Preparation and characterization of black cobalt solar selective coatings

Black cobalt (cobalt oxide) solar selective coatings were prepared on copper substrates by using an electroplating technique. The changes in the physical properties of the cobalt coating due to thermal degradation were characterized by using Auger electron spectroscopy, glow discharge spectrometry, ultraviolet-visible-near infrared spectrophotometer and atom force microscopy. The optimum black cobalt solar selective coating was prepared on a copper substrate by using a synthesized electrolyte with CoCl2 and KSCN at a current density of about 0.5 A/dm2 for 45 s ∼ 60 s. Its optical properties were a solar absorptance of the order of 0.80 ∼ 0.84 and a thermal emittance of 0.01. From the depth profile analysis, the concentration of cobalt particles near the film-substrate interface was higher than that at the surface, but the oxygen concentration at the surface was higher than that at the interface. These results suggest that the selective absorption was dominated by this chemical composition variation in the cobalt coating. The surface morphology of this film, as observed by using atomic force microscopy, showed that the surface was composed of a nearly uniform coating with a small rough surface and pores. We also found that the thermal degradation of the black cobalt coating annealed for 43 hours at temperatures of 250 ∼ 350 .deg. C occurred primarily due to diffusion of copper substrate materials into the cobalt film. The results of the thermal stability test showed that the black cobalt selective coatings on copper substrates were stable during use at temperatures under 250 .deg. C.