The optical properties of electrolytic black cobalt coatings with different film thicknesses are reviewed.
Although the intrinsic selectivity of black cobalt is only moderate, efficient selective surfaces can be prepared on highly infrared reflecting metals like smooth copper. The optical and structural investigations show that the porous material consists of a granular distribution of a cobalt-rich phase, which causes a significant infrared absorption.
Due to this, the angular dependence of the thermal emission is strongly dependent on film thickness. The influence of different annealing steps on the film properties is discussed.
It is shown that black cobalt can be used in high-temperature designs of flat plate evacuated tubular collectors, provided the annealing process is performed under inert conditions. If the coating is used in air, degradation may occur due to oxidation of cobalt metal above 200°C.
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