The Pt/Al2O3 cermet system is extremely stable against oxidation. Therefore, effective high temperature selective absorber coatings can be formed by depositing Pt/Al2O3 cermets onto Pt low emittance base layers. Three Pt/Al2O3 cermet configurations have been investigated for photothermal receiver applications: (1) cermets with graded Pt composition and Al2O3 antireflection layers; (2) cermets with uniform Pt compositions and Al2O3 antireflection layers; and (3) Al2O3-M-Al2O3 multilayer coatings where the M-layer is a Pt/Al2O3 mixture containing about 20 volume percent Al2O3. The coatings were deposited by cosputtering from Pt and Al2O3 sources. Both direct rf sputtering from an alumina target and Ar-O2 dc reactive sputtering from an aluminum target were investigated as methods for depositing the Al2O3 components of the cermet and the antireflection layers. Cr, Mo, Ta, W, and ZrB2 were investigated as alternative low emittance base layer materials. Most of the coatings were deposited onto flat plates of glass, although type 316 stainless steel, type 1020 low carbon steel, and Incoloy 800 substrates were also examined. Hemispherical absorptances as high as 0.97, with room temperature emittances in the 0.06 to 0.08 range, were achieved. The optical properties of the coatings with rf-sputtered Al2O3, and Pt low emittance base layers were stable at 600° C in air for as long as 2000 hr, consistent with data for evaporated Pt/Al2O3 coatings reported by Cornell University. The coatings with reactive sputtered aluminum oxide, or alternative base layers such as Mo, exhibited somewhat reduced thermal stability but should be adequate for intermediate temperature applications (up to 400° C in air).

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