All entries for Sean Bishop

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Ionic Conduction Studies in TlBr Radiation Detector Material

• Authors: S. R. Bishop, H. L. Tuller
• Sponsorship: Domestic Nuclear Detection Office, Radiation Monitoring Devices, Inc.

Detection of high-energy radiation (e.g., γ-rays) is key in nuclear non-proliferation strategies.  When a wide-band gap semiconductor detector intercepts a γ-ray, electron–hole pairs are formed, resulting in an increase in electrical conductivity. This change in conductivity, or sensitivity, is maximized if the conductivity in the non-illuminated (dark) state is very low.  In order to achieve high sensitivity, current semiconductor technologies require device cooling to very low temperature, which adds to cost and reduces portability.  TlBr is an attractive detector material because of its low dark conductivity at room temperature as well as its high mass density, leading to higher radiation absorption.

In this project, we have characterized the dominant ionic conduction properties in TlBr using impedance spectroscopy.  Through doping techniques, we have determined that TlBr is primarily a Schottky-type ionic conductor, meaning that Tl and Br move through the material by ionic vacancy motion.  These measurements have led us to predict a doping strategy to minimize the dark conductivity, and we are collaborating with a local company (RMD) to implement this technology as well as developing it further by studying new TlBr-based material systems.  In addition, our newfound understanding of TlBr has led us to investigate novel device designs never before used in ionic conducting systems.

Chemomechanics of Fuel Cell Related Materials

• Authors: S. R. Bishop, J.-J. Kim, H. L. Tuller (in collaboration with K. Van Vliet, S. Yip)
• Sponsorship: Department of Energy

Solid oxide fuel cells (SOFCs) directly convert chemical to electrical energy with high efficiency and can operate using a wide variety of fuels from hydrogen to kerosene and gasified coal.  Many of the more advanced materials in SOFCs experience significant changes in oxygen content, or oxygen stoichiometry, during operation, resulting in changes in volume and elastic properties termed chemomechanics.  This chemical expansion, analogous to temperature-induced thermal expansion, is oxygen nonstoichiometry-induced and can have a negative impact on SOFC performance. In this project, we are studying the chemical expansion coefficient, elastic properties, and oxygen stoichiometry of thin film and bulk SOFC materials. Thin films are of particular interest since they allow for control of strain and increase the surface-to-volume ratio, particularly important for electrode performance.  The chemomechanical properties are being investigated using high temperature, atmosphere controlled nanoindentation, high resolution x-ray diffraction, dilatometry, impedance spectroscopy, and thermo-gravimetry techniques.

Catalytic Oxygen Storage Materials

• Authors: S. R. Bishop, D. Chen, P.-S. Cho, H. L. Tuller
• Sponsorship: NSF

Combustion of fossil fuels, essential for electricity generation and vehicular propulsion, is generally incomplete, leading to harmful NOx, CO, and unburned hydrocarbons emissions.  Great progress in minimizing such emissions has relied on the operation of “three-way catalysts” (TWCs), which utilize a combination of precious metals and metal oxides with the ability to take up or release oxygen for reduction/oxidation of pollutants (NOx to N₂ plus CO and HC to CO₂ and H₂O, respectively).  In this project, we are investigating the rate at which oxygen storage materials (OSM) exchange oxygen with the atmosphere and the magnitude of oxygen they store with the aid of geometrically well-defined thin film structures. Impedance spectroscopy, Kelvin probe, and thermogravimetric methods are used to determine electrochemical performance and oxygen storage capabilities.  These properties, when correlated to actual TWC performance using a differential flow reactor, will allow for a more detailed understanding of performance criteria.