Effect of surface treatments on absorptance and morphology of molybdenum and silica absorbing thin films

Abstract

The thermal performance of solar collectors depends on the selective coverage over their absorbing plate, known as selective surfaces, which determines the heat gain and loss of the equipment. Different materials and layering arrangements can be used for the production of selective surfaces. This work proposes the production of selective surfaces of molybdenum (Mo) and silica (SiO2) evaluating the influence of the type of substrate treatment (chemical cleaning, passivation and electropolishing) on this performance. The results obtained in UV–Visible and Near Infrared Spectrophotometry showed that the surface absorptance is higher for Mo and Mo/SiO2 films on electropolished surfaces compared to treatments with acid and hexane. The highest absorptance reached was 98.10% for a Mo/SiO2 film (45.023 nm), on electropolished substrate. However, the highest solar selectivity was obtained from the Mo/SiO2 film on a passivated substrate. X-ray diffraction (XRD) exhibited peaks characteristic of the metallic Mo phase, and the presence of silica in the amorphous and crystalline phases in the form of quartz was also verified. The results obtained in Optical Profilometry and Atomic Force Microscopy (AFM) indicate that the absorptance of the samples is influenced by the roughness of the substrates.