Thermal stability of high entropy graded solar selective absorbers deposited by non-equilibrium RF magnetron sputtering

In this paper, we evaluate the thermal stability of high-entropy AlCrNbSiTi-based selective solar absorbers (SSAs) in air and vacuum medium. The SSAs have been prepared by a recently developed non-equilibrium reactive RF magnetron sputtering route. This method has synthesized the effective one-layer SSAs with gradient metal and oxygen composition. A few different targets are being used to evaluate the elemental composition effect on the high-temperature stability of the SSAs. AlCrNbSiTi target with an element ratio of 3:2:1:1:2 shows the highest stability after heating at 500 degrees C for 1 h in air. The emittance and absorptance of the absorbers on Mo and W infrared mirrors are in the range of 0.05-0.12 and 0.11-0.23, and 0.91-0.92 and 0.93-0.96, respectively. High-resolution transmission electron microscopy exhibits the amorphous struc-ture of the absorbing layers with gradient metal and oxygen composition. The graded high-entropy SSAs demonstrate stability with PC < 0.05 in the air at 500 degrees C for 4 h and in a vacuum at 700 degrees C for at least 1 h. Heat treatment at 500 degrees C for 14 h in the air leads to equalization of the element concentrations and increase of the oxygen concentration at the bottom of the absorbing layer, which results in deterioration of SSA optical performance.(c) 2023 Elsevier B.V. All rights reserved.