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

Black nickel coatings electroplated in a nickel- and sodium chlorine aqueou s solution have been prepared and parameters in the process have been optim ized to achieve optimal solar selectivity. The best result is a solar absor ptance of 0.96 and a thermal emittance of 0.10, as also has been obtained p reviously 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 backscatte ring (RBS) and H-1(N-15, alpha gamma)C-12 nuclear resonance reaction (NRR) have contributed with new information about surface chemical composition, m orphology and atomic composition in depth profile. These investigations rev eal that the coating is porous and contains mainly metallic nickel at the s ubstrate-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 absorptanc e changes initially, during the first hour of exposure at high temperature, but will then stabilize. Condensation causes a more severe attack on the c oating by cracking it. This is contradictory to the results from previous t ests 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 reser ved.