Optical properties and thermal stability of Al/Al 2 O 3 cermet solar absorbers

A broad spectrum of metal/dielectric composite materials for solar absorber coatings has been investigated in the past. In order to find a cost-effective coating which is easy to manufacture, the material combination Al/A1203 was investigated. The coatings were produced by simultaneous evaporation of Al and A12O3 from two separate electron gun sources as well as reactively by evaporation of Al in oxygen. The results of both procedures are compared for layers with a homogeneous and a graded filling factor of the metal. A solar absorptance of 0.96 at a thermal emittance (373 K) of 0. 12 was achieved. The thermal stability of the coatings was investigated by annealing in air and under high vacuum conditions. The stability was poor in both cases. The changes of the material composition due to the degradation was investigated by Auger depth profiling.