introduction of gadolinium creates lots of oxygen vacancies that will be able to incorporate water molecules, without trapping the hydrogen. Both these conditions are necessary to get a good electrolyte for protonic ceramic fuel cell.

Neste trabalho, utilizamos o desenvolvimento recente do método DFT/LDA-1/2 para cálculos de estados excitados em materiais. Começamos com um resumo da teoria do funcional da densidade (DFT) e incluímos uma introdução ao método LDA-1/2 para cálculos de excitações em sólidos. Na compilação dos resultados esperamos ter demonstrado a utilidade do LDA-1/2 para cálculos de alinhamentos de bandas em junções semicondutor/semicondutor e semicondutor/isolante. A aplicação do método envolve o conhecimento da química básica dos sistemas. Para tanto, escolhemos sistemas importantes para diversas aplicações, e cujos modelos de simulação estão o limite ou fora do alcance de metodologias que envolvem alto custo computacional, mas que foram bem caracterizados experimentalmente. Concentramos nossas ações no estudo da capacidade preditiva do LDA-1/2 para alinhamentos de bandas, os chamados band offsets, particularmente importantes para a micro e optoeletrônica. Quando não foi possível compararmos nossos resultados com o experimento, procuramos a comparação com métodos estado-da-arte como GW. Bons resultados foram obtidos para band gaps e band offsets de interfaces A1As/GaAs, Si/SiO2, A1N/GaN e CdSe/CdTe, que representam os diferentes tipos de jun_c~oes poss__veis, com (e.g. A1As/GaAs, A1N/GaN) e sem (e.g. Si/SiO2, CdSe/CdTe) ^anions omuns, com (e.g. A1As/GaAs) e sem (e.g. CdSe/CdTe, Si/SiO2) casamento de parâmetros de rede e diferentes tipos de alinhamentos (\"straddling\", e.g. A1As/GaAs ou \"staggered\"e.g. CdSe/CdTe). Analisamos de maneira sistemática o comportamento do entorno do bandgap ao longo da interface, verificando plano a plano atômico o comportamento das bordas de valência e condução com LDA-1/2 em comparação com o LDA, ou comparando diferentes modelos dentro do LDA-1/2, como o caso do CdSe/CdTe e do Si/SiO2. Para o caso A1As/GaAs, aproveitamos o casamento de parâmetros de rede dos semicondutores constituintes e tentamos um modelo de interface de ligas A1xGa1-x As/GaAs para estudar a variação de valência, condução e bandgap em função da composição x. No AlN/GaN, estudamos também os offsets com as contribuições dos orbitais separadamente. Em todos os casos o LDA-1/2 levou-nos a resultados interessantes com modelos simples. A exploração de novas fronteiras de aplicação do método fez-se necessária com a diminuição da dimensionalidade dos sistemas, de 3D (bulk ) para 2D (interfaces) e depois para 1D, ou seja, _os quânticos (\"nanofios\"). Nosso material de estudo para os foi o ZnO que, além da motivação oriunda de conhecidas aplicações em optoeletrônica, apresenta desafios para simulações bulk com qualquer método, e que foi abordado com certo sucesso usando o LDA-1/2 anteriormente, sendo que para fios quânticos encontramos resultados interessantes em geometrias triangulares que facilitaram os modelos. Calculamos o bandgap ZnO bulk e de nanofios passivados e não passivados com hidrogênios usando LDA e LDA-1/2 sem polarização de spin. As estruturas de bandas e o bandgap como função do diâmetro do ano_o foram calculados e ajustes com funções de decaimento foram feitos para comparação, por extrapolação, dos bandgaps com valores experimentais. Foi possível comparar nossos resultados de fios com o bulk, e predizer uma faixa de variaação de bandgaps que os experimentais podem encontrar para nanofios triangulares de ZnO. Também foi feita análise de energias de confinamento em fios quânticos de ZnO, comparando o LDA com LDA-1/2. Finalmente, mostramos os resultados de uma oportunidade de aplicação do método a um material com defeitos, recentemente descoberto e promissor, e com enorme mercado potencial em fotocatálise, o Ti1-O4N. Nosso trabalho envolveu a aplicação do LDA-1/2 a um problema muito desafiador, e.g. a geração de energia limpa, especificamente a separação da molécula de água para produção de hidrogênio. O desafio maior vem da dificuldade de predição de bandgaps teoricamente, em particular para sistemas grandes como é o caso de modelos atomísticos com defeitos, devido aos altos custos computacionais envolvidos. Tais dificuldades forçam os pesquisadores a usarem parâmetros ajustáveis ou métodos semi-empíricos, ou modelos simplificados demais para descrever precisamente resultados experimentais. Isto dificulta o estudo dos sistemas fotocatalíticos potencialmente eficientes e que não foram ainda caracterizados ou otimizados. O LDA-1/2 é aqui validado para esta classe de materiais, abrindo assim a oportunidade para estudar sistemas mais realísticos e complexos para cálculos ainda mais precisos, particularmente para geração de energia limpa. Em particular, modelamos o TiO2 na estrutura rutile com nitrogênio substitucional, cuja estrutura eletrônica é ainda debatida. Foi a primeira aplicação do LDA-1/2 a sistemas com algum tipo de defeito, com ótimos resultados para o novo sistema Ti1- _O4N com vacâncias de Ti. / In this work, we used the recent development of DFT/LDA-1/2 method for calculations of excited states in materials. We begin with a summary of the density functional theory (DFT) and included an introduction to the method LDA-1/2 for calculations of excitations in solids. In compiling the results we hope to have demonstrated the usefulness of the LDA-1/2 for calculating alignments of bands at junctions semiconductor / semiconductor and semiconductor / insulator. The method involves the knowledge of basic chemical systems. To do this we chose systems important for several applications, and simulation models which are the limit or beyond the reach of methodologies involving high computational cost, but have been well characterized experimentally. We focus our actions in the study of the predictive capability of the LDA-1/2 for alignments of bands, the band called offsets, particularly important for micro and optoelectronics. When it was not possible to compare our results with experiment, we compared the methods with state of the art as GW. Good results were obtained for band gaps and band offsets of interfaces A1As/GaAs, Si/SiO2, A1N/GaN and CdSe / CdTe, which represent the different types of jun_c poss__veis-tions, with (eg A1As/GaAs, A1N/GaN) and without (eg Si/SiO2, CdSe / CdTe) ^ omuns anions with (eg A1As/GaAs) and without (eg CdSe / CdTe, Si/SiO2) matching network parameters and different types of alignments (\"straddling\" eg A1As/GaAs or \"staggered\" eg CdSe / CdTe). Systematically analyze the behavior of the environment along the interface bandgap, plane by plane scanning behavior of the edges atomic valence and conduction with LDA-half in comparison with LDA, or comparing templates within the LDA-1 / 2, as the case of CdSe / CdTe and Si/SiO2. For the case A1As/GaAs, we take the marriage of network parameters of semiconductor components and try an interface model alloys A1xGa1-x As / GaAs to study the variation of valence, conduction and bandgap as a function of composition x. In the AlN / GaN, we also studied the offsets with the contributions of the orbitals separately. In all cases the LDA-half led us to interesting results from simple models. The exploration of new frontiers of the method was necessary to decrease the dimensionality of the systems, the 3D (bulk) for 2D (interfaces) and then to 1D, ie, quantum _os (\"nanowires\"). Our study material for the ZnO was that, apart from the motivation coming from known applications in optoelectronics, presents challenges for bulk simulations with any method, and that was addressed with some success using the LDA-half earlier, and for wireless find interesting results in quantum triangular geometries that facilitated models. We calculate the bandgap and bulk ZnO nanowires passivated and not passivated with hydrogen using LDA and LDA-1/2 without spin polarization. The bandgap structures and strips as a function of the diameter of ano_o adjustments are calculated and decay functions for comparison were made by extrapolation of the bandgaps with experimental values. It was possible to compare our results with the bulk of wires, and predict a range of bandgaps that variaação can find experimental triangular ZnO nanowires. It was also made analysis of energy confinement in ZnO quantum wires, comparing LDA with LDA-1/2. Finally, we show the results of an opportunity to apply the method to a material with defects, newly discovered and promising, and with huge market potential in photocatalysis, the Ti1-O4N. Our work involved the application of LDA-1/2 to a very challenging problem, eg the generation of clean energy, specifically the separation of the water molecule for hydrogen production. The main challenge has been the difficulty of predicting bandgaps theoretically, in particular for large systems such as the model atomistic defects because of the high computational costs involved. These difficulties force the researchers to use adjustable parameters or semi-empirical methods, or other simplified models to accurately describe experimental results. This complicates the study of potentially efficient photocatalytic systems which have not yet been characterized or optimized. The LDA-1/2 is here validated for this class of materials, thus opening the opportunity to study more realistic and complex systems for more accurate calculations, particularly for clean energy generation. In particular, we modeled the structure of TiO2 in the rutile with substitutional nitrogen, whose electronic structure is still debated. It was the first application of the LDA-1/2 systems with some kind of defect, with excellent results for the new system Ti1-_O4N with Ti vacancies.

Orientador: Rogerio Custodio / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-11T20:32:26Z (GMT). No. of bitstreams: 1 Souza_KelyFerreirade_M.pdf: 2708513 bytes, checksum: 018efeaf228676a57ca3b884dc55ca80 (MD5) Previous issue date: 2008 / Resumo: Piroxicam é um fármaco com atividade analgésica, antiinflamatória e antipirética de uso muito difundido. Apresenta algumas reações adversas, dentre as quais a possibilidade de foto-sensibilidade cutânea após exposição do paciente à radiação solar. Por este motivo, o fármaco tem sido alvo de inúmeros estudos, mas até os dias atuais não se conhece o mecanismo envolvido na foto-sensibilidade. Com o objetivo de buscar maiores informações sobre o fármaco, o presente trabalho partiu de estudos preliminares tanto da forma ceto quanto da enólica, através do método AM1. Os resultados, em conjunto com dados experimentais, apontaram para o Piroxicam enol como principal tautômero envolvido nos mecanismos de foto-toxicidade. Construiu-se então uma superfície de potencial para o Piroxicam enol, agora com o método DFT/B3LYP/CEP-31G(d,p). Estudaram-se as barreiras de interconversão entre os confôrmeros mais estáveis através do método QST2. Calculou-se o espectro eletrônico destas espécies empregando o método TD-DFT/B3LYP/CEP-31G(d,p) utilizando-se o modelo PCM para inclusão do efeito de solvente. De posse do confôrmero de maior interesse, partiu-se para o estudo de uma proposta de mecanismo obtida da literatura para a reação entre o Piroxicam enol e a molécula de oxigênio no primeiro estado excitado singlete. Partindo-se da otimização em DFT/B3LYP/CEP-31G(d,p) das geometrias das espécies envolvidas no mecanismo, realizou-se a busca por estados de transição entre as mesmas através dos métodos QST2 e QST3. Estruturas de transição entre os caminhos testados foram obtidas, mostrando a possibilidade do mecanismo ocorrer. Os resultados do trabalho apontam, ainda, para um dos confôrmeros como o mais provável de se envolver em foto-reações / Abstract: Piroxicam is a widely used drug with analgesic, antiinflamatory and antipiretic properties. Undesirable side effects are observed in some patients, among which the photosensitivity skin after exposure to solar radiation. For that reason the drug has been widely studied, but until the present days the mechanism involved in the photosensitivity is still unknown. Looking for more information about the drug, preliminary studies were carried out on two tautomers of Piroxicam ¿ ceto and enol structures, using the AM1 method. The calculated and experimental data suggest that Piroxicam Enol is the main tautomer involved in the phototoxicity mechanisms. A new potential surface, now using the DFT/B3LYP/CEP- 31G(d,p) method, was calculated. The interconversion barriers were studied using the QST2 method. The electronic spectrum was calculated with the TD-DFT/B3LYP/CEP- 31G(d,p) method and the solvent effect was investigated applying the PCM model. The predominant conformer was studied in combination with the singlet excited oxygen molecule and a mechanism proposed in the literature was investigated. From the optimized geometries of the species involved in this mechanism, the transition states between them were studied using the QST2 and QST3 methods indicating a feasible reaction path. The results also show one of the conformers as the most important agent responsible by the photochemical sensitivity / Mestrado / Físico-Química / Mestre em Química

Neste trabalho estudamos propriedades estruturais, magnéticas e eletrônicas de complexos de cobre em duas classes de ligantes semelhantes com potenciais aplicações nas áreas de nanotecnologia e farmacologia. A primeira classe de ligantes é do tipo bases de Schiff, conhecidas por sua vasta gama de aplicações em química e biologia, aqui estudadas como miméticos de sítios de proteínas e também como potenciais metalo-fármacos. Procuramos investigar as características estruturais e eletrônicas buscando correlacionar as informações obtidas experimentalmente com as obtidas através de cálculos de estrutura eletrônica. Em especial, no estudo da competição pelo íon Cu entre as bases de Schiff e a proteína albumina, mostramos a influência de diversos fatores como, por exemplo, a geometria dos ligantes e a sua estrutura eletrônica. A segunda classe estudada é constituída de bases modificadas de DNA com a habilidade de complexar metais (espécie [M-DNA]) e formar estruturas com acoplamento ferromagnético. Observamos para as espécies [M-DNA] que o acoplamento ferromagnético é estabilizado através de dois efeitos: a) diferentes estados de carga que podem gerar distorções na coordenação quadrado planar; b) a adição do backbone do DNA. Utilizamos neste estudo o método Projector Augmented Wave (PAW) e também bases locais dentro da Teoria do Funcional da Densidade, atrav´es dos c´odigos computacionais CP-PAW e Gaussian03. / We studied structural, electronic and magnetic properties of copper complexes with two similar classes of ligands with potential applications in nanotechnology or pharmacology. The first class of compounds consists of Schiff bases and corresponding copper complexes. We investigated structural and electronic properties of these complexes searching for correlations among the information obtained experimentally and by electronic structure calculations. In special, in the study of the competition for the Cu ion between Schiff bases and the albumin protein, we show the influence of different factors such as the geometry of the ligands and their electronic structure. The second class of ligands focused in our studies are modified DNA bases coordinated to copper, where EPR studies have shown ferromagnetic interactions among the metal centers. We studied in this case the influence of the charge state and of the backbone in the magnetization of the Cu chain. In all cases presented here, we used ab-initio electronic structure calculations in the framework of the Density Functional Theory (DFT). This has been done using the CP-PAW code and the Gaussian03 code.

A maior parte da energia consumida no mundo provém da queima de combustíveis fósseis, responsáveis pelo desenvolvimento da sociedade moderna, mas, também, por diversos danos ao meio ambiente. Desse modo, a exploração de fontes limpas e alternativas de energia renovável, tais como a solar e a eólica, tem ganhado grande importância e atenção dos pesquisadores nos últimos anos. Entretanto, o armazenamento destas formas de energia precisa ser eficiente, para que possam ser utilizadas nos períodos de indisponibilidade da fonte. Neste sentido, as baterias são uma das mais eficazes formas de geração e armazenamento de energias renováveis. No entanto, seu uso ainda é limitado, principalmente para aplicação em veículos automotores, que são um dos responsáveis pela emissão de gases poluentes. O principal fator limitante está relacionado às baixas densidades energéticas nos materiais utilizados para compor os elementos das baterias modernas. O desenvolvimento experimental de novos materiais, para este fim, é um trabalho demorado e caro, de tal forma que métodos teóricos têm sido usados com sucesso para prever o comportamento de novos materiais, apropriados para utilização como componentes básicos das baterias. Neste projeto serão investigados, teoricamente, materiais nanoestruturados para aplicação como eletrodos de baterias de íons de lítio, uma das formas mais populares de baterias atualmente. Neste trabalho apresentamos as propriedades estruturais, eletrônicas e vibracionais da grafite e dos sistemas bidimensionais grafeno, SiC e CN utilizando cálculos ab initio baseados na teoria do funcional da densidade, onde se utilizou o método de pseudopotencial PAW (Projector Augmented Wave) e aproximação van der Waals (optB88-vdW) para o termo de troca-correlação. A grafite e o grafeno foram estudados para validar o método aplicado, o qual foi usado para estudar as estruturas de SiC (bicamadas) e CN (monocamada e bicamada) como possíveis nanomateriais para compor ânodos de baterias de íons de lítio. Estudamos a adsorção de Li nas bicamadas de SiC em diferentes sítios encontrando a posição estável e as possíveis vantagens e desvantagens de utilizá-las como ânodos de baterias de íons de lítio. As estruturas CN não se mostraram ser sistemas viáveis, uma vez que são dinamicamente instáveis. No entanto, estudamos as propriedades de camadas de CN hidrogenadas, que são dinamicamente estáveis e possíveis candidatas para aplicação em baterias de lítio. / Most of the energy consumed in the world comes from the burning of fossil fuels, responsible for the development of modern society, but also for various damages to the environment. Thus, the exploration of clean and alternative sources of renewable energy, such as solar and wind, has gained great importance and attention from researchers in recent years. However, the storage of these forms of energy must be efficient, so that they can be used during periods of unavailability of the source. In this sense, batteries are one of the most effective forms of generation and storage of renewable energies. However, its use is still limited, mainly for application in automotive vehicles, which are one of the responsible for the emission of polluting gases. The main limiting factor is related to the low energy densities in the materials used to compose the elements of modern batteries. The experimental development of new materials for this purpose is a time consuming and expensive work, so that theoretical methods have been used successfully to predict the behavior of new materials suitable for use as basic components of the batteries. In this project it will be investigated, theoretically, nanostructured materials for application as electrodes of lithium-ion batteries, currently, one of the most popular forms of batteries. In this work we present the structural, electronic and vibrational properties of graphite and two-dimensional graphene, SiC and CN systems using ab initio calculations based on the density functional theory, where the pseudopotential PAW (Projector Augmented Wave) method and van der Waals approximation (optB88-vdW) for the exchange-correlation were used. Graphite and graphene were studied to validate the applied method, which was used to study the SiC (bilayer) and CN (monolayer and bilayer) structures as possible nanomaterials to compose anodes of lithium-ion batteries. We studied the adsorption of Li in the SiC bilayers at different sites, finding the stable position and the possible advantages and disadvantages of using them as anodes of lithium-ion batteries. The CN structures have not been shown to be viable systems, since they are dynamically unstable. However, we have studied the properties of hydrogenated CN layers, which are dynamically stable and possible candidates for application in lithium batteries.

Desde a identificação do vírus da imunodeficiência humana (HIV, do inglês Human Immunodeficiency Virus) como agente causador da Síndrome da Imunodeficiência Adquirida (AIDS ? do inglês Acquired Immunodeficiency Syndrome), a busca para tratamentos seguros e eficazes contra o HIV transformou-se no principal foco para a descoberta de uma nova droga em todo o mundo. A AIDS aparece como um dos principais problemas de saúde pública para as próximas décadas, onde será o maior determinante de mortalidade na faixa etária entre 20 e 50 anos em praticamente todos os países do mundo. Tendo como objetivo relacionar a atividade de compostos biflavonóides anti-HIV-1 com algumas de suas propriedades moleculares, serão utilizados métodos de Mecânica Molecular e Química Quântica. O método de cálculo semi-empírico AM1 foi empregado para calcular um conjunto de propriedades moleculares dos 14 compostos biflavonóides com atividade anti-HIV-1. A seguir utilizar-se-á métodos estatísticos com a finalidade de separar os 14 compostos em duas classes, ativos e não ativos, de forma que se relacione qual as propriedades, dentre as calculadas, são responsáveis pela atividade dos compostos biflavonóides estudados. As técnicas estatísticas utilizadas foram a Análise de Componentes Principais (PCA: Principal Components Analysis), Análise Hierárquica de Agrupamentos (HCA: Hierarquical Clusters Analysis) e Análise de Discriminates por Passos (SDA: Stepwise Discriminant Analysis). Os estudos com PCA, HCA, e SDA mostraram que as variáveis HOMO (Highest Occupied Molecular Orbital - Orbital Molecular Ocupado de Maior Energia), LUMO (Lowest Unoccupied Molecular Orbital ? Orbital Molecular Desocupado de Menor Energia), e Área superficial são responsáveis pela separação dos compostos com alta e baixa atividade anti-HIV-1. O comportamento destas três propriedades pode ser útil na tentativa de se obter outros compostos biflavonóides com elevada atividade inibidora anti-HIV-1. / A set of 25 quinone compounds with anti-trypanocidal activity was studied by using the Density Functional Theory (DFT) method in order to calculate electronic atomic and molecular properties to be correlated with the biological activity. The chemometric methods Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA), Stepwise Discriminant Analysis (SDA), Kth nearest neighbor (KNN) and Soft Independent Modeling of Class Analogy (SIMCA) were used to obtain possible relationships between the calculated descriptors and the biological activity studied and predict the anti-trypanocidal activity of new quinone compounds from a test set. Four descriptors were responsible for the separation between the active and inactive compounds: T5 (torsion angle), HOMO-1 (energy of the first molecular orbital below HOMO), QTS1 (sum of absolute values of the atomic charges) and VOLS2 (volume of the substituent at region B). These descriptors give information on the kind of interaction that occurs between the compounds and the biological receptor. The prediction study was done with a set of three new quinone compounds by using the PCA, HCA, SDA, KNN and SIMCA. Beside the five chemometric methods, the neural network method was used by employing the backpropagation algorithm. The four variables (T5, QTS1, VOLS2 and HOMO-1) were employed to validate the models constructed previously. The architecture of networks consisting of four neurons at input layers, ten neurons at intermediary layers and two neurons at output layers was adopted to observe the root mean square error between the true and desired output over the entire training set. The percentage of correct classification was 87.5%, and only one compound was predicted wrong in the test set, which indicates that the model is robust and could be able to make predictions. The docking studies were carried out with two different programs in the approach of ligands: the Autodock and FlexX. The docking results on trypanothione reductase enzyme showed that all studied compounds stay at second hydrophobic pocket in the outer region of the active site called the Z-site. The residues that could be specifically involved in the binding of ligands are Lys62, Thr66, Thr397, Thr463, Leu399, Ser464, Glu466 and Glu467, where the residues Thr66, Thr463 and Leu399 are conserved in different trypanothiones and could be used for the development of selective inhibitors against to the parasite enzyme.