Sol–gel process

Sol gel is a colloidal suspension of inorganic, catalytic silicon oxide particles that is gelled to form a solid. The resulting porous gel can be chemically purified and consolidated at high temperatures into high purity silica. With the sol gel process, silica can be modified with a variety of dopants to produce novel glass properties unattainable by other means. It can be used in ceramics manufacturing processes, as a mold to be washed away, or as a means of producing very thin films of metal oxides for various purposes, including a form superior to teflon.

The sol-gel process

The sol-gel process is a process for making glass/ceramic materials. The sol-gel process involves the transition of a system from a liquid (the colloidal “sol") into a solid (the "gel") phase. The sol-gel process, allows the fabrication of materials with a large variety of properties: ultra-fine powders, monolithic ceramics and glasses, ceramic fibers, inorganic membranes, thin film coatings, and of course, aerogels.

The sol is made of solid particles of a diameter of few hundred of nm, usually inorganic metal salts, suspended in a liquid phase. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymeration reactions to form a colloidal suspension. Then the particles condense in a new phase,the gel, in which a solid macromolecule is immersed in a solvent.

Forms

Further processing enables the creation of ceramic materials in different forms. Thin films,for example, can be produced on a piece of substrate by spin-coating or dip-coating. Cast into a mold, and with further drying and heat-treatment, dense ceramic or glass articles can be formed. With the viscosity of a sol adjusted into a proper range, ceramic fibers can be drawn, while ultra-fine and uniform ceramic powders can be formed by precipitation.

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. Drying the gel by means of low temperature treatments (25-100 C), it is possible to obtain porous solid matrices called xerogels.

Reference

Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing by C. Jeffrey Brinker, George W. Scherer