Biomedical Engineering Reference
In-Depth Information
By definition, a sol is a suspension of colloidal particles in a liquid (Floch et al. 1995).
A sol differs from a solution in that a sol is a two-phase, solid-liquid system, whereas a
solution is a single-phase system. Colloidal particles can be in the approximate size range
of 1 to 1000 nm; for this reason, gravitational forces on these colloidal particles are negli-
gible and interactions are dominated by short-range forces such as van der Waals forces
and surface charges. Diffusion of the colloids by Brownian motion leads to a low-energy
arrangement, thus imparting stability to the system (Brinker and Scherer 1990).
The stability of the sol particles can be modified by reducing their surface charge. If the
surface charge is significantly reduced, then gelation is induced and the resultant prod-
uct is able to maintain its shape without the assistance of a mould. Gels are regarded as
composites, since gels consist of a solid skeleton or network that encloses a liquid phase
or excess of solvent. Depending on their chemistry, gels can be soft and have a low elastic
modulus, usually obtained through controlled polymerization of the hydrolyzed start-
ing compound. In this case, a three-dimensional network forms, resulting ultimately in a
high molecular weight polymeric gel. The resultant gel can be thought of as a macroscopic
molecule that extends throughout the solution. The gelation point is the time taken for the
last bond in this network to form. This gelation can be used to produce a nanostructured
monolith or nanosized coatings, depending on the process applied.
Advantages and Disadvantages
Sol-gel processing is unique in that it can be used to produce different forms, for exam-
ple coatings, fibers, powders, platelets, and monoliths of the same composition, simply
by varying the chemistry, viscosity, and other factors of a given solution. In addition, the
range of different compositions that can be produced includes pure single oxides, mixed
oxides, and nonoxides, such as borides, chlorides, and nitrides.
The advantages of the sol-gel process are numerous:
1. It is of the nanoscale.
2. It has the ability to produce uniform fine-grained structures.
3. High purity can be maintained as grinding can be avoided.
4. It results in a stoichiometric, homogeneous, and pure product owing to mixing on
the molecular scale.
5. It allows reduced firing or sintering temperatures due to its small particle sizes
with high surface areas.
6. It allows the use of different chemical routes (alkoxide or aqueous-based).
7. It is easily applied to complex shapes with a range of coating techniques, including
dip, spin, and spray deposition.
In addition, sol-gel coatings have the added advantage that the costs of precursors are
relatively unimportant, owing to small amounts of material requirements. Shrinkage in
coatings, depending on the chemistry, is fairly uniform perpendicular to the substrate and
the coating can normally dry without cracking. Shrinkage-generated cracking is an impor-
tant issue in monolith production and thicker than 500-nm multilayered coatings.
Sol-gel film deposition also offers the significant advantage over other film deposition
techniques such as CVD, PVD, and sputtering, in that properties such as surface area,
pore volume, and size can be carefully controlled by chemistry. These are particularly
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