Effect of multilayered CoO-CoAl2O4 films on improving solar absorptance of Co-WC solar selective absorbing coatings

The multilayered CoO-CoAl2O4 solar selective films were prepared on Co-WC coatings with different mole ratio of Co and Al by sol-gel process. The thin film has high and low refractive indices, respectively. The inner layer can improve surface condition of Co-WC coating as a high absorbing film and buffer lay, annealed at 650 °C in vacuum. The outer layer was annealed at the same condition. The absorptance (α) of the multilayer structure coating increased from 0.873 to 0.932. The properties of the sols were characterized by thermogravimetry-differential scanning calorimetry (TG-DSC). The phase compositions and structure of samples annealed at 650 °C were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure and elements distribution of thin films were investigated by field emission scanning electron microscopy (FE-SEM) and energy dispersive spectrometer (EDS), respectively. The absorptance of coatings indirectly indicate the binding force between the Co-WC substrate, durability, long-time stability and corrosion resistance are fine. The results illustrated that defects on surface of the Co-WC coating gradually decrease with the deposition of sol films. And the good element gradients in multilayer structures and the structure of thin films might be the reasons for its improvement in absorptance.