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

Coatings with a double absorbing layer based on AlSiN/AlSiON were deposited on stainless steel substrates by magnetron sputtering technique, with different Al:Si ratios. A tungsten layer was used as a back reflector and AlSiOy or SiOx thin films were used as antireflection top layers. Prior the structure design, several single layers were deposited on glass substrates by varying the reactive gases flows, which allowed the stacking of a series of layers with different optical properties. Experimental transmittance and reflectance were modelled for the assessment of the spectral optical constants, which were then used to design a coating stack with optimized solar absorptance and thermal emittance. Optical properties, microstructure, morphology, composition and chemical bonding were investigated by employing optical spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The samples were annealed in air at 400 °C and vacuum at 580 °C with the purpose to evaluate their oxidation resistance and thermal stability, which was subsequently 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 absorbance and thermal emittance for the optimized multilayer selective coatings is 93-94% and 7-10% (at 400 °C), respectively.