Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 355
where T = 21°C. Using values from table (8.7), gives stress in the fibre made from this
core / clad pair around 72 MPa, which is relatively low [9]. Cherbanov et al. have
reported 170 µm TeO2-WO3 fibre with tensile strength as high as 5.8 GPa [13].
8.3.2 Viscosity (TMA)
The viscosity / temperature curves of both glasses was studied using TMA. Modelling
was performed to extrapolate the viscosity over a small temperature range either side of
the validated TMA data, to predict the temperature at which fibre drawing should occur
(10 4.5 Pa.s). These models are generally useful over a narrow compositional and
temperature range. Fig. (8.4) shows the TMA data for core and clad compositions. For a
given temperature, the viscosity increases with decreasing ZnF2 content. ZnF2 breaks up
the TeO2 network, enabling the glass to flow more easily at lower temperatures [9]. This
is desirable for fibre drawing, as stability increases with ZnF2 content. Figure (8.5) shows
the viscosity modelling for the clad glass. The Cohen-Grest model is shown [4], with the
Arrhenius [2] for comparison. Moynihan showed the Cohen-Grest equation models the
viscosity / temperature behaviour of fluorozirconate glasses (fragile and non-Arrhenian,
see [14]) more accurately. Stronger glass formers (such as silicates and phosphates [15,
16]) exhibit Arrhenian behaviour, and are better modelled by the Vogel-Fulcher-Tamman
equation [3, 17]. On heating above Tg, structural break down is more rapid for fragile
glass formers due their ionic nature, resulting in a steeper viscosity / temperature curve.
The presence of oxygen and fluorine in these glasses, will result in a fragility somewhere
in between oxide tellurite glasses (stronger), and fluorozirconates (more fragile). This is