Spectrally selective composite coatings of Cr–Cr 2O 3 and Mo–Al 2O 3 for solar energy applications

Abstract

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 is presented. Since magnetron sputtering is a promising method to produce thin, solar selective films, a study of cermet Cr–Cr 2O 3 and Mo–Al 2O 3 coatings obtained by this technique in a reactive atmosphere is presented. The multilayered cermets produced have a thickness of approximately 300 nm and were based on metallic chromium (molybdenum) in a matrix of a chromium oxide (aluminium oxide) with a gradient in oxygen composition. The selective cermet graded films were produced by a reactive DC magnetron sputtering of pure chromium (aluminium with molybdenum) target in a plasma of argon–oxygen at different sputtering pressures (ranging from 5×10 −3 to 1.2×10 −2 mbar) and substrate temperatures (150 and 250°C). The microstructure, surface 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 with 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.