Stuttgart, Univ., Diss., 2005.

Stuttgart, Univ., Diss., 2007.

Techn. University, Diss., 2001--Berlin.

Universiẗat, Diss., 1999--Dortmund.

Techn. Hochsch., Diss., 1999--Aachen.

Techn. Hochsch., Diss., 2001--Aachen.

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

This thesis presents i-PI, a new path integral molecular dynamics code designed to capture nuclear quantum effects in ab initio electronic structure calculations of condensed phase systems. This software has an implementation of estimators used to calculate a wide range of static and dynamical properties and of state-of-the-art techniques used to increase the computational efficiency of path integral simulations. i-PI has been designed in a highly modular fashion, to ensure that it is as simple as possible to develop and implement new algorithms to keep up with the research frontier, and so that users can take maximum advantage of the numerous electronic structure programs which are freely available without needing to rewrite large amounts of code. Among the functionality of the i-PI code is a novel integrator for constant pressure dynamics, which is used to investigate the properties of liquid water at 750 K and 10 GPa, and efficient estimators for the calculation of single particle momentum distri- butions, which are used to study the properties of solid and liquid ammonia. These show respectively that i-PI can be used to make predictions about systems which are both difficult to study experimentally and highly non-classical in nature, and that it can illustrate the relative advantages and disadvantages of different theoretical methods and their ability to reproduce experimental data.

Made available in DSpace on 2015-05-14T13:21:20Z (GMT). No. of bitstreams: 1 ArquivoTotal.pdf: 4842604 bytes, checksum: 4468b46dc88edcbb8cca74341b5e5b91 (MD5) Previous issue date: 2013-03-05 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / A research of nitrosamines has attracted attention, because its complexation with the heme group of cytochrome P450 may be associated with its carcinogenic potential. To characterize this problem is realized quantum-chemical treatment of some molecules from the class of metalloporphyrins, more specifically porphyrins Iron III, similar to the heme group mentioned above, with two nitrosamines ligand in axial positions. In literature it is reported that, experimentally these ferroporfirinas containing nitrosamines ligand, have been synthesized and structurally characterized and properly paramagneticamente (sextet spin state). As the Fe (III), this complex presents three possible spin state (doublet, quartet and sextet) the motivation of this dissertation is to know theoretically what spin state most likely to be the ground state, with the goal of describing the relative energy difference between these spin states of Fe (III), as well as characterize the order of energy d orbitals belonging to unpaired electrons alpha iron, these three electronic states. Thus, we performed single-point calculations analyzing guess in control from the beginning of SCF calculation using the program Gaussian09, using ab initio methods at the Hartree-Fock version restricted and unrestricted layer to open with charges calculations NBO, with a set base 6-31 + g *, 6-311 + g * and cc-pVDZ. The method UHF / cc-pVDZ, with Harris and guess Rdscale, showed the best results, providing the lowest energies and contamination in spin states. For the method using the ROHF wave function generated by calculating the spin state in the method sextet UHF wave function as initial results showed parallel to the UHF method. The Mulliken population analysis and NBO, the shape of the spin density and the electrostatic potential map show the location of the positive charge on the iron. The d orbitals, the unpaired electrons, show no overlap with the orbitals of dimetilnitrosaminas. The main contribution of the theoretical results are in agreement with the experimental results available in the literature about the spin state of Fe (III) heme group in the interaction with the dimetilnitrosaminas. / A pesquisa envolvendo nitrosaminas tem despertado interesse, pois a sua complexação com o Fe do grupo heme do Citocromo P450 pode estar associada ao seu potencial carcinogênico. Para caracterização deste problema, realiza-se o tratamento químico-quântico de algumas moléculas da classe das metaloporfirinas, mais especificamente porfirinas de Ferro III, similares às do grupo heme citado acima, com duas nitrosaminas ligadas nas posições axiais. Na literatura é relatado que, experimentalmente, estas ferroporfirinas contendo ligantes nitrosaminas, já foram sintetizadas e devidamente caracterizadas estruturalmente e paramagneticamente (estado de spin sexteto). Como o Fe(III), neste complexo, apresenta três possibilidades de estado de spin (dubleto, quarteto e sexteto) a motivação desta dissertação é saber teoricamente qual o estado de spin mais provável para ser o estado fundamental, com o objetivo de descrever a diferença de energia relativa entre esses estados de spin do Fe(III), assim como caracterizar a ordem de energia dos orbitais d, pertencentes aos elétrons alfa desemparelhados do ferro, nesses três estados eletrônicos. Para isso, foram realizados cálculos single-point analisando os chutes iniciais no controle do início do cálculo SCF utilizando o programa Gaussian09, empregando métodos ab initio a nível Hartree-Fock na versão restrita e irrestrita para camada aberta com cálculos de carga NBO, com um conjunto de base 6-31+G*, 6-311+G* e cc-pVDZ. O método UHF/cc-pVDZ, com o chute inicial para função de onda Harris e Rdscale, apresentaram os melhores resultados, fornecendo as menores energias e contaminações nos estados de spin. Para o método ROHF utilizando a função de onda gerada pelo cálculo no estado de spin sexteto no método UHF, como função de onda inicial, mostrou resultados paralelos ao do método UHF. A análise populacional de Mulliken e NBO, a forma da densidade de spin e o mapa do potencial eletrostático evidenciam a localização da carga positiva no ferro. Os orbitais d, dos elétrons desemparelhados, mostram nenhuma sobreposição com os orbitais das dimetilnitrosaminas. A principal contribuição dos resultados teóricos, está na concordância com o resultado experimental disponível na literatura, sobre o estado de spin do Fe(III) do grupo Heme na interação com as dimetilnitrosaminas.

Les propriétés des matériaux ferroélectriques proviennent principalement de l’influencedes conditions aux limites et des déformations sur la polarisation. Cette influence est encoreplus grande à de petites échelles ou des structures particulières de la polarisation apparaissent,comme les vortex dans les cubes quantiques ou des structures en rayures dans lescouches minces. Pour le calcul, à très basses échelles, de telles structures de polarisation, lesHamiltonien effectifs, basés sur les calculs ab-initio sont les plus utilisés. Parallèlement Lesmodèles continus sont préconisés à plus grandes échelles. Néanmoins, il n’existe pas de lienentre ces deux modèles. Le but de cette thèse est alors de construire une approche permettantde relier ces deux modèles et par cela même ces différentes échelles.Notre modèle se base sur un Hamiltonien effectif écrit pour le titanate de baryum enfonction de la polarisation et des déformations. Cet Hamiltonien est reformulé de façon àdécrire un milieu continu. Les difficultés de cette reformulation proviennent des interactionsnon locales. Le résultat est alors un système d’équations aux dérivées partielles, décrivantl’équilibre et les conditions aux limites. La température est ensuite introduite de façon effectivedans les coefficients de ces équations. Notre modèle ressemble fortement aux modèlesde Landau.Une telle approche est appliquée dans les cubes quantiques et les couches minces óu l’organisationdes domaines dépend de la taille. Les résultats montrent l’implication de la méthodedes éléments finis sur la précision. La formation de vortex dans les cubes quantiquesest bien reproduite. L’agencement en domaines de polarisation alternée dans les couchesminces est elle aussi bien reproduite pour les couches minces. De plus en augmentant l’épaisseurde ces couches minces, la périodicité de cet agencement alterné est modifié, comportementdécrit par la loi de Kittel qui est ici calculée et comparée aux résultats expérimentaux. / Physicals properties of ferroelectric materials mainly arise from the fact that the polarizationis strongly influenced by strain and electrical boundary conditions, which may changeits orientation and magnitude. At small scales, this influence is even stronger and unusualdomain structures are produced like vortices in quantum dots or stripes in thin films. For thecalculation of domain structures, at small scales, first-principle-based effective Hamiltonianare widely used whereas at higher scales, continuum models are predominants. Nevertheless,in between there is no computational method connecting both scales. Therefore„ thegoal of this dissertation is to develop and build new approaches in order to bridge these twoseparated scales.Our model stems for classical effective Hamiltonian, written for barium titanate as afunction of the polarization and strain. This Hamiltonian is then formulated in order tocorrespond to a continuous description. Difficulties arise from non local interactions. In theend, the Hamiltonian is transformed into a set of partial differential equations describing theequilibrium and the boundary conditions. The temperature is then introduced in such a waythat makes evolve the coefficients of those sets of equations. We therefore reconstructed aLandu-like model.Such approach can be applied in quantum dots and thin films where the domain organizationdepend on the size. The results show how to apply finite element in order to obtainpatterns of polarizations with the wanted precision. The vortices shapes of domain patternin quantum dots is well reproduced. The stripes-like polarization pattern is also well reproducedin thin films. Besides expanding thickness of those films change the periodicity ofthose stripes, behaviour described by the Kittel law. This law is calculated and compared tomeasurements.