Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 118
10Na2O-10ZnO mol. % (n = 2.046 [28]) as Pb +2 has a higher polarisability than Te +4
[32].
Glass MOD011, 70TeO2-10ZnO-10Na2O-10PbO mol. %, which contained 10 mol. %
PbO was less stable than MOD010 (Tx-Tg = 77°C compared to no observable Tx),
indicating higher levels of PbO in the glass resulted in destabilisation. MOD001, which
contained 10 mol. % PbO was also unstable (55°C), however compositions with 3 to 5
mol. % PbO (MOD002, 6, 8 and 10) showed greater resistance to devitrification (Tx-Tg of
85°C for MOD002 and no observable Tx for MOD006, 008 and 010).
Glass MOD012 (75TeO2-10ZnO-10Na2O-5GeO2 mol. %) was identical to MOD010,
but the PbO was substituted with GeO2. Glass MOD012 was also highly stable (Tx-Tg =
164°C), which is expected as GeO2 is a glass former and will reinforce the covalent
glassy network. Composition MOD012 would be suitable for the cladding in an optical
fibre as it is thermally stable and GeO2 would tend to lower the refractive index of the
ternary composition 80TeO2-10Na2O-10ZnO, as Ge +4 has a lower polarisability than Te +4
[32]. Glass MOD013, 80TeO2-10ZnO-10Na2O mol. %, is stable (Tx-Tg = 108°C), as
expected for a ternary composition with a 1:1 molar ratio of ZnO:Na2O [28].
Effect of WO3, Nb2O5 and Bi2O3
Glasses MOD014 to 16 inclusively (fig. (4.13)), 90TeO2-5WO3-5Nb2O5, 82.5TeO2-
7.5WO3-10Nb2O5 and 70TeO2-25WO3-5Bi2O3 mol. %, contained WO3 and were all
relatively thermally stable, with Tx-Tg values >110°C; moreover, glass MOD016 did not
display a crystallisation exotherm over the region recorded (50 to 700°C). Several