Graded selective coatings based on chromium and titanium oxynitride

Abstract

In order to improve the performance of thermal solar collectors, the development of good, durable and reproducible spectrally selective solar absorber surfaces for photothermal conversion is of greatest importance. Spectrally selective cermet coatings were produced by DC magnetron sputtering using metallic chromium and titanium targets at constant target current, substrate bias and substrate temperature. These coatings are graded cermets with a metal concentration decreasing from the substrate to the coating surface. Basically, these coatings are formed with a layer structure consisting of a cermet layer enriched in metal, one or more cermet layers with less metal than the first one, and on top a pure ceramic layer as an antireflection layer. The metal layer was deposited by non-reactive DC sputtering, while the ceramic layers were deposited by DC reactive sputtering in argon–nitrogen or argon–oxygen atmosphere. The metallic and ceramic layers were deposited as a sub-layer system, consisting of alternating metallic and nitride or oxide sub-layers. The effect of the optical properties in dependence on the metallic fraction in the layers and the number of sub-layers in the film is discussed. For coatings based on chromium, the optimum selectivity achieved was a solar absorptance of 94% and a thermal emittance of 6% at 82 °C. For titanium oxynitride based coatings, the best selectivity achieved was a solar absorptance of 91% and a thermal emittance of 4%. The microstructure and thickness were studied by scanning electron microscopy (SEM). The surface microtopography was analysed by atomic force microscopy (AFM) and the metal concentration profile by Rutherford backscattering spectrometry (RBS).