Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 357
and low temperatures (T < T0) the behaviour is more Arrhenian [20]. As T approaches T0,
the viscosity is proportional to T 1/2 , and therefore non-linear in this region. If T0 is close
to Tg the viscosity behaviour around the transformation range will be non-Arrhenian, and
linear over a small region (≈ 60°C above Tg). This modelling has been shown to yield T0
values > Tg for fluoride glasses [20], which was also seen in this study (see table (8.4)).
The CG model has been shown to fit the behaviour of fluoride melts where T0 was close
to the liquidus temperature [19]. T0 for both glasses from table (8.4) is > Tg, and closer to
the melting temperatures than Tg in both samples.
Fig. (8.16) shows a fragility plot (η versus Tg / T) for glass MOF001 (25 % mol. ZnF2)
and for a hypothetical strong glass former such as a silicate or phosphate. The plot for the
fluorotellurite glass was generated using the CG parameters in table (8.4), and the
hypothetical plot for a strong glass was generated using a linear relationship. It can be
seen the viscosity-temperature behaviour of a fragile glass is significantly different to a
strong glass, with almost linear behaviour for Tg / T > 0.6. However this linear behaviour
still deviates from the strong glass former, with viscosity decreasing at a higher rate with
increasing temperature. For Tg / T < 0.6 (approaching the melting temperature), the
viscosity-temperature behaviour begins to plateau, and tends towards stronger behaviour.
At these high temperatures (Tg / T 0), there will be sufficient thermal energy for
virtually complete structural breakdown of the glass regardless of fragility, resulting in
very low viscosities.