Made available in DSpace on 2015-04-01T12:17:41Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 2728849 bytes, checksum: 473329771da433558aad326b78ebbb89 (MD5) Previous issue date: 2013-09-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / A model was used to simulate solute distribution during rapid solidification of a system Ag-15%massCu with initial undercooling of 5 K, 10 K and 30 K. Boettinger et al (1987) measured the microsegregation profile in Ag-Cu alloys superficially treated with a focused electron beam (resolidification). In that work, they found an irregular variation for Cu concentration in cross sections of the resolidified microstructures. In some cases, the profile of concentration versus radius presented a W shape. Current models of microsegregation predict a monotonic variation the concentration depending on the cell radius, which are apparently not in accordance with the acquired measurements. The present study used a numeric model to evaluate the solidification of Ag-Cu alloy under same conditions to the Boettinger´s experiment. Numerical results are for a metallic spherical drop, initially liquid, cooled by convection. Solidification starts in the center of the droplet in a given undercooling. The algorithm used here is based on a model in volumetric average of the quantities and the balance equations in a representative elementary volume for treating a solid/liquid mixture. Inside the droplet transport of energy and mass are repurely diffusive processes of average quantities. The algorithm used permitted a global analysis of the phenomena occurring during the solidification process, as well as a local analysis of the quantities involved, which has been shown to be a prerequisite for the prediction of non-monotonic profiles of microsegregation, such as those measured in the experiment mentioned above. / Foi utilizado um modelo para simular a distribuição de soluto durante a solidificação rápida de um sistema Ag-15%Cu com superresfriamentos iniciais de 5 K, 10 K e 30 K. Boettinger et al (1987) mediram o perfil de microssegregação em ligas Ag-Cu tratadas superficialmente com um feixe de laser (resolidificação). Nesse trabalho, os autores constataram uma variação irregular para a concentração de cobre em seções transversais de células nas microestruturas resolidificadas. Em alguns casos um perfil de concentração ao longo do raio em forma de W foi medido. Os modelos de microssegregação atuais, em geral, preveem uma variação monotônica da concentração em função do raio da célula, o que aparentemente não está de acordo com as medições obtidas. No presente trabalho, foi usado um modelo numérico para avaliar a solidificação de uma liga Ag-Cu em condições análogas às do experimento de Boettinger. Os resultados numéricos são para uma gota metálica esférica, inicialmente líquida, resfriada por convecção. A solidificação tem início no centro da gota em um superresfriamento dado. O algoritmo usado foi desenvolvido previamente para um modelo baseado na tomada de médias volumétricas das grandezas e das equações de balanço descritivas em um volume elementar representativo para tratar com uma mistura sólido/líquido. No interior da gota, os transportes de energia e massa são processos puramente difusivos de grandezas médias. O algoritmo usado permite não apenas uma análise global dos fenômenos que ocorrem durante o processo de solidificação, mas também uma análise local das grandezas envolvidas, o que demonstrou ser condição necessária para a previsão dos perfis não monotônicos de microssegregação, tais como os medidos no experimento mencionado acima.

Através do processo de evaporação térmica é possível produzir filmes de KCl e KBr altamente dopados com íons Cu+. Investigamos o efeito de altas concentrações de Cu+ (1020 - 1021 cm -3) nestes filmes policristalinos, onde, associado com o efeito fora de centro do Cu+, origina uma forte banda de absorção propícia para aplicação ótica destes filmes como filtros passa banda na região do UV. Os filmes estudados, de 1 UM, foram preparados por coevaporação resistiva de KCl (Br), como hospedeiro, com 1 a 10% de CuCl(Br), como dopante, sobre diferentes substratos. Diversas técnicas foram usadas para obter as propriedades estrutural e ótica, tais como: microscopia eletrônica de varredura (MEV), difração de raios-X, elipsometria, absorção ótica e transmitância. Do estudo da morfologia é observado um aumento na densidade dos grãos com a concentração nominal de Cu+ de até 1021 átomos/cm-3. Este comportamento também resulta em um aumento na transmitância na região do visível. A banda de absorção em 266 nm (278 nm) para KCl ()KBr): Cu+ é atribuída à transição eletrônica parcialmente permitida de 3d10 3d94s e sua área integrada em função da temperatura mostra o comportamento da força do oscilador com a temperatura, que é propriedade do efeito fora de centro. A este efeito fora de centro está associado um dipolo elétrico devido ao deslocamento da carga da posição substitucional. Para verificar a existência de um sistema de relaxação dipolar elétrica, usamos o método de Corrente de Despolarização Termicamente Estimulada (CDTE). As medidas de CDTE mostram uma fraca e alargada banda em 49K, com energia de ativação média de 147 meV e um fator pré-exponencial de 2x10-14 seg, para amostras de KCl + 1% CuCl, com 3 UM de espessura. Nossos resultados são discutidos e comparados com outros observados em monocristais do mesmo material / Through the thermal evaporation procedure it is possible to obtain highly Cu+ doped KCl and KBr films. We have investigated the effect of 1020 - 1021 cm-3 Cu<sup+ concentration in these polycristalline films, where beside the Cu+ off-center effect it is detected a strong absorption band suitable for UV band pass optical filter application. The studied films are APROXIMADAMENTE 1 UM thick and are prepared from coevaporated KCl (KBr), as host, jointly with 1 to 10% of CuCl (CuBr) nominal doping value. Several techniques were used to obtain the optical and structural properties, such as optical absorption and transmittance, scanning electronic microscopy (SEM), X-ray diffraction and ellipsometry. From the morphology study it is observed that the grain density increases as the Cu+ concentration increase. This behavior results in an increasing transmittance in the visible region tôo. The absorption banda t 266 nm (276 nm) for KCl (KBr):Cu+ is attributed to the Cu+ partially allowed electronic transition 3d10 3d94s and its integrated área shows a temperature independent oscillator strenght behavior which is a property of the off-center effect. To this off-center effect it is associated a electric dipole due to the íon charge displaced from the substitutional position. To verify the existence of a relaxing electric dipole system, we use the thermally stimulated depolarization current (TSDC) method to find out for the Cu+ off-center electric dipole system. The TSDC measurements show a very weak result for our KCl + 1% CuCl samples, resulting in a meanactivation energy of 147 meVwith a peak position at 49K and a pré-exponential Arrhenius time of 2x10-14 sec. We compare and discuss the present results with others found in the same single crystal materials

Com base em trabalhos tanto teóricos como experimentais, tem-se concluído que o Cu+ fica em posição fora de centro em alguns halogenetos alcalinos. Neste, trabalho apresentamos resultados obtidos por técnicas de correntes termoestimuladas(ITC), para, verificar propriedades de dipolos elétricos devido a Íons de Cu+ em posição fora da centro em cristais de KCl. Ampliou-se o estudo do comportamento do Cu+ em cristais de KBr e KI. As bandas de ITC observadas nestes cristais, apresentam máximos em 62, 70 e 83 &#176K, e as energias de ativação são de 0.177, 0.196 e 0.232 eV em KCl, KBr e KI, respectivamente. A partir da integração das bandas de ITC e as concentrações de Cu+ achados por absorção ótica, o momento, dipolar elétrico é determinado, conduzindo a valores de 1.5, 1.9 e 2.6 e &#197 no KCl, KBr e KI, respectivamente. Dada a forma da banda de ITC, comprova-se que a reorientação dos dipo1os segue o modelo clássico com um tempo de relaxação simples, e que o Cu+ fica congelado em posição fora de centro abaixo daquelas temperaturas em que ocorre o máximo da banda de ITC. / Theoretical and experimental investigations have concluded that Cu+ occupies an off-canter position in several alkali halides. In this work we report results obtained by ionic thermal-currents technique (ITC) to verify the electric dipole properties due to Cu+ ions in off-center position in KC1 crystals. This was developed to study the Cu+ ion behaviour in KBr and KI crystals. The ITC band observed in these crystals geve maxima at 62, 70 and 83 &#176K, and the activation energies were 0.177, 0.196 end 0.232 eV in KCI, KBr and KI, respectively. From the integration of the ITC bands and the concentration of Cu+ obtained by optical absorption, the dipole moment is determined, giving values of 1.5, 1.9 and 2.6 e &#197 in KC1, KBr and KI, respectively. Due to the shape of the ITC band, we conclude that the dipole reorientation follows the classical model with a single relaxation time, and that the Cu+ freeze in off-center position below those temperatures where maximum occur at the ITC band.

nicialmente descreve-se o Sistema Cu-A1 e as fases que ocorrem do lado rico em cobre; em seguida faz-se uma revisão sobre as reações de decomposição da fase ? de alta temperatura no resfriamento e sobre as reações de decomposição da martensita ?\'? no revenido. Faz-se também a revisão de alguns conceitos teóricos sobre interfaces e estabilidade das microestruturas. Estudou-se, na parte experimental, a decomposição isotérmica (revenido) da martensita ?\'? de duas ligas hipoeutetóides em temperaturas que correspondem aos campos ? + ?? (520 ºC) e ? + ? (650 ºC) do diagrama Cu-A1. Realizaram-se ainda experiências de solubilização no campo ? + ? a 576 ºC da estrutura ? + ?? obtida pelo revenido por 90 minutos a 520 ºC. No revenido a 520 ºC da martensita ?\'? de ambas as ligas observou-se inicialmente a precipitação de alotriomorfos de fase ? em contornos de grão, a reversão parcial da martensita para fase ?? , e transformação in situ da martensita em fase ? supersaturada, que gradualmente passa a fase ? de equilíbrio. Com tempos maiores, observou-se a decomposição da fase ?? em ?? e? . A fase ? tem a forma de plaquetas que mantém planos de hábito herdados da martensita. A fase ?? precipita como glóbulos que coalescem sem formar contornos de grão, formando lamelas de interface lisas entre as plaquetas de ? . A estrutura ? + ?? não coalesce nem esferoidiza para os tempos estudados, o que pode ser explicado pela existência de interfaces ? / ?? , e ? / ?? de baixa energia. Para se estudar a existência de interfaces de baixa energia entre a fase ? da transformação in situ e a fase ? da reversão da martensita, realizou-se a decomposição da martensita no campo ? + ? .Observou-se que a fase ? resultante de transformação in situ mantém os planos de hábito da martensita; as plaquetas de fase ? coalescem sem esferoidizar, mantendo interfaces retas com a fase ? e os mesmos planos de hábito que a martensita obtida pelo resfriamento da fase ? de reversão. Cada grão de ? original dá origem a apenas um grão de ? , com a mesma orientação cristalográfica. Para estudar a existência de interfaces de baixa energia entre as fases ?? e ?? e de ambas com fase ? , realizou-se a solubilização no campo ? + ? da estrutura ? + ?? obtida por revenido. Verificou-se que a fase ? precipita no interior da fase ?? na forma de glóbulos, que depois crescem dissolvendo parcialmente a fase ? . As interfaces ? / ? mantém-se planas e as plaquetas de ? mantém os planos de hábito herdados da martensita. Cada antigo grão de ? dá origem a um grão de ? com a mesma orientação. Discutiu-se os resultados sob o ponto de vista da relação entre morfologia e a natureza das interfaces. Concluiu-se que os resultados podem ser explicados pela existência de interfaces ? / ?? , ? / ?? e ? / ? de baixa energia em algumas direções e interfaces ?? / ?? e ?? / ? em todas as direções. Discutiu-se ainda a origem de relações de orientação entre os reticulados que correspondem a interfaces de baixa energia. / The Cu-Al system, in the copper-rich side, is described. The reactions that occur during decomposition of the high temperature ? phase, as well as the decomposition of the ?\'? martensite during tempering, are reviewed. Some theoretical concepts about interfaces and microstructural stability are also reviewed. In the experimental part of this dissertation, the isothermal decomposition (tempering) of the ?\'? martensite is studied in two hipoeutectoid alloys, at temperature in the ? + ?? field (520 ºC) and in the ? + ? field (650 ºC) of the Cu-Al equilibrium diagram. Experiments on the solubilization of the ? + ?? structure obtained by tempering, in the ? + ? field of the equilibrium diagram at 576 ºC, are also made. During tempering at 520 ºC of the ?\'? martensite, for both alloys, it was initially observed the precipitation of ? phase alotriomorphs in grain boundaries, the partial reversion of the martensite ?? phase, and the transformation \"in situ\" of the martensite to supersaturated ? phase, which gradually becomes equilibrium ? phase. Later it was observed the decomposition of the ?? phase to ?? and ? phases. The \"in situ\" formed ? phase is plate-like, and maintain the habit plane of the martensite. The ?? phase precipitates as globules, which coalesce without leaving grain boundaries, forming lamellas between ? plates, with smoth inbterfaces. The ? + ?? structure neither coarsen nor spheroidizes for the studied periods of times. The results can be explained by the existence of low energy ? / ?? and ? / ?? interfaces. Experiments on the decomposition of the martensite in the ? + ? field were made to check the existence of low energy interfaces between the \"in situ\" transformed ? phase and the ? phase from the martensite reversion. It was observed that ? phase plates maintain the martensite habit planes. The plates coarsen without spheroidizing, but maintaining straight interfaces with the ? phase, and with the same habit plane of the martensite that results from the quenching of the ? phase from the reversion. Each ? grain originates only one new ? grain, with the same crystallographic orientation. Experiments on the solubilization in the ? + ? field of the ? + ?? structures obtained by tempering were made in order to study the existence of low energy interfaces between the ?? and ?? phases, and between both and the ? phase. It was observed that the ? phase precipitates inside the ?? as globules which later grow, dissolving partially the ? phase. The ? / ? interfaces stay straight and the ? plates maintain their habit planes. Each original ? grain originates one new ? grain with the same orientation. The results were discussed by relating the morphologies to the nature of the interfaces. It was concluded that the results could be explained by the existence of low energy ? / ?? , ? / ?? and ? / ? interfaces on some directions, and ?? / ?? and ?? / ? interfaces in all directions. The origins of the lattices orientation relationships that gives low energy interfaces are discussed.

Ressonância Paramagnética Eletrônica (RPE) e Absorção Ótica Eletrônica foram utilizadas para estudar-se os complexos de Cobre (II) com tris-hidroximetil aminometano em solução. A análise dos parâmetros de RPE e óticos medidos na solução a diversos pHs foi feita tendo-se à mão as curvas de formação dos complexos. Estas curvas foram construídas em função do pH da solução, usando os dados de equilíbrio químico publicado por K. S. Bai e Martell (J. Inorg. Nucl. Chem. 31 (1969) 1697-1707), numa faixa de concentração de tris na qual este reagente é usando como tampão em estudos biológicos e químicos. Determinou-se cinco conjunto de parâmetros de RPE (go, ao, a11 e A11, correspondentes a diferentes complexos de íon metálico. Cada um desses espectros é encontrado em determinados intervalos de pH, sendo possível associá-los aos complexos previstos nas curvas de formação. A maior estabilidade dos complexos que se formam a pHs mais altos segundo as constantes de equilíbrio é evidenciada pelos valores e go isotrópico (RPE) menores pelos desdobramentos hiperfino isotrópico ao (RPE) e de campo cristalino (ótica) maiores do que os encontrados nas soluções menos alcalinas. A formação do complexo quelato neutro a pH alcalino foi observada, e obteve-se seu tensor g, das medidas do cristal simples. Este tensor encontra-se em bom acordo com o valor de g isotrópico determinado em solução (go=1/3(gxx+gyy+gzz)). Existe evidência de que este complexo seja pentacoordenado). Observou-se similaridade entre os parâmetros para os complexos coordenados por: 2N e 1N+1 0&#8254; 3N e 2N+1 0&#8254 ; 4N e 2N+2 0&#8254. Isto leva à conclusão de que do ponto de vista energético a coordenação de nitrogênio Cu2+ e a coordenação do oxigênio negativo ao Cu2+ nestes complexos são equivalentes, pelo menos dentro de nossa resolução experimental. / Electron Paramagnetic Ressonance (EPR) and optical visible spectroscopies were used to study the complexes of Cu (II) íon with tris-hydroxymethylamino methane (tris) in solution. Analyses of EPR and optical parameters measured in solution were made using the calculated formation curvesof the complexes. These formation curves were calculated as function of the solution pH, on the basis of the equilibrium constants published by K.S. Bai and Martell (J. Inorg.Nucl. Chem. 31 (1969)-1967-1707), for a concentration of this normally employed in a buffer in biochemical studies. Five sets of EPR parameters ((go, ao, a11 e A11) were determined corresponding to different metal complexes. Each det is found in a characteristic pH range and it was possible to make a correlation of these complexes with the appear in the formation curve plot. The greater stability of the complexes in the high pH range is indicated by the values of go, wich are smaller and the values of the hyperfine splittings ao and the crystal field splittings (from optical measurements) which are greater tahn at the less alcaline solutions. The formation of the neural chelate complexa t alcaline pH was observed, and its g-factors were obtained from measurements with the single crystal. These g-factors agree very well with the solution value (go=1/3(gxx+gyy+gzz)) and the orthorombic symmetriy is obtained. Evidence is present that this complex is penta-coordinated. It was observed a similarity between the EPR parameters for the complexes coordinated by: : 2N e 1N+1 0&#8254; 3N e 2N+1 0&#8254 ; 4N e 2N+2 0&#8254. This lead us the conclusion that from the energetical point of view the coordination of Cu2+ to nitrogen or to negative oxigen in these cases are equivalent, at or our resolution is not sufficent to differentiate between them.

[pt] O aumento do interesse em materiais nanoestruturados, nos anos recentes, tem incentivado o desenvolvimento de materiais compósitos de matriz metálica reforçados com nanotubos de carbono. No presente estudo foi produzido um material nano compósito de matriz de cobre contendo nanotubos de carbono (CNT 2% peso), a partir de síntese por métodos químicos. O procedimento começa pela dissociação do nitrato de cobre na presença de CNT e um tensoactivo aniônico a 250°C e sua posterior redução in-situ com atmosfera de Hidrogênio sobre pressão de 1 atm. a 350°C. A análise por difração de Raios X confirmou a formação de CuO puro no momento da dissociação, assim como de cobre metálico após a redução. A presença dos CNT foi detectada nas duas etapas por essa técnica. Análises por Microscopia Eletrônica de Transmissão (MET)mostram que o tamanho médio de partícula do óxido e de 30nm em quanto que para o material reduzido está na faixa de 150-300nm, apresentando-se boa dispersão dos nanotubos. O material reduzido foi compactado, em forma de pastilhas, por pressão uniaxial a frio sob 25MPa e, posteriormente, por pressão isostática a 150MPa. O material compactado foi sinterizado em atmosfera de Argônio a 650°C por 15 min. Análise por Microscopia Eletrônica de Varredura (MEV) assim como TEM do material sinterizado, mostrou uma distribuição heterogênea de tamanho de grão na faixa de 100nm a 4 μm. Medidas de resistividade elétrica mostram que o compósito apresenta uma resistividade sensivelmente menor a baixa temperatura (2x10(-6) ? .cm) a 83°K que o cobre sem nanotubos (5.9x10(-6) ? .cm). / [en] The increasing interest in nanostructure materials in recent years has provided incentive to develop nanostructure composite materials with metal matrix, reinforced with carbon nanotubes. In the present work, copper matrix nano composite with carbon nanotubos (2% wt) was produced by chemical synthesis method. The procedure begins by the copper nitrate dissociation containing SWCNT and anionic tensoactive agent at 250°C, followed by in-situ reduction at 350°C, under hydrogen atmosphere at pressure of 1atm. CuO and Cu formation was confirmed by X ray diffraction at the moment of dissociation and reduction respectively. CNTs presence was detected at both steps by this characterization method. Transmission Electron Microscopy analysis, estimate particles grain size of 30nm for CuO powder while Cu powder particles were observed to be in the 100-300nm range, showing good dispersion of CNT. Bulk nano-composite pellets of the reduced material were obtained by pre-compactation under uniaxial pressure of 17 MPa followed by issostatic pressure of 150MPa. Sinterizing of the compacted material was carry out at 650°C under Argon atmosphere by 15 min. Scanning Electron Microscopy and Transmission Electron Microscopy analysis of the sinterized material showed an heterogeneous grain size distribution in the 100nm to 4 ìm range. Electric resistivity measures show that the nanocomposite material has lower resistivity at low temperature (2x10(-6) ? .cm) at 83°K than the copper without carbon nanotubes (5.9x10(-6) ? .cm).

Ce travail de thèse est dédié à l’étude théorique de l’effet de l'environnement sur les nano-catalyseurs AuCu. Ainsi dans le cadre de la théorie de la fonctionnelle de la densité (DFT), la stabilité du système bimétallique AuCu, modélisé sous forme de surfaces semi-infinies, de nanoparticules libres et de nanoparticules supportées, a été examinée en présence et en absence de gaz réactif. Du point de vue thermodynamique, et sous conditions de vide, la ségrégation de l’or en surface est favorisée, en partie grâce à sa plus faible énergie de surface comparée à celle du cuivre. En revanche, en présence de molécules de gaz (CO, NO ou O2), les résultats montrent une ségrégation inversée, du cuivre vers la surface et de l’or vers le volume. Ces résultats confirment et expliquent les observations expérimentales et permettent aussi de prédire la localisation du cuivre à la surface de l’alliage en présence de chaque environnement gazeux. Sur les surfaces, l’étude énergétique de l’anisotropie de ségrégation (c'est-à-dire la force de ségrégation pour chaque type de terminaison de la surface) montre une ségrégation préférentielle de Cu vers la surface (110) en présence de gaz. L’analyse de la structure électronique issue des calculs de la densité d’état et des distributions de charges met en évidence des caractères très différents pour les molécules de gaz et donc des effets différents sur l’alliage AuCu, à savoir un caractère local pour CO, semi-local pour NO et complètement singulier dans le cas de O2.Sous forme de nanoparticule (cuboctaèdre de 38 atomes), l’étude de l’évolution de la stabilité de AuCu en fonction de la teneur en Au et pour différents types d’alliages (cœur-coquille, alliage régulier, peau-cœur) a été effectuée grâce aux calculs de l’énergie de surface. Les résultats révèlent l’existence d’une relation linéaire entre la composition chimique et la stabilité de AuCu. En outre, à partir d’une teneur en Au de 20%, les nano-alliages bimétalliques AuCu se révèlent plus stables que les composantes Au et Cu pures. En présence de gaz, un modèle thermodynamique qui prend en compte l’adsorption des molécules de CO a été développé pour prédire le diagramme de stabilité en fonction de la température et de la pression de CO. Les résultats montrent l’instabilité des nanoparticules de Cu sous pression de gaz et prédisent une composition critique en Au (15%) à partir de laquelle l’alliage AuCu devient stable sous gaz réactif. Ces résultats sont en parfait accord avec des résultats expérimentaux récemment publiés.Enfin, l'effet du support oxyde a été soigneusement étudié en considérant l’interaction des nanoparticules AuCu sur la surface de TiO2(100) anatase. L’analyse des composantes énergétiques (énergies de dépôt, énergies d’interaction, etc ..), des effets géométriques (déformations de la nanoparticule et du support) et des propriétés électroniques (transferts de charges) ainsi que l’étude de la réactivité ont permis de comprendre le comportement de l’alliage supporté et d’évaluer l’effet du support anatase, pas du tout le même en fonction de la structure et de l’ordre chimique de la nanoparticule considérée. / This thesis is devoted to the theoretical description of the effect of the environment on AuCu nanocatalysts. In the framework of density functional theory (DFT), the stability of the AuCu bimetallic system, modeled as semi-infinite surfaces, free nanoparticles and supported nanoparticles, was studied in the presence and in the absence of reactive gas. From a thermodynamic point of view, and under vacuum conditions, the segregation of gold at the surface is favored, which is due, in part, to its lower surface energy compared to that of copper. However, in the presence of gas molecules (CO, NO or O2), the results show an inverted segregation, of copper towards the surface and of gold towards the bulk. These results confirm and explain the experimental observations and also make it possible to predict the localization of the copper on the surface of the alloy in the presence of gas environment.Over the AuCu surfaces, the energetic study of segregation anisotropy (i.e. the segregation forces versus surface terminations) shows preferential segregation of Cu toward the (110) surface, in the presence of gas. The analyses of the electronic structure from the calculation of density of states and charge distributions reveal very different characters of gas molecules and therefore different effects on the AuCu alloy, namely a local character for CO, semi-local for NO and a completely different behavior for the case of O2.In the form of nanosized particle (truncated cuboctahedron of 38 atoms), the AuCu stability as a function of Au content and for different alloy types (core-shell, regular alloy, skin-heart) was investigated by considering surface energy calculations. The results reveal the existence of a linear relationship between the chemical composition and the stability of AuCu. In addition, for Au content equal and beyond 20%, the AuCu bimetallic nano-alloys are found to be more stable than the pure Au and Cu components. In the presence of gas, a thermodynamic model that takes into account the adsorption of CO molecules was developed to predict the stability diagram as a function of temperature and CO pressure. The results show the instability of Cu nanoparticles under gas pressure and predict a critical composition in Au of about 15% from which the AuCu nanoalloys become stable. These results are in full agreement with recent experimental reports.Finally, the effect of the oxide support has been carefully investigated by considering the adsorption of AuCu nanoparticles over TiO2 (100) anatase surface. The analysis of energetic components (deposition and interaction energies etc.), geometric effects (nanoparticle and support deformations) and electronic properties (charge transfers) as well as the study of the reactivity, made it possible to understand the behavior of the supported nanoalloys and to evaluate the effect of the anatase support; not at all the same depending on the structure and the chemical order of the considered nanoparticle.

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Investigations on size dependent phase stability and transformations in isolated nanoparticles have gained momentum in recent times. Size dependent phase stability generates size specific particle microstructure which consequently yields size specific functionality. One important prerequisite for conducting studies on nanoparticles is their synthesis. A substantial amount of research effort has therefore been focused on devising methodologies for synthesizing nanoparticles with controlled shapes and sizes. The present thesis deals with both these two aspects: (a) synthesis of nanoparticles and (b) phase transformations in nanoparticles. The system chosen in this study is AuCu intermetallic nanoparticles. The choice of AuCu nanoparticle was due to the fact that the literature contains abundance of structural and thermodynamic data on Au–Cu system which makes it a model system for investigating size dependence of phase transformations. With respect to synthesis, the present thesis provides methodologies for synthesizing alloyed Au–Cu nanoparticles of different sizes, Au–Cu nano-chain network structures and uniform Au–Cu2S hybrid nanoparticles. For every type, results are obtained from a detailed investigation of their formation mechanisms which are also presented in the thesis. With respect to phase transformation, the thesis presents results on the size dependence of fcc to L10 transformation onset in Au–Cu nanoparticles under isothermal annealing conditions. The present thesis is divided into eight chapters. A summary of results and key conclusions of work presented in each chapter are as follows. The ‘introduction’ chapter (chapter I) describes the organization of the thesis. Chapter II (literature study) presents a review of the research work reported in the literature on the various methodologies used for synthesizing Au–Cu based nanoparticles of different shapes and sizes and on ordering transformation in AuCu nanoparticles. The chapter also presents a brief discussion on the reaction variables that control the process of nucleation and growth of the nanoparticles in solution. Chapter III titled ‘experimental details and instrumentation’ describes the synthesis procedures that were used for producing various nanoparticles in the present work. The chapter also briefly describes the various characterization techniques that were used to investigate the nanoparticles. The fourth chapter titled ‘synthesis and mechanistic study of different sizes of AuCu nanoparticles’ provides two different methodologies for synthesis, referred as ‘two-stage process’ and ‘two-step process’ that have been used for producing alloyed AuCu nanoparticles of different sizes (5, 7, 10, 14, 17, 25 nm). The ‘two-stage’ process involved sequential reduction of Au and Cu precursors in a one pot synthesis process. Whereas, the ‘two-step’ process involved a two-pot synthesis in which separately synthesized Au nanoparticles were coated with Cu to generate alloyed AuCu nanoparticles. In the two-stage synthesis process it was observed that by changing the total surfactant-to-metal precursor molar ratio, sizes of the alloyed AuCu nanoparticles can be varied. ‘Total surfactants’ here include equal molar amounts of oleic acid and oleylamine surfactants. Interestingly, it was observed that there exists a limitation with respect to the minimum nanoparticle size that can be achieved by using the two-stage process. The minimum AuCu nanoparticle size achieved using the two-stage synthesis process was 14 nm. Mechanism of formation of AuCu nanoparticles in the two-stage synthesis process was investigated to find out the reason for this size limitation and also to determine how the synthesis process can be engineered to synthesize alloyed AuCu nanoparticles with smaller (<14nm) sizes. Studies to evaluate mechanism of synthesis were conducted by investigating phase and size of nanoparticles present in the reaction mixture extracted at various stages of the synthesis process. Their studies revealed that (a) the nanoparticle formation mechanism in the two-stage synthesis process involves initial formation of Au nanoparticles followed by a heterogeneous nucleation and diffusion of Cu atoms into these Au rich seeds to form Au–Cu intermetallic nanoparticles and (b) by increasing the relative molar amount of the oleylamine surfactant, size of the initial Au seed nanoparticles can be further reduced from the minimum size that can be achieved in the case when equal molar amounts of oleylamine and oleic acid surfactants are used. The information obtained from the mechanistic study was then utilized to design the two-step synthesis process. In the two-step process, Au nanoparticles were synthesized in a reaction mixture containing only the oleylamine surfactant. Use of only oleylamine resulted in production of pure Au nanoparticles with sizes that were well below 10 nm. These Au nanoparticles were washed and dispersed in a solution containing Cu precursor. Introduction of a reducing agent into this reaction mixture led to the heterogeneous nucleation of Cu onto the Au seed particles and their subsequent diffusion into them to form alloyed AuCu nanoparticles with sizes of ~5, 7 and 10 nm. The study present in this chapter essentially signified that the surfactants used in the reaction mixture not only prevent nanoparticles from agglomerating in the final dispersion but also control their nucleation and growth and therefore can be used as a tool to tune nanoparticle sizes. The fifth chapter titled ‘size dependent onset of FCC-to-L10 transformations in AuCu alloy nanoparticles’ illustrates the effect of AuCu nanoparticle size on the onset of ordering under isothermal annealing conditions. Nanoparticles in this study were annealed in-situ in a transmission electron microscope. Samples were prepared by drop drying a highly dilute dispersion of as-synthesized nanoparticles onto an electron transparent TEM grid. Nanoparticles sitting on the TEM grid were well separated from each other to minimize particle sintering during the annealing operation. It was however observed that during the isothermal annealing, particle coarsening due to atomic diffusion was appreciable for 5 nm particles but negligible for 7 and 10 nm particles. Therefore for this study only 7 nm and 10 nm sized particles were considered. Onset of ordering was determined from the time when first sign of the diffraction spot, corresponding to the ordered phase, appears in the selected area electron diffraction pattern from a region containing large number of AuCu nanoparticles. Through a series of isothermal experiments it was observed that the time for onset of ordering increased with decrease in size of the nanoparticles. It is speculated that the delay in onset of ordering may be due to the fact that with a decrease in nanoparticle size the probability of a nanoparticle containing a fluctuation that shall generate a thermodynamically stable nuclei of the ordered phase decreases. A sharp interface between the ordered and the disordered phase inside the particle was also observed which suggested that the ordering transformation in as-synthesized fcc AuCu nanoparticles is a first order transformation. The sixth chapter titled ‘synthesis and characterization of Au1-xCux–Cu2S hybrid nanostructures: morphology control by reaction engineering’ provides a modified polyol method based synthesis strategy for producing uniform Au–Cu2S hybrid nanoparticles. Detailed compositional and structural characterization revealed that the hybrid nanoparticles are composed of cube shaped Au-rich, Au–Cu solid solution phase and hemispherical shaped Cu2S phase. Interestingly, the hemispherical Cu2S phase was attached to only one facet of the cube shaped phase. A study on the formation mechanism of hybrid nanoparticles was also conducted by characterizing specimens extracted from the reaction mixture at different stages of the synthesis process. The study revealed that the mechanism of formation of hybrid nanoparticles involved initial formation of isolated cube shaped pure Au nanoparticles and Cu–thiolate complex with a sheet morphology. With increase in time at 180°C, the Cu–thiolate complex decomposed and one part of the Cu atoms that were produced from the decomposition were utilized in forming the spherical Cu2S and other part diffused into the Au nanoparticles to form Au–Cu solid solution phase. The chapter also presents a study on the effect of dodecanethiol (DDT) on achieving the hemisphere-on-cube hybrid morphology. In this study it is illustrated that an optimum concentration of dodecanethiol is required both for achieving size and morphological uniformity of the participating phases and for their attachment to form a hybrid nanoparticle. The seventh chapter titled ‘synthesis of Au–Cu nano-chains network and effect of temperature on morphological evolution’ provides methodology for synthesizing fcc Au– Cu nano-chain network structures using polyvinylprrolidone (PVP) surfactant. It was observed that with increase in the molar amount of PVP in the reaction mixture, morphology of the as-synthesized product gradually changed from isolated nanoparticles to branched nano-chain like. The nano-chains contained twins which indicated an absence of continuous growth and possibility of growth by oriented attachment of initially formed Au–Cu nanoparticles. Both in-situ and ex-situ annealing of the nano-chains led to their decomposition into isolated nanoparticles of varying sizes. Annealing also caused fcc-to¬L10 phase transformation. Investigation of the wave length of perturbation leading to breaking of a nano-chain into particles indicated that the surface energy anisotropy affects the splitting of nano-chain network structure into nano-sized particles. The thesis ends with a last chapter where we have presented possible future extension of current work.