Spectrally selective coatings are used in solar collectors to concentrate the incident radiation. Their function is to enhance the operating efficiency of systems used for the production of electricity. These coatings have to be able to withstand high operating temperatures for long periods of time, and even higher temperatures for shorter times while the collectors are not in use.

Many materials and combinations of materials possess suitable optical properties and sufficient thermal stability for use at temperatures below 300°C. To improve the efficiencies of these systems, however, the coatings would be required to withstand significantly higher temperatures and few absorbers are stable in air at temperatures above 400°C.

For this reason researchers at the Universität Konstanz, Germany, have studied two systems, at temperatures up to 1000°C for potential use as solar selective absorber surfaces, these being platinum on alumina and three molybdenum silicide modifications on alumina (J. H. Schön, G. Binder and E. Bucher, “Performance and Stability of Some New High-Temperature Selective Absorber Systems Based on Metal/Dielectric Multilayers”, Solar Energy Mater. Solar Cells, 1994, 33, (4), 403–416).

The optical properties of thin sputtered films of platinum on alumina were evaluated and found to agree with existing data. Thermal sta-bilities were tested for up to 400 hours at temperatures of 550 to 825°C. Platinum/alumina multilayers on quartz were stable in air up to approximately 700°C. The solar absorptance changed during a 300 hour experiment from α = 0.92 to 0.90. On the actual metal used as a support in such collectors the coatings degraded faster, and at over 640°C the optical changes became more obvious.

Absorptances up to α = 0.95 and emittances of ε ~ 0.1 were achieved for platinum/alumina, and the coatings are thermally stable in air up to 600°C for a 400 hour testing period.