Multi-component oxide powder processing dynamics & synergy towards multi-functionality
Heywood, Stephen Kevin
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Multi-component or multi-cation ceramic oxides are particularly sensitive to processing-properties variation, in which a single defined chemical stoichiometry can embody dramatic variability in measured properties simply through the steps of synthesis and processing to reach the desired form. Hence, the tailoring of complex oxides is often convoluted by chemical doping and changes in stoichiometry when the influence of processing is not understood. Mixed conducting, multi-valent double perovskite Sr 2-x V Mo O 6-delta (SVMO) shows extraordinary electrical conductivity relative to comparable double perovskites. The technical hurdles of synthesizing and processing bulk powders of SVMO to optimize studies of fundamental electrical transport mechanisms otherwise convoluted by porosity in prior literature were overcome. The basis of various synthesis conditions via rapid microwave assisted sol-gel synthesis were discerned for their contribution to either open porosity of SVMO or enhanced densification. Enhanced resistance to particle coarsening under reducing contrast to inert atmosphere and a means to generate electrical percolation via solid-solution stability of SVMO were two key discoveries among fundamental breakthroughs understanding particle consolidation behaviors. It was discovered that SVMO's elastic modulus was well in excess of other oxide materials, approaching 300 GPa and in correspondence with the mixed V/Mo valency system provides an explanation for low thermal diffusion during sintering. The advanced solid lithium conducting garnet Li 6.25 La 3 Zr 2 Al 0.25 O 12 (LLZO) demonstrates high ionic conductivity for all solid-state batteries, however, it must be paired with an active cathode and anode to enable high energy storage capacity. The study presented here identifies methods to process LLZO materials into dense and porous constituents to satisfy the design architecture of a solid-state battery emphasizing the sensitivity of LLZO performance to lithium content and the desired cubic phase. The aim was to calibrate synthesis techniques towards minimizing sensitivity to thermal processing that contributes towards lithium loss. Studies of sintering optimization and excess lithium content in conjunction with novel freeze based tape casting methods to generate low tortuosity pores were explored. Development of these novel microstructures represents a backbone of processing methodology necessary to incorporate multivalent double perovskites in fuel-electrolysis cells and improve solid state lithium battery technologies.