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Show 40 g-meters, which translates into mantles with mechanical impact resistances about two orders of magnitude greater than conventional mantles. OPTICAL PROPERTIES Although a light-emitting, thoria-ceria composition has been used as an example of a strong fiber fabrication technique, it must be emphasized that the same synthesis procedure can be applied to many oxide ceramics. For example, rare earth oxide ceramics are refractory and can be made in strong fibrous form. In addition, some rare earths exhibit selective emissions in the visible and near IR portions of the optical spectrum. Two examples, which have been reported earlier,4 are included here. The normalized ytterbia exitance (Figure 1) exhibits a single emission at 0.98 ~m with a full width at half maximum (FWHM) of about 150 nm. The normalized exitance of an erbia specimen in fibrous form is included in Figure 2. Although erbia has a number of emissions including a very small emission in the visible (green) that is not evident in Figure 2, the significant erbia emission is at 1.55 ~ which can be very selective (FWHM is about 65 nm). These two examples are included because they are candidate emitters for a thermophotovoltaic energy conversion system where a primary energy source is used to heat an emitter to incandescence. The radiant energy is converted directly to electricity in a photoconverter. Erbia is well matched spectrally to a germanium photoconverter, and ytterbia, of course, is an ideal illuminator for a silicon photocell. Note the limited off-band emission in these two specimens. The small (5-10 ~m diameter) fibers in these emitters are almost optically transparent over the optical spectrum investigated except for the selective emissions which arise from electronic transitions among inner electron shell vacancies peculiar to the rare earth elements. There is a role also for very emissive, broad-band fibers in radiant burners where a high emittance will lead to low burner operating temperatures with consequential low NOx emission and long burner life. Although there are some rare earth oxides which are very emissive in the near IR region because of multiple electronic transitions that occur energetically in this optical region, there are some much lower cost alternative materials. In particular, a perovskite, lanthanum chromite, is being investigated by us for this application. Others have investigated the application of lanthanum chromite as an electrode material in magnetohydrodynamic 4R. E . Nelson, "Rare Earth Oxide TPV Emitters," Proceedings of the 32nd International Power Sources Symposium, 9-12 June 1986, Published by the Electrochemical Society, Pennington, NJ 08534. pp. 95-100. |