the applications of sol–gel processing derive from the specialty shapes obtained directly from the gel state (e.g., films, fibers, monoliths and particles), coupled with compositional and microstructural control, and low temperature processing. sol–gel processing also has its problems. many alkoxides are expensive and most require special handling in a dry inert atmosphere. drying is often a limiting step for articles thicker than ~1 mm, and the shrinkage during drying and sintering are large. some advantages and disadvantages of the process.
thin films and coatings
prior to gelation, the sol or solution can be used for preparing thin films by common methods such as dipping, spinning, and spraying. we shall consider the techniques of (1) dip coating, where the object to be coated is lowered into the solution and withdrawn at a suitable speed, and (2) spin coating, where the solution is dropingped onto the object, which is spinning at a high speed. in practice, dip coating is currently the more widely used.
fibers
fibers can be produced by two sol–gel routes. in one route, a dense monolithic perform is produced by sol–gel processing, drying, and sintering, followed by conventional fiber drawing above the glass softening temperature.
monoliths
monoliths are defined as bulk gels (smallest dimension ? 1 mm) cast into shape and processed without cracking. the ability to form complex shapes by casting the gel coupled with relatively low temperature sintering are attractive features of the route, but cracking during drying and the significant shrinkage during sintering can present significant challenges in achieving the final shape.
porous materials
sol–gel processing is an attractive route for the production of porous ceramics and glasses because the characteristic properties of gels such as high surface area, high porosity, and small pore size are not easily attainable by more conventional methods.