On the origin of the aging process of porous SiO2 antireflection coatings

Broad-band and low-cost antireflection coatings are necessary for solar collector covers. Porous SiO2 coatings of ≈100 nm thickness, prepared by a dip-coating process with silica sol as a precursor, can increase the transmittance of float glass up to 99%. However, exposure of the coated glass to ambient atmosphere or even storage in a desiccator leads to an aging with a successive loss in transmittance. This aging process is reversible and may be removed either by a repeated heat treatment at 500°C or by a cleaning with a polar solvent such as water or acetone. The aging process was investigated and it could be shown that there are two overlapping mechanisms. In a faster process with a reaction time of weeks, residual Na+ ions from the silica sol react with H2O and/or CO2 from the air. In a slower process with a reaction time of months, Ca2+ ions diffuse from the float glass substrate into and even through the coating and form products such as CaCO3. In both cases, the pores are filled and the refractive index of the SiO2 coating is increased.