Characterization of black nickel solar absorber coatings electroplated in a nickel chlorine aqueous solution

Black nickel coatings electroplated in a nickel- and sodium chlorine aqueous solution have been prepared and parameters in the process have been optimized to achieve optimal solar selectivity. The best result is a solar absorptance of 0.96 and a thermal emittance of 0.10, as also has been obtained previously for the same type of coating [1]. Characterization by means of X-ray photo-electron spectroscopy (XPS), scanning electron microscopy (SEM), glow discharge optical emission spectroscopy (GDOES), Rutherford backscattering (RBS) and H-1(N-15, alpha gamma)C-12 nuclear resonance reaction (NRR) have contributed with new information about surface chemical composition, morphology and atomic composition in depth profile. These investigations reveal that the coating is porous and contains mainly metallic nickel at the substrate-coating interface and mainly nickel hydroxide at the front surface. The stability of the coating has been tested regarding high-temperature and condensation at high humidity. It has been found that the solar absorptance changes initially, during the first hour of exposure at high temperature, but will then stabilize. Condensation causes a more severe attack on the coating by cracking it. This is contradictory to the results from previous tests at high temperature and humidity in which the black nickel coating was found to be resistant [1]. (C) 1998 Elsevier Science B.V. All rights reserved.