Biomedical Engineering Reference
In-Depth Information
Importantly, the gel is sitting in its aqueous by-products during ageing.
This process strengthens the gel by consolidation as the condensa-
tion reaction continues. Without the ageing stage, the gel will crack
during drying.
3.4.5 Stage 5: Drying
The aim of the drying stage is to remove the by-products of the conden-
sation reaction. As the water and alcohol evaporate, they leave behind an
interconnected pore network. The pores have diameters in the nanome-
tre range, typically 1-30 nm in diameter. Figure 3.4 shows that -OH
groups are left behind on the surface of the particles.
3.4.6 Stage 6: Stabilisation
Although the drying process removes water, hydroxyl (OH) groups
are left on the pore walls. In a silica sol-gel glass, the H + actsasa
network modifier, disrupting the silica network and reducing network
connectivity (Chapter 2 explains the concept of network connectivity
and its relation to bioactivity). A chemically stable porous solid can
be obtained by removing surface silanol (Si-OH) bonds from the pore
network, usually by thermal processing. Thermal processing drives
off the -OH groups, causing further formation of O-Si-O bonds. If
other precursors, such as nitrates, have been used in the process, for
example calcium nitrate to introduce calcium into the composition, the
nitrates will also be removed during stabilisation. The usual method
is to heat the dried gel to temperatures above 700 C to produce a
porous glass.
3.4.7 Stage 7: Densification
Some applications require a glass without the interconnected nanoporous
network. This can be achieved by heating the glass at higher temperatures
(
1000 C for silica), causing densification (sintering) to occur. The
pores are eliminated, and the density ultimately becomes equivalent to
that of the melt-quenched glass. The densification temperature depends
considerably on the morphology of the pore network, the surface area
and the glass composition. Temperatures must be raised above the
glass transition for sintering to occur, but they must be kept below the
crystallisation temperature, otherwise a glass-ceramic will form.
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