Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 331
applied to the sample. The sample was then heated at 10°C.min. -1 under a helium
atmosphere to above its softening temperature. The change in sample height (hs), with
time (t), was recorded, and the viscosity in Pa.s, η, calculated using equation (8.1).
5
2πFhs
η =
(8.1)
dhs
3
3V
( 2πhs
+ V )
dt
where V = sample volume. The viscosity data obtained was modelled using a number of
equations, shown in table (8.1), where parameters A, B, and C are materials constants,
and T is temperature. The meaning of T0 depends on the equation used, and will be
discussed later.
Table (8.1): Viscosity models used in this study, where parameters A, B, C and T0 are
materials constants, and T is temperature.
Model Equation Reference
Arrhenius
1
logη = A + B ⋅
T
[2]
Vogel-Fulcher-
B
logη
= A +
T − T
[3]
Tamman 0
Cohen-Grest
Macedo-Litovitz
2 ⋅ B
logη
= A +
[4]
{ ( ) [ ( ) ] } 5 . 0
2
T − T0
+ T − T0
+ 4 ⋅ C ⋅ T
B C
logη
= A + +
[5]
T T − T
0