Development of a protective coating for TAGS-85 thermoelectric material

Date of Award

2013

Degree Name

M.S. in Materials Engineering

Department

Department of Chemical and Materials Engineering

Advisor/Chair

Advisor: Chadwick Douglas Barklay

Abstract

Radioisotope Power Systems (RPSs), employed by the National Aeronautics and Space Administration (NASA) since the 1960s, convert some of the heat of decaying nuclear fuel into electricity. These power systems are commonly used in space technology due to their reliability and long operational lifespan. TAGS-85, a telluride of antimony, silver, and germanium, is a heritage p-type thermoelectric material employed in RPSs that have powered the NASA Pioneer, Viking, and most recently Mars Science Laboratory missions. Previous studies have found that the power output of RPSs degrade over time under field conditions. This power degradation has been found to be partially attributable to the sublimation-induced deterioration of TAGS-85. This thesis seeks to develop a diffusion-inhibitive coating that can be applied to TAGS-85 elements prior to their installation in order to inhibit TAGS-85 sublimation. Silica (SiO2), alumina (Al2O3), and titania (TiO2) were chosen as the experimental protective coatings. Coatings were deposited upon TAGS-85 samples via chemical solution deposition. Coated and uncoated TAGS-85 samples were exposed to conditions similar to those experienced within the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), the most recent RPS. Samples were removed and analyzed after 1000, 3000, and 5000 hours of exposure. Coating effectiveness was analyzed via sample mass loss and Auger electron spectroscopy (AES) surface scanning and depth profiling. Chemical solution deposition was found to be able to deposit coatings that could effectively inhibit the diffusion and sublimation of TAGS-85 constituent elements. Coating effectiveness at inhibiting thermoelement sublimation was shown to be dependent upon both the coating thickness and the coating material choice. AES analysis revealed that germanium was the primary element subliming from the TAGS-85 samples after exposure to MMRTG operational conditions. Based on the results of this experiment, titania (TiO2) and alumina (Al2O3) are recommended as protective ceramic coating materials for TAGS-85. The titania coating was found to preferentially inhibit the diffusion of tellurium. Discussed in detail are an in-depth analysis of the effectiveness of each coating, the effect of exposure upon uncoated TAGS-85 samples, and the effect of the coating deposition process upon the TAGS-85 samples.

Keywords

Thermoelectric materials, Diffusion coatings, Radionuclide generators Power supply, Materials science; space power; thermoelectric; MMRTG; TAGS

Rights Statement

Copyright © 2013, author

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