By means of molecular simulation, we propose possible packing models for £\ and £\¡¬ phases in poly(9,9-di-n-hexyl-2,7-fluorene) (PFH). Simulated multi-chain unit cell structures are compared with experimental diffraction patterns of PFH where the unit cell structure (and the space group) of the high-temperature £\ crystals was identified as monoclinic (C2) and that of £\¡¬ phase (kinetically favored upon programmed cooling) triclinic (P1). Results show that £\ phase prefers to adopt bi-radial side-chain conformation whereas the £\¡¬ phase prefers tetra-radial one. Both models exhibit embracing behavior between adjacent chains in spite of differences in inter-chain distance. A group of embracing chains aligned along b-axis in £\ phase and the comparatively greater inter-chain distance in £\¡¬ phase are consistent with the observed faceting along (100) planes and the tensile cracking along the a-axis. A qualitative analysis of co-existing £\ and £\¡¬ phases reproduce the [001] SAED pattern quite well. In addition, we also show that arbitrary alternation of 40o and 140o in dihedral angle between neighboring monomers generates equally stable single-chain conformations in this case of linear alkyl side-chains.

Aluminum and its alloys are used in many aspects of modern life, from soda cans and household foil to the automobiles and aircraft in which we travel. Aluminum alloy systems are characterized by good workability that enables these alloys to be economically rolled, extruded, or forged into useful shapes. Mechanical properties such as strength are altered significantly with cold working, annealing, precipitation-hardening, and/or heat-treatments. Heat-treatable aluminum alloys contain one or more soluble constituents such as copper, lithium, magnesium, silicon and zinc that individually, or with other elements, can form phases that strengthen the alloy. Microstructure development is highly dependent on all of the processing steps the alloy experiences. Ultimately, the macroscopic properties of the alloy depend strongly on the microstructure. Therefore, a quantitative understanding of the microstructural changes that occur during thermal and mechanical processing is fundamental to predicting alloy properties. In particular, the microstructure becomes more homogeneous and secondary phases are dissolved during thermal treatments. Robust physical models for the kinetics of particle dissolution are necessary to predict the most efficient thermal treatment. A general dissolution model for multi-component alloys has been developed using the front-tracking method to study the dissolution of precipitates in an aluminum alloy matrix. This technique is applicable to any alloy system, provided thermodynamic and diffusion data are available. Treatment of the precipitate interface is explored using two techniques: the immersed-boundary method and a new technique, termed here the sharp-interface method. The sharp-interface technique is based on a variation of the ghost fluid method and eliminates the need for corrective source terms in the characteristic equations. In addition, the sharp-interface method is shown to predict the dissolution behavior of precipitates in aluminum alloys when compared with published experimental results. The influence of inter-particle spacing is examined and shown to have a significant effect on dissolution kinetics. Finally, the impact of multiple particles of various sizes interacting in an aluminum matrix is investigated. It is shown that smaller particles dissolve faster, as expected, but influence the dissolution of larger particles through soft-impingement, even after the smaller particles have disappeared.

Chemical reactions can be understood as transitions from basin to basin on a high dimensional potential energy landscape. Varying temperature only changes the average kinetic energy of the system. While applying voltages or external pressures directly tilts the landscape and drives the reactions in desired directions. In solids at relatively low temperature, where the entropy term is approximately invariant, the reaction spontaneity is determined by the energy difference between the reactant and product basins and the reaction rate can be calculated from the barriers in between. To achieve sufficient accuracy to explain experimental observations we are interested in, density functional theory (DFT) is usually employed to calculate energies. There are two types of reactions I have studied: the first type of reaction only involves a few number of individual atoms, corresponding to traveling in a small volume in the high dimensional configuration space; the other type involves a large amount of atoms moving in a concerted pattern, and the distance traveled in the configuration space is significantly longer. The scopes of these two in the energy landscapes are in different scales and thus proper metrics for distance measurements are required. In the first case, I have mainly studied Li/Na behaviors in the cathode materials of secondary batteries. Here resolving the energy landscape step by step with detailed information is possible and useful. By analyzing the energy landscapes with DFT plus the Hubbard U correction, I have explained several phenomena related to the degradation of lithium-rich layered oxides, rate performance of surface modified LiFePO₄, and capacity of vanadium-based fluorophosphates. Predictions on both thermodynamic and kinetic properties of materials are also made based on the calculation results and some are confirmed by experiments. In the second case, my focus is on solid-solid phase transitions. With a tremendous long reaction pathway, examining every possible atomic step is too expensive. By adopting periodic boundary conditions, a small supercell can represent the main feature of the energy landscape in a coarse grained way, where the connection between phases is easier to explore. After the big picture of a phase transition mechanism learned from this simplified model, details along the reaction pathway, like new phase nucleation and growth, could be resolved by using a larger supercell. In the above treatment, two types of variables, the cell vectors and atomic positions, span a generalized configuration space. Special consideration is required to balance these two to keep consistency under different supercells and avoid biases. A solid-state NEB (SSNEB) and a solid-state dimer (SSD) method are then developed to locate saddle points in the generalized configuration space. With the methodology well justified, we are able to efficiently find possible nucleation mechanisms, for examples the CdSe rock salt to wurtzite and Mo A15 to BCC phase transitions. SSNEB is also applied in studying phases transitions under pressures, including the graphite to diamond, and CaIrO₃ perovskite to post-perovskite transitions. Combined with the adaptive kinetic Monte Carlo (AKMC) algorithm, SSD shows the ability to find new polymorphs of CdSe and the connecting barriers between them. / text

No description available.

ZnO nanowires were grown using carbothermal reduction and convective vapor phase transport in a tube furnace. Si <100> substrates that were 20 mm x 76.2 mm were sputter coated with 2 nm to 50 nm gold which formed nanoparticles on the order of 50 nm in diameter through a process of Ostwald ripening upon being heated. Growth temperatures were varied from 800ºC to 1000ºC, flow rates were varied from 24 sccm to 3300 sccm, and growth durations were varied from 8 minutes to 5 hours. Vapor phase Zn, CO, and CO2, produced by carbothermal reduction and suspended in an Ar atmosphere, were flowed over the Si substrates. The Au nanoparticles formed an eutectic alloy with Zn, causing them to become liquid nanodroplets which catalyzed vapor-liquid-solid nanowire growth. The nanowires were also synthesized by self-catalyzing vapor-solid growth in some cases. Using the tube furnace never resulted in more than 50% of the substrate being covered by nanowires. It was found that a bench-top furnace could achieve nearly 100% nanowire coverage by placing the 20 mm x 76.2 mm sample face down in a quartz boat less than 2 mm above the source powder. This was because minimizing the distance between the sample and the source powder was critical to achieve macroscopically uniform growth consistently. / text

L’alliage germanium-étain est un semiconducteur qui suscite une grande attention en raison de ses propriétés électriques et optiques. L’incorporation de Sn dans le germanium permet d’ajuster la largeur de bande interdite (gap) et d’améliorer la mobilité des électrons et des trous, et pour une quantité suffisante d’étain, le matériau passe d’un gap indirect à direct. Cet alliage est versatile parce qu’il peut être intégré d’une façon monolithique sur le Si, c’est ce qui en fait un matériau idéal dans les domaines de l'optoélectronique à base de silicium. Cette thèse est sur la fabrication et la caractérisation de nanofils cristallins Ge1-xSnx à haute concentration en Sn. Des nouvelles stratégies ont été employées pour fabriquer de nombreux types de nanofils GeSn. Les résultats ont été expliqués en fonction des modèles cinétiques existants. Un nouveau mécanisme de croissance y est décrit: le mécanisme solide-solide-solide – SSS. Il consiste à faire croître des nanofils de GeSn dans le plan du substrat à l’aide de catalyseurs d’étain à une température inférieure au point de fusion de Sn. Quatre modèles de transport de masse sont proposés pour le mécanisme de croissance du SSS. Diverses caractérisations (par exemple TEM et APT) ont été effectuées pour étudier les propriétés physiques, et chimiques des nanofils. / Germanium-Tin alloy is a unique class semiconductor gaining a strong attention because of its significant electrical and optical properties. Sn incorporation in Ge allows straightforward band-gap engineering enabling to enhance the electron and hole mobilities, and for a sufficient Sn amount an indirect-to-direct band-gap transition occurs. Its versatility rises due the possible monolithic integration on Si-platforms making it an ideal material in domains of optoelectronics, and high speed electronic devices. This thesis has focused on the fabrication and characterization of crystalline Ge1-xSnx nanowires with high Sn concentrations. New strategies were designed to fabricate many types of GeSn nanowires. The results have been explained as function of the existing kinetic models. A new growth mechanism was reported (i.e. Solid-Solid-Solid mechanism – SSS), it consists of growing in-plane GeSn nanowires using Sn catalysts below the melting point of Sn. Four mass transport models were proposed for the SSS growth mechanism. Various characterizations (e.g. TEM and APT) were done to investigate the physical and chemical properties of the obtained nanowires.

A produção de brita para construção civil na região metropolitana de São Paulo (RMSP) é uma tarefa desafiadora: de um lado, as crescentes pressões da sociedade e das questões ambientais e, de outro, os baixos preços e parâmetros de qualidade impostos pelo mercado fazem a indústria de agregados perseguir, cada vez mais, custos de operação baixos que se traduzam em vantagem competitiva. Seja na otimização de intalações existentes, seja em novos projetos de instalações, vem se acentuando o uso de simuladores de circuitos de britagem. A automação das instalações produtoras de agregados da RMSP também se mostra como uma tendência irreversível. Neste contexto, este trabalho apresenta um simulador dinâmico de circuitos de britagem, desenvolvido para auxiliar as tarefas de otimização, automação e mesmo projeto de instalações produtoras de agregados. AggXtream, um novo simulador dinâmico de circuitos de britagem, foi desenvolvido com os mais modernos modelos matemáticos de britagem atualmente disponíveis, e traz consigo um conjunto de rotinas de calibração desses modelos que utiliza técnicas de inteligência artificial. / The production of aggregates for civil construction in the metropolitan region of São Paulo (RMSP) is a challenging task: from one side, growing pressures of society and environmental issues and, from another, low prices and quality standards demanded by the market make the aggregate industry pursue, even more, low operating costs that turn into competitive advantage. Either in the optimization of existing plants or new projects, the use of simulators of mineral comminution circuits is becoming widespread. The automation of RMSPs aggregates operations has also become an irreversible trend. Within this context, the present work describes the development of a dynamic simulator for crushing circuits, aiming to be used in the optimization, automation and project of aggregates plants. AggXtream, a new dynamic simulator of crushing circuits, has been built with the most modern mathematical models of crushing currently available. It also incorporates model calibration routines that use artificial intelligence techniques.

A produção de brita para construção civil na região metropolitana de São Paulo (RMSP) é uma tarefa desafiadora: de um lado, as crescentes pressões da sociedade e das questões ambientais e, de outro, os baixos preços e parâmetros de qualidade impostos pelo mercado fazem a indústria de agregados perseguir, cada vez mais, custos de operação baixos que se traduzam em vantagem competitiva. Seja na otimização de intalações existentes, seja em novos projetos de instalações, vem se acentuando o uso de simuladores de circuitos de britagem. A automação das instalações produtoras de agregados da RMSP também se mostra como uma tendência irreversível. Neste contexto, este trabalho apresenta um simulador dinâmico de circuitos de britagem, desenvolvido para auxiliar as tarefas de otimização, automação e mesmo projeto de instalações produtoras de agregados. AggXtream, um novo simulador dinâmico de circuitos de britagem, foi desenvolvido com os mais modernos modelos matemáticos de britagem atualmente disponíveis, e traz consigo um conjunto de rotinas de calibração desses modelos que utiliza técnicas de inteligência artificial. / The production of aggregates for civil construction in the metropolitan region of São Paulo (RMSP) is a challenging task: from one side, growing pressures of society and environmental issues and, from another, low prices and quality standards demanded by the market make the aggregate industry pursue, even more, low operating costs that turn into competitive advantage. Either in the optimization of existing plants or new projects, the use of simulators of mineral comminution circuits is becoming widespread. The automation of RMSPs aggregates operations has also become an irreversible trend. Within this context, the present work describes the development of a dynamic simulator for crushing circuits, aiming to be used in the optimization, automation and project of aggregates plants. AggXtream, a new dynamic simulator of crushing circuits, has been built with the most modern mathematical models of crushing currently available. It also incorporates model calibration routines that use artificial intelligence techniques.

[pt] Sistemas Mo/Al2O3 e W/ZrO2 foram estudados a fim de evidenciar se a reação WO3+ZrO2 ocorre no estado sólido, e se a reação MoO3+Al2O3, também no estado sólido, ocorre com aluminas porosas. Para tanto, essas misturas foram preparadas utilizando-se dois teores de Mo e de W, respectivamente: 5 e 10 µmolMo/m2 de suporte alumina e 10 e 16 µmolW/m2 de suporte zircônia sendo tratadas a 773 K. Para o caso dos sistemas Mo/Al2O3, foram utilizadas três aluminas com diferentes propriedades, tais como: área específica, porosidade, grau de hidroxilação e distribuição de grupos OH superficiais. Para os sistemas W/ZrO2, verificou-se também o efeito de temperatura de tratamento mais elevada: 973 K. Todas as amostras obtidas foram caracterizadas pelas técnicas de: difração de raios-X, adsorção de N2, espectroscopia de absorção na região do infravermelho e de reflectância difusa no UV-visível. Considerando-se os sistemas Mo/Al2O3, verificou-se que o MoO3 reagiu com as três aluminas, apesar da maior dificuldade de difusão de massa devido à textura das aluminas porosas, formando estruturas de Mo diferentes do óxido original, sendo que a maior dispersão, sobre o suporte, das espécies de Mo geradas parece estar relacionada à presença de grupos hidroxila mais básicos. Para os sistemas W/ZrO2, os resultados também evidenciaram que foram geradas espécies de tungstênio diferentes do WO3 original, sendo que a utilização da maior temperatura de calcinação levou à formação de mais espécies de W dispersas sobre o suporte. / [en] Mo/Al2O3 and W/ZrO2 systems have been studied in order to verify if the WO3+ZrO2 reaction occurs in solid state, and if the MoO3+Al2O3 reaction also occurs with porous alumina in the solid state. These mixtures were prepared using two Mo and W loadings: 5 and 10 µmolMo/m2 of support alumina and 10 and 16 µmolW/m2 of support zirconia, and they were calcined at 773 K. For Mo/Al2O3 systems three aluminas with different properties, such: specific area, porosity, hydroxylation degree, and OH groups distribution were used. For W/ZrO2 systems the effect of a higher treatment temperature: 973 K was also evaluated. All samples were characterized by the following techniques: X- ray diffraction, N2 adsorption, infrared absorption spectroscopy, and UV-vis diffuse reflectance spectroscopy. For Mo/Al2O3 systems it has been verified that molibdenum oxide reacted with the aluminas even though the mass transfer difficulties related to the porosity, forming Mo structures distinct from MoO3, which higher Mo dispersion may be related to more basic hydroxyl groups. For W/ZrO2 systems, the results have also shown that W species different from WO3 were obtained, and higher calcination temperature promoted the formation of more dispersed species onto the support.

No description available.