No description available.

No description available.

We present a method of constructing the phase diagram at low temperatures, using the low temperature expansions. We consider spin Iattice systems described by a Hamiltonian with a d-dimensional perturbation space. We prove that there is a one-one correspondence between subsets of the phase diagram and extremal elements of some family of convex sets. We also solve a linear programming problem of the phase diagram for a set of affine functionals. / Ph. D.

No description available.

Ce travail de thèse s'inscrit dans le cadre du développement d'une solution innovante d'empoisonnement neutronique homogène, par insertion d'erbium, au cœur des gaines de combustible en alliage de zirconium utilisées dans les réacteurs à eau pressurisée. Dans cette étude réalisée par le CEA, la géométrie envisagée est une gaine triplex constituée d'un liner interne faiblement enrichi en erbium, compris entre deux couches d'alliage industriel base zirconium. Dans le cœur du réacteur, l'eau se dissocie à la surface de la gaine. Il est donc intéressant d'évaluer les interactions potentielles entre l'hydrogène et l'alliage Zr-Er du liner. Cela nécessite de connaître le système ternaire Er-H-Zr ainsi que les systèmes binaires associés. Ceci peut être obtenu par détermination expérimentale et grâce à la modélisation thermodynamique. Les deux techniques ont été utilisées au cours de cette thèse. Les systèmes binaires Er-Zr et H-Zr ont été déterminés expérimentalement et modélisés dans la littérature. Le système binaire Er-H en revanche était très mal connu. Nous avons étudié ce système expérimentalement. Il a ensuite été modélisé avec l'approche Calphad. Nous obtenons une nouvelle évaluation du système binaire Er-H, avec des limites de phases différentes de ce qui avait été proposé précédemment. Dans le but de déterminer les limites des domaines de phases et de mettre en évidence l'éventuelle existence d'un composé ternaire dans le système ternaire Er-H-Zr, une étude expérimentale a été menée. Une technique originale a été utilisée : le dosage chimique des éléments d'alliage a été fait par ERDA combinée à la RBS. Cette étude nous a permis de proposer une coupe isotherme expérimentale à 350°C de ce système ternaire. Concernant la modélisation, les trois systèmes binaires ont été rendus compatibles dans le but de calculer le diagramme de phases ternaire par projection des binaires. Le calcul obtenu est en bon accord avec la coupe isotherme expérimentale. Enfin, par le biais de campagnes d'essais de traction, nous avons évalué l'impact de l'ajout d'erbium et/ou d'hydrogène sur le comportement mécanique du zirconium de pureté industrielle. Nous avons ainsi mis en évidence un effet durcissant de l'erbium et de l'hydrogène sans pour autant que ces effets soient corrélés. Aucun de ces résultats n'est rédhibitoire pour l'utilisation de cet alliage Er-Zr en tant que liner dans le concept triplex / This work at CEA is being achieved in the framework of the development of an innovating concept including the neutronic solid burnable poison, such as erbium, inside the cladding of pressurized water reactors. These new claddings are constituted by a liner of a zirconium base alloy slightly enriched in erbium between two liners of industrial zirconium alloys. Into the reactor core, the water dissociates at the surface of the cladding. So it is interesting to evaluate the interactions between the hydrogen released and the Zr-Er alloy. To do so, the Er-H-Zr ternary system has to be determined such similarly to its associated binaries. This can be done by experimental determination and by thermodynamic modelling. Both techniques were used in this work. Er-Zr and H-Zr have already been studied experimentally and modelled, but the Er-H binary system is almost unknown. So, we studied it experimentally. Then, it has been modelled using the Calphad method. We obtain a new evaluation of the Er-H binary system with phases limits rather different than what has been proposed in the literature. In order to determine the phase limits and, the potential existence of a ternary compound in the Er-H-Zr ternary system, an experimental study has been carried out. An original technique has been used to obtain the chemical compositions: ERDA combined with RBS. In this study, we propose a new isothermal section at 350°C of the Er H-Zr ternary system. About the modelling, the compatibility of the three modelled binaries has been checked in order to optimize the ternary system by the projection of the three binaries. The calculation obtained is in good agreement with the experimental isothermal section at 350°C determined in our work. Finally, uniaxial tensile test campaigns have been conducted to evaluate the impact of erbium and/or hydrogen on the mechanical properties of an industrial zirconium pure alloy. We evidenced a hardening effect of erbium and hydrogen but these effects are not correlated. None of these results is prohibitive for the use of this Er Zr alloy as a liner in the triplex concept

Parmi les combustibles fossiles, le gaz naturel est le plus propre, en termes d'émissions de CO2, d'efficacité énergétique et de quantité de polluants atmosphériques émis. Le méthane est l'élément principal du gaz naturel; néanmoins, il contient des quantités considérables de gaz acides (CO2, H2S) qui peuvent entraîner la corrosion des équipements et des pipelines si de l'eau est présente. Les mercaptans sont d’autres composés soufrés présents dans le gaz naturel dont combustion peut produire du SO2 qui est un produit chimique indésirables causant des problèmes environnementaux. Les gaz acides et les mercaptans doivent être retirés du gaz naturel jusqu'à une norme acceptable. Le gaz naturel traité contient jusqu'à 2% de CO2, 2-4 ppm de H2S et 5-30 ppm de mercaptans. L'absorption chimique avec des solvants aqueuses comportant des alcanolamines [3] (comme la monoéthanolamine (MEA), la diéthanolamine (DEA), la méthyldiéthanolamine (MDEA)) est la méthode la plus bien malteuse pour séparer les gaz acides du gaz naturel. Les gaz acides réagissent selon une réaction acide base dans l'absorbeur pour former des espèces électrolytes. Les mercaptans et les hydrocarbures ne réagissent pas avec les molécules d'alcanolamines, et sont physiquement absorbés.Le modèle thermodynamique a une grande importance pour la conception du procédé de traitement des gaz acide, car il va permettre de déterminer l'Equilibre Liquide Vapeur et faire les bilans d’énergie. Des modèles thermodynamiques fiables peuvent permettre aux concepteurs non seulement de confirmer leurs limites réglementaires, mais aussi de minimiser la perte de composants précieux comme les hydrocarbures.Dans ce travail, un modèle thermodynamique a été développé pour prédire:•Les solubilités des alcanes (méthane, éthane, propane, n-butane, n-pentane, n-hexane), aromatiques (ethylbenzène, benzène, toluène) et mercaptans (MM, EM) dans une solution aqueuse d'alcanolamine• Les solubilités des gaz acides (CO2, H2S) dans des solutions aqueuses d'alcanolamine et d'autres propriétés cruciales telles que: la concentration d'électrolyte, la composition en phase vapeur (principalement le conteneur d'eau)• Les diagrammes de phase pour les systèmes multi-composants contenant du CO2-H2S-alcanolamine-eau-hydrocabon-mercaptan.Les paramètres du model ont été déterminés avec les données expérimentales disponibles dans la littérature et les nouvelles données mesurées. / Among fossil fuels, natural gas is the cleanest, in terms of CO2 emission, burn efficiency and amount of air pollutant. Methane is the prevailing element of natural gas; therefore, there are also a variety of impurities. In fact, it contains usually considerable amounts of acid gases (CO2, H2S) which can lead to corrosion in equipments and pipelines if water is present. Mercaptans are known as toxic molecules with undesirable odor, and fuel combustion of mercaptan molecules can produce SO2 which is undesirable chemical, they can cause environmental issues. Acid gases and mercaptans are needed to be removed from natural gas until acceptable standard. The treated natural gas contains as maximum as 2% of CO2, 2–4 ppm of H2S and 5–30 ppm of total mercaptans. Chemical absorption with alkanolamines [3] (such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA)) is the most well-established method to separate acid gas from natural gas. Acid gases react with alkanolamines in the absorber to form electrolyte species, mercaptans and hydrocarbons do not react with alkanolamines molecules, and they are physically absorbed by aqueous alkanolamine solution. Then the loaded solution can be regenerated by heating in the stripper.Thermodynamic model is of high importance for the conception of the process, as it is linked directly to the accurate determination of the Vapor-Liquid Equilibrium and energy balances. Reliable thermodynamic models can allow designers not only to confirm their regulatory limits, but also to minimize the loss of valuable hydrocarbons components.In this work a thermodynamic model has been developed to describe:• Alkane (methane, ethane, propane, n-butane, n-pentane, n-hexane), aromatic (ethylbenzene, benzene, toluene) and mercaptans (MM,EM) in aqueous alkanolamine solution• Acid gases (CO2,H2S) solubilities in aqueous alkanolamine solutions, and other crucial properties like: electrolyte concentration, vapor phase composition(mostly water contant)• The phase diagram for multi-component system containing CO2-H2S-alkanolamine-water-hydrocabon-mercaptan.The parameters of the model were determined with the experimental data available in the literature and the new measured data.

O desenho planar para as células a Combustível de Óxido Sólido (SOFC) é melhor do que o tubular devido a sua maior densidade de corrente e menor custo de fabricação. No entanto, o projeto de SOFC planar requer selantes para evitar o vazamento de combustível e a mistura de gases em altas temperaturas. Os vidros e os vitrocerâmicos têm demonstrado serem os mais adequados por apresentarem boa compatibilidade com outros componentes da célula nas temperaturas de trabalho das SOFCs (700-1000°C). No presente estudo, uma série de composições pertencentes ao sistema BaO-Al2O3-SiO2 (BAS) com a adição de B2O3 foram sintetizados tomando as proporções apropriadas de cada óxido constituinte. Propôs-se melhorar este sistema utilizando-se formadores e teores relevantes de modificadores estruturais, de forma a compatibilizar tanto o desempenho térmico por meio do coeficiente de expansão térmica (CET) como a compatibilidade química com os demais componentes da célula. A originalidade deste estudo está na busca destas características em regiões de composições ainda não exploradas, localizadas dentro do triangulo de compatibilidade BS-B2S-BAS2 na região rica em bário do sistema ternário. Entre estes vidros sintetizados quatro composições (BAS-4, BAS-5, BAS-6 e BAS-7) foram escolhidas porque são as mais adequadas às solicitações termomecânicas exigidas para um material vítreo atuar como selante em SOFC. / The design for planar cells Fuel Solid Oxide (SOFC) is better than the tubular due to its higher current density and lower manufacturing cost. However, the design of planar SOFC requires sealant to prevent leakage of fuel and the mixture of gases at high temperatures. Glasses and glass-ceramics have proven to be the most suitable because they have good compatibility with the other components of the cell at working temperature (700-1000°C). In the present study, a series of compositions belonging to the BaO-Al2O3-SiO2 (BAS) system with the addition of B2O3 were synthesized having the appropriate proportions of each component oxide. It was proposed to improve this system using relevant levels of formers and structural modifiers oxides, in order to match both the thermal performance of thermal expansion coefficient (TEC) and chemical compatibility with other components of the cell. The originality of this study is to search for these characteristics in regions of compositions not yet explored, located inside the compatibility triangle BS-B2S-BAS2 at the barium rich part of the ternary diagram. Among the synthesized glasses four batch compositions (BAS-4, BAS-5, 6-BAS, BAS-7) were chosen because best matched the thermo-mechanical required for a glassy material to act as SOFCs sealant.

Thesis: Ph. D., Massachusetts Institute of Technology. Department of Materials Science and Engineering, 1975. / Vita. / Includes bibliographical references. / The discrepancy between two recent phase diagram determinations of the Fe-Al system is resolved experimentally. Both diagrams are correct, but-one is a metastable coherent phase diagram. The iron-aluminum system possesses a tricritical point where a line of [lambda]- transitions ends at a miscibility gap at about 23 atom percent aluminum and 615°C. Rules of general applicability governing phase separation within the miscibility gap are developed. Application of the rules to the iron-aluminum system results in detailed predictions about the mechanisms of decomposition and their sequences in this system. Electron microscopy is used to study the reactions experimentally and the results are in agreement with theoretical predictions. / by Samuel Miller Allen. / Ph. D.

O CVM - Cluster Variation Method, ou Método Variacional de Clusters é um método para cálculos termodinâmicos, baseado na aproximação de campo médio para a energia livre. Em sua concepção original, o CVM dispõe apenas de contribuições configuracionais. A proposta do presente trabalho é neste sentido aprimorar o método com a incorporação de outras componentes à energia livre. Acreditamos que as contribuições volumétricas, tanto dilatacionais quanto vibracionais, são de grande importância, e estas contribuições à energia livre foram aqui adicionadas ao método. Outro objetivo do presente trabalho foi verificar se esta abordagem solucionaria o problema da escala de temperaturas de cálculos ab initio aliados ao CVM. Nestes cálculos, a escala de temperaturas dos diagramas de fases geralmente é duas a três vezes maior que o verificado experimentalmente ou, equivalentemente, que os resultados CVM a partir de dados experimentais. Um novo algoritmo de minimização foi deste modo proposto para levar em conta os efeitos vibracionais e a inclusão do volume como variável. O algoritmo é baseado no NIM (Natural Iteration Method), que é utilizado para a minimização no caso configuracional. Um método para os cálculos vibracionais foi elaborado a partir do modelo de Debye-Grüneisen, com considerações adicionais elásticas, adaptado para sistemas multicomponentes (isto é, ligas). O modelo é baseado naquele desenvolvido por Anderson, quando existem dados ab initio de constantes elásticas, ou Moruzzi-Janak-Schwarz caso contrário. Em cálculos ab initio é possível determinar as constantes elásticas de cada estrutura considerada. Com estes dados, a temperatura de Debye e o módulo de volume a 0K são determinados com maior precisão. Dados de primeiros princípios relativos a energias de coesão/formação em função do volume para o sistema prototípico Fe-Al CCC (ferro-alumínio cúbico de corpo centrado) foram utilizados como exemplo para a obtenção de parâmetros para o modelo. / The Cluster Variation Method (CVM), used in thermodynamical calculations, is based in the mean-field approximation to the free energy. The CVM was originally devised to treat configurational-only cases. The scope of the present work is to enhance the method's capabilities, introducing other free energy components. The volumetric contributions, either dilatacional or vibrational, are believed to be of great importance, and are therefore incorporated here in the method. Another aim is to verify whether this approach would solve the temperature range calculated with CVM using ab initio data. In this kind of calculation, the phase diagram temperature range is usually twice or three times as large as experimentally verified or, equivalently, as the ones obtained in CVM calculations using experimental data. Therefore, a new minimisation algorithm was proposed to handle with the vibrational effects and the volume as a variable. The algorithm is based on the NIM (Natural Iteration Method), which is used for the minimization in the configurational case. The Debye-Grüneisen approximation has been adapted, with elastical considerations, for multicomponent systems (i.e., alloys). The method is based on Anderson model, when ab initio elastic constants are available, or on Moruzzi-Janak-Schwarz model otherwise. In ab initio calculations it is possible to determine the elastic constants for each structure considered. Using these data, Debye temperature and bulk modulus at 0K are determined with greater accuracy. First-principles cohesion/formation energies in function of volume for the b.c.c. Fe-Al (body-centered cubic iron-aluminum) system were used as an example to derive parameters to the model.

Understanding of the interrelationship between structure, thermodynamic properties and phase diagrams is very useful for rationalizing the behavior of materials and development of predictive models, which can be used to optimize the composition of materials and their fabrication processes. The properties of materials are governed by its electronic and crystallographic structure. Chemical bonding determines the electronic structure of materials. Furthermore, the electronic structure plays a predominant role in determining the physical, electrical, magnetic, thermal and optical properties of materials. Crystal structure also influences most properties of materials. Since changes in thermodynamic variables such as temperature, pressure, and composition dramatically alter the physical properties of materials and its structure, it is desirable to study the thermodynamic stability of materials in conjunction with phase relations. Phase diagrams can indicate the ranges of pressure, temperature and chemical composition where specific phases and mixtures of phases are stable. If the Gibbs energies of all the phases involved are known, phase diagram can be computed using Gibbs energy minimization algorithms. In recent times, one of the important uses of thermodynamics in materials science has been in the computation of phase diagrams. To materials scientists phase diagrams are like maps to travelers. They guide the path through the composition space to find phases, fulfilling specific materials performance requirements. As phase diagrams are the graphic representations of minimizations of Gibbs energy under given constraints, computational thermodynamics significantly expands our capability to walk in the multi-component space of engineering materials. High-temperature phase-equilibrium studies, thermodynamics and materials processing have had a close relationship over a number of decades. Successful utilization of ceramic materials under different environmental conditions at high temperatures requires accurate thermodynamic data. Focus of the present investigation is to obtain correct phase relations and accurate thermodynamic data in selected technologically important ceramic oxide systems in which the data are either not available or are inconsistent. Based on the experimental data, different types of phase diagrams are computed for the systems of contemporary relevance. After a brief introduction, Chapter 1 discusses the brief overview of the experimental techniques available for determining the phase relations and thermodynamic properties at high temperatures and the methods used in this study. The chapter reviews the possible sources of errors in experimental techniques and tests for correct functioning. In Chapter 2, systematic studies on high-temperature phase equilibria and thermodynamic properties of compounds in the ternary systems Ln-Pd-O (Ln = La, Pr, Eu, Gd, Tb, Dy, Ho and Er) are presented. Some of the ternary oxides on the Ln-Pd-O systems have potential application in catalysis and electrochemistry. To optimize the parameters for the synthesis and to understand the behavior of the catalysts, it is useful to have information on the thermodynamic stability domain of each compound. Quantitative information on the stability of the ternary oxides is also useful for assessing the interaction of metal Pd with ceramic compounds containing rare-earth elements under different environments. Furthermore, the thermodynamic data are beneficial for the design of processes for the recovery of rare earth and precious metals from scrap. There is very little thermodynamic and phase diagram information on the Ln-Pd-O systems. Isothermal sections of phase diagram for the ternary system La-Pd-O at 1200 K and for the systems Ln-Pd-O (Ln = Pr, Eu, Gd, Tb, Dy, Ho and Er) at 1223 K, were established by the isothermal equilibration technique at high temperatures. Phases were identified after quenching by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS). Based on the phase relations, the thermodynamic properties of ternary interoxide compounds were determined by the solid-state galvanic cell technique over a range of temperature between 925 - 1400 K. An advanced version of the solid-state cell incorporating a buffer electrode was used for high temperature thermodynamic measurements. The function of the buffer electrode, placed between reference and working electrodes, was to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. These novel features enhanced the accuracy of thermodynamic data. From electrochemical measurements, the standard enthalpies of formation of these oxides from elements and their standard entropies at 298.15 K were also evaluated. The variation of the lattice parameters and unit cell volume as a function of rare earth atomic number for the three ternary compounds Ln4PdO7, Ln2PdO4 (Ln = La, Pr, Nd, Sm, Eu, Gd) and Ln2Pd2O5 (Ln = La to Er) are discussed. The systematic variations of thermodynamic properties of all the ternary compounds as a function of rare earth atomic number are presented and correlated with structural features. Thermodynamic and structural parameters of uninvestigated Ln-Pd-O systems (Ln = Ce, Pm) can be obtained by interpolation. Based on the thermodynamic information obtained in this study and auxiliary data on binary compounds available in the literature, different types of phase diagrams, isothermal oxygen potential diagrams, isobaric phase diagrams, isothermal two dimensional and three-dimensional chemical potential diagrams for the systems Ln-Pd-O (Ln = La, Pr, Eu, Gd, Tb, Dy, Ho and Er) are constructed. Chapter 3 contains the studies on partial phase diagrams of the systems M-Ru-O (M = Ca and Sr) at 1300 K and determination of Gibbs energies of formation of calcium and stronsium ruthenates in the temperature range from 925 to 1350 K using solid-state cells with yttria-stabilized zirconia as the electrolyte and Ru + RuO2 as the reference electrode. Gibbs energies, enthalpies and entropies of formation of calcium and strontium ruthenates from their component binary oxides were deduced. The standard enthalpies of formation of these oxides from elements and their standard entropies at 298.15 K were also evaluated. Based on the thermodynamic data obtained in this study and auxiliary information from the literature, the three dimensional representation of oxygen potential diagram for the M-Ru-O systems (M = Ca and Sr) as a function of composition and temperature are computed. The purpose of this chapter is to determine the thermodynamic stability of alkaline earth metal ruthenates in the perovskite related layered system Mn+1RunO3n+1 (n = 1, 2, and ¥ for Ca-Ru-O system and n = 1, 2, 3 and µ for Sr-Ru-O system) since these calcium and stronsium ruthenates have interesting magnetic and electronic device applications. Moreover, there is no literature available for thermodynamic properties on first and second members of the Ruddelsdon-Popper (R-P) series in Ca-Ru-O system, Ca2RuO4, Ca3Ru2O7 and third member of R-P series in Sr-Ru-O system, Sr4Ru3O10. Some of the available literature information on thermodynamic properties for other compounds of R-P series in Mn+1RunO3n+1 (M = Ca, Sr) are found to be based on incorrect assumptions and erroneous calculation. Thus, this chapter provides the complete thermodynamic information for all the electronically and magnetically applicable alkaline earth metal ruthenates for optimizing the deposition condition in device fabrications. Chapter 4 gives the structure-properties correlations of 2-3 spinel compounds and spinel-corundum equilibria for the system NiO-Al2O3-Cr2O3 at 1373 K. Nickel, aluminum and chromium are important base-constituent elements of high-temperature oxidation-resistant alloys. A spinel phase is usually found in the protective scale formed on the surface of the alloys. There is no thermodynamic data on spinel solid solution NiAl2O4-NiCr2O4. Thus, the phase relations and mixing properties of the spinel solid solution have been determined in this chapter. The inter-crystalline ion-exchange equilibrium between NiAl2+2xO4+3x-NiCr2O4 spinel solid solution and Al2O3-Cr2O3 solid solution with corundum structure in pseudo-ternary system NiO-Al2O3-Cr2O3 have been determined by the conventional tie-line rotation method at 1373 K. The nonstoichiometry of NiAl2+2xO4+3x has been taken into consideration. Lattice parameters were used to obtain the compositions of the corundum and spinel solid solutions at equilibrium. Formation of homogeneous solid solutions and attainment of equilibrium were confirmed by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). From the experimental tie-line information and thermodynamic data on Al2O3-Cr2O3 solid solution available in the literature, the activities in the spinel solid solution were derived by using a modified Gibbs-Duhem integration technique. Gibbs energy of mixing of the spinel solid solution has been calculated from the derived activity data. Since high temperature data generation is expensive and time consuming, it is useful to develop models, which relate thermodynamic properties to electronic and crystallographic structure, leading to predictive modeling of mixing properties. By comparing the results from models with experimental information, one can evolve methodologies for the prediction of the properties of uninvestigated system. A model can be used to discriminate among conflicting experimental data and extrapolate the data into regions where direct measurements are lacking or difficult to perform. In this chapter, a model approach has also been considered to analyze the activity-composition relationship in the NiAl2O4-NiCr2O4 spinel solid solution in terms of the intra-crystalline exchange of cations between the tetrahedral and octahedral sites of the spinel structure governed by site preference energies of the cations. Since Ni2+ and Cr3+ ion in tetrahedral coordination exhibits Jahn-Teller distortion, an entropy corresponding to randomization of the distortion in the cubic phase has been incorporated in the cation distribution model. The thermodynamic mixing properties of stoichiometric spinel solid solution NiAl2O4-NiCr2O4 in terms of one mole of mixing species were computed at 1373 K. The strain energy caused by size mismatch was added as a separate term to the Gibbs energy of mixing using empirical relationship between enthalpy of mixing for a pair of ions and the difference in their ionic volumes. Madelung constant and electrostatic contribution of energy of mixing of the spinel solid solution have also been computed. Comparison of Gibbs energy of mixing calculated using the cation mixing model for the stoichiometric spinel solid solution NiAl2O4-NiCr2O4 with that of the experimental tie-line data for nonstoichiometric spinel solid solution NiAl2+2xO4+3x-NiCr2O4 were included in this chapter. The thermodynamic mixing properties obtained in this study would be helpful in understanding the formation of complex spinel protective layers on alloys containing nickel, aluminium and chromium in high-temperature applications. The summary of the important finding and the conclusions arrived at on the basis of results obtained from the present investigations are presented in Chapter 5.