TIN-OXIDE-COATED ANODIZED ALUMINUM SELECTIVE ABSORBER SURFACES .2. AGING AND DURABILITY

Accelerated aging tests on tin-oxide-coated anodized aluminium selective absorber surfaces have been performed. It is shown that the tin oxide layer provides excellent protection for the anodized surface. The thermal stability of the double layer is equal to, or better than, the stability of the bare anodized surface. Both surfaces degrade very slowly at 450-degrees-C. The anodic layer is porous and therefore not very scratch resistant and it degrades when exposed to humidity, both acidic and alcaline. It is shown that the tin oxide film, which is hard and chemically very stable, almost completely protects the anodized surface from chemical degradation. The mechanical wear resistance is also considerably improved. The optical selectivity does not quite match that of the the anodized surface, but the remarkable stability together with the solar parameters of alpha = 0.91 and hemispherical emittance epsilon = 0.16 (normal emittance epsilon-n = 0.12) make this surface an interesting selective absorber surface.