Ni-Al2O3 selective cermet coatings for photothermal conversion up to 500°C

Spectrally selective Ni-Al₂O₃ composite films were prepared by r.f. planar magnetron sputtering using hot-pressed targets of two different compositions. The composition of the films were varied by co-sputtering the target with additional nickel pellets distributed uniformly on the target. The composition of the films were analysed by energy-dispersive X-ray analysis. Optical simulations were carried out with the experimentally measured n and k and the published n and k of the metallic substrate. R.f.-sputtered SiO₂ and Al₂O₃ were used as antireflection coatings. From the computer optimization studies we found that α = 0.94 and ε(100°C) = 0.07 could be obtained with 650 Å of Ni-Al₂O₃ (f = 0.61) antireflected with 780 Å SiO₂ on a nickel-coated glass substrate. When molybdenum-coated nickel-plated stainless steel substrates were used, the films were found to be stable up to 500°C in air.