Sol-Gel Technology

Sol-gel, colloidal system in which a porous network of interconnected particles spans the volume of a liquid medium.

NOTE. Particles in a sol-gel are often nanoparticles.
sol-gel processing

Production process involving the conversion of a sol to a gel, which is then desiccated to produce particles or a film.

NOTE. A thin layer of the sol deposited on a substrate continues to react until the degree of cross-linking transforms the sample into a solid-like gel. The latter maybe heated to remove volatile products so forming a hard solid film.

Source:
BSI. PAS 135:2007. Terminology for nanofabrication.

The sol-gel process is a wet-chemical technique (Chemical Solution Deposition) for the fabrication of materials (typically a metal oxide) starting either from a chemical solution (sol short for solution) or colloidal particles (sol for nanoscale particle) to produce an integrated network (gel). Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloid, a system composed of solid particles (size ranging from 1 nm to 1 μm) dispersed in a solvent. The sol evolves then towards the formation of an inorganic continuous network containing a liquid phase (gel). Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. The drying process serves to remove the liquid phase from the gel thus forming a porous material, then a thermal treatment (firing) may be performed in order to favor further polycondensation and enhance mechanical properties.

The precursor sol can be either deposited on a substrate to form a film (e.g. by dip-coating or spin-coating), cast into a suitable container with the desired shape (e.g. to obtain a monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g. microspheres, nanospheres). The sol-gel approach is interesting in that it is a cheap and low-temperature technique that allows for the fine control on the product's chemical composition, as even small quantities of dopants, such as organic dyes and rare earth metals, can be introduced in the sol and end up in the final product finely dispersed. It can be used in ceramics manufacturing processes, as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g. controlled drug release) and separation (e.g. chromatography) technology.

Source:
Wikipedia. Sol-gel.