Cobalt oxide films for solar selective surfaces, obtained by spray pyrolisis

Cobalt oxide films upon stainless steel substrates were deposited by using the pneumatic spray pyrolisis technique, starting from an inorganic salt (CoNO3·3H2O) dissolved in a water–alcohol mixture. Stainless steel and nickeled stainless steel substrates were used. Absorptance and emittance, for selective surface applications, were evaluated from reflectance measurements in the UV-Vis and infrared ranges. X-ray diffraction, XPS and AFM measurements were done. The predominant cobalt phase is Co3O4, but also CoO and Co2O3 phases, besides metallic cobalt, were detected. Films upon nickeled steel substrates at 400°C exhibit high absorptances (0.86), but also the emittance is high (0.43), yielding a selectivity of 2.0. A similar film on steel substrate reaches only a figure of 0.77 absorptance, but the thermal emittance remains low (0.20), giving a selectivity of 3.85. These films are good prospects for selective solar absorption coatings.