Spectral Selective Solar Energy Absorbanck of Electrolytically Coloured Black Anodic Films on Aluminium

Aluminium sheet of commercial purity was anodized and electro-lytically coloured in nickel base solution, forming light/thermal conversion coatings. It was found that the colouring of films formed in sulphuric acid was more difficult, if not impossible, as compared with films formed in phosphoric acid. Morphology observations, using ultramicrotomy allied with transmission electronmicroscopy, suggest that the easiness of colouring is related to the diameter of the pore base, independent of the pore size in the surface layer of the film. Good solar energy absorbtive anodic films were obtained by anodizing in phosphoric acid and electrolytically colouring, with absorbance α=0.93 at λ<2.5μm and emittance ε=0.26 at λ>2.5μm.