Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 205

4.4 Thermal Decomposition of Solids

…and nothing can hide itself from Thy heat
Saint Augustine, The Confessions of St. Augustine

4.4.1 Background

Solid substances are very diverse. They can be made of atoms, ions, or molecules
and can exist in the crystalline or amorphous form. However, when it comes to the
thermal decomposition of solids, the solid usually means [ 92 - 95 ] an ionic crystal-
line compound. This is the largest class of solid compounds that on heating can
decompose before they melt. Therefore, the thermal decomposition of most of ionic
solids can proceed entirely in the solid state at least in some temperature region.
This is in contrast to molecular solids whose solid structure is held together by
weak van der Waals forces that break easily on heating causing a solid to melt. As a
result, the thermal decomposition of molecular solids typically occurs in the liquid
state. As discussed in Sect. 4.3, most of polymers, being molecular solids, undergo
thermal degradation in the liquid state.
Ionic solids in which the cation is a metallic ion usually decompose to form at
least one solid and one gaseous product in accord with the following general equa-
tion:

(4.64)

ABC
s

→+.

s g

Equation 4.64 suggests that the solid reactant A transforms directly into the solid
product B or, in other words, the new phase B grows inside the reactant phase A. The
process is reminiscent of the solid-solid phase transitions discussed in Sect. 3.8, ex-
cept that in decomposition the phases obviously have different molecular composi-
tions. As discussed in Chap. 3, the formation of new phases is treated customarily in
terms of nucleation. In the area of the thermal decomposition of solids, the concept
of nucleation was introduced by MacDonald and Hinshelwood [ 96 ]. The general
idea is that the solid product B appears in the form of individual nuclei that succes-
sively grow on the surface of the solid reactant A . This concept has given rise to a
multitude of the reaction kinetics models, some of which are listed in Table 1.1. An
overview of these models is given elsewhere [ 93 , 94 , 97 ].
For thermolysis, equations for the critical nucleus size and the free energy bar-
rier to nucleation were derived by Jacobs and Tompkins [ 98 ]. Their approach is
similar to the traditional approach to the nucleation kinetics of phase transitions
(see Sect. 3.5). They start by assuming that the free energy of the formation of the
spherical nucleus of the solid reaction product B is determined by the sum of the
volume and surface free energies:

(4.65)

4 πσ, ∆ GmGr = ∆ +

B