Biomedical Engineering Reference
In-Depth Information
Negative charge
Si
4+
O
2-
AI
3+
Figure 12.2.
Structure of tetrahedral [SiO
4
]
4
−
with a Si/Al substitution ([AlO
4
]
5
−
) yielding a
negative surplus charge [74].
(BO)
(NBO)
Si
O
Al
Si
Ca
O
Ca
Al
Al
Si
O l
O
(T)
(BO)
Si
Figure 12.3.
Cartoon of transformation of bridging oxygen (BO) to non-bridging oxygen
(NBO) [21].
at least one of them must have a coordination number higher than four, that is,
fi ve or six, towards oxygen. The Al/Si ratio plays also an important role in the
glass composition. The substitution of Si
4+
by Al
3+
can only happen up to a limit of
1 : 1 ratio, above which aluminium is no longer forced to adopt the tetrahedral
coordination. All oxygens in the alumino-silicate glass network are bridging
oxygens (BO). Inclusion of species such as CaO in the glass leads to the develop-
ment of non-bridging oxygens (NBO). Stebbins and Zhu [21] reported the trans-
formation of BO to NBO in simple aluminosilicate glass structures. They
considered that if all Al remains as AlO
4
tetrahedra the new structural unit should
be a tricluster notated as T in Figure 12.3.
On the other hand, if AlO
5
and AlO
6
are formed, the tricluster oxygen may
itself behave as an NBO. CaO is considered therefore to be a network modifi er as
opposed to the network formers silicon and aluminium. However, aluminium can
easily play both roles adopting a six-fold coordination and forcing the creation of
sites in the glass structure, where additional oxygen atoms need to be present.
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