are two main ways to synthesize gels at room temperature.
The first one
consists of a common reaction which occurs in nature where silica
chemical species diluted in aqueous solutions condense to lead to the
formation of silica network. Such a condensation may occur
in various aqueous solutions depending on pH and salt concentration.
Different morphologies may be obtained and for silica the most known is
the precious "opal".
The other way to produce silica from
solution corresponds to a chemical reaction implying metal alkoxides
and water in an alcoholic solvent. The first reaction is an hydrolysis
which induces the substitution of OR groups linked to silicon by silanol
Si-OH groups. As previously, these chemical species may react together
to form Si-O-Si (siloxane) bonds which lead to the silica network
formation. This phase establishes a 3D network which invades the whole
volume of the container. Of course, for these two syntheses the liquid
used as solvent to perform the different chemical reactions remains
within the pores of the solid network. A gel is thus obtained. This two
phases material consists of shaped solid exhibiting specific
Drying of GELS
Removing the liquid located within the pores
leads to a dried gel named "xerogel", a word
issued from the Greek word "xeros" and which means dry.
The different drying processes are listed below.
A "cryogel" results from a freeze-drying process. usually a
material which is hydrophilic. It may react very quickly
with water to lead again a solution identical to that from which it was
An aerogel results
from a supercritical drying process. The drying step is performed inside
an autoclave which allows to overpass the critical point (PC,
TC) of the solvent. There are different schedules to reach
the critical point of the solvent while the solvent itself may be chosen
with respect to the nature of the solid part. Strong inorganic solids
are commonly dried using alcohol (or acetone) as solvent. Organic solids
which may decompose at temperature above 100°C will be dried using CO2
Excepted these two unusual
drying ways which are precisely named, other dryings lead to xerogels.
Consequently we can say that
xerogels refer to gels dried at temperature close to room temperature
and under atmospheric pressure. Xerogel is the result of a gentle drying
to avoid cracking associated to the very low permeability of the solid
network. Such a process is time consuming.
particles and Photonic crystals
Pure inorganic xerogels are
rarely used as they are obtained because of their residual porosity
which implies quite low mechanical properties and low durability.
of the xerogels show high specific surface which favours chemical
reactions with atmosphere. However a new application is of interest. It
is related to the synthesis of sub-micronic dense particles of silica.
Particles have about the same size (Stober process).
Particle sedimentation gives rise to a perfect hexagonal packed array of
particles (see work made at Bell-Labs). As natural opal, a great insight has been recently done in
the direction of these new synthetic opals. Their optical properties
depend on the refractive index difference between the matter
constituting the spheres and the inter sphere volume and on the size of
spheres (for a given hexagonal compact arrangement). These gels exhibit
3D photonic band gap. They should found applications in the field of
Inorganic gel is rarely used
as obtained after a simple drying. It is often heat treated to be
transformed into a material having a smaller porosity and consequently
better mechanical properties. At this stage one can separate inorganic
gels in two large families. The first one concerns those for which the
densification thermal treatment operates via a viscous flow
avoiding the gel crystallization. The second one refers to materials
that crystallize during an increasing temperature treatment. A viscous
flow sintering is a convenient way to densify silica compound.
obtained from supercritical drying offer the best example of a viscous
flow sintering which provides dense silica glass at low temperature
(1200°C) and for a short duration of treatment (10 minutes).
It is worth noticing that the
nature and the quantity of impurities and more precisely alkali ions
play a very important role on the tendency to crystallize.
The other family of xerogels
concerns those that crystallize during heat treatment.
occurs, sintering is difficult to achieve and generally
the shrinkage stops. On the other hand, crystallization induces a sudden
structure shrinkage at some location in the amorphous matrix.
Consequently most of previously monolithic xerogels are transformed into
a fine powder. If the gels are previously
seeded with suitable crystals, further crystallization may be oriented
to lead to high performance abrasive grains. It is likely the largest
industrial application of crystallized xerogels with respect to the
volume of matter prepared by year. A
similar fast sintering is obtained for xerogels issued from dipping of
spinning method on to a variety of substrates.
coatings may be prepared from such a process. Sintering at low
temperature causes a densification of the xerogel film which becomes
dense. Densification by closing the pores hinders the condensation of
atmospheric water molecules to condense in smallest pores, a phenomenon
which modifies the refractive index and consequently the quality of the
composition of coatings may be modified with suitable chemical species :
to develop colours
planar guiding structures
to provide optical functions
(switching, amplification etc.)
mechanically and chemically the surface of the substrate
to improve scratch resistance.
Fibers of gels drawn from the
solution can be easily converted into xerogels which are further
sintered into continuous glass or ceramic fiber.
applications of gel technologies concern both fibers and matrices.
However, presently, synthetic mullite, alumina and others are replaced
by SiC or C fibres. But matrices made of MAS (MgO-Al2O3-SiO2),
AS (Al2O3-SiO2), MASL (MgO-Al2O3-SiO2-Li2O)
are still prepared from gels.
A pressure aid is often required to
achieve fully dense material which can be then subjected to a nucleation
- crystallization heat treatment that results in an enhancement of mechanical properties.
Full organic resistant gels (a
variety far to gelatine and other very compliant organic material) have
been investigated. Among them, gels which are dried without cracking and
which are pyrolysed to lead to an inorganic structured ultraporous
carbon, offer a variety of industrial applications. Thermal insulator
and super capacitor carbon electrodes are two examples of such a material
which is generally CO2 supercritically dried.
formaldehyde, melamine - formaldehyde and others derived compounds are
the starting compounds.
Since 1980, a new family of
gels, named hybrids, has been studied. Thanks to the wide potentialities
of organic chemistry, it was possible to synthesize suitable chemical
precursors which consist of inorganic and organic groups. According to
the morphology of the starting molecule (like hyper-branched precursors)
or the nature of some bonds (hydrophobic Si-CH3 or Si-H
groups), it is possible to build up a network with controlled pore size. Moreover
Si-R groups lead to more compliant xerogels. During drying the shrinkage
of the network is of great extent and stresses which act on the solid
part are lowered. Finally a nearly fully dense xerogel is obtained. The
pore size decreases and reaches low values. The material is transparent
and has properties close to those of glass. Accordingly, the diffusion
of oxygen molecules is very low. Consequently specific organic molecules
may be incorporated to this matrix.
By changing the nature of R,
it is possible to control the interaction between the xerogel matrix
which plays the role of a host material and the optically active
molecules embedded in such a transparent matrix. The field of optically
organic molecules covers a lot of applications. These new hybrid
xerogels show very attractive applications (tunable lasers using dye
molecules, photochromism using spiropyrane molecules...).
The behaviour of gels during
the different steps of preparation (forming, drying, firing) is now well
understood. It is presently possible to select starting molecules to
tailor physical and chemical properties of xerogels.
A such adaptability
must merely result in applications in the field of mechanical properties
improvements, optics and sensors.