Optical, structural and thermal characteristics of Cu–CuAl2O4 hybrids deposited in anodic aluminum oxide as selective solar absorber

A new selective solar absorber coating of Cu–CuAl2O4 hybrids deposited in anodic aluminum oxide (AAO) on aluminum substrate has been prepared through an electrochemical process. The structure, clarified by computer modeling, featured graded layers with increasing refractive index from the surface to the bottom, of which an overall optical performance of absorptance (α=0.923) and emittance (ε=0.06) was achieved. The stability of the absorber was tested at high temperature and condensation at high humidity. While α decreased after annealing at 200 °C in air for 24 h, ε was slightly influenced; the absorber showed high selectivity (α/ε) of 0.87/0.04. Even after exposing to 300 °C for 48 h, the optical property did not change. XRD analysis showed that crystalline structure of the hybrids did not change after annealing treatment; probable reasons for the optical degradation are presented. Condensation also caused a decrease of absorptance by cracking the surface, but a good selectivity of 0.86/0.03 was obtained.