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Underwood, M.L.

Publications and source records attributed to Underwood, M.L..

4 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Component Selection, Accelerated Testing, and Improved Modeling of AMTEC Systems for Space Power

Alkali metal thermal to electric converter (AMTEC) designs for space power are numerous, but the selection of materials for construction of long-lived AMTEC devices has been limited to electrodes, current collectors, and the solid electrolyte. AMTEC devices with lifetimes greater than 5 years require careful selection and life testing of all hot-side components. The likely selection of a remote condensed design for initial flight test and probable use with a radioisotope heat source in AMTEC powered planet probes requires the device to be constructed to tolerate operating T greater than 1150K, as well as exposure to Na (g) , and Na (liq) on the high pressure side. The temperatures involved make the characterization of high strength and chemical resistance to Na containing Na 2 O critical. Selection among materials which can be worked should not be driven by ease of fabricablity, as high temperature stability is the critical issue. These concepts drive the selecton of Mo alloys for Na (liq) containment in AMTEC cells for T to 1150K operation, as they are significantly stronger than comparable Nb or Ta alloys, are less soluble in Na (liq) containing dissolved Na 2 O, are workable compared with W alloys (which might be used for certain components), and are ductile at the T greater than 500K of proposed AMTEC modules in space applications.

AMTEC↗
NASA NTRS1993

Advances in Studies of Electrode Kinetics and Mass Transport in AMTEC Cells

Previous work reported from JPL has included characterization of electrode kinetics and alkali atom transport from electrodes including Mo, W, WRh x (Mn), in sodium AMTEC cells and vapor exposure cells; and Mo in potassium vapor exposure cells. These studies were generally performed in cells with small area electrodes (about 1 to 5 cm 2 ), and device geometry had little effect on transport. Alkali metal diffusion coefficients through these electrodes have been characterized, and approximate surface diffusion coefficients were derived in cases of activated transport. A basic model of electrode kinetics and transport at the alkali metal vapor/pourous metal electrode/alkali beta"-alumina solid electrolyte (BASE) three phase boundary has been proposed which accounts for electrochemical reaction rates with a collision frequency near the three phase boundary and tunneling from the porous electrode partially covered with adsorbed alkali metal atoms. The small electrode effect in AMTEC cells has been discussed in several papers, but quantitative investigations have described only the overall effect and the important contribution of electrolyte resistance. The quantitative characterization of transport losses in cells with large area electrodes has been limited to simulations of large area electrode effects, or characterization of transport losses from large area electrodes with significant longitudinal temperature gradients. This paper describes new investigations of electrochemical kinetics and transport, with four 14.4 cm 2 WPt 3.5 electrodes, including the influence of electrode size on the mass transport loss in the AMTEC cell. These electrodes exhibit very slow sintering, as well as excellent sodium transport properties, making them attractive candidates for AMTEC power conversion use. However, the facile sodium transport in WPt 3.5 electrodes makes characterization of the transport process difficult.

electrode↗