Biomedical Engineering Reference
In-Depth Information
of Sr
2
+
is only slightly larger than that of Ca
2
+
(1.18
˚
A compared to
1.00
˚
A), incorporation of the larger cation into the phosphate structure
results in a network expansion, which makes the strontium glasses crys-
tallise more readily and reduces the PW, an effect that is similar to that
found in bioactive silicate glasses (Chapter 2).
Another way of controlling the crystallisation tendency of a glass
is by increasing the number of glass components. This increases the
entropy of mixing and subsequently increases the energy barrier for
atomic rearrangement, which is necessary for crystals to form. Take
the example of glass Ti-3, which contains the components P
2
O
5
-CaO-
MgO-Na
2
O-TiO
2
. Figure 4.11 shows that when more components are
added to the glass composition to form the multi-component Ti-3(MC)
glass P
2
O
5
-CaO-SrO-MgO-ZnO-Na
2
O-K
2
O-TiO
2
, the processing
window increases. In this example, the partial substitution of stron-
tium for calcium, zinc for magnesium and potassium for sodium was
performed, improving its processing properties.
glass Ti-3 (MC)
PW
Ti-3 (MC)
glass Ti-3
170
165
160
155
PW
Ti-3
150
Ti-3 Ti-3 (MC)
Glasses
300
350
400
450
500
550
600
650
700
Temperature (
C)
°
Figure 4.11
Processing window (difference between the onset temperature of
crystallisation,
T
o
, and the glass transition temperature,
T
g
, obtained from
differential scanning calorimetry, DSC) for a base glass (Ti-3) in the system
P
2
O
5
-CaO-MgO-Na
2
O-TiO
2
with 37 mol% P
2
O
5
. To obtain glass Ti-3 (MC),
the number of glass components was increased by partially substituting strontium
for calcium, zinc for magnesium and potassium for sodium.
