The application of antireflective coatings to the glass covers of solar thermal collectors, allows increasing the efficiency of the whole system. Among the methods for applying nanostructure coatings, sol-gel method was selected in this study. Via this method, an antireflective silica coating deposited on glass and the solar transmittance was investigated. The precursor solutions were made by mixing tetraethylorthosilicate (TEOS), ethanol, water and alkali (NH3) or acid (HNO3) catalyst in the molar ratio of 1:3.5:0.35:0.005. In addition, the role of two types of catalysts was examined. UV-visible spectroscopy, Fourier-transformed infrared spectrophotometer, Scanning electron and Optical microscopy were used for the characterization of silica thin films. Results showed that nanoporous silica layers cause to considerable reduction of these light reflections compared with uncoated glasses. It was found that the amount of reduction as well as the adhesive properties is depending on the type of catalyst. Results revealed that not only solar transmittance of acidic-catalyzed coatings is higher than of alkali-catalyzed coatings, but also acidic-catalyzed coatings possess adhesive-resistance higher than alkali-catalyzed coatings. However, at high pH condition, the condensation rate is very fast, and the growth tends to form spherically expanding clusters cause to increasing porosity. The dense and strong binding acidic-catalyzed films have a good antireflective property; because of that, the nanoporousity appears on the surface.
Preparation of single- and double-layer antireflective coatings by sol-gel method
