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

Titanium-doped and undoped CuCoMnO_x spinel films were deposited on Al substrates from sols which were made from the following: Co-acetate, Cu-chloride and Mn-acetate (Ti:CoCuMnO_x-I); and Co-acetate, Cu-nitrate and Mn-acetate (CoCuMnO_x-II). The precursors’ ratio Co:Cu:Mn was equal to 1:3:3. The solar absorptance (α_s) and the thermal emittance (_T) of the films, which were annealed at 450°C for 15 or 30 min, were determined from the corresponding diffuse reflectance spectra in the 0.32–20 μm range. The results show that the CoCuMnO_x-II films with SiO_x protective over-coatings exhibited values of α_s=0.85–0.91 and _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 CoCuMnO_x. 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 140°C for 30 min.