Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 88
← Endothermic ∆T / °C Exothermic →
T x
300 320 340 360 380 400 420 440
Temperature / °C
Fig. (4.4): DTA trace of glass MOF005 (70TeO2-10Na2O-20ZnF2 (mol. %)), showing the
‘lift-off’ point (Tx) from the baseline before the crystallisation event.
Irreversible transformations are those in which reactants do not reform from products on
cooling. Here one of the reactants is a metastable state (i.e. glass) and only requires heat
to raise it above Tg to the undercooled liquid state to initiate the transformation.
Devitrification of glass is a classic example of an irreversible transformation. In these
types of reactions the sample temperature will not remain constant during the
transformation. As glass devitrification proceeds, heat released at the glass-crystal
interface raises the temperature of the sample. The rates of such transformations
generally have an exponential temperature dependence, causing them to proceed more
rapidly, which in turn causes a more rapid temperature rise, and so on. Therefore, these
‘self-feeding’ reactions show irregular temperature-time profiles when heated at a
constant rate [3]. This is why the peak crystallisation temperature on a DTA or DSC trace