Spectrally selective composite coatings of Cr-Cr2O3 and Mo-Al2O3 for solarenergy applications

Efficient solar photothermal conversion benefits from spectrally selective absorber surfaces. In this paper, a numerical model that allows correlation of the selectivity of the absorbers produced to the collector efficiency i s presented. Since magnetron sputtering is a promising method to produce th in, solar selective films, a study of cermet Cr-Cr2O3 and Mo-Al2O3 coatings obtained by this technique in a reactive atmosphere is presented. The mult ilayered cermets produced have a thickness of approximately 300 nm and were based on metallic chromium (molybdenum) in a matrix of a chromium oxide (a luminium oxide) with a gradient in oxygen composition. The selective cermet graded films were produced by a reactive DC magnetron sputtering of pure c hromium (aluminium with molybdenum) target in a plasma of argon-oxygen at d ifferent sputtering pressures (ranging from 5 x 10(-3) to 1.2 X 10(-2) mbar ) and substrate temperatures (150 and 250 degreesC). The microstructure, su rface roughness, crystallographic phases, composition and chemical analysis were determined by X-ray photoelectron spectroscopy, reflectivity spectra in the vis/NIR region were analysed, and thermal emissivity was measured wi th an emissometer. The coatings have high spectral selectivity, with solar absorption ranging from 0.88 to 0.94 and thermal emissivity ranging from 0. 15 to 0.04, depending on the coating materials and sputtering conditions. ( C) 2001 Elsevier Science B.V. All rights reserved.