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

One requirement of a TPV radiator is to efficiently emit photons at hi,oh t emperatures to TPV cells for conversion to electric power. Since many candi date radiator materials with adequate structural properties display low emi ssivity, coatings or other surface modifications are required for enhanceme nt of emissivity. Coatings or other surface modifications using vacuum plas ma 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, an d Al2O3 + TiO2), are texturing (carbon and SiC electrodes), slurry fusion ( SiO2 + C), laser ablation texturing, atomic oxygen beam texturing, and chem ical vapor deposition (CVD) of rhenium whiskers have been evaluated as mean s of increasing the emissivity of molybdenum, niobium, and Haynes 230 (nick el-base alloy). Characterization and emissivity testing of these surfaces i n the as-deposited condition and after a 500 h vacuum anneal are used to ev aluate coating performance. Thermal cycling tests were also completed. Six plasma spray coatings (ZrO2 + 18% TiO2 + 10% Y2O3, ZrC, Fe2TiO5, ZrTiO4, Zr O2 + 8% Y2O3 + 2% HfO2, and Al2O3-TiO2) and a CVD rhenium whisker coating d id have pre- and post-anneal emittance values that were higher than or clos e 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 emittanc e of molybdenum, niobium, or nickel base metals for at least 500 h at 1100 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.