SOL-GEL TECHNOLOGY

The sol-gel process is a versatile solution process for making ceramic and glass materials.  In general, the sol-gel process involves the transition of a system from a liquid "sol" (mostly colloidal) into a solid "gel" phase.  Applying the sol-gel process, it is possible to fabricate ceramic or glass materials in a wide variety of forms: ultra-fine or spherical shaped powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials.  An overview of the sol-gel process is presented in a simple graphic work below.

The starting materials used in the preparation of the "sol" are usually inorganic metal salts or metal organic compounds such as metal alkoxides.  In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymeration reactions to form a colloidal suspension, or a "sol".  Further processing of the "sol" enables one to make ceramic materials in different forms.  Thin films can be produced on a piece of substrate by spin-coating or dip-coating.  When the "sol" is cast into a mold, a wet "gel" will form.  With further drying and heat-treatment, the "gel" is converted into dense ceramic or glass articles.  If the liquid in a wet "gel" is removed under a supercritical condition, a highly porous and extremely low density material called "aerogel" is obtained.  As the viscosity of a "sol" is  adjusted into a proper viscosity range, ceramic fibers can be drawn from the "sol".  Ultra-fine and uniform ceramic powders are formed by precipitation, spray pyrolysis, or emulsion techniques.