The development and testing of emissivity enhancement coatings for themophotovoltaic (TPV) radiator applications

Abstract

One requirement of a TPV radiator is to efficiently emit photons at high temperatures to TPV cells for conversion to electric power. Since many candidate radiator materials with adequate structural properties display low emissivity, coatings or other surface modifications are required for enhancement of emissivity. Coatings or other surface modifications using vacuum plasma spray (ZrO2+18% TiO2+10% Y2O3, Cr2O3, ZrC, Fe2TiO5, ZrTiO4, ZrO2+8% Y2O3+2% HfO2, TiC, TiC+5% Al2O3+5% TiO2, ZrB2, ZrB2+10% MoSi2, and Al2O3+TiO2), arc texturing (carbon and SiC electrodes), slurry fusion (SiO2+C), laser ablation texturing, atomic oxygen beam texturing, and chemical vapor deposition (CVD) of rhenium whiskers have been evaluated as means of increasing the emissivity of molybdenum, niobium, and Haynes 230 (nickel-base alloy). Characterization and emissivity testing of these surfaces in the as-deposited condition and after a 500 h vacuum anneal are used to evaluate coating performance. Thermal cycling tests were also completed. Six plasma spray coatings (ZrO2+18% TiO2+10% Y2O3, ZrC, Fe2TiO5, ZrTiO4, ZrO2+8% Y2O3+2% HfO2, and Al2O3-TiO2) and a CVD rhenium whisker coating did have pre- and post-anneal emittance values that were higher than or close to a value of 0.8. These coatings generally exhibited favorable stability during vacuum annealing, and excellent resistance to thermal cycling. Any of these coatings could potentially be used to improve the surface emittance of molybdenum, niobium, or nickel base metals for at least 500 h at 1100°C.