Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 358
Log10(η) η) / Pa.s
12
10
8
6
4
2
0
-2
-4
Strong
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
T g / T
Fragile
MOF001 (25 mol. % ZnF2) CG model Strong glass former MOF001 (25 mol. % ZnF2) TMA data
Fig. (8.16): Fragility plot of glass MOF001 (25 % mol. ZnF2) and a hypothetical strong
glass former for comparison.
Komatsu et al. [21-23] have shown that TeO2 based glasses fall into the category of
fragile glasses (but stronger than fluorozirconates), due to their steep viscosity-
temperature profile, and large heat capacity change (∆Cp) at Tg. A discrepancy has been
shown by this group in tellurite glasses between the activation energy for viscous flow,
Eη, and the enthalpy of relaxation, ∆H, around Tg. This is thought to be due to decoupling
between the activation energies for enthalpy relaxation and viscous flow in the glass
transition region [21].
Watanabe et al. [24] related the Vicker’s hardness of tellurite glasses to fragility and
relaxation processes in the glass. There are two types of relaxation processes in glasses:
α-relaxation which is observed above Tg due to atomic displacement / rearrangement, and