Refinement of solar absorbing black chrome microstructure and its relationship to optical degradation mechanisms

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Abstract
An in depth characterization of the electrodeposited solar absorber black chrome resulting in refinement of the microstructural model of the coating is presented in light of current studies in valence band Auger electron spectroscopy, x‐ray photoelectron spectroscopy, thermal desorption spectroscopy, secondary ion mass spectroscopy, and the chromium electrodeposition mechanism. Through the presented model, the structural and chemical parameters of the coating are linked to its optical response with the degradation of the optical response at elevated temperatures related to changes in these parameters. The degradation process is divided into two distinct phases. First, upon heating to low temperatures (≲300 °C) the chromium hydroxide localized on the surface and within the microstructure of the film decomposes producing H2O and chromium oxide leaving an expected microvoid‐metallic particle structure. Heating to temperatures in excess of 300 °C leads to the second degradation phase. The fine chromium crystallites comprising the film undergo oxidation and the metallic chromium component, responsible for the intrinsic optical absorption, is diminished. The optical model of the solar absorbing film has been refined within the effective medium approach to account for the flat response in the visible part of the spectrum.