Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 119

Fig. 3.47 Hysteresis in
transition between phase I
and II of ammonium nitrate
measured on heating ( solid
line ) and cooling ( dash-dot
line ) at 20 ﾰC min −1

100

80

60

15 o C

40

20

0

100 110 120 130 140

T / o C

occur via nucleation (Sect. 3.7) in the same manner as the reverse transition from
melt to crystal, i.e., crystallization (Sect. 3.6). However, the superheating during
melting is normally negligibly small. Melting starts from the surface, and because
the surface possesses higher molecular mobility, the surface layer melts before tem-
perature reaches the bulk melting temperature. In a sense, the crystal surface is pre-
nucleated so that the melt layer grows very rapidly when temperature reaches the
equilibrium temperature of melting, T m . This is the only reason why T m provides an
accurate estimate for the temperature at which a crystal can coexist in equilibrium
with its melt.
The kinetics of the solid-solid transitions is generally consistent with the Turn-
bull-Fisher nucleation model [ 92 ]. The temperature dependence of the transition
rate has a rather complex form shown in Fig. 3.48 . One can obtain this dependence

Fig. 3.48 Nucleation rate
dependence derived from the
Turnbull-Fisher equation
(Eq. 3.44)

phase I

phase II

supercooled
phase I

T