Thermally and air–plasma-oxidized titanium and stainless steel plates as solar selective absorbers

Solar selective surfaces can be constructed in many ways. In particular, low emissivity (ε) is currently sought for by starting with surfaces bearing high infrared reflectance. It is well known that metallic surfaces behave in this way. Then, high solar absorptance (α) can be achieved by adding an appropriate thin layer through a wide variety of possibilities. In this work, a simple direct method to produce such type of coatings on titanium and 304 stainless steel plates is assessed: thermal oxidation. Good selectivities S(=α/ε) 10 or higher in some cases were obtained mainly on steel substrates. An alternative is to expose these metallic surfaces to ionized oxygen species, rather than to neutral oxygen molecules. This was accomplished by oxidizing some plates in a typical glow discharge capacitive system. In this case, acceptable but not so high selectivities, as in the first case, were obtained. Some comments on the metallic surface morphology and the stability of the oxidized surfaces are also presented.