Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 117
a crystallisation peak. Glass MOD006 contained slightly less PbO than MOD002, no
rare-earth oxide (Er 3+ and Yb 3+ ), and contained 1:1 molar ratio of ZnO:Na2O. Aida et al.
[28] found that ternary tellurite glass compositions with a 1:1 molar ratio of R2O:R’O
(where R is an alkali metal ion, e.g. Na + , Li + , K + ; and R’ is a metal ion, e.g. Zn +2 , Mg +2 ,
Ba +2 ) have a high resistance to crystallisation, particularly 10 to 15 mol. % R2O and R’O.
The DTA trace of glass MOD007 (fig. (4.11)), 78TeO2-12ZnO-10Na2O mol. %,
showed a broad, low intensity crystallisation peak. This ternary composition 78TeO2-
12ZnO-10Na2O (MOD007) contained a 1.2:1.0 molar ratio of ZnO:Na2O (close to a 1:1
molar ratio of ZnO:Na2O [28]) and was far more stable than glasses MOD001 and 002
(Tx-Tg = 161°C compared to 55°C and 85°C respectively).
Glasses MOD008 and 009 (74TeO2-12ZnO-10Na2O-3PbO-1Yb2O3 and 77TeO2-
12ZnO-10Na2O-1Yb2O3 mol. %), were also thermally stable. These compositions had the
same molar ratio of ZnO:Na2O (1.2:1.0) as MOD007, but some of the TeO2 was
substituted by PbO and Yb2O3 (3 and 1 mol. % respectively for MOD008) and Yb2O3 (1
mol. % for MOD009). These substitutions improved the stability of the glasses in both
cases compared to that of MOD007. No crystallisation exotherm was observed for
MOD008 and 009, as opposed to MOD007 which had Tx-Tg value of 161°C.
Fig. (4.12) depicts the DTA trace of glass MOD010, 75TeO2-10ZnO-10Na2O-5PbO
mol. %, which was highly thermally stable. This composition had a 1:1 molar ratio of
ZnO:Na2O and 5 mol. % PbO. This glass would be suitable for the core of an optical
fibre as it was thermally stable (no Tx peak on the DTA trace) and the addition of PbO
would tend to increase the refractive index from the ternary composition 80TeO2-