Aging of solar selective Cr-SiO absorbers
Ageing of Solar Selective Cr-SiO-AbsorbersM. Djahanbakhsh, J. Eldring, B. Kiuge, L. K. ThomasInstitut fur Metallische Werkstoffe-Metallphysik-,Technische UniversitatBerlin,Hardenbergstr. 36, 10623 Berlin, FRGABSTRACTThin films of Cr-SiO-cermets produced by evaporation on metal substrates show spectral selectivity. This propertydepends not only on the substrate material, the average composition and the thickness of the film but also on theconditions ofthe preparation. In particular the temperature ofthe substrate dunng preparation has a strong influence onthe change of selectivity during ageing. Ageing processes have been investigated in the temperature range 400 to 800°Cin Aratmosphere for time periods up to 42 days by means of optical measurements, Xray analysis and other properties.The main effect is the decrease ofthe thickness of the film and a change of the structure. By dissociation of the SiO intoSi and °2 all chromium-suicides have been observed which are known from the binary CrSi-constitution diagram withthe exception of CrSi. Some formation of Cr203 was observed not because of the development of oxygen by the reduc-tion of $102 but by residual oxygen in the Ar-atmosphere.I . INTRODUCTIONSelective absorber coatings change their properties because they are not in thermodynamic equilibrium. Usually onetries to use components with high melting points and stable oxides so that changes of properties remain small duringageing even at higher temperatures which are necessary to provide heat for industrial processes. In earlier investigationswe tested the optical properties of thin films of Cr-SiO-cermets on metal substrates12. But there is missing a betterunderstanding of the long-time behaviour and the ageing process of these coatings in the temperature range of theintended industrial use.In this paper we were interested in the ageing of these Cr-SiO-cermets and in some parameters which influence theirproperties. We did not consider reflectivity only but included also the geometry of the films, the hardness and particularlythe structure. For the ageing expenments the samples were kept in Ar-atmosphere in the temperature range 400 to800°C for periods up to 1000 h (42 days).