Spectral properties and thermal stability of solar selective absorbing AlNi-Al₂O₃ cermet coating

A new solar selective absorbing coating of AlNi-Al₂O₃ cermet multilayer is designed and prepared on Cu or stainless steel (SS) substrate by magnetron sputtering. The coating consists of an IR-reflective metallic layer, an absorption bilayer and an anti-reflection layer from substrate to top. The IR-reflective metallic layer is Cu or Mo. The absorption bilayer consists of a high metal volume fraction (HMVF) AlNi-Al₂O₃ cermet sublayer and a low metal volume fraction (LMVF) AlNi-Al₂O₃ cermet sublayer. The anti-reflection layer is Al₂O₃. The optical performance (α/ε) of the coating is 0.94/0.07 on Cu substrate and 0.95/0.078 on SS substrate by optimizing the metal volume fraction and thickness of each layer. The samples deposited on SS substrate were heated at 500°C for 138h and 284h in vacuum to evaluate the optical stability. Experimental results showed that the high temperature stable AlNi intermetallic compound, instead of Mo in the traditional Mo-Al₂O₃ coatings, displayed a good thermal stability at 500°C. There is no significant change appearing in the optical properties of the coating after the heat treatment. It means that the new coating is a good potential candidate as a high temperature solar selective absorbing coating for parabolic trough concentrated solar power (CSP).