Phosphate Glasses - Bio-Glasses: An Introduction - page 49

modifier cation, which determines how strong the ionic cross-links
between two NBOs are: an increase in charge-to-size ratio results in
stronger cross-linking. Therefore, cross-linking would be expected to
increase in the order Na + → Ti 4 + ; similarly for cations

of the same charge but with decreasing ionic radius, as for the series
Ba 2 + →

Ca 2 + → Fe 3 + →

Mg 2 + .
Besides network formers and network modifiers, there is a third class
of glass components, called intermediate oxides [5]. Intermediate oxides
can switch their role, either acting like a network modifier, that is,
disrupting P-O-P linkages and creating NBOs, or acting more like a
network former, entering the glass network (i.e. the phosphate network),
creating P-O-M-O-P bonds (M

Sr 2 + → Ca 2 + →

intermediate metal atom) with a more
covalent character. Magnesium (Mg), iron (Fe), aluminium (Al), zinc
(Zn) and titanium (Ti), among others, are typical intermediates.

=

4.4 TEMPERATURE BEHAVIOUR AND
CRYSTALLISATION

Understanding the thermal behaviour of glasses is very important if the
glasses are to be manipulated or processed into complex shapes. Besides
their amorphous structure, their temperature behaviour is another char-
acteristic of glassy materials. While crystalline materials show a distinct
melting point, where they transform from a solid material to a liquid,
glasses show a temperature range of glass transition or transforma-
tion, in which they soften (i.e. their viscosity decreases) gradually. The
glass transition temperature ( T g ) is usually defined as the onset of this
transformation temperature range. The value of T g can be determined
using methods such as differential thermal analysis (DTA) or differential
scanning calorimetry (DSC), and in both methods T g appears as a shift
in the baseline (Figure 4.7).
Glass transition and crystallisation temperatures decrease with increas-
ing phosphate content (i.e. from invert to ultraphosphate glasses), as
shown for binary polyphosphate glasses in the system P 2 O 5 -MgO
(Figure 4.8) [6]. This trend can be explained by decreasing ionic cross-
linking for less disrupted glasses, resulting in lower T g .
In general, the more disrupted the phosphate structure is (i.e. the
smaller the phosphate units), the more easily the glass crystallises. The
tendency to crystallise is closely connected to the viscosity of the melt,
with lower viscosities allowing for the components to arrange into an
ordered crystalline structure more easily. This means that phosphate