Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 121
such as TeO2-PbO-PbF2 [48], TeO2-PbO-ZnF2 [20, 44-46] and TeO2-ZnO-ZnF2 [40, 41]
have been studied; however the ternary TeO2-Na2O-ZnF2 has not been reported in the
literature, except for work published from this study to the author’s knowledge [10].
Fig. (4.14) shows DTA traces of glasses MOF001, MOF004 to 008 inclusively, of the
composition series (90-x)TeO2.10Na2O.xZnF2 (x = 5, 10, 15, 20, 25 and 30 mol. %). The
DTA traces show Tg decreased with increasing batched zinc fluoride in the glasses (see
fig. (4.15)). At 5 mol. % added ZnF2, Tg was around 273°C, and it decreased to 235°C at
30 mol. % ZnF2. This is to be expected as fluoride tends to break up the strong TeO2
covalent network of the glass by forming ionic, non-bridging M-F bonds (where M is a
cation), enabling the glass-forming liquid to flow more easily at lower temperatures;
hence isoviscous points occur at lower temperatures as fluoride increases. Fig. (4.15)
shows that the glass stability on reheating, expressed as Tx –Tg, increased to a maximum
of around 161°C at 25 mol. % ZnF2, making glasses around this composition favourable
for fibre drawing as the tendency for devitrification will be suppressed. This stability
increase with increasing ZnF2 addition may have occurred because the more fluoride
added, the more competition there is between different phases to crystallise, which would
tend to stabilise the glass. Also, as the eutectic composition is approached across the
notional fluoride/oxide tie-line, glass forming ability should be enhanced. It should also
be noted here that the batched amount of fluoride was not equal to the retained fluoride in
the glass, however, around 95 atomic % F was retained in the glass after melting,
calculated from the weight % of the batch which volatilised. The glasses reported here
are the most stable ZnF2-containing reported in the literature to this author’s knowledge,
with Tx-Tg gaps exceeding 160°C (MOF001 and 4). Durga et al. [47] reported a glass of