Sputter-deposited Al 2O 3/Mo/Al 2O 3 selective absorber coatings

Abstract

The development of large-area magnetron sputtering sources has made sputtering attractive for selective absorber applications. In this paper we describe an investigation of Al 2O 3/Mo/Al 2 O 3 (AMA) interference-type selective absorber coatings deposited by cylindrical magnetron sputtering onto low emittance molybdenum-coated glass and stainless steel substrates. Both post and hollow cathode magnetrons were used. The Al 2O 3 layers were formed by reactive sputtering from aluminum and by r.f. sputtering from alumina targets. The semitransparent molybdenum intermediate layers were deposited with and without oxygen injection. The optical constants for the individual sputtered layers were determined from transmission and reflectance measurements and were used to calculate the influence of these layers on the solar absorptance of the complete AMA coating. The optical properties of the sputtered AMA layers were in reasonable agreement with theory, yielding hemisperical solar absorptances α H of 0.92–0.95 with total hemispherical emittances ε H of 0.06–0.10 at 20°C. The highest absorptions and the lowest emittances were obtained for coatings in which the center molybdenum layer had been deposited with oxygen addition. The thermal stabilities of coatings with r.f. sputtered Al 2O 3 were superior to with reactively sputtered Al 2O 3. AMA coatings on stainless steel with an Al 2O 3 diffusion barrier were stable (less than 2% loss in α) at 700°C in vacuum and at 550°C in air. These coatings are therefore attractive for a range of selective absorber applications including high temperature collectors for use between 300 and 600°C.