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La réalisation expérimentale de la condensation de Bose-Einstein (BEC) a ouvert un nouveau champ d'investigation très fertile dans l'étude des atomes froids. En particulier, la possibilité de synthétiser des gaz de fermions piégés dans des réseaux optiques représente un développement de la plus haute importance pour la physique de la matière condensée. Ceci ouvre notamment sur la perspective d'étudier des phases quantiques exotiques stabilisées dans des systèmes d'électrons fortement corrélés.Récemment, les gaz atomiques d'alcalino-terreux ou d'ytterbium ont suscité un vif intérêt et ont été refroidis jusqu'à la dégénérescence quantique. La structure atomique particulière de ces systèmes leur confère de très hauts degrés de symétrie, grâce au découplage entre le spin nucléaire et le moment angulaire électronique. Une physique exotique conduisant à de multiple applications peut résulter de ces systèmes de hautes symétries qui ne peut être sondée que par les solides bases de la matière condensée.Dans cette thèse, on se propose d'étudier les propriétés physiques de basse énergie d'un gaz de fermions de type alcalino-terreux, piégé dans un réseau optique à une dimension. À une dimension, il est possible d'analyser les effets des interactions de manière non-perturbative par des approches de théorie des champs comme la bosonisation ou la théorie des champs conformes, et numériquement par le groupe de renormalisation de la matrice densité (DMRG). L'ensemble de ces outils sera notamment utilisé pour déterminer le diagramme de phase des gaz de fermions d'alcalino-terreux ou d'ytterbium à une dimension. / Experimental realization of Bose-Einstein condensate (BEC) opened a new and rich field of investigation for the study of the cold atoms. In particular, the possibility of creating trapped fermionic gases in optical lattices represent one of the most important development for the condensed matter physics. This open the outlook of studying exotic and stabilized quantum phases in strongly correlated systems of electrons.Recently, alkline-earth or ytterbuim atomic gases have given rise to great interest and have been cooled down up to quantum degenaracy. The specific atomic structure of these systems confer them very high degrees of symetry, thanks to the decoupling beetwin the nuclear spin and the electronic angular momentum. An exotic physics which is only probe thanks to the strong fundament of the condensed matter.In this thesis, we propose to study the physical properties at low energy of a alkaline-earth-like fermionic gas, trapped in a one dimensional optical lattice. In one dimension, we are able to analyse effects of interactions in a non-pertubative way with conformal field theory or bosonization, and numerically with Density Matrix Renormalization Group (DMRG) approach. All of these tools will be used to provide the phase diagram of these alkaline-earth-like fermionic gases in one dimension.

Part I: The present work is to calculate dimensionless three-dimensional universal phase and lift force diagrams for a microbubble or pendant drop in a static liquid on a solid surface or orifice. Studying microbubble dynamics is important due to its controlling mass, momentum, energy and concentration transfer rates encountered in micro- and nano-sciences and technologies. In this work, dimensionless phase and force diagrams are presented by applying an equation for microbubble shape to accuracy of the second order of small Bond number provided by O¡¦Brien (1991). Two dimensionless independent parameters, Bond number and contact angle (or base radius), are required to determine dimensionless phase and force diagrams governing static and dynamic states of a microbubble. The phase diagram divides the microbubble surface into three regions, the apex to inflection, inflection to neck, and neck to the edge of microbubble. The growth, collapse, departure and entrapment of a microbubble on a surface thus can be described. The lift forces include hydrostatic buoyancy, difference in gas and hydrostatic pressures at the microbubble base, capillary pressure and surface tension resulted from variation of circumference. The force to attach the microbubble to solid surface is the surface tension resulted from variation of circumference, which is not accounted for in literature. Adjusting the base radius to control static and dynamic behaviors of a microbubble is more effective than Bond number. Part II: Controlling states and growth of a microscale bubble (or pendant drop) in a static liquid on a surface by introducing general phase diagrams is proposed. Microbubbles are often used to affect transport phenomena in micro- and nano-technologies. In this work, a general phase diagram is provided by applying a perturbation solution of Young-Laplace equation for bubble shape with truncation errors of the second power of small Bond number. The three-dimensional phase diagram for a given Bond number is uniquely described by the dimensionless radius of curvature at the apex, contact angle and base radius of the microbubble. Provided that initial and end states are chosen, adjusting two of them gives the desired states and growth, decay and departure of the bubble described by path lines in the phase diagram. A universal three-dimensional phase diagram for a microbubble is also introduced.

QCD at non-zero baryon density is expected to have a critical point where the finite temperature crossover at zero density turns into a first order phase transition. To identify this point, we use the canonical ensemble approach to scan the temperaturedensity plane through lattice QCD simulations with Wilson-type fermions. In order to scan a wide range of the phase diagram, we develop an algorithm, the ”winding number expansion method” (WNEM) to fix the numerical instability problem due to the discrete Fourier transform for calculating the projected determinant. For a given temperature, we measure the chemical potential as a function of the baryon number and look for the signal of a first order phase transition. We carry out simulations using clover fermions with mπ ≈ 800MeV on 63 × 4 lattices. As a benchmark, we run simulations for the four degenerate flavor case where we observe a clear signal of the first order phase transition. In the two flavor case we do not see any signal for temperatures as low as 0.83 Tc. To gauge the discretization errors, we also run a set of simulations using Wilson fermions and compare the results to those from the clover fermion. The three flavor case is close to realistic QCD with two light u and d quarks and one heavier s quark. Any hint of the existence of the first order phase transition and, particularly, its critical end point will be valuable for the planned relativistic heavy-ion experiments to search for such a point. In the three flavor case we found a clear signal for the first order phase transition, the critical point is located at a temperature of 0.93(2) Tc and a baryon chemical potential of 3.25(7) Tc. Since the quark mass in our present simulation is relatively heavy, we would like to repeat it with lighter quark masses and larger volumes.

E système Mo-Ni-Re est un sous-système majeur des superalliages à base de Ni conçus pour une utilisation dans les applications à haute température. Compte tenu des contradictions entre les informations publiées antérieurement, cette étude a été consacrée à la détermination expérimentale complète des équilibres entre phases dans le système Mo-Ni-Re, à la caractérisation structurale des phases intermétalliques et à la modélisation thermodynamique du système à l'aide de la méthode CALPHAD. L'étude expérimentale des équilibres entre phases a été effectuée en utilisant des alliages à l'équilibre, et les diagrammes de phases du système Ni-Re et Mo-Ni-Re (à 1200°C et à 1600°C) ont été proposées. En comparaison avec les études précédentes, le diagramme de phases Ni-Re déterminé au cours de ce travail a montré des différences significatives en termes de domaines d'homogénéité, des domaines de cristallisation et de température de réaction péritectique. La coupe isotherme à 1200°C du système Mo-Ni-Re proposée lors de cette étude a montré l'extension importante de la phase σ du système binaire Mo-Re et de la phase δ du système Mo-Ni dans le diagramme ternaire. En outre, la présence de deux phases ternaires jusque-là inconnues a également été observée. La coupe isotherme du système Mo-Ni-Re à 1600°C a également montré une grande extension de la phase σ dans le diagramme ternaire tandis que l'extension de la phase χ du système Mo-Re dans les deux coupes isothermes est limitée à un étroit domaine de composition. Les phases ternaires observées dans la coupe isotherme à 1200°C ne sont pas présentes dans la coupe isotherme à 1600°C. D'autre part, les études partielles des domaines de composition des phases dans les systèmes binaires Mo-Ni et Mo-Re ainsi que la détermination de la projection du liquidus du système Mo-Ni-Re ont également été effectuées. La projection du liquidus du système Mo-Ni-Re proposée lors de la présente étude a également montré des champs de cristallisation primaire de la phase σ de système Mo-Re et la solution solide de Re dans la région ternaire qui sont largement étendus. Puisque la coupe isotherme du système Mo-Ni-Re a montré un domaine d'homogénéité de la phase σ très étendu, la caractérisation structurale de la phase σ du système ternaire Mo-Ni-Re a été effectuée en mettant l'accent sur la détermination de l'occupation des sites en fonction de la composition par affinement de Rietveld combiné des données de diffraction des rayons X et des neutrons. Les résultats expérimentaux recueillis au cours de cette étude ainsi que les informations disponibles dans la littérature ont été utilisés comme données d'entrée pour la modélisation thermodynamique du système Mo-Ni-Re. La description thermodynamique du système Mo-Re a été prise de la littérature tandis que les optimisations thermodynamiques des systèmes Mo-Ni, Ni-Re et Mo-Ni-Re ont été effectuées au cours de ce travail avec la méthode CALHAD.Mots-clés: Mo-Ni, Mo-Re, Ni-Re, Mo-Ni-Re; diagramme de phases; coupe isotherme; caractérisation structurale; modélisation thermodynamique; méthode CALPHAD / The Mo-Ni-Re system is one of the important subsystems of the Ni based superalloys engineered for use in high temperature applications. Considering the contradictions among previously reported information, the present study was devoted to the complete experimental determination of the phase equilibria in the Mo-Ni-Re system, structural characterization of its intermetallic phases and thermodynamic modeling of the system with the help of the CALPHAD method. The experimental investigation of phase equilibria was carried out with the help of equilibrated alloys and phase diagrams of the Ni-Re and Mo-Ni-Re system (at 1200°C and 1600°C) were proposed. In comparison with previous investigations, the Ni-Re phase diagram determined during the present study showed significant difference in terms of homogeneity domains, freezing ranges and peritectic reaction temperature. The 1200°C isothermal section of the Mo-Ni-Re system proposed during the present study showed large extension of the Mo-Re σ phase and Mo-Ni δ phase in the ternary region. In addition, presence of two previously unknown ternary phases was also observed. The isothermal section of the Mo-Ni-Re system at 1600°C also showed large extension of σ phase in the ternary region whereas extension of the Mo-Re χ phase in both isothermal sections was restricted to narrow composition range. The presence of the ternary phases observed in the 1200°C isothermal was not evidenced in 1600°C isothermal section. On the other hand, partial investigations of phase boundaries in the Mo-Ni and Mo-Re binary systems and determination of liquidus projection of the Mo-Ni-Re system was also carried out. The liquidus projection of the Mo-Ni-Re system proposed during present study also showed largely extended primary crystallization fields of the Mo-Re σ phase and Re solid solution in the ternary region. Since isothermal sections of the Mo-Ni-Re system showed largely extended homogeneity domain of σ, structural characterization of the Mo-Ni-Re σ with particular emphasis on determination of site occupancy trends as a function of composition was carried out by combined Rietveld refinement of the X-ray and neutron diffraction data. The experimental results gathered during the present study along with the information available in the literature were used as input for thermodynamic modeling of the Mo-Ni-Re system. The thermodynamic description of the Mo-Re system was taken from literature whereas thermodynamic modeling of the Mo-Ni, Ni-Re and Mo-Ni-Re system was carried out during the present study with the help of the CALHAD method.Keywords: Mo-Ni; Mo-Re; Ni-Re; Mo-Ni-Re; phase diagram; isothermal section; structural characterization; thermodynamic modeling; CALPHAD method

Cette thèse concerne l'étude du diagramme de phase du fer en conditions extrêmes de pression et température. La Terre possède un noyau interne solide et un noyau externe liquide, qui sont principalement composés de fer. Une détermination fiable de la température de fusion du fer à 330 GPa, pression au-delà de laquelle le noyau terrestre est solide, permet de contraindre la température du noyau, ce qui est essentiel pour comprendre la dynamique terrestre. Le diagramme de phase du fer a été étudié jusqu'à 200 GPa en cellule à enclumes de diamant chauffée par laser utilisant la diffraction par rayon X comme diagnostic de l¿apparition de la fusion. Les températures obtenues sont en accord avec celles mesurées par compression dynamique, aux incertitudes expérimentales près, et sont plus élevées que celles obtenues lors de précédentes expériences statiques utilisant un critère de fusion différent. L'appareil, les méthodes et la métrologie utilisés pour les expériences en cellule à enclume de diamant chauffée par laser sont présentées ainsi que les problèmes rencontrés dans les expériences statiques à de telles conditions extrêmes. La possibilité d'utiliser le signal de diffraction des rayons X du joint en Re à des fins d'étalonnage de la pression pour l'expérimentation dans le domaine du multi-Mbar est aussi abordée. Dans ce but, l'équation d¿état du Re a été mesurée à 144 GPa. En fin, un test préliminaire a été effectué pour vérifier la possibilité d'utiliser la spectroscopie d'absorption des rayons X en dispersion d'énergie comme une technique complémentaire à la diffraction des rayons X pour la détermination de la courbe de fusion du fer. / This thesis concerns the study of the phase diagram of iron at extreme conditions of pressure and temperature. Iron is the main constituent of the terrestrial planetary cores. In particular, the Earth has a solid inner core and a liquid outer core which are mainly composed of iron. The accurate determination of the melting temperature of iron at the inner core boundary pressure, 330 GPa, would provide an important constraint on the temperature of the core, which is essential to understand how the dynamic Earth works. The phase diagram of iron has been investigated in laser-heated diamond anvil cell experiments up to 200 GPa using synchrotron-based fast X-ray Diffraction as a primary melting diagnostic. The obtained melting temperatures agree within the experimental uncertainties with the ones obtained from shock wave experiments and are higher than those reported by previous static experiments, where a different melting criterion was used. The apparatus, methods and metrology used in the static laser heated diamond anvil cell are discussed together with the issues encountered in static experiments at such extreme conditions. The possibility of using the X-ray diffraction signal of Re gasket for pressure calibration purpose for experiment in the multi-Mbar range is also discussed. For this purpose, Re equation of state has been measured up to 144 GPa. Finally, a preliminary test has been performed to check the possibility of using energy dispersive X-ray absorption spectroscopy as a technique complementary to fast X-ray diffraction in the investigation of the melting curve of iron.

Superalloys based on platinum-group metals (PGMs) are being developed for high temperature applications. Currently, the optimum alloy is Pt84:Al11:Ru2:Cr3, and work is ongoing. Niobium is a possible addition to increase the melting point, but limited phase diagram data are available. Although work has been done on the Pt-Al-Nb system, there are no reported data for Pt-Cr-Nb. This is a study of the Pt-Cr-Nb system. As-cast samples of the Pt-Cr-Nb ternary system were investigated using scanning electron microscopy with energy-dispersive X-ray spectroscopy, and X-ray diffraction. The results were used to plot a solidification projection and all binary phases extended into the ternary, except for α´Pt, which was not identified in this investigation. The extensions of the binary phases were: (Cr) ~2 at.% Nb; (Pt) ~24 at.% Nb and 65 at.% Cr; (Nb): ~12 at.% Pt and ~17 at.% Cr; ~NbPt2: ~20 at.% Cr; ~NbCr2: ~18 at.% Pt; ~Cr3Pt: ~10 at.% Nb; ~βNbPt3: ~4 at.% Cr and ~Nb1-XPt1+X: ~13 at.% Cr. Five ternary phases were identified: τ1:~Nb17:Cr64:Pt19; τ2:~Nb28:Cr55:Pt17; τ3:~Nb30:Cr30:Pt40; τ4:~Nb45:Cr27:Pt28 and τ5:~Nb40:Cr18:Pt42. The liquidus surface was derived using the binary phase diagrams, identified primary phases, solidification sequences and the solidification projection, together with the eutectic compositions. Nineteen ternary invariant reactions were identified. Hardness measurements were made on all samples, and also toughness and fracture toughness were deduced. Alloys with (Pt) and ~NbPt2 were ductile with reasonable hardness. Alloys containing (Cr), (Nb), ~Cr3Pt, ~NbCr2, ~Nb3Pt and ~Nb2Pt were extremely brittle with cracks, whereas alloys containing ~βNbPt3 and ~Nb1-XPt1+X phases were slightly brittle.

Atualmente, existe uma grande demanda por materiais caracterizados por um balanço adequado de propriedades, para aplicações estruturais em altas temperaturas. Superligas de última geração a base de níquel podem ser usadas sob carregamento em temperaturas próximas a 1150 ºC, mas informações indicam que materiais constituídos de microestruturas multifásicas apresentam maior potencial para estas aplicações, dentre os quais, os que contêm fase(s) intermetálica(s) em equilíbrio com um metal ou liga refratária. Na última década foram avaliados pelo Grupo Diagrama de Fases e Termodinâmica Computacional da Escola de Engenharia de Lorena os sistemas ternários metal refratário (molibdênio, nióbio, tântalo, vanádio, titânio, zircônio)-silício-boro, com o objetivo de serem determinadas as relações de fases em altas temperaturas e de ser desenvolvido um banco de dados termodinâmicos. Com o estudo experimental do sistema háfnio-silício- boro na seção isotérmica a 1600 ºC e sua Projeção Liquidus, na região rica em Háfnio, completa-se este ciclo de trabalhos. Saliente-se que o estudo deste sistema ternário exigiu a revisão dos sistemas binários háfnio-silício e háfnio-boro, através de sua avaliação experimental. Foram utilizadas no trabalho matérias-primas de elevada pureza (háfnio - mínimo de 99,8%, silício - mínimo de 99,998% e boro - mínimo de 99,5%). A metodologia experimental envolveu basicamente as seguintes etapas: (i) produção das ligas em forno a arco; (ii) tratamento térmico das ligas na temperatura de 1600 ºC; (iii) caracterização por difração de raios-X, microscopia eletrônica de varredura das ligas no estado bruto de fusão e tratadas termicamente e espectroscopia de energia dispersiva. Como resultado do estudo observou-se: (i) no sistema binário háfnio-silício a reação eutética L _HfSS + Hf2Si, na região rica em háfnio, as reações peritéticas L + Hf5Si3 Hf2Si, L + Hf3Si2 Hf5Si3, L + Hf3Si2 Hf5Si4, L + Hf5Si4 HfSi, L + HfSi HfSi2, a transformação congruente L Hf3Si2, e a reação eutética L SiSS + HfSi2, na região rica em silício, a estabilidade das fases intermediárias Hf2Si, Hf5Si3, Hf3Si2, Hf5Si4 a 1600 ºC, e a estabilidade de HfSi e HfSi2 a 1200 ºC, o que sugere alterações significativas em relação ao diagrama de fases atualmente aceito pela literatura; (ii) no sistema binário háfnio-boro a reação eutética L _HfSS + HfB, na região rica em háfnio, a reação peritética L + HfB2 HfB, a transformação congruente L HfB2 e a reação eutética L B-RhomSS + HfB2, na região rica em boro, e a estabilidade das fases intermediárias HfB e HfB2 a 1600 ºC, o que está de acordo com o diagrama de fases atualmente aceito pela literatura; (iii) no sistema ternário háfnio-silício-boro na região rica em háfnio, na projeção Liquidus, verificou-se as reações L _HfSS + Hf2Si + HfB, L HfB + Hf2Si, L HfB2 + Hf2Si, L HfB2 + Hf5Si3, L HfB2 + Hf3Si2, L HfB2 + Hf5Si4, L HfB2 + HfSi, L _HfSS + Hf2Si, L _HfSS + HfB, L + Hf5Si3 Hf2Si, L + Hf3Si2 Hf5Si3, L + Hf3Si2 Hf5Si4, L + Hf5Si4 HfSi e L + HfB2 HfB e na seção isotérmica a 1600 ºC, verificou-se a estabilidade das fases _HfSS, Hf2Si, Hf5Si3, Hf3Si2, Hf5Si4, HfSi, HfSi2, HfB e HfB2, e a existência dos campos trifásicos _HfSS + HfB + Hf2Si, HfB2 + HfB + Hf2Si, HfB2 + Hf2Si + Hf5Si3, HfB2 + Hf5Si3 + Hf3Si2, HfB2 + Hf3Si2 + Hf5Si4, HfB2 + Hf5Si4 + HfSi e HfB2 + HfSi + HfSi2. / Nowadays, there is a big demand for materials for structural applications at high temperatures. These materials must present a good properties balance. The last generation of the nickel-base superalloys can be used at temperatures close to 1150oC. However, information available so far shows that multiphase microstructure materials are potentially better for such application. Among these materials, the Group of Phase Diagrams and Computational thermodynamics in the Escola de Engenharia de Lorena has chosen those systems, which contains intermetallic(s) phase(s) in equilibrium with refractory metal or alloy for evaluation. Recently we have evaluated the phase stability at high temperature in the refractory metal (molybdenum, niobium, tantalum, vanadium, titanium, zirconium)- silicon-boron system, aiming at the development of a thermodynamic data base. The experimental study of the isothermal section at 1600 ºC and the Liquidus projection of the hafnium-silicon-boron system completes this cycle of works. The study of this ternary system demanded the revision of the hafnium-silicon and hafnium-boron binary systems, through its experimental evaluation. Alloys had been produced with blades of hafnium (minimum 99.8%), silicon (minimum 99.998%) and boron (minimum 99.5%), in the voltaic arc furnace under argon atmosphere, and heat treated at 1600 ºC under argon atmosphere. The phases had been identified by X-ray diffraction and contrast in backscattered electron imaging mode and spectroscopy of dispersive energy. The study determined: (i) in the binary system hafnium-boron the eutectic reaction L _HfSS + HfB, in the rich region of hafnium, the peritectic reaction L + HfB2 HfB, the congruent transformation L HfB2 and the eutectic reaction L B-Rhom + HfB2, in the rich region of boron, and the stability of the intermediate phases HfB and HfB2 at 1600 ºC, what is in agreement to the currently accepted diagram; (ii) in the binary system hafnium-silicon the eutectic reaction L _HfSS + Hf2Si, in the rich region of hafnium, the peritectic reactions L + Hf5Si3 Hf2Si, L + Hf3Si2 Hf5Si3, L + Hf3Si2 Hf5Si4, L + Hf5Si4 HfSi and L + HfSi HfSi2, the congruent transformation L Hf3Si2, and the eutectic reaction L SiSS + HfSi2, in the rich region of silicon, the stability of the intermediate phases Hf2Si, Hf5Si3, Hf3Si2, Hf5Si4 at 1600 ºC, and the stability of HfSi and HfSi2 at 1200 ºC, what suggests significant alterations in relation to the currently accepted diagram; (iii) in the ternary system hafnium-silicon-boron, in the rich region in hafnium, in the Liquidus projection, the reactions L _HfSS + Hf2Si + HfB, L HfB + Hf2Si, L HfB2 + Hf2Si, L HfB2 + Hf5Si3, L HfB2 + Hf3Si2, L HfB2 + Hf5Si4, L HfB2 + HfSi, L _HfSS + Hf2Si, L _HfSS + HfB, L + Hf5Si3 Hf2Si, L + Hf3Si2 Hf5Si3, L + Hf3Si2 Hf5Si4, L + Hf5Si4 HfSi e L + HfB2 HfB and in the isothermal section at 1600 ºC, the stability of the phases _HfSS, Hf2Si, Hf5Si3, Hf3Si2, Hf5Si4, HfSi, HfSi2, HfB and HfB2, and the threephase fields _HfSS + HfB + Hf2Si, HfB2 + HfB + Hf2Si, HfB2 + Hf2Si + Hf5Si3, HfB2 + Hf5Si3 + Hf3Si2, HfB2 + Hf3Si2 + Hf5Si4, HfB2 + Hf5Si4 + HfSi and HfB2 + HfSi + HfSi2.

Thesis advisor: Stephen D. Wilson / Iridium-based members of the Ruddlesden-Popper family of oxide compounds are characterized by a unique combination of energetically comparable effects: crystal-field splitting, spin-orbit coupling, and electron-electron interactions are all present, and the combine to produce a Jeff = 1/2 ground state. In the bilayer member of this series, Sr3Ir2O7, this state manifests as electrically insulating, with unpaired Ir4+ spins aligned along the long axis of the unit cell to produce a G-type antiferromagnet with an ordered moment of 0.36 uB. In this work, this Mott state is destabilized by electron doping via La3+ substitution on the Sr-site to produce (Sr1−x Lax)3Ir2O7. The introduction of carriers initially causes nano-scale phase-separated regions to develop before driving a global insulator-to-metal transition at x=0.04. Coinciding with this transition is the disappearance of evidence of magnetic order in the system in either bulk magnetization or magnetic scattering experiments. The doping also enhances a structural order parameter observed in the parent compound at forbidden reciprocal lattice vectors. A more complete structural solution is proposed to account for this previously unresolved distortion, and also offers an explanation as to the anomalous net ferromagnetism seen prior in bulk measurements. Finally, spin dynamics are probed via a resonant x-ray technique to reveal evidence of spin-dimer-like behavior dominated by inter-plane interactions. This result supports a bond-operator treatment of the interaction Hamiltonian, and also explains the doping dependence of high temperature magnetic susceptibility. / Thesis (PhD) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The equilibrium diagrams are the starting point and the guideline to predict and control the microstructure that will form during processing materials. Despite experiments being necessary in binaries and ternaries systems, it is difficult to experimentally determine phase diagrams of higher orders systems over wide ranges of compositions and temperature. The CALPHAD (CALculation of PHAse Diagrams) method was developed in order to solve this problem. The essence is to optimize the parameters of thermodynamic models that describe the Gibbs free energies of each phase aiming to reproduce the experimental and estimated (abinitio) data. The compound energy formalism (CEF) is widely used in order to describe phases which present several sublattices. It allows the modeling of a large variety of phases and numerous methods have been developed to treat different situations. The activities in this work developed a new approach of the CEF (NACEF) based on a mathematic analysis of the parameters which leads to a new formulation of the Gibbs free energy function evolving new independent parameters in which new independent parameters are obtained to express the Gibbs free energy. This approach was used in this work to describe the intermetallic phases with two-sublattice in which the only defect type is anti-sites (A,B)a(A,B)b. The Al-Fe-Nb system was chosen due to its importance for the manufacturing process of several families of alloys currently used, e.g. steels, light alloys, and also for the development of new materials for high temperatures application. The binaries Al-Nb and Fe-Nb were reassessed and the Al-Fe-Nb system was assessed for the first time using literature information and new experimental data. / Os diagramas de equilíbrio são o ponto de partida e a diretriz para prever e controlar a microestrutura ao final do processamento de um material. Apesar de experimentos serem necessários em sistemas binários e ternários, é muito difícil determinar experimentalmente diagramas de fase de sistemas de ordens superiores numa vasta amplitude de composições e temperatura. A fim de solucionar este problema, o método CALPHAD (CALculation of PHAse Diagrams) foi desenvolvido. A essência consiste em aperfeiçoar os parâmetros de modelos termodinâmicos que descrevem as energias livres de Gibbs de cada fase de modo a reproduzir as informações experimentais ou estimadas (ab-initio). O compound energy formalism (CEF) é amplamente utilizado para descrever fases que apresentam várias sub-redes. Ele permite a modelagem de uma grande variedade de fases e vários métodos têm sido desenvolvidos para o tratamento de diferentes situações. As atividades deste trabalho ajudaram a desenvolver uma nova abordagem para o CEF (NACEF) com base em um estudo matemático dos seus parâmetros termodinâmicos que levou a uma nova formulação para função da energia livre de Gibbs envolvendo novos parâmetros independentes. Esta nova abordagem tem sido utilizado como parte do presente trabalho para modelar fases intermetálicas binárias constituídas de sub-redes cujo único defeito é do tipo anti-sítio (A,B)a(A,B)b. O sistema Al-Fe-Nb foi escolhido devido a sua importância para o processo de fabricação de diversas famílias de ligas usadas atualmente, e.g. aços, ligas leves e, além disto, é um sistema importante para o desenvolvimento de materiais para aplicações em altas temperaturas. Neste trabalho os binários Al-Nb e Fe-Nb foram reavaliados e o sistema Al-Fe-Nb foi modelado pela primeira vez utilizando as informações da literatura e novos dados experimentais.