MSE 298 Seminar: Professor Sossina Haile, Northwestern University

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Relaxing in a time of crisis: Extracting material properties from relaxation experiments

Abstract: Redox active oxides, with mixed ionic and electronic conductivity, are critical components in a wide range of energy technologies, serving as electrodes in fuel cells and batteries, and as reactive substrates in solar-driven thermochemical reactors. Accurate knowledge of the surface reaction rate constant (or equivalently, surface exchange coencient), kS, is essential for both optimal design of components using existing materials and rational discovery of new materials with enhanced catalytic activity. A variety of relaxation methods have been used extensively to determine kS. Such approaches rely on the change in some measurable property, most commonly conductivity, upon application of a step change in gas-phase oxygen partial pressure. Under the appropriate experimental conditions, the rate at which the property changes in response to the change in gas-phase oxygen chemical potential provides a direct measure of the material kinetic parameters. Here, we present several considerations relevant to accurate extraction of these parameters, with particular focus on identifying relaxation occurring due to thermodynamic rather than material kinetic reasons. Furthermore, while the electrical conductivity relaxation (ECR) method is one of the most widely employed relaxation techniques because of the ease with which high precision conductivity measurements can be made using samples of almost arbitrary dimensions, ECR is impractical for the evaluation of a material in which the change in conductivity in response to a change in oxygen partial pressure is extremely small. For such materials, mass relaxation emerges as a viable alternative measurement approach. To this end, we describe a high-temperature mass elaxation apparatus based on a gallium phosphate piezocrystal microbalance that enables measurements at temperatures as high 700°C.

Bio: Sossina M. Haile is the Walter P. Murphy Professor of Materials Science and Engineering at Northwestern University, where she also holds appointments in applied physics and in (by courtesy) chemistry. She assumed her position at Northwestern University in 2015 after serving 18 years on the faculty at the California Institute of Technology, prior to which she had been on the faculty of the University of Washington. She earned her bachelor's degree in materials science and engineering in 1986 from the Massachusetts Institute of Technology and her master's degree in the same field in 1988 from UC Berkeley. She returned to MIT to earn her doctorate in materials science and engineering in 1992. As part of her studies, she spent two years at the Max Plank Institute for Solid State Research, Stuttgart, Germany, first as a Fulbright Fellow then as a Humboldt Postdoctoral Fellow. Her research broadly encompasses oxide materials for energy technologies. She has established new classes of fuel cells with record performance for clean and encient electricity generation, and created new avenues for harnessing sunlight to meet rising energy demands.