abstract: The passivity of metals is a phenomenon of vast importance as it prevents many materials in important applications from rapid deterioration by corrosion. Alloying with a sufficient quantity of passivating elements (Cr, Al, Si), typically in the range of 10% - 20%, is commonly employed to improve the corrosion resistance of elemental metals. However, the compositional criteria for enhanced corrosion resistance have been a long-standing unanswered question for alloys design. With the emerging interest in multi-principal element alloy design, a percolation model is developed herein for the initial stage of passive film formation, termed primary passivation. The successful validation of the assumptions and predictions of the model in three corrosion-resistant binary alloys, Fe-Cr, Ni-Cr, and Cu-Rh supports that the model which can be used to provide a quantitative design strategy for designing corrosion-resistant alloys. To date, this is the only model that can provide such criteria for alloy design.The model relates alloy passivation to site percolation of the passivating elements in the alloy matrix. In the initial passivation stage, Fe (Ni in Ni-Cr or Cu in Cu-Rh) is selectively dissolved, destroying the passive network built up by Cr (or Rh) oxides and undercutting isolated incipient Cr (Rh) oxide nuclei. The only way to prevent undercutting and form a stable protective passive film is if the concentration of Cr (Rh) is high enough to realize site percolation within the thickness of the passive film or the dissolution depth. This 2D-3D percolation cross-over transition explains the compositional dependent passivation of these alloys. The theoretical description of the transition and its assumptions is examined via experiments and kinetic Monte Carlo simulations. The initial passivation scenario of the dissolution selectivity is validated by the inductively coupled plasma mass spectrum (ICP-MS). The electronic effect not considered in the kinetic Monte Carlo simulations is addressed by density functional theory (DFT). Additionally, the impact of the atomic configuration parameter on alloy passivation is experimentally measured, which turns out to agree well with the model predictions developed using Monte Carlo renormalization group (MC-RNG) methods. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Biofuel additives have been designed to reduce vehicular emissions to the atmosphere to limit the effects of greenhouse gases on global climate change. The chemical properties of common biofuel additives exhibit ideal characteristics for use in gasoline and diesel, while limiting emissions from exhaust. As biofuel additives begin to be administered regularly to gasoline and fuel sources, the compounds will appear in spill sites, posing a risk to groundwater sources. The interactions that occur between common biofuel additives and trapped gases below the water table were analyzed in this work to further understand the potential consequences on quasi-saturated groundwater zones. The behaviour of trapped gases contaminated with different biofuel additives were analyzed in laboratory experiments conducted in a two-dimensional flow cell to demonstrate the mechanisms of gas flow through a capillary barrier resulting from modified interfacial properties in the presence of a chemical surfactant. Contamination of gas-fluid interfaces by applied biofuel additives at the pore scale resulted in the breakthrough of gas through the capillary barrier. Gas migration terminated at a critical pool height proportional to the reduction in interfacial tension induced by the administered biofuel additives. To further demonstrate the relationship between interfacial tension and critical gas pool height, an interfacial tension-macroscopic invasion percolation model was developed to simulate the transport mechanisms and behaviours of gas flow when an immobile pool is contaminated with 1-Butanol. The findings in this study provide a fundamental understanding of the mechanisms and behaviours of gas mobilization in the presence of common biofuel additives. / Thesis / Master of Science (MSc) / The use of biofuel additives in gasoline and diesel fuels has become an attractive alternative to fully petroleum-based fuels to reduce the release of vehicular greenhouse gases to the atmosphere. As fuel spills and storage tank leaks continue to be a primary source of groundwater contamination, the appearance of biofuel additives in contaminated systems will appear below the subsurface as they continue to be administered to modern gasoline and diesel fuels. This work investigated the consequences of biofuel contamination of groundwater systems containing gas trapped within pore spaces through the use of laboratory experiments and numerical modelling. Contamination of these systems with different biofuel additives displayed a similar response, in which gas had mobilized from within pore spaces and released to the atmosphere. Mobilization of trapped gas in groundwater can alter the primary hydraulic properties that characterize a particular hydrogeologic system.

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Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2018-03-05T11:48:46Z No. of bitstreams: 2 Dissertação - Joan Jesus Amaya Triana - 2018.pdf: 11320340 bytes, checksum: daee9afd4ae2db3c36dee6d85ae3be27 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2018-03-05T11:50:52Z (GMT) No. of bitstreams: 2 Dissertação - Joan Jesus Amaya Triana - 2018.pdf: 11320340 bytes, checksum: daee9afd4ae2db3c36dee6d85ae3be27 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2018-03-05T11:50:52Z (GMT). No. of bitstreams: 2 Dissertação - Joan Jesus Amaya Triana - 2018.pdf: 11320340 bytes, checksum: daee9afd4ae2db3c36dee6d85ae3be27 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2018-02-23 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / In this work, we study the information transmission models in infinite graphs introduced in \cite{Thecone} and \cite{article}, that is, models of transmission of information on infinite graphs subject to the following rules: (1) at time zero, only the root of the graph has the information, (2) in a time greater than or equal to one, a new vertex is informed and transmits the information to neighbors that are within a finite random neighborhood, and (3) informed vertices remain forever informed. They are considered variants of this process in the spherically symmetrical tree that includes as particular cases the periodic tree and the homogeneous tree. In addition, the model is considered in random trees. In this model, we study phase transition, probability of survival, among other important numerical characteristics for this process. It is also considered the particular case in which the influence radius has a Bernoulli distribution. The proofs are based on comparisons with branching processes. / Neste trabalho, são estudados modelos de transmissão de informação em grafos infinitos introduzidos em \cite{Thecone} e \cite{article}, isto é, modelos de transmissão de infomação sobre grafos infinitos sujeitos as seguintes regras: (1) no tempo zero, somente a raiz do grafo possui a informação, (2) em um tempo maior ou igual a um, um novo vértice é informado e transmite a informação para vizinhos que estejam dentro de uma vizinhança aleatória finita, e (3) vértices informados permanecem informados para sempre. Serão consideradas variantes deste processo na árvore esfericamente simétrica que inclui como casos particulares a árvore periódica e a árvore homogênea. Além disso, é considerado o modelo em árvores aleatórias. Para este modelo são estudados transição de fase, probabilidade de sobrevivência, dentre outros característicos numéricos importantes para este processo. Também é considerado o caso particular em que o raio de influência tem uma distribuição de Bernoulli. As provas são baseadas fazendo comparações com processos de ramificação.

L'émergence des nanotubes de carbone a ouvert de nouveaux champs d'application dans le domaine des matériaux polymères. L'ajout de ces charges carbonées au sein de polymères permet la mise en œuvre de composites aux propriétés électriques optimisées. La conductivité de ces matériaux dépend en grande partie de l'organisation des charges dans la matrice, notamment de la présence de réseaux percolants. L'objectif du présent travail de thèse est de comprendre les mécanismes de structuration des nanotubes de carbone au sein de différents milieux. L'architecture de ces réseaux de charges a principalement été révélée par le biais de mesures électriques et diélectriques. L'originalité de nos travaux réside dans l'utilisation de matrices liquides, notamment des huiles de silicone, afin de s'affranchir des contraintes présentes dans les plastiques d'une part, et de simplifier les processus de mise en œuvre d'autre part. Le manuscrit de thèse est articulé autour de six chapitres. Une première partie bibliographique aborde les propriétés des nanotubes de carbone ainsi que les phénomènes que sont la percolation et la percolation dynamique. Le second chapitre, matériel & méthode, présente les matériaux employés ainsi que les différentes techniques de caractérisation utilisées au cours de la thèse. Le troisième chapitre de la thèse aborde, à travers des mesures de conductivité, la percolation dynamique des nanotubes de carbone sein d'huiles de silicone. Le chapitre 4 propose une modification la loi de puissance de Kirkpatrick, afin de décrire la conductivité en fonction du temps et du taux de charge. L'exposant critique de percolation, caractérisant la transition isolant conducteur, se révèle être un indicateur de l'état de dispersion des nanotubes à travers la matrice. Le chapitre 5 démontre la possibilité de contrôler l'organisation des charges par l'application d'un champ électrique. L'application d'un champ élevé permet une augmentation de plusieurs ordres de grandeur de la conductivité ainsi qu'une diminution des charges nécessaire à la formation d'un réseau percolant. Nous avons notamment déterminé des seuils de percolation de l'ordre de 0.005% massique en nanotube de carbone. Enfin, l'influence des propriétés intrinsèques de la matrice, telles la viscosité et la tension de surface, est étudié dans le chapitre 6. La dispersion des nanotubes de carbone s'avère être favorisée au sein de liquides ayant des tensions de surface proches de celle des tubes. Au contraire, une agrégation de charge est rapidement observée dans le cas ou la différence de tension de surface charge-matrice est importante. Nous avons également observé que la percolation des nanotubes est défavorisée au sein de milieux visqueux. / The rise of carbon nanotube has open possibility for composites polymers. Mixing this carbonaceous filler with polymer medias leads to an optimization of the electrical properties. Then, conductivity mainly depends of the filler architecture, especially the presence of percolating networks. The objective of this work is to understand the percolation mechanisms of the carbon nanotubes in different media. During this study, filler network has been revealed by the mean of electrical and dielectrical measurements. The originality of our work lies in the use of liquid matrices, such as silicone oils, in order to overcome the stresses in the plastic on the one hand, and to simplify the processing in other hand. This thesis is organized around six chapters. The first bibliographic part discusses the carbon nanotubes properties as well as percolation and dynamic percolation phenomena. The second chapter, matériel & méthode, presents the materials used and the different characterization techniques employed. The third chapter of the thesis talks about dynamic percolation of carbon nanotubes in silicone oil, probed by conductivity measurements. Chapter 4 provides a change of the power law Kirkpatrick to describe the conductivity as a function of time and filler content. The critical exponent of percolation is proving to be an indicator of the dispersion state of nanotubes throughout the matrix. In the Chapter 5, electric field is depicted as a tool to control the organization of fillers. The application of a high field increases the conductivity of several orders of magnitude and decreases the percolation threshold. Percolation thresholds close to 0.005 wt % have been determined. At last, the influence of the intrinsic properties of the matrix, such as viscosity and surface tension, is discussed in Chapter 6. Carbon nanotubes dispersion appears to be favored if the difference of surface tension between filler and liquid is low. In contrast, a filler aggregation is rapidly observed in the case where the difference in surface tension is important. We also observed that the percolation of the nanotubes is favored in viscous media.