Geoscience Reference
In-Depth Information
Figure 4.12 Relation of log R to 1/T
for the filling factors—x: (1011); o:
(1120)
[67]
.
log R
0.6
.4
×
×
0.65
.2
×
×
0.55
×
0.0
×
×
.8
0.6
×
×
.6
.4
×
-
1.2
T
°
K 10
3
1.14
1.18
1.22
1.26
on the curves—x:
ð
1011
Þ
;
o:
ð
1
12
0
Þ:
The activation energies ca
lc
ulated from
Figure 4.12
are 32 kcal/mol for
face
[67]
.
Riman and coworkers
[30,31,76,77]
have investigated the crystallization kinetics
of ABO
3
(A
ð
1011
Þ
face and 17.5 kcal/mol for
ð
1120
Þ
Ti, Zr, Hf) and their solid solutions.
The fundamental role of temperature, pressure, precursor, and time on crystalliza-
tion kinetics of perovskite oxides have been studied in detail. Early work by
Battelle Laboratories set the stage for the development of hydrothermal processes
that provide excellent morphological control for a variety of ceramic chemistries
[78]
. The morphology control and size of lead zirconate titanate and lead titanate
precipitated from organic mineralizer solutions have been worked out in detail.
Kaneko and Imoto
[79]
have investigated the effects of pressure, temperature, time,
and Ba:Ti ratio on the kinetics of a hydrothermal reaction between barium hydrox-
ide and titania gels to produce barium titanate powders. Ovramenko et al.
[80]
con-
ducted kinetic studies to compute an activation energy (E
a
) of 21 kJ/mol. In
contrast, Hertl
[81]
calculated activation energy of 105.5 kJ/mol. However, this dis-
crepancy may be due to the difference in source of titania precursor as well as other
reaction conditions (e.g., temperature). Computations performed in the composition
temperature
5
alkaline-earth elements, B
5
pressure space facilitate the construction of stability diagrams, specia-
tion diagrams, and yield diagrams (
Figure 4.13
)
[82]
. In addition, the authors have
carried out kinetic analysis based on reaction progress, yielded into the reaction-
rate regime for various perovskite-type oxides.
Other materials of commercial interest in the present-day context whose kinetics
of crystallization have been studied in detail are zeolites and HAp
[83
85]
.
Various approaches or models have been proposed to understand the reaction
mechanism of these compounds. The models based on the dissolution process,
growth process, surface diffusion, structure directing chelates, or amines in the syn-
thesis of zeolites, etc., have been used as the parameters in the study of crystalliza-
tion kinetics. These studies have helped in the preparation of phase-pure zeolites
and HAp particles with a perfect control over the morphology, grain size, rate of
crystallization, and so on.
