Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 122

In conclusion, we should mention that the aforementioned nucleation analysis
is identical to that considered earlier for the solid-liquid (melting) and liquid-solid
(crystallization) phase transitions. Since first-order transitions are accompanied by a
change in volume, the molar volumes of the solid and liquid phase are generally dif-
ferent. Because of the mismatch of the volumes, the formation of a new phase nucle-
us would have to overcome an energy barrier associated with elastic strain energy.
When one of the phases is liquid, the strain energy can be neglected. However, if
both phases are solid, a more accurate representation of nucleation needs to account
for the strain energy term as a part of the free energy barrier of the process [ 143 ].

3.8.2 Isoconversional Treatment

The kinetics of the solid-solid phase transitions is conveniently measured by DSC.
Generally, one can produce a set of data at several heating rates for the transition
from the low- to high-temperature phase and a set of data at several cooling rates
for the reverse transition. Obtaining the high-temperature phase in the supercooled
metastable state may be difficult for many substances. However, if obtained, a da-
taset should be generated at multiple heating rates to be able to study the kinetics
of the transition from the metastable high-temperature phase into the stable low-
temperature phase. The datasets are then treated by an isoconversional method. It
should be emphasized that not every isoconversional method is applicable to the
data obtained on cooling. The methods suitable to this task have been discussed in
Sect. 2.1.2.
The application of an isoconversional method to the datasets obtained on heat-
ing and cooling results in the E ʱ versus ʱ dependencies such as those shown in
Fig. 3.51 for the transition between the form I (cubic) and II (tetragonal) of ammo-
nium nitrate. The dependencies are in agreement with the E versus T dependence

Fig. 3.51 Isoconversional
activation energies estimated
for the phase transition
between tetragonal ( phase II )
and cubic ( phase I ) forms of
ammonium nitrate

500

Heating:
phase II → phase I

400

300

200

100

Cooling
phase I → phase II

0

-100

0.0 0.2 0.4 0.6 0.8 1.0

α