Optical properties and thermal stability of Al/Al2O3 cermet solar absorbers

A mixture of dispersed metal in a dielectric matrix, a so-called cermet, offers the possibility of tailoring the refractive index. Combined with a reflector on the back side, an interference coating with a spectrally selective reflectance suitable for solar absorbers can be made high 1,2. The solar absorptance of a cermet with a homogeneous metal distribution is limited by the reflectance losses due to its high refractive index. These losses can be reduced in different ways. The so-called graded cermets have a monotone varying filling factor of the metal, decreasing from the substrate mirror to the front of the coating, causing a gradient in the refractive index. Such coatings can be thicker without increasing the thermal emittance too much high 3, because of the relatively low content of dielectric. Another method for the reduction of the reflectance losses is the roughening of the interfaces high 4,5. The dimension of such a micro roughness should be optimized in order to achieve a h igh solar absorptance without a high enhancement of the IR-emittance and a strong disturbance of the interference. The optical properties of aluminium and aluminium oxide as well as the costs of these materials seem very promising for application in solar energy conversion.