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relaxation modes in the glass are no longer active.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