Preparation and Characterization of Black Chrome Solar Selective Coatings

Abstract

Black chrome coatings (Cr-Cr2O3) were prepared on nickel-coated copper substrates by using an electroplating technique. The changes in the physical properties of the sample due to thermal degradation were characterized by Auger electron spectroscopy, glow discharge spectrometry, and UV-Vis-NIR spectrophotometry. The optimum black chrome coating was made on a nickel-coated copper substrate by using a newly synthesized electrolyte at a current density of 260 270 mA/cm 2 for 1.5 2.0 min. Its optical properties were a solar absorptance of 0.80 0.90 and a thermal emittance of 0.01 0.03. From the surface analysis, the concentration of chrome granules near the nickel layer was higher than at the surface, which suggests that the selective absorption was dominated by finely divided metallic chrome particles. The surface of this film exhibited a morphology with root-mean-square roughness of 55.83 nm. We also found that the thermal aging characteristics of the black chrome coating annealed at temperature of above 450 C occurred primarily due to both oxidation of the metallic chrome particles and diusion of the copper substrate materials. Atomic force microscopy showed that black chrome layers with increasing annealing temperatures became very rough and that their grain sizes became very large. X-ray photoelectron spectroscopy measurements showed that the binding energy of the major component in the as-prepared black chrome films was chrome in the hydroxide form. The result of the thermal stability test showed that the black chrome selective coating was stable for use at temperatures under 350 C.