Optimization and evaluation of black zinc selective solar absorber surfaces

Abstract

Black zinc oxide surfaces produced by anodix oxidation in an alkaline electrolyte have been studied for potential application as solar-selective absorber surfaces for low temperature photothermal solar energy conversion. Multiple linear regression analysis has been used to estimate the functional dependence of the surface optical properties and thermal stability on the anodizing parameters. Anodizing parameters studied, at various stages of the investigation, include NaOH, NaNO 2, NaNO 3 and zinc concentrations, bath temperature, current density, anodizing time (as charge transfer) and substrate form. Reproducibility of the coating surfaces was demonstrated and the models obtained were used to specify optimized ranges of values for the parameters in terms of the production of surfaces exhibiting high solar absorptance and good thermal stability. The effects of aging on optical properties as a function of temperature (up to 350 °C) and time (up to 320 h) were evaluated. Structural features of the coatings were evaluated utilizing the techniques of scanning electron microscopy, transmission electron microscopy, Auger electron spectroscopy and reflection high energy electron diffraction. Possible mechanisms for solar absorptance and thermal degradation are suggested on the basis of the structural observations.