Preparation and thermal stability of CrAlON based spectrally selective absorbing coatings

Abstract

Spectrally selective absorbing coating is the core component of the utilization of solar energy. The spectral properties of selectively absorbing coating directly determine the conversion efficiency of constructing solar power plants. To enhance the selective absorbability and thermal stability, we propose an idea that these metal particles are replaced with transition-metal nitrides, and then coated with periodic nanocrystalline-amorphous heterogeneous structures. Double-absorbing layer Cr/CrAlN/CrAlON/CrAlN/CrAlON/CrAlO solar selective absorbing coatings with a high solar absorptance of 0.90 and a relatively low emittance of 0.15 are obtained by the cathodic arc ion plating technique. After the coating is aged at 500 °C in air for 220 h, its absorptance increases to 0.94 and the emittance decreases to 0.10. More importantly, the coating exhibits an outstanding thermal stability with a selectivity of 0.94/0.11 even after being aged at 500 °C for 1000 h in air. The microstructure analysis indicates that the multilayer coating consists of aperiodic CrAlN and CrAlON layers in addition to the Cr and CrAlO layers. Through the long-term aging, a small number of AlN, CrN and Cr₂N nanocrystallites are observed to be homogeneously embedded in the CrAlN and CrAlON amorphous matrices. The nanoparticles in the CrAlN and CrAlON layers can effectively scatter the incident light into a broadband wavelength range, increasing the optical path length in the absorbing layers, and thus resulting in a pronounced enhancement in the absorptivity. A handful of Cr₂O₃ and Al₂O₃ nanograins are observed to be embedded in the amorphous CrAlO antireflection layer, which can effectively reflect the solar infrared radiation and the thermal emittance from the substrate, and thus resulting in pretty low infrared emissivity. The good thermal stability is attributed to the excellent thermal stability of the dielectric amorphous matrices and the sluggish atomic diffusion in the nanoparticles, which could effectively slow down the inward diffusion of oxygen and avoid agglomerating the nanoparticles. These results are of great importance for enhancing the overall performance of cermet spectrally selective absorption coating and also for improving the conversion efficiency of solar energy photo-thermal utilization.