Biomedical Engineering Reference
In-Depth Information
invert glasses, which have a highly disrupted structure consisting of
short phosphate units (orthophosphate and pyrophosphate groups, see
above) have a very low viscosity of the melt and show a high tendency
to crystallise. Phosphate glasses consisting of a chain structure crystallise
less easily, as the entangled chains increase the viscosity of the melt
and impede crystallisation. Ultraphosphate glasses, which consist of an
actual phosphate network, show an even lower crystallisation tendency.
The charge-to-size ratio of the network modifier cation also affects
the crystallisation tendency, with a larger charge-to-size ratio result-
ing in a lower crystallisation tendency due to stronger ionic cross-
linking of the phosphate units. Therefore, a sodium phosphate glass,
x
P
2
O
5
-(100 -
x
)Na
2
O, crystallises more readily than the corresponding
calcium phosphate glass,
x
P
2
O
5
-(100 -
x
)CaO, while incorporation of
calcium oxide into a sodium phosphate glass will reduce the crystallisa-
tion tendency of the glass.
In order to quantify the tendency of a glass to undergo crystallisation,
its processing window (PW) can be determined. The PW (sometimes
referred to as the sintering window) is the temperature range between
glass transition temperature (
T
g
) and the onset of crystallisation (
T
o
,
Figure 4.7). A large PW (
100 K, ideally around 200 K) is preferred for
sintering and fibre drawing. During sintering, crystallisation is undesired
because it inhibits sintering by a high-temperature viscous flow mecha-
nism, while in glass fibres crystals represent defects, which affect fibre
mechanical properties.
Owing to the large charge-to-size ratio of Ti
4
+
, TiO
2
was shown to
effectively increase the PW of phosphate invert glasses in the system
P
2
O
5
-CaO-MgO-Na
2
O. DSC traces of the titanium-free base glass
(Ti-0) and a composition containing 10 mol% TiO
2
(Ti-10) are shown
in Figure 4.9; the PWs are indicated and show a significant increase
upon introduction of TiO
2
. Incorporation of TiO
2
allows for synthesis
of phosphate glasses consisting of orthophosphate units, which usually
crystallise, so that they are easily obtained in a glassy state. However,
ionic cross-linking by Ti
4
+
inhibits crystallisation and allows for glass
formation in this composition range [7].
Even subtle changes to the glass composition can have pronounced
effects on temperature behaviour and crystallisation. Strontium release
from a glass is thought to be highly beneficial, as it suppresses osteo-
porosis. If, in a P
2
O
5
-CaO-MgO-Na
2
O invert glass, calcium is partially
replaced by strontium, the PW decreases from about 145 K in the
all-calcium glass to 125 K in the all-strontium glass (Figure 4.10).
Although the ionic radius (and subsequently the charge-to-size ratio)
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