Emissivity enhancement coatings for thermophotovoltaic (TPV) radiator applications

Ten emissivity enhancing coatings (ZrO{sub 2} + 18% TiO{sub 2} + 10% Y{sub 2}O{sub 3}, Cr{sub 2}O{sub 3}, ZrC, Fe{sub 2}TiO{sub 5}, ZrTiO{sub 4}, ZrO{sub 2} + 8% Y{sub 2}O{sub 3} + 2% HfO{sub 2}, TiC, TiC + 5% Al{sub 2}O{sub 3} + 5% TiO{sub 2}, ZrB{sub 2}, and ZrB{sub 2} + 10% MoSi{sub 2}) deposited on Mo, Nb, and Haynes 230 substrates were evaluated for potential use in thermophotovoltaic (TPV) radiator applications. Emissivity testing of as-coated and annealed coupons indicate that 5 of the 10 Vacuum Plasma Spray (VPS) coating candidates have promise (ZrO{sub 2} + 18% TiO{sub 2} + 10% Y{sub 2}O{sub 3}, ZrC, Fe{sub 2}TiO{sub 5}, ZrTiO{sub 4}, ZrO{sub 2} + 8% Y{sub 2}O{sub 3} + 2% HfO{sub 2}). Four of the ten coatings have emissivity values that are too low to be of further interest (TiC, TiC + 5% Al{sub 2}O{sub 3} + 5% TiO{sub 2}, ZrB{sub 2}, and ZrB{sub 2} + 10% MoSi{sub 2}). The final coating was mostly Cr{sub 2}O{sub 3}, and exhibited excessive evaporation during vacuum annealing with a significant decrease in emissivity. Base metal powder, which was added to the bond layer of all coatings to improve coating adhesion, was detected in the top layer of the coatings. Differences in reactive interaction between the base metal powder and coating during vacuum annealing produced varying changes in emissivity. A small decrease in emissivity was observed for the ZrC coating deposited on niobium, which agrees with the limited interaction between the niobium base metal particles and the ZrC coating detected in SEM/EDS examinations. A large decrease in emissivity was observed for the ZrC coating deposited on Haynes 230, and significant interaction between the base metal particles and ZrC coating was observed.