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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program
ciency, and the explanation and understanding of the surface physics are incomplete.
MTC Electrode Materials
The rationale for the MTC configuration depends not only on the presumed low manufacturing cost using integrated circuit manufacturing techniques, but also on very substantial advances in durable electrode work function properties. The search for such materials has been thoughtfully and extensively pursued in a number of laboratories over the years without the constraint of compatibility with integrated circuit fabrication methods and materials. Even if low work function electrode materials can be fabricated inexpensively and made compatible with the integrated circuit industry fabrication methods used to form the main structure of the MTCs, the same electrode materials should also be helpful in forming more conventional thermionic fuel elements. In the case of conventional thermionic fuel elements (TFEs), the space charge limitations of higher gap spacing are compensated for by the use of Cs vapor. Therefore, even if an MTC device became practical, the gains in low work function materials would probably allow conventional TFEs to outperform the MTC device.
Even if it is assumed that the several major technical hurdles identified above can be overcome, very small scale (on the scale of a chip or radioisotope heater unit) MTCs still face stiff competition from thermal electric generators. At 1 to 100 watts, MTCs also face strong competition from AMTEC and free-piston Stirling, which appear to have fewer material problems at the temperature levels proposed for the Sandia converters.
Ultimately, the experimental data from the MTC program do not support the theoretical predictions. Not only are postulated low work function emitters not yet functional, they also are not expected to be so in the near future. Finally, while it is assumed that integrated circuit fabrication methods will lead to low cost production, the fact remains that fabrication of stable, small gap converters has not been demonstrated for near-term experiments, and these devices do not have the right characteristics for long-term, low cost production given material constraints such as directional etching, compatible layer chemistry, and so on.
Recommendation 6. The sponsoring agency should discontinue the microminiature thermionic converter (MTC) program, the close-spaced vacuum converter tasks, the oxygenation effects research, and all current theory and theory validation work.
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