This thesis focus on the effect that selection has on the ancestry of a spatially structured population. In the absence of selection, the ancestry of a sample from the population behaves as a system of random walks that coalesce upon meeting. Backwards in time, each ancestral lineage jumps, at the time of its birth, to the location of its parent, and whenever two ancestral lineages have the same parent they jump to the same location and coalesce. Introducing selective forces to the evolution of a population translates into branching when we follow ancestral lineages, a by-product of biased sampling forwards in time. We study populations that evolve according to the Spatial Lambda-Fleming-Viot process with selection. In order to assess whether the picture under selection differs from the neutral case we must consider the timescale dictated by the neutral mutation rate Theta. Thus we look at the rescaled dual process with n=1/Theta. Our goal is to find a non-trivial rescaling limit for the system of branching and coalescing random walks that describe the ancestral process of a population. We show that the strength of selection (relative to the mutation rate) required to do so depends on the dimension; in one and two dimensions selection needs to be stronger in order to leave a detectable trace in the population. The main results in this thesis can be summarised as follows. In dimensions three and higher we take the selection coefficient to be proportional to 1/n, in dimension two we take it to be proportional to log(n)/n and finally, in dimension one we take the selection coefficient to be proportional to 1/sqrt(n). We then proceed to prove that in two and higher dimensions the ancestral process of a sample of the population converges to branching Brownian motion. In one dimension, provided we do not allow ancestral lineages to jump over each other, the ancestral process converges to a subset of the Brownian net. We also provide numerical results that show that the non-crossing restriction in one dimension cannot be lifted without a qualitative change in the behaviour of the process. Finally, through simulations, we study the rate of convergence in the two-dimensional case.

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Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Sintering is an important processing step for obtaining the necessary mechanical stability and rigidity of ceramic bulk materials. Both mass and heat transfer are essential in the sintering process. The importance of radiation heat transfer is significantly enhanced at high temperatures according to the well-known Stefan-Boltzmann’s law. In this thesis, we modified the pressure-less spark plasma sintering set-up to generate intense thermal radiation, aiming at rapid consolidation of ceramic bulk materials. This approach was named as “Sintering by Intense Thermal Radiation (SITR)” as only thermal radiation contributed. Firstly, the heat and mass transfer mechanisms during the SITR process were studied by choosing zirconia ceramics as references. The results revealed that the multiple scattering and absorption of radiation by the materials contributed to the heat diffusion. The observed enhanced densification and grain growth can be explained by a multiple ordered coalescence of zirconia nanocrystals using high heating rates. Secondly, the temperature distribution during the SITR process was investigated by both numerical simulation and experimental verifications. It showed that the radiator geometry, sample geometry and radiating area were influencing factors. Besides, the change of material and geometry of the radiators resulted in an asymmetric temperature distribution that favored the formation of SiC foams. The foams had gradient structures with different open porosity levels and pore sizes and size distributions. Finally, ceramic bulk materials were successfully fabricated by the SITR method within minutes. These materials included dense and strong ZrO2 ceramics, Si3N4 foams decorated with one-dimensional nanostructures, and nasal cavity-like SiC-Si3N4 foams with hierarchical heterogeneities. Sufficient densification or formed strong necks were used for tailoring these unique microstructures. The SITR approach is well applicable for fast manufacture of ceramic bulk materials because it is clean and requires low energy consumption and properties can be controlled and tuned by selective heating, heating speed or temperature distribution. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.</p><p> </p>

O presente trabalho foi realizado para estudar o efeito sobre a propagação de fraturas em juntas não-persistentes quando existe uma variação da rugosidade nas juntas. Para esta abordagem, foram analisados os modos de coalescência, o ângulo de início das fraturas, a resistência à compressão e deformação de modelos de argamassa contendo juntas não-persistentes e submetidos a estados biaxiais de tensão. Na literatura especializada encontram-se vários trabalhos que estudam o comportamento de juntas lisas não-persistentes. Infelizmente, este tipo de juntas não leva em conta os efeitos da dilatância e do aumento do atrito. Neste estudo, desenvolveu-se um método para produzir juntas rugosas não-persistentes dentro dos moldes de argamassa. Desta forma, conseguiu-se comparar os diferentes modos de ruptura, tanto para as juntas lisas como as rugosas. A configuração geométrica usada foi de 15 juntas com 'alfa' = 54 graus; 'beta' = 60 graus; Lj = Lb = 50mm e d = 25mm. Onde 'alfa' é o ângulo gerado entre o plano de uma junta e o plano formado entre as pontas de juntas não-coplanares, 'beta' é a inclinação da junta com respeito ao plano de tensão maior, Lb denota à distância entre juntas paralelas coplanares, Lj é o comprimento da junta e d é a distância entre juntas paralelas não coplanares. Estes parâmetros mantiveram-se constantes em todos os ensaios, tanto para as juntas lisas (JRC = 0) como as rugosas (JRC = 6,96 e JRC = 12,25). Usando os parâmetros acima mencionados, observou-se que o tipo de ruptura sempre aconteceu por escalonamento. Para as amostras contendo as juntas lisas, JRC = 0, o mecanismo principal de coalescência entre as juntas é a tração. Para estas juntas, os ângulos de início das fraturas, em média, resultaram de 64º e 65º para esquerda e direita, respectivamente. Em média, a resistência a compressão normalizada ('sigma'nor = 'sigma'1 - 'sigma'2 / 'sigma'cs; onde 'sigma'1 e 'sigma'2 são as tensões principais e 'sigma'cs é a resistência média à compressão simples) destes corpos de prova resultou ter o valor mais baixo, 'sigma'nor = 0,52, e a maior deformação, 'épsilon' = 0,0057. No caso das juntas com JRC = 6,7, a coalescência ocorreu tanto por tração como por cisalhamento, descrevendo um caminho ondulante entre as pontas das juntas. O ângulo médio de início das fraturas foi de 40º para o lado esquerdo e 48º para o lado direito. A resistência média normalizada destes corpos a compressão foi de 'sigma'nor = 0,54 e uma deformação de 'épsilon' = 0,0053. Com as juntas com JRC = 12,3, a coalescência também apresentou ambos os mecanismos, tração e cisalhamento, seguindo uma direção inclinada no início e trajetória reta no meio. Os ângulos de início obtidos foram 5º no lado esquerdo e 20º do lado direito. Os valores maiores de resistência média normalizada e o valor médio menor de deformação foram obtidos nestes tipo de juntas, sendo estes de 'sigma'nor = 0,59 e 'épsilon' = 0,0045. Verificou-se então que a rugosidade tem uma grande influência sobre a propagação da fratura, afetando grandemente o modo de coalescência, o ângulo de início de propagação das fraturas e a resistência e deformação total dos corpos de prova / The present experimental study was conducted to investigate the effect of joint roughness on the fracture propagation of models with non persistent joints. For this approach the parameters investigated are : coalescence, the crack initiation angle, the compressive strength and deformation of the mortar samples containing non-persistent rough joints under biaxial loading. There are many works in specialized literature that study the behavior of non-persistent smooth joints, unfortunately, this kind of approach does not take into account the effect of the dilation and the increase of friction due to the joint roughness. In this study, a new method was developed to produce non-persistent rough joints inside the mortar models. With this method it was possible to compare the different paths of rupture generated for the smooth and for the rough joints. Each sample had 15 joints with 'alfa' = 54º; 'beta' = 60º; Lj = Lb = 50mm and d = 25mm. Where 'alfa' is the formed angle between the joint plane and the plane generated by two non coplanar joint tips, 'beta' is the joint inclination angle with the principal plane stress, Lb is the distance between coplanar joints, Lj is the length of the joint, d is the distance between two non-coplanar joints. These parameters remained constant in all the tests; the only variation permitted was in the joint roughness : from smooth joints (JRC = 0) to rough joints (JRC = 6,96 and JRC = 12,25). Using the mentioned parameters before it was always obtained the stepping failure. For the samples containing smooth joints, JRC = 0, the main mechanism of coalescence is tension. For these joints the average crack initiation angles, had resulted of 64º and 65º for left and right side, respectively. The average normalized compression strength ('sigma'nor = 'sigma'1 - 'sigma'2 / 'sigma'cs; where 'sigma'1 and 'sigma'2 are the principal stresses and 'sigma'cs is the average compression strength) of these tests resulted to have the lowest value, 'sigma'nor = 0,52, and the highest deformation, 'épsilon' = 0,0057. In the case of joints with JRC = 6,7; the mechanisms of coalescence are tension and shear, growing in a waving path between the joint tips. The average crack initiation angle was of 40º for the left side and 48º for the right side of the joint. The average normalized compression strength of these tests was of 'sigma'nor = 0,54 and deformation 'épsilon' = 0,0053. With joints having JRC = 12,3; the coalescence also presents both mechanisms, tension and shear, following a direction inclined in the beginning and a straight line in the middle of the way. The crack initiation angles had been : 5º in the left side and 20º of the right side. The highest values of average normalized strength resistant and the lowest average value of deformation were found in this type of joint, being of 'sigma'nor = 0,59 e 'épsilon' = 0,0045 respectively. With this approach, it was verified that the joint roughness influence the fracture propagation, affecting the coalescence, the crack initiation angle, the resistance and total deformation of the tested specimens

The period of ice break-off in spring is a key event for many biogeochemical processes in lakes globallly. The biogeochemical processes occurring at ice break-off have the potential of influencing characteristics of lakes throughout spring and summer, including algal blooms and greenhouse gas emission. This makes it important to study lakes in the period of ice break-off. At ice break-off, soil bacteria from the catchment area usually enter the lake via spring floods and mix with the bacteria already occurring in the lake water. In this study, the effects of mixing soil- and lake microbial communities during ice break-off-like conditions were tested by performing an experiment under controlled conditions in the laboratory. In the experiment, light, microbial community composition and concentration of soil-derived organic matter were manipulated to simulate different conditions associated with ice break-off. The variables investigated were bacterial activity and functionality, measured as cell abundance and enzymatic activity, as well as primary production and concentration of dissolved organic matter. The results showed that a mix of soil and lake microbial communities had enzymatic activity patterns resembling lake communities, and then shifted to being more similar to soil communities. The experiment also showed that degradation of measured dissolved organic matter was not linked to biotic processes, and that the observed decrease was most likely due to photo degradation. Finally, the experiment showed that primary production, here measured as chlorophyll a, was only stimulated by the mixed community with light and added soil dissolved organic matter. The results found in this study are important as they show that microbial communities do alter their function and enzymatic activity based on composition. Furthermore, the result that primary production was only seen in the presence of light, soilderived organic matter and a mixed community of lake and soil bacteria may be seen as an indication that primary producers in lake ecosystems to some extent depend on the inflow of terrestrial microbes and organic matter. It also possible that the coalescence of microbial communities enables the communities to perform tasks they were unable to prior to coalescence (i.e. perform tasks that allows primary production to take place). These results give the basis for further, more detailed studies.

Dans cette thèse, nous nous intéressons à deux problèmes de reconnaissance de formes : l'option de rejet en classification supervisée, et la détermination du nombre de classes en classification non supervisée. Le premier problème consiste à déterminer les zones de l'espace des attributs où les observations n'appartiennent pas clairement à une seule classe. Le second problème repose sur l'analyse d'un nuage d'observations pour lesquelles on ne connait pas les classes d'appartenance. L'objectif est de dégager des structures permettant de distinguer les différentes classes, et en particulier de trouver leur nombre. Pour résoudre ces problèmes, nous fondons nos propositions sur des opérateurs d'agrégation, en particulier des normes triangulaires. Nous définissons de nouvelles mesures de similarité permettant la caractérisation de situations variées. En particulier, nous proposons de nouveaux types de mesures de similarité : la similarité d'ordre, la similarité par blocs, et enfin la similarité par une approche logique. Ces différentes mesures de similarité sont ensuite appliquées aux problèmes évoqués précédemment. Le caractère générique des mesures proposées permet de retrouver de nombreuses propositions de la littérature, ainsi qu'une grande souplesse d'utilisation en pratique. Des résultats expérimentaux sur des jeux de données standard des domaines considérés viennent valider notre approche.

L'objectif de cette thèse est d'étudier les corrélations entre les propriétés morphologiques, structurales et magnétiques de nanoparticules à base de cobalt et platine. L'importance de cette étude tient au fait que les nanoparticules de CoPt constituent un matériau potentiellement utilisable dans le domaine de l'enregistrement magnétique. Il est donc essentiel de déterminer les liens entre les propriétés magnétiques et l'organisation atomique des nanoparticules. Pour cela, nous avons d'abord étudié la structure et la morphologie de ces objets à l'aide de techniques complémentaires : la microscopie électronique, la diffusion et l'absorption des rayons X. Les résultats montrent des transitions structurales dépendantes de paramètres comme la température, la taille ou le mode de croissance des nanoparticules. Ainsi, lors de la croissance atome par atome, des transitions de structure icosaédrique vers cubique à faces centrées sont observées. En revanche, la coalescence permet la formation d'une structure intermédiaire décaédrique. La structure chimiquement ordonnée qui est la plus intéressante pour le stockage de l'information a été obtenue par recuit à une température de 630 °C. Dans un second temps nous avons étudié les propriétés magnétiques des nanoparticules par magnétométrie SQUID et par dichroïsme magnétique circulaire des rayons X. L'analyse des résultats fait apparaître des liens clairs entre leurs structures et leurs propriétés magnétiques. Ces études ont été menées sur des particules alliées et coeur / coquille. Des effets d'interface ont été mis en évidence et les échantillons recuits ont montré une très forte augmentation du moment magnétique.

Le diagnostic par une méthode optique telle que l'anémométrie phase Doppler, des particules présentes dans les milieux diphasiques, nécessite l'emploi de faisceaux laser focalisés. La compréhension et l'élimination des effets engendrés sur les mesures de taille, par les gradients d'éclairage ou les « effets de trajectoire », est au centre de la première partie de cette thèse. Différentes solutions, testées numériquement et expérimentalement, sont proposées pour éliminer les biais constatés. L'extension de l'anémométrie phase Doppler à la mesure de la partie réelle et complexe (absorption) de l'indice de réfraction des particules est ensuite considérée. Les méthodes originales proposées autorisent, en plus des mesures de taille et de vitesse, la reconnaissance des particules par leur indice, l'étude de la coalescence de gouttes (liquides transparents ou absorbants) ou la détection des fortes variations de température de particules. La dernière partie de ce travail propose diverses solutions pour étendre l'anémomètrie phase Doppler à la mesure de particules cylindriques (fibres, jets liquides), ovoïdes (oblates/problates), sphériques non­homogènes (multicouches, à coeur, hétérogènes: diphasiques ou non) et irrégulières. Le cas des particules multicouches est particulièrement détaillé, à partir de simulations basées sur le travail théorique effectué pour étendre la théorie de Lorenz-­Mie généralisée à ce type de particules.

Le détecteur interférométrique d'ondes gravitationnelles Virgo est en phase de mise au point, il devrait atteindre une sensibilité lui permettant de prendre des données scientifiques dès la deuxième moitié de l'année 2006. La préparation à l'analyse de ces données est en cours et le mémoire traite de plusieurs aspects :<br />- Un environnement d'analyse, VEGA, a été mis au point. Il permet à un utilisateur physicien d'accéder et de gérer les données provenant de Virgo, de développer un code d'analyse spécifique, de disposer d'outils de traitement du signal et de manipuler et visualiser des données dépendantes du temps. L'ensemble est intégré et basé sur ROOT.<br />- Un ensemble de pages web a été développé permettant l'affichage de données de contrôle en quasi temps-réel. Il tire parti de VEGA pour la création des graphiques.<br />- Un algorithme de tessellation d'un espace de paramètres a été développé dans le cadre des recherches de signaux provenant de la coalescence de binaires d'objets compacts (étoiles à neutrons ou trous noirs). Ces recherches utilisent communément des méthodes d'extraction de signal par filtrage optimal qui rendent nécessaire l'utilisation d'un grand nombre de calques (formes d'onde théoriques) dans un espace de paramètres. La tessellation de cet espace détermine les calques qui seront utilisés. Le mémoire traite également des performances de l'algorithme observées lors de son utilisation dans la collaboration Virgo, ainsi que ses possibles extensions.