Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 356
confirmed by viscosity data reported by Wang et al. [18], which shows TeO2-Na2O-ZnO
and ZBLAL viscosity of 10 6 Pa.s to occur at 352 and 295°C respectively. From fig. (8.4)
it can be seen the viscosity of 10 6 Pa.s occurs at 296 and 306°C for the 25 and 20 mol. %
ZnF2 glasses respectively, closer in fragility to fluorozirconate glasses, but slightly
stronger. The models predict the fibre drawing viscosity (10 4.5 Pa.s) of the clad glass to
occur around 324°C, 74°C below Tx. Figure (8.6) shows the modelling for the core glass.
As expected, the predicted fibre drawing viscosity occurred at a higher temperature than
the value for the clad glass, at 333°C, 61°C below Tx. Table (8.4) shows the parameters
from the viscosity models for both glasses, and predicted fibre drawing temperatures.
These values indicate successful fibre drawing is possible without devitrification.
The VFT and ML models generated negative Kelvin values for T0 in these models for
the core (MOF005) glass, therefore these models were not used on the plots shown in fig.
(8.5) and (8.6). In the VFT model, T0 is believed to correspond to an ideal glass transition
temperature below Tg, where the free volume of the glass tends to zero [19]. Therefore,
some researchers have suggested this justifies the theoretical basis for an otherwise
empirically derived equation. However, as this equation models the viscosity-temperature
behaviour of strong glass formers, such as silicates, more accurately, it will not be
considered further here. The CG model was also developed by the considering the free
volume of the glass [4], although T0 takes on a different meaning. Here it represents the
transition between high and low temperature behaviour, and below the transition certain
relaxation modes in the glass are no longer active. For high (h) and low (l) regions,
different values of T0 exist, with the free volume proportional to T-T0l and T-T0h. At high
temperatures, the CG model approximates VFT behaviour (i.e. when (T-T0) 2 >> 4CT),