Isoconversional Methodology - Isoconversional Kinetics of Thermally Stimulated Processes - page 40

It should be noted that single-heating-rate methods had dominated the field for
decades, which was not very surprising. Given a choice, not many people would
pick an isoconversional method that requires multiple runs in favor of a method
that promises to produce the same information from just a single run. As a matter
of fact, the need to perform more than a single run was considered [ 11 ] as a major
disadvantage of isoconversional methods. The other point of critique typically was
that isoconversional methods did not offer direct ways of determining the other two
components of the kinetic triplet, i.e., the reaction model and preexponential factor.
Computational techniques for estimating these two components are discussed in the
next section.

2.2 Estimating Reaction Models and Preexponential

Factors

Tao creates the One.
The One creates the Two.
The Two creates the Three.
The Three creates all things.

Lao Tzu, Tao Te Ching: 42

2.2.1 Prelude

The ability of isoconversional methods to estimate the activation energy without
estimating the reaction model has long been considered as one of their advantages.
However, it is sometimes believed that the methods are not capable of estimating
the reaction model as well as the preexponential factor. This is complete fallacy.
As a matter of fact, the seminal paper [ 13 ] by Friedman describes not only the
isoconversional method of estimating the activation energy but also a way of de-
termining the preexponential factor and the reaction order model. Since then, some
more sophisticated and accurate methods for estimating the preexponential factor
and reaction model have been developed. Two very popular methods are discussed
in this section.
Before we get to discussion of these methods, we need to forewarn one against
some rather unsound approach to the problem that unfortunately is not quite uncom-
mon. In it, the reaction model and preexponential factor are estimated by matching
the activation energy estimated by an isoconversional method with the activation
energy determined by some method that uses model fitting of single-heating-rate
data. For example, one can fit d ʱ /d t versus T data obtained at one heating rate to
Eq. 2.27

E
RT

1 d

d

j

ln

=

ln A − . (2.27)

j

f () α t

j