Progress in sol-gel process

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

While considerable progress has been made in the preparation of ceramic membranes by sol-gel processing, the development of membranes from hybrid polymers is in its infancy (see also Section V). This is, nevertheless, a very promising area of development, because the possibility of forming mechanically stable membranes by inorganic polycondensation is implemented by the possibility to incorporate organic functions. 

In this chapter, recent progress in several key areas is reviewed. These areas are catagorized by material classification rather than by end-use application (1) bulk silica optics (2) optically active doped silica glasses (3) gel-polymer composites (4) organically modified silicates (ormosils) and (5) gradient-index glasses. These represent the five most significant developments in the area of bulk optical materials by the sol-gel process to date. 

Sol-gel processing makes possible the synthesis of solid products by gelation rather than by crystallization or precipitation. This process can be described as the creation of an oxide network by progressive polycondensation reactions of molecular precursors in a liquid medium, or as a process to form materials via a sol, gelation of the sol, and finally removal of the solvent. Publications dealing with sol-gel processing have appeared. " ... 

In addition to classical surface modification techniques (chemisorption, covalent bonding, and film deposition) a few other methods have become progressively popular or have found a renewed interest here, emphasis is put on the sol-gel process and the layer-by-layer (LBL) technique. Only a few illustrative recent references wQl be cited, from which most of the older extensive literature can be traced out. 

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