Solar selective absorbing coatings based on AlSiN/AlSiON/AlSiOy layers

Abstract

Coatings with a double absorbing layer based on AlSiN/AlSiON were deposited on stainless steel sub-strates by magnetron sputtering technique, with different Al:Si ratios. A tungsten layer was used asa back reflector and AlSiOyor SiOxthin films were used as antireflection top layers. Prior the structuredesign, several single layers were deposited on glass substrates by varying the reactive gases flows, whichallowed the stacking of a series of layers with different optical properties. Experimental transmittanceand reflectance were modelled for the assessment of the spectral optical constants, which were thenused to design a coating stack with optimized solar absorptance and thermal emittance. Optical prop-erties, microstructure, morphology, composition and chemical bonding were investigated by employingoptical spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffrac-tion and X-ray photoelectron spectroscopy. The samples were annealed in air at 400◦C and vacuum at580◦C with the purpose to evaluate their oxidation resistance and thermal stability, which was sub-sequently correlated with the Al:Si ratio. Optimum results were achieved for an Al:Si ratio of 2.3:1,whereas for significantly higher Si content resulted in detrimental performance. The solar absorbanceand thermal emittance for the optimized multilayer selective coatings is 93–94% and 7–10% (at 400◦C),respectively.