A new selective film for a stainless-steel solar collector has been developed. The selective film is produced by a chemical conversion process, the bath of which is a mixed acid of Na2Cr2O7 and H2SO4. Examining the conversion process, it was found that large amounts of Fe and Cr were embedded in an oxide film as the metallic state. A theoretical analysis shows that metals within an oxide layer form a film with a graded optical index and that selectivity is realized. When the concentration of metals within an oxide film increases linearly, excellent selectivity is obtained. The selectivity was evaluated by the reflectance and the temperature rise of samples using a hot box placed in the sunlight. The color of the film treted by the new bath is black; this new black film shows a selectivity that is as good as that of black chrome film formed on a Cu substrate.
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OHNO et al. (1987) studied this question.
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