No description available.

Doctor of Philosophy(PhD) / This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.

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No description available.

Este trabalho é parte da cooperação internacional BRAGECRIM (Iniciativa Brasil-Alemanha para Pesquisa em Tecnologia de Fabricação) do projeto "Investigation and improvement of a manufacturing process chain from cold drawing processes to induction hardening – reducing dimensional changes and distortion" (Investigação e Melhoramento de uma Rota do Processo de Trefilação Combinada e Endurecimento por Indução para Minimização de Distorções e Mudanças de Forma) e está diretamente relacionado ao tema de estudo. As tensões residuais são aquelas presentes em um material quando não há atuação de forças externas ou presença de gradientes de temperatura, podendo gerar distorções de forma e empenamentos. Elas podem atuar de forma benéfica, para o caso de possuírem sentido contrário ao da carga de trabalho; ou maléfica, no caso do seu sinal ser o mesmo da carga aplicada em regime, o que gera, por exemplo, redução da vida em fadiga. Com a evolução dos softwares de modelagem e simulação numérica dos últimos 40 anos, a simulação computacional tem se mostrado uma importante ferramenta para prever o comportamento, as características geométricas e os parâmetros (velocidade, força, etc) dos processos de fabricação. Neste trabalho, foram simuladas numericamente através do método de elementos finitos as tensões residuais geradas durante a trefilação de barras redondas de aço AISI 1045, os resultados numéricos foram comparados com medições de tensões residuais via difração de nêutrons e com equações fornecidas pela literatura para avaliação de forças e deformações. Foram criados três modelos axissimétricos (mecânico axissimétrico, mecânico axissimétrico com fieira elástica, termomecânico axissimétrico) e outro tridimensional mecânico com o objetivo de reproduzir o processo de fabricação. Os parâmetros que alimentaram o software foram adquiridos de processos industriais em uso corrente e na literatura, dentre eles: o coeficiente de atrito do processo, a curva tensão-deformação das barras, as dimensões das barras e a velocidade de trefilação. A necessidade do conhecimento prévio do valor do coeficiente de atrito como um dado de entrada para o software impôs a realização de simulação computacional para o ensaio do anel, criando uma curva de calibração posteriormente confrontada com os resultados experimentais do mesmo ensaio. A curva de escoamento do material foi obtida através de ensaio de tração e as tensões residuais foram medidas via difração de nêutrons e comparadas com os resultados simulados. O modelo que indicou maior aproximação com os valores de força teórica foi escolhido para a variação de alguns parâmetros como: taxas de deformação, ângulos de fieira e configuração geométrica das reduções para se estimar a influência destes na geração de tensões residuais. A comparação entre os resultados de força e deformação analíticos e numéricos demonstrou que os modelos desenvolvidos descrevem corretamente o processo. Observou-se uma grande diferença entre os resultados de tensões residuais simulados e medidos via difração de nêutrons para o centro das barras na direção axial, principal responsável pelo surgimento de tensões residuais, que podem ser originados de erros da medição experimental e da não consideração de importantes fenômenos físicos que ocorrem no comportamento do material como o efeito Bauschinger e a anisotropia do material. A magnitude das tensões residuais na direção axial aumenta com o aumento do semi-ângulo de fieira; e as taxas de deformação não modificaram significativamente os resultados, demonstrando não afetar em grande escala o desenvolvimento das tensões residuais. O modelo com duas reduções de feira mostrou-se o que gera menores gradientes de tensão residual, diminuindo os valores compressivos no centro da barra, sendo este o sugerido como melhoramento do processo. / This work is part of international cooperation BRAGECRIM (Brazil-German Initiative for Research in Manufacturing Technology) of the project "Investigation and improvement of the manufacturing process chain from cold drawing to induction hardening processes - Reducing dimensional changes and distortion" is directly related to the topic of this study. Residual stresses are those present in a material when there is no action of external forces or presence of temperature gradients, it may lead distortions of form and warping. They can act in a beneficial way, in case they have opposite direction of the loading, or evil, in case your signal is the same under the applied stresses, which generates, for example, reduction of life in fadigue. The evolution of software modeling and numerical simulation of the last 40 years, computer simulation has become an important tool to predict the behavior, geometric characteristics and parameters (speed, strength, etc.) of manufacturing processes. In this work, the residual stresses generated during the drawing of round bars of steel AISI 1045 were numerically simulated using the finite element method, the numerical results were compared with measurements of residual stresses via neutron diffraction and equations given in the literature for evaluating forces and deformations. It was carried out three axisymmetric models (mechanical axisymmetric, axisymmetric with elastic die, thermomechanical axisymmetric) and other three-dimensional mechanical aiming to replicate the manufacturing process. The parameters that fed the software were acquired from industrial processes in current use and literature, among them: the coefficient of friction of the process, the stress-strain curve of the bars, the dimensions of the bars and wire drawing speed. The necessity of knowing the coefficient of friction as an input for the software imposed the implementation of computer simulation to ring test, to create a calibration curve to compare with the experimental results of the same test. The flow curve of the material was taken by tensile test and the residual stresses were measured via neutron diffraction and compared with the simulated results. The model that indicated closer values with theoretical force was chosen for the variation of some parameters such: strain rates, die angle and geometric configuration of the reductions to estimate their influence on the generation of residual stresses. The comparison between the results of theric force and deformation and numerical models developed showed that the models correctly describe the process. There was a big difference between the results of simulated and measured residual stresses via neutron diffraction in the center of the bars to the axial direction, the main responsible to the level of residual stresses, which can be derived from errors in the experimental measurement and the for not consider important physical phenomena occurring in the material behavior as the Bauschinger effect and anisotropy of the material. The magnitude of residual stresses in the axial direction increases with increasing die angle, and the strain rate did not change significantly the results showing no affect on a large field development of residual stresses. The model with two reductions showed lower gradients of residual stress, low compressive values in the center of the bar, which is suggested as a process improvement.

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