Graded solar selective absorbers deposited by non-equilibrium RF magnetron sputtering

Abstract

A new approach to deposit graded solar selective absorbers (SSA) is proposed. High entropy composite SSAs with antireflection layers have been prepared by non-equilibrium reactive RF magnetron sputtering. In order to obtain the gradient composition a relaxation process of target poisoning in the reactive O 2 /Ar atmosphere is employed. After introduction of oxygen the target gradually poisoned, resulting in a smooth change of the absorber composition from pure metals to their oxides. The poisoning relaxation time controls the thickness of SSA and the profile distribution of elements, and can be adjusted by selecting the reactive gas flow rate and RF power. Cintered Al:Cr:Nb:Si:Ta:Ti:Y mixture of powders is used as a target material. The graded SSAs demonstrate encouraging results: a coating with maximal selectivity of 13.4 has the solar absorptivity of 0.884 and emissivity of 0.066. In order to test the deposition method, we also try another target materials (AlTi and AlSc), which show better optical properties with the absorptivity of 0.949 (0.978 for textured surface) and selectivity of 18.6. The non-equilibrium reactive sputtering method proposed in this study has the advantages of simplicity and high production yield compared with the existing methods to deposit effective SSAs including double absorbing layer and multilayer coatings. • A new approach to fabricate graded solar selective absorbers (SSA) is developed. • High entropy composite SSAs was synthesized using nonequilibrium RF sputtering. • Graded composition is obtained using a relaxation process of target poisoning. • The relaxation time of the poisoning defines thickness and profile distribution. • Absorbers demonstrate selectivity of 13.4 and absorptivity of 0.909