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Sol-Gels and Aerogels Explained

Dr Khalil Khan
19 October 2017

Aerogel

Many industries benefit from the use of sol-gel chemistry, due to its versatility in fabricating a wide range of materials with different properties. Examples can be 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. The technology can be used to prepare fibres, microspheres, thin films, fine powders and monoliths. Applications for sol-gel technology include protective coatings, catalysts, piezoelectric devices, wave-guides, lenses, high-strength ceramics, superconductors, synthesis of nanoparticles and insulating materials.

There are two things most people in the Western world own: a refrigerator and a mobile phone - aerogels could revolutionise the manufacture of both.

An aerogel is a material that is full of tiny holes. Made by extracting all the liquid from a gel, it can be up to 95% pores. Those pores are so small - between 20 and 50 nanometres - that gas molecules can't squeeze through them. As a result, aerogels can't transport heat, making for a material with incredible insulating properties. The unusual electrical properties of aerogels also make them suitable as lightweight antennae for mobile phones, satellites and aircraft.

What is Sol-Gel process?

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).

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) which are used.

 

Sols, Gels and Aerogels are Colloids...

A colloid is a mixture in which at least two different phases are intimately mixed at the nanoscale. The term “phase” generally refers to a solid, liquid, or gas form of some substance. A colloid typically has a continuous phase in which something else with a different phase is dispersed (the "dispersed phase"). Different phases can still be the same phase of matter, for example, two different phases could both be liquids, just not miscible liquids.

Colloids are different from homogeneous solutions, in which a substance is dissolved or mixed with another substance and does not separate out, in that the components of colloids are nanoparticles or macromolecules (giant molecules), typically with a length or diameter ranging from a few nm to several hundred nm in diameter.

A sol is a liquid. The continuous phase in a sol is a liquid and the dispersed phase is a solid. The difference between a sol and a non-colloidal liquid is that solid nanoparticles are dispersed throughout the liquid in a sol. If you put a sol in a centrifuge, you can force the nanoparticles dispersed in the liquid to precipitate out. This will not happen for a non-colloidal liquid solution, for example, salt dissolved in water. An example of a sol is black inkjet ink (carbon black dispersed in water).

A gel is a wet solid-like material in which a solid network of interconnected nanostructures spans the volume of a liquid medium. The continuous phase is a solid network and the dispersed phase is a liquid. Gels tend to be mostly liquid in composition and typically exhibit the density of a liquid as result but have cohesiveness like a solid.

An aerogel is solid with air pockets dispersed throughout. Aerogels are essentially the solid framework of a gel isolated from the gel's liquid medium. Some aerogels, such as carbon aerogels and iron aerogels, are derived from other types of aerogels, but the aerogels they are derived from came from a gel directly.

For more on aerogels and their uses, follow the links below:

 

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Sol-Gel Processing Market show exponential growth by 2018