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This thesis focuses on the issue of heating of medium-voltage switchgears. The paper contains descriptions of the individual devices which the switchgear consists of. Furthermore, the paper examine the theoretical relations which describe the heating of the conductor. It also present the normative regulations regarding the heading of MV switchgears as well as specific impacts which influence the final temperature-rise. This is followed by chapters that describe the refining of a 3D model of specific switchgear for purposes of simulating temperature-rise, further followed by chapters that look at the actual simulation environment. The thesis further focuses on the actual simulation of MV switchgear temperature-rise and it will compare the results of the simulation with measurements taken under real conditions. The thesis also includes results of simulated temperature-rise while taking into consideration the forced cooling of the switchgear. In the last part, it focuses on the creation of an excel file, which enables the prediction of final results for the temperature-rise of disconnector based on its contact resistance.

Le graphène est un matériau carboné exceptionnel de par ses propriétés électriques (mobilité électronique à température ambiante de 200 000 cm²V-¹s-¹), mécaniques (module d'Young de 1,5 TPA) et sa capacité à protéger un substrat de l’atmosphère. Cela en fait un candidat idéal comme revêtement de protection pour les dispositifs de connexion dont la partie active (le contact électrique) peut subir des dégradations sévères au cours du temps.. Un tel revêtement de protection doit en effet avoir des propriétés de robustesse et de conduction la plupart du temps antinomiques. La première étape de ce travail a consisté à mettre en œuvre un procède d’exfoliation en phase liquide afin de produire des suspensions de graphène ou de matériaux de type graphène. Différents type de caractérisations structurelles et électriques à l’échelle microscopique ont permis de déterminer des conditions d’exfoliation favorables en variant la nature du solvant et les conditions de sonication. Il s’est agi ensuite d’évaluer différents modes de dépôts permettant d’obtenir un revêtement de protection à partir de feuillets individuels. Les méthodes de trempé, évaporation, spray et filtration ont été caractérisées et ont montré leurs avantages et inconvénients. La méthode de filtration donne les films les plus couvrants mais pose le problème du transfert vers la surface à protéger : on a mis en évidence une contamination résiduelle difficile à éliminer. Les premiers résultats avec une méthode de spray industriel sans buse ont montré une grande réduction du frottement mais aussi la complexité des mécanismes régissant la qualité des dépôts. Les limites des différentes méthodes de dépôt pourraient être dépassées par l’utilisation de films composites polymère-graphène. / Graphene is a special carbon material due to its electrical properties (electron mobility at room temperature 200 000 cm²V-¹s-¹), mechanical (Young modulus of 1.5 TPA) and ability to protect a substrate of atmosphere. This makes it an ideal candidate as a protective coating for connecting devices including the active portion (electrical contact) may suffer severe damage over time .. Such a protective coating must indeed have properties and robustness conducting most of the contradictory time. The first step of this work was to implement an exfoliation proceeds in the liquid phase to produce suspensions of graphene and graphene-like materials.Different types of structural and electrical properties at the microscopic scale have determined favorable conditions for exfoliation by varying the nature of the solvent and conditions of sonication. He then acted deposits evaluate different methods to obtain a protective coating from individual sheets. The methods of dip coating, drop casting, spray and filtration have been characterized and have shown their advantages and disadvantages. The filtration method gives the most covering films, but the problem of the transfer to the surface to be protected: it showed residual contamination difficult to remove. The first results with an industrial spray without nozzle method showed a large reduction in friction but also the complexity of the mechanisms governing the quality of deposits. Limitations of different deposit methods could be exceeded by the use of graphene-polymer composite films.

Rechargeable lithium-ion batteries are a staple of modern society, providing power to a significant portion of the world's electronics and rapidly replacing older power sources. The advent of widely available electric cars with batteries of up to 200 kWh, with an increasing emphasis on fast charging, has only increased their importance. Lithium-ion battery electronic and ionic properties are largely determined by the microstructure of the battery electrode film and can be heavily influenced by relatively small variations in film makeup, including the formation of voids or distribution of carbon and binder. Prior to this research, electrical properties, which are some of the most important characteristics to battery cost, performance, and safety, were either difficult or, in the case of contact resistance, impossible to directly measure. This dissertation focuses on the development and use of a micro-surface probe for measurement and mapping of lithium-ion battery film electronic characteristics. The measurement apparatus, inversion and mapping routines, and experimental data presented provide manufacturers and researchers with a better understanding of battery heterogeneity and the influence of microstructure on electrical properties. The micro-surface probe was used to map spatial variation on both a macro and micro scale; compare physical, electrical, and ionic properties; and validate tests that were previously used to estimate electronic parameters. Experiments on commercial-quality battery electrode films showed higher micro-heterogeneity than was previously assumed by a significant margin. Additionally, electronic and ionic properties were shown to not always be inversely related and some physical explanations for observed variation were explored. Macro-variations were measured and shown to exist across electrode films which were previously assumed to be uniform. Finally a comparison to the mechanical peel test, a common test used in industry as a proxy measurement of electrical contact resistance, proved the peel test to be inconclusive and showed that it will not always accurately reflect electrical properties of films. Direct measurements of both electrical conductivity and contact resistance provide a new and important tool to advance understanding and development of lithium-ion batteries. The magnitude of the measured resistivities and their significant variation demonstrates that a better understanding of film properties is needed and will significantly influence our understanding of modern battery parameters and the effects of manufacturing techniques on battery performance.

Thermal systems often feature composite regions that are mechanically mated. In general, there exists a significant temperature drop across the interface between such regions which may be composed of similar or different materials. The parameter characterizing this temperature drop is the thermal contact resistance, which is defined as the ratio of the temperature drop to the heat flux normal to the interface. The thermal contact resistance is due to roughness effects between mating surfaces which cause certain regions of the mating surfaces to loose contact thereby creating gaps. In these gap regions, the principal modes of heat transfer are conduction across the contacting regions of the interface, conduction or natural convection in the fluid filling the gap regions of the interface, and radiation across the gap surfaces. Moreover, the contact resistance is a function of contact pressure as this can significantly alter the topology of the contact region. The thermal contact resistance is a phenomenologically complex function and can significantly alter prediction of thermal models of complex multi-component structures. Accurate estimates of thermal contact resistances are important in engineering calculations and find application in thermal analysis ranging from relatively simple layered and composite materials to more complex biomaterials. There have been many studies devoted to the theoretical predictions of thermal contact resistance and although general theories have been somewhat successful in predicting thermal contact resistances, most reliable results have been obtained experimentally. This is due to the fact that the nature of thermal contact resistance is quite complex and depends on many parameters including types of mating materials, surface characteristics of the interfacial region such as roughness and hardness, and contact pressure distribution. In experiments, temperatures are measured at a certain number of locations, usually close to the contact surface, and these measurements are used as inputs to a parameter estimation procedure to arrive at the sought-after thermal contact resistance. Most studies seek a single value for the contact resistance, while the resistance may in fact also vary spatially. In this thesis, an inverse problem (IP) is formulated to estimate the spatial variation of the thermal contact resistance along an interface in a two-dimensional configuration. Temperatures measured at discrete locations using embedded sensors appropriately placed in proximity to the interface provide the additional information required to solve the inverse problem. A superposition method serves to determine sensitivity coefficients and provides guidance in the location of the measuring points. Temperature measurements are then used to define a regularized quadratic functional that is minimized to yield the contact resistance between the two mating surfaces. A boundary element method analysis (BEM) provides the temperature field under current estimates of the contact resistance in the solution of the inverse problem when the geometry of interest is not regular, while an analytical solution can be used for regular geometries. Minimization of the IP functional is carried out by the Levenberg-Marquadt method or by a Genetic Algorithm depending on the problem under consideration. The L-curve method of Hansen is used to choose the optimal regularization parameter. A series of numerical examples are provided to demonstrate and validate the approach.

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yes / This work outlines a novel approach for determining thermal contact resistance (TCR) in micromoulding. The proposed technique aims to produce TCR predictions with known confidence values and combines experimental evidence (temperature fields and contact angle measurements) with various mathematical modelling procedures (parametric representation of surfaces, finite element analysis and stochastic processes). Here, emphasis is made on the mathematical aspects of the project. In particular, we focus on the description of the parametric surface representation technique based on the use of partial differential equations, known as the PDE method, which will be responsible for characterizing and compressing micro features in either moulds or surface tools. / EPSRC

L’étude du contact électrique quasi statique à plusieurs échelles permet de comprendre celui des micro-relais MEMS. Au cours de ce travail, une modélisation fine du contact est développée pour valider des lois de comportement établies à partir des mesures obtenues grâce à la mise au point de deux dispositifs expérimentaux originaux : la balance de précision, qui permet de réaliser un contact à l’échelle macroscopique entre barreaux croisés recouverts des films minces des matériaux à tester, et un nanoindenteur instrumenté pour la mesure électrique reproduisant un micro-contact identique à celui des micro-relais. Ils permettent tous deux de mener une étude comparative de différents échantillons en fonction de la force (de la dizaine de µN à quelques N), du courant (du µA à l’A), de l’état de surface (rugosité) ou encore du temps ; le contact étant caractérisé par sa résistance électrique. Ce travail concerne principalement le contact réalisé entre films minces en Au, matériau de contact de référence pour les applications micro-relais MEMS. L’étude des contacts de grande dimension a néanmoins été élargie à Ru, Rh, Pt et à l’alliage Au-Ni.Les résultats obtenus à l’aide de la balance de précision ont démontré la nécessité de prendre en compte l’influence de la configuration en film mince des matériaux de contact, tant du point de vue mécanique (rugosité) qu’électrique (répartition des lignes de courant). Leur comparaison à une modélisation statistique du contact rugueux donne des résultats satisfaisants. Cette approche a par ailleurs nécessité le développement d’un algorithme d’analyse d’image des relevés topographiques réalisés au microscope à force atomique, permettant ainsi de quantifier précisément les positions, taille et rayon de courbure de chaque aspérité de la surface.Les mesures effectuées à l’aide du nanoindenteur ont mis en évidence l’effet de la durée de fermeture des microcontacts sur la valeur de la résistance électrique. Le fluage des aspérités serait en partie responsable de la décroissance temporelle observée, aboutissant à des valeurs de résistance limite comparables à celles calculées à l’aide d'une modélisation numérique du contact entre des aspérités discrétisées et une sphère lisse. / The multi scale study of quasi static electrical contact is aimed at understanding those in MEMS microswitches. In this work, an accurate modeling of contact is developed to validate constitutive relations based on measurements obtained through the development of two original experimental set ups: a precision balance, which enables to perform a macroscopic contact between crossed roads coated with thin films of the materials to be tested, and a nanoindenter instrumented for electrical measurements reproducing microswitches contacts. They both allow a comparative study of different samples depending on the force (from μN to N), current (µA to A), surface condition (roughness) or time, while the contact is characterized through its electrical resistance. The measurements are obtained in the first place for gold, the reference contact material for MEMS microswitches applications. The study of large contacts was nevertheless extended to Ru, Rh, Pt and Au-Ni alloy.The results obtained using the precision balance showed the need to take into account the influence of the thin film configuration of contact materials, both in terms of mechanical (roughness) and electrical (distribution of current lines). Their comparison to a statistical model of rough contact gives satisfactory results. This approach also required the development of an image analysis algorithm of topographic maps obtained through atomic force microscopy. It allows quantifying precisely the position, height and radius of curvature of each surface asperity.Measurements made using the nanoindenter showed the effect of the time of closure of the micro contact on electrical resistance values. The creep of asperities may be partly responsible for the observed time decay, leading to limit resistance values comparable to those calculated using a numerical modeling of the contact between discretized asperities and a smooth sphere.