Everything about The Sol-gel totally explained
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 (for example by dip-coating or spin-coating), cast into a suitable container with the desired shape (for example to obtain a monolithic ceramics, glasses, fibers, membranes,
aerogels), or used to synthesize powders (for example microspheres, nanospheres). The sol-gel approach is interesting in that it's 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 (for example controlled drug release) and separation (for example chromatography) technology.
The interest in sol-gel processing can be traced back in the mid-1880s with the observation that the hydrolysis of
tetraethyl orthosilicate (TEOS) under acidic conditions led to the formation of SiO
2 in the form of fibers and monoliths. Sol-gel research grew to be so important that in the 1990s more than 35,000 papers were published worldwide on the process.
Applications
Scientists have used it to produce the world’s lightest materials and some of its
toughest ceramics.
The applications for sol gel-derived products are numerous. One of the largest application areas is
thin films, which can be produced on a piece of substrate by
spin-coating or
dip-coating. Other methods include spraying,
electrophoresis,
inkjet printing or
roll coating.
Optical coatings, protective and decorative coatings, and electro-optic components can be applied to glass, metal and other types of substrates with these methods.
Cast into a mold, and with further drying and heat-treatment, dense ceramic or glass articles with novel properties can be formed that can't be created by any other method. Macroscopic
optical elements and active optical components as well as large area
hot mirrors,
cold mirrors,
lenses and
beam splitters all with optimal geometry can be made quickly and at low cost via the sol-gel route.
With the
viscosity of a sol adjusted into a proper range, both
optical and
refractory ceramic fibers can be drawn which are used for fiber optic sensors and
thermal insulation, respectively.
Ultra-fine and uniform ceramic powders can be formed by precipitation. These powders of single- and multicomponent compositions can be made in submicrometre particle size for dental and
biomedical applications. Composite powders have been patented for use as
agrochemicals and
herbicides. Also powder
abrasives, used in a variety of finishing operations, are made using a sol-gel type process.
One of the more important applications of sol-gel processing is to carry out
zeolite synthesis. Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicalite sol formed by this method is very stable.
Other products fabricated with this process include various ceramic
membranes for
microfiltration,
ultrafiltration,
nanofiltration,
pervaporation and
reverse osmosis.
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's possible to obtain porous solid matrices called
xerogels.
Finally of historical note, a sol-gel process was developed in the 1950s for the production of
radioactive powders of
UO2 and
ThO2 for
nuclear fuels, without generation of large quantities of dust.
Further Information
Get more info on 'Sol-gel'.
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