Selective black absorbers using rf-sputtered Cr2O3/Cr cermet films

Excellent selective black absorbers for solar radiation, with solar absorptivity of over 0.9 and infrared emissivity of less than 0.1, have been obtained by using rf sputtering to coat metal substrates with a Cr2O3/Cr cermet film, then with a Cr2O3 antireflection coating. By determining the optical constants of Cr2O3/Cr films over the composition range up to 35 vol% Cr, it was found that films containing 29 vol% Cr are nearly optimum for this application. The optical and chemical properties of absorbers made with such films are quite similar to those obtained with electroplated Cr-black coatings. This similarity suggests that the electroplated coatings are actually Cr2O3/Cr cermets comparable in composition and microstructure to the sputtered films.