Geoscience Reference
In-Depth Information
aspects for preparation of these perovskite-type oxides owing to their potential
applications in modern technology. However, it is impossible to discuss every
aspect of these perovskite types of oxides in this handbook which is devoted to all
general aspects of hydrothermal technology.
The general formula for perovskite can be written as ABO
3
, where the A cation is
relatively large and of low valence (such as Ba
2
1
,Sr
2
1
,Ca
2
1
,Pb
2
1
,La
3
1
,Sm
3
1
,
Nd
3
1
,Bi
3
1
,andK
1
), and the B cation is relatively small (such as Ti
4
1
,Zr
4
1
,Sn
4
1
,
W
6
1
,Nb
5
1
,Ta
5
1
,Fe
3
1
,Mn
3
1
,Mg
2
1
,Zn
2
1
,andNi
2
1
), and for the solid solution
A(B
x
C
1
2
X
)O
3
,where0
Zr.
Kutty and Balachandran
[164]
have synthesized crystallization of perovskite
x
1, A
Ca, Sr, Ba, Pb, and Bi; B
Ti; and C
,
,
5
5
5
(x
Ti gels,
obtained by hydrolyzing TiOCl
2
and ZrOCl
2
and NH
3
(aq.). Beal
[165]
obtained
perovskite (x
0.5) at 573 K starting with crystallization of PbO and mixed Zr
5
titania gels, crystalline
PbO, and various mineralizers. The author found that the purity and morphology of
the product depended on the chemical identity of the mineralizers.
Dawson and Swartz
[166]
have synthesized several solid solutions (0
0.5) at 573 K from mixtures of zirconia
5
1)
from the PZT family using aqueous gels obtained by hydrolyzing TiCl
4
and ZrOCl
2
in basic solutions. The slurry obtained, along with lead oxide and precursors of other
elements (dopants), was hydrothermally treated at 573 K. It was found that the
obtained solids had the same metal ion stoichiometry as the feeding material.
Riman and coworkers
[167
,
x
,
169]
have done excellent work on the prepara-
tion of these PZT types of ceramics and have studied in detail the thermodynam-
ics and kinetics of these systems. The thermodynamic model proposed by them
has been well accepted for this perovskite system. Also, they have developed a
new approach—intelligent engineering—in order to transform hydrothermal
synthesis from an empirically based technology to one that revolves around
engineering principles. They approached this problem from a multidisciplinary
perspective of chemistry, chemical engineering, and physical chemistry,
which all embrace principles of thermodynamics and kinetics
[170,171]
.
Thermodynamic principles enable one to determine how to design a reaction to
yield phase-pure materials. Without this knowledge, it is impossible to distin-
guish a process that is being controlled by thermodynamics versus kinetics.
These authors have studied all the possible reactions that may occur in the
hydrothermal medium, more of a typical PZT system, for example,
in the
Ba
Ti and Pb
Ti systems. The following are the relevant equilibria in the
Ba
Ti and Pb
Ti hydrothermal systems
[167]
:
i. H
2
O
5
H
1
1
OH
2
ii. H
2
O
(g)
5
H
2
O
iii. TiO
2
ð
s
Þ
5
OH
2
2
HTiO
3
iv. Ba(OH)
2(s)
5
Ba
2
1
1
2OH
2
v. BaOH
1
5
Ba
2
1
1
OH
2
vi. BaTiO
3(s)
1
Ba
2
1
1
H
2
O
2OH
2
1
TiO
2(s)
5
Ba
2
1
1
vii. Ba(OH)
2
8H
2
O
(s)
5
2OH
2
1
8H
2
O
Ba
2
1
1
viii. BaO
(s)
1
H
2
O
ix. Ba
2
TiO
4(s)
1
2H
2
O
5
2Ba
2
1
1
4OH
2
1
TiO
2(s)
2H
1
5
