Biomedical Engineering Reference
In-Depth Information
The precursors for silicate-based glass production are usually
alkoxides (Si(OR) 4 ,whereRisCH 3 ,C 2 H 5 or C 3 H 7 ) and nitrates.
Alkoxides are usually used as precursors of the network formers, such
as silica and phosphate, whereas salts (e.g. nitrates) are usually used to
introduce network modifiers such as calcium.
3.4.1 Stage 1: Mixing
Sols are created by hydrolysis of the precursors. Sols are dispersions of
colloidal particles (diameters of 1-100 nm) in a liquid. The combina-
tion and quantities of the precursors mixed with water to create the
sol will determine the composition of the glass. In the case of TEOS
(Si(OC 2 H 5 ) 4 ), it is insoluble in water, but a catalyst can trigger hydroly-
sis, where it reacts with water to effectively produce silica tetrahedra in
water (Equation 3.1 and Figure 3.3).
OC 2 H 5
Si
OH
Si
C 2 H 5 O
OC 2 H 5
+
HO
OH
+
4(H 2 O)
4(C 2 H 5 OH)
(3.1)
OC 2 H 5
OH
The catalyst can be either an acid (e.g. HNO 3 ) or a base, and this will
affect the final properties of the glass. The tetrahedra have surfaces of
Si-OH groups and are floating in the water.
At pH
2 (usually using a nitric acid catalyst) the tetrahedra will
come together and the Si-OH groups will condense, forming O-Si-O
bridging bonds, leaving water as a by-product (Equation 3.2).
<
OH
OH
OH
OH
+
Si
OH
+
HO
Si
OH
HO
Si
Si
OH
H 2 O
HO
(3.2)
OH
OH
OH
OH
H + −
− + H
TEOS
+
H 2 O
+
catalyst
− + H
H + −
Figure 3.3 A silica tetrahedron, immediately after hydrolysis of a TEOS molecule.
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