L'alliage Si-Ge est utilisé depuis de nombreuses années dans les modules thermoélectriques dans les sondes spatiales de la NASA. Ils convertissent la chaleur résultant de la désintégration radioactive de matériaux riches en un ou plusieurs radio-isotopes en électricité. Cet alliage est performant à haute température (à partir de 700 °C), c'est pourquoi il trouve également un fort intérêt dans l'industrie automobile. De nombreuses recherches dans ce secteur s'orientent vers la thermoélectricité, notamment vers des modules fonctionnant à haute température pour permettre la réduction de consommation de carburant.La meilleure composition de l'alliage en thermoélectricité est Si0,8Ge0,2. Le facteur de mérite réduit (ZT) de ces matériaux est généralement proche de 0,75 et de 0,45 à 700 °C pour les types n et p respectivement. Le germanium étant très onéreux, la composition retenue dans cette étude est Si0,92Ge0,08. Pour améliorer les performances de la composition choisie et se rapprocher de celles de la meilleure composition, la voie retenue est l'incorporation de nanoinclusions à base de siliciures de molybdène dans le matériau, permettant la diminution de la conductivité thermique.L'alliage Si-Ge est synthétisé par mécanosynthèse, et densifié par SPS. Les dopants utilisés sont le phosphore et le bore pour les types n et p respectivement. Le taux de dopage optimal est de 0,7 %. Ainsi, les ZT obtenus à 700 °C sont égaux à 0,7 et 0,5 pour les types n et p respectivement. La nature des inclusions stables dans la matrice est déterminée par la méthode CalPhad qui permet l'obtention du diagramme ternaire Mo-Si-Ge. La phase MoSi2 apparait alors comme étant la seule phase stable dans la matrice Si0,92Ge0,08. La fraction volumique optimale de molybdène est de 1,3 % lorsque les matériaux sont densifiés à 1280 °C. Le ZT obtenu est supérieur à 1 à 700 °C pour le type n, et proche de 0,8 pour le type p. L'ajout de nanoinclusions a permis d'augmenter les performances de 43 % et de 60 % à 700 °C. / Si-Ge alloys has been used for many years in the thermoelectric modules in the NASA space probes in which they convert heat produced by the radioactive decay of a heat source into electricity. This alloy is effective at high temperature (from 700 °C), so it is also a strong interest in the automotive industry. The strong incentive in this area to reduce fuel consumption leads researchers to develop thermoelectric modules that can operate at high temperatures. The composition at which SiGe alloys are the most thermoelectrically efficient is Si0.8Ge0.2. Their figure of merit (ZT) is generally close to 0.75 and 0.45 at 700 °C for type n and p respectively. As Germanium is very expensive, this study aims to develop a Si0.92Ge0.08 alloy that can compare to the existing Si0.8Ge0.2 alloys. To get to a higher level of performance, the thermal conductivity of the chosen composition has to be decreased, which is done by incorporating molybdenum silicides in the Si0.92Ge0.08 alloys.The Si-Ge alloy was synthesized by mechanical alloying, and sintered by SPS. The dopants used are phosphorous and boron for the n and p types respectively. The optimal doping level is 0.7%. ZT obtained for Si0.92Ge0.08 base alloys at 700 °C are equal to 0.7 and 0.5 for n and p types respectively. The nature of stable inclusions in the matrix has been determined by the CALPHAD method to obtain the ternary diagram Mo-Si-Ge. Then, the MoSi2 phase appears to be the only stable phase in the matrix Si0.92Ge0.08. The optimum volume fraction of molybdenum was 1.3% when the materials are sintered at 1280 °C. Therefore, the ZT obtained is higher than 1 at 700 °C for n-type and close to 0.8 for p-type. Adding nanoinclusions has increased performance by 43% (n-type) and 60% (p-type) at 700 °C.

Made available in DSpace on 2017-07-21T20:42:33Z (GMT). No. of bitstreams: 1 Ariane Emi Yonekubo.pdf: 14387414 bytes, checksum: 915c8e68747eb2fccc91d9829eb93469 (MD5) Previous issue date: 2010-09-10 / Fundação Araucária de Apoio ao Desenvolvimento Científico e Tecnológico do Paraná / Superduplex stainless steels are often used in applications where is necessary high mechanical strength combined with corrosion resistance. They are formed by a ferrite (a) BCC and austenite (g) FCC mixture, have a yield stress about of the double of the austenitic and ferritic stainless steels, larger plasticity that the one of the martensitic and precipitation hardening, high resistance to the intergranular corrosion, pitting and the stress-corrosion cracking. However, during hot deformation, these steels recrystallize forming a lamellar microstructure with anisotropic properties. Through the high energy milling, accomplished with Spex, Attritor and Planetary ball mills, it was aimed to obtain a homogeneous and with fine grains superduplex stainless steel. Initially, in this work, chips were made by machining of a UR 52N+ (UNS S32520) commercial superduplex stainless steel rolled sheet, without previous heat treatment, which will be submitted to the high energy milling adjusting the milling parameters and subsequently it was accomplished the heat treatment at temperatures of 1200 °C e 1250 °C for 1 hour in argon atmosphere and 1200 °C for 1 hour in argon with 2% of hydrogen. The obtained powders were analyzed by scanning electron microscopy, observing that the smaller and more homogeneous particles were obtained by Spex milling for 16 and 32 hour; the compressibility curve showed that after a long time milling, the material can present compaction difficulty and this way, great final porosity. The x ray diffraction presented peaks with a bottom widening due to the small size of the obtained particles. The porosity percentage determined by optical microscopy, reduced after heat treatment at 1200 °C in argon atmosphere with 2% hydrogen, however the scanning electron microscopy showed a fine porosity that could not be observed only in the optical microscopy analysis. The differential scanning calorimetry and the differential thermal analysis showed formation and revertion of strain induced martensite peaks and using atomic force microscopy, magnetic ferrite and paramagnetic austenite phases were observed by a non destructive test. The milling of the chips in Spex milling for 16 hours provided a refined microstructure with equiaxial grains and size of 11 μm after heat treatment at 1200 °C for 1 hour in argon atmosphere. / Aços inoxidáveis dúplex são frequentemente utilizados em aplicações onde é necessária elevada resistência mecânica combinada a resistência à corrosão. São formados por uma mistura de ferrita (a) CCC e austenita (g) CFC, apresentam limite de escoamento cerca de o dobro dos aços inoxidáveis austeníticos e ferríticos, maior plasticidade que a dos martensíticos e dos endurecíveis por precipitação, elevada resistência à corrosão intergranular, à corrosão localizada e à corrosão sob tensão. Porém, durante a deformação a quente, estes aços recristalizam-se formando uma microestrutura lamelar com propriedades anisotrópicas. Através da moagem de alta energia, realizada com moinhos de bolas dos tipos Spex, Attritor e Planetário, objetivou-se obter um aço inoxidável superdúplex de grãos homogêneos equiaxiais e finos. Inicialmente, neste trabalho, foram feitos cavacos por usinagem da chapa laminada de um aço inoxidável superdúplex comercial UNS S32520 (UR 52N+), sem tratamento térmico prévio, os quais foram submetidos à moagem de alta energia acertando-se os parâmetros de moagem e posteriormente realizou-se o tratamento térmico em temperaturas de 1200 °C e 1250 °C, por tempo de 1 hora em atmosfera de argônio, e a 1200 °C por 1 hora em atmosfera de argônio com 2% de hidrogênio. Analisaram-se os pós obtidos por microscopia eletrônica de varredura, observando que as partículas menores e mais homogêneas foram obtidas após as moagens no Spex por 16 e 32 horas; a curva de compressibilidade mostrou que após um tempo muito grande de moagem, o material pode apresentar dificuldade de compactação e desta maneira, maior porosidade final. A difração de raios x apresentou picos com alargamento na base devido ao tamanho pequeno de partículas obtidas. A porcentagem de porosidade, determinada por micrografia óptica, diminuiu após tratamento térmico a 1200 °C em atmosfera de argônio com 2% de hidrogênio,porém a microscopia eletrônica de varredura mostrou uma porosidade fina que não pôde ser observada somente através da microscopia óptica. As análises térmicas por calorimetria exploratória diferencial e por análise térmica diferencial mostraram picos de formação e reversão de martensita induzida por deformação e pela microscopia de força atômica, foram observadas as fases ferrita magnética e austenita paramagnética por ensaio não destrutivo. A moagem dos cavacos em moinho do tipo Spex por 16 horas forneceu uma microestrutura refinada com grãos equiaxiais e tamanho de 11 μm após tratamento térmico a 1200 °C por 1 hora em atmosfera de argônio.

Les travaux de recherches développés dans cette thèse concernent la réalisation de couchesminces, à partir de l’alliage à mémoire de forme (AMF) NiTi mécanoélaboré et de structurenanocristalline, en vue de leur intégration dans des microsystèmes. Le but est d’améliorer lespropriétés AMF de leurs homologues de structure microcristalline, dits conventionnels.Les techniques de caractérisation physico-chimiques (DRX, MET, MEB) nous ont permisd’une part, de suivre le mécanisme de formation de l’intermétallique B2-NiTi en fonction dutemps de broyage et d’autre part, de déterminer les paramètres microstructuraux à savoir, lataille des cristallites, le taux de microdéformations et la densité de dislocations des élémentspurs ainsi que ceux de la phase B2-NiTi. Ces paramètres révèlent le caractère nanocristallin etdésordonné des poudres broyées.Par ailleurs, nous avons fabriqué pour la première fois une cible B2-NiTi de structurenanocristalline, par l’intermédiaire d’une méthode alternative (mécanosynthèse et procédé deprojection à froid).Nous avons montré également qu’il est possible de déposer sous forme de couche mincel’intermétallique NiTi nanostructuré. Ce film mince d’épaisseur 447 nm a été déposé parpulvérisation cathodique à magnétron à partir de la cible élaborée par projection à froid (coldspray). / The research work developed in this thesis is related to preparing a thin film of NiTi shapememory alloy used to integrate into microsystems. The goal is to improve the properties oftheir counterparts of microcrystalline structure (conventional target).Physical and chemical techniques of characterization (XRD, TEM and SEM) have allowed onthe one hand, to follow the mechanism of intermetallic NiTi formation as a function ofmilling time and on the other hand, to determine the microstructural parameters : crystallitesize, the microstrain and dislocation density of the pure elements and the B2-NiTi phase.These parameters reveal the character of the disordered nanocrystalline of the milled powders.Furthermore, we fabricated a target of B2-NiTi nanocrystalline structure by an alternativemethod (mechanical alloying and cold spray).We also showed that it is possible to deposit the nanocrystalline NiTi intermetallic thin film.This film with a thickness of about 447 nm was deposited by magnetron sputtering techniquefrom the NiTi target.

L’utilisation des combustibles fossiles (énergies non renouvelables) est responsable de l’augmentation de la concentration en gaz à effet de serre dans l’atmosphère. Parmi les solutions de remplacement envisagées, l’hydrogène apparaît comme le vecteur énergétique le plus séduisant. Son stockage dans des intermétalliques permet d’obtenir des capacités massiques et volumiques (e.g. 140 g/L) supérieures à celles obtenues en voie liquide ou sous pression (respectivement 71 et 40 g/L). Dans les accumulateurs Nickel-Métal Hydrure (Ni-MH), l’électrode négative est constituée d’un composé intermétallique qui absorbe l’hydrogène de façon réversible dans des conditions normales de pression et de température. Ce travail de thèse vise d’une part, à synthétiser les alliages Mg2Ni1-xMnx (x =0, 0.125, 0.25, 0.375) et les alliages Mg2-xAlxNi (x = 0, 0.25) avec ou sans nanotubes de carbone (MWCNTs) par mécanosynthèse et d’autre part, d’étudier les effets des substitutions/additions sur la composition et la microstructure des alliages Mg2Ni afin d’améliorer leurs propriétés de stockage d’hydrogène.Les résultats obtenus montrent que les capacités de décharge des alliages Mg2Ni1-xMnx(x = 0, 0.125, 0.25, 0.375) augmentent avec le temps de broyage. Pour l’alliage Mg2Ni0.625Mn0.375 broyé durant 48 h, nous avons mis en évidence la formation d’une nouvelle phase Mg3MnNi2 qui est relativement stable. Par conséquent, Mg3MnNi2 est capable d’améliorer de manière significative la stabilité des cycles tout en maintenant une capacité de décharge relativement élevée.Les résultats obtenus par la théorie de la fonctionnelle de la densité (DFT) en utilisant le programme CASTEP montrent d’une part, que les paramètres de maille et les coordinations atomiques sont en parfait accord avec les résultats expérimentaux. D’autre part, que la stabilité des phases décroit graduellement selon l’ordre suivant : Mg2Ni sans aucune substitution >Mg3MnNi2 > Mg2Ni avec substitution par Mn.L’addition de nanotubes de carbone et de Al ont des effets synergétiques sur la capacité de stockage d’hydrogène électrochimique dans le cas des alliages Mg2-xAlxNi (x = 0, 0.25) + 5 wt.% MWCNTs. / The use of fossil fuels (non-renewable energy) is responsible for increasing the concentration of greenhouse gases in the atmosphere. Among the considered alternatives, hydrogen is seen as the most attractive energy vector. The storage in intermetallics makes it possible to obtain mass and volume capacities (e.g. 140 g/L) higher than those obtained by liquid form or under pressure (respectively 71 and 40 g/L). The negative electrode of Nickel-Metal Hydride (NiMH) batteries, is constituted by an intermetallic compound which is able to reversibly absorb hydrogen under normal conditions. In this work, on the one hand, Mg2Ni1-xMnx(x=0, 0.125, 0.25, 0.375) and Mg2-xAlxNi (x = 0, 0.25) electrode alloys with and without multiwalled carbon nanotubes (MWCNTs) have been prepared by Mechanical Alloying. On the other hand, influence of the partial elements substitution on the microstructure and electrochemical hydrogen storage properties of Mg2Ni-type alloy has been studied.The results show that the discharge capacities of Mg2Ni1-xMnx (x =0, 0.125, 0.25, 0.375) alloys increase with the prolongation of milling time. The new phase Mg3MnNi2 is formed only when x=0.375 after 48 h of milling. Mg3MnNi2 phase is relatively stable during charge/discharge cycles and therefore can significantly enhance the cycle stability under simultaneously maintaining a high discharge capacity.Based on the calculated results of first principles, the lattice parameters and atomic coordinates are in good agreement with the experimental results and the stability of phases gradually decreases along the sequence pure Mg2Ni phase > Mg3MnNi2 phase > Mn-substitution doped Mg2Ni phase.When Al and MWCNTs are added simultaneously, the highest discharge capacity is obtained for Mg1.75Al0.25Ni-MWCNTs composite, which implies that MWCNTs and Al have synergistic effects on electrochemical hydrogen storage capacity of milled alloys.

Les accumulateurs Ni-MH (Nickel-Métal-Hydrure) sont un sujet prometteur et largement étudié dans les recherches d’une énergie propre et durable. Trouver le matériau idéal pour l'électrode négative à haute densité volumétrique et gravimétrique est la clé pour l’application de cette technologie. Les hydrures métalliques à base de Ti-Ni ont des propriétés équilibrées entre la capacité d’hydrogène et les performances électrochimiques.L’objectif de cette thèse est d’étudier les effets de substitutions/additions d’éléments et de la mécanosynthèse sur la structure et les propriétés d’hydrogène des alliages Ti-Ni. Dans cette étude, une série d’alliages à base de Ti-Ni avec des substitutions/additions de Mg ou de Zr ont été systématiquement étudiés.Les alliages (TiNi)1-xMgx, (TiH2)1.5Mg0.5Ni, and Ti2-xZrxNi ont été synthétisés par mécanosynthèse à partir de poudres élémentaires. Dans un premier temps, l’influence du temps de broyage et les effets de substitutions/additions sur les microstructures ont été caractérisés par des techniques telles que la DRX, le MEB et le MET. Dans un second temps, les propriétés d’hydrogénation des différents alliages ont été mesurées par des réactions solid-gaz et par cyclage électrochimique.La théorie de la fonctionnelle de la densité (DFT) en utilisant le programme CASTEP a permis de calculer les enthalpies de formation afin de comparer la stabilité thermodynamique des alliages obtenus. Dans ces travaux de recherche, nous avons identifié les priorités d’alliage des ternaires Ni-Ti-Mg et Ti-Ni-Zr dans des conditions de broyage. La transformation structurale du Ti en phase CFC, induite par l’introduction d’éléments étrangers, a été mise en évidence.Les courbes PCI (Pression-Composition-Isothermes) et les capacités de décharge en fonction du nombre de cycles indiquent les propriétés d’hydrogène des alliages obtenus, y compris TiNi, Ti2Ni (amorphe), Ti-Mg et Ti-Zr. / Ni-MH (Nickel-Metal-Hydride) batteries have been a promising and extensively studied topic among clean and sustainable energy researches. Finding the ideal material for the negative electrode with high volumetric and gravimetric densities is the key to apply this technology on broader applications. Metal hydrides based on Ti-Ni have balanced properties between hydrogen capacity and electrochemical performances in cycling.The objective of this thesis is to study the effects of element substitution/doping and mechanical alloying on the structural and hydrogen properties of Ti-Ni alloys. In this study, a series of Ti-Ni based systems with Mg or Zr doping/substitution have been systematically investigated.The metallic compounds (TiNi)1-xMgx, (TiH2)1.5Mg0.5Ni, and Ti2-xZrxNi were synthesized by mechanically alloying from elemental powders.The milling time and effects of Mg, Zr substitution/doping were studied firstly in respect of their microstructures, using characterization techniques including XRD, SEM, TEM (EDX support), followed by the hydrogen properties measurements of the samples by hydrogen solid-gas reaction and electrochemical cycling.A first principle calculation tool based on DFT (Density Functional Theory) was carried out to further investigate the enthalpy of formation in order to compare the thermodynamical stability of the obtained compounds. In the study, we have found the alloying priorities in the ternary alloys Ti-Ni-Mg and Ti-Ni-Zr under milling conditions.A structure transformation of Ti to FCC induced by foreign elements is reported and investigated. Enthalpy of formation per atom of the compounds were obtained by DFT calculations, which helped interpreting the experimental results. PCI (Pressure Composition Isotherms) curves and discharge capacities as the function of cycling numbers revealed the hydrogen properties of the obtained compounds, including TiNi, Ti2Ni (amorphous), Ti-Mg and Ti-Zr.

Le stockage de l'hydrogène est l'un des plus grands problèmes techniques qui restreignent l'application pratique de l'hydrogène. Les hydrures métalliques sont considérés comme la solution principale à ce problème puisqu'ils peuvent absorber et désorber de façon réversible une grande quantité d'hydrogène sous une température et une pression modérées. Par ailleurs, les hydrures métalliques utilisés comme électrodes négatives dans les accumulateurs Nickel-Métal Hydrure (Ni-MH) sont également les composants clés des performances de ces derniers.Dans cette thèse, les alliages métalliques TiMgNix, MgTi1-xNix et TiMg1-xNix ont été synthétisés par broyage mécanique à partir de poudres élémentaires. La microstructure et les transformations de phase des échantillons préparés ont été caractérisées par DRX, MEB et MET (avec microanalyse EDS).Les propriétés d'hydrogénation ont été mesurées par réaction d'hydrogène solide-gaz et par des essais électrochimiques. Un diagramme de composition-capacité 3D a été établi sur la base du diagramme de phase ternaire Ti-Mg-Ni. Un procédé de broyage en deux étapes a été mis en œuvre pour améliorer les performances électrochimiques des alliages Ti-Mg-Ni.De plus, les alliages TiNi1-xCux ont été synthétisés par broyage mécanique et ensuite recuits. L 'influence de la substitution du nickel par le cuivre sur la structure et les propriétés électrochimiques est étudiée en utilisant une double approche: expérimentale et par simulation.Les résultats obtenus par la théorie de la fonctionnelle de la densité (DFT) en utilisant le programme CASTEP montrent que l'enthalpie de formation et l'énergie d'adsorption de l¿hydrogène de la phase pseudo-binaire Ti(Ni, Cu) sont en bon accord avec les résultats expérimentaux. / The storage of hydrogen is one of the biggest technical problem that restrict the practical application of hydrogen. Metal hydrides are mainly regarded as the solution facing to this issue since it can reversibly absorb and desorb big amount of hydrogen under moderate temperature and pressure. Meanwhile, metal hydrides used as the negative electrodes of Ni-MH batteries are also the key components to the battery performance.In this thesis, the metallic composite TiMgNix, MgTi1-xNix and TiMg1-xNix were synthesized by mechanical alloying from elemental powder. The microstructure and phase transformation of prepared samples were characterized by XRD, SEM, TEM (EDS support). The hydrogenation properties were measured by hydrogen solid-gas reaction and electrochemical tests. Based on the Ti-Mg-Ni ternary phase diagram, a 3D composition-capacity diagram have been established. Two-step mill process was proposed for meliorating the electrochemical performance of Ti-Mg-Ni alloys.Additionally, TiNi1-xCux alloys had been synthesized by mechanical alloying and subsequent annealing and studied using experimental and computational approaches. The influence of Cu substitution for Ni on the phase structure and electrochemical properties are investigated. The first principle calculation was carried out to study the formation enthalpy and hydrogen adsorption energy of pseudo-binary Ti(Ni, Cu) phase. The computational results are in good agreement with experimental results.

L’hydrogène est la solution potentielle pour réussir la transition énergétique d’un système actuel basé en grande partie sur les combustibles fossiles vers un système non émetteur de gaz toxiques et respectueux de l’environnement. Cependant, le stockage de l’hydrogène est un grand défi qui freine son application pratique dans les différents domaines. Les hydrures métalliques permettent de stocker une grande quantité d’hydrogène de façon réversible dans de bonnes conditions (Température, pression, sécurité…) comparée aux autres modes de stockage (gazeux et liquide). En plus, ces mêmes matériaux sont utilisés comme électrode négative dans les batteries Nickel-Métal Hydrure.Dans la première partie de cette thèse, les alliages Ti-Fe ont été synthétisés parmécanosynthèse pour différents temps de broyage et différents rapports massiquesbilles/poudre. Afin d’optimiser les paramètres d’élaboration, ces alliages ont été caractérisés par différentes techniques telles que la diffraction des rayons X, la microscopie électronique à balayage, la chronopotentiométrie, la chronoampérométrie et la voltamétrie cyclique.Dans une seconde partie, les alliages TiFe+4%MWNTs, TiFe0.95-xMx, TiFe0.90M0.10 etTiFe0.90Mn0.05V0.05 (x=0.05, 0.15) (M : Mn ou V) ont été élaborés selon les paramètres optimaux déterminés précédemment. L’influence de l’additif Nanotubes de Carbone à multiparois (MWNTs), de la substitution partielle du Fe par Mn et/ou V et de l’excès de Titane sur les propriétés structurales, morphologiques et électrochimiques telles que l’activation, la capacité de décharge électrochimique, la réversibilité, la tenue au cyclage, le coefficient de diffusion ont ensuite été étudiés. Les propriétés redox des électrodes, le potentiel de Nernst et la densité du courant d’échange, ont été déterminés, en se basant sur la première loi de Sternet le modèle théorique de Bulter -Volmer.Les résultats électrochimiques obtenus montrent que l’alliage TiFe+4 wt.% MWNTs présente les meilleures performances : une activation rapide (au 1er cycle) et une meilleure capacité maximale de décharge (266 mAh g-1) avec une réversibilité qui reste inchangée. / Hydrogen is the potential solution to make a success of the energy transition of a current system basically based on fossil fuels towards a system friendly to environment. However, the storage of hydrogen is a big challenge that hinders its practical application in different areas.. Metal hydrides can store a large amount of hydrogen reversibly under good conditions (temperature, pressure, safety ...) compared to other storage modes (gaseous and liquid). In addition, these same materials are used as negative electrode in Nickel-Metal Hydride batteriesIn the first part of this thesis, Ti-Fe alloys were synthesized using mechanical alloying (MA) under argon atmosphere at room temperature, with different ball to powder weight ratio and at different milling times. In order to determine the optimal parameters of the elaboration the metallic composite were investigated using different techniques such as X-ray diffraction, scanning electron microscopy (EDS support), chronopotentiometry, chronoamperometry and cyclic voltammetry,In the second part, the metallic compounds, TiFe+4%MWNTs, TiFe0.95-xMx, TiFe0.90M0.10 and TiFe0.90Mn0.05V0.05 (x=0.05, 0.15) (M : Mn or V), which are used as the negative electrode material for Ni-MH secondary batteries, were synthesized by mechanical alloying according to optimal parameters, previously determined.The effect of MWNT addition, the Mn and/or V partial substitution for Fe and the excess of titanium on the structural, morphological and electrochemical parameters such as activation, electrochemical discharge capacity, reversibility, cycle life time and hydrogen diffusion coefficient were investigated.The redox properties of the electrodes such as the Nernst potential and the exchange current density were studied based on Stern’s first law and the theoretical model of Bulter-Volmer.The electrochemical properties of studied samples show the best performance for TiFe+4% MWNTs alloy. Indeed, this alloy presents a rapid activation (1st cycle) and a best discharge capacity (266 mAhg-1) with a reversibility remaining unchanged

In this thesis, novel bulk Al-based alloys with high content of Al have been produced by powder metallurgy methods from amorphous and partially amorphous materials. Different processing routes, i.e. mechanical alloying of elemental powder mixtures, controlled pulverization of melt-spun glassy ribbons and gas atomization, have been employed for the production of the Al-based powders. Among the different processing routes, gas atomization is the best choice for the production of Al-based amorphous and partially amorphous powders as precursors for the subsequent consolidation step because it allows the production of large quantities of powders with homogeneous properties (e.g. structure and thermal stability) along with a uniform size distribution of particles. Amorphous and nanocrystalline powders have to be consolidated to achieve dense bulk specimens. However, consolidation of these phases is not an easy task and special care has to be taken with respect to accurate control of the consolidation parameters in order to achieve dense bulk specimens without inducing undesirable microstructural transformations (e.g. crystallization and grain coarsening) or insufficient particle bonding. Consequently, the effect of temperature on viscosity as well as on phase formation has been studied in detail in order to select the proper consolidation parameters. Following their characterization, the Al-based powders have been consolidated into bulk specimens by hot pressing (HP), hot extrusion and spark plasma sintering (SPS) and their microstructure and mechanical properties have been extensively investigated. Consolidation into highly-dense bulk samples cannot be achieved without extended crystallization of the glassy precursors. Nevertheless, partial or full crystallization during consolidation leads to remarkable mechanical properties. For example, HP Al84Gd6Ni7Co3 samples display a remarkably high strength of about 1500 MPa, which is three times larger than the conventional high-strength Al-based alloys, along with a limited but distinct plastic deformability (3.5 – 4%). Lower strength (930 MPa) but remarkably larger plastic strain exceeding 25 % has been achieved for the Al87Ni8La5 gas-atomized powders consolidated by SPS above their crystallization temperature. Similarly, HP Al90.4Y4.4Ni4.3Co0.9 bulk samples display high compression strength ranging between 820 and 925 MPa combined with plastic strain in the range 14 – 30%. Finally, preliminary tensile tests for the Al90.4Y4.3Ni4.4Co0.9 alloy reveal promising tensile properties comparable to commercial high-strength Al-based alloys. The mechanical behavior of the consolidated specimens is strictly linked with their microstructure. High strength and reduced plasticity are observed when a residual amorphous phase is present. On the other hand, reduced strength but enhanced plastic deformation is a result of the complete crystallization of the glass and of the formation of a partially or fully interconnected network of deformable fcc Al. These results indicate that the combined devitrification and consolidation of glassy precursors is a particularly suitable method for the production of Al-based materials characterized by high strength combined with considerable plastic strain. Through this method, the mechanical properties of the consolidated samples can be varied within a wide range of strength and ductility depending on the microstructure and the consolidation techniques used. This might open a new route for the development of innovative high-performance Al-based materials for transport applications.

Submitted by Maryse Santos (maryseeu4@gmail.com) on 2016-08-26T14:49:04Z No. of bitstreams: 1 Dissertacao de Mestrado-Carla de Albuquerque Dias.pdf: 13161482 bytes, checksum: 7dae370ff2aeed1ca51c5f46b1701ffe (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2016-08-29T13:45:00Z (GMT) No. of bitstreams: 1 Dissertacao de Mestrado-Carla de Albuquerque Dias.pdf: 13161482 bytes, checksum: 7dae370ff2aeed1ca51c5f46b1701ffe (MD5) / Approved for entry into archive by Divisão de Documentação/BC Biblioteca Central (ddbc@ufam.edu.br) on 2016-08-29T13:47:30Z (GMT) No. of bitstreams: 1 Dissertacao de Mestrado-Carla de Albuquerque Dias.pdf: 13161482 bytes, checksum: 7dae370ff2aeed1ca51c5f46b1701ffe (MD5) / Made available in DSpace on 2016-08-29T13:47:30Z (GMT). No. of bitstreams: 1 Dissertacao de Mestrado-Carla de Albuquerque Dias.pdf: 13161482 bytes, checksum: 7dae370ff2aeed1ca51c5f46b1701ffe (MD5) Previous issue date: 2016-04-15 / The motivation for this research was the synthesis and production of nanostructured materials ZnO (oxide of zinc) starting from Zn (pure zinc) using the high-energy mechanochemical technique (mechanochemistry or mechanochemical), applying the glycerin (C3H8O3) and the stearic acid (C18H36O2) as controlling agents of the process (PCA’s), both considered materials of interest in green chemistry. Through the measuring of X-Ray Diffraction (DRX) allied to the Rietveld Method it was possible to observe and to analyze in function of the times of grinding (0 min, 225 min, 525 min, 825 min, 855 min, 885 min, 915 min, 945 min, 975 min and 1005 min) the structural evolution of the sample. After 525 min a single phase of ZnO was synthesized, with a hexagonal structure of wurtzite type (or B4 type) with an average size of nanometric crystallites. It was also observed that the mechanochemical process does not modify the solubility of Zn according to the equilibrium diagram phase of the material. With the increase of milling time it was observed a polymorphic transition from the hexagonal structure of wurtzite type for a cubic rocksalt structure type (type B1 and type NaCl). The kinetics of the transition was measured by linewidth analysis and variation of the crystallographic parameters obtained in this study. / A motivação deste trabalho foi a síntese e produção de material nanoetruturado ZnO (óxido de zinco) a partir do Zn (zinco puro) utilizando-se a técnica mecanoquímica de alta energia (mechanochemistry ou mechanochemical), empregando-se como agente s controladores de processo (ACP’s) a glicerina (C3H8O3) e o ácido esteárico (C18H36O2), ambos considerados materiais de interesse na química verde. Através da realização de medidas de Difração de Raios-X (DRX) aliadas ao Método de Rietveld foi possível observar e analisar a evolução estrutural da amostra em função dos tempos de moagem (0 min, 225 min, 525 min, 825 min, 855 min, 885 min, 915 min, 945 min, 975 min e 1005 min). Após 525 min foi sintetizada uma fase única de ZnO, com estrutura hexagonal do tipo wurtzita (ou tipo B4) com tamanho médio de cristalitos nanométricos. Foi observado ainda que o processo de mecanoquímica não altera a solubilidade do Zn, conforme diagrama de fases em equilíbrio do material. Conforme houve o aumento do tempo de moagem observou- se uma transição polimórfica da estrutura hexagonal do tipo wurtzita para uma estrutura cúbica do tipo rocksalt (ou tipo B1 ou tipo NaCl). A cinética da transição foi quantificada pela análise de largura de linha e variação dos parâmetros cristalográficos obtidos no estudo.

La récupération de chaleur perdue lors des procédés industriels grâce à la thermoélectricité peut contribuer au développement des économies d'énergie. Pour une plus large diffusion des convertisseurs thermoélectriques, le facteur de mérite ZT n'est pas le seul critère à prendre en compte ; le coût et l'éco-compatibilité des éléments utilisés, la facilité de synthèse et de mise en forme sont des aspects également importants. Deux familles de matériaux, encore peu présentes dans la littérature, répondent assez bien à toutes ces exigences : les siliciures et les composites. Nous nous sommes intéressés au siliciure de cobalt CoSi, ainsi que aux alliages et aux composites obtenus à partir de cette phase.Nous avons tout d'abord étudié la nanostructuration. La fusion à arc suivie d'un broyage mécanique ou la mécanosynthèse ont permis de synthétiser des cristallites de CoSi avec une taille de 13 nm. Ces poudres présentent une bonne stabilité chimique et un grossissement de grain limité jusqu'à 400 °C. Pour des températures plus élevées, une croissance cristalline importante s'accompagne d'une perte de silicium et de la formation de Co2Si. La nanostructuration diminue la conductivité thermique de CoSi de 35% par rapport au matériau massif. Le facteur de mérite ZT=0,15 obtenu à une température T=300 °C est supérieur à celui de CoSi synthétisé par four à arc mais légèrement inférieures à celui d'un monocristal.Différents éléments ont été envisagés pour former des alliages avec CoSi sur la base de considérations pratiques ainsi que sur les résultats de calculs ab initio. Certains se sont montrés insolubles dans CoSi (Ca, Zr, Nb, Mo, Sn, Ta, W et Pb), d'autres très peu solubles (Ti, V ou Cu). Des solutions solides de composition Co0,85Cr0,15Si, Co0,90Mn0,10Si et CoSi0,92Zn0,08 ont aussi été synthétisées, mais le facteur de puissance des deux premières est inférieure à celui de CoSi.Nous avons comparé trois polymères intrinsèquement conducteurs et commercialement disponibles : la polyaniline (PANI), le polypyrrole (PPy) et le polyéthylène(3,4dioxythiophéne) dopé avec le polystyrène sulfonate (PEDOT:PSS). Pour former des composites contenants l'alliage Co0.85Fe0.15Si, le PPy s'est avéré être le polymère le plus adapté. Le meilleur facteur de puissance PF=2,5 μW/m⋅K2 a été obtenu avec une fraction volumique de polymère ϕ=10%, toutefois la tenue mécanique pour cette composition est limitée. Les performances de ces composites sont toujours inférieures à celles de la phase la plus performante ; l'intérêt de ces matériaux réside donc surtout dans leur grande facilité de mise en forme. / Wasted heat recovery through thermoelectricity can contribute to a more sustainable energetic model. For a large-scale application of thermoelectric devices, their efficiency is not the only criterion to consider; materials should be easy to synthesize and made of abundant, cheap and environmental friendly elements. Silicides and composites are little known thermoelectric materials that meet all these requirements. We studied the cobalt silicide CoSi and some alloys and composites obtained using this phase.Firstly we investigate whether nanostructuration allows increasing the thermoelectric properties of CoSi. CoSi crystallites with a size of 13 nm were synthesized by arc melting followed by mechanical milling or by mechanical alloying. These powders showed good chemical stability and a limited grain growth up to 400 °C. At higher temperatures grain coarsening is accompanied by a loss of silicon and the formation of Co2Si. The thermal conductivity of CoSi was reduced by 35% by nanostructuring. A ZT=0.15 was obtained at T=300 °C, which is higher than that of CoSi synthesized by arc melting but slightly lower than that of CoSi single crystals.Various elements were considered in order to form a solid solution with CoSi, taking into account common sense considerations and the results of ab initio calculations. We observed that Ca, Zr, Nb, Mo, Sn, Ta, W and Pb are not soluble while Ti, V and Cu have a limited solubility. The phases Co0.85Cr0.15Si, Co0.90Mn0.10Si and CoSi0.92Zn0.08 were also synthesized but the first two have a lower power factor than CoSi.We compared three commercially available intrinsically conducting polymers: polyaniline (PANI), polypyrrole (PPy) and polyethylene (3,4dioxythiophene) doped with polystyrene sulfonate (PEDOT: PSS). PPy demonstrated to be the best polymer to form composites containing Co0.85Fe0.15Si. The highest power factor, PF=2.5 μW/m.K2, is obtained for a volume fraction of polymer ϕ=10%, nonetheless this composition induces poor mechanical strength. The thermoelectric performances we measured were always inferior to the ones of inorganic phase, thus the main advantage of these composites is their ease of shaping.