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Selective absorbers from substituted magnetite-based coatings

Selective solar absorbers have been prepared for industrial applications with an original treatment -- chemical conversion of the surface -- of ferritic stainless steels. They present a high solar absorbance ((alpha) s approximately equals 0.95), a low emittance ((xi) approximately equals 0.20), a high thermal stability, and a high resistance to corrosion by atmospheric agents. Characteristics of the material have been thoroughly analyzed. Results are given and discussed. Physical chemical composition and texture of the coatings are very complex. The coating is a thin, microporous and rough medium presenting a fractal structure. The main component is Cr3+ substituted magnetite. Optical properties have been studied with the help of a simple model. In spite of the complexity of the material, the spectral selectivity can be explained and predicted taking chemical composition, texture, and depth of the coating into account.