The name sol-gel derives from the fact that micro particles or molecules in a solution (sols) agglomerate and under controlled conditions eventually link together to form a coherent network (gel).
Although sol-gel is generally viewed as a relatively new industrial technology, it is used extensively in a number of industries, such as abrasive powder manufacture, optical fibres, etc. Like all new technologies, however, the scaling-up to full industrial exploitation has been slow.
There are two generic variations of the sol-gel technique. One is called the colloidal method, the other is called the polymeric (or alkoxide) route. The differences between the two stem from the types of starting materials (precursors) that are used.
Both routes involve suspending or dissolving the precursor(s) in a suitable liquid, usually water for the colloidal route and alcohol for the polymeric route. The precursor is then activated by the addition of an acid (such as hydrochloric acid) or a base (such as potassium hydroxide). The activated precursors then react together to form a network. The network grows and ages with time and temperature until it is the size of the container. At this point the viscosity of the liquid increases at an exponential rate until gelation occurs, that is, no more flow is observed.
Processing can be undertaken before gelation using a range of methods including spraying to produce coatings and powders, drawing from a liquid to produce fibres and processing in a vessel to produce aerogels and monoliths.
The most widespread commercial use of sol-gel is in the fabrication and deposition of coatings. The role of the coating may be to provide one or more of a number of functions:
- chemical protection (e.e. corrosion protection)
- mechanical protection (e.g. abrasion resistance)
- optical properties (e.g. anti-reflective, optoelectronic)
- electronic properties (e.g. ferroelectric, conductive)
- catalytic activity (usually associated with high surface area)
Using sol-gel methods, protective coatings can be made at modest (less than 150°C) temperatures. These coatings can be tailored for specific applications including: mechanical protection for soft substrates (such as plastics and metals), chemical protection for reactive substrates, and environmental protection for exposed surfaces. Coatings can also be made that provide a substrate with further functional properties; these include anti-reflection and anti-soiling for glass and plastics, and UV opacity for plastics.
Many different industries already benefit from the use of sol-gel chemistry due to its versatility in fabricating a wide range of materials with different properties. Current examples are found in the construction, electronics, communications, automotive and biomedical sectors. Sol-gel processing is at the heart of the emerging bio-mimetic and multifunctional materials development.
Glasses and ceramics fabricated by sol-gel have identical material properties to those produced by more conventional routes. The advantages of using sol-gel processing instead of high temperature processing methods are:
- lower synthesis temperature
- high purity
- novel materials
- low capital costs
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