NiAl₂O₃ cermet solar absorbers by RF planar magnetron sputtering for high temperature applications

NiAl₂O₃ cermet films were sputtered from a hot pressed NiAl₂O₃ composite target, in a RF planar magnetron sputtering system. Coatings of various thickness were deposited on different substrates and their absorptance (α) and emittance (ε{lunate}) were obtained in the solar and thermal infrared regions, respectively, to evaluate their use as spectrally selective surfaces. Films were also deposited on different substrates with SiO₂ anti-reflection coatings and an α of 0.88 to 0.92 and ε{lunate} of 0.07 to 0.33 were obtained. Films of about 700 Å thick deposited on molybdenum-coated nickel-plated stainless steel substrates with 600 Å SiO₂ AR coating and annealed at 550°C in air for six hours have best selective properties with α=0.91 and ε{lunate}=0.07. These coatings are stable up to 500°C in air for more than 1000 h. ESCA and Auger depth profilings were done to estimate the composition and its variation with depth.