Plasma spraying has been shown to be an effective means of depositing thin films of various oxides for use as wavelength-selective emitters for thermophotovoltaic devices (see Figure 1). Wavelength-selective emitters are needed to increase the overall efficiencies of such devices. A suitable emitter material for a given thermophotovoltaic device is one that, when heated to incandescence, emits light with a spectral peak or peaks that preferably lie within the wavelength band of peak response of the photovoltaic cell in the device; more specifically, what is desired is low emissivity at photon energies less than the band gap and high emissivity at photon energies above the band gap of the photovoltaic cell.
Oxides that have shown promise as wavelength-selective-emitter materials include some rare-earth oxides (erbia, thulia, and holmia, both singly and in combination), plus cobalt-doped spinel. Previous techniques for fabricating emitters of these and other materials include casting from slurries and doping crystals with rare-earth oxides. The major disadvantage of cast emitters is mechanical weakness - especially the inability to withstand stresses caused by temperature gradients and thermal cycling. The major disadvantage of doped crystals is high cost.
To fabricate specimens for testing, the various oxide emitter materials were plasma-sprayed onto silicon carbide, alumina, and yttria substrates. Preparation of substrate surfaces by cleaning and roughening was found to be necessary to ensure adhesion and integrity of the deposited materials (see Figure 2). Some of the substrate surfaces were coated with reflective layers of refractory metals (platinum or rhodium) to suppress unwanted out-of-band radiation from the substrates. Emitter deposits with thicknesses from 15 to 250 µm were built up, a few microns per pass, by multiple passes of a plasma-spray apparatus.
The emitters (that is, the coated substrates) were tested in a bench-top thermophotovoltaic apparatus capable of reproducing prototypical operating conditions. The apparatus included an electrical heater for the emitter, and a 0.70-eV GaSb photo a voltaic cell equipped with an integral filter. This apparatus was designed to enable the comparison of the cell output power, total power emitted, and conversion efficiency for various emitters under similar conditions.
In the tests, the plasma-sprayed emitter materials were found to retain their spectral emission characteristics through the deposition process. They were found to survive prototypical operating conditions, including temperatures up to 1,760 K and associated temperature gradients. The plasma-sprayed emitter materials did not exhibit any observable degradation during long-term operation at high temperature (3,600 hours at 1,500 K). Under the given test conditions, the best of the emitter materials tested was found to be a mixture of erbia and thulia; in comparison with a single oxide, the mixture provided stronger emission and hence more useful power within the wavelength band of the photovoltaic cell.
This work was done by Christopher J. Crowley and Nabil A. Elkouh of Creare, Inc., for Glenn Research Center.
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Refer to LEW-16809