The reduction of light reflection is desirable in many applications such as photovoltaic and photothermic elements, architectural glass or optical elements. The reflection of visible light passing through an optically transparent substrate (e. g., glass) can be reduced by coating the substrate with an optical active thin layer exhibiting a refractive index laying between the refractive index of the substrate (ns˜1.5 in case of glass) and that of air (nair=1). An ideal single layer coating material would have a refractive index around nc˜1.23, resulting in a nearly 100% optically transparent system (see FIG. 1).
Multi-layer systems (interference layers, usually alternating layers of high reflective TiO2— and low reflective SiO2-films) are known in the art. They suffer from high production costs and complex methods of manufacture.
Anti-reflective (AR) oxide monolayers are known in the art. The oxide material with the lowest refractive index is SiO2 (nSiO2˜1.46). To achieve lower n values, porosity has to be introduced into such layers. Up to more than 50% porosity, however, is needed to provide porous layers with n=1.23, resulting in low mechanic stability of such layers.
Some metal fluorides exhibit refractive indexes significantly lower than SiO2. Magnesium fluoride is the most investigated material (nMgF2=1.38). Other properties such as scratch resistance, mechanic stability, thermal stability and hydrolysis resistance are important for applications such as coatings of glass or polymers.
Vapor phase deposition methods such as sputtering can be employed for film deposition. Thin layers obtained thereby do usually not show significant porosity, resulting in a refractive index of close to or corresponding to that of the bulk material. Vapor phase deposited MgF2-layers suffer from non-stoichiometric adjustment of metal to fluorine ratios, leading to point defects (“F-center”) formation, resulting in impaired optical quality of the layer. Evaporation methods may aid in overcoming this problem, however large area depositions are not facilitated thereby because of point defect formation. The introduction of the necessary porosity is insufficient in both methods.
Metal fluoride layers generated from liquid phase deposition were described first based on metal trifluoroacetate sols (S. Fujihara, in Handbook of Sol-Gel Science and Technology, ed. S. Sakka, Kluwer, Boston, 2005, vol. 1, pp. 203-224). In a first step, the metal fluoride trifluoroactetates are deposited onto the substrate and are subsequently decomposed thermally, resulting in very porous metal fluoride layers. Due to the formation of hydrogen fluoride during this thermal decomposition process and a drastic shrinking of the layer thickness, an adjustment of the parameters of such layers is difficult. Moreover, the coated substrate as well as the equipment can undergo corrosion caused by evaporated hydrogen fluoride gas. Insufficient mechanical performance of the resulting layer is a further drawback of this technology.
U.S. Pat. No. 6,880,602B2 (EP 1 315 005 B1) shows a process for obtaining sol solutions of magnesium fluoride by reacting magnesium acetate or methoxide with aqueous hydrofluoric acid in methanol at elevated temperatures under high pressure. This process suffers from significant disadvantages when applied in technical scale, such as the need for high pressure batch reactions and the use of methanol.
US 2011/0122497 A1 (EP 1 791 002 A1) shows MgF2-sols obtained by a high pressure process, with added SiO2-sols as “binders”, which results in acceptable optical and mechanical characteristics of the layers.
Clear magnesium fluoride based sol solutions that are suitable for coating glasses have not been accessible so far from alkoxides other than from magnesium methoxide. Furthermore, the methoxide has only been accessible in methanol, a solvent that poses problems related to its toxicity and workplace safety profile. Magnesium methoxide is not soluble in ethanol or isopropanol, thus blocking the path to clear sol solutions. Magnesium ethoxide, which is available commercially at lower prices, does not dissolve directly in methanol, ethanol or isopropanol.