Biomedical Engineering Reference
In-Depth Information
Non-bridging oxygen (NBO)
Network modifier oxide
O
O
O
O
2
+
−
−
P
O
P
+
CaO
PO
+
Ca
+
O
P
Bridging oxygen (BO)
Figure 4.2
Schematic of cleavage of P-O-P linkages by network modifier oxides,
turning bridging oxygens (BOs) into non-bridging oxygens (NBOs).
linked to each other by covalent POP bonds. The oxygens in these POP
linkages are commonly called bridging oxygens (BOs). Orthophosphate
tetrahedra can thereby be arranged to form chains, rings or branching
networks.
P
2
O
5
is called a network former because it is possible to make a
glass using P
2
O
5
only. Pure P
2
O
5
glass can be produced under certain
conditions, but, as it is very reactive and hygroscopic, it is of scientific
interest only. The durability can be increased, however, by adding other
components, called network modifiers, such as calcium oxide (CaO).
The addition of network modifier oxides such as Na
2
O and CaO to
the glass results in the cleavage of POP linkages and the creation of
non-bridging oxygens (NBOs) in the glass, as shown in Figure 4.2. The
resulting glass structure is disrupted, containing not only covalent POP
bonds but also ionic cross-linkages between NBOs, and this process is
called depolymerisation of the network (Figure 4.3).
O
O
O
O
O
O
O
O
P
P
P
P
O
O
O
P
Ca
2
+
P
O
O
O
O
O
O
O
O
O
O
−
O
−
Na
+
O
−
P
P
O
O
Na
+
P
O
P
O
O
O
P
O
P
O
P
P
O
O
O
O
Na
+
O
−
O
_
Na
+
O
O
O
Ca
2
+
O
O
O
−
O
O
O
O
−
P
P
O
O
P
P
P
P
O
O
O
−
O
O
P
O
O
P
O
O
Ca
2
+
O
O
O
O
O
P
P
O
O
O
O
O
O
(a)
(b)
Figure 4.3
Schematic of phosphate glass structure: (a) P
2
O
5
glass consisting of
a phosphate network and (b) phosphate glass after addition of modifier cations
and cleavage of P-O-P bonds, turning bridging oxygens (BOs) into non-bridging
oxygens (NBOs).
