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669 <043> --- academic collection --- Metallurgy--Dissertaties --- Theses --- 669 <043> Metallurgy--Dissertaties

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Increased energy demands combined with depleting fossil fuel reserves has fuelled the need for energy conversion technologies such as fuel cells. Fuel cells directly and efficiently convert chemical energy to electrical energy. Of the various fuel cell types, solid-oxide fuel cells (SOFC) combine the benefits of environmentally friendly power generation with fuel flexibility. However, the necessity for high operating temperatures (800-1000 &#176;C) has resulted in high costs and materials compatibility challenges. This doctoral research focuses on the synthesis, sintering and processing of apatite type lanthanum silicate (ATLS) electrolytes and half-cells (anode- electrolyte) based on ATLS electrolyte for solid oxide fuel cell operating at intermediate temperatures (600-800 &#176;C). A modified sol-gel approach for the synthesis of nanometric, phase pure electrolyte powders of varying compositions was developed. Using this method five ATLS compositions - La9.33Si6O26(LSO), La9.83Al1.5Si4.5O26(LASO), La9.83Fe1.5Si4.5O26 (LFSO), La9.83Al1.0Fe0.5Si4.5O26 (LAFSO) and La9.83Mg0.9Si5.1O26 (LMSO) - were synthesized, with stoichiometric oxygen content, a lower valence cation dopant on the silicon site and vacancies on the lanthanum site. The powders were sintered to closed porosity using conventional pressureless sintering and an advanced pressure assisted sintering technique – pulsed electric current sintering (PECS). In comparison to the more conventional powder processing routes like solid state synthesis, the sol-gel method helped in reducing the sintering temperature and dwell time required. Ionic conductivity studies on dense electrolyte ceramics (> 96% of the theoretical density) with different grain size and dopants revealed that doping with lower valence cations improved the conductivity of ATLS when compared to the undoped equivalents. Ceramics with a lower grain boundary area, i.e., a larger grain size, have higher ionic conductivity when compared to fine grained ceramics of the same composition. Neutron diffraction studies carried out at different temperatures revealed the changes that occur within the unit cell as a function of temperature and doping. The structural changes within the unit cell were correlated to the observed conductivity. The atoms surrounding the conduction channel influence the ionic conductivity and the ionic conduction path depends on the composition of the electrolyte.Anode-ATLS electrolyte half-cells were processed using electrophoretic deposition (EPD) to produce green half cells on porous anode substrates using appropriate suspension parameters. Alternatively, PECS was also used to fabricate half cells which made co-sintering half cells at significantly lower temperatures and yet retaining the desired microstructure in each layer needed for operation of the fuel cell possible.

669 <043> --- academic collection --- Metallurgy--Dissertaties --- Theses --- 669 <043> Metallurgy--Dissertaties

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Gas injection reactors in general, and bubble column reactors in particular, are key elements of many pyrometallurgical flow charts. Nevertheless, the phenomena and interactions that govern these reactors are not yet fully understood. From a multiscale point of view, the main bottleneck is situated at a mesoscopic level on which individual bubbles are considered. While simulations and water models can be very helpful to widen this bottleneck, experimental observations of gas bubbles in real pyrometallurgical systems remain indispensable for validation and fine-tuning of mesoscopic system descriptions. Unfortunately, the opacity of these systems enforces the use of indirect imaging techniques with limited temporal or spatial resolution. In addition, accurate tracking of the gas-liquid interface requires tomography, further complicating the design of an experimental setup. In this doctoral study an alternative and innovative approach is suggested that circumvents these two main restrictions. By injecting gas in a thin sheet of liquid entrapped between two flat and closely spaced plates, bubbles in a Hele-Shaw flow regime are generated. The resulting quasi-two-dimensional multiphase flow phenomena can be fully captured from a single point of view. Moreover, when using a transparent plate material that is not wetted by the liquid, even bubbles in opaque liquids can be visualized directly. This approach is explored for inert gas injection in liquid metals. To demonstrate the feasibility of the suggested Hele-Shaw based approach, buoyancy driven nitrogen bubbles in liquid mercury are observed at room temperature in a Hele-Shaw cell of 1 mm thickness. By using a moving high speed camera to make continuous close up recordings of individual bubbles, the position and geometry of these bubbles are quantified with a high resolution along their entire path. After a thorough evaluation of the experimental accuracy, this information is used for a detailed analysis of bubble volume variations. It is clear that a hydrostatic pressure gradient accounts for the most of the observed bubble growth. Yet, a careful assessment of the variations for smaller bubbles suggests that an accurate bubble description should also account for significant dynamic pressure variations that are largely flow regime dependent. Contrary to aqueous systems, in mercury transitions between these regimes can readily be observed along the trajectory of individual, expanding bubbles. A mapping for bubbles with diameters between 4 mm and 20 mm shows the existence of two different regimes: smaller bubbles adopt a constant round-to-elliptical shape and travel along a linear path while larger bubbles exhibit a periodically distorted shape and follow a swirling trajectory. The transition between this linear and periodic regime can be attributed to a shift from capillary to inertia dominated flow with increasing bubble size. Furthermore, a quantitative mapping of the bubble velocity shows that the transition goes hand in hand with a steep acceleration, and that the linear regime is marked by a negative correlation between the Eötvös number and the Reynolds number while the opposite is true for the periodic regime. To demonstrate the applicability of the suggested Hele-Shaw based approach for industrially relevant systems, a high-temperature experimental setup is developed. With this setup, nitrogen bubbles are observed in liquid zinc at 700&#176;C in a fused quartz cell with a thickness of 1.5 mm. At low oxygen levels, cell walls are not wetted by the liquid zinc and bubbles can be visualized directly through the transparent cell walls, using the same moving high speed camera as for mercury. In the range of diameters between 5.9 and 9.0 mm, this reveals a single periodic flow regime in which bubbles follow a sinusoidal path with a characteristic frequency of 3.31 Hz. In addition, systematic intermediate accelerations are observed of which the exact origin remains unexplained. A direct extrapolation of the observations in a Hele-Shaw cell to industrially relevant geometries is not straightforward. As a consequence, such experiments will never be able to completely replace three-dimensional observations. Nevertheless, the suggested approach seems very promising as a first step in the study of gas bubbles in liquid metals. After all, in a Hele-Shaw cell, the same effects and interactions that govern unconfined bubbles can be studied. In addition, the resolution of the observations is unseen for bubbles in liquid metals, especially at high temperatures. This makes them highly suitable for detailed validations of simulation results. Therefore it is expected that this approach can contribute to a better understanding of the mechanisms that govern gas injection in pyrometallurgy and support future developments in this field.

669 <043> --- Metallurgy--Dissertaties --- Academische collection --- Theses --- 669 <043> Metallurgy--Dissertaties

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669 <043> --- Academische collection --- Metallurgy--Dissertaties --- Theses --- 669 <043> Metallurgy--Dissertaties

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The materials under consideration in the present research are a glass fibre reinforced PU-foam sandwich structure and its constituents. These constituents are: a flexible open cell PU-foam, a rigid closed cell PU-foam and a continuous glass fibre mat. The general goal of this research is to build different tools to analyse and predict the mechanical properties of the constituents and the final sandwich structure. Therefore, this research combines experimental work with numerical modelling techniques.The first part of this dissertation discusses the open cell PU-foam. Although the open cell foam is not the main load carrying material in the sandwich panel, characterizing and modelling it is of great importance in other research areas (e.g. surface functionalization or pressure drop calculations). The cellular structure of this foam is characterized by means of optical microscopy, SEM and CT. Based on the outcome of the experimental observations, the Kelvin cell and Weaire-Phelan structure were selected as RVE for the FE-modelling. Up to now, the use of the latter structure to build a FE-model has not been reported in literature.In correspondence to real foams, the material distribution in the cell edges of the RVE based FE-models is completely governed by a minimization of the surface energy. The influence of the cell size, solid PU material stiffness, relative density and shape anisotropy on the mechanical properties of the open cell foam, is investigated by means of these models. The Weaire-Phelan based FE-model proved to represent and predict the mechanical properties of the open cell foam in a better way than the standard Kelvin cell based FE-model. In order to decrease the large spread on the available data regarding the solid PU stiffness, special attention is given to this parameter.The skins of the investigated sandwich structures consist of a GF/PU-foam composite. Because the skins are formed in situ, they cannot be characterized in advance which hinders a pre-production prediction of the mechanical properties of the sandwich panel. Therefore, X-ray CT-imaging combined with image processing algorithms are used in the current study to determine the skin thickness and fibre orientation distribution function. This allows to calculate the stiffness of the skins based on the rules of mixture and the Mori-Tanaka inclusion model. The resulting data were successfully validated by experimental work.In the final part of this research the knowledge on the stiffness of PU-foam core and GF/PU-foam composite skin is joined into a simple calculation tool to predict the bending stiffness of the sandwich panels. A comparison of the calculated values to experimental stiffness data, measured on industrially produced plated, revealed an accuracy of +/-10%. Moreover, the influence of different parameters like the weight and orientation of the applied fibre reinforcement mats is indicated by this tool and allows to identify directions for future material developments.

669 <043> --- Metallurgy--Dissertaties --- Academische collection --- Theses --- 669 <043> Metallurgy--Dissertaties