Tesis sobre el tema "Thermodynamic phase"

Thermodynamic modelling is a relatively new technique with which to study the interactions of ionic species in solution. Using the computer program PHREEQE, and the modified version PHRQPITZ, the effects of a range of reactive anions and cations on the components of cementitious systems are studied with respect to cement barrier performance in a nuclear waste repository. A review is made of the underlying thermodynamic theory, and of the development of the technique of thermodynamic modelling, before concentrating on the program PHREEQE and the method by which it performs calculations. The technique is then applied to the CaO-SiO₂-H₂O system under the influence of sulfate, carbonate, chloride, arsenite, Na₂O, K₂O, NaCl and MgSO₄. The effects of NaCl and MgSO₄ on certain sub-systems within the CaO-Al₂O₃-H₂O system are also studied. It is calculated that Ca(OH)₂ and CSH are the main pH maintaining phases, of which CSH has the greater stability under the influence of the species considered. AFm and AFt phases also act as pH buffers, but they are quantitatively less important. A review is made of solid solutions between members of the AFm and AFt groups of minerals. Experimental methods are used to study the extent of solid solution between monosulfoaluminate and Friedel's salt, and between monosulfoaluminate and C₄AH₁₃. In the case of the former, no solid solution is observed, but an intermediate phase, designated Kuzel's salt, is observed. In the latter case, two areas of solid solution at low and high sulfate content, separated by a miscibility gap, are detected. No evidence is found to support the existence of calcium hemisulfoaluminate.

Based on theoretical considerations an explanation for the temperature dependence of the thermal expansion and the bulk modulus is proposed. A new equation state is also derived. Additionally a physical explanation for the latent heat of fusion is presented. These theoretical predictions are tested against experiments on highly symmetrical monoatomic structures. The volume is not an independent variable and must be broken down into its fundamental components when the relationships to the pressure and temperature are defined. Using zero pressure and temperature reference frame, the initial parameters, volume at zero pressure and temperature[VJ, bulk modulus at zero temperature[Ko], and volume coefficient of thermal expansion at zero pressure [a J are defined. The new derived EoS is tested against the experiments on perovskite and epsilon iron. The Root-mean-square-deviations (RMSD) of the residuals of the molar volume, pressure, and temperature are in the range of the uncertainty of the experiments. Separating the experiments into 200 K ranges, the new EoS was compared to the most widely used finite strain, interatomic potential, and empirical isothermal EoSs such as the Burch-Murnaghan, the Vinet, and the Roy-Roy respectively. Correlation coefficients, RMSD’s of the residuals, and Akaike Information Criteria were used for evaluating the fitting. Based on these fitting parameters, the new p-V-T EoS is superior in every temperature range relative to the investigated conventional isothermal EoS. The new EoS for epsilon iron reproduces the preliminary-reference earth-model (PREM) densities at 6100-7400 K indicating that the presence of light elements might not be necessary to explain the Earth’s inner core densities. It is suggested that the latent heat of fusion supplies the energy required for overcoming on the viscous drag resistance of the atoms. The calculated energies for melts formed from highly symmetrical packing arrangements correlate very well with experimentally determined latent heat values. The optical investigation of carbonado-diamond is also part of the dissertation. The collected first complete infrared FTIR absorption spectra for carbonado-diamond confirm the interstellar origin for the most enigmatic diamonds known as carbonado.

We study in this thesis work the spectroscopic and thermodynamic quantities of some substances such as ammonium halides (NH4Cl, NH4I), ferroelectric crystals of tris-sarcosine calcium chloride (TSCC), tris-sarcosine calcium bromide (TSCB), organic compounds of carbon tetrachloride (CCl4) and s-triazine (C3N3H3) close to the phase transitions. Various physical and chemical properties of those materials have been measured near the critical points and have been reported in the literature. In this study, the spectroscopic parameters of the frequency shifts, intensity and bandwidths are calculated as functions of temperature or pressure near the phase transitions in ammonium halides using the experimental data from the literature. The spectroscopic parameters are related to the crystal volume and the specific heat in these compounds. The thermodynamic quantities of the specific heat, thermal expansion and the isothermal compressibility are also calculated in the solid and liquid phases of carbon tetrachloride using the experimental data. In another part of this thesis work, we analyze the temperature dependence of the spontaneous polarization and the dielectric susceptibility at fixed pressures for TSCC and TSCB by using the experimental data from the literature. The temperature dependence of the damping constant for the s-triazine is also calculated here close to the I-II transition. We use the theoretical models on the basis of the observations in the literature to calculate the critical behaviour of these physical quantities and we compare the results with the observed data. Various experimental studies in the literature give us the opportunity to find the proper way of fitting the calculated and observed results. This study gives us the chance of a better understanding of the critical behavior of the studied materials by verifying the values of some critical exponents and the types of transitions as expected by different theoretical models.

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

Thesis (M. Eng. and S.B.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.
Includes bibliographical references (p. 177-200).
The chaotic behavior of dynamical systems underlies the foundations of statistical mechanics through ergodic theory. This putative connection is made more concrete in Part I of this thesis, where we show how to quantify certain chaotic properties of a system that are of relevance to statistical mechanics and kinetic theory. We consider the motion of a particle trapped in a double-well potential coupled to a noisy environment. By use of the classic Langevin and Fokker-Planck equations, we investigate Kramers' escape rate problem. We show that there is a deep analogy between kinetic rate theory and stochastic chaos, for which we propose a novel definition. In Part II, we develop techniques based on Volterra series modeling and Bayesian non-linear filtering to distinguish between dynamic noise and measurement noise. We quantify how much of the system's ergodic behavior can be attributed to intrinsic deterministic dynamical properties vis-a-vis inevitable extrinsic noise perturbations.
by Zhi-De Deng.
M.Eng.and S.B.

Aqueous-polymer two-phase systems consist of various combinations of water, polymer(s), low molecular weight component(s), and salts. These aqueous-polymer systems are comprised of two phases, each of which contains about 90 percent (by weight) water. Due to some very unique properties, these systems have been applied to separations involving biological molecules for at least a quarter of a century. In particular, these systems are inexpensive, efficient, and provide a mild (aqueous) and possibly stabilizing environment for fragile biologically-active molecules. These systems may also be designed for a high degree of selectivity. Although much effort has been expended in the area of polymer solution theory, the theory of why these systems exhibit this extraordinary two-phase behavior that characterizes them as viable liquid-liquid extraction systems for use with biologically-active molecules is not completely understood. A thermodynamic model which could accurately represent the phase equilibria exhibited by these systems would be useful for the design of systems for use in many different applications. A potpourri of thermodynamic models and their underlying theoretical structure have been critically studied for their particular application to predicting the phase behavior of aqueouspolymer two-phase systems. In particular, the Flory-Huggins model is reviewed (with discussion of its inadequacies and subsequent modifications); the theory of Ogston; the model by Heil; several local composition models (NRTL, Wilson, and UNIQUAC); and two group-contribution models (ASOG and UNIFAC) are all discussed. The development of a solvent-electrolyte model (Chen's model) based on local composition theory (in particular the NRTL model) is reviewed, and the subsequent possible modification of this theory for solvent-polymer-electrolyte systems is discussed. The pros and cons of each model are discussed and qualitative results are given. Quantitative comparisons with experimental data are made with several of these models when appropriate data are available. The main conclusions of this work are: 1. A major limitation to the modeling of these aqueous-polymer two-phase systems is the lack of experimental data. Sufficient, accurate data is needed for the reduction of meaningful thermodynamic parameters by which thermodynamic models can be tested for their applicability. There exists a definite need for the generation of accurate, meaningful thermodynamic data from well characterized systems. 2. The most promising model identified in this work is the theory of Ogston. First, the model is based on the virial expansion and is thus quite suitable for dilute solutions. The Ogston model is the simplest theoretically-relevant dilute-solution model. Second, it appears to be easily extended to solvent-polymer-electrolyte solutions. 3. The Flory equation of state approach appears to be promising for representing polymer solutions. The free volume dissimilarity effect on which it is based is extremely important for solvent-polymer solutions. The most important aspect of this theory is its ability to predict lower critical solution temperature (LCST) behavior -- for which the Flory-Huggins theory is totally inadequate.
M.S.

One major source of uncertainty in current climate projections is the effect of clouds on the Earth’s climate system due to a direct influence on the radiative energy budget as well as through complex feedback mechanisms. The radiative properties of clouds are governed by their microphysical properties, e.g. the thermodynamic phase. However, the interactions of cloud and aerosol particles leading to complex vertical structures of the thermodynamic phase and their influence on the optical properties of clouds are not yet fully understood. Therefore, measurements with the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) are utilized during the StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) campaign in the Asian monsoon to detect cloud thermodynamic phase in the terrestrial infrared wavelength range for cloud-side observations.
Der Einfluss von Wolken auf das Klimasystem der Erde bildet einen großen Unsicherheitsfaktor in aktuellen Klimaprojektionen durch einen direkten Effekt auf den Strahlungshaushalt sowie über komplexe Feedbackmechanismen. Die Streueigenschaften von Wolken werden durch deren mikrophysikalische Eigenschaften (z.B. die thermodynamische Phase) bestimmt. Allerdings sind die Wechselwirkungen zwischen Wolken- und Aerosolpartikeln, welche zu komplexen vertikalen Strukturen der thermodynamischen Phase führen und damit die optischen Eigenschaften derWolke beeinflussen, noch nicht endgültig verstanden. Deshalb werden in dieser Studie Messungen mit dem Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) während der Messkampagne StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) in der asiatischen Monsunregion verwendet, um die thermodynamische Phase von Wolken aus Wolkenseitenbeobachtungen im terrestrischen infraroten Wellenlängenbereich abzuleiten.

In dieser Arbeit wird mit Hilfe der Gitter-Methode der Phasenübergang/Crossover bei nicht verschwindender Temperatur der Quantenchromodynamik mit zwei Quark Flavour untersucht sowie die thermodynamische Zustandsgleichung berechnet. Es wird dabei die Wilson twisted-mass Formulierung der Quark-Wirkung verwendet, welche hinsichtlich des Kontinuum-Limes eine automatische Verbesserung birgt. Erste belastbare Resultate mit dieser Wirkung bei endlicher Temperatur werden in dieser Arbeit gezeigt. Mehrere kleine Werte der Pion-Masse werden betrachtet mit dem Ziel, Aufschluss über die Ordnung des Phasenüberganges im chiralen Limes zu erhalten. Im Bereich der von uns simulierten Pion-Massen zwischen 300 und 700 MeV wird hierbei lediglich ein Crossover-Übergang beobachtet. Die Abhängigkeit der gemessenen Crossover-Temperatur von der Masse wird für eine Extrapolation zu verschwindender Masse hin verwendet unter der Annahme verschiedener Szenarien für den chiralen Limes. Dazu komplementär wird das chirale Kondensat, der Ordnungsparameter der spontanen Brechung der chiralen Symmetrie, vor dem Hintergrund der so genannten magnetischen Zustandsgleichung untersucht, welche das universelle Verhalten in der Nähe des Phasenüberganges für die Universalitätsklasse des O(4) Modells angibt. Hinsichtlich der Thermodynamik wird ausgehend von der Spur-Anomalie und unter Benutzung der Temperatur-Integral Methode der Druck und die Energiedichte im Crossover-Gebiet berechnet. Der Kontinuum-Limes der Spur-Anomalie wird mit mehreren Gitterdiskretisierungen der Temperatur Nt sowie unter Zuhilfenahme einer tree-level Korrektur untersucht.
In this thesis we report about an investigation of the finite temperature crossover/phase transition of quantum chromodynamics and the evaluation of the thermodynamic equation of state. To this end the lattice method and the Wilson twisted mass discretisation of the quark action are used. This formulation is known to have an automatic improvement of lattice artifacts and thus an improved continuum limit behaviour. This work presents first robust results using this action for the non-vanishing temperature case. We investigate the chiral limit of the two flavour phase transition with several small values of the pion mass in order to address the open question of the order of the transition in the limit of vanishing quark mass. For the currently simulated pion masses in the range of 300 to 700 MeV we present evidence that the finite temperature transition is a crossover transition rather than a genuine phase transition. The chiral limit is investigated by comparing the scaling of the observed crossover temperature with the mass including several possible scenarios. Complementary to this approach the chiral condensate as the order parameter for the spontaneous breaking of chiral symmetry is analysed in comparison with the O(4) universal scaling function which characterises a second order transition. With respect to thermodynamics the equation of state is obtained from the trace anomaly employing the temperature integral method which provides the pressure and energy density in the crossover region. The continuum limit of the trace anomaly is studied by considering several values of Nt and the tree-level correction technique.

Realistic simulations of clouds are of uppermost importance for climate modelling using general circulation models. Satellite data are well suited to evaluate model parametrizations. In this study we use the Laboratoire de Me´te´orologie Dynamique general circulation model (LMDZ). We evaluate the current LMDZ cloud phase parametrization, in which the repartition of condensed cloud water between liquid and ice is a function of the local temperature. Three parameters are used to derive a relation between liquid cloud water content and temperature, two of which are not physically based. We use the POLDER-1 satellite data to infer more realistic parameters by establishing statistical relationships between cloud top thermodynamical phase and cloud top temperature, consistently in both satellite data and model results. We then perform a multitude of short model integrations and derive a best estimate for the lowest local temperature where liquid water can exist in a cloud (Tice = -32°C in our parametrization). The other parameter which describes the shape of the transition between ice and liquid water is also estimated. A longer simulation has then been performed with the new parameters, resulting in an improvement in the representation of the shortwave cloud radiative forcing.

Les diagrammes d’équilibre sont le point de départ et la ligne directrice qui permet de prévoir et contrôler les phases pouvant se former au cours de différents processus industriels. Bien que l’étude expérimentale soit nécessaire pour les systèmes binaires et ternaires, elle est difficilement envisageable pour déterminer les diagrammes de phases des systèmes d’ordre supérieur sur de larges gammes de composition et de température. Afin de contourner ce problème, la méthode dite CALPHAD (CALculation of PHAse Diagram) a été développée. Son principe consiste à optimiser les paramètres des modèles thermodynamiques utilisés pour décrire l´énergie libre de Gibbs de chaque phase à partir d’informations expérimentales ou estimées (ab-initio). Le modèle appelé « Compound Energy Formalism » (CEF) est largement utilisé pour décrire les phases qui présentent plusieurs sous-réseaux. Ce modèle et ceux qui en dérivent permettent la modélisation d'une grande variété de composés. Les activités menées au cours de ce travail ont permis de développer une nouvelle approche du CEF (NACEF) basée sur une étude mathématique de ses paramètres thermodynamiques. Elle a conduit à une nouvelle formulation de la fonction d'énergie libre de Gibbs faisant intervenir de nouveaux paramètres indépendants. Cette nouvelle approche a été utilisée dans le cadre de ce travail afin de modéliser les phases intermétalliques binaires constituée de deux sous-réseaux présentant des défauts uniquement de type anti-sites (A,B)a(A,B)b. Le système Al-Fe-Nb sur lequel porte notre étude a été choisi en raison de son importance dans la fabrication de nombreuses familles d'alliages tels que les aciers, les alliages légers et plus récemment dans le développement de nouveaux matériaux réfractaires à base Nb pour des applications à hautes températures. Dans ce travail, de nouvelles modélisations des bordures binaires Al-Nb et Fe-Nb et pour la première fois du ternaire Al-Fe-Nb sont proposées en utilisant la NACEF et en s’appuyant sur les informations issues de la littérature ou obtenues dans cette étude
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 (ab-initio) 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

We correlate in the first part of this study the specific heat and thermal expansivity to the temperature-and pressure-dependent frequency shifts, respectively, in ammonia solid I, solid II, hexagonal ice and ice close to their melting points. This is carried out for some fixed pressures for the two translational and one librational modes in ammonia solid I. By obtaining linear plots of specific heat and thermal expansivity against temperature-and pressure-dependent frequency shifts, the values of slope were deduced and compared with experimental values. The correlation between the thermal expansivity and frequency shifts was constructed in the ammonia solid II by calculating the Raman frequencies of the translational and the librational modes for some fixed pressures. Calculated values of slope were compared with experimental values. Temperature and pressure dependent frequency shifts of the translational modes in hexagonal and ice are correlated to the specific heat and the thermal expansivity, respectively. When the mode Grü
neisen parameter depends on temperature and pressure, correlations among the specific heat, thermal expansivity and, temperature-and pressure-dependent frequency shifts, respectively, are reexamined in hexagonal ice. When the mode Grü
neisen parameter depends on temperature, correlation between the specific heat and the frequency shifts is reexamined using translational modes in NH4Cl. In the second part of this study, we predict the damping constant for ammonium halides (NH4Cl and NH4Br) for zero pressure, and for the tricritical and second order phase transitions for a lattice mode of NH4Cl. Also, the observed Raman intensities of this mode are analyzed at those two pressures.

Solvation-thermodynamics models based on computational quantum mechanics, such as the conductor-like screening model (COSMO), provide a good alternative to traditional group-contribution methods for predicting thermodynamic phase behavior. Two COSMO-based thermodynamic models are COSMO-RS (real solvents) and COSMO-SAC (segment activity coefficient). The main molecule-specific input for these models is the sigma profile, or the probability distribution of a molecular surface segment having a specific charge density. Generating the sigma profiles represents the most time-consuming and computationally expensive aspect of using COSMO-based methods. A growing number of scientists and engineers are interested in the COSMO-based thermodynamic models, but are intimidated by the complexity of generating the sigma profiles. This thesis presents the first free, open-literature database of 1,513 self-consistent sigma profiles, together with two validation examples. The offer of these profiles will enable interested scientists and engineers to use the quantum-mechanics-based, COSMO methods without having to do quantum mechanics. This thesis summarizes the application experiences reported up to October 2004 to guide the use of the COSMO-based methods. Finally, this thesis also provides a FORTRAN program and a procedure to generate additional sigma profiles consistent with those presented here, as well as a FORTRAN program to generate binary phase-equilibrium predictions using the COSMO-SAC model.
Master of Science

In this study, the iron-titanium-vanadium-oxygen (Fe-Ti-V-O) system in equilibrium with air was studied experimentally by high-temperature equilibration, quenching, scanning electron microscope and microprobe analysis coupled with critical assessment and thermodynamic evaluation. The thermodynamic evaluation was performed with FactSage 7.0. The purpose of the study was to develop a set of Gibbs equations for all compounds and solutions of the Fe-Ti-V-O system in equilibrium with air, using the well-known calculation of phase diagram (CALPHAD) technique. The study was categorically divided into three separate investigations. The lower order Fe-V-O and Ti-V-O systems in equilibrium with air were st experimentally investigated and thermodynamically assessed. This was then followed by an experimental investigation and thermodynamic assessment of the Fe-Ti-V-O system in equilibrium with air. The Fe-V-O and Ti-V-O systems in equilibrium with air were studied experimentally in temperatures ranging from 700 C to 1500 C. The measured concentration of Fe in the V-O slag is 35 weight % at 1400 C, and the measured V concentration in the hematite phase reached a maximum of 4.4 weight % at 1350 C. A signi cant amount of precipitation was observed for Fe-V-O samples quenched at 1400 C, causing calculated standard deviations of Fe and V to be more than 1 weight %. The measured concentration of Ti in the V-O slag is less than 6 weight % at 1500 C, and the measured V concentration in the rutile phase reached a maximum of 15 weight % at 1400 C. The solubility of V2O5(s) in the hematite and rutile phases was described with the compound energy formalism. The properties of the liquid phase were described with both the modi ed quasichemical model and the associate species model. A set of self-consistent thermodynamic parameters was estimated within acceptable error limits. The calculated phase diagrams of Fe-V-O and Ti-V-O in equilibrium with air are presented and compared to experimental observations and other literature data. Before experiments in the Fe-Ti-V-O system in equilibrium with air were conducted, the Fe-Ti-O system in equilibrium with air was critically assessed and thermodynamically evaluated. This was due to the slag phase and solid solutions of the Fe-Ti-O system that were previously thermodynamically evaluated only under reducing conditions. However, limited data were available in literature, hence assumptions were required for the evaluation. Nevertheless, an improved phase diagram of the Fe-Ti-O system in equilibrium with air was calculated. Thereafter, isothermal planes were calculated from optimized binary parameters to estimate a range of plausible starting compositions for experiments of the Fe-Ti-V-O system in equilibrium with air. The Gibbs phase rule was carefully applied to avoid redundant experiments. The Fe-Ti-V-O system in equilibrium with air was studied experimentally, ranging from 1000 C to 1400 C. The properties of the liquid phase were successfully described with the quasichemical model by optimizing parameters only related to the Fe-Ti-O system. The model for the rutile solid solution was extended to describe the solubility of Fe2O3(s) and V2O5(s) simultaneously. The model for the hematite solid solution was similarly extended to describe the solubility of TiO2(s) and V2O5(s) simultaneously. The ferropseudobrookite solid solution was modelled with a simple polynomial model to include a small solubility region of V2O5(s). A nal set of self-consistent thermodynamic parameters was estimated within acceptable error limits. Calculated isothermal projections at 1000 C, 1100 C, 1200 C, 1300 C, and 1400 C are presented and compared to experimental observations.
Thesis (PhD)--University of Pretoria, 2018.
Materials Science and Metallurgical Engineering
PhD (Metallurgical Engineering)
Unrestricted

Deposition of protein fibers with a characteristic cross-β sheet structure is the molecular marker associated with human disorders ranging from Alzheimer's disease to type II diabetes and spongiform encephalopathy. Given the large number of non-disease related proteins and peptides that have been shown to form amyloid fibrils in vitro, it has been suggested that amyloid fibril formation represents a generic protein phase transition. In the last two decades it has become clear that the same protein/peptide can assemble into distinct morphologically and structurally amyloid aggregates depending on the solution conditions. Moreover, recent studies have shown that the early stage, oligomeric amyloid assemblies are the main culprit in vivo. We have investigated the amyloid assemblies formed under denaturing conditions for Hen Egg White Lysozyme (HewL) whose human homologue is directly implicated in hereditary non-neuropathic systemic amyloidosis. Our early investigations showed that HewL can aggregate via at least two distinct assembly pathways depending on solution ionic strength at fixed pH, temperature, and protein concentration. By combining Dynamic Light Scattering (DLS), Static Light Scattering (SLS) and Atomic Force Microscopy (AFM) we showed that at low ionic strength, the pathway is characterized by the nucleation and growth of long (several micron), rigid fibrils (RF) via monomers assembly. A second, high ionic strength pathway is characterized by the rapid assembly of monomers into globular oligomers that further polymerize into curvilinear fibrils (aO/CF). At NaCl concentrations above 400 mM, aggregation resulted in precipitate formation. Next, we used Foureir Transform Infrared spectroscopy (FTIR) and an amyloid-specific dye, Thioflavin T (ThT), to show that both RF and (a)O/CF are amyloidogenic species, but they have detectable structural differences. Moreover, we have determined that each assembly pathway has unique SLS, DLS, FTIR and ThT response signatures that help determine the assembly type prior to AFM imaging of aggregates. Taking advantage of the morphological, structural and kinetic signatures for the two distinct HewL amyloid aggregates I mapped out their amyloid aggregates phase diagram spanning over two orders of magnitude in protein concentration and from 50 to 800 mM NaCl in ionic strength. This is the most complete phase diagram for amyloid aggregates of a given protein up to date. The phase diagram has three distinct regions delineated by sharp boundaries. The RF- aO/CF was called Critical Oligomer Concentration, and we commonly refer to “above the COC” as the region were aO/CF are kinetically favored.. In the region of low salt/high protein concentrations, RF were the only amyloid species to nucleate and grow. As both salt and protein concentrations increase, aO/CF become the kinetically favored species, and RF nucleate and grow after several days of incubation. At high protein and high salt concentrations, aO/CF form very fast and eventually lose solubility forming a precipitate (Ppt). Cross-seeding experiments showed that RF is the thermodynamically stable aggregate phase, while the O/CF are the metastable species. Finally, we used the phase diagram to design experiments that would allow us to reveal the RF nucleation mechanism in presence of aO/CF. RF nucleation above the COC can undergo either via internal restructuring of aO/CF (NCC) or through a random coalescence of monomers into a nucleus (NP). The experimental results obtained so far strongly indicate that RF nucleate via NP mechanism both below and above the COC.

This work is a thermodynamic and kinetic study of the Fe-Cr-Ni system as the core of stainless steels. The Fe-Cr, Fe-Ni and Cr-Ni systems were studied intensively using both computational and experimental techniques, including CALPHAD (CALculation of PHAse Diagrams), phase field simulation, ab initio modeling, calorimetry, and atom probe tomography. The purpose of this thesis is to reveal the complexity of the phase transformations in the Fe-Cr-Ni system via the integrated techniques. Due to the importance of the binary Fe-Cr system, it was fully reassessed using the CALPHAD technique by incorporating an updated description of the lattice stability for Fe down to zero kelvin. The improved thermodynamic description was later adopted in a phase field simulation for studying the spinodal decomposition in a series of Fe-Cr binary alloys. Using atom probe tomography and phase field simulation, a new approach to analyze the composition amplitude of the spinodal decomposition was proposed by constructing an amplitude density spectrum. The magnetic phase diagram of the Fe-Ni system was reconstructed according to the results from both ab initio calculations and reported experiments. Based on the Inden-Hillert-Jarl magnetic model, the thermodynamic reassessment of the Fe-Ni system demonstrated the importance of magnetism in thermodynamic and kinetic investigations. Following this, the current magnetic model adopted in the CALPHAD community was further improved. Case studies were performed showing the advantages of the improved magnetic model. Additionally, the phase equilibria of the Fe-Cr-Ni ternary were discussed briefly showing the need of thermodynamic and kinetic studies at low temperatures. The “low temperature CALPHAD” concept was proposed and elucidated in this work showing the importance of low temperature thermodynamics and kinetics for designing the new generation of stainless steels.

QC 20120612

Hero-m

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.

The current understanding of the mechanical properties and deformation behavior of some individual phases in titanium alloys is limited due to the fine scale at which these phases precipitate within the β-phase matrix. The α and ω phases represent the most widely observed phases in titanium alloys depending on the alloy composition and also the heat treatment procedure adopted during processing. The possibility of precipitating ω-phase depends on the content of the β-stabilizers within the system. Although a significant compositional partitioning occurs within ω-phase upon aging treatment, the knowledge of ω-phase mechanical properties as a function of composition is very limited. The initial part of the current work focuses on the effect of common β-stabilizers elements on the phase stability and mechanical properties of the ω-phase using first-principles calculations. A relation between the bonding nature, the phase stability, and elastic properties was proposed. Thereafter αʺ martensitic phase was investigated in Ti-Nb and Ti-Nb-O alloys. The phase stability and martensitic start temperature of αʺ-phase was studied as a function of Nb and oxygen content. Also, the effect of the lattice shear distortion induced by oxygen atom on stabilizing β-phase was investigated. Subsequently the effect of the β-stabilizers' elements on stacking faults energy and ductility in α-Ti alloys was studied. Both prismatic and basal slip system were investigated with different concentration of β-stabilizers at the slip plane. Lastly, while the Tadmor and Bernstein model was employed to predict the partial dislocation emission and twinning propensity, the Rice criterion was used to estimate the effect of different β-stabilizers on the ductility of α-Ti alloys.

En raison de leurs propriétés chimiques et physiques, les oxydes mixtes d'uranium et de plutonium sont considérés comme combustibles pour les réacteurs nucléaires de quatrième génération. Dans ce cadre, des études expérimentales complémentaires sont nécessaire, notamment pour mieux comprendre les phénomènes mis en jeu lors de la fabrication ou sous irradiation. L'objet de ce travail est d'étudier le diagramme de phase U-Pu-O dans une large gamme de composition et de températures afin d'améliorer notre connaissance de ce système. La plupart des expériences ont été réalisées par diffraction des rayons X en fonction de la température. La contrôle in situ de la pression partielle en oxygène a permis de faire varier la stœchiométrie en oxygène dans le matériau. L'approche expérimentale a été couplée avec la modélisation thermodynamique par la méthode CALPHAD afin de mieux dimensionner les expériences et interpréter les résultats. Cette méthodologie a permis d'améliorer notre connaissance des équilibres de phase dans le système U-Pu-O
Due to their physical and chemical properties, mixed uranium-plutonium oxides are considered for fuel in 4th generation nuclear reactors. In this frame, complementary experimental studies are necessary to develop a better understanding of the phenomena that take place during fabrication and operation in the reactor. The focus of this work was to study the U Pu–O phase diagram in a wide range of compositions and temperatures to ameliorate our knowledge of the phase equilibria in this system. Most of experiments were done using in situ X-ray diffraction at elevated temperatures. The control of the oxygen partial pressure during the treatments made it possible to change the oxygen stoichiometry of the sample, which gave us an opportunity to study rapidly different compositions and the processes involved. The experimental approach was coupled with thermodynamic modeling using the CALPHAD method, to precisely plan the experiments and interpret the obtained results. This approach enabled us to enhance the knowledge of phase equilibria in the U–Pu–O system

Realistic simulations of clouds are of uppermost importance for climate modelling using general circulation models. Satellite data are well suited to evaluate model parametrizations. In this study we use the Laboratoire de Me´te´orologie Dynamique general circulation model (LMDZ). We evaluate the current LMDZ cloud phase parametrization, in which the repartition of condensed cloud water between liquid and ice is a function of the local temperature. Three parameters are used to derive a relation between liquid cloud water content and temperature, two of which are not physically based. We use the POLDER-1 satellite data to infer more realistic parameters by establishing statistical relationships between cloud top thermodynamical phase and cloud top temperature, consistently in both satellite data and model results. We then perform a multitude of short model integrations and derive a best estimate for the lowest local temperature where liquid water can exist in a cloud (Tice = -32°C in our parametrization). The other parameter which describes the shape of the transition between ice and liquid water is also estimated. A longer simulation has then been performed with the new parameters, resulting in an improvement in the representation of the shortwave cloud radiative forcing.

This study gives our calculations for the temperature-pressure and temperature-concentration phase diagrams using the mean field models applied to ammonium halides (NH4Cl, ND4Cl), ammonium sulfate ((NH4)2SO4/H2O), lithium potassium rubidium sulfate (LiK1-xRbxSO4), potassium pyrosulfate-potassium hydrogensulfate (K2S2O7-KHSO4), cholestanyl myristate-cholesteryl myristate (CnM-CrM), cholestanyl myristate-cholesteryl oleate (CnM-CO), benzene (C6H6) and ice. The phase line equations are derived from the free energies expanded in terms of the order parameters and they are fitted to the experimental data. Some thermodynamic quantities are calculated close to phase transitions in these crystalline systems. We also calculate the specific heat CV using the Raman frequency shifts for NH4Br on the basis of an Ising model close to the lambda-phase transition. A linear relationship is obtained between the specific heat CP and the frequency shifts (1/v)(dv/dT)P near the lambda-point in NH4Br.

Materials in proximity to quantum critical points (QCPs) experience strong fluctuations in the order parameter associated with the transition and often, as a result, display interesting properties. In this dissertation, we have used a variety of experimental probes such as Shubnikov-de Haas quantum oscillations, thermal conductivity and heat capacity, to better understand two such materials — $A_3T_4$Sn$_{13}$ and YFe$_2$Ge$_2$. $A_3T_4$Sn$_{13}$ ($A$ = Ca, Sr; $T$ = Ir, Rh) is a family of quasi-skutterudite superconductors with moderate $T_c$’s between 4 and 8 K. Although the superconductivity is believed to be phonon-mediated with s-wave pairing symmetry, an unusual second-order structural transition makes this material family fascinating to study. Whether this structural transition is a result of three distortions with perpendicular wavevectors resulting in a cubic-to-cubic transformation, or each wavevector acting independently giving rise to cubic-to-tetragonal transformations and formation of twinned domains is a disputed issue. We have measured quantum oscillations in the resistivity of Sr3Ir4Sn13 and compared it to density functional theory (DFT) calculations for both scenarios. Our results strongly suggest that the former interpretation is correct. The structural transition temperature $T^*$ in $A_3T_4$Sn$_{13}$ can be suppressed to zero by tuning with physical or chemical pressure. In (Ca$_x$Sr$_{1−x}$)$_3$Rh$_4$Sn$_13$, the quantum critical point can be accessed purely by chemical substitution at x ~ 0.9. In the vicinity of the QCP, we expect large fluctuations of the order parameter at low temperatures, which for a structural transition could manifest as a structural disorder. We have measured thermal conductivity at temperatures much lower than $T_c$ and found that it is well described by a single power law with suppressed exponents near the QCP. The heat capacity, however, remains ~ $T^3$. After excluding conventional phonon scattering mechanisms, we propose the possibility of intrinsic quasi-static spatial disorder that is related to the structural QCP. YFe$_2$Ge$_2$ is closely linked to the “122” family of iron-based superconductors like KFe$_2$As$_2$, although it has a significantly lower $T_c$ ~ 1 K. It has a rather three-dimensional Fermi surface which closely resembles that of KFe$_2$As$_2$ in the pressure-induced collapsed tetragonal phase. YFe$_2$Ge$_2$ is in proximity to several types of magnetic order which are predicted by DFT calculations to have lower energy than the non-spin polarised case. Even though YFe$_2$Ge$_2$ is non-magnetic, its superconductivity could be strongly affected by magnetic fluctuations. Through a collaboration with researchers at the University of Waterloo, we have measured the thermal conductivity of YFe$_2$Ge$_2$ down to millikelvin temperatures and up to 2.5 T in field. Our results suggest that YFe$_2$Ge$_2$ is a nodal superconductor. This result could assist in the explanation of the unconventional superconductivity in iron-based superconductors.

U-Pu-Zr nuclear fuels experience a redistribution of constituents and a number of phase transformations when subjected to the thermal gradient present in nuclear reactors. This redistribution and phase separation leads to several undesirable fuel performance issues. In an effort to better understand how different alloys compositions are affected by this thermal gradient, we utilize the recently introduced Hybrid Potts-phase Field Method to study the U-Pu-Zr system. The recently introduced Hybrid method couples microstructural and compositional evolutions of a system so that the two phenomena can be studied together rather than separately, as is frequently done. However, simulation of the U-Pu-Zr system required several adaptations to the modeling framework. First the model was adapted to incorporate a thermodynamic database for free energy calculations, as well as thermal diffusion (the Soret effect). These abilities were tested in the Al-Si system. Second, the modeling framework was expanded to simulate three component systems such that ternary U-Pu-Zr alloys could be studied.Simulations capture constituent redistribution and the appropriate phase transformations as compared to experimentally irradiated a U-16Pu-23Zr (at%) nuclear fuel. Additional simulations analyze constituent redistribution over the entire spectrum of U-Pu-Zr compositions. Analysis of these simulation results indicate alloys that are likely to experience minimal constituent redistribution and fewer phase boundaries, such that their fuel performance should be improved. The outcomes of the work include a coupled microstructural-compositional modeling framework for ternary alloys and suggestions of U-Pu-Zr alloys that could lead to improved fuel performance.

Orientador: Wei-Liang Qian
Resumo: Uma característica vital da cromodinâmica quântica (QCD) está relacionada à simetria quiral. Isso é particularmente intrigante devido ao papel crítico da simetria quiral não abeliana dos spinores de Lorentz na física teórica moderna. Muitos esforços teóricos foram dedicados à sua quebra espontânea no vácuo, bem como a restauração da mesma no ambiente extremamente quente ou denso. Além disso, quarks e glúons tornam-se os graus de liberdade relevantes por meio da transição de desconfinamento do estado dos hádrons. O significado desta última está intimamente ligado às implicações da equação de Callan-Symanzik e à teoria do grupo renormalizado. No entanto, em princípio, ambas as transições acima podem ser descritas pela QCD. Os estudos da QCD na rede demonstraram que a transição do sistema é um cruzamento suave com a densidade bariônica nula e a massa de quarks estranhos grandes. No potencial químico finito, por outro lado, uma variedade de modelos prevê a ocorrência de uma transição de fase de primeira ordem entre a fase hadrônica e o plasma de quarks e glúons (QGP). Esses resultados indicam que um ponto crítico (CEP) pode estar localizado em algum lugar no diagrama de fases da QCD no qual a linha de transições de fase de primeira ordem termina. Espera-se que a transição seja de segunda ordem neste caso. De fato, entre outros objetivos estabelecidos, o programa Beam Energy Scan (BES) em andamento no Relativistic Heavy Ion Collider (RHIC) é impulsionado pela busca do CEP. Nesta t... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: One vital characteristic of the quantum chromodynamics (QCD) is regarding the chiral symmetry. This is particularly intriguing owing to the critical role of non-abelian gauge symmetry of Lorentz spinors in modern theoretical physics. Many theoretical efforts have been devoted concerning its spontaneously breaking in the vacuum, as well as the restoration at the extremely hot or dense environment. Furthermore, quarks and gluons become the relevant degrees of freedom through the deconfinement transition from the hadron state of matter. The significance of the latter is closely connected to the implications of the Callan-Symanzik equation and the theory of the renormalized group. Nonetheless, in principle, both of the above transitions can be described by the QCD. Lattice QCD studies demonstrated that the transition of the system is a smooth crossover at vanishing baryon density and large strange quark mass. At finite chemical potential, on the other hand, a variety of models predict the occurrence of a first-order transition between the hadronic phase and quark-gluon plasma (QGP). These results indicate that a critical endpoint (CEP) might be located somewhere on the QCD phase diagram at which the line of first-order phase transitions terminates. The transition is expected to be of second-order at this point. As a matter of fact, among other established goals, the ongoing Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC) is driven by the search for th... (Complete abstract click electronic access below)
Doutor

The yttria-stabilized zirconia (YSZ) system has been extensively studied because of its critical applications, like solid oxide fuel cells (SOFCs), oxygen sensors, and jet engines. However, there are still important questions that need to be answered and significant thermodynamic information that needs to be provided for this system. There is no predictive tool for the ionic conductivity of the cubic-YSZ (c-YSZ), as an electrolyte in SOFCs. In addition, no quantitative diagram is available regarding the oxygen ion mobility in c-YSZ, which is highly effective on its ionic conductivity. Moreover, there is no applicable phase stability diagram for the nano-YSZ, which is applied in oxygen sensors. Phase diagrams are critical tools to design new applications of materials. Furthermore, even after extensive studies on the thermodynamic database of the YSZ system, the zirconia-rich side of the system shows considerable uncertainties regarding the phase equilibria, which can make the application designs unreliable. During this dissertation, the CALPHAD (CALculation of PHase Diagrams) approach was applied to provide a predictive diagram for the ionic conductivity of the c-YSZ system. The oxygen ion mobility, activation energy, and pre-exponential factor were also predicted. In addition, the CALPHAD approach was utilized to predict the Gibbs energy of bulk YSZ at different temperatures. The surface energy of each polymorph was then added to the predicted Gibbs energy of bulk YSZ to obtain the total Gibbs energy of nano-YSZ. Therefore, a 3-D phase stability diagram for the nano-YSZ system was provided, by which the stability range of each polymorph versus temperature and particle size are presented. Re-assessment of the thermodynamic database of the YSZ system was done by applying the CALPHAD approach. All of the available thermochemical and phase equilibria data were evaluated carefully and the most reliable ones were selected for the Gibbs energy optimization process. The results calculated by the optimized thermodynamic database showed good agreement with the selected experimental data, particularly on the zirconia-rich side of the system.

Cold Spray is a solid-state additive manufacturing process that uses metallic feedstock powders to create layers on a substrate through plastic deformation. This process can be used for the repair of mechanical parts in the aerospace industry as well as for structural applications. Aluminum alloy powders, including Al 6061, 7075, 2024, and 5056, are typically used in this process as feedstock material. Since this process takes place all in the solid state, the properties and microstructure of the initial feedstock powder directly influence the properties of the final consolidated Cold Spray part. Given this, it is important to fully understand the internal powder microstructure, specifically the secondary phases as a function of thermal treatment. This work focuses on the understanding of the internal microstructure of Al 6061, 7075, 2024, and 5056 through the use of light microscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, electron backscatter diffraction, and differential scanning calorimetry. Thermodynamic models were used to predict the phase stability in these powders and were calibrated using the experimental results to give a more complete understanding of the phase transformations during thermal processing.

Glass-ceramics are of growing interest due to their enhanced properties compared to the base glasses. More specifically, the control of microstructures is a major challenge as the properties of glass-ceramics are the direct consequences of microstructures. Microstructures can be modified by forming specific crystal phases or by using a prior amorphous phase separation before crystallization. The PhD thesis objectives are to demonstrate that the properties of silicate glasses can be enhanced by controlling their microstructure genesis with composition and thermal process parameters. More specifically, two systems were studied and compared: the BaO-TiO2-SiO2 system and the soda-lime silica Na2O-CaO-SiO2 used industrially. Both systems exhibit a large zone of immiscibility allowing the study of the influence of phase separation on crystallization.

The first system BaO-TiO2-SiO2 has gathered interest from the interesting properties of fresnoite (Ba2TiSi2O8): piezo and pyroelectricity, second harmonic generation and blue/white photoluminescence. Many studies on the stoichiometric composition were conducted to understand and improve those promising properties. However, it was recently suggested that the photoluminescence can be improved with composition exhibiting phase separation. This indicates that the photoluminescence intensity can be improved through a microstructural control. The possible role of a prior amorphous phase separation on the subsequent crystallization has been however the topic of vigorous debates over the last decades and has not yet been clarified, especially regarding the role of the interfaces created by the phase separation. In this PhD, the effect of phase separation on fresnoite crystallization was studied. This had to pass through the calculation of the liquid-liquid immiscibility in the phase diagram in order to select suitable compositions to compare in a systematic study. The systematic study concludes to a surface crystallization mechanism for all non- stoichiometric compositions and shows no influence between amorphous droplets and matrix crystallization. This study was also completed with the investigation of the effect of composition (i.e. SiO2-excess), annealing temperature and prior heat treatment, i.e. heating rate, cooling rate or a prior isothermal step before annealing. It is shown that specific microstructures are obtained depending on the process parameters. Finally, selected compositions and heat treatment show how photoluminescence intensity can be improved by a microstructural control. The highest intensity is obtained with a high crystallization fraction and a maximization of the number of interfaces.

The results obtained in the study of the BaO-TiO2-SiO2 system are extended to the soda-lime-silica system in order to study the effect of phase separation on crystallization. It is shown that cristobalite forma- tion from the surface cannot be avoided and that the involved composition shift inhibits phase separation. It is consequently difficult to observe an interplay. Those studies lead to a general discussion about the criteria allowing to observe an interplay between phase separation and crystallization in oxide glasses.
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

The following thesis details the structural and thermo-dynamic analysis of melittin, mutant tryptophan cage constructs and a series of truncated murine β-defensin 14 derivatives in solution, in vacuo and in silico. These experiments are used to identify correlations between their solvated and desolvated structural isoforms and, in doing so, investigate the role of mass spectrometry in the field of structural proteomics. The integrated experimental approach described in this thesis employs an array of analytical techniques including charge partition calculations, capillary-induced thermal degradation, gas phase hydrogen/deuterium exchange and collision induced dissociation to obtain a consensus view of the structural organisation of each analyte. Supporting evidence is presented from antimicrobial assays, molecular modelling and dynamics calculations, circular dichroism and fluorescence spectroscopy and ion mobility mass spectrometry. In addition, a novel gas inlet system is detailed which allows gas phase ion/molecule reactions, such as hydrogen/deuterium exchange, to be performed within the quadrupole ion traps of a Finnigan LCQ Classic. Significant differences are observed between the solvated and desolvated structures of most of the analyte ions examined, unless they exhibit a very stable structural fold. This suggests an important role for mass spectrometry in the field of structural proteomics, but, one that is limited to the comparative study of different solvation states, the analysis of biologically important membrane interactions and the analysis of peptides and proteins that are expressed or secreted into hydrophobic environments.

In view of the important applications of carbides and carbo-nitrides of transition metals in the heat-resistant and hard materials industries, the thermodynamic activities of Cr and Mn in the Cr-C, Fe-Cr-C, Mn-Ni-C and Mn-Ni-C-N systems have been studied in the present work by the use of the galvanic cell technique. CaF₂ single crystals were used as the solid electrolyte. The phase relationships in selected regions of the systems in question were investigated by the use of the equilibration technique. The phase compositions and microstructures of the alloys were analysed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).

In the Cr-C system, the Gibbs energy of formation of Cr3C2 were obtained from ElectroMotive Force (EMF) measurements conducted in the temperature range 950-1150 K. The values of the enthalpy of formation of Cr₃C₂ were evaluated by the third-law method. The ground-state energy of the hypothetic end-member compound CrC3, in the bcc structure at 0 K, was calculated by use of the Ab-initio method. Based on the obtained results the Cr-C system was reassessed by use of the CALPHAD approach.

In the Fe-Cr-C system, 16 different alloys were quenched at 1223 K and their equilibrium phases identified by XRD. The experimental results show that the substitution of Cr by Fe in the (Cr,Fe)₇C₃ carbide changes the lattice parameters of the phase. A slight decrease of the lattice parameters with an increase in the Fe content was established. The lattice parameters of the γ-phase in the Fe-Cr solid solution did also show a decrease with an increase of the Fe content. The activities of chromium in Fe-Cr-C alloys were investigated in the temperature range 940-1155 K. The activity of chromium decreases with an increase in the Fe content when the ratio of C/(Cr+C) was constant. It was also established that the activity of chromium decreases with an increase of the carbon content when the iron content was constant. The experimental results obtained were compared with the data calculated by use of the Thermo-Calc software.

In the Mn-Ni-C system the phase relationships were investigated at 1073 K as well as at 1223 K. The experimental results obtained showed that the site fraction of Ni in the metallic sublattice of the carbides M₂₃C₆, M₇C₃ and M₅C₂ (M=Mn and Ni) was quite low (approximately 2~3 percent). The activities of manganese in Mn-Ni-C alloys were investigated in the temperature range 940-1165 K. The three-phase region γ/M₇C₃/graphite was partly constructed at 1073 K.

In the Mn-Ni-C-N system, nitrogen was introduced into Mn-Ni-C alloys by equilibrating with N2 gas. It was established that the solubility of nitrogen in the investigated alloys was effected by the carbon content, and that a (Mn,Ni)₄(N,C) compound was formed in the nitrided alloys. EMF measurements were performed on Mn-Ni-C-N alloys in the temperature interval 940-1127 K. The addition of nitrogen to Mn-Ni-C alloys was found to decrease the activity of manganese. The negative effect of nitrogen on the activity of manganese was found to decrease as the carbon content increased.

Keywords: Thermodynamic activity; Galvanic cell technique; Transition metal carbides; Transition metal nitrides; Phase equilibrium; Thermodynamics; Differential thermal analysis; Scanning electron microscopy; Transmission electron microscopy; Ab-initio calculations; CALPHAD approach;

Thesis (MScEng)--Stellenbosch University, 2004.
ENGLISH ABSTRACT: During the production of pure Ti02 for the pigment industry, ilmenite, containing 35 - 60 % Ti02, is reduced to high titania slag, containing 85 - 95 % Ti02 and pig iron. These ilmenite smelters are operated in very tight operating windows. Over reducing the slag may lead to the formation of TiC and reducing much of the Ti02 to Th03. According to Namakwa Sands furnace operators, this does not only affect the grade of the product, but it can cause slag foaming and furnace eruptions. In under reducing conditions, the liquid slag is fluxed by the FeO and may corrode the furnace lining and consequently lead to run-outs. The reducing conditions in the furnace are not only controlled by carbon addition, but also by temperature. Standard practise in industrial ilmenite smelters is to operate the furnace with a slag freeze lining to protect the refractory lining from chemical and physical attack by the slag. It is therefore clear that it is of great importance to be able to predict the slag liquidus temperature at different compositions. This can help the operator to avoid dangerous operating conditions. Over the past few decades, a number of solution models have been developed to describe non-ideal solutions. With the rapid increase in computer power, these models became more valuable and practical to use in advanced control and decisionsupport. In this study, some of the better-known models are discussed and evaluated for the Ti02 - Th03 - FeO system, based on a critical review of properties and measurements published in literature. Two of these models, the "modified quasi-chemical" model and the "cell" model were chosen to be applied to the high-titania slag system. Both these models are based on statistical thermodynamics with some differences in the initial assumptions. In this study, the model parameters for the cell model were regressed from experimental data. The high-titania slag produced, consists mainly of titanium in different oxidation states and FeO, placing its composition inside the Ti02 - Th03 - FeO ternary system. Reliable experimental data for this system are very limited. All three binary systems contained in the Ti02 - ThO) - FeO system were considered, namely FeO - Ti02, Ti02 - ThO) and FeO - ThO). Only liquidus data for these three binaries were used to regress the model parameters. Accuracy of the models was determined by calculating the root mean square (RMS) error between the experimental data point and the value calculated using the model and the newly determined model parameters. These errors corresponded weil with the reported experimental error of the datasets for both the models and all the binary systems. Due to the fact that this study focussed on the liquidus surface of the system, the results were also plotted in the form of binary phase diagrams and ternary liquidus isotherms. The cell model uses only binary interaction parameters to describe the ternary system. These parameters are not expanded to higher order polynomials, which makes this model more robust, but also less accurate than other models such as the modified quasi-chemical model.
AFRIKAANSE OPSOMMING: Tydens die produksie van suiwer Ti02 vir die pigmentbedryf, word ilmeniet, wat 35 tot 60 % Ti02 bevat, gereduseer tot 'n hoë titaan slak, met 'n Ti02 inhoud van 85 tot 95 % Ti02, en potyster. Hierdie ilmeniet smeltoonde word binne baie nou bedryfskondisies beheer. Oor-redusering van die slak kan lei tot the formasie van TiC en die redusering van Ti02 tot Th03. Dit affekteer nie net die produk se kwaliteit nie, maar kan volgens Namakwa Sands oond operateurs ook slak skuiming en ontploffings tot gevolg hê. Gedurende onder-reduserende omstandighede in die oond, word die vloeibaarheid van die slak verhoog deur die hoër FeO inhoud in die slak. Dit maak die slak meer korrosief en kan lei tot faling van die vuurvaste stene. Die mate van redusering in die oond word nie net bepaal deur die toevoeging van koolstof nie, maar ook deur die temperatuur van die slak. Dit is 'n standaard praktyk van die industrie om die oond te bedryf met 'n gevriesde slak laag om sodoende die vuurvaste stene te beskerm teen chemiese en fisiese aanval van die slak. Dit is dus duidelik dat dit baie belangrik is om die slak se smeltpunt by verskillende samestellings te kan voorspel. Dit kan die operateur help om die oond binne veilige bedryfskondisies te hou. 'n Hele aantaloplossingsmodelle is oor die afgelope paar dekades ontwikkel vir die beskrywing van nie-ideale oplossings. Hierdie modelle het oor die afgelope paar jaar baie toegeneem in praktiese waarde as gevolg van die snelle toename in rekenaarkapasiteit en -spoed. Dit het veral groot waarde in gevorderde beheerstelsels en besluitneming steun. Sommige van die meer bekende modelle word in hierdie studie bespreek en ge-evalueer vir die Ti02 - Th03 - FeO stelsel, gebaseer op 'n kritiese evaluasie van eienskappe en eksperimentele data gepubliseer in die literatuur. Twee van hierdie modelle, die "gemodifiseerde kwasi-chemiese" model en die "sel" model, is gebruik om die hoë titaan slak stelsel te beskryf. Beide hierdie modelle is gebaseer op statistiese termodinamika en het klein verskille m.b.t. die aanvanklike aannames. Die model veranderlikes vir die sel model is in hierdie studie afgelei vanaf die eksperimentele data. Die hoë titaan slak wat tydens hierdie proses geproduseer word, bestaan hoofsaaklik uit FeO en titaan in sy verskillende oksidasie toestande. Dit plaas die samestelling van die slak reg binne die Ti02 - Th03 - FeO temêre stelsel. Betroubare eksperimentele data vir hierdie stelsel is baie beperk. In hierdie studie word daar gekyk al drie binêre stelsels binne die Ti02 - Th03 - FeO temêre stelsel, naamlik: FeO - Ti02, Ti02 - Th03 en FeO - Th03. Slegs die smeltpunt temperatuur data vir hierdie twee binêre is gebruik in die afskatting van die model veranderlikes. Die akkuraatheid van die modelle is bepaal deur die wortel van die gemiddelde kwadraat van die fout tussen die eksperimentele waardes en die berekende waardes te bepaal. Albei die modelle het 'n relatiewe klein fout in vergelyking met die geraporteerde eksprimentele fout gehad vir al die binêre stelsels. Hierdie studie het gefokus op die smeltpunt temperatuur van die slak en die resultate is daarom ook in die vorm van binêre fasediagramme en isoterme projeksies op die temêre fasediagramme gestip. Die "sel" model gebruik slegs binêre interaksie parameters om die temêre stelsel te beskryf Hierdie parameters word vir die "sel" model nie uitgebrei tot hoër order polinome en dit maak die "sel" model meer robuust, maar minder akkuraat as ander modelle soos byvoorbeeld die "kwasi-chemiese" model.

In this dissertation, we will study solvent-vapor induced spherulitic growth in multicomponent thin films modeled as prestressed elastic solids. The interface between the crystalline phase and the amorphous phase will be treated as an evolving thermodynamic system and no diffusion of any component will be considered. The dissertation is divided into three parts. In Part I we will determine necessary conditions of thermodynamic equilibrium between the two solid phases, the inter- face, and the vapor. In Part II we will derive the thermodynamic driving force for spherulitic growth in multicomponent elastic thin films. In Part III we will investigate the effect of prestress on the directional dependence of the growth. There a formula that delineates how the prestress affects the shape of the spherulite will be proposed.

The specific heat Cp has been showed at various temperatures in the literature, which shows a sharp increase labeled as the lambda-transition at the critical temperature. This transition has been observed previously among the phases of solid-nematic-isotropic liquid in p-azoxyanisole (PAA) and anisaldazine (AAD), and among the phases of solid-smectic-cholesteric-isotropic liquid in cholesteryl myristate (CM). In this thesis work, we analyze the experimental data for the temperature dependence of Cp and the thermal expansion alpha_p and also pressure dependence of alpha_p by a power-law formula. From the analysis of pressure dependence of alpha_p, we calculate the temperature dependencies of specific heat Cp and of the isothermal compressibility kappa_T for the phase transitions considered in PAA, AAD and CM. Our calculations for the temperature dependence of the p and kappa_T can be compared with the experimental data when available in the literature. Polarization, tilt angle and the dielectric constant have been reported in the literature at various temperatures close to the solid-smectic C*-smectic A-isotropic liquid transition in the ferroelectric liquid crystals of A7 and C7. The mean field model with the free energy expanded in terms of the order parameters (polarization and tilt angle) has been reported in the literature previously. In this thesis work, we apply the mean field model first time by fitting the expressions derived for the temperature dependence of the polarization, tilt angle and the dielectric constant to the experimental data for A7 and C7 from the literature. Since the mean field model studied here describes adequately the observed behaviour of A7 and C7, the expressions for the temperature dependence of the polarization, tilt angle and the dielectric constant which we derive, can also be applied to some other ferroelectric liquid crystals to explain their observed behaviour.