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.
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 (Tg)
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
The envisioned advantages of this approach are many :
|| Less energy consumption,
(mixing at the molecular level),
Liquids can by successive distillation be purified at ppb or even
whereas such purity is much difficult to achieve in the solid state.
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.
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