Optical simulation, characterization and thermal stability of Cr2O3/Cr/Cr2O3multilayer solar selective absorber coatings

Abstract Cr 2 O 3 /Cr/Cr 2 O 3 multilayered selective solar absorber coating was deposited onto stainless steel substrates using electron beam vacuum evaporation at room temperature. The optical constants of the individual layers were obtained from reflectance measurements fitted by CODE software using the suitable dielectric function model, and showed that both Cr 2 O 3 layers were dielectric in nature, while the Cr layer was semi-transparent due to its low thickness. Then, the experimental reflectance spectrum of the SS/Cr 2 O 3 /Cr/Cr 2 O 3 solar absorber was successfully simulated using CODE software. It was found that this coating exhibited a good spectral selectivity of 0.82/0.21 at 300 °C. The effect of annealing on the structural, morphological, mechanical and optical properties of the SS/Cr 2 O 3 /Cr/Cr 2 O 3 solar absorber was investigated. SEM revealed an increase in Cr 2 O 3 crystal size after annealing at 400 °C for 24 h in air, this leads to a surface smoothing illustrated by a decrease in roughness. The structural investigations of the annealed multilayered coatings showed a decrease in intensity of the stainless steel XRD peak linked to the increase in crystallinity of Cr 2 O 3 . Nanoindentation measurements showed that after annealing the samples were more resistant to mechanical solicitations as compared to as-deposited samples. Optical analysis showed that the sample annealed at 400 °C for 24 h exhibited higher absorptivity α  = 0.89 and lower emissivity e at 300 °C = 0.15than the as-deposited sample due to material structuring with temperature, which is promising for mid-to high temperature concentrated solar power applications in air. Coating optimization in CODE also indicated that α could be further improved to 0.916 without increasing emissivity.