The sol-gel process, also known as chemical solution deposition, is a wet-chemical technique widely used in the fields of
materials science and
ceramic engineering. Such methods are used primarily for the
fabrication of
materials (typically a
metal oxide) starting from a chemical
solution which acts as the precursor for an integrated network (or gel) of either
discrete particles or network
polymers. Typical
precursors are
metal alkoxides and
metal chlorides, which undergo various forms of
hydrolysis and
polycondensation reactions. The formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. Thus, the sol evolves towards the formation of a gel-like diphasic system containing both a
liquid phase and
solid phase whose morphologies range from
discrete particles to continuous polymer networks.
In the case of the
colloid, the volume fraction of particles (or particle density) may be so low that a significant amount of fluid may need to be removed initially for the gel-like properties to be recognized. This can be accomplished in any number of ways. The simplest method is to allow time for
sedimentation to occur, and then pour off the remaining liquid.
Centrifugation can also be used to accelerate the process of
phase separation.
SEM micrograph of amorphous colloidal silica particles (average particle diameter 600 nm) precipitated in basic solution from
TEOS using ammonium hydroxide as a morphological catalyst.
Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significant amount of
shrinkage and densification. The rate at which the solvent can be removed is ultimately determined by the distribution of
porosity in the gel. The ultimate
microstructure of the final component will clearly be strongly influenced by changes imposed upon the structural template during this phase of processing. Afterwards, a thermal treatment, or firing process, is often necessary in order to favor further polycondensation and enhance mechanical properties and structural stability via final
sintering, densification and
grain growth. One of the distinct advantages of using this methodology as opposed to the more traditional processing techniques is that densification is often achieved at a much lower temperature.
SEM micrograph of surface of colloidal solid. Structure and morphology consists of ordered domains with both interdomain and intradomain lattice defects.(Amorphous colloidal silica particles of average particle diameter 600 nm).
The
precursor sol can be either deposited on a
substrate to form a film (e.g., by
dip coating or
spin coating),
cast into a suitable container with the desired shape (e.g., to obtain monolithic
ceramics,
glasses,
fibers,
membranes,
aerogels), or used to synthesize powders (e.g.,
microspheres,
nanospheres). The sol-gel approach is a cheap and low-temperature technique that allows for the fine control of the product's chemical composition. Even small quantities of dopants, such as
organic dyes and
rare earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. It can be used in
ceramics processing and manufacturing as an
investment casting material, or as a means of producing very
thin films of metal
oxides for various purposes. Sol-gel derived materials have diverse applications in
optics,
electronics,
energy,
space, (bio)
sensors,
medicine (e.g.,
controlled drug release),
reactive material and separation (e.g.,
chromatography) technology.
Highlighted image of surface of colloidal solid. Emphasis on microstructural defects to illustrate the defect/domain morphology typical of a simple one-component colloidal solid.
The interest in sol-gel processing can be traced back in the mid-1880s with the observation that the hydrolysis of
tetraethyl orthosilicate (TEOS) under acidic conditions led to the formation of
SiO2 in the form of fibers and monoliths. Sol-gel research grew to be so important that in the 1990s more than 35,000 papers were published worldwide on the process.
Uniformity
In the processing of fine ceramics, the irregular particle sizes and shapes in a typical powder often lead to non-uniform packing morphologies that result in packing
density variations in the powder compact. Uncontrolled
agglomeration of powders due to attractive
van der Waals forces can also give rise to microstructural inhomogeneities.
Differential stresses that develop as a result of non-uniform drying shrinkage are directly related to the rate at which the
solvent can be removed, and thus highly dependent upon the distribution of
porosity. Such stresses have been associated with a plastic-to-brittle transition in consolidated bodies, and can yield to
crack propagation in the unfired body if not relieved.
In addition, any fluctuations in packing density in the compact as it is prepared for the kiln are often amplified during the
sintering process, yielding inhomogeneous densification. Some pores and other structural defects associated with density variations have been shown to play a detrimental role in the sintering process by growing and thus limiting end-point densities. Differential stresses arising from inhomogeneous densification have also been shown to result in the propagation of internal cracks, thus becoming the strength-controlling flaws.
It would therefore appear desirable to process a material in such a way that it is physically uniform with regard to the distribution of components and porosity, rather than using particle size distributions which will maximize the green density. The containment of a uniformly dispersed assembly of strongly interacting particles in suspension requires total control over particle-particle interactions.
Monodisperse colloids provide this potential.
Monodisperse powders of colloidal
silica, for example, may therefore be stabilized sufficiently to ensure a high degree of order in the
colloidal crystal or
polycrystalline colloidal solid which results from aggregation. The degree of order appears to be limited by the time and space allowed for longer-range correlations to be established. Such defective polycrystalline colloidal structures would appear to be the basic elements of submicrometre colloidal
materials science, and, therefore, provide the first step in developing a more rigorous understanding of the mechanisms involved in microstructural evolution in inorganic systems such as polycrystalline ceramics.
Particles and polymers
The sol-gel process is a wet-chemical technique used for the fabrication of both glassy and ceramic materials. In this process, the sol (or solution) evolves gradually towards the formation of a gel-like network containing both a liquid phase and a solid phase. Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloid. The basic structure or morphology of the solid phase can range anywhere from discrete colloidal particles to continuous chain-like polymer networks.
The term
colloid is used primarily to describe a broad range of solid-liquid (and/or liquid-liquid) mixtures, all of which contain distinct solid (and/or liquid) particles which are dispersed to various degrees in a liquid medium. The term is specific to the size of the individual particles, which are larger than atomic dimensions but small enough to exhibit Brownian motion. If the particles are large enough, then their dynamic behavior in any given period of time in suspension would be governed by forces of gravity and sedimentation. But if they are small enough to be colloids, then their irregular motion in suspension can be attributed to the collective bombardment of a myriad of thermally agitated molecules in the liquid suspending medium, as described originally by
Albert Einstein in his
dissertation. Einstein concluded that this erratic behavior could adequately be described using the theory of
Brownian motion, with sedimentation being a possible long term result. This critical size range (or particle diameter) typically ranges from tens of angstroms (10−10 m) to a few micrometres (10−6 m).
Under certain chemical conditions (typically in base-catalyzed sols), the particles may grow to sufficient size to become colloids, which are affected both by sedimentation and forces of gravity. Stabilized suspensions of such sub-micrometre spherical particles may eventually result in their self-assembly—yielding highly ordered microstructures reminiscent of the prototype colloidal crystal: precious
opal.
Under certain chemical conditions (typically in acid-catalyzed sols), the interparticle forces have sufficient strength to cause considerable aggregation and/or
flocculation prior to their growth. The formation of a more open continuous network of low density polymers exhibits certain advantages with regard to physical properties in the formation of high performance glass and glass/ceramic components in 2 and 3 dimensions.
In either case (discrete particles or continuous polymer network) the
sol evolves then towards the formation of an inorganic network containing a liquid phase (
gel). Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution.
In both cases (discrete particles or continuous polymer network), the drying process serves to remove the liquid phase from the gel, yielding a micro-porous
amorphous glass or micro-crystalline ceramic. Subsequent thermal treatment (firing) may be performed in order to favor further polycondensation and enhance mechanical properties.
With the viscosity of a sol adjusted into a proper range, both optical quality
glass fiber and refractory ceramic fiber can be drawn which are used for fiber optic sensors and
thermal insulation, respectively. In addition, uniform ceramic powders of a wide range of chemical composition can be formed by
precipitation.
Polymerization
Metal
alkoxides are members of the family of
organometallic compounds, which are organic compounds which have one or more metal atoms in the molecule. Metal alkoxides (R-O-M) are like
alcohols (R-OH) with a metal atom, M, replacing the hydrogen H in the hydroxyl group. They constitute the class of chemical precursors most widely used in sol-gel synthesis.
The most common mineral in the earth's crust is
silicon dioxide (or
silica), SiO2. There are at least seven different crystalline forms of silica, including
quartz. The basic building block of all of these crystalline forms of silica is the SiO4
tetrahedron. Since each tetrahedron shares 2 of its edges with other SiO4 tetrahedra, the overall ratio of oxygen to silicon is 2:1 instead of 4:1 (thus SiO2). The intricate and highly specific geometry of this network of tetrahedra takes years to form under incredible terrestrial pressures at great depth. That is why SiO2 is such a good glass former. Crystallization in a reasonable amount of time under the most ideal laboratory conditions is highly unlikely. Thus, amorphous silica is the major component of nearly all window glass.
A well studied alkoxide is silicon tetraethoxide, or
tetraethyl orthosilicate (TEOS). The chemical formula for TEOS is given by: Si(OC2H5)4, or Si(OR)4 where the alkyl group R = C2H5. Alkoxides are ideal chemical precursors for sol-gel synthesis because they react readily with water. The reaction is called hydrolysis, because a
hydroxyl ion becomes attached to the silicon atom as follows:
Si(OR)4 + H2O → HO-Si(OR)3 + R-OH
Depending on the amount of water and catalyst present, hydrolysis may proceed to completion, so that all of the OR groups are replaced by OH groups, as follows:
Si(OR)4 + 4 H2O → Si(OH)4 + 4 R-OH
Any intermediate species [(OR)2–Si-(OH)2] or [(OR)3–Si-(OH)] would be considered the result of partial hydrolysis. In addition, two partially hydrolyzed molecules can link together in a condensation reaction to form a
siloxane [Si–O–Si] bond:
(OR)3–Si-OH + HO–Si-(OR)3 → [(OR)3Si–O–Si(OR)3] + H-O-H
or
(OR)3–Si-OR + HO–Si-(OR)3 → [(OR)3Si–O–Si(OR)3] + R-OH
Thus,
polymerization is associated with the formation of a 1, 2, or 3- dimensional network of siloxane [Si–O–Si] bonds accompanied by the production of H-O-H and R-O-H species.
By definition, condensation liberates a small molecule, such as water or alcohol. This type of reaction can continue to build larger and larger silicon-containing molecules by the process of polymerization. Thus, a polymer is a huge molecule (or
macromolecule) formed from hundreds or thousands of units called
monomers. The number of bonds that a monomer can form is called its functionality. Polymerization of silicon alkoxide, for instance, can lead to complex branching of the polymer, because a fully hydrolyzed monomer Si(OH)4 is tetrafunctional (can branch or bond in 4 different directions). Alternatively, under certain conditions (e.g., low water concentration) fewer than 4 of the OR or OH groups (
ligands) will be capable of condensation, so relatively little branching will occur. The mechanisms of hydrolysis and condensation, and the factors that bias the structure toward linear or branched structures are the most critical issues of sol-gel science and technology.
Applications
The applications for sol gel-derived products are numerous. For example, scientists have used it to produce the world's lightest materials and also some of its
toughest ceramics. One of the largest application areas is
thin films, which can be produced on a piece of substrate by
spin coating or
dip coating. Other methods include spraying,
electrophoresis,
inkjet printing or roll coating.
Optical coatings, protective and decorative coatings, and electro-optic components can be applied to glass, metal and other types of substrates with these methods.
Cast into a mold, and with further drying and heat-treatment, dense ceramic or glass articles with novel properties can be formed that cannot be created by any other method. Macroscopic
optical elements and active optical components as well as large area
hot mirrors,
cold mirrors,
lenses and
beam splitters all with optimal geometry can be made quickly and at low cost via the sol-gel route.
One of the more important applications of sol-gel processing is to carry out
zeolite synthesis. Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicate sol formed by this method is very stable.
Another application in research is to entrap
biomolecules for sensory (
biosensors) or catalytic purposes, by physically or chemically preventing them from leaching out and, in the case of
protein or chemically-linked
small molecules, by shielding them from the external environment yet allowing small molecules to be monitored. The major disadvantages are that the change in local environment may alter the functionality of the protein or small molecule entrapped and that the synthesis step may damage the protein. To circumvent this, various strategies have been explored, such as monomers with protein friendly leaving groups (e.g.
glycerol) and the inclusion of polymers which stabilize protein (e.g.
PEG).
Finally of historical note, a sol-gel process was developed in the 1950s for the production of
radioactive powders of
UO2 and
ThO2 for
nuclear fuels, without generation of large quantities of dust.
Asignatura: CRF
Publicado por Sthefany Raga
