The Spectral Emittance and Stability of Coatings and Textured Surfaces for Thermophotovoltaic (TPV) Radiator Applications

摘要

Coatings or surface modifications are needed to improve the surface emissivity of materials under consideration for TPV radiator applications to a value of 0.8 or higher. Vacuum plasma spray coatings (ZrO_2 + 18 percent TiO_2 + 10 percent Y_2O_3, ZrC, Fe_2TiO_5, ZrTiO_4, ZrO_2 + 8 percent Y_2O_3 + 2 percent HfO_2, and Al_2O_3 + TiO_2) and a chemical vapor deposited coating of rhenium whiskers were used to increase the surface emissivity of refractory metal and nickel-base materials. Emittance measurements following 4000 hours of vacuum annealing at 1100 deg C show that only the ZrO_2 + 18 percent TiO_2 + 10 percent Y_2O_3, ZrC, and Al_2O_3 + TiO_2 coatings have the desired thermal stability, and maintain emissivity values higher than 0.8. These coatings are graybody emitters, and provide a high emissivity value in the wavelength range that is relevant to the TPV cells. The highest emissivity values were observed for the Al_2O_3 + TiO_2 coatings, with post-anneal values higher than graphite.