Performance and impedance studies of thin, porous molybdenum and tungsten electrodes for the alkali metal thermoelectric converter

Wheeler, B. L.; Williams, Roger; Jeffries‐Nakamura, B.; Lamb, James L.; Loveland, M. E.; Bankston, C. P.; Cole, T.

doi:10.1007/bf01093756

Cited by 23 publications

(25 citation statements)

References 6 publications

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“…The interpretation of ac impedance data of mature Mo and W electrodes is discussed in detail in another paper (11). Fresh BASE tubes with minimal exposure to air contribute about 0. show a collection of ac impedance data for Mo and Na2MoO4-treated Mo electrodes.…”

Section: Resultsmentioning

confidence: 99%

“…Individual columns have a microstructure which results in the appearance of wedgeshaped structures on the exterior surface of the film. It is somewhat more porous than similarly prepared and operated, mature, untreated W electrodes (11). The briefly annealed films show a substantial loss of microstructural detail and separation of columns within the groups but an increase in the size of pores and crevices between groups.…”

Section: ~M Porous Molybdenum Film Surface As Deposited On Rotating Bmentioning

confidence: 90%

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Effects of Na2MoO4 and Na2 WO 4 on Molybdenum and Tungsten Electrodes for the Alkali Metal Thermoelectric Converter (AMTEC)

Williams

Wheeler²,

Jeffries‐Nakamura³

et al. 1988

J. Electrochem. Soc.

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Performance and impedance characteristics of porous molybdenum and tungsten electrodes, to which Na2MoO4 and Na2WO4, respectively, have been added, are significantly altered from the characteristics of similar but untreated electrodes. High initial power levels are more persistent in treated electrodes, and regeneration during discharge is observed during initial stages of degradation. Sheet resistances of treated electrodes at T > 1000 K are higher but decrease as performance degradation proceeds. AC impedance complex plane plots show an enhanced, high frequency, potentialindependent RC loop due to the ionic conducting molten salt and a large capacitance in the low frequency, potentialdependent loop due to a greatly increased interfacial area, as well as a reaction at the electrode/solid electrolyte and molten salt interface in the case of Na2MoOgMo electrodes. Corrosion of the porous electrode by Na2MoO4 or Na2WO4 to form Na2M0306 and WO2, respectively, and recrystallization of the molybdenum or tungsten as the salt evaporates, results in major morphological changes including loss of columnar structure and a significant increase in porosity. This effect is more pronounced in Na2MoO4/Mo electrodes due to the lower stability of Na2MoO4 relative to compounds with intermediate oxidation states of Mo. If there is not a very large accompanying increase in sheet resistance, the resulting electrode may retain significant improved performance after loss of the molten salt.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.11.21.2 Downloaded on 2014-11-01 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.11.21.2 Downloaded on 2014-11-01 to IP

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Section: Resultsmentioning

confidence: 99%

Section: ~M Porous Molybdenum Film Surface As Deposited On Rotating Bmentioning

confidence: 90%

Effects of Na2MoO4 and Na2 WO 4 on Molybdenum and Tungsten Electrodes for the Alkali Metal Thermoelectric Converter (AMTEC)

Williams

Wheeler²,

Jeffries‐Nakamura³

et al. 1988

J. Electrochem. Soc.

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“…(i-3) Mo electrodes, -2.5 ~m thick containing Na2MoO 4 have performed for short periods of time (~i0 hours) at nearly ideal levels, delivering ~I.0 W/cm 2 at -1200 K; but until recently, reproducible sustained power levels greater than 0.3 W/cm 2 were not achieved. Other electrodes as div~ as 0.5-~n Mm, TiN, liquid Sn, and WPt or WRh have recently demonstrated sustained performance at ~0.5 W/cm 2 at Ii00 to 1200 K. (4)(5)(6) We have achieved sustained power densities of ~0.7 W/cm 2 through partial optimization of WPt and WRh alloy electrodes.…”

mentioning

confidence: 96%

“…(4) In a practical device where the electrodes cover the entire BASE tube surface, electrode area-normalized values of …”

mentioning

confidence: 99%

High Power Density Performance of WPt and WRh Electrodes in the Alkali Metal Thermoelectric Converter

Williams

Jeffries‐Nakamura²,

Underwood³

et al. 1989

J. Electrochem. Soc.

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“…[3,6] Surface diffusion is also likely to provide the dominant Na transport pathway in W/Pt and W/Rh electrodes, which show little open pore area but have excellent Na transport properties at AMTEC operating temperatures from 1100-1200 K. [5] In Na vapor exposure cells, W/Rh and W/Pt electrodes on BASE substrates show activated Na transport, determined from anodic limiting currents, over ten^jerature from 700-1100 K. [7] Experiments run for >1000 hours with W/Pt electrodes have been strongly contaminated with Mn from the steel chamber, heated externally in a fiimace, but the resultant electrodes, while not practicable for AMTEC electrodes, have quite good electrochemical characteristics and only slightly increased grain size. This paper will include results from Pt/W/Mn as a model electrode with well defined behavior.…”

Section: Disclaimermentioning

confidence: 99%

Activated Transport in AMTEC Electrodes

Williams

Jeffries‐Nakamura

Ryan

et al. 1992

SAE Technical Paper Series

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Transport of alkali metal atoms through porous cathodes of alkali metal thermal-to-electric converter (AMTEQ cells is responsible for significant, reducible losses in the electrical performance of these cells. Experimental evidence for activated transport of metal atoms at grain surfaces and boundaries within some AMTEC electrodes has been derived from temperature dependent studies as well as from analysis of the detailed frequency dependence of ac impedance results for other electrodes, including thin, mature molybdenum electrodes which exhibit transport dominated by free molecular flow of sodium gas at low frequencies or dc conditions. Activated surface transport will almost always exist in parallel with free molecular flow transport, and the process of alkali atom adsorption/desoiption from the electrode surface will invariably be part of the transport process, and possibly a dominant part in some cases. Little can be learned about the detailed mass transport process from the ac impedance or current voltage curves of an electrode at one set of operating parameters, because the transport process includes a number of important physical parameters Uiat are not all uniquely determined by one experiment. The temperature dependence of diffusion coefHcient of the alkali metal through the electrode in several cases provides an activation energy and pre-exponential, but at least two activated processes may be operative, and the activation parameters should also be expected to depend on the alkali metal activity gradient that the electrode experiences. Variation in the electrode's physical and morphological parameters, by control during deposition and as morphology evolves with electrode aging, may help to distinguish the influence of activated diffusion from adsorption/ desorption, for exanq>le. In the case of Pt/W/Mn electrodes operated for 2S00 hours, limiting currents varied with electrode thickness, and the activation parameters could be assigned primarily to the surface/grain boundary diffusion process.

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Performance and impedance studies of thin, porous molybdenum and tungsten electrodes for the alkali metal thermoelectric converter

Cited by 23 publications

References 6 publications

Effects of Na2MoO4 and Na2 WO 4 on Molybdenum and Tungsten Electrodes for the Alkali Metal Thermoelectric Converter (AMTEC)

Effects of Na2MoO4 and Na2 WO 4 on Molybdenum and Tungsten Electrodes for the Alkali Metal Thermoelectric Converter (AMTEC)

High Power Density Performance of WPt and WRh Electrodes in the Alkali Metal Thermoelectric Converter

Activated Transport in AMTEC Electrodes

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