Structured selective coatings

Abstract

Nickel-impregnated anodized Al₂O₃ films on aluminium substrates were investigated as possible selective coatings for the photothermal conversion of solar energy. A purely chemical dip process for producing these spectrally selective coatings was developed as an alternative to the electrochemical process reported earlier. The process parameters were optimized to give α = 0.92 and ε = 0.19. These coatings are cheap, efficient and durable. X-ray diffraction, scanning electron microscopy and electron spectroscopy for chemical analysis investigations were performed at various stages during the formation of these coatings and their structure was determined from the data obtained. The proposed structure agrees with that reported earlier. Maxwell Garnett theory and the concept of the equivalent homogeneous medium were used to determine the theoretical optical properties of these films as a function of (1) the thickness and porosity of the Al₂O₃ layer and (2) the thickness of nickel deposited in the pores. The optimum values of the theoretical parameters are in good agreement with those determined experimentally.