Sol-gel deposition and characterization of low refractive index nanocomposite Mg:F:Si:O thin films for solar and protective coatings

Coatings for thermal solar energy conversion have to be abrasion-resistant, stable at elevated temperatures, and often have to protect the substrate against corrosion. Due to the conditions of refractive index matching between the substrate and the ambient air, low refractive index materials are desired regarding a large variety of optical applications. Novel nanocomposite thin films based on the elements Mg, F, Si and O have been deposited by a sol-gel dip-coating process starting from trifluoroacetic acid, magnesium acetate and alcoxide precursors. Optical properties of this new material have been characterised by spectrophotometry and spectral ellipsometry, and show surprisingly low refractive index values. The experimental data obtained for varying chemical composition have been compared to the optical properties as predicted by Bruggeman and Ping-Sheng effective medium theories. Strong deviations from both theories are reported, suggesting modifications of the involved crystalline phases and/or formation of nanopores. Evidence for embedded crystalline nanoparticles has been found by Transmission Electron Microscopy (TEM). Compared to well known existing low refractive index materials such as porous SiO2 and MgF2, the quaternary Mg:F:Si:O thin films can be expected to show higher hardness and improved aging stability with respect to pore filling by hydrocarbons. The novel thin film material opens promising perspectives for new solar and protective coatings