Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 128
crystallisation exotherm on DTA traces and was identified as cubic in structure by XRD
[57].
Effect of melting time for glass 70TeO2-10Na2O-20ZnF2
Fig. (4.20) shows the change in DTA trace with increasing melting time if glass
MOF005, 70TeO2-10Na2O-20ZnF2 mol. %. Tg and Tx-Tg were plotted against melting
time in fig. (4.21). With increasing melting time, the fluorine content of the glass will be
reduced, due to increased volatilisation, resulting in a more rigid oxide network, hence as
expected Tg increased. Glass stability also increased with increasing melting time,
indicating lower fluorine containing compositions are more stable, or OH in the glass
results in destabilisation, providing sites for nucleation. The latter would seem more
plausible as fig. (4.15) shows lower fluoride containing compositions are less stable; for
the compositional series (90-x)TeO2.xZnF2.10Na2O (mol. %), Tx-Tg = 118°C for x = 5
increasing to Tx-Tg = 161°C for x = 30. It is also interesting to note that as the glass
melting time increases, a higher temperature shoulder to the melting endotherm becomes
more distinguishable, most likely due to the presence of more oxide fraction in the glass.
4.4. Summary
The ternary system TeO2-ZnO-Na2O (TZN) can be melted to form a range of stable
glasses, and doped with additions such as PbO to increase the refractive index, GeO2 to
lower the index, and rare-earths (La +3 , Er +3 , Yb +3 ) for active applications. These additions