Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 124
The DTA trace of glass MOF014, 65TeO2-10WO3-25ZnF2 mol. % (fig. (4.16)), shows
the WO3-containing composition to be highly stable, without exhibiting a significant
crystallisation exotherm over the temperature range recorded (50 to 600°C). It can be
seen from table (4.4) that W +6 has a similar radius to Zn +2 , lower than Te +4 (60, 74 and
97 pm respectively) [53], therefore tungsten oxide will enter the network as an
intermediate with less disruption than lead oxide, for example.
Effect of ZnO:ZnF2 ratio on the series 65TeO2.(25-x)ZnO.xZnF2.10Na2O (mol. %), for 0
≤ x ≤ 20
Fig. (4.17) shows DTA traces of glasses MOF009 to 13, which obeyed the compositional
formula 65TeO2.(25-x)ZnO.xZnF2.10Na2O (mol. %), for 0 ≤ x ≤ 20. The effect of the
molar ratio of ZnO:ZnF2 in this series on Tg and Tx-Tg is shown in fig. (4.18). As
expected, Tg fell with increasing fluoride in the glass as fluorine tends to break up the
network due to its ionic character, enabling the glass to flow more easily at lower
temperatures. Stability increased with addition of fluoride and then increased to a
maximum of 168°C at around ZnO:ZnF2 = 0.55, which corresponds to an interpolated
composition of 65TeO2-10Na2O-13.75ZnO-11.25ZnF2 (mol. %). Stability significantly
decreased when the molar ratio of ZnO:ZnF2 was > 0.6. The gradient of Tg versus
ZnO:ZnF2 ratio also showed a marked increase here (fig. (4.18)), indicating some sort of
structural change possibly occurred in the glass, perhaps a change of co-ordination of the
Zn +2 ions due to the change in the surrounding anionic environment (i.e. population and
position of O -2 and F - ions). Rao et al. [55] found that in the binary system PbO-PbF2,