Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 219

ticle size of potassium iodate from ~ 140 to 70 m causes about 5 % increase in the
rate constants and no systematic change in the isoconversional activation energy of
decomposition [ 128 ]. Nevertheless, the effect on the rate and activation energy can
be quite dramatic when the particle size is reduced significantly, especially to nano-
meter dimensions. This is illustrated in Fig. 4.44 that presents the isoconversional
values of the activation energy estimated for the thermal decomposition of calcium
carbonate nanoparticles [ 129 ]. It is seen that for decomposition of the larger-size
(~ 88 nm) nanoparticles the E ʱ values are practically independent of ʱ and average
to ~ 180 kJ mol − 1 that is quite similar to what is observed (Fig. 4.42 ) for decompo-
sition of regular microparticles. However, as the particle size is brought down to
36 nm, the effective activation energy drops below 140 kJ mol − 1 .
The defects of the crystalline structure can be induced by mechanical treatment
of solid reactants, e.g., by grinding. The stress field produced by mechanical treat-
ment is relaxed by the solid via a number of pathways that include the release of
heat, creation of new surface, accumulation of the crystalline lattice defects, poly-
morphic transitions, amorphization, and chemical reactions [ 130 ]. The creation of a
new surface is possible as long as solid particles continue to break. As the particles
approach a certain minimum size, the solid undergoes a transition from the brittle to
ductile state and, as a result, can only undergo plastic deformation [ 131 ]. The latter
gives rise to various crystalline defects that include dislocations as well as vacan-
cies and interstitial ions in the crystalline lattice [ 130 ].
It is important to recognize that the phase transitions and chemical reactions can
be initiated by simple manual grinding [ 132 , 133 ]. Thus, one should be mindful of
this ubiquitous procedure as a factor affecting the reactivity of a solid. The use of a
more intense mechanical treatment such as high-energy ball milling can lead to the
massive creation of defects, which is the purpose of mechanical activation of solids.
This activation usually results in a significant decrease in the activation energy of
the reaction involving the solids [ 130 ]. An illustration of this effect is the thermal
Fig. 4.44 Isoconversional
activation energies for the
thermal decomposition of
calcium carbonate powders of
different particle size ([ 129 ])
QP
QP
QP
QP
QP
α