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Sensitive thermogravimetric equipment was used to monitor the oxidation rates of A1-1 to 9% Mg alloys in 0.21 oxygen/0.79 helium mixtures with and without 0.03 atm water vapour pressure, in the temperature range 500-725°C which include their respective solid, semi-liquid and liquid states. These measurements were supported by structural and topographical studies of oxide films using SEM, X-ray diffraction, and TEM with electron diffraction and EDX techniques. The objective was to provide information in the context of dross formation during melting operations. The only oxidation product observed was MgO. The oxidation rates do not follow simple rate laws. The rate normally increases as the temperature is raised except for a curious inverse relationship for liquid alloys just above the liquidus temperatures due to the formation of undulating surfaces. An amorphous MgO film formed on liquid alloys restricted the initial oxidation but, following an incubation period, crystallisation of the film induced breakaway oxidation. The crystallisation was promoted by high magnesium contents, high temperatures and moist atmospheres. The oxidation rates for alloys in the semi-liquid state were as follows: (i) for < 50% liquid, the rates were faster than those for wholly solid alloys due to preferential oxidation of magnesium-enriched liquid fractions along grain boundaries; (ii) for > 50% liquid, the rates were initially faster than those for wholly liquid alloys due to the presence of solid phase particles which acted both as disruptive stress-raisers and as sources of nuclei for crystallisation of the amorphous oxide film over the liquid phase. For the moist atmosphere: (i) wholly solid alloys and semi-liquid alloys with < 50% liquid oxidised initially faster but eventually slower than in the dry atmosphere. These effects are explained by enhanced initial oxidation but suppressed subsequent nucleation of tertiary MgO particles and by the injection of OH - ions into the oxide; (ii) wholy liquid alloys and semi-liquid alloys with > 50% liquid oxidised faster than in the dry atmosphere because of the injection of OH- ions into the oxide with consequent disruptive effects caused by hydrogen absorption by the alloy. The addition of 0.003% Be to Al-8% Mg alloy decreased the oxidation rate markedly by toughening the MgO films preventing cracking and hence the nucleation of tertiary MgO. Small additions of Mn or Zr offset the effect of Be and probably enhanced the Mg 2+ ion conductivity in the oxide by the 4+ injection of Mn 2+ , Mn 4+ or Zr ions. For alloys with Be, although breakaway eventually occurred it was not initiated by crystallisation of the amorphous film and moisture in the atmosphere increased the oxidation throughout the temperature range, 500-725°C.

Dans cette étude, nous proposons des procédures basées sur des calculs informatiques et des modèles théoriques qui peuvent être utilisés pour prévoir le comportement de certains matériaux membranaires d'intérêt pour les applications de séparation de gaz. En particulier, nous nous sommes concentrés sur: i) des alliages VNiTi de structure cubique centrée, considérés comme de nouveaux matériaux pour les membranes denses sélectives à H2 et ii) sur des matériaux poreux cristallins qui sont des systèmes attractifs pour la séparation de gaz légers tels que H2, O2, CO, CO2, CH4 et N2. Les deux groupes de matériaux sont traités en utilisant une méthodologie différente, adaptée aux besoins des recherches associées à ces matériaux.Dans le cas des membranes métalliques denses, nous nous sommes intéressés à la controverse, connue de longue date, concernant l'occupation de l'hydrogène interstitiel dans les alliages à base de vanadium. Le système V-Ni-Ti est en effet particulièrement intéressant grâce à sa perméabilité élevée pour H2 et à ses propriétés mécaniques accrues par rapport au vanadium pur. Ce travail cible la compréhension de la structure ces alliages à l'échelle atomique, de façon à les optimiser et à activer la conception et le développement de ces matériaux comme nouvelles membranes pour la séparation de H2. Notre approche, basées sur les premiers principes, donne un aperçu des sites préférentiels de l'hydrogène et évalue le rôle des solutés de substitution Ti et Ni, sur l'affinité d'absorption de l'hydrogène. La méthode basée sur la théorie de la fonctionnelle de la densité (DFT) ne nécessite aucune donnée expérimentale autre que l'information sur la structure cristalline. En outre, cette méthode n’utilise aucun paramètre empirique ou d’ajustement, contrairement à d'autres techniques de calcul. Ainsi, cette approche est une voie alternative pour explorer de nouveaux alliages métalliques utilisables comme membranes de séparation de H2. La méthodologie appliquée pourra être utilisé ensuite dans des calculs à haut débit pour cribler diverses compositions d'alliage. Les résultats reportés ici seront utilisés comme guide pour adapter la formulation des solutions solides VNiTi et préparer des membranes en alliage denses à faible coût (par rapport aux membranes à base de palladium) dans le cadre d’autres projets (e.g. Projet Européen FP7- DEMCAMER).Dans le cadre de notre étude sur les membranes cristallines microporeuses, nous démontrons comment les données sur un composant unique peuvent être utilisées pour prédire la sélectivité idéale de ces membranes envers les gaz légers. Des modèles théoriques sont ainsi proposés pour décrire les propriétés de séparation de gaz de matériaux de type zéolithiques (« zeotypes ») en fonction de leurs paramètres structuraux et des conditions de fonctionnement. Les paramètres du modèle peuvent être obtenus expérimentalement ainsi que par le calcul. Afin d’analyser le degré de validité et les limites des modèles, les sélectivités idéales de deux membranes zéolithes classiquement étudiées (NaA, CaA) et d’une membrane MOF (ZIF-8) ont été évaluées. Les résultats démontrent que les expressions théoriques peuvent être utilisées pour le criblage de séries de matériaux microporeux cristallins sous réserve que des données fiables sur l'adsorption de gaz purs soient disponibles pour ces matériaux. Cependant, étant donné que les modèles n’intègrent pas tous les paramètres (notamment liés au design des membranes) et mécanismes impliqués dans le transport de gaz à travers ces membranes, les prédictions doivent être considérées comme celles correspondant à un cas idéal. / In this study, we propose procedures based on computational calculations and theoretical models that can be used to predict the behaviour of some of the membrane materials of interest for gas separation applications. In particular, we focus on: i) body-centred cubic VNiTi alloys as novel materials for H2-selective dense membranes and ii) crystalline porous materials that are attractive media for separation of light gases such as H2, O2, CO, CO2, CH4 and N2. These two types of materials are treated using different methodologies, adapted to the needs of our research objectives associated to each material.In the case of dense metal membranes, the long-standing controversy over occupancy of interstitial hydrogen in V-based alloys is addressed. The V-Ni-Ti system is of particular interest here, exhibiting high H2 permeability and improved mechanical properties relative to pure V. This work intends to gain understanding of hydrogen-metal interactions as function of alloy composition and thereby to optimize these new materials and advance their development as novel membranes for H2 separation. We use a first-principles approach that gives insights into the sites preference of hydrogen and assesses the role of Ti and Ni substitutional solutes for the hydrogen absorption affinity. The method based on Density Functional Theory requires no experimental input except crystal structure information. Furthermore, it uses no empirical or fitting parameters in contrast to other computational techniques. Hence this approach provides an alternative way to explore new metal alloys for H2 separation membranes. The applied methodology can be used further in high-throughput calculations to screen various alloy compositions. The hereto-reported results will be used as guidance for tailoring the formulation of VNiTi solid solutions and preparation of low cost dense alloy membranes in the frame of other projects (e.g. European DEMCAMER project).Further, we explore how single-component inputs can be used to forecast the ideal selectivity towards light gases of crystalline porous materials, used for membrane preparation. Theoretical models for describing gas separation properties of zeotype materials as function of structural characteristics and operation conditions are proposed. The model parameters can be obtained as experimentally as well as computationally. To analyse the extent of validity and limitations of the models, ideal selectivities of few crystalline porous materials are evaluated, including widely used zeolites (NaA, CaA) and a metal organic framework structure (ZIF-8). The results verified that the theoretical expressions could be used for screening series of zeotype materials when reliable single gas adsorption data are available. However, since the models don’t take into account all parameters (namely related to the membrane design) and mechanisms involved in gas transport through porous membranes, their predictions should be considered as values referring to an ideal case.

During the incorporation of ceramic particles into metallic alloy melts for the production of metal matrix composites, the particles tend to float or sink, depending on their density. In order to study the sedimentation patterns, a novel electrical resistance technique has been developed. A current is passed between two electrodes, and the potential over a fixed distance is measured with two other electrodes. Experiments were carried out in an aluminum foundry alloy(A356) containing up to 30 volume percent 88 μm silicon carbide particles. The particles’ behaviour was compared with sedimentation patterns in aqueous systems. The implications for fabrication and remelting of metal matrix composite material are discussed. / Thesis / Master of Engineering (ME)

Plusieurs verrous scientifiques et technologiques empêchent aujourd’hui de développer une technique et/ou un matériau qui permette de stocker une quantité importante d’hydrogène à pression et température ambiante dans un volume et un poids acceptable pour des applications embarquées. Une possible solution consiste à synthétiser des matériaux hybrides (matériaux poreux/métaux ou alliages) où les processus d’adsorption et d’absorption pourraient coopérer pour obtenir une capacité de stockage d’hydrogène en adéquation avec les besoins des applications. Notre travail a consisté à identifier et caractériser différents matériaux poreux ayant une organisation de pores bien définie et une taille de l’ordre de quelques nanomètres. Parmi eux, ont été choisis : une réplique de carbone (CT) et un réseau organométallique (MOF-5). De plus, plusieurs métaux nobles (Ni, Pd et Pt) ont été choisis pour leur facilité à dissocier l’hydrogène et à former des alliages (Pd-Ni) avec différentes compositions en milieu aqueux (oxydant). Une méthode d’imprégnation par voie chimique ainsi que le broyage mécanique ont été utilisés pour la synthèse des hybrides. L’étude des propriétés structurales, texturales et thermodynamiques (hydrogénation) des composites CT/Pd a montré qu’un effet coopératif existe entre les pores du CT et les nanoparticules métalliques pendant le processus d’ad/absorption d’hydrogène. Cette interaction entraîne une amélioration de la capacité d’hydrogénation par rapport à chacun des constituants de l’hybride. / Nowadays many technological and scientific constraints have limited the finding of a suitable system and/or material able to reversibly store hydrogen at room temperature and ambient pressure for automotive application. An interesting way to overcome such limits could be the synthesis of hybrid materials (porous materials/metals or alloys composites) for which the adsorption and absorption processes can be combined in order to get higher hydrogen storage capacity. In this work, several porous materials displaying a well defined nanometric pore structure have been investigated. Among them a carbon template (CT) and a metal organic framework (MOF-5) have been chosen. In addition, several noble metals (Ni, Pd and Pt) have been used due to their ability to dissociate hydrogen and to form alloys. Two synthesis routes have been followed in order to synthesize hybrid composites: metal salts infiltration and mechanical grinding. In particular, the investigation of the structural, textural and hydrogen storage properties of the CT/metal composites has proven that a synergic mechanism between the CT pores and the metallic nanoparticles takes place during the hydrogen ad/absorption process. This interaction leads to an enhancement of the hydrogen storage capacity of each hybrid component taken separately.

Recently, efforts have been made for self-powering the batteries and portable electronic devices by piezoelectric nanogenerators. The piezoelectric nanogenerators can work as a power source for nano-systems and also as an active sensor. The piezoelectric nanogenerator is a device that converts random mechanical energy into electrical energy by utilizing the semiconducting and piezoelectric properties. Also, the mechanical energy is always available in and around us for powering these nano devices. The aim of the present thesis work is to explore 1D and 2D ZnO nanostructures (nanorods and nanosheets) on metal alloy substrates for the development of piezoelectric nanogenerators in energy harvesting and sensors applications. Hydrothermal synthesis method was adopted for the growth of ZnO nanostructures. The nanogenerators were fabricated by using the optimized synthesis parameters and subsequently studied their performance for power generation and as an active speed sensor. These 1D and 2D nanostructures based nanogenerators have opened up a new window for the energy harvesting applications and sensors development. The thesis is divided into following six chapters. Chapter 1: This chapter gives a general introduction about energy harvesting devices such as nanogenerators, available energy sources, mechanical energy harvesting, ZnO material and the details on hydrothermal synthesis process. A brief literature survey on different applications of piezoelectric nanogenerators is also included. Chapter 2: A novel flexible metal alloy (Phynox) and its properties along with its applications are discussed in this chapter. Details on the synthesis of 1D ZnO nanorods on Phynox alloy substrate by hydrothermal method are presented. Further, the optimization of parameters such as growth temperature, seed layer annealing and substrate temperature effects on the synthesis of ZnO nanorods are discussed in detail. As-synthesized ZnO nanorods have been characterized using XRD, FE-SEM, TEM and XPS. Chapter 3: It reports on the fabrication of piezoelectric nanogenerator on Phynox alloy substrate as power generating device by harvesting the mechanical energy. Initially, the performance of the nanogenerator for power generation due to finger tip impacts was studied and subsequently its switching polarity test was also carried out. Output voltage measurements were carried out using the in-house developed experimental setup. Stability test was also carried out to see the robustness of the nanogenerator. Finally, the output voltage response of the nanogenerator was studied for its use as an active speed sensor. Chapter 4: Synthesis of Al doped 2D ZnO nanorsheets on Aluminum alloy (AA-6061) substrate by hydrothermal method is reported in this chapter. The optimized parameters such as growth temperature and growth time effects on the synthesis of ZnO nanosheets are discussed. As-synthesized ZnO nanosheets were characterized using XRD, FE-SEM, TEM and XPS. The Al doping in ZnO is confirmed by EDXS and XPS analysis. Chapter 5: Cost effective fabrication of Al doped 2D ZnO nanosheets based nanogenerator for direct current (DC) power generation is reported in this chapter. The performance of the nanogenerator for DC power generation due to finger tip impacts was studied and subsequently its switching polarity test was also carried out. Output voltage measurements were carried out using the in-house developed experimental setup. Stability test was also carried out to see the robustness of the nanogenerator. Finally, the DC output voltage response of the nanogenerator was studied for its use as an active speed sensor. Chapter 6: The first section summarizes the significant features of the work presented in this thesis. In the second section the scope for carrying out the further work is given.

>Magister Scientiae - MSc / Organic-inorganic lead-based perovskite has exhibited great performance in the past few years. However, the lead (Pb) embedded in those compounds is a significant drawback to further progress, due to its environmental toxicity. As an alternative, tin (Sn) based-perovskites have demonstrated promising results in terms of electrical and optical properties for photovoltaic devices, but the oxidation of tin ion- from stannous ion (Sn2+) to stannic ion (Sn4+) presents a problem in terms of performance and stability when exposed to ambient conditions. A more feasible approach may be in a Pb-Sn binary metal perovskite in pursuit of efficient, stable perovskite solar cells (PSCs) with reduced Pb-content, as compared to pure Pb- or Sn-based PSCs. Here, we report on the deposition of a Pb-Sn binary perovskite by sequential chemical vapor deposition.

A fotoelasticidade é um método analítico usado para avaliar e calcular as tensões existentes em qualquer ponto da superfície de um material. É uma ferramenta para análise de tensões bidimensionais, usa luz monocromática polarizada e modelos de resina. O objetivo do presente estudo foi comparar as tensões induzidas na interface osso-implante em diferentes planejamentos de próteses fixas totais implantossuportadas protocolo de Branemark. As variáveis nos modelos foram: número de implantes, tipo de liga, presença ou ausência de cantilever, inclinação dos implantes distais e secção transversal da infraestrutura. Foram confeccionados 32 modelos fotoelásticos e foram realizadas análises qualitativa e quantitativa das tensões induzidas. Os modelos sofreram carga estática no valor de 50 N na porção central e distal da barra. Os implantes central e distal foram analisados sob as cargas central e distal individualmente. Os pilares receberam torque de 20 N.cm e os parafusos protéticos de fixação da barra receberam 10 N.cm. Antes de qualquer avaliação, foi certificado que os modelos fotoelásticos encontravam-se livres de tensões residuais. A tensão (MPa) acumulada nos implantes distais sob carga distal foi aproximadamente 7 vezes maior que nas outras situações de carga. Além disso, 69% de toda tensão acumulada no sistema se manifestou no implante distal sob carga distal. Houve uma clara vantagem para as estruturas fundidas em cobalto-cromo em relação às fundidas em titânio concernente à distribuição de tensões. A análise da secção transversal demonstrou que na comparação entre viga convencional e viga I, houve distribuição aleatória de resultados. Também ficou evidente que o número maior de implantes favorece a uma melhor distribuição de tensões. Já quando foram comparadas barras de protocolo apresentando ou não cantilever, as infraestruturas sem cantilever apresentaram um resultado marcadamente superior àquelas com cantilever. Finalmente, as infraestruturas com implantes distais retos apresentaram uma melhor distribuição de tensões. Com base nos resultados obtidos, pode-se concluir que o método de fotoelasticidade é um método laboratorial apropriado para análise de tensões; próteses tipo protocolo de Branemark com 5 implantes provêm uma melhor condição de distribuição de tensões do que com 4 implantes; a secção transversal da supraestrutura metálica e o tipo de liga utilizado na fundição não apresentaram relação direta com o padrão de distribuição de tensões nos implantes; a distribuição de tensões nos implantes dentários em uma prótese tipo protocolo é mais eficiente com implantes distais retos e sem extensão em cantilever. / Photoelastic analysis is a method used to evaluate and calculate stresses at a material surface. It is a tool for bidimensional stresses analysis. Polarized monochromatic light and resin models for stresses evaluation were used. The purpose of this study was to compare the induced stresses in the bone-implant interface using different implant supporting complete fixed prostheses Branemark protocol. The variables studied in the models were: the number of implants, cast metal alloy, presence or absence of cantilever, angulation of distal implants and superstructure cross-sectional shape. It were prepared thirty-two photoelastic models and perceived quantitative and qualitative analysis of the induced stresses. 50 Newtons static load was exerted on the central and the distal part of the superstructure. Central and distal implants were analyzed under central and distal loads individually. The abutments were tightened with a 20 N.cm torque and the prosthetic screws with a 10 N.cm torque. Before the evaluations took place, it was checked if the implants presented residual stresses. Stresses accumulated in the distal implants under distal load were nine times greater than in the other loading situations and received approximately 69% of all stresses scattered in the whole system. There was a clear advantage for cobalto-chromium cast superstructures over titanium ones about stress transfer. The analysis of the cross-sectional superstructure shape evinced that when comparing conventional beam with i-beam there was an aleatoric results distribution. It was also evident that a superstructure with more implants collaborates to a better stresses distribution. Comparing Branemark protocol bars presenting or not cantilever extension, those with no cantilever presented remarked superior results than those with cantilever. Finally, superstructures with straight implants had a better stress transfer than those with angulated ones. Based upon the obtained results, it was concluded that the photoelastic method is a laboratorial evaluation method suitable for stress transfer analysis. Five implants complete fixed prostheses provide a better stress than those with four implants. Cross-sectional metal superstructure shape and the type of cast alloy demonstrate no direct correlation about the stress transfer pattern over the implants. The stress distribution over dental implants in a Branemark procotol prosthese is more efficient with straight distal implants and a bar with no cantilever extension.

La demande croissante de poudres d‟alliages métalliques aux propriétés spécifiques utilisées en particulier en projection thermique et fabrication rapide pousse les chercheurs à améliorer et à optimiser sans cesse les procédés de production de ces poudres. L‟objectif affiché sur ces procédés est de maîtriser à la fois morphologie/ distribution de taille des particules produites et coût de fabrication. Actuellement, la majorité de ces poudres est produite par des procédés d‟atomisation par fluide et essentiellement par le procédé d‟atomisation gazeuse. Parmi ces procédés, le procédé Nanoval utilisant une buse «De Laval» est l‟un des plus performants en termes de distribution granulométrique et de rendement.L‟objectif principal de ce travail de thèse vise à améliorer la compréhension des phénomènes physiques mis en jeu par le procédé Nanoval afin d‟en optimiser le fonctionnement. Deux approches composent ce travail :- une partie numérique de modélisation sous Fluent. Deux modèles ont été étudiés, un modèle monophasique relatif à l‟écoulement gazeux dans l‟unité d‟atomisation (passage de l‟autoclave à la chambre d‟atomisation) et un modèle diphasique relatif à la constriction du filament de métal liquide en sortie de buse de coulée. Cette étude numérique a permis de mettre en évidence l‟effet des paramètres opératoires tels que la pression d‟atomisation et le diamètre de la buse de coulée sur la dynamique du jet de gaz, sur la striction du filament de métal liquide ainsi que les zones de forte pression et de haute vitesse avant, pendant et après la désintégration du filament métallique.- une partie expérimentale pour laquelle la mise en place d‟outils de diagnostic in–situ a été nécessaire pour la caractérisation du procédé en cours de fonctionnement. Trois analyses ont été conduites. La première renseigne de la dynamique du jet d‟atomisation évaluée à partir de mesures de Vélocimétrie par Images de Particules (PIV) à proximité de la sortie de la buse De Laval. La deuxième concerne les caractéristiques à l‟écrasement des particules sur un substrat placé dans la chambre d‟atomisation. La troisième et dernière analyse porte sur les propriétés des particules produites et la comparaison avec la matière récupérée après refroidissement dans l‟autoclave. Différents paramètres opératoires ont été explorés (pression d‟atomisation, diamètre de la buse de coulée, pression dans la chambre d‟atomisation, nature du métal) et reliés à leur influence sur la vitesse et le diamètre des particules. Des relations directes entre les résultats de ces trois analyses ont pu être démontrées ainsi qu‟une bonne adéquation entre résultats expérimentaux et résultats issus de la modélisation. / The growing demand for metal alloy powders with specific properties used in thermal spray application and rapid manufacturing encourages researchers to improve and optimize their manufacturing processes. The aim of these processes is to master both morphology/particle size distribution and manufacturing cost. Today the vast majority of powders are produced by fluid atomization and mainly gas atomization process. Among them, the Nanoval process, consisting of a De Laval nozzle is one of the most outstanding process in terms of granulometric distribution and output.The main objective of this thesis is to improve the understanding of the physical phenomena occurring in the Nanoval process to optimize the way it operates. Two approaches will be developed:- A numerical study using Fluent. The two following models were studied, one monophasic concerning gas flow in the atomization unit form autoclave to atomization chamber and the other, a diphasic model concerning the finest part of the filament in the exit of the melt nozzle. This numerical study has highlighted the effect of parameters such as atomization pressure, nozzle diameter on the gas dynamics, fine filament, high-pressure and high speed areas before, during the process and after the disintegration of the metallic filament.- And an experimental study which required the implementation of the in-situ diagnosticTools to characterize the process under working conditions. Three analyses were carried out. The first concerns the dynamics of the atomization jet from Velocimetry measures by Particle Image Velocimetry (PIV) close to De Laval nozzle exit. The second deals with the characteristics obtained when particles impact the substrate in the atomization chamber. The third describes the particle properties and deals with the comparison with the matter in the autoclave after cooling process. Different operating parameters were explored (atomization pressure, melt nozzle diameter, pressure in the atomization chamber, nature of metal) and linked to their influence on the particle velocity and diameter. Narrow links between the analysis results were demonstrated as well as a good adequacy between experimental and modeling results.

In this study, the OMCVD method is demonstrated as a powerful, fast, economic and environmental friendly method to produce a set of PGMelectrocatalysts with different supports, metal content and metal alloys in one step and without the multiple processing stages of impregnation, washing, drying, calcinationsand activation.