Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 122
composition, 50TeO2-40ZnF2-9.6As2O3-0.4Cr2O3 mol. %, with a Tx-Tg gap of 104°C (see
table (2.6)). Nazbal et al. [42] reported two compositions, 75TeO2-18ZnO-7ZnF2 and
69TeO2-9ZnO-22ZnF2 mol. %, with Tx-Tg gaps of 120°C (see table (2.7)). The inclusion
of 10 mol. % Na2O in the glass series, (90-x)TeO2.xZnF2.10Na2O (mol. %), resulted in
resistance to devitrification over a wide compositional range, with Tx-Tg gaps > 110°C for
5 ≤ x ≤ 30 mol. % ZnF2.
Effect of PbF2 and WO3
Fig. (4.16) shows DTA traces of MOF002 and 003, 70TeO2-10Na2O-10ZnF2-10PbF2 and
65TeO2-10Na2O-20ZnF2-5PbF2 mol. %, which contained PbF2. Both compositions
formed glasses but exhibited two distinguishable crystallisation exothems and were less
stable than the ternary series (90-x)TeO2.xZnF2.10Na2O (mol. %) for 5 ≤ x ≤ 30 mol. %
ZnF2 (i.e. MOF001, 004 to 008). However MOF002 and 003 had Tx-Tg gaps > 100°C,
which is comparable to some of the stable oxide glasses reported in section 4.1.2.1. (e.g.
MOD013, 80TeO2-10Na2O-10ZnO, Tx-Tg = 108°C). Table (4.4) shows van der Waals
(rvdw), ionic (rion), covalent (rcov) and Bragg-Slater (rb-s) ionic radii of elements used in this
study [53, 54].