The microstructure and reflective properties of "CHROM-ONYX" type of black chrome/metal selective absorber was investigated to obtain a better understanding of their influence upon the mechanism of wavelength selectivity. Hemispherical reflectance measurements were performed on seven samples. In this research, the best selectivity a/e ratio was found by these measurements to be 0.7 micron of black chrome on nickel and 1.0 micron of black chrome on copper. Both scanning and transmission electron microscopy were employed to study micro-structure and chemical composition. As a result of the combined studies, some consequences of black chrome thickness and the metallic substrate were determined. It was determined that black chrome consisted of a very fine metallic distribution of particles of chromium in the 100A range, suspended within a matrix of a chromium oxide phase. This assembly was, in turn, agglomerated into larger particles within the 0.05-0.3 micron size range. These larger particles formed a continuous network which constituted the surface coating.
Carl M. Lampert,
"Microstructural Characterization Of A Black Chrome Solar Selective Absorber", Proc. SPIE 0161, Optics Applied to Solar Energy IV, (17 November 1978); doi: 10.1117/12.956876; https://doi.org/10.1117/12.956876