Lesson 9.6ShapeChapter 9 · Powders and ceramics

Sol-gel processing

A liquid sets into a gel, then is dried and fired

Sol-gel processing makes glass and ceramics from a liquid instead of a melt or a powder. Tiny particles form in a liquid, link into a jelly-like gel, and the gel is dried and gently heated into a solid. The anti-reflection coating on some camera lenses and the ghostly blue aerogel used as insulation are made this way.

Photo: NASA/JPL-Caltech, Public domain, via Wikimedia Commons

Sol-gel processing · sol, gel, dry, fire, and ways to shape itThe top row follows the material from liquid to solid; the bottom row shows how it is shaped while still liquid. Circled numbers match the steps.

How it works

Make the sol. Liquid chemicals that contain a metal or silicon (such as TEOS, a silicon compound) are mixed with water, alcohol and a little acid or base. Reactions build particles a few nanometers across, giving a sol: a liquid with tiny solid particles floating in it.
Shape it. While still liquid, the sol is dip-coated or spin-coated onto a part, sprayed, cast into a mold, or drawn into fibers.
Let it gel. The particles link up into a network that spans the whole volume and traps the liquid, like gelatin setting. This takes minutes to days; resting it longer (aging) makes the network stronger.
Dry it. Slow evaporation pulls the network together, so the gel shrinks into a dense, glassy xerogel and can crack if rushed. Drying above the liquid's critical point (supercritical drying, where there is no surface tension) keeps the network open, giving an aerogel that is mostly air.
Fire it. Heating to typically 400–1,200 °C burns off leftover organics and sinters the pores shut, giving dense glass, ceramic or a hard coating. That is far below the roughly 1,700 °C needed to melt silica glass. Aerogels are often used without firing.

Key facts

Materials: Silica, titania, zirconia, alumina and mixed oxides; hybrids of glass and plastic
Typical parts: Optical and anti-reflection coatings, protective films, aerogel insulation, fine ceramic powders, fibers
Part size: Films from nanometers to a few micrometers thick on parts of almost any size; solid pieces are usually only centimeters across, because drying cracks big ones
Precision: Film thickness controllable to roughly tens of nanometers
Surface finish: Very smooth, optically clear films
Batch size it suits: From single lab samples to continuous coating lines for glass
Tooling and machine cost: Low for coatings: glassware and dip or spin coaters from a few thousand dollars; supercritical dryers cost much more
Speed: Coating takes minutes; gelling hours to days; drying solid pieces can take days

Strengths and limits

Strengths

Low temperatures compared with melting glass or sintering powder
Very pure, even materials mixed at the molecular level
Easy thin, even coatings on glass, metal or plastic
Unique products like aerogels, the lightest solids made

Limits

Large solid pieces crack while drying
Big shrinkage, often more than half the volume
Chemicals (alkoxides, solvents) are costly and some are flammable
Slow: gelling and drying take hours to days

Where it's used

OpticsAnti-reflection and filter coatings on lenses and glass
Buildings and energyAerogel insulation blankets, coatings on solar glass
Electronics and sensorsThin oxide films, gas and humidity sensors
Protective coatingsScratch- and corrosion-resistant films on metal and plastic
Space scienceAerogel dust collectors, such as on NASA's Stardust mission
Ceramics and catalystsVery fine, pure powders and porous catalyst supports

How to recognize it

Thin coatings show faint rainbow or bluish tints from light interference, like a soap bubble.
Anti-reflection lenses look slightly colored when tilted and reflect little light.
Aerogel looks like frozen blue smoke and feels almost weightless.
Air-dried bulk pieces are small and may show fine cracks.

Compare with

Powder metallurgy9.1also ends with sintering, but starts from a pressed powder, not a liquid.CVD coating11.9builds thin films from reacting gases, usually hotter and in a vacuum chamber.Atomic layer deposition13.4grows films one atomic layer at a time, more precise but much slower.

Watch it

University of California Television (UCTV) · Sol-gel chemistry from a lab expert, with a live demo of the first steps (from about 5:00)

NileRed · The whole route at home: mixing, gelling, solvent exchange, supercritical drying

nottinghamscience · How the special drying step works

Veritasium · What finished aerogel is like and what it is used for

See real photos

Wikipedia: Sol–gel process — process diagrams from sol to film, xerogel and aerogel
Wikimedia Commons photo search

Remember

Sol-gel builds a solid from a liquid: sol, then gel, then dry, then fire.
It works at low temperature, which makes it ideal for thin coatings and pure materials.
How you dry the gel decides the result: dense xerogel or feather-light aerogel.