Biomedical Engineering Reference
In-Depth Information
species that undergo polycondensation to form silica nanoparticles,
which then coalesce; further condensation links them together during
gelation (under acidic catalysis). Calcium is usually incorporated using
calcium nitrate. The gels are then dried and heated to at least 600
◦
C
to remove the nitrates from the calcium nitrate. During the thermal
processing, the coalesced nanoparticles sinter together, leaving intersti-
tial nanoporosity. The nanopores are usually in the range of 1-20 nm
diameter and can be tailored during processing by controlling the pH
of the catalyst, the nominal composition and the final temperature. It
is, however, difficult to produce large crack-free monoliths (greater than
10mm thickness) because driving off the water, organics and nitrates
causes capillary stresses that result in cracking.
Larger pieces can be made using the sol-gel foaming process, as the
pores that are introduced reduce the distance that water molecules
need to travel. To produce a porous glass, the sol is foamed under
vigorous agitation (Figure 12.11). The viscosity of the sol increases
until it becomes a solid gel, and therefore no polymeric gelation agent
is needed. However, an extra catalyst is needed. Gelation normally
Stage 1
:
Mixing
precursors
+
catalyst
Hydrolysis & condensation to produce sol
Stage 2
:
Foaming
with surfactant and
gelling agent
Stage 3
:
Casting and Gelation
Stage 4
:
Ageing
- strengthening
Stage 5
:
Drying
- removal of byproducts
Stage 6
:
Stabilisation
- removal of
nitrates/silanols
Figure 12.11
Schematic of the sol-gel foaming process.
