Experiments have demonstrated the feasibility of using flame-heated refractory dielectric solid bodies as wavelength-selective sources of light for special applications; in particular, for powering thermophotovoltaic (TPV) devices and for pumping lasers. A refractory dielectric material suitable for this purpose is one that, when heated by a flame, emits intense light with a spectral peak or peaks at one or more visible and/or infrared wavelengths. For a given laser or TPV application, one would choose such a material with an emission peak or peaks to match the corresponding laser absorption or TPV response peak(s). The emissive material can be in the form of a mantle or a felt, or it can be one of the chemical constituents of a solid crystal. To increase the effectiveness of a light source of this type, one can join a crystalline rod containing the emissive material with another rod (which serves as an optical waveguide) to form a device called a "superemissive light pipe" (SELP).
In one set of experiments, ytterbia, erbia, and thulia were tested as candidate emissive materials in an effort to match the absorption peak of Nd3+ in neodymium: yttrium aluminum garnet (Nd:YAG) lasers and to match the photovoltaic response peaks of Si and GaSb TPV devices. Figure 1 shows the measured emission spectra of mantles of these materials heated by propane flames. Thulia could be chosen for pumping Nd:YAG lasers because its spectral peak at a wavelength of 818 nm lies in the absorption band (790 to 890 nm) of Nd3+. The erbia spectrum shows only weak emission in the Nd3+ absorption band but is well matched to the spectral response of GaSb devices.
SELPs made of various material combinations were tested in another set of experiments. For example, Figure 2 depicts a test setup for measuring the output of a GaSb photovoltaic cell under illumination from a SELP that comprised an emissive crystalline rod of erbium aluminum garnet (Er3Al5O12, also known as "ErAG") bonded to a YAG light pipe. In one experiment in which the ErAG emitter was heated to an estimated temperature of 1,350 °C by torches burning stabilized methacetylene propadiene (commonly called "MAPP gas"), the photovoltaic-power density was found to be 1.56 W/cm2, corresponding to a photon-to-electron conversion efficiency of 29 percent. It has been estimated that if the emitter temperature were raised into the range of 1,500 to 1,600 °C, and if the conversion efficiency were to remain the same, then the photovoltaic-power density would rise to about 5 W/cm2.
This work was done by L. G. DeShazer, A. S. Kushch, and K. C. Chen of Quantum Group, Inc., for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category.
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