Selective electroplated chromium blacks

The electrodeposition of chrome black selective surfaces on polished metal substrates has been studied by measuring the properties of samples as a function of plating time. Optical measurements show that in the first 50 sec a lossy dielectric film (apparently Cr(2)O(3)) of 0.6-microm thickness is deposited. The weights of deposits and their emittances, as well as the surface profiles of the samples, show that most (0.4-0.5 microm) of this dielectric film falls off between 50-sec and 60-sec plating time. An absorbing layer of chromium metal particles is then deposited on the remaining dielectric film to produce the high solar absorptance characteristic of these films. Calculations based on a particulate chromium deposit linearly graded with air using a recent theory of McKenzie and McPhedran are able to reproduce the film properties with deposit weights similar to those actually measured. It is shown that the thickness of the dielectric layer left under the particulate chromium deposit is important in obtaining the optimum performance in the trade-off between solar absorptance and thermal emittance.