Biomedical Engineering Reference
In-Depth Information
(a)
(b)
Figure 5.2
(a) Two different six-membered rings in the cristobalite structure and
(b) a six-membered ring (pale gray/yellow) and an eight-membered ring (mid-
gray/green) in the quartz structure. For a better understanding of the figure, please
refer to the colour section (Figure 5).
When a glass contains components other than silica, as in the bioactive
glasses, the network structure is modified; essentially, the network is
disrupted (Figure 5.3). Some of the oxygen atoms are no longer bonded
to two silicon atoms and are said to be non-bridging oxygens (NBOs).
Essentially, these are created to charge-compensate for the addition of
the modifier cations. In modified glasses, one now has to consider the
distribution of NBOs. Is it possible for a tetrahedron to have only one
NBO, or may some have more than one - perhaps as many as four? And
is the corresponding distribution of the modifier cations homogeneous
or heterogeneous? Since the presence of NBOs disrupts the links between
nodes in the network (that is, some tetrahedra are no longer connected
to each other), the ring size distribution will also change as modifiers
are introduced into the structures. The distribution of the NBOs over
the tetrahedral structural units is characterized in terms of a quantity
denoted Q
n
. Here
n
refers to the number of bridging oxygen atoms
on the tetrahedron, so Q
3
represents a tetrahedron with one NBO, Q
2
units have two NBOs, and so on. Another way to characterize this is
in terms of network connectivity (NC), which measures the average
number of bridging oxygens on each tetrahedron (Chapter 2). Thus, at
one extreme, silica has an NC value of four, and a structure comprising
strings of tetrahedra would have an NC value of two. Because NC is an
average quantity, it masks some actual variations in the distribution of
