Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 96
k
c
= k
⎛ − E
exp⎜
⎝ RT
⎞
⎟
⎠
a
0 (4.4)
where k0 is the pre-exponential rate constant and Ea the activation energy. If natural logs
are taken on both sides:
Ea
ln kc
= ln k0
−
(4.5)
RT
Values of lnkc (obtained from isothermal experiments) can be plotted against 1/T and the
slope of this straight line is equal to –Ea/R. Therefore, Ea, the activation energy for
crystallisation, for a particular glass composition can be calculated.
4.1.4. Experimental considerations
Good mechanical contact between the sample and the bottom of the crucible will improve
instrument sensitivity to transformations. Surface contact can be optimised by crushing
granules finely (of equal particle size for reproducible results), however with glasses this
can result in the crystal nucleation and growth mechanism changing from the bulk to the
surface due to the increased surface energy [3]. Also, gas entrapment may result in poor
heat transfer when using a powder compared to a single particle. The particle size also
affects the rate of glass crystallisation. For spherical particles the surface to volume ratio
decreases by 3/r (r = particle radius) with increasing particle size. Heat flow from the
surroundings to the interior of the larger particles is relatively suppressed due to reduced