Dissertations / Theses on the topic 'Boundary triples'

Un polycristal est constitué par un agglomérat de monocristaux micrométriques accolés les uns aux autres. L’interface entre deux monocristaux tournés constitue un joint de grains (JdG). Les JdGs jouent un rôle important dans le comportement plastique des matériaux cristallins. Sous certaines conditions de traitement thermique (TT) ou de thermomécanique (TM), les JdGs peuvent migrer et font évoluer profondément les microstructures. Ces phénomènes anciens restent toutefois mal connus et il n’existe pas de modèle quantitatif décrivant la migration. Dans ce contexte, nous avons étudié expérimentalement la migration des JdGs dans deux nuances de cuivre électrolytique de même pureté. L’un livré sous la forme d’une tôle obtenue par le laminage à froid suivi d’un recuit continu à 700°C (que l’on nommera cuivre I), est recristallisé. L’autre, fourni sous forme de cylindre (cuivre II) est partiellement recristallisé. Un traitement thermique initial permet d’obtenir un état de référence puis différents TT et TM sont effectués afin d’étudier la migration des joints.Nous recherchons les conditions de l’écrouissage critique (croissance de grains lors de recuit sans germination après très faible déformation plastique) afin d’observer la migration induite par déformation (Strain Induced Boundary Migration SIBM). Le déplacement de JdG par ce mécanisme a été observé in-situ sous le MEB. Une technique de mesure combinée EBSD et AFM est utilisé pour suivre de manière quantitative les évolutions microstructurales et évaluer les différentes forces motrices en présence. Le déplacement des JdGs est quantifié précisément grâce aux phénomènes de gravure thermique et l’existence de points fixes dans les clichées EBSD. L’énergie stockée, associée aux microstructures de dislocations dans chaque grain a été estimée par deux méthodes, la mesure de la désorientation locale à l’aide de cartographie de KAM et la simulation par modèle de Taylor relâché incluant un modèle de plasticité cristalline justifié physiquement et incluant une restauration isotrope ou anisotrope. La courbure des JdGs, 2nde force motrice en présence, est évaluée à partir des contours des JdGs expérimentaux. Un diagramme de migration a été tracé en fonction des déplacements de JdGs liant à l’énergie de courbure de JdG et à la différence d’énergie stockée. Il ressort des différences significative avec le comportement du polycristal d’Al, notamment liées à la différence d’énergie de faute d’empilement et l’existence de nombreuse macles. Le joint triple semble fortement influer le déplacement des JdGs qui a été étudié statistiquement pour le cuivre I à l’aide de donnée EBSD
A polycrystal is made up of an agglomerate of micrometric single crystals adjoining each other. The interface between two rotated single crystals forms a grain boundary (GB). GBs play an important role in the plastic behaviour of crystalline materials. Under certain conditions of heat (HT) or thermomechanical (TT) treatments, GBs can migrate and cause microstructures to evolve radically. However, these ancient phenomena remain poorly understood and there is no quantitative model describing migration. In this context, we experimentally studied the migration of GB in two grades of electrolytic copper of the same purity. One is delivered in the form of a sheet obtained by cold rolling followed by continuous annealing at 700°C (called copper I), and is recrystallized. The other, supplied as a cylinder (copper II), is partially recrystallized. An initial heat treatment allows to obtain a reference state then different HT and TM are carried out in order to study the migration of the grains boundaries.Our interest lies in the conditions of critical strain hardening (grain growth during germination-free annealing after very low plastic deformation) in order to observe strain-induced migration (Strain Induced Boundary Migration SIBM). The displacement of GB by the mechanism was observed in-situ in SEM. A combined EBSD and AFM measurement technique is used to quantitatively monitor microstructural changes and evaluate the different driving forces involved. The displacement of the GB is precisely quantified thanks to the phenomenon of thermal etching and the existence of stationary points in the EBSD pictures. The stored energy, associated with the dislocation microstructures in each grain, was estimated by two methods, the measurement of local disorientation using KAM mapping and the simulation by a relaxed Taylor model including a physically justified crystal plasticity law and accounting for an isotropic or anisotropic restoration. The curvature of GBs, the 2nd driving force involved, is evaluated from the observed contours of GBs. A migration diagram is drawn and represents the displacements of GBs as a function of estimates of the curvature energy of GB and the difference in stored energy. Significant differences with the behavior of Al polycrystals are noted, certainly due to the difference in stacking fault energy and the existence of numerous twins. From our statistical analysis of copper I using EBSD data, triple junctions seem to strongly influence the migration of GBs

The first part of this thesis is concerned with the synthesis, characterisation and applications of surface functionalised carbon nanoparticles. Synthetic techniques are used to modify the existing surface architecture of carbon nanoparticles towards high surface area modified electrodes and pH sensing applications. Electrochemical and synthetic techniques have been used to study triple phase boundaries and enhance their properties towards a bulk synthetic technique, in which an electrolyte phase and redox probe phase are held separate. A salt matrix, ultrasound, high shear force and a carbon fibre membrane have all been used to form unique triple phase boundary environments in which electron and ion transfer processes can be studied and enhanced towards analytical and electrosynthetic applications. A number of electro-reduction reactions have been shown to be feasible using the triple phase boundary methodology. Alkenes, aldehydes and imines have all been successfully electro-reduced, analysed and optimised to elucidate the synthetic triple phase boundary mechanism.

The main aim and ultimate final goal of the work carried out in this thesis is a drive towards a feasible system for light harvesting, which is in short, using the Sun’s energy to create electricity or a fuel for our energy requirements here on Earth. This work will see an approach using the triple phase boundary afforded by a microdroplet array. Although light harvesting is an ambition which has seen decades of work and uncountable man-hours, approaching it from the angle of utilizing the triple phase boundary between two immiscible liquids and a solid electrode is a new, and novel concept. Before any attempts towards a light harvesting technique can be made, we will need to have characterized and fully understood the mechanisms and nuances, both for dark and light processes, that are observed at the triple phase boundary. This initial process will start by selection of a suitable redox molecule, and exploring its reactivity in microdroplets under dark conditions. Once this has been achieved, an attempt can be made to use this knowledge, and implement it towards light harvesting. This will eventually include an attempt to couple photo-excited states with other molecules, this will be an important step if energy is ever able to be stored from such a system. This early phase will also see the need to employ many other techniques other than electrochemistry in an effort to aid in the understanding and characterization of the triple phase boundary at microdroplets. This will include travelling to other laboratories in search of specialized scientific skills and apparatus, such as electron paramagnetic resonance, or photocurrent spectroscopy. It will also see the need to build new equipment needed to conduct tests such as surface tension visualization, or new electrochemical cells for photocurrent measurement. In summary, this report will see initial characterization of the processes, both light and dark, that occur within the triple phase boundary of a microdroplet for a given redox molecule dissolved within. Early attempts at coupling excited states with other molecules are also explored. Serendipity has always played a part in scientific discovery and the work outlined in this report was no different. The choice of oil used for the organic phase microdroplet deposits yielded some interesting and unexpected results, and has been implicated as one of the key aspects of the photoreactions that have been explored.

Présentation du domaine: Il s'agit d'étudier des opérateurs différentiels sur des variétés riemanniennes singulières et leurs applications. Parmi les opérateurs les plus importants, on trouve les opérateurs de Laplace et de Dirac. Il y a beaucoup de connexions entre les deux types d'opérateurs, à cause de la formule de Lichnerowicz, un mathématicien français du dernier siècle. Pourtant, les opérateurs de Laplace ont été beaucoup plus étudiés que les opérateurs de Dirac. Les opérateurs de Dirac, aussi appelé opérateurs d'Atiyah--Singer, sont des opérateurs fondamentaux dans la géométrie riemannienne et dans la théorie de l'indice. Ce sont des opérateurs associés à une métrique et à un fibré de Clifford doté d'une connexion admissible. Leurs généralisations est l'objet principal dans la théorie de Kasparov, qui est un utile fondamental dans les algèbres d'opérateurs. Il y a beaucoup de gens qui pensent que les opérateurs de Dirac joueront un rôle central dans le programme de Grothendieck: généraliser le théorème de Riemann--Roch aux variétés algébriques singulières. Les opérateurs de Dirac ont donc été beaucoup étudiés dans les mathématiques fondamentales, ainsi que dans ces applications. Les opérateurs de Maxwell et de de Rham sont des cas particuliers des opérateurs de Dirac. Les opérateurs de Dirac apparaissent dans beaucoup d'applications dans d'autres domaines des mathématiques et physique théorique, comme la théorie des champs dans l'espace-temps courbe ou la théorie de la relativité générale. Ces opérateurs constituent donc un lien entre les mathématiques fondamentales et ces applications. Sujet de thèse: Il y a beaucoup de résultats sur l'analyse des opérateurs de Dirac, mais la plupart d'eux sont sur des variétés compactes lisses, avec ou sans bord. Cependant, il est important d'étudier ces opérateurs pour des variétés non compactes ou non lisses. Par exemple, les applications aux variétés algébriques et au programme de Grothendieck nécessitent le cas non lisse. Le sujet que nous proposons est d'utiliser les résultats et les techniques introduites par Monique Dauge et ses collaborateurs pour étudier les singularités des opérateurs de Dirac dans un domaine polyédrique et d'autres domaines singuliers. Un problème particulier est d'obtenir l'application au calcul de l'homologie de Rham avec des complexes finis, comme dans les travaux récents de Douglas Arnold. Pour la régularité des solutions de l'équation de Dirac, nous proposons d'utiliser les méthodes introduites récemment par Bernd Amman et Nadine Grosse ou par Victor Nistor et Nadine Grosse dans des articles récents. Nous allons étudier aussi les opérateurs de Dirac avec des potentiels et terms non linéaires. Un problème concret ici est d'étudier de modèles non linéaires couplés avec Maxwell, par exemple les modèles de magnéto-hydrodynamique et l'équation de Vlasov--Maxwell
General description of the domain. The general question that will be pursued as part of the thesis will be to study differential operators on Riemannian spaces and their applications. The Laplace and Dirac operators are among the most important differential operators arising in applications. There are many connections between these two types of operators, due to Lichnerowicz' formula, a French mathematician of the last century. However, Laplace operators have been much more studied than the Dirac operators. Dirac operators, also called Atiyah--Singer operators, are fundamental operators in Riemannian geometry and in index theory. These operators are associated to a metric and a Clifford bundle with an admissible connection. Their generalizations are the main object in Kasparov's theory, which is a fundamental theory in Operator Algebras. It is believed that the Dirac operators will play a central role in Grothendieck's program to generalize the Riemann--Roch theorem to singular algebraic varieties. Dirac operators have therefore been much studied in theoretical mathematics, as well as in its applications. The Maxwell and de Rham operators are special cases of Dirac operators. Dirac operators appear in many applications in other domains of mathematics and theoretical physics, such as field theory in curved space-time or the theory of general relativity. These operators are thus a link between the fundamental mathematics and its applications. Theses subject: There are many results on the analysis of the Dirac operators, but most of them are on smooth compact varieties, with or without boundary. However, it is important to study these operators for non-compact or non-smooth spaces (or varieties). For example, applications to algebraic varieties and to the Grothendieck program require the case of non smooth varieties. The subject we propose to use the results and the techniques introduced by Monique Dauge and her collaborators to study the singularities of the Dirac operators in a polyhedral domain and other singular domains. A particular problem is to obtain the application to the calculation of de Rham's homology with finite complexes, as in the recent works of Douglas Arnold. For the regularity of the solutions of the Dirac equation we propose to use the methods recently introduced by Bernd Amman and Nadine Grosse or by Victor Nistor and Nadine Grosse in recent articles. The thesis will also study the Dirac operators with nonlinear terms and potentials. A concrete problem here is to study nonlinear models coupled with Maxwell's equation, which arrise, for example in magneto-hydrodynamic models and in the Vlasov-Maxwell equation

The idea of having the triple phase boundary (TPB) is extensively used in the fuel cell literature, especially with respect to solid oxide fuel cells (SOFC). The TPB concept indicates that the hydrogen oxidation reaction and the oxygen reduction reaction, which produce electric current, can actually occur at special sites, called "triple phase boundaries" where the gaseous fuel phase, ion conducting phase: electrolyte, and electron conducting phase, come into contact. Recent study shows that despite the common assumption about TPB, it is not just a point, but a zone that consists of two lines. The kinetic reaction often introduces a significant limitation to fuel cell performance. Therefore, understanding, characterizing, and optimizing the TPB content in fuel cells provides excellent opportunities for performance improvement. Studying the kinetics of the reaction that takes place at the triple phase boundary is one aspect of this paper. It includes the study of all kinds of chemical and electrochemical reactions as well as their reaction rates, the surface species, and the electrochemical parameters, such as reaction rate constants and conductivity. A mathematical model is developed to describe a simplified anodic solid oxide fuel cell (SOFC) system, Ni/ H2--H 2O/YSZ, and its reaction occurring in the vicinity of the triple phase boundary (TPB). The model incorporates coupled diffusion, migration and reaction phenomena of the chemical components in the gas phase, Ni particle and zirconia solid state. The kinetic constants necessary for the simulations are estimated on the basis of literature data.

In this thesis, we investigate flow past an array of micro-electro-mechanical-type (MEMS-type) heating elements placed on a flat surface, where MEMS devices have hump-shaped surfaces, using the triple deck theory. In this work we start by investigating the problem with a single heating element. MEMS devices can be used to control the fluid dynamics over the surface. Hence, we present a review of the boundary layer and the triple deck theories, followed by a literature review of the problem of flow past an array of MEMS devices. Next, we formulate our problem with the aid of the method of matched expansions for supersonic and subsonic flows. Thirdly, we solve analytically the linear version of the problem for supersonic flows. Thereafter, the non-linear problem is solved numerically where a detailed description of a hybrid method to solve the formulated non-linear problem for supersonic flow is exhibited. Fourthly, for subsonic flows we continue investigating flow past a heating element placed on a flat surface. Linear analysis of this problem is conducted. A novel numerical method to solve the non-linear problem for subsonic flows is described. The results are then discussed. In a similar context, we formulate a problem which can be considered as an the extension of previous subsonic flow problem to the three dimensional case. Analytical results are obtained using the Fourier transform where the linear approximation of the problem is considered and numerical results are then obtained using the Fast Fourier Transform. Finally, we consider a case of transonic flow past a heating element placed on a flat surface, where MEMS device has a hump-shaped surface. This transonic flow problem is non-linear in the upper deck and the lower deck equations where they should be solved simultaneously. Hence, a numerical method is required where we will use a finite difference method in stream-wise direction and Chebyshev collocation method in the wall normal direction. The results are then analysed. In conclusion, the use of localised heating elements in boundary layers for flow types considered in the thesis can contribute to the possibility of favourably controlling the fluid flow perturbations.

The extension of the theory for two dimensional turbulent boundary layers into three dimensional flows has met with limited success. The failure of the extended models is attributed to the anisotropy of the turbulence. This is seen by the turbulent shear stress angle lagging the flow gradient angle and by the behavior of the Reynolds shear stresses lagging that of the mean flow. Transport equations for the turbulent shear stresses were proposed to be included in a modeling effort capable of accounting for the lags seen in the flow. This study is aimed at developing algebraic relationships between the various Reynolds-averaged terms in these modeling equations. Particular emphasis was placed on the triple products that appear in the transport equations. Eleven existing experimental data sets were acquired from the original authors and re-examined with respect to developed and existing parameters. A variety of flow geometries were collected for comparison. Emphasis was placed on experiments that included all six components of the Reynolds stress tensor and triple products. Parameters involving the triple products are presented that appear to maintain a relatively constant value across regions of the boundary layer. The variation of these parameters from station to station and from flow to flow is discussed. Part of this study was dedicated to parameters that were previously introduced, but never examined with respect to the data that was collected. Results of these parameters are presented and discussed with respect to agreement or disagreement with the previous results. The parameters presented will aid in the modeling of three dimensional turbulent boundary layers especially with models that employ the transport equations for the Reynolds stresses.
Master of Science

Pour anticiper la fragilisation par l'hydrogène, il faut connaître l'effet des différentes échelles temporelles et spatiales sur les processus de diffusion et de piégeage dans la microstructure du matériau. Dans ce travail, nous examinons l'impact de différents types de jonctions triples (TJ) et de joints de grains (GB) et de leur connectivité sur la diffusion de l'hydrogène. Tout d'abord, nous proposons quatre algorithmes pour contrôler la distribution des fractions de TJ en fonction d'une fraction donnée de joints de grains aléatoires. Ensuite, à l'aide de la modélisation par éléments finis, nous étudions les effets de la distribution des jonctions triples sur la diffusivité de l'hydrogène. Nous découvrons des relations remarquables entre la connectivité des joints de grains et le coefficient de diffusion effectif en quantifiant les réseaux de joints de grains dans des microstructures idéalisées en 2D. De plus, une plus grande diffusivité est reflétée dans les matériaux nanocristallins. Nos résultats prouvent qu'ils reproduisent les chemins de connectivité des GB en contraignant la distribution. En outre, des microstructures EBSD reconstruites en 2D ont été simulées et comparées aux données expérimentales. Ensuite, des simulations de microstructures idéalisées en 3D ont été réalisées, et les effets de la connectivité des jonctions triple ont été étudiés. Nous montrons qu'il existe de forte corrélation entre la connectivité des jonctions triple et le coefficient de diffusion effectif, en particulier dans les matériaux nanocristallins. Les simulations 3D s'approchent plus précisément des résultats expérimentaux tant que l'impact de la connectivité des TJ est inclus
Anticipating hydrogen embrittlement requires knowledge of the effect of different time and spatial scales within diffusion and trapping processes in the material's microstructure. In this work, we examine the impact of various types of triple junctions (TJ) and grain boundaries (GB) and their connectivity on the diffusion of hydrogen. First, we propose four algorithms to control the distribution of TJ fractions depending on a given fraction of Random grain boundaries. Then, using finite element modeling, we investigate the effects of triple junction distribution on hydrogen diffusivity. We discover remarkable relationships between grain boundary connectivity and the effective diffusion coefficient by quantifying grain boundary networks in 2D idealized microstructures. In addition, greater diffusivity is reflected in nanocrystalline materials. Our results prove to reproduce GB connectivity paths by compelling the distribution. Furthermore, 2D reconstructed EBSD microstructures were simulated and compared to experimental data. Following that, 3D simulations of idealized microstructures were performed, and the effects of triple junction connectivity were investigated. We show that there is strong correlation between the connectivity of the triple junction and the effective diffusion coefficient, particularly in nanocrystalline materials. 3D simulations approach the experimental results more accurately as the impact of the connectivity of TJs is included

L’archipel des Açores dans l’océan Atlantique est édifiées sur un épais plateau océanique, à proximité de la jonction triple entre les plaques Nord-américaine (Na), Nubienne (Nu) et Eurasienne (Eu). La formation du plateau et l’origine du volcanisme ont été le plus souvent attribués à la présence d’une instabilité mantellique. Cependant, la répartition et la morphologie des édifices volcaniques semblent avoir été grandement influencés par la déformation régionale liée à la migration de la frontière de plaque (Eu/Nu). En effet, la frontière serait passée d’une faille transformante aujourd’hui inactive, la zone de fracture est des Açores (EAFZ), à un rift ultra lent actif appelé le Rift de Terceira (TR).Lors de ce travail, nous utilisons le volcanisme comme marqueur de la déformation régionale. Nous nous intéressons particulièrement aux îles de S. Miguel et Graciosa, qui sont localisées à l’intérieur du TR, et à Santa Maria, une île volcanique éteinte qui se situe entre la EAFZ et le TR. De par leur position, ces trois îles constituent donc des cibles particulièrement appropriées afin d’étudier l’architecture et l’évolution de la frontière de plaque Eu/Nu durant les dernier Millions d’années. A partir de nouvelles données géomorphologiques, stratigraphiques, géochronologiques et tectoniques, couplées aux données bathymétriques et géophysiques disponibles, nous reconstruisons les étapes successives de construction et de démantèlement de ces îles puis discutons de leur signification géodynamique. Ces données sont ensuite complétées par des expériences de mécanique des fluides afin d’investiguer les liens possibles entre un panache mantellique, la migration de la frontière de plaque sur plusieurs échelles d’espace et de temps.Les résultats montrent que les édifices localisés dans le TR se construisent via des pulses volcaniques courts (<100 kyr) et relativement synchrones, séparés par des épisodes d’effondrements catastrophiques. Nous proposons qu’une telle évolution reflète des épisodes brefs et intenses de déformation régionale le long de la frontière de plaque active. La distribution des marqueurs tectoniques ainsi que leurs orientations N110 et N150 dans la partie Est de S. Miguel, nous conduit à proposer que l’extension oblique du TR est principalement accommodée par les failles bordières majeures du rift. Nous identifions une nouvelle tendance tectonique orientée N50° qui pourrait représenter des failles transformantes accommodant les variations d’obliquité du TR. L’activité de île de Santa Maria est ici datée entre 5.7 et 2.8 Ma. S. Maria a été façonnée par plusieurs effondrements sectoriels catastrophiques, le plus probablement déclenchés par les mouvements tectoniques régionaux. Nous identifions également une nouvelle structure de type graben reliant les îles de S. Maria et S. Jorge plus loin au NW. La forme de ce graben est semblable au TR et est située entre l’ancienne et la nouvelle frontière Eu/Nu. Nous interprétons ce graben comme un ancien rift transitionnel et donc comme une ancienne frontière de plaque Eu/Nu. A partir de nos données géochronologiques, nous proposons que la partie Est de ce rift transitionnel aurait migré vers la partie Est du TR entre 2.8 et 1.7 Ma.La migration de la frontière Eu/Nu a été interprétées par Vogt and Jung (2004) comme résultant de sauts successifs vers le NE de l’axe du Rift afin de maintenir sa position au dessus d’un point chaud fixe. Nos expériences de mécanique des fluides suggèrent que l’archipel des Açores, comme celui des Canaries, du Cap Vert, de Madère ainsi que les volcans sous marins de Great Meteor sont la signature en surface d’un groupe d’instabilités mantellique prenant naissance et remontant à partir du sommet d’un dôme thermochimique situé dans le manteau inférieur. De plus, Ces panaches secondaires pourraient être suffisamment faibles pour adapter leurs mouvements aux équilibres de forces pré-existants, notamment la structure et la morphologie de la lithosphère
The Azores archipelago in the Atlantic comprises nine volcanic islands which developed on a thick oceanic plateau close to the Triple Junction between the North American (Na), the Nubian (Nu), and the Eurasian (Eu) lithospheric plates. The formation of the plateau and the origin of the volcanism remain controversial, but have been generally attributed to a plume-like mantle instability. However, the distribution of the volcanic edifices east of the Mid-Atlantic Ridge (MAR) appears greatly influenced by regional deformation associated with the northward migration of the Eu/Nu plate boundary from an extinct old transform fault, the East Azores Fracture Zone (EAFZ), up to the presently active ultra-slow Terceira Rift (TR). In this thesis, we use the volcanism as a marker for regional deformation. We especially focus on S. Miguel and Graciosa, which are located within the TR, and on S. Maria, an old volcanically extinct island located between the EAFZ and the TR. These three islands thus constitute particularly suitable targets to track the architecture and the evolution of the Eu/Nu plate boundary during the last few Myr. From new geomorphological, stratigraphic, geochronologic, structural/tectonic data, and existing bathymetric and geophysical data, we reconstruct the successive stages of growth and destruction of the islands, and discuss their geodynamic meaning. These data are then complemented by fluid dynamic modelling using laboratory experiments to examine the possible links between mantle instability, plate boundary migration and the development of the volcanism on various spatial and temporal scales.The new results on the islands show that the edifices located within the TR grew through short (<100 kyr) and partly synchronous volcanic pulses, separated by catastrophic sector collapses. We propose that such evolution reflects brief and intense episodes of regional deformation along the still active Eu/Nu plate boundary. The distribution of tectonic markers and the recognition of N110 and N150 tectonic structures in eastern S. Miguel leads us to propose that oblique extension in the TR is mainly accommodated by the master faults of the rift, and that the TR is presently not the locus of appreciable sea-floor spreading. Furthermore, we identify a new N050 trend, which may represent transform faults accommodating the variation in obliquity of the TR. The activity of S. Maria is here dated between 5.7 and 2.8 Ma. Like the recent islands, S. Maria experienced catastrophic flank collapses, most probably triggered by regional tectonics. We identify a new graben structure linking Santa Maria to the island of S. Jorge further NW. The shape of this graben is similar to the TR and it is located between the EAFZ and the current plate boundary. We interpret this graben as a former transient rift, and therefore an old Eu/Nu plate boundary. From the new data, we propose that the eastern part of the transient rift migrated to the eastern part of the TR between 2.8 Ma and 1.7 Ma.The overall migration of the Eu/Nu plate boundary to the north and the creation of the Azores plateau has been interpreted by Vogt and Jung (2004) as resulting from successive NE jumps of the rift axis to maintain its position over a fixed ‘hotspot’. Our fluid mechanics experiments suggest that the Azores, as Canary, Cape Verde, Madeira Islands and Great Meteor seamounts might be the surface signature of a cluster of mantle instabilities rising from the top of a large thermochemical dome located in the lower mantle. However, such secondary plumes present a strong time-dependence 5-40 Myr time scale. Moreover, they could be sufficiently weak to adapt their motions to the pre-existing force balances and morphology of the lithosphere. We therefore present a scenario of the Azores area evolution combining a triple junction and decompression melting buoyant material (i.e. such in volatiles and/or temperature) under a thickening lithosphere

This thesis is a study of the effects of distributed roughness elements on a two-dimensional turbulent boundary layer. Measurements were taken on a total of ten rough wall configurations: four involving Gaussian spikes, and six with circular cylindrical posts. Rough wall flows are particularly suited to study with Laser Doppler Velocimetry (LDV) due to the fact that measurements are required near a solid surface, as well has in highly turbulent fluid. The LDV system used in this study is a fine resolution (~50 micron), three-component, fiber optic system. All mean velocities, Reynolds stresses, and triple products are measured. This study is unique in the range and variety of roughness cases for which data was taken.

The data show that the flow over a rough wall is characterized by high levels of turbulence near the roughness element peaks at the interface between low-speed, near-wall fluid and the higher speed fluid above. Behind an element, high-momentum fluid sweeps toward the wall, and there is a small region of ejection of low-momentum fluid. Cylindrical elements typically have larger magnitudes of turbulent stresses at their peaks compared to Gaussian elements. Trends in mean velocity profile parameters such as displacement height, roughness effect, and wake parameter are examined with respect to roughness element geometry and spacing.
Master of Science

Die vorliegende Arbeit behandelt die Modellierung einer Quantenpunkt-LED und die Berechnung des elektrischen Stromes und der Lichtproduktion im Landauer-Büttiker-Formalismus. Die Elektron-Photon-Wechselwirkung kann im Landauer-Büttiker-Formalismus behandelt werden, indem wir annehmen, dass jedes Elektron mit einem separaten Photonenfeld interagiert. Dies erlaubt es uns, ein Elektron zusammen mit seinem Photonenfeld als „einzelnes, nicht wechselwirkendes Teilchen“ im Sinne des Landauer-Büttiker-Formalismusses zu betrachten. Wir entwickeln ein Modell einer QP-LED, dessen Elektron-Photon-Wechselwirkung auf dem Jaynes-Cummings-Modell basiert, das die Interaktion eines Quantenpunkts mit einer Mode des elektromagnetischen Feldes beschreibt. Um auch die Energieverteilung der emittierten Photonen analysieren zu können, schlagen wir ein auf einem Pauli-Fierz-Modell basiertes Modell vor. Anstelle einer einzelnen Mode modelliert es Photonen beliebiger Energie, allerdings beschränken wir uns auf den Unterraum mit maximal einem Photon. Wir beweisen eine abstrakte Landauer-Büttiker-Formel, die für alle relativ nuklearen Streusysteme gilt. Sie ist ähnlich zu dem Ergebnis von Aschbacher et al. (2007), unterscheidet sich aber in der Regularisierung des Stroms. Wir wenden das abstrakte Ergebnis auf die Jaynes-Cummings-QP-LED an. Als Startpunkt für die Berechnung der Streumatrix verallgemeinern wir die Darstellung der Streumatrix durch die Weyl-Funktion eines Randwert-Triplets von Behrndt et al. (2010) vom Fall für Störungen endlichen Ranges auf den Fall relativ nuklearer Störungen. Dies deckt insbesondere den Fall der Jaynes-Cummings-QP-LED ab. Die Resolventendifferenz der Pauli-Fierz-QP-LED ist nicht nuklear, weshalb wir eine verallgemeinerte Landauer-Büttiker-Formel für eine gewisse Klasse von Multiplikationsoperatoren beweisen, die in der Faser nuklear sind. Dieses abstrakte Resultat liefert uns auch für die Pauli-Fierz-QP-LED eine Landauer-Büttiker-Formel.
This thesis treats the modeling of a quantum dot LED and the calculation of the electric current and the light production in the Landauer-Büttiker framework. The electron-photon interaction is fitted into the Landauer-Büttiker framework by assuming that every electron interacts with a separate photon field. It allows us to consider an electron together with its photon field as a ''single non-interacting particle'' in the sense of the Landauer-Büttiker formalism. We develop a model of a QD-LED with an electron-photon interaction that is based on the Jaynes-Cummings model, which describes the interaction of a quantum dot with a single mode of the electromagnetic field. To be able to analyze the energy distribution of the emitted photons, we propose a second model of a QD-LED that is based on a one-dimensional Pauli-Fierz model. It models photons of arbitrary positive energy instead of just a single mode, but we restrict it to the subspace of at most one photon. We prove an abstract Landauer-Büttiker formula that applies to all relatively trace class interactions. It is similar to the result by Aschbacher et al. (2007), but differs in the regularization of the flux. We apply this formula to the Jaynes-Cummings QD-LED. Since knowing of the scattering matrix is essential for explicit calculations with the Landauer-Büttiker formula, we generalize a result by Behrndt et al. (2010) on a representation of the scattering matrix in terms of the Weyl function of a boundary triplet from the finite rank case to relatively trace class perturbations, which covers the case of the Jaynes-Cummings QD-LED. The resolvent difference of the Pauli-Fierz QD-LED is not trace class, whence we prove a generalized Landauer-Büttiker formula for a certain multiplication operators that are trace class in the fiber. This abstract result gives us a Landauer-Büttiker formula also for the Pauli-Fierz QD-LED.

Les travaux menés, depuis plusieurs années, au CORIA ont abouti à la construction d’un outil numérique (ARCHER) permettant la simulation numérique directe d’écoulements diphasiques et notamment l’atomisation d’un jet liquide à haute vitesse. Ce type de simulation permet de capturer les phénomènes d’atomisation au voisinage de l’injecteur difficilement caractérisables par les outils expérimentaux actuels. Ces simulations requièrent des conditions d’injection délicates à évaluer a priori car elles dépendent des caractéristiques de l’écoulement au sein de l’injecteur. Or, certains jets présentent une grande sensibilité à ces conditions d’injection. Dès lors, il est nécessaire de simuler l’écoulement au sein de l’injecteur afin d’appréhender la nature de cette sensibilité. L’utilisation d’un maillage cartésien par le code ARCHER conjuguée à la volonté de simuler le système d’atomisation dans son ensemble ont orienté ces travaux vers l’utilisation d’une méthode de frontière immergée. Ces travaux ont ainsi permis de reproduire des écoulements au sein d’injecteurs de forme quelconque tout en conservant le maillage cartésien d’origine, précieux tant pour l’efficacité du solveur que pour sa précision. Dans un premier temps, l’implantation dans le code ARCHER d’une méthode de frontière immergée a été réalisée et testée sur des configurations de canal et de conduite et de l’écoulement autour d’un cylindre. L’application de cette méthode a porté sur la simulation de l’écoulement au sein d’un injecteur triple disque mono-trou et a notamment permis de caractériser l’origine de l’écoulement secondaire formé dans l’orifice de décharge. Afin d’évoluer vers la construction d’un outil numérique capable de simuler le système d’atomisation dans son ensemble, un couplage entre la méthode de frontière immergée et la méthode Ghost fluid a été nécessaire. La version bi-dimensionnelle développée a été testée sur la relaxation d’une goutte posée sur une paroi. Cette version a permis de simuler des écoulements au sein de canaux à différents rapports de longueur sur diamètre et l’écoulement au sein d’une buse convergente. La simulation simultanée de l’écoulement interne et externe a permis de lier les fluctuations de vitesses des écoulements internes à la création de surface engendrée sur les écoulements externes
Since several years, the research conducted at the CORIA laboratory led to the development of a numerical tool (ARCHER) alllowing direct numerical simulations of two phase flows. In particular, the simulations of high speed liquid jet primary break-up have been strongly investigated. These simulations are able to capture primary break-up phenomena near the nozzle exit where experimental characterisations are difficult to conduct. These simulations need injection conditions tricky to gauge a priori, since they depend on the flow characteristics inside the nozzle. Moreover, some jets are highly sensitive to these injection conditions. Therefore, it becomes necessary to simulate the flow inside the nozzle to better understand this sensitive nature. The objective to simulate the whole atomization system guided the present work dedicated to the use of an immersed boundary method (IBM). Such an approach allows reproducing flows inside nozzles of arbitrary shape while keeping the original cartesian mesh valuable for numerical efficiency and accuracy. As a first step, the implementation of an IBM in ARCHER was carried out and tested on channels, pipes and uniform flows past a circular cylinder. An industrial application focused on the flow inside a triple disk compound injector. This work led to a refined description of the secondary flow origin in the discharge hole. In order to move towards the design of a numerical tool able to simulate the whole injection system, a coupling between IBM and the Ghost Fluid Method (GFM) has been found necessary. This allows accounting for two phase flows inside the nozzle where the dynamics of the triple line has to be considered. The bidimensional developments have been tested on drops released on walls. This version enabled to simulate flows inside channels with different ratios of length over diameter and the flow inside a convergent nozzle. The simultaneous computation of flows inside and outside nozzle has enabled to link the velocity fluctuations of internals flows to the surface setting-up gene-rated on external flows

This paper explores the extent to which corporations currently increase the voluntary disclosures in triple bottom line (TBL) reports. Although research already has provided substantial contributions as to why and how firms apply TBL reporting, there remains limited understanding of the motivations for and against making voluntary disclosures. Drawing from literature in environmental management and accounting as well as international auditing, this work focuses on guidance, boundarysetting and external assurance for TBL reports. An inductive case study approach is applied to investigate the credibility of TBL reporting by contrasting the competing predictions from legitimacy theory and voluntary disclosure theory on voluntary disclosures. A set of firm and industry specific factors are identified that are expected to determine a firm’s level and extent of reporting. The sample comprises nine US and EU red biotechnology corporations which aligned their TBL reporting to the GRI reporting standards in at least part of their TBL reports that were published between 2000 and 2009. An initial attempt is made to systematically investigate the credibility of TBL reporting to develop the model of voluntary disclosures. The empirical findings of this case study suggest that current guidance, boundary-setting and assurance for TBL reports are not sufficient to increase the credibility, comparability and reliability of reporting. Voluntary disclosure theory can serve to provide economic motivations, while legitimacy theory is helpful to provide a legitimating motivation. The findings support the notion that the economics-based factors better explain the secrecy strategy of providing mainly soft disclosures. The increase of disclosure levels maybe is considered sufficient to respond to public pressure. This work concludes by suggesting some directions of research in the areas of boundary-setting and assurance that have academic and practical implications.

This paper explores the extent to which corporations currently increase the voluntary disclosures in triple bottom line (TBL) reports. Although research already has provided substantial contributions as to why and how firms apply TBL reporting, there remains limited understanding of the motivations for and against making voluntary disclosures. Drawing from literature in environmental management and accounting as well as international auditing, this work focuses on guidance, boundarysetting and external assurance for TBL reports. An inductive case study approach is applied to investigate the credibility of TBL reporting by contrasting the competing predictions from legitimacy theory and voluntary disclosure theory on voluntary disclosures. A set of firm and industry specific factors are identified that are expected to determine a firm’s level and extent of reporting. The sample comprises nine US and EU red biotechnology corporations which aligned their TBL reporting to the GRI reporting standards in at least part of their TBL reports that were published between 2000 and 2009. An initial attempt is made to systematically investigate the credibility of TBL reporting to develop the model of voluntary disclosures. The empirical findings of this case study suggest that current guidance, boundary-setting and assurance for TBL reports are not sufficient to increase the credibility, comparability and reliability of reporting. Voluntary disclosure theory can serve to provide economic motivations, while legitimacy theory is helpful to provide a legitimating motivation. The findings support the notion that the economics-based factors better explain the secrecy strategy of providing mainly soft disclosures. The increase of disclosure levels maybe is considered sufficient to respond to public pressure. This work concludes by suggesting some directions of research in the areas of boundary-setting and assurance that have academic and practical implications.

Doutoramento em Nanociências e Nanotecnologia
Solid oxide fuel (SOFCs) and electrolyzer (SOECs) cells have been promoted as promising technologies for the stabilization of fuel supply and usage in future green energy systems. SOFCs are devices that produce electricity by the oxidation of hydrogen or hydrocarbon fuels with high efficiency. Conversely, SOECs can offer the reverse reaction, where synthetic fuels can be generated by the input of renewable electricity. Due to this similar but inverse nature of SOFCs and SOECs, these devices have traditionally been constructed from comparable materials. Nonetheless, several limitations have hindered the entry of SOFCs and SOECs into the marketplace. One of the most debilitating is associated with chemical interreactions between cell components that can lead to poor longevities at high working temperatures and/or depleted electrochemcial performance. Normally such interreactions are countered by the introduction of thin, purely ionic conducting, buffer layers between the electrode and electrolyte interface. The objective of this thesis is to assess if possible improvements in electrode kinetics can also be obtained by modifying the transport properties of these buffer layers by the introduction of multivalent cations. The introduction of minor electronic conductivity in the surface of the electrolyte material has previously been shown to radically enhance the electrochemically active area for oxygen exchange, reducing polarization resistance losses. Hence, the current thesis aims to extend this knowledge to tailor a bi-functional buffer layer that can prevent chemical interreaction while also enhancing electrode kinetics.The thesis selects a typical scenario of an yttria stabilized zirconia electrolyte combined with a lanthanide containing oxygen electrode. Gadolinium, terbium and praseodymium doped cerium oxide materials have been investigated as potential buffer layers. The mixed ionic electronic conducting (MIEC) properties of the doped-cerium materials have been analyzed and collated. A detailed analysis is further presented of the impact of the buffer layers on the kinetics of the oxygen electrode in SOFC and SOEC devices. Special focus is made to assess for potential links between the transport properties of the buffer layer and subsequent electrode performance. The work also evaluates the electrochemical performance of different K2NiF4 structure cathodes deposited onto a peak performing Pr doped-cerium buffer layer, the influence of buffer layer thickness and the Pr content of the ceria buffer layer. It is shown that dramatic increases in electrode performance can be obtained by the introduction of MIEC buffer layers, where the best performances are shown to be offered by buffer layers of highest ambipolar conductivity. These buffer layers are also shown to continue to offer the bifunctional role to protect from unwanted chemical interactions at the electrode/electrolyte interface.
As pilhas de combustível e eletrolisadores de óxido sólido (PCOSs e EOSs) têm sido promovidas a tecnologias promissoras para estabelecer o abastecimento de combustível e sua utilização futura em sistemas de energia limpa. As PCOSs são dispositivos que produzem energia elétrica pela oxidação de combustíveis como o hidrogénio ou de hidrocarbonetos de elevada eficiência. Alternativamente, as EOSs funcionam de maneira inversa, na qual podem ser gerados combustíveis sintéticos ao fornecer energia eléctrica renovável ao sistema. É, pois, devido a esta natureza semelhante e ainda que inversa, que estes dispositivos têm sido tradicionalmente construídos a partir de materiais compatíveis. No entanto, a entrada no mercado destas tecnologias encontra-se ainda condicionada por diversos factores. Um dos mais limitantes, está associado a problemas de estabilidade química entre os constituintes da célula, que podem reduzir a longevidade a elevadas temperaturas de operação e/ou a um desempenho eletroquímico insuficiente. Normalmente, tais problemas de compatibilidade são minimizados pela introdução de uma camada de proteção muito fina constituída por um material condutor puramente iónico, na interface elétrodo/eletrólito. Deste modo, o objetivo deste trabalho é avaliar se modificando as propriedades de transporte destas camadas de proteção se pode conduzir ao aumento das propriedades de cinética do elétrodo, através da introdução de catiões polivalentes. A introdução de condutividade eletrónica menor na superfície do electrólito foi anteriormente relatada apresentando uma melhoria muito considerável das zonas eletroquimicamente activas para a permuta de oxigénio, reduzindo, desta forma, as perdas de resistência de polarização.Assim, esta dissertação tem por objetivo desenvolver este conhecimento para adaptar uma camada de proteção bifuncional que consiga evitar os problemas de interação química e ao mesmo tempo aumentar a cinética dos elétrodos. Esta dissertação apresenta um cenário típico de um eletrólito à base de zircónia estabilizada com ítrio combinado com um elétrodo de oxigénio contendo lantanídeos. Foram investigados como materiais de proteção, os sistemas de céria dopada com gadolínio, térbio e praseodímio. As propriedades inerentes à condução eletrónica e iónica mista (MIEC) dos materiais dopados foram analisadas e agrupadas. Posteriormente, foi realizada uma análise detalhada sobre o impacto das camadas de proteção na cinética do elétrodo de oxigénio em dispositivos PCOS e EOS. Foi dada especial atenção às potenciais relações entre as propriedades de transporte da camada proteção e subsequente desempenho do elétrodo. O trabalho também avalia o desempenho eletroquímico de cátodos de K2NiF4 com diferentes estruturas, depositadas sobre a camada de proteção que apresentou melhor desempenho, isto é, a céria dopada com praseodímio, assim como a influência da espessura da camada e da fração de Pr presente na céria. Demonstrou-se que a introdução de camadas de proteção à base de MIECs levou a um aumento drástico no desempenho do elétrodo, nomeadamente pelos MIECs de maior condutividade ambipolar. Estas camadas de proteção utlizadas provaram ser também eficazes em manter o papel de inibidores de interactividade química na interface elétrodo/eletrólito.

La presente tesi tratta due argomenti distinti. Il Capitolo 1 e il Capitolo 2 riguardano problemi di evoluzione di interfacce nel piano. Nel Capitolo 1 viene considerata l’evoluzione di un materiale policristallino con tre (o più) fasi, in presenza di un’anisotropia cristallina (pari) ϕo la cui linea di livello 1, Fϕ :={ϕo ≤1} (Frank diagram), è un poligono regolare di n lati. La funzione duale ϕ : R2 →R definita da ϕ(ξ) := sup{ξ·η : ϕo(η)≤1}´e anch’essa un’anisotropia cristallina e Wϕ := {ϕ ≤ 1} è detta Wulff shape. In particolare, viene studiato il moto per curvatura cristallina di triodi elementari, ossia speciali reti piane di curve che sono frontiere regolari di insiemi rappresentanti tre fasi distinte di un materiale. Un triodo elementare è formato dall’unione di tre curve Lipschitziane, le interfacce, che si intersecano in un unico punto detto giunzione tripla. Ogni interfaccia è l’unione di un segmento di lunghezza finita e di una semiretta che riproduce due lati consecutivi della Wulff shape Wϕ. Viene analizzata l’esitenza locale e globale e la stabilità del flusso. Si dimostra l’esistenza locale di un unico flusso regolare stabile a partire da un dato iniziale regolare stabile: se n, il numero dei lati della Wulff shapeWϕ, è un multiplo di 6 allora il flusso è globale e converge a un flusso omotetico per t →+∞. L’analisi del comportamento del flusso per tempi grandi richiede lo studio della stabilità. La stabilità è l’ingrediente che assicura che nessun segmento si sviluppa dalla giunzione tripla durante il flusso. In generale, il flusso può diventare instabile in un tempo finito: se ciò accade e tutte le lunghezze dei segmenti finiti sono strettamente positive per tale tempo,è possibile costruire un flusso regolare per tempi successivi aggiungendo in corrispondenza della giunzione tripla in una delle tre interfacce un segmento infinitesimo opportuno (o addirittura un arco di curva a curvatura cristallina nulla). ´E anche possibile che durante il flusso uno dei tre segmenti scompaia in un tempo finito. In tal caso, in tale tempo il campo vettoriale di Cahn-Hoffman ha un salto di discontinuità e ai tempi successivi la giunzione tripla si muove traslando lungo la semiretta adiacente. Ognuno di questi flussi ha la proprietà che tutte le curvature cristalline rimangono limitate (persino se un segmento appare o scompare). ´E importante sottolineare che Taylor aveva già predetto la nascita di nuovi segmenti dalla giunzione tripla (senza però dimostrarlo). Viene inoltre considerato il flusso per curvatura cristalina di una partizione regolare stabile formata da due triodi elementari adiacenti. Vengono discussi alcuni esempi di situazioni di colasso che portano a cambi di topologia, come ad esempio la collisione di due giunzioni triple. Questi esempi (come anche il risultato di esistenza per tempi piccoli) mostrano uno dei vantaggi del flusso per curvatura cristallino rispetto, ad esempio, all’usuale moto per curvatura: calcoli espliciti possono essere fatti, e nel caso di non unicità, è possibile confrontare le energie delle diverse evoluzioni (difficile nel caso euclideo). Nel Capitolo 2 viene introdotta, usando la teoria delle funzioni a variazione limitata a valori in S1, la sfera diR2, una nuova classe di funzionali energia definiti su partizioni. Attraverso la variazione prima del funzionale energia, viene fornito un nuovo modello per l’evoluzione di interfacce che parzialmente estende quello introdotto nel Capitolo 1 e che consiste in un problema di frontiera libera definito sulle funzioni a variazione limitata a valori in S1. Questo modello è legato all’evoluzione di materiali policristallini dove è consentito alla Wulff shape di ruotare. Assumendo l’esitenza locale del flusso, si dimostra che durante il flusso curve chiuse convesse rimangono convesse e curve chiuse embedded rimangono embedded. Il secondo argomento della tesi è trattato nel Capitolo 3: l’obiettivo è quello di estendere il metodo delle linee di livello a sistemi di equazioni differenziali alle derivate parziali. Il metodo che viene proposto è consistente con la precedente ricerca portata avanti da Evans per l’equazione del calore e da Giga e Sato per equazioni di Hamilton-Jacobi. Il nostro approccio segue una costruzione geometrica che è legate alla nozione di barriera introdotta da De Giorgi. L’idea principale è quella di forzare un principio di confronto tra varietà di diversa codimensione e richiedere che ogni sottolivello di una soluzione dell’equazione per le linee di livello, detta level set equation, sia una barriera per i grafici di soluzioni del corrispondente sistema. Tale metodo ben si applica a una classe di sistemi di equazioni quasi-lineari del primo ordine. Viene fornita la level set equation associata ad opportuni sitemi di leggi di conservazione del primo ordine, al flusso per curvatura media di una varietà di codimensione arbitraria e a sitemi di equazioni di reazione-diffusione. Infine, viene calcolata la level set equation associata al sistema soddisfatto dalle parametrizzazioni di curve piane che si muovono per curvatura.
The present thesis deals with two different subjects. Chapter 1 and Chapter 2 concern interfaces evolution problems in the plane. In Chapter 1 I consider the evolution of a polycrystalline material with three (or more) phases, in presence of for an even crystalline anisotropy ϕo whose one-sublevel set Fϕ := {ϕo ≤ 1} (the Frank diagram) is a regular polygon of n sides. The dual function ϕ : R2 → R defined by ϕ(ξ) := sup{ξ ·η : ϕo(η) ≤ 1} is crystalline too and Wϕ := {ϕ ≤ 1} is called the Wulff shape. I am particularly interested in the motion by crystalline curvature of special planar networks called elementary triods, namely a regular three-phase boundary given by the union of three Lipschitz curves, the interfaces, intersecting at a point called triple junction. Each interface is the union of a segment of finite length and a half-line, reproducing two consecutive sides of Wϕ. I analyze local and global existence and stability of the flow. I prove that there exists, locally in time, a unique stable regular flow starting from a stable regular initial datum. I show that if n, the number of sides of Wϕ, is a multiple of 6 then the flow is global and converge to a homothetic flow as t → +∞. The analysis of the long time behavior requires the study of the stability. Stability is the ingredient that ensures that no additional segments develop at the triple junction during the flow. In general, the flow may become unstable at a finite time: if this occurs and none of the segments desappears, it is possible to construct a regular flow at subsequent times by adding an infinitesimal segment (or even an arc with zero crystalline curvature) at the triple junction. I also show that a segment may desappear. In such a case, the Cahn-Hoffman vector field Nmin has a jump discontinuity and the triple junction translates along the remaining adjacent half-line at subsequent times. Each of these flows has the property that all crystalline curvatures remain bounded (even if a segment appears or disappears). I want to stress that Taylor already predicted the appearance of new edges from a triple junction. I also consider the crystalline curvature flow starting from a stable ϕ-regular partition formed by two adjacent elementary triods. I discuss some examples of collapsing situations that lead to changes of topology, such as for instance the collision of two triple junctions. These examples (as well as the local in time existence result) show one of the advantages of crystalline flows with respect, for instance, to the usual mean curvature flow: explicit computations can be performed to some extent, and in case of nonuniqueness, a comparison between the energies of different evolutions (difficult in the euclidean case) can be made. In Chapter 2 we introduce, using the theory of S1-valued functions of bounded variations, a class of energy functionals defined on partitions and we produce, through the first variation, a new model for the evolution of interfaces which partially extends the one in Chapter 1 and which consists of a free boundary problem defined on S1-valued functions of bounded variation. This model is related to the evolution of polycrystals where the Wulff shape is allowed to rotate. Assuming the local existence of the flow, we show convexity preserving and embeddedness preserving properties. The second subject of the thesis is considered in Chapter 3 where we aim to extend the level set method to systems of PDEs. The method we propose is consistent with the previous research pursued by Evans for the heat equation and by Giga and Sato for Hamilton-Jacobi equations. Our approach follows a geometric construction related to the notion of barriers introduced by De Giorgi. The main idea is to force a comparison principle between manifolds of different codimension and require each sub-level of a solution of the level set equation to be a barrier for the graph of a solution of the corresponding system. We apply the method for a class of systems of first order quasi-linear equations. We compute the level set equation associated with suitable first order systems of conservation laws, with the mean curvature flow of a manifold of arbitrary codimension and with systems of reaction-diffusion equations. Finally, we provide a level set equation associated with the parametric curvature flow of planar curves.

Les piles à combustible à membrane échangeuse d'anions (AEMFC) ont récemment attiré l'attention en tant que piles à combustible alternatives à faible coût aux piles à combustible à membrane échangeuse de protons traditionnelles en raison de l'utilisation possible d'électrocatalyseurs non-nobles. Bien que l'AEMFC ressemble à la PEMFC, les problèmes de gestion de l'eau sont plus prégnants dans une AEMFC car l'ORR en milieu alcalin nécessite de l'eau, tandis qu'en même temps, de l'eau est produite en grande quantité du côté de l'anode. Pour mieux comprendre la gestion de l'eau dans ce type de pile à combustible, il faut d'abord développer et acquérir de l'expérience avec ce type de pile à combustible à l'échelle du laboratoire. Puisque les matériaux prêts à l'emploi n'existaient pas au commencement de la thèse, nous avons dû fabriquer nos propres assemblages électrode-membranes (AMEs) à partir des matériaux disponibles dans le commerce. Etant donné que la thématique de fabrication des AMEs est nouvelle pour les chercheurs du LEMTA, cette thèse est articulée en deux parties, une dédiée à la formulation, la préparation et l'optimisation des AMEs pour PEMFC ; et une autre dédiée au développement d'AEMFC. Les résultats ont indiqué que la composition et préparation de l'encre, ainsi que la manière de déposer l'encre modifient systématiquement la structure de l'électrode, de même que ses performances en piles à combustible. En outre, l'étude fournit des informations sur les procédures et les méthodes pour les tests en AEMFC. Ici, nous souhaiterions partager notre savoir-faire avec les nouveaux venus dans le domaine de la préparation des AMEs pour piles à combustibles à membranes échangeuses d'ions
Anion exchange membrane fuel cells (AEMFCs) have recently attracted significant attention as low-cost alternative fuel cells to traditional proton exchange membrane fuel cells as a result of the possible use of platinum-group metal-free electrocatalysts. Although AEMFC is a mimic of PEMFC but working in an alkaline medium, water management issues are more severe in AEMFC because ORR in alkaline media requires water, while at the same time water is produced at the anode side. To better understand water management in this type of fuel cell, it is necessary first to develop and gain experience with this kind of fuel cell on the laboratory scale. Since no ready-to-use materials are available at the beginning of the project, the necessity of fabricating homemade MEAs from commercially available materials becomes a reality that we must face. As MEA fabrication is a new topic to LEMTA's researchers, this is why this thesis was divided into two parts: one part dedicated to the formulation, preparation, and optimization of MEAs for PEMFC through physico-chemical and electrochemical characterizations; another part dedicated to the development of AEMFC. The results indicated that ink deposition, composition, and preparation systematically change the electrode structure and thus affect fuel cells performance. Furthermore, the study provides information on the AEMFC procedures and methods. Here, we would like to share our know-how with newcomers in the field of preparation of MEA in ion exchange membrane fuel cells

碩士
國立成功大學
材料科學及工程學系碩博士班
95
Deformation heterogeneity at the mesoscale plays a central role at understanding the Hall-Petch relationship, strain hardening, and recrystallization. This heterogeneity is not predicted by the classical theory of Taylor model in which the features of crystallographic texture are explained. In past, crystal plasticity finite-element (CPFEM) simulations were used in modeling polycrystalline plasticity by considering both compatibility of strain and stress at interfaces. However, the effects of the microstrcutre features, such as grain boundaries and triple junctions, are not still well established on plastic deformation. Therefore, the purpose of this investigation is to understand the interaction between microstructure and plane strain compression. In this study, experimental and numerical approaches are investigated on the influence of grain boundary and the triple junction during plane strain compression. In the experimental approaches, electron back scattering diffraction (EBSD) and Digital Image Correlation method (DIC) were used to characterize the orientation image mapping and the strain distribution, respectively. With the help of the orientation and strain mappings, the effects of grain boundary and triple junction during deformation can be compared with the results predicted from CPFEM. The results of the systematic analysis are able to contribute the further development of crystal plasticity including the microstructure influence.

碩士
淡江大學
數學學系
88
We consider the existence of triple solutions for nth order ordinary differential equation : where and is fixed where is a nonnegative continuous function on . We also consider the (n, p) difference equation where and is a fixed integer satisfying We assume there exist a continuous functions and such that for By employing Leggett-Williams fixed point theorem for operators on a cone, existence criteria are developed for multiple (at least three) positive solutions of the boundary value value problems. 第2章 先備知識 ......................................... 3 第3章 微分方程邊界值問題的三個正解的存在 .............. 6 第4章 差分方程 (n, p) 邊界值問題的三個正解的存在性 .... 17 參考文獻 ............................................... 23

Nanocrystalline (NC) metals with averaged grain size smaller than 100 nm have shown promising mechanical properties such as higher hardness and toughness than conventional coarse-grained metals. Unlike conventional metals in which the deformation is controlled by dislocation activities, the microstructural evolution in NC metals is mainly dominated by grain rotation and stress-driven grain boundary motion (SDGBM) due to the high density of grain boundaries (GBs). SDGBM is thus among the most studied modes of microstructural evolution in NC materials with particular interests on their fundamental atomistic mechanisms. In the first part of this thesis, molecular dynamics simulations were used to investigate the influences of Triple Junctions (TJs) on SDGBM of symmetric tilt GBs in copper by considering a honeycomb NC model. TJs exhibited asymmetric pinning effects to the GB migration and the constraints by the TJs and neighboring grains led to remarkable non-linear GB motion in directions both parallel and normal to the applied shear. Based on these findings, a generalized model for SDGBM in NC Cu was proposed. In the second part, the interaction of SDGBM with crack, voids and precipitates was investigated. It was found that depending on the GB structure, material type and temperature, there is a competition between different atomistic mechanisms such as crack healing, recrystallization and GB decohesion. It is hoped that the findings of this work could clarify the micro-mechanisms of various experimental phenomena such as grain refinement in metals during severe plastic deformation, which can be used to design optimized route of making stabilized bulk NC metals.
February 2016

As the demand for green energy and fuel cells grows, more attention is drawn towards Solid Oxide Fuel Cells (SOFCs). Random and complex structure of composite electrodes and underlying electrochemical process has not been completely unveiled yet and further study is required to acquire more understanding. Modeling in this regard plays an important role as it pinpoints key parameters in optimum design of the cell without resorting to costly and uncertain experiments which might even lead to misinterpretations due to random nature of experimental data. The aim of this work is to develop a new rigorous model to study the structure performance relationship of (SOFC) composite electrodes. The work has been conducted in two phases, a two-dimensional continuous approach and three-dimensional discrete model. A new two-dimensional, geometrical model which captures the inhomogeneous nature of the location of electrochemical reactions based on random packing of electronic and ionic conducting particles has been developed. The results show that the concentration of oxygen inside the cathode in the two-dimensional model is not only a function of the electrode depth but also changes along the width of the electrode. Furthermore the effect of composition of the electrode on the length of three phase boundary (TPB) and total polarization resistance has been demonstrated. A parametric study of the effect of the conductivity of ionic conductor and diffusion coefficient on the performance of the electrode has been given. To make a more realistic analysis, a three-dimensional reconstruction of (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of electrodes. To enhance connectivity between particles and increase the length of TPB, sintering process is mimicked by enlarging particles to certain degree. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler-Volmer equation is used to deal with charge-transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not homogeneous. Effects of particle size, electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated.
Chemical Engineering

A numerical model was developed to evaluate the performance of detailed solid oxide fuel cell (SOFC) anode microstructures obtained from experimental reconstruction techniques or generated from synthetic computational techniques. The model is also capable of identifying the linear triple phase boundary (TPB) reaction sites and evaluating the effective transport within the detailed structures, allowing a comparison between the structural properties and performance to be conducted. To simulate the cell performance, a novel numerical coupling technique was developed in OpenFOAM and validated. The computational grid type and mesh properties were also evaluated to establish appropriate mesh resolutions to employ when studying the performance. The performance of a baseline synthetic electrode structure was evaluated using the model and under the applied conditions it was observed that the ionic potential had the largest influence over the performance. The model was used in conjunction with a computational synthetic electrode manufacturing algorithm to conduct a numerical powder to power parametric study investigating the effects of the manufacturing properties on the performance. An improvement in the overall performance was observed in structures which maximized the number of reaction sites and had well established transport networks in the ion phase. From the manufacturing parameters studied a performance increase was observed in structures with low porosity and ionic solid volume fractions near the percolation threshold, and when the anodes were manufactured from small monosized particles or binary mixtures comprising of smaller oxygen ion conductive particles. Insight into the anode thickness was also provided and it was observed that the current distribution within the anode was a function of the applied overpotential and an increase in the overpotential resulted in the majority of the current production to increase and shift closer to the electrode-electrolyte interface.
Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-10-01 09:41:47.617

Text in Afrikaans
In hierdie navorsingstuk word 'n totaal nuwe Radikale en gedifferensieerde Universumgerigte aanloop tot die teologie, die peumato/ogie en veral die ou klassieke pentekosta/isme se eng partikularistiese en sogenaamde unieke elitistiese geestesdoopmodel krities teologies-filosofies ondersoek. Hoewel die moderner neopentekostalistiese oftewel charismatiese weergawe van klassieke pentekostalisme aandag kry om as kriterium van vergelyking le dien, val die hoofklem op klassieke pentekosta/isme. Uiteraard kom die verskillende Geesdoopmodelle pertinent aan die orde. En in die lig hiervan poneer hierdie studie dan 'n kraaknuwe globalistiese Geesdoopmodel, genaamd die RGU-Geesdoopmodel wat dui op 'n Radikale en Gedifferensieerde Universumgerigte Geesdoopmodel. Hierdie nuwe RGU-Geesdoopmodel impliseer logiesgewys dan ook 'n radikale revolusionere paradigmatiese ommeswaai van en verandering in die ou klassieke Geesdoopperspektiewe. Noodwendig impliseer dit ook 'n grater oopspanning van die teologies-wetenskaplike navorsingspeelveld en die interverwante relasies daarvan met alle teologiese en nie-teologiese wetenskappe en bedrywe, met alle kerklike en nie-kerklike kaders, met alle religieuse en nie-religieuse kringe, asook met alle teoretiese en praksiologiese ingesteldhede. Gebiedsafbakening is dus ook gebiedend noodsaaklik en onontbeerlik. Die sentrifigale vernouing van die studieskopus hanteer dan uiteraard net die essensieile teologies-wysgerige en praksiologiese impakte, nie net op die ou klassieke pentekostalisme nie, maar ook op alle teologiee en alle filosofiee en alle wetenskappe, wat op 'n samehangende holisties-geintegreerde wyse universumgerig tot een groot globale geheel omskep word
In this research a totally new Radical and Differentiated Universal-directed approach to theology, pneumatologyand particularly the old classical pentecostalism's narrow particularistically and so called unique elitistic model of Spiritbatism is critically investigated on theological-philosophical levels. Though the moderner neo-pentecostalistic or rather charismatic rendering of classical pentecostalism receives attention to serve as criterion for comparison, the main focus is on classical pentecostalism. Naturally the different models of Spiritbaptism comes pertinently on the table. And in the light of this, our study propose a brand new globalistic model for Spiritbaptism, namely the RDU-Spiritbaptism-mode lwhich signifies a Radical and Differentiated Universal-directed model of Spiritbaptism. This new RDU-Spiritbaptism-model also logically implies a radical revolutionary paradigmshift and tum-about in the old classical Spiritbaptism perspectives. This inevitably also implies a widening of the theological-scientific field and the interrelatedness thereof with all theological and non-theological sciences and professions, with all ecclesiological and non-ecclesiological scopes, with all religious and non-religious circles, as well as all theoretical and praxiological inclinations. Confinement of territory is thus also imperatively essential and indispensable. The centrifical narrowing of the studyscope handles naturally only the essential theological-philosophical and praxiological impacts, not only on the old classical pentecostalism, but also on all theologies and all philosophies and all sciences which are all transformed in a coherent holistically interconnected fashion to one great universal whole
Philosophy, Practical & Systematic Theology
D. Th. (Systematic Theology)

Seafloor compliance is a non-intrusive geophysical method sensitive to the shear modulus of the sediments below the seafloor. A compliance analysis requires the computation of the frequency dependent transfer function between the vertical stress, produced at the seafloor by the ultra low frequency passive source-infra-gravity waves, and the resulting displacement, related to velocity through the frequency. The displacement of the ocean floor is dependent on the elastic structure of the sediments and the compliance function is tuned to different depths, i.e., a change in the elastic parameters at a given depth is sensed by the compliance function at a particular frequency. In a gas hydrate system, the magnitude of the stiffness is a measure of the quantity of gas hydrates present. Gas hydrates contain immense stores of greenhouse gases making them relevant to climate change science, and represent an important potential alternative source of energy. Bullseye Vent is a gas hydrate system located in an area that has been intensively studied for over 2 decades and research results suggest that this system is evolving over time. A partnership with NEPTUNE Canada allowed for the investigation of this possible evolution. This thesis describes a compliance experiment configured for NEPTUNE Canada’s seafloor observatory and its failure. It also describes the use of 203 days of simultaneously logged pressure and velocity time-series data, measured by a Scripps differential pressure gauge, and a Güralp CMG-1T broadband seismometer on NEPTUNE Canada’s seismic station, respectively, to evaluate variations in sediment stiffness near Bullseye. The evaluation resulted in a (- 4.49 x10-3± 3.52 x 10-3) % change of the transfer function of 3rd October, 2010 and represents a 2.88% decrease in the stiffness of the sediments over the period. This thesis also outlines a new algorithm for calculating the static compliance of isotropic layered sediments.