Biomedical Engineering Reference
In-Depth Information
Sol-gel
technologies
Gel
Coating
Aerogel
Solid
Platelet
Powder
FIGURE 2.5
Sol-gel processing methods and related products, nanocoatings, aerogels, solid ceramics, platelets and
powders.
To produce homogeneous gels from a liquid, which is a pertinent requirement to produce
homogeneous coatings and powders, it is essential to avoid precipitation. This becomes
increasingly important in multicomponent systems. In alkoxide systems, precipitation can
be induced by physical agglomeration or by chemical reaction (Zheng et al. 1988).
Chemical precipitation from sol-gel solutions is a result of the alkoxide reacting with
water or a chelating agent to form an insoluble product such as a hydroxide or organic salt.
Factors affecting precipitation include alkoxide precursor chemistry, temperature of the
environment, and water concentration.
Another important factor to be considered on gelation is the gelation environment that
influences the reaction rate. If gelation takes place under tightly capped controlled envi-
ronments, the resultant gel retains all solvents and reaction products remains within the
gel. When it is under open conditions, it produces a material called xerogel with less sol-
vent molecules in it. Both structures behave differently in further processing, like sinter-
ing, to yield dried gels or sintered gels with different properties (Figure 2.5).
The chemical reactions involved in a sol-gel process are hydrolysis and polymerization.
Figure 2.6a and b shows the processes involved with the production of a thin film coating
via the sol-gel process. At high temperatures, the rate of polymerization is greater than the
rate of hydrolysis, and precipitates such as hydroxides do not have a chance to form. When
the rate of hydrolysis is less than the rate of polymerization, that is, when the temperature
is low, hydroxides form by nucleation and growth. This general rule applies for neutral
solutions, but can be overshadowed by changes in pH to acidic or basic solutions.
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