Sol-gel yesterday, today and tomorrow

Abstract

7000 years ago, the Assyrians made a multicomponent oxide glass. In the 19th century, silica glass was produced from the melt. The origin of the sol-gel process, which almost led to silica glass, stems from the same time, but soon after fell into oblivion. The first sol-gel patent in 1939 demonstrates the formation of SiO2 and TiO2 layers, and in 1953 the first layers were introduced into the market. The further industrial development of the sol-gel dip-coating process led to the present products like the blue car rear-view mirror, the antireflex coatings Mirogard® and the sun-shielding coatings IROX®. It was demonstrated in 1968 that it is possible to make glasses, glass ceramics and ceramics in a well-defined way from multicomponent alkoxides without going through the melt. The first example was the borosilicate glass Duran. After the first publication [1] and an incubation period of several years, a world-wide sol-gel development began, which still lasts, and which is probably best characterized by the slogan "better ceramics through chemistry". Besides thin layers - which probably still are the major sol-gel product - powders, fibres and objects like hollow spheres, plates, rods and tubes were developed. Often the development started with silica glass and then continued with multicomponent oxides. New products are the non-reflecting shop-window glazing Amiran® and the contrast-enhancing screens Conturan® for computer displays. The development of organically modified silicates was stimulated by the rich chemistry which is so typical for sol-gel processes. The future of the sol-gel process is characterized by the manifold chemical possibilities, like PLTZ (Pb-La-Ti-Zr oxides) for optoelectronics, PbTiO3 for piezoelectrics, NASICON (Na3Zr2Si2PO12, e.g. as a superionic conductor) or V2O5 because of its electrical conductivity, and Sb2S3 because of its photosensitivity. Enormous efforts are made for oxidic superconductors (fibres, layers). Furthermore, there ist the combination of inorganic and organic materials with a multitude of applications in photochromics, electrochromics, non-linear optics and much more.