Optical properties and thermal stability of solar selective absorbers based on Co-Al2O3 cermets

Cobalt alumina cermet thin films were deposited on stainless steel substrates and the related solar selective absorption was studied. The prepared cermet thin films were characterized optically, structurally, and morphologically. The measured reflectance spectra of the cermet thin films were used to calculate the pertinent normal solar absorptance and normal thermal emittance. The structure of the absorbing coating was determined by X-ray diffraction (XRD). The related surface structures were observed by a scanning electron microscope (SEM). The sample with a Co/Al = 40/60 ratio showed the lowest thermal emittance. Energy-dispersive X-ray spectroscopy (EDX) was used for elemental analysis. A multilayer structure (high and low Co/Al ratio layers and an antireflection coating of Al2O3) was prepared to inhibit optical degradation. The thermal stability of the solar selective absorbing coating was tested by annealing at high temperature in air.