Solar absorber coatings based on CoCuMnOx spinels prepared via the sol-gel process:: structural and optical properties

Titanium-doped and undoped CuCoMnOx spinel films were deposited on At substrates front sols which were made from the following: Co-acetate, Cu-chloride and Mn-acetate (Ti:CoCuMnOx-I); and Co-acetate, Cu-nitrate and Mn-acetate (CoCuMnOx-II). The precursors' ratio Co:Cu:Mn was equal to 1:33. The solar absorptance (alpha(s)) and the thermal emittance (epsilon(T)) of the films, which were annealed at 450degreesC for 15 or 30 min, were determined from the corresponding diffuse reflectance spectra in the 0.32-20 mum range. The results show that the CoCuMnOx-II films with SiOx protective over-coatings exhibited values of alpha(s) = 0.85-0.91 and epsilon(T) below 0.036 after just a single dipping/annealing cycle. The structure of the films was studied with X-ray diffraction (XRD), infrared (IR) absorbance and near-grazing incidence angle (NGIA) reflection-absorption spectroscopy. Our results suggested that the films have a spinel structure with the composition CoCuMnOx. The stability of the films was tested by soaking them in boiling water for 2 h. NGIA IR spectra of the treated films confirmed the formation of the hydrated mixed oxide (Mn-, Co-, Cu-) phases. To improve the stability of the films two kinds of protective over-coatings were tested: one over-coating was based on polysiloxane resin and the other on high-density silica (T-resin). Films that were resistant to boiling water were obtained by applying the high-density silica protective over-coating, which was cured at 140degreesC for 30 min. (C) 2002 Elsevier Science B.V. All rights reserved.