Dissertations / Theses on the topic 'Solid state gas phase equilibria'

Die ternären Systeme Ti/P/Te, Ce/P/Te und Si/P/Te wurden in der vorliegenden Dissertation hinsichtlich der Existenz neuer Verbindungen untersucht. Diese Verbindungen wurden insbesondere in Bezug auf ihre thermochemischen Eigenschaften charakterisiert. Durch Kombination von experimentellen Untersuchungen und thermodynamischen Modellierungen der ablaufenden Festkörper-Gasphasen-Gleichgewichte konnten die Synthesen der neuen Verbindungen optimiert werden. Abschließend erfolgte zudem die physikalische und kristallographische Charakterisierung der gefundenen Phosphidtelluride.

Die ternären Systeme Ti/P/Te, Ce/P/Te und Si/P/Te wurden in der vorliegenden Dissertation hinsichtlich der Existenz neuer Verbindungen untersucht. Diese Verbindungen wurden insbesondere in Bezug auf ihre thermochemischen Eigenschaften charakterisiert. Durch Kombination von experimentellen Untersuchungen und thermodynamischen Modellierungen der ablaufenden Festkörper-Gasphasen-Gleichgewichte konnten die Synthesen der neuen Verbindungen optimiert werden. Abschließend erfolgte zudem die physikalische und kristallographische Charakterisierung der gefundenen Phosphidtelluride.

The Ph.D. project compiled in this thesis has focused on the role of the solvent in solid-liquid phase equilibria and in nucleation kinetics. Six organic substances have been selected as model compounds, viz. ortho-, meta- and para-hydroxybenzoic acid, salicylamide, meta- and para-aminobenzoic acid. The different types of crystal phases of these compounds have been explored, and their respective solid-state properties have been determined experimentally. The solubility of these crystal phases has been determined in various solvents between 10 and 50 oC. The kinetics of nucleation has been investigated for salicylamide by measuring the metastable zone width, in five different solvents under different experimental conditions. A total of 15 different crystal phases were identified among the six model compounds. Only one crystal form was found for the ortho-substituted compounds, whereas the meta-isomeric compounds crystallized as two unsolvated polymorphs. The para-substituted isomers crystallized as two unsolvated polymorphs and as several solvates in different solvents. It was discovered that the molar solubility of the different crystal phases was linked to the temperature dependence of solubility. In general, a greater molar solubility corresponds to a smaller temperature dependence of solubility. The generality of this relation for organic compounds was investigated using a test set of 41 organic solutes comprising a total of 115 solubility curves. A semi-empirical solubility model was developed based on how thermodynamic properties relate to concentration and temperature. The model was fitted to the 115 solubility curves and used to predict the temperature dependence of solubility. The model allows for entire solubility curves to be constructed in new solvents based on the melting properties of the solute and the solubility in that solvent at a single temperature. Based on the test set comprising the 115 solubility curves it was also found that the melting temperature of the solute can readily be predicted from solubility data in organic solvents. The activity of the solid phase (or ideal solubility) of four of the investigated crystal phases was determined within a rigorous thermodynamic framework, by combining experimental data at the melting temperature and solubility in different solvents and temperatures. The results show that the assumptions normally used in the literature to determine the activity of the solid phase may give rise to errors up to a factor of 12. An extensive variation in the metastable zone width of salicylamide was obtained during repeated experiments performed under identical experimental conditions. Only small or negligible effects on the onset of nucleation were observed by changing the saturation temperature or increasing the solution volume. The onset of nucleation was instead considerably influenced by different cooling rates and different solvents. A correlation was found between the supersaturation ratio at the average onset of nucleation and the viscosity of the solvent divided by the solubility of the solute. The trends suggest that an increased molecular mobility and a higher concentration of the solute reduce the metastable zone width of salicylamide.
QC 20100831

Thesis (Ph. D.)--School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 2005. Directed by Charles A. Eckert.
Charles L. Liotta, Committee Co-Chair ; Charles A. Eckert, Committee Chair ; Amyn S. Teja, Committee Member ; J. Carson Meredith, Committee Member ; Rigoberto Hernandez, Committee Member. Vita. Includes bibliographical references.

xiv, 195 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.
We explore the reconstruction of B-state vibrational wave packets in I 2 from simulated two-color nonlinear wave packet interferometry data. As a simplification of earlier proposals, we make use of different vibrational energy ranges in the B-state--rather than different electronic potential surfaces--for the short-pulse preparation and propagation of both target and reference wave packets. Numerical results from noisy interferograms indicate that experimental reconstruction should be possible with high fidelity (>0.99). Time-resolved coherent nonlinear optical experiments on small molecules in low-temperature host crystals are exposing valuable information on quantum mechanical dynamics in condensed media. We make use of generic features of these systems to frame two simple, comprehensive theories that will enable the efficient calculation of their ultrafast spectroscopic signals and support their interpretation in terms of the underlying chemical dynamics. Both treatments rely on the identification of normal coordinates to unambiguously partition the well-structured guest-host complex into a system and a bath and expand the overall wave function as a sum of product states between fully anharmonic vibrational basis states for the system and approximate Gaussian wave packets for the bath degrees of freedom. The theories exploit the fact that ultrafast experiments typically drive large-amplitude motion in a few intramolecular degrees of freedom of higher frequency than the crystal phonons, while these intramolecular vibrations indirectly induce smaller-amplitude--but still perhaps coherent--motion among the lattice modes. The equations of motion for the time-dependent parameters of the bath wave packets are fairly compact in a fixed vibrational basis/Gaussian bath (FVB/GB) approach. An alternative adiabatic vibrational basis/Gaussian bath (AVB/GB) treatment leads to more complicated equations of motion involving adiabatic and nonadiabatic vector potentials. Numerical tests of the FVB/GB are presented. We consider two bilinearly coupled harmonic oscillators with varying coupling strengths and initial conditions and show that the mixed quantum/semiclassical theory compares favorably with the exact results. Linear absorption spectra and wave-packet interferometry signals calculated using the theory are presented. This dissertation includes previously published coauthored material.
Committee in charge: David Herrick, Chairperson, Chemistry; Jeffrey Cina, Advisor, Chemistry; Thomas Dyke, Member, Chemistry Michael Kellman, Member, Chemistry; Hailin Wang, Outside Member, Physics

Los materiales conductores mixtos de electrones e iones (oxígeno o protones) son capaces de separar oxígeno o hidrógeno de los gases de combustión o de corrientes de reformado a alta temperatura. La selectividad de este proceso es del 100%. Estos materiales, óxidos sólidos densos, pueden usarse en la producción de electricidad a partir de combustibles fósiles, así como formar parte de los procesos que forman parte del sistema de captura y almacenamiento de CO2. Las membranas de transporte de oxígeno (MTO) se pueden utilizar en las plantas energéticas con procesos de oxicombustión, así como en reactores catalíticos de membrana (RCM), mientras que las membranas de transporte de hidrógeno (MTH) se aplican en procesos de precombustión. Además, estos materiales encuentran aplicación en componentes de sistemas energéticos, como electrodos o electrolitos de pilas de combustible de óxido sólido, de ambas clases iónicas y protónicas (SOFC y PC-SOFC). Los procesos mencionados implican condiciones de operación muy severas, como altas temperaturas y grandes gradientes de presión parcial de oxígeno (pO2), probablemente combinadas con la presencia de CO2 and SO2. Los materiales más que mayor rendimiento de separación presentan y más ampliamente investigados en este campo son inestables en estas condiciones. Por tanto, existe la necesidad de encontrar nuevos materiales inorgánicos estables que proporcionen alta conductividad electrónica e iónica. La presente tesis propone una búsqueda sistemática de nuevos conductores iónicos-electrónicos mixtos (MIEC, del inglés) con diferente estructura cristalina y/o diferente composición, variando la naturaleza de los elementos y la estequiometría del cristal. La investigación ha dado lugar a materiales capaces de transportar iones oxígeno, protones o cargas electrónicas y que son estables en las condiciones de operación. La caracterización de una amplia serie de cerias (CeO2) dopadas con lantánidos proporciona una comprensión general de las propiedades estructurales y de transporte, así como la relación entre ellas. Además, se estudia el efecto de la adición de cobalto a dicho sistema. Se ha completado el análisis con la optimización de las propiedades de trasporte a partir de la microestructura. Todo esto permite hacer una clasificación inicial de los materiales basada en el comportamiento de transporte principal y permite adecuar la estructura y las condiciones de operación para obtener las propiedades deseadas para cada aplicación. Algunos de los materiales extraídos de este estudio alcanzaron las expectativas. Las familias de materiales basadas en Ce1-x Tbx O2-¿ y Ce1-x Tbx O2-¿ +2 mol% Co proporcionan flujos de oxígeno bajos pero competitivos, ya que son estables en atmósferas con CO2. Además, la inclusión de estos materiales en membranas de dos fases aumenta el flujo de oxígeno. La combinación con una espinela libre de cobalto y de metales alcalinotérreos como es el Fe2 NiO4, ha dado lugar a un material prometedor en cuanto a flujo de oxígeno y estabilidad en CO2 y en SO2, que podría ser integrado en el proceso de oxicombustión. Por otra parte, se ha añadido metales como codopantes en el sistema Ce0.9-x Mx Gd0.1O1.95. Estos materiales, en combinación con la perovskita La1- x Srx MnO3 usada comúnmente como cátodo de SOFC, han sido capaces de disminuir la resistencia de polarización del cátodo. La mejora es consecuencia de la introducción de conductividad iónica por parte de la ceria. Las perovskitas dopadas basadas en CaTiO3 forman el segundo grupo de materiales investigados. La dificultad de obtener perovskitas estables y que presenten conducción mixta iónica y electrónica se ha hecho evidente. De entre los dopantes utilizados, el hierro y la combinación hierro-magnesio han sido los mejores candidatos. Ambos materiales presentan conductividad principalmente iónica a alta temperatura, mientras que a baja predomina la conductividad electrónica tipo p. CaTi0.73Fe0.18Mg0.09O3-¿ se ha mostrado como un material competente en la fabricación de membranas de oxígeno, que proporciona flujos adecuados a la par que estabilidad en CO2. Finalmente, la perovskita La0.87Sr0.13CrO3 (LSC) ha sido dopada con el objetivo de aumentar la conductividad mixta protónica electrónica. Este estudio ha llevado al desarrollo de una nueva generación de ánodos para PC-SOFC basadas en electrolitos de LWO. Las perovskitas dopadas con Ce en el sitio del La (LSCCe) y con Ni en el sitio del Cr (LSCN) son estables en condiciones de operación reductoras, así como en contacto con el electrolito. El uso de ambos materiales como ánodo disminuye la resistencia de polarización con respecto al LSC. El LSCCe está limitado por los procesos que ocurren a baja frecuencia (BF), relacionados con los procesos superficiales, y que son atenuados en el caso del LSCN debido a la formación de nanopartículas de Ni metálico en la superficie. La infiltración posterior con nanopartículas de Ni permite disminuir la resistencia a BF lo que sugiere que la reacción superficial de oxidación del H2 está siendo catalizada. La infiltración más concentrada en Ni (5Ni) elimina completamente la resistencia a BF en ambos ánodos, de forma que los procesos que ocurren a altas frecuencias son ahora limitantes. El ánodo constituido por LSCNi20+5Ni dio una resistencia de polarización de 0.26 ¿·cm 2 at 750 ºC en H2 húmedo.
Mixed ionic (oxygen ions or protons) and electronic conducting materials (MIEC) separate oxygen or hydrogen from flue gas or reforming streams at high temperature in a process 100% selective to the ion. These solid oxide materials may be used in the production of electricity from fossil fuels (coal or natural gas), taking part of the CO2 separation and storage system. Dense oxygen transport membranes (OTM) can be used in oxyfuel combustion plants or in catalytic membrane reactors (CMR), while hydrogen transport membranes (HTM) would be applied in precombustion plants. Furthermore, these materials may also be used in components for energy systems, as advanced electrodes or electrolytes for solid oxide fuel cells (SOFC) and proton conducting solid oxide fuel cells (PCSOFC) working at high and moderate temperature. The harsh working conditions stablished by the targeted processes include high temperatures and low O2 partial pressures (pO2), probably combined with CO2 and SO2 containing gases. The instability disadvantages presented by the most widely studied materials for these purposes make them impractical for application to gas separation. Thus, the need to discover new stable inorganic materials providing high electronic and ionic conductivity is still present. This thesis presents a systematic search for new mixed ionic-electronic conductors. It includes different crystalline structures and/or composition of the crystal lattice, varying the nature of the elements and the stoichiometry of the crystal. The research has yielded new materials capable to transport oxygen ions or protons and electronic carriers that are stable in the working condition to which they are submitted.
Balaguer Ramírez, M. (2013). New solid state oxygen and hydrogen conducting materials. Towards their applications as high temperature electrochemical devices and gas separation membranes [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/31654
TESIS
Premiado

Les composites Cu-Cr sont couramment utilisés comme matériaux de contact électrique pour ampoules à vide des disjoncteurs de moyenne tension. Pourtant très répandu, le frittage en phase solide de ces matériaux a été relativement peu étudié. L’optimisation du procédé passe par la compréhension des mécanismes de frittage. Cette étude est focalisée sur deux aspects importants du frittage : les processus d’oxydo-réduction liés aux oxydes de surfaces des poudres et la compétition entre mécanismes de densification et de gonflement au cours du frittage.L’oxydo-réduction a été étudiée par analyse thermogravimétrique couplée à différentes techniques de spectroscopie d’abord sur les matériaux purs puis sur les composites. Des analyses des interfaces par des coupes réalisées au FIB ont permis de préciser la localisation de l’oxyde dans les matériaux frittés. Un transfert d’oxygène a lieu entre les poudres de cuivre et de chrome. L’intensité de ce transfert dépend de la nature réductrice de l’atmosphère utilisée.La densification a été analysée par dilatométrie sur les matériaux purs et sur les composites. Ces analyses ont été appuyées par des observations microstructurales, notamment par tomographie des rayons X. L’effet des paramètres du procédé (atmosphère, vitesse de chauffage, poudres…) a été étudié. Les résultats montrent le lien entre la désoxydation des poudres de cuivre et le frittage. Un phénomène de gonflement du cuivre seul s’explique par le dégazage du cuivre à haute température lors de la fermeture des pores. Ce gonflement n’a pas lieu dans les composites Cu-Cr car le chrome retarde la fermeture des pores et piège les gaz émis par le cuivre en formant l’oxyde Cr2O3. L’atmosphère de frittage, la morphologie et la taille des poudres de chrome influent sur la densification. Le frittage sous vide permet de réduire la porosité. Une morphologie sphérique des particules de chrome limite l’effet inhibiteur de celui-ci sur la densification. Pour de faibles tailles de particules, le chrome participe à la densification, ce qui permet de mieux densifier le matériau. Ces résultats ouvrent des voies d’optimisation du procédé de frittage des matériaux.Les matériaux élaborés ont été testés dans leurs conditions d’utilisation, c'est-à-dire lors de coupures sur court-circuit en ampoule à vide. Ces essais ont montré l’intérêt de réduire la quantité d’oxyde de chrome et ont permis de déterminer l’effet des impuretés rencontrées usuellement sur les poudres de cuivre et de chrome
Cu-Cr composites are commonly used as contact materials for medium voltage circuit breakers vacuum bottles. Solid state sintering process of Cu-Cr composites is widespread but has been relatively little studied. Optimizing the process requires understanding the sintering mechanisms. This study was focused on two important aspects of sintering: the redox reactions associated to oxides on the powder surface and the competition between densification and swelling mechanisms during sintering.The redox reactions were studied by thermogravimetric analysis coupled to various spectroscopic techniques, first on isolated Cu and Cr, then on Cu-Cr composites. Interfaces analyses obtained by FIB clarified the location of the oxide inside the sintered materials. Oxygen transfer takes place between copper and chromium powders. This phenomenon strongly depends on the reducing character of the sintering atmosphere.Densification was analyzed by dilatometry on Cu, Cr and Cu-Cr composites. This analysis was supported by microstructural observations, including X-ray tomography .The effect of process parameters (atmosphere, heating rate, powders ...) was studied. The results show the relationship between sintering and copper oxide reduction. The swelling phenomenon of copper compacts is explained by high temperature degassing of copper during pore closure. This swelling does not occur in Cu-Cr composites as chromium delays pore closing and entraps the gases released by copper. Sintering atmosphere, chromium morphology and chromium particle size affect densification. Vacuum sintering reduces porosity. Chromium particles with spherical shape limit its inhibiting effect on densification. For small particle sizes, chromium participates to densification, leading to better densification of the material. These results open the route for optimizing the sintering of Cu-Cr composites.Cu-Cr composites were tested for short circuit performance in vacuum interrupters. The result of these tests showed the importance of reducing the chromium oxide amount. The effect of impurities commonly encountered on the powders copper and chromium powders was also determined

La cristallisation est un processus d’auto-assemblement de molécules à partir d’une phase désordonnée (liquide, amorphe ou gazeuse). De façon générale, la miscibilité à l’état solide entre un constituant d’intérêt et son impureté dépendent d’une part, de la structure moléculaire de ces deux espèces, et d’autre part, de la structure cristalline du réseau hôte, c’est-à-dire celui du composé d’intérêt. En cas d’absence de miscibilité, l’optimisation des conditions de cristallisation permet un retrait total de l’impureté du composé visé. Cette thèse vise à démontrer que, lorsque deux molécules sont semblables et que leur structure cristalline est peu dense, des solutions solides de substitutions peuvent se former et empêcher le retrait de l’impureté de l’espèce cible. Inversement, lorsque les impuretés sont très diférentes de l’espèce à purifer, l’absence de solution solide est observée et permet leur élimination totale grâce à une bonne discrimination à l’état solide. Le système du phénanthrène, un composé servant entre autres de base pour la synthèse de composésmorphiniques, a été choisi pour illustrer cette thèse. Diverses méthodes de cristallisation (fusion de zone, co-cristallisation, recristallisation en solution, sublimation-condensation) ont été testées et ont permis, après optimisation et combinaison, l’obtention à l’échelle préparative de phénanthrène pur à plus de 99,999 % en moles, ce qui le place sur l’échelle de l’ultra-pureté (pureté > 99,9 % en moles)
Crystallization is a process during which molecules self assembly from a disordered (liquid, amorphous or gaseous) phase. The miscibility in the solid state between a component of interest and its impurity depends on: (i) their molecular structures, (ii) the crystal structure of the host lattice (namely, that of the component of interest). When no solid solution exists, the impurity can be completely eliminated from the target product after optimization of the crystallization conditions.The present thesis intends to demonstrate that, when two compounds present similar molecular structures with low-density crystal structures, solid solution formation can occur which prevents from their complete separation. Conversely, in case of sufcient dissimilarity, no solid solution is stable and their separation is possible thanks to a large discrimination in the solid state. The phenanthrene system – a model compound used as base for the synthesis of morphine derivatives – was chosen to illustrate this thesis. Several crystallization methods (zone melting, co-crystallization, solvent assisted crystallization and sublimation-condensation) were investigated and permitted, after optimization and combination, to obtain 99.999(9) mole % purity phenanthrene (which is above the ultrapure grade of 99.9 mole %)

The objective of the present work is to better understand chemical evolution of both solid and gaseous phases during torrefaction of various biomasses. Torrefaction experiments were carried out with a dynamic profile of temperatures between 200 and 300°C, under inert atmosphere, for pine, ash-wood, miscanthus and wheat straw. Mass loss and formation of condensable species were analyzed by TGA-GC-MS, and chemical evolution of solid phase was characterized by 13C CP/MAS solid-state NMR. Thirty condensable species could be detected; a half of these species were formed during the whole temperature range, and a third were formed by all biomass types. The main phenomena that occurred in solid phase were found to be decrystallization of cellulose, severe degradation of hemicellulose, devolatilization of acetyl groups, conservation of methoxyl groups and charring. It was also found that mass loss and chemical evolution of solid were not directly correlated for different biomasses. Based on the experimental results, a conceptual model was developed to describe biomass degradation duringtorrefaction. Thirty reactions were determined for the three major macromolecular constituents, namely cellulose, hemicellulose – represented by C5 and C6 sugars – and lignin – represented by H, G and S units. The main innovations of this model are in thedetailed approach of hemicellulose and lignin compositions, as well as in the prediction of sixteen condensable and five permanent species, and six forms of solid char, through chemically meaningful and stoichiometrically valid reactions.

Das Phasendiagramm Sn - Mo - O wurde für 500°C und 1000°C experimentell abgeleitet. Als ternäre Phasen konnten SnMo2O8 und Sn1-xMo4-yO6-2y dargestellt werden. Die Phasen SnMo5O8 und Sn4,4Mo24O38 konnten unterhalb von 1000°C nicht dargestellt werden. Die Phase SnMo2O8 wurde über Festkörper - Gasphasenreaktion einkristallin dargestellt und auf ihr thermisches Verhalten untersucht. Anhand dieser Untersuchungen konnte das Phasendiagramm SnO2 - MoO3 abgeleitet werden. An dieser Phase wurden Einkristall-untersuchungen durchgeführt. Die Struktur der Phase SnMo2O8 wurde in der Raumgruppe Pa (a = 8,967 Å) gelöst. Die Zinnatome besetzen die Flächen - und Kantenmitten der Elementarzelle. Die Lage der Molybdän - und Sauerstoffpositionen kann über ein Fehlordnungsmodell beschrieben werden. Die Phase Sn1-xMo4-yO6-2y konnte über chemischen Transport mit Wasser einkristallin hergestellt werden. Einkristalluntersuchungen bestätigten eine Überstruktur zur NaMo4O6 - Struktur. Die Phase weist mit hoher Wahrscheinlichkeit eine Unterstöchiometrie im Zinngehalt, sowie im Molybdän - bzw. Sauerstoffgehalt auf. Diese Tatsache konnte durch Mößbaueruntersuchungen am Pulver von Sn1-xMo4-yO6-2y bestätigt werden. Eine Lösung der Struktur von Sn1-xMo4-yO6-2y war bisher nicht möglich. Die thermodynamischen Daten der Phasen SnMo2O8 und SnMo4O6 (vereinfacht für Sn1-xMo4-yO6-2y) konnten theoretisch abgeleitet werden. Das Zustandsdiagramm Sn - Mo - O wurde berechnet. Die Überprüfung der Daten erfolgte durch die Berechnung der Bodenkörper - Gasphasengleichgewichte mit dem Programm TRAGMIN. Mit den abgeleiteten Daten wurden Berechnungen zum chemischen Transportverhalten durchgeführt. Dabei wurde gefunden, dass sich die Phase Sn1-xMo4-yO6-2y über chemischen Transport abscheiden lässt. Das Phasendiagramm Pb - Mo - O wurde bis 1000°C experimentell abgeleitet. Die Phasen PbMoO4 und Pb2MoO5 konnten als einphasige Pulver hergestellt werden. Die Phase Pb0,75Mo4O6 konnte über chemischen Transport einkristallin abgeschieden werden. Beim Erhitzen auf 1250°C wurde die Zersetzung dieser Phase in die Phase PbMo5O8 und Mo beobachtet. Die thermodynamischen Daten der Phasen PbMoO4, Pb2MoO5, Pb5MoO8 und Pb0,75Mo4O6 konnten theoretisch abgeleitet werden. Das Zustandsdiagramm Pb - Mo - O wurde berechnet. Im Verlauf der Rechnungen wurde das Zustandsdiagramm PbO - MoO3 mit dem Programm CHEMSAGE berechnet und mit den Literaturdaten verglichen. Die Überprüfung der Daten erfolgte durch die Berechnung der Bodenkörper - Gasphasengleichgewichte mit dem Programm TRAGMIN. Mit den abgeleiteten Daten wurden Berechnungen zum chemischen Transportverhalten durchgeführt. Dabei wurde gefunden, dass sich die Phase Pb0,75Mo4O6 über chemischen Transport abscheiden lässt. Das Phasendiagramm Sb - Mo - O wurde bei 500°C bzw. 700°C experimentell abgeleitet. Im System existieren die Phasen Sb2MoO6 und Sb2Mo10O31. Sb2Mo10O31 konnte einphasig als Pulver hergestellt werden. Die Existenz einer Phase mit der Zusammensetzung Sb4Mo10O31 konnte nicht bestätigt werden. Es wurden Hinweise auf eine dritte ternäre Phase im System Sb - Mo - O gefunden. Die Phasen Sb2MoO6 und Sb2Mo10O31 konnten über chemischen Transport einkristallin dargestellt werden. Mößbaueruntersuchungen an Sb2Mo10O31 ergaben, dass in der Verbindung ausschliesslich dreiwertiges Sb vorliegt und somit Mo gemischtvalent sein muss. Die thermodynamischen Daten der Phasen Sb2MoO6 und Sb2Mo10O31 konnten theoretisch abgeleitet werden. Das Zustandsdiagramm Sb - Mo - O wurde berechnet. Die Überprüfung der Daten erfolgte durch die Berechnung der Bodenkörper - Gasphasengleichgewichte mit dem Programm TRAGMIN. Mit den abgeleiteten Daten wurden Berechnungen zum chemischen Transportverhalten durchgeführt. Dabei wurde gefunden, dass sich beide ternäre Phasen über chemischen Transport abscheiden lassen.

Although some physiological effects of toxic metal poisoning have been known for centuries, the specific chemical interactions between biological molecules and mercury(I), mercury(II) or lead(II) are not well understood. To date, only thirteen crystal structures of inorganic mercury-amino acid complexes and six crystal structures of lead-amino acid complexes have been reported with varying degrees of characterization. In order to improve our understanding of the coordination chemistry of mercury and lead in biological environments, a systematic method for the isolation of inorganic metal-amino acid complexes from acidic aqueous solutions has been developed. With this method we have prepared five new lead-amino acid complexes (with L-valine, L-isoleucine, L-phenylalanine, and L-arginine) and four new mercury-amino acid complexes (with L-alanine, D-alanine, L-proline, and N-methyl-L-alanine). These metal-amino acid complexes have been comprehensively characterized in the solid state, solution state and gas phase. The development of this isolation technique in conjunction with the exploration of a number of characterization techniques for studying metal-amino acid interactions greatly enhances the known methods by which metal-biological molecule systems are studied.

The present thesis reports computer simulation studies of several phase transition related phenomena in a range of soft-condensed matter systems. A coherent unifying theme of the thesis is the understanding of dynamics of phase transitions through free energy calculations using recently developed efficient non-Boltzmann sampling methods. Based on the system/phenomena of interest, the thesis has been classified into four major parts: I. Isotropic-nematic (IN) phase transition in liquid crystals. II. Nucleation phenomena in gas-liquid transition with particular emphasis on the systems close to the spinodal curve. III. Collapse transition in linear hydrocarbon (n-alkane) chains for a varying range of length, solvent and temperature. IV. Crystallization of unbranched polymer chains in dilute solution, with particular emphasis on the temperature dependent crossover between the rod-like crystalline state and spherical molten globule state. The thesis has been further divided into ten chapters running through the four parts mentioned before. In the following we provide a brief chapter-wise outline of the thesis. Part I deals with the power law relaxation and glassy dynamics in thermotropic liquid crystals close to the IN transition and consists of two chapters. To start with, Chapter I.1 provides an introduction to thermotropic liquid crystals. Here we briefly introduce various liquid crystalline phases, the order parameter used to characterize the IN transition, a few well established theoretical models, and we conclude with describing the recent experimental and computer simulation studies that have motivated the work described in the next chapter. In Chapter I.2, we present our molecular dynamics simulation studies on single particle and collective orientational dynamics across the IN transition for Lebwohl Lasher model, which is a well-known lattice model for thermotropic liquid crystals. Even this simplified model without any translational degrees of freedom successfully captures the short-tointermediate time power law decay recently observed in optical heterodyne detected optical Kerr effect (OHDOKE) measurements near the IN transition. The angular velocity time correlation function also exhibits a rather pronounced power law decay near the IN boundary. In the mean squared angular displacement at comparable time scales, we observe the emergence of a sub-diffusive regime which is followed by a super-diffusive regime before the onset of the longtime diffusive behavior. We observe signature of dynamical heterogeneity through pronounced non-Gaussian behavior in the orientational motion particularly at lower temperatures. Interestingly, this behavior closely resembles what is usually observed in supercooled liquids. We obtain the free energy as a function of orientational order parameter by the use of recently developed transition matrix Monte Carlo (TMMC) method. The free energy surface is flat for the system considered here and the barrier between isotropic and nematic phases is vanishingly small for this weakly first-order transition, hence allowing for large scale, collective, and correlated orientational density fluctuations. We attribute this large scale fluctuations as the reason for the observed power law decay of the orientational time correlation functions. Part II consists of three chapters, where we focus on the age old problem of nucleation and growth, both from the perspective of thermodynamics and kinetics. We account for the rich history of the problem in the introductory Chapter II.1. In this chapter we describe various types and examples of the nucleation phenomena, and a brief account of the major theoretical approaches used so far. We begin with the most successful Classical Nucleation Theory (CNT), and then move on to more recent applications of Density Functional Theory (DFT) and other mean-field types of models. We conclude with a comparison between the experiments, theories and computational studies. In the next chapter (Chapter II.2) we attempt to elucidate the mechanism of nucleation near the gas-liquid spinodal from a microscopic point of view. Here we construct a multidimensional free energy surface of nucleation of the liquid phase from the parent supercooled and supersaturated vapor phase near the gas-liquid spinodal. In particular, we remove the Becker-Doring constraint of having only one growing cluster in the system. The free energy, as a function of the size of the largest cluster, develops a pronounced minimum at a subcritical cluster size close to the spinodal. This signifies a two step nature of the process of nucleation, where the rapid formation of subcritical nuclei is followed by further growth by slower density fluctuations on an uphill free energy surface. An alternative free energy pathway involving the participation of many subcritical clusters is envisaged near the spinodal where the growth of the nucleus is found to be promoted by a coalescence mechanism in contrast to the single particle addition assumption within CNT. The growth of the stable phase becomes progressively collective and spatially diffuse, and the significance of a “critical nucleus” is lost for deeper quenches. In this chapter we present our studies both in 3dimensional Lennard-Jones (LJ) system and Ising model (both 2and 3dimensions). Our general findings seem to be independent of the model chosen. While the previous chapter focuses on relatively well-studied 3-dimensional (3D) LJ system, in Chapter II.3 we present our studies on the characteristics of the nucleation phenomena in 2dimensional (2D) Lennard-Jones fluid. To the best of our knowledge this is the first extensive computer simulation study to check the accuracy of CNT in 2D. Using various Monte Carlo methods, we calculate the free energy barrier for nucleation, line tension, and bulk densities of equilibrium liquid and vapor phases, and also investigate the size and shape of the critical nucleus. The study is carried out at an intermediate level of supersaturation (away from the spinoidal limit). In 2D, a surprisingly large cutoff (rc ≥ 7.0σ where σ is the diameter of LJ particles) in the truncation of the LJ potential is required to obtain converged results. A lower cutoff leads to a substantial error in the values of the line tension, nucleation barrier, and characteristics of the critical cluster. Note that typically 2.5σ is sufficient for 3D LJ fluids. We observe that in 2D system CNT fails to provide a reliable estimate of the free energy barrier. While it is known to slightly overestimate the nucleation barrier in 3D, it underestimates the barrier by as much as 50% at the saturation ratio S = 1.1(defined as S = P/Pc, where Pc is the coexistence pressure) and at the reduced temperature T* = 0.427(defined as T* = KBT/ ε, where ε is the depth of the potential well). The reason for the marked inadequacy of the CNT in 2D can be attributed to the non-circular nature of the critical clusters. Although the shape becomes increasingly circular and the clusters become more compact with increase in cutoff radius, an appreciable non-circular nature remains even without any cutoff to make the simple CNT inaccurate. Part III again consists of three chapters and focuses on the conformational equilibria. Collapse transition and self-organized structures of n-alkanes in solution. In Chapter III.1 we carry out a brief survey of the existing theories of polymer in solution, with particular emphasis on the collapse process in poor solvents. We also introduce the concept of “hydrophobicity” and “hydrophobic collapse”, which is now a subject enormous interest, partly because it my help in understanding the initial processes involved in protein folding. We briefly discuss the subject of formation of beautiful self-organized structures by block copolymers, and also simple homopolymers which is essentially the focus of the work embodied in the next two chapters. In Chapter III.2 we demonstrated a chain length dependent crossover in the structural properties of linear hydrocarbon (n-alkane) chains using detailed atomistic simulations in explicit water. We identify a number of exotic structures o the polymer chain through energy minimization of representative snapshots collected from molecular dynamics trajectory. While the collapsed state is ring-like(circular) for small chains(CnH2n+2; n ≤ 20) and spherical for very long ones( n = 100), we find the emergence of ordered helical structures at intermediate lengths (n ~ 40). We find different types of disordered helices and toroid-like structures at n = 60. We also report a sharp transition in the stability of the collapsed state as a function of the chain length through relevant free energy calculations. While the collapsed state is only marginally metastable for C20H42, a clear bistable free energy surface emerges only when the chain is about 30 monomers long. For n = 30, the polymer exhibits an intermittent oscillation(characterized by well-developed 1/f noise, where f is the frequency ) between the collapsed and the coil structures, characteristic of two stable states separated by a small barrier. This appears to support a weakly first order phase transition between the extended and the collapsed states. Chapter III.3 extends the study of previous chapter to much longer chains (n ≥ 100), which irreversibly collapse in water into globular forms. Even though the collapsed form has a nearly spherical shape, close inspection shows a propensity towards local ordering in the alignment of the polymer segments. This tendency to maintain alignment in order to maximize the number of contacts leads to a core-shell like structure, where the shell is often characterized by a bent rod-like shape consisting of two adjacent segments running in parallel. A key event associated with the initial stage of collapse seems to be the formation of a skewed ring (or loop) that serves as a “nucleation center” for rest of the chain to collapse into. Time evolution of the radial distribution function of water surrounding the polymer, shows that the density of neighboring water decreases by only about 15-20% from that of bulk water. Even though interior of the ting-like structures is fully devoid of water, solvent accessible surface representation shows that these regions are geometrically/spatially inaccessible to water molecules. We suggest that the role of water is to stabilize such ring-like structures once formed by natural conformational fluctuations of the polymer chain. This view is confirmed by observation of spontaneous formation and melting away of such ring-like entities in a polar aprotic solvent(DMSO). We also comment on the role of the flexibility of polymer chains in determining the collapse kinetics. The last part(Part IV) of the thesis consists of two chapters that deal with the crystallization of linear polymer chains from dilute solution. The way long chain polymers crystallize is drastically different from their small molecule counterparts due to their topological connectivity. Linear polymers often crystallize from dilute solution in the form of thin lamellae with well-defined crystallographic features. In Chapter IV.1 we briefly survey the current theoretical understanding and confusions associated with the highly debated field of polymer crystallization. While the last few decades have seen the development of many successful phenomenological theories, the molecular mechanism of formation of such self-organized lamellae is extremely complex and very poorly understood. There are clearly two distinct steps in polymer crystallization. Firstly, the individual linear polymers must self-organize into bundles of somewhat regular structures. These structures then further aggregate to lamellar form and crystallize into a lattice. In this respect , it has marked similarity to the problem of protein crystallization. In chapter IV.2 we present Brownian dynamics simulation studies of a single polythelene chain of length 500. Such systems can reasonably mimic the process of crystallization from dilute solutions. Our simulations could successfully reproduce some of the interesting phenomena observed in experiments and very recent computer simulation studies, including multi-center nucleation of rod-like structures within a single polymer chain, an inverse relation between lamellar thickness and temperature etc. But our primary focus has been to understand the nature of the phase transition as one traverses along the melting temperature and the underlying free energy surface. Near the melting temperature we observe a very intriguing fluctuation between the disordered molten globule state and the ordered rod-like crystalline, where these two forms have highly different shape and structure. These fluctuations have strong signature of 1/f noise or intermittency. This clearly indicates the existence of a weakly first order transition, where two widely different states with large difference in values of order parameter are separated by a rather small free energy barrier. This can be related to the experimentally observed density fluctuations that resemble spinodal decomposition. It is important to note that very similar fluctuations have been observed in our previous studies on liquid crystals (Chapter 1.2) and intermediate sized alkalines in water(Chapter III.2) that signifies a universal underlying energy landscape for these systems. We have discussed the scope of future work at the end of each chapter whenever appropriate.

Unrestricted Kohn-Sham (broken symmetry) density functional calculations have been used to determine the low-energy geometries of the chromium dihalide molecules (CrX2) and their clusters, Cr2X4, Cr3X6, and Cr4X8. The monomers are also investigated at a higher level, including coupled-cluster and state-average CASSCF computations. Our calculations show that the monomers have a 5B2 ground state arising from the Renner-Teller distorted 5IIg transition state, leading to a bent geometry. The global minima of the gas-phase clusters of CrF2 and CrCl2 consist of two-dimensional, anti-ferromagnetically coupled chains of CrX2 units forming four-membered, doubly bridged Cr2X2 rings, closely resembling their solid-state structures. The global minima of the CrBr2 and CrI2 clusters consist of the same two-dimensional chain-like structures for their dimers, but their trimers and tetramers consist of three-dimensional ’triangular’ structures which contain two capping ligands bound to three chromium atoms along with a Cr-Cr bond. Each Cr atom within these clusters has spin quantum number S=2. There is approximately a constant change in energy, between 45-55 kcal/mol, with every new CrX2 unit during cluster formation. Information about the structure of the CrCl2 clusters is used in the reanalysis of high-temperature electron diffraction data. The vapor at 1170 K contains about 77% monomeric molecules, 19% dimers, and a small amount of trimers. Monomeric CrCl2 is found to be bent with a bond angle of 149(10)degrees, in good agreement with our computations. Solid-state DFT calculations are performed on alpha-CrCl2 to determine the lattice structure and spin-coupling constants for the Cr atoms within the crystals. The GGA (PW91) method produces a structure in good agreement with the literature. In the lowest energy structure, the spins of the Cr atoms within the chains along the crystallographic c-axis are anti-ferromagnetically coupled with four parallel spins situated almost exclusively in the d-bands of Cr along these chains. This anti-ferromagnetic coupling is also seen in the CrX2 clusters.