Chromium-based thin sputtered composite coatings for solar thermal collectors

Abstract

One of the fundamental components of a solar thermal collector is the absorber surface that should be spectrally selective for solar radiation. Efficient solar photothermal conversion benefits from spectrally selective absorber surfaces. Most solar selective coatings use metal-dielectric composites, known as cermets, as the absorber of solar energy. In this study we will present a numerical model that allow us to correlate the selectivity of the produced absorbers to the collector efficiency. The cathodic magnetron sputtering being a promising method to produce thin solar selective films, a study of some cermet Cr–Cr 2O 3–CrO 3 coatings obtained by this technique in reactive atmosphere and using a DC power unit will be presented. The multilayered composites produced were based in metallic chromium in a matrix of a chromium oxide with a gradient in oxygen composition. The selective graded films were produced by a reactive DC magnetron sputtering of pure chromium target in a plasma of argon–oxygen at different sputtering pressures and substrate temperatures. The microstructure, surface roughness, crystallographic phases, composition and chemical analysis by X-ray photoelectron spectroscopy and reflectivity spectra in vis-NIR were analysed.