Optical characterization of Zn coatings deposited on low carbon steel substrates

Abstract

We studied the optical reflectance profile of zinc layers with thickness of 2-10 μm electrodeposited on low carbon steel in the spectral range of solar radiation (0.3 - 2.5μm) and in the thermal infrared (2.5 - 15μm). Spectral selectivity is an important parameter of the solar absorbers used in solar thermal collectors. It is defined by low reflectance in the range 0.3 - 2.5 μ, and high reflectance in the thermal infrared, 2.5 - 15 μm. The Zn coatings are thick and their surface is rough. If the surface features size is in the order of the visible wavelengths, then absorption of light at the surface is possible. The investigations revealed the formation of ZnO on the Zn coating surface. The Raman study showed a weak peak around 512 cm-1, which can be attributed to the ZnO oxide phase. The longer wavelengths penetrate the oxide surface and reach the bulk part of the Zn coating where they are reflected. As a result, a step-wise profile is expected in the reflectance spectrum within the 0.3 -15 μm range, with a change from a comparatively low reflectance in the visible to a high reflectance in the infrared. © Published under licence by IOP Publishing Ltd.