The well-known lanthanide cuprates exist in two principal forms, T and T´, which behave as p-type and n-type conductors, respectively. In order to understand the structural properties and crystal chemistry from the T to T´ phase, the Nd₁.₈₋ₓLaₓPr₀.₂CuO₄ (NLPCO) system was studied varying the La substitution ratio (0≤x≤1.8) and then characterised using high temperature X-ray powder diffraction. From analysis of the X-ray diffraction patterns obtained at room temperature, there are clearly five distinguishable regions for the NLPCO system. They are, (1) monophasic T´ solid–solution (2) two phase mixture T´ + T´´ (3) monophasic T´´solid–solution (4) two phase mixture T´´ + O and finally (5) monophasic O phase solid–solution. The T´´ form has previously been suggested as an ordered form of T´; however here we show via high temperature X-ray diffraction studies that it is a non-transformable metastable phase formed on quenching of the T phase via an orthorhombically distorted variant. Also neutron diffraction and selected area electron diffraction (SAED) studies confirmed that the T ´´phase is 4- fold Cu coordinated. The structural, magnetic and electrical properties of this NLPCO series have been investigated for the selected compositions using X-ray diffraction, magnetization measurements, thermal analysis and conductivity measurements. The aim of the second half of this work was to discover the basic high temperature electrical characteristics of Nd₂₋ₓPrₓCuO₄ and investigate how this matches with those required for components on the SOFC cathode side to identify which dopant level shows highest conductivity and whether it is stable at different temperatures. The idea was to make a new concept in SOFC cathodes and current collector development, using n-type conductors instead of p- type conductors and to try to produce a high conductivity material which is stable under the chemical and thermal stresses that exist while under load that can be used in cathode or current collector applications. The Nd₂₋ₓPrₓCuO₄ (NPCO) series has been studied over a range of dopant levels (x=0.15 - 0.25) and maximum conductivity of 86.7 Scm⁻¹ has been obtained for the composition where x = 0.25. Also NPCO shows n-type semiconductor behaviour which gives operational advantages when operating at mild oxygen deficiency. AC impedance studies have been carried out on symmetrical cells to investigate the performance of NPCO as a cathode material. These studies mainly focused on polarization resistance and the activation energies of the cells. Low Rp values and low activation energies are obtained for a composite cathode compared to pure cathode material. Two configurations of NPCO as cathode materials were tested, pre-fired and in-siu fired. Pre-fired NPCO exhibited better performance than in-situ fired NPCO. Both in-situ and pre-fired current collecting NPCO still showed lowest activation energies which suggest good catalytic activity. From all of these studies, it is evident that the praseodymium doped neodymium cuprate material shows considerable promise as a potential cathode material for solid oxide fuel cell applications.

Le système zirconium-hydrogène est très étudié dans le cadre de la sûreté nucléaire car la précipitation d'hydrures entraîne la fragilisation des gainages, à base d'alliage de zirconium. Il s'agit de la première barrière de confinement des produits radioactifs : son intégrité doit être maintenue tout au long de la vie des assemblages combustible, en centrale y compris en cas d'accident et post-centrale (transport et entreposage). De nombreuses incertitudes demeurent quant aux cinétiques de précipitation des hydrures et à l'impact des contraintes sur leur précipitation. La modélisation à l'échelle atomique de ce système permettrait d'apporter des clarifications sur les mécanismes en jeu. Les méthodes traditionnelles de modélisation atomistique sont basées sur des approches thermostatistiques, dont la précision et la fiabilité dépendent du potentiel interatomique qui les alimente. Or il n'existe pas de potentiel rendant possible une étude rigoureuse du système Zr-H. Cette thèse a permis de développer cet outil manquant en utilisant l'approximation des liaisons fortes. Au-delà de ce nouveau potentiel, ce travail donne un guide détaillé des nombreuses étapes d'une dérivation de tels potentiels avec la prise en compte de l'hybridation spd, ajustés ici sur des calculs DFT. Ce guide est établi tant pour un métal de transition pur que dans la perspective d'un couplage métal-covalent (carbures, nitrures et siliciures métalliques). / The zirconium-hydrogen system is of nuclear safety interest, as the hydride precipitation leads to the cladding embrittlement, which is made of zirconium-based alloys. The cladding is the first safety barrier confining the radioactive products: its integrity shall be kept during the entire fuel-assemblies life, in reactor, including accidental situation, and post-operation (transport and storage). Many uncertainties remain regarding the hydrides precipitation kinectics and the local stress impact on their precipitation. The atomic scale modeling of this system would bring clarifications on the relevant mechanisms. The usual atomistic modeling methods are based on thermostatistic approaches, whose precision and reliability depend on the interatomic potential used. However, there was no potential allowing a rigorous study of the Zr-H system. The present work has indeed addressed this issue: a new tight-binding potential for zirconium hydrides modeling is now available. Moreover, this thesis provides a detailed manual for deriving such potentials accounting for spd hybridization, and fitted here on DFT results. This guidebook has be written in light of modeling a pure transition metal followed by a metal-covalent coupling (metallic carbides, nitrides and silicides).

O presente trabalho está relacionado a estudos de superligas à base de Ni, cujas microestruturas multifásicas apresentam propriedades satisfatórias para aplicações em altas temperaturas. O Nb e o Ta são componentes importantes nesta classe de materiais, estando normalmente associados às fases endurecedoras ?\' e \'\'. Desta forma, a determinação das relações de fases no sistema Ni-Nb-Ta na região rica em Ni corresponde a uma importante contribuição para a literatura referente, especialmente para os trabalhos ligados à construção de base de dados termodinâmicos. Assim, foi definido como objetivo principal a proposição das relações de fases na região rica em Ni do sistema Ni-Nb-Ta para a temperatura de 1050oC. As ligas foram obtidas por fusão a arco e tradas termicamente a 1050ºC por no mínimo 120 h. Amostras no estado bruto de fusão e tratadas termicamente foram caracterizadas microestruturalmente (MEV e DRX) e por microanálise química (EDS). Na região de estudo, observou-se as fases Niss, Ni8Ta, Ni3(Nb,Ta), Ni2(Nb,Ta), e ? (NiTa; Ni6Nb7), sem a presença de qualquer fase ternária. Observou-se uma solubilidade completa de Nb e Ta nas fases Ni3(Nb,Ta) e ?. A fase Ni2Ta apresentou uma solubilidade de Nb próxima de 13% at. Não foi possível determinar a região de solubilidade de Nb na fase Ni8Ta. / This work is related to studies of Ni-based superalloys, which multiphase microstructures have satisfactory properties for high temperature applications. The Nb and Ta are important components in this class of materials and is typically associated with the hardening phases ? \' and .\'\' Thus, the determination of phase relationships in the system Ni-Nb-Ta in the Ni-rich region represents an important contribution to the literature, especially for determination of related thermodynamic data. Thus, the main objective of this work was to propose the phase relationships in the Ni-rich region of the system Ni-Nb-Ta at 1050oC. The alloys were obtained by arc melting and heat-treated at 1050 º C for at least 120 h. Samples in the a-scast and heat treated states were characterized via (SEM and XRD) and chemical microanalysis (EDS). In the region of study, the observed phases were Niss, Ni8Ta, Ni3(Nb, Ta), Ni2(Nb,Ta), and ? (NiTa; Ni6Nb7) without the presence of any ternary phase. There was a complete solubility of Nb and Ta in the Ni3(Nb,Ta) and ? phases. The Ni2Ta showed a solubility of Nb close to 13% at. It was not possible to determine the region of solubility of Nb in the Ni8Ta phase.

Si les diagrammes de phase des alliages binaires massifs sont désormais bien connus, il en va tout autrement de ceux des nanoalliages, particules constituées de quelques centaines à quelques milliers d’atomes. Le développement des nanotechnologies est un puissant moteur pour développer la connaissance des diagrammes de phase des nanoparticules. Dans ce but, nous avons étudié le polyèdre de Wulff de 405 atomes (PW405) dans le système Cu-Ag caractérisé par une forte différence de rayons atomiques entre les constituants et une tendance à la démixtion, du moins dans sa forme massive. L’étude est réalisée par simulations Monte Carlo utilisant un potentiel interatomique à N-corps, en tirant parti de la richesse de deux ensembles thermodynamiques, à savoir les ensembles pseudo-grand canonique, i.e. à différence de potentiel chimique fixée, et canonique, i.e. à concentration nominale fixée. Nous montrons tout d’abord qu’un diagramme de phase de nanoalliage est constitué d’un ensemble de diagrammes de phase attachés à des classes de sites de surface ou des couches internes. Ainsi, pour le PW405, nous distinguons trois diagrammes de phase : ceux des facettes (100) et (111) et celui des couches de cœur. Chacun de ces trois diagrammes est attaché à des phénomènes physiques très différents. Ainsi, le diagramme de phase des facettes (100) est relatif à une transition structuro-chimique, alors que celui des facettes (111) caractérise une transition démixtion – désordre sans évolution structurale. Dans l’ensemble p-GC, cela se traduit par une bistabilité collective pour les facettes (100) et individuelle pour les facettes (111). Une conséquence de ce dernier point est la possibilité d’observer des facettes (111) pures en cuivre et d’autres pures en argent au sein d’un même agrégat !L’utilisation de la méthode de Widom pour déterminer dans l’ensemble canonique s’est révélée déterminante pour pouvoir séparer les régimes monophasés de ségrégation superficielle ou sub-superficielle des régimes biphasés. La différence de rayons atomiques joue un rôle important, tant dans la stabilisation dans l’état biphasé de la configuration Janus dissimulée par une coquille d’argent, que dans l’abaissement de la température critique par un facteur d’environ 2 par rapport au diagramme de phase massif.Une analyse des simulations Monte Carlo par une approche en champ moyen sur réseau effectif s’est révélée d’une richesse insoupçonnée pour déterminer les fores motrices de chaque type de transition, tant à la surface que dans les couches de cœur. Cela nous a permis de plus de montrer que les arêtes jouent le rôle d’une véritable nano-armature pour l’agrégat. Ce travail s’achève par ce qui constitue, à notre connaissance, la proposition du premier diagramme de phase de nanoalliage. / If bulk phase diagrams of binary alloys are now well known, it is not the case for nanoalloys, which are particles consisting in a few hundred to a few thousand of atoms. The development of nanotechnologies is a powerful driving force to develop the interest in the phase diagrams of nanoparticles. For this purpose, we have studied the Wulff polyhedron of 405 atoms (PW405) in the Cu-Ag system characterized by a large difference in atomic radii between the components and a tendency to phase separation, at least in the bulk. The study is carried out by Monte Carlo simulations using N-body interatomic potentials, taking advantage of the complementarity of two thermodynamic ensembles, namely the semigrand canonical ensemble (sGC), i.e. at fixed difference in chemical potentials and the canonical ensemble, i.e. at fixed nominal concentration.We first show that a phase diagram of nanoalloy consists in a set of phase diagrams related to the various classes of surface sites or to the internal layers. Thus, for the PW405, we distinguish three phase diagrams: the (100) facets diagram, the (111) facets diagram and the phase diagram for the layers of the core. Each of these diagrams is linked to very different physical phenomena. Thus, the phase diagram of the (100) facets is related to a structural and chemical transition, while the one of the (111) facets characterizes a transition between phase separation and disorder, without structural evolution. In the sGC ensemble, this results in a collective bistability for the (100) facets and an individual bistability for the (111) facets. A consequence of this last point is the possibility to observe some (111) facets pure in copper and other (111) facets pure in silver within the same nanoparticle !The use of the Widom method to determine in the canonical ensemble is decisive in order to discriminate between a single-phase regime (with surface or subsurface segregation) and a two-phase regime. Furthermore, the difference of atomic radii between Cu and Ag plays an important role, both for stabilizing the Janus configuration (with an Ag shell) in the two-phase state, and for lowering the critical temperature of the core layers by a factor of about 2 with respect to the bulk phase diagram.An analysis of the Monte Carlo simulations within an effective lattice formalism and a mean-field approximation is very powerful to determine the driving forces at the origin of each type of transition, both for the different facets of the surface layers and for the core. Moreover, this allowed us to show that the edges act as a nano-armature for the nanoparticles. This work ends with the establishment of what should be considered as the first phase diagram of nanoalloys, to the best of our knowledge.

Amostras policristalinas de La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; foram produzidas pelo método da mistura estequiométrica de óxidos e tratadas termicamente ao ar a 1400 oC. As amostras foram caracterizadas através de medidas de difração de raios-X, resistividade elétrica rho(T), susceptibilidade magnética chi(T) e magnetorresistividade rho(T, H = 50 kOe). Os resultados das análises dos diagramas de raios-X indicaram que os materiais são de fase única e que houve uma substituição efetiva de La por Pr no sítio A ao longo da série. Medidas de rho(T) e chi(T) revelaram que a temperatura de transição de fase metal-isolante TMI e temperatura de Curie TC decrescem com o aumento da concentração y e que a resistividade residual rho0 (rho(T = 10 K)) é consideravelmente alta em amostras com y 0.35. Ainda, com o decréscimo de T, as amostras com y 0.35 transitam para uma fase de ordenamento de carga em TOC ~ 194 K e, em seguida, para uma fase metálica em TMI. Essas medidas também sugerem a coexistência de fases ferromagnética-metálica FMM e de ordenamento de carga isolante OCI nesses materiais. Nas propriedades físicas macroscópicas, a fase FMM mostrou ser a dominante para os compostos com pequenas concentrações de Pr (y 0.25) e a fase OCI dominante para os compostos com altas concentrações de Pr (y 0.40). As medidas de rho(T, H = 50 kOe) mostram que a magnitude da resistividade elétrica decresce drasticamente nas vizinhanças de TMI sob a aplicação de um campo magnético externo. A magnitude de MR (MR = (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe)) entre os extremos da série (y = 0 e 0.625) varia até sete ordens de grandeza, sendo que o máximo valor de MR para amostras com y = 0 é de ~ 0.75 e naquelas com y = 0.625 é ~ 3.4x106 . O diagrama de fases deste composto evidencia uma região crítica (0.30 y 0.40) onde os valores de TMI, TC, MR e 0 variam abruptamente como função de y, sendo que em outras regiões tal variação é mais suave. A variação significativa desses quatro valores indica uma competição mais forte entre as fases coexistentes ocorre na região crítica. Algumas características marcantes podem ser observadas nas amostras da região crítica tais como: a presença de um segundo pico, abaixo de TMI, em ~ 90 K e ~ 72 K na curva de rho(T) de amostras com y = 0.30 e 0.35, histerese térmica mais pronunciada em rho(T) e chi(T), MR torna-se colossal, relaxação significativa da resistividade elétrica com o tempo, entre outras. Assim, as propriedades de transporte e magnéticas nessa região crítica são dominadas pela forte competição entre as fases coexistentes. / Polycrystalline samples of La(5/8-y)Pr(y)Ca(3/8)MnO(3); 0 y 0.625; were synthesized by the solid-state reaction method and sintered in air at 1400 oC. These compounds were studied by measurements of X-ray powder diffraction, magnetic susceptibility chi(T), and electrical resistivity rho(T, H). X-ray powder diffraction measurements indicated single phase materials and an effective substitution of La by Pr. Results from rho(T) and chi(T) revealed that increasing y in this series results in a rapid reduction of both the insulator to metal transition temperature TMI and the Curie temperature TC. Such a rapid decrease in TMI with increasing y is also accompanied by the occurrence of a new transition temperature, termed TCO, which is related to the transition to the charge ordered CO state. Such a temperature, which is essentially independent of y, occurs at TCO ~ 194 K and is mainly observed in samples with y 0.35. The other feature is the presence of a large residual resistivity electrical rho(0 = (10 K)) for large y (y 0.35) at low-T even though rho(T) suggests a metallic behavior below TMI. The temperature for the maximum magnetoresistance effect occurs near TMI, that shifts to higher T with increasing field. The MR is defined here as (rho(H = 0)-rho(H = 50 kOe))/rho(H = 50 kOe) and is enhanced by seven orders of magnitude from ~ 0.75 up to ~ 3.4x106 in samples with y = 0 and y = 0.625, respectively. Some features like the thermal hysteresis observed in both rho(T) and chi(T) curves indicate the coexistence of different phases in a range of y concentration, i. e., the ferromagnetic-metallic FMM and the charge ordered-insulating COI domains. The FMM is stable for y 0.25, but the COI state becomes dominant for y 0.40. There is a critical region in the phase diagram, ranging from y = 0.30 to 0.40, where the magnitude of the TMI, TC, MR, and 0 were found to display abrupt changes with increasing y. Some anomalous features like a second peak in rho(T) below TMI, a two-step increasing in chi(T), a colossal MR effect and others are observed for compositions belonging to this critical region. Our combined data suggest that the general physical properties of these compounds in such a critical region are dominated by the strong competition between coexisting ferromagnetic-metallic and charge ordered-insulating phases.

L'objectif de cette étude est de comprendre l'influence du silicium sur la microstructure d'alliages base TiAl. En effet, de faibles additions de silicium peuvent améliorer la tenue à chaud de ces intermétalliques. Nous montrons que le silicium a tendance à ségréger fortement durant la solidification, à l'échelle microscopique, provoquant l'apparition de siliciures primaires dans les zones interdendritiques. Après étude de cette ségrégation, nous avons procédé à des traitements thermiques d'homogénéisation afin d'étudier quantitativement les transformations solide-solide ayant lieu dans ces alliages. Ainsi, nous avons pu observer la précipitation de siliciures aux interfaces gamma/alpha2 dans des structures lamellaires homogènes. Mais, la structure lamellaire tend à se modifier en même temps que les siliciures germent et croissent. Les liens entre ces deux transformations simultanées sont mis en évidence expérimentalement, avant d'être modélisés à partir d'hypothèses simples. La réalisation d'essais mécaniques sur des microstructures contrôlées permet, en guise de conclusion, de donner des tendances quant à l'influence du silicium sur le comportement à chaud des alliages TiAl / The goal of this study is the understanding of the influence of silicon on the microstructure of TiAl-based alloys. Small additions of silicon are actually known to improve the heat resistance of these intermetallics. It is shown here that silicon segregates strongly at the microscopic scale during solidification, leading to the apparition of primary silicides in the interdendritic regions. After a study of this segregation, homogenization heat treatments were performed in order to focus on a quantitative study of the solid-solid transformations occuring in these alloys. Thus, silicide precipitation was observed at the gamma/alpha2 interfaces in homogeneous lamellar structures. However, the lamellar structure undergoes its own evolution as the silicides nucleate and grow. The links between these simultaneous transformations are shown by our experimental results, then modelled through simple considerations. As a conclusion, mechanical tests on controlled microstructures give some trends on the influence of silicon on the high temperature mechanical properties of TiAl alloys

O uso do cálculo termodinâmico e consequentemente base de dados desenvolvidas para esse propósito é hoje uma das ferramentas fundamentais para o aprimoramento e desenvolvimento de superligas a base de níquel. A proposta deste trabalho foi avaliar a confiabilidade de uma base de dados comercial, específica para superligas a base de níquel, utilizada em softwares de cálculos termodinâmicos computacionais. Comparou-se os dados experimentais da superliga Inconel 713LC com informações obtidas através do calculo termodinâmico para a composição química específica da superliga estudada. As amostras da superliga comercial Inconel 713LC foram retiradas de barras da liga fundida a vácuo (VIM). Os tratamentos térmicos foram realizados sob atmosfera de argônio em cápsulas de quartzo nas temperaturas de 600?C, 900?C e 1250 ?C nos tempos de 10h, 24h, 50h, 100h, 250h, 500h, 1000h e 2000 horas, de acordo com a temperatura de tratamento utilizada, visando acompanhar o desenvolvimento microestrutural até alcançar a condição de equilíbrio termodinâmico. Previamente, a amostra no estado bruto de fusão e as amostras após os tratamentos de equilíbrio foram caracterizadas microestruturalmente via microscopia eletrônica de varredura (MEV) e a composição das fases medidas por microanálise de energia dispersiva (EDS). As temperaturas das transformações de fases foram medidas por analise térmica diferencial (DTA). Nas amostras também foram realizadas medidas de microdureza para acompanhar indiretamente a evolução microestrutural da fase gama prime. Para os cálculos termodinâmicos foram utilizados o software Thermo-Calc® e a base de dados TTNi8 da empresa Thermotech Ltd®. / The computational thermodynamic simulation and the databases designed for this purpose are today one the most important tools to develop new nickel based superalloys. The aim of this work was to check the reliability of a commercial database, designed specifically for Ni-based superalloys. The experimental phase diagram data of Inconel 713LC, such as, liquidus and solidus temperatures as well as chemical phase compositions were compared with thermodynamic information extracted from computational simulations. Samples of a commercial superalloy Inconel 713LC were extracted from an ingot produced by Vacuum Induction Melting (VIM) and heat treated under argon in quartz capsules at 600?C, 900 ?C and 1250?C for 10h, 24h, 50h, 100h, 250h, 500h, 1000h and 2000h, according to the treatment temperature used, aiming at reaching the thermodynamic equilibrium conditions. The as-cast and all heat-treated samples were characterized by Scanning Electron Microscopy (SEM) and the chemical compositions of the phases were measured via Energy Dispersive X-ray Spectroscopy (EDS). Temperatures of phase transformations were determined by Differential Thermal Analysis (DTA). Microhardness measurements were performed to indirectly follow the microstructural evolution of gamma prime. The Thermo-Calc® software and the TTNi8 database from Thermotech Ltd® were used for thermodynamic calculations.

Estudou-se neste trabalho a preparação de LiLa(WO4)2:TR3+ (LLW:TR) nas formas de fibras monocristalinas (micro-pulling-down) e de microcristais pó cerâmico (método dos precursores poliméricos). No que se refere às fibras: a taxa de puxamento no crescimento de LiLa(1-x)Eux(WO4)2 para 0≤x≤1 é influenciada pela diferença de raio iônico do Li e das TR; a estrutura tetragonal da scheelita descreve as composições 0≤x≤1; a incorporação do Eu gera distorções na estrutura sem reduzir a simetria local do íon; uma queda de luminescência para x>0,20 é observada; e as estruturas para 0≤x≤1 foram modeladas através de simulação atomística com bastante precisão. No que se refere aos microcristais: o efeito do controle de pH na preparação de LLW:Nd 1,0 mol% foi avaliado; obteve-se aglomerados de morfologia irregular e com tamanhos médios entre 22-48 μm, cujo aumento é maior em função da temperatura do que do tempo de calcinação. No que se refere à construção experimental do diagrama de fases do sistema xLi2W2O7-(1-x)La2W2O9: confirmou-se que o LLW (formado numa região de homogeneidade de 0,48≤x≤0,55) se decompõe peritéticamente a 1000°C; a dopagem por TR influencia sua fusão; dados do crescimento, de DTA e de DRX de fibras de LLW, crescidas com composições baseadas neste diagrama, corroboram o mesmo. O processo de obtenção de fibras foi otimizado, com um excesso mínimo de 1,5 mol% de Li2W2O7. / In this work, the preparation and characterization of LiLa(WO4)2:RE3+ (LLW:RE) microcrystals as single crystal fibers (micro-pulling-down technique) and as powder (polymerizable complex method) was studied. Concerning the fibers: the pulling rate for LiLa(1-x)Eux(WO4)2 (0≤x≤1) is influenced by the difference between the constituents ionic radii; the tetragonal scheelite-like structure describes all compositions; the Eu incorporation distorts the LLW lattice without reducing the dopant local symmetry; a luminescence quenching for x>0.20 was observed; and the structure modelling for 0≤x≤1 by atomistic simulation was carried out with good precision. Concerning the powder microcrystals: the pH control effect on the preparation of Nd:LLW 1,0 mol% was evaluated; they presented irregular morphology and agglomerates with average sizes of 22-48 μm with higher increase for the calcination temperature than for the time. Concerning the experimental construction of the phase diagram of the system xLi2W2O7-(1-x)La2W2O9: it was confirmed that the LLW (formed in the a homogeneity region of 0.48≤x≤0.55) decomposes peritectically at 1000°C; RE-doping influences the LLW melting; growth, DTA and XRD data of LLW fibers (grown from the compositions based on this diagram) confirmed it. The process of fiber obtaining was optimized with a minimum excess of 1.5 mol% of Li2W2O7.

Estudou-se neste trabalho a preparação de LiLa(WO4)2:TR3+ (LLW:TR) nas formas de fibras monocristalinas (micro-pulling-down) e de microcristais pó cerâmico (método dos precursores poliméricos). No que se refere às fibras: a taxa de puxamento no crescimento de LiLa(1-x)Eux(WO4)2 para 0≤x≤1 é influenciada pela diferença de raio iônico do Li e das TR; a estrutura tetragonal da scheelita descreve as composições 0≤x≤1; a incorporação do Eu gera distorções na estrutura sem reduzir a simetria local do íon; uma queda de luminescência para x>0,20 é observada; e as estruturas para 0≤x≤1 foram modeladas através de simulação atomística com bastante precisão. No que se refere aos microcristais: o efeito do controle de pH na preparação de LLW:Nd 1,0 mol% foi avaliado; obteve-se aglomerados de morfologia irregular e com tamanhos médios entre 22-48 μm, cujo aumento é maior em função da temperatura do que do tempo de calcinação. No que se refere à construção experimental do diagrama de fases do sistema xLi2W2O7-(1-x)La2W2O9: confirmou-se que o LLW (formado numa região de homogeneidade de 0,48≤x≤0,55) se decompõe peritéticamente a 1000°C; a dopagem por TR influencia sua fusão; dados do crescimento, de DTA e de DRX de fibras de LLW, crescidas com composições baseadas neste diagrama, corroboram o mesmo. O processo de obtenção de fibras foi otimizado, com um excesso mínimo de 1,5 mol% de Li2W2O7. / In this work, the preparation and characterization of LiLa(WO4)2:RE3+ (LLW:RE) microcrystals as single crystal fibers (micro-pulling-down technique) and as powder (polymerizable complex method) was studied. Concerning the fibers: the pulling rate for LiLa(1-x)Eux(WO4)2 (0≤x≤1) is influenced by the difference between the constituents ionic radii; the tetragonal scheelite-like structure describes all compositions; the Eu incorporation distorts the LLW lattice without reducing the dopant local symmetry; a luminescence quenching for x>0.20 was observed; and the structure modelling for 0≤x≤1 by atomistic simulation was carried out with good precision. Concerning the powder microcrystals: the pH control effect on the preparation of Nd:LLW 1,0 mol% was evaluated; they presented irregular morphology and agglomerates with average sizes of 22-48 μm with higher increase for the calcination temperature than for the time. Concerning the experimental construction of the phase diagram of the system xLi2W2O7-(1-x)La2W2O9: it was confirmed that the LLW (formed in the a homogeneity region of 0.48≤x≤0.55) decomposes peritectically at 1000°C; RE-doping influences the LLW melting; growth, DTA and XRD data of LLW fibers (grown from the compositions based on this diagram) confirmed it. The process of fiber obtaining was optimized with a minimum excess of 1.5 mol% of Li2W2O7.

O presente trabalho está relacionado a estudos de superligas à base de Ni, cujas microestruturas multifásicas apresentam propriedades satisfatórias para aplicações em altas temperaturas. O Nb e o Ta são componentes importantes nesta classe de materiais, estando normalmente associados às fases endurecedoras ?\' e \'\'. Desta forma, a determinação das relações de fases no sistema Ni-Nb-Ta na região rica em Ni corresponde a uma importante contribuição para a literatura referente, especialmente para os trabalhos ligados à construção de base de dados termodinâmicos. Assim, foi definido como objetivo principal a proposição das relações de fases na região rica em Ni do sistema Ni-Nb-Ta para a temperatura de 1050oC. As ligas foram obtidas por fusão a arco e tradas termicamente a 1050ºC por no mínimo 120 h. Amostras no estado bruto de fusão e tratadas termicamente foram caracterizadas microestruturalmente (MEV e DRX) e por microanálise química (EDS). Na região de estudo, observou-se as fases Niss, Ni8Ta, Ni3(Nb,Ta), Ni2(Nb,Ta), e ? (NiTa; Ni6Nb7), sem a presença de qualquer fase ternária. Observou-se uma solubilidade completa de Nb e Ta nas fases Ni3(Nb,Ta) e ?. A fase Ni2Ta apresentou uma solubilidade de Nb próxima de 13% at. Não foi possível determinar a região de solubilidade de Nb na fase Ni8Ta. / This work is related to studies of Ni-based superalloys, which multiphase microstructures have satisfactory properties for high temperature applications. The Nb and Ta are important components in this class of materials and is typically associated with the hardening phases ? \' and .\'\' Thus, the determination of phase relationships in the system Ni-Nb-Ta in the Ni-rich region represents an important contribution to the literature, especially for determination of related thermodynamic data. Thus, the main objective of this work was to propose the phase relationships in the Ni-rich region of the system Ni-Nb-Ta at 1050oC. The alloys were obtained by arc melting and heat-treated at 1050 º C for at least 120 h. Samples in the a-scast and heat treated states were characterized via (SEM and XRD) and chemical microanalysis (EDS). In the region of study, the observed phases were Niss, Ni8Ta, Ni3(Nb, Ta), Ni2(Nb,Ta), and ? (NiTa; Ni6Nb7) without the presence of any ternary phase. There was a complete solubility of Nb and Ta in the Ni3(Nb,Ta) and ? phases. The Ni2Ta showed a solubility of Nb close to 13% at. It was not possible to determine the region of solubility of Nb in the Ni8Ta phase.