Development of Sputter-Deposited Pt/Al₂O₃ Selective Absorber Coatings

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).