Phase transformations and chemical interactions in materials exposed to high temperatures
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Phase transformations and chemical interactions occur in many materials systems exposed to elevated temperatures. In this study, materials exposed to high temperatures in three distinctive applications, have been examined. The first application involves the fabrication of semiconductor-core optical fibers for mid-infrared transmission. Such fibers can be used for chemical sensing, threat detection, and bio-imaging. In this study, germanium-core borosilicate glass cladded fibers were fabricated using rod-in-tube drawing. An analytical model for the deformation and heat transfer in the fiber preform during the high temperature fabrication process was developed. The solidification of the germanium core was experimentally studied using a proxy system of melting ice in a tube. The relative roles of conductive and convective heat transfer in determining the melting mechanism was analyzed. The fabricated fibers were characterized by various electron microscopy based techniques to understand impurity diffusion from the cladding to the core, as well as to study the crystalline quality of the Ge core. The second application involves solid oxide membrane (SOM) based electrolytic production of silicon, where the interaction between the ceramic membrane and the molten salt is the key in determining the lifetime of the membrane. The yttria-stabilized zirconia (YSZ) membrane was found to degrade over time due to chemical interactions with the silica-containing molten oxy-fluoride flux. These interactions led to the formation of a yttria depletion layer in the YSZ in contact with the molten salt. A series of flux compositions were designed to systematically test the correlation between flux optical basicity, yttria activity and YSZ membrane degradation. The results provide a guideline for eliminating membrane degradation during the production of silicon using the SOM electrolysis process. The third application involves molten mixtures of lithium chloride and metallic lithium for metal oxide reduction application. These mixtures exhibit anomalous physical properties that lack a comprehensive explanation. In this study, the structures of bulk molten LiCl and LiCl-Li mixtures were investigated using an in-situ high-energy x-ray diffraction (HEXRD) technique. The structure factors and the pair distribution functions (PDF) of LiCl-Li mixtures were compared with those of pure LiCl. The results suggest Li disperses in LiCl as nano-clusters.
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