Chemical Expansion of Mixed Conducting Ceramics

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0074539AdlerCeramics used in high-temperature applications must match closely in coefficient of thermal expansion (CTE) to avoid excessive stress associated with strain differences over large changes in temperature. Electrochemical ceramics (used as ion-transport membranes or in solid oxide fuel cells) present yet an additional challenge. These materials not only expand with temperature, but also with changes in chemical oxidation state. The majority of expansion at high temperatures in these materials may result from chemical rather than thermal effects. Yet today there is little knowledge or understanding of chemical expansion, or its consequences and technological applications. This project will enlarge our understanding of expansion in ceramics to include these chemical effects. A new continuum theory that defines expansion thermodynamically in terms of temperature and oxygen content will be presented and then an experimental study of expansion in several mixed conducting oxides as a function of cation stoichiometry, temperature, and oxygen partial pressure will be performed. By examining this data in the framework of the theory, a more complete and general understanding of expansion in ceramics will be gained, as well as into how expansion relates to electronic and defect structure. As a secondary goal, new experimental techniques will be developed involving non-equilibrium expansion for directly measuring oxygen surface exchange kinetics and chemical diffusion. The material systems under study include lanthanum-strontium-cobalt-iron-oxide (LSCF) and strontium-iron-cobalt-oxide (SFC). LSCF is of direct relevance to solid-oxide fuel cells and electrically driven air separation, while SFC is of particular interest to workers developing dense membranes for partial oxidation of methane to syngas. LSCF is also a good model material system because its defect thermodynamics are well understood, and its cation composition covers an enormous range of defect and electronic structure (both ionic and electronic). Electrochemical ceramics are of importance in a broad spectrum of applications, including fuels processing, electricity production, and air separation. A full understanding of expansion in is critical to advancing knowledge and application of electrochemical ceramics. This project will also provide a framework for approaching chemical expansion in all solids, thus extending the impact of it much further.