Thermal Stability and Degeneration Behavior of Solar Selective Absorber Based on WTi-Al2O3 Cermet

A WTi-Al2O3 cermet-based solar selective absorber was prepared to investigate the atomic diffusion induced spectral selectivity degeneration. The as-deposited coating exhibits superior absorptance (0.934) and low thermal emittance (0.098), as well as excellent thermal stability with a selectivity of 0.900/0.07 even after annealing at 923 K for 400 h in Ar ambient. However, the multilayer coating failed after being subjected to annealing at 923 K for 400 h in an air environment, as indicated by a decrease in solar absorptance to 0.912 and an increase in thermal emittance to 0.634. The microstructure characterizations reveal that the annealed coating exhibits a columnar morphology along the vertical direction of the substrate. The presence of abundant grain boundaries in the multilayer coating promotes the outward diffusion of Cr and Mn atoms in the stainless-steel substrate. The Mn atoms, in particular, possess the capability to migrate towards the surface of the coating and undergo an oxidation reaction with oxygen, facilitating the formation of a thick Mn2O3 layer. The roughness of the coating surface was significantly increased in this case, adversely affecting solar absorptance due to amplified sunlight reflection. In addition, the rocketing of thermal emittance is attributed to the destabilization of W infrared reflective layer during the annealing. These findings highlight the importance of considering the outward diffusion of Mn and Cr elements in the stainless-steel substrate when optimizing solar selective absorbers.