Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 354
Table (8.7): Thermal and mechanical properties of a possible core / clad pair, where Tg =
onset of glass transition temperature, TEC = thermal expansion coefficient, Y = Young’s
modulus, rof = outer fibre / preform diameter (5 mm), rco = core diameter (2.5 mm), µp =
Poisson’s ratio, and Kb = bulk modulus [10]. Subscripts 1 and 2 denote core and cladding
respectively, and *, properties just above Tg [9].
Composition /
mol. %
65TeO -10Na O-
2 2
25ZnF 2 (clad)
70TeO 2 -10Na 2 O-
20ZnF 2 (core)
T g /
°C
TEC /
×10 -6
°C -1
TEC * /
×10 -6
°C -1
Y /
GPa
µµµµp
Kb /
GPa
Kb * /
GPa
r /
mm
237 49.6 - - - - - 5.0
241 47.7 83.9 42.1 0.289 32.5 16.3 2.5
Thermal expansion coefficients (TEC) were measured at 225°C (just below Tg) for both
glasses, and TEC * at 250°C. Values of Y, µp, Kb are from a study on 80TeO2-20ZnCl2
mol. % glass [10] (assuming Kb1 ≈ Kb2, and bulk modulus above Tg is 0.5 times that
below [11]). Close TEC, will mean minimal thermal expansion mismatch in the fibre.
Below Tg, on cooling, the cladding will contract more than the core (higher TEC), putting
the core in compression, and the cladding in tension of equal and opposing magnitude.
Tensile stress in the cladding would tend to open up Griffith flaws at the fibre surface,
therefore coating would be necessary. The stress, σ, in the fibre can be calculated using
equation (8.2) [11, 12].
*
( TEC1
− TEC2
)( T − Tg1)
Y ( TEC1
− TEC2
)( Tg
2 − Tg1
)
σ =
+
(8.2)
2 2
2 2 *
( r / r )( 1−
µ ) (( 1−
r / r ) / 3K
) − ( 1/(
3K
))
of
co
p
of
co
b1
b2