Biomedical Engineering Reference
In-Depth Information
chlorides and sulfates are more thermally stable and may be difficult to remove (Wallace
and Hench 1984; Schwart et al. 1986). On the other hand, the use of nitrates in amounts
greater than a few hundred grams should be avoided, as nitrates are very strong oxidizing
agents and can lead to uncontrollable exothermic reactions and even explosions during
drying (Thomas 1988). Acetates are a suitable alternative to nitrates, eliminating the pos-
sibility of explosions; however, they do not thermally degrade as readily as nitrates, often
leaving behind organic residues.
Other Organic Additives
Owing to the large quantity of organic material required in the processing of sol-gel
derived forms, cracking during the production stages can be a problem. Shrinkage during
drying is common in the case of monoliths. Cracking in thicker coatings is often due to
phenomena such as inhomogeneities resulting from phase separation, thermal mismatch
with the substrate used, or a number of other factors related to the drying process.
Modification of sols through the addition of organics, sometimes referred to as dry-
ing control chemical additives or DCCA, can lessen these problems through the control
of evaporation rates of volatile species, which in turn, control hydrolysis rates. They also
influence properties such as micro- and nanoporosity (Orcel and Hench 1986; Schmidt et
al. 1988). Examples include glycol, acetylacetone, carboxylic acids, or β-diketones.
Process
Sol synthesis involves preparation of a solution from alkoxides, metal salts, or other suitable
precursors. Coatings are produced by a number of deposition methods from the solutions.
The coated substrate is then exposed to water for hydrolysis. During this process, hydrox-
ides or hydrated oxides form and gelation occurs to form a three-dimensional network.
Control of this gelation and the three-dimensional structure determines the final densifi-
cation and the shape of the product (Figure 2.5). Further heat treating of the resultant gel
removes residual organic material and induces conversion to the oxide state under oxidiz-
ing conditions (Hench and West 1990; Ben-Nissan et al. 1994; Chai and Ben-Nissan 1999).
Gelation
The simplest picture of gelation is that clusters grow by condensation of polymers or
aggregation of particles until the clusters collide. Links then develop between the clusters
to generate a single giant cluster that is referred to as a gel (Figure 2.6a and b). The gel point
can only be estimated by an increase in viscosity as no latent heat is released. The gel is
composed of a solid skeleton surrounding a continuous liquid phase.
There are a number of factors that decreases the gel time:
1. Increasing the concentration and pH of the solution
2. Increasing the gelling temperature
3. Increasing the water/alkoxide ratio
4. Decreasing the size of the alkoxyl groups
In essence, it means appropriate control of the material chemistry (Gottardi et al. 1984;
Colby et al. 1988).
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