Isoconversional Methodology - Isoconversional Kinetics of Thermally Stimulated Processes - page 45

Fig. 2.14 The g ( ʱ ) values estimated for the first ( diamonds ) and second ( triangles ) steps of dehy-
dration of nedocromil sodium trihydrate. The solid lines represent the theoretical models. (Repro-
duced from Zhu et al. [ 64 ] with permission of Wiley)

following two problems. First, the selection of the models is always subjective and
incomplete so that the proper model may simply be missing from the list. Second,
the theoretical models were developed for largely idealized processes that often-
times do not represent accurately the reality. For this reason, the theoretical reaction
model is frequently used in the form with adjustable parameters, most popular being
the so-called truncated Sestak-Berggren (SB) model [ 37 ]:

m n

f () ( αα α = 1 − ) .

(2.32)

Although this equation is frequently referred to simply as the SB model, the latter
has one term more [ 65 ] than Eq. 2.32. On the other hand, Eq. 2.32 has been used in
the heterogeneous kinetics years before [ 66 , 67 ] the influential work [ 65 ] by Sestak
and Berggren. A great advantage of Eq. 2.32 is that depending on the values of n
and m it can imitate the three major types of kinetic curves, i.e., decelerating, ac-
celerating, and sigmoid ones. Furthermore, Perez-Maqueda et al. have shown [ 68 ]
that Eq. 2.32 can accurately match a number of theoretical models. The parameters
n and m are easy to estimate by fitting the right-hand side of Eq. 2.32 to the numeri-
cal f ( ʱ ) values determined by Eq. 2.31. Unfortunately, the model does not exist in
the integral form because the integral