Silica Glass from Aerogels

by Michel Prassas

Aerogel evolution during sintering

Glass from Gels Glass researchers in the late 70's start investigating intensively low temperature processing routes for oxides glass manufacturing. Glasses are usually  prepared by mixing at solid state the different  oxides precursors (Carbonates, nitrates, sulfates, oxides..) and then melt at relatively high temperature (1300-2000°C) the mixture to obtain a liquid.

Browse Article Glass from gels Chemistry Hypercritical drying Gel to glass transformation Conclusions References & Links

The high temperature structure of the liquid is characterized by an amorphous state (no order at long distance) which is preserved by cooling the melt rapidly at room temperature. The resulted solid is structurally amorphous and presents, when heated, the characteristic glass transition temperature (T_g)
Network randomness is what we try to create and preserve in order to obtain the glass. And by doing that we consume a lot of energy to destroy the order initially present on the various crystalline precursors. Creating this random network  by using liquid precursors, in the same way macromolecular chemist do to elaborate polymers, is what glass scientists dreamed and achieved 25 years ago. They reconstruct the glass amorphous state using a  "bottom-up" approach. No need any more to go at the melting temperature since the network structure is already elaborated at relatively low temperatures (20-80°C). 
This  chemical approach of making a material starting with molecular precursors and elementary building blocks (here SiO2 tetrahedra) pass beyond the initial restricted field of glass and ceramics science  intimate circle of researchers. Is widely used  today  for tailoring structures at the molecular level to create new materials with enhanced performances.

The envisioned advantages of this approach are many : 

	Less energy consumption,
	Better homogeneity  (mixing at the molecular level),
	Higher purity  Liquids can by successive distillation be purified at ppb or even lower levels,  whereas such purity is much difficult to achieve in the solid state.
	Extended composition range

However making a glass from a gel is useless unless the technique offers significant advantages over the traditional melting technique. Homogeneity , purity and low temperature synthesis are the main attributes which can favorably compete if the cost is not prohibitively high. Therefore the first attempt to synthesize glass from gels was focused on silica (a simple from fundamental point of view system but extremely difficult to melt oxide). 

That was the beginning of an extraordinary research effort started in the late 70s' known today as sol-gel process.  A chemical route for obtaining glassy and ceramic materials at relatively low temperatures starting from liquids .
Although the sol-gel technique has drastically evolved since, and is now used for a variety of materials in any imaginable form ( thin and thick films, fibers, membranes, nanopowders, bulk porous or dense ceramics), composition and structure (Inorganic, organic-inorganic hybrids, semiconductors, nanotextured materials,etc..), it is worthwhile to recall the basics of the initial challenges the researchers were facing in the starting of this exciting technological field.