Optical Design Of Al-AlN Solar Selective Absorbing Coatings

The fabrication of solar selective absorbing coatings with high-performance is a key technology for solar thermal utility. The improvement of the performance of solar selective absorbing coatings by optimizing their coating structure design and their fabrication technology is one of the main research directions in the field of solar selective absorbing coatings.Medium frequency reactive magnetron sputtering was used to deposit Al-AlN composite films and the influence of nitrogen flow rate on the microstructure and optical constants of Al-AlN composite films was studied. It was found that the nitrogen concentration in Al-AlN composite films is increased and the films are denser with the increase of nitrogen flow rate. With the volume fraction of aluminum in Al-AlN composite films, the largest refractive index of Al-AlN composite films with a aluminum content of 0.2-0.3 is found, and the extinction coefficient of Al-AlN composite films is increased with the increasing of metal volume fraction.Recursive method was used to study the influence of the film structure on the optical properties of Al-AlN solar selective absorption coatings, the results as follows were found: for the Al/Al-AlN/AlN solar selective absorbing coatings, the absorbtivity of Al/Al-AlN/AlN solar selective absorbing coatings is increased first with the increase of aluminum volume fraction in Al-AlN absorbing layer, and then is decreased after a largest absorptivity is found. The visible light and infrared reflectance of the Al/Al-AlN/AlN solar selective absorbing coatings is decreased and the critical wavelength of the reflectance curve is moved to a longer wavelength with the increase of thickness of Al-AlN absorbing layer. For Al/Al-AlN(Ⅱ)/Al-AlN(I)/ AlN solar selective absorbing coatings, when the aluminum volume fraction of Al-AlN(I) layer (f1) and the aluminum volume fraction of Al-AlN(Ⅱ) layer (f2) are changed in the range of 0.19-0.26 and 0.35-0.55 respectively, a high absorptance is found; the critical wavelength of the reflectance curve is moved to a longer wavelength with the increase of f1; the spectrum selectivity can be improved and the the critical wavelength of the reflectance curve is moved to a longer wavelength with the increase of f2. With the increase of the thickness of Al-AlN(I) layer or Al-AlN(Ⅱ) layer, the critical wavelength of the reflectance curve is moved to a longer wavelength, but their influence is not so obvious as that of f1 and f2.A high absorptance and low emissivity can be obtained for the selective absorption coatings whose d0, d1, d2, f1 and f2 are 60nm, 40nm, 100nm, 0.23 and 0.4 respectively.