Geoscience Reference
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10.6.4 Hydrothermal Preparation of Simple Oxide Ceramics
Zirconia ceramics are remarkable materials due to their excellent mechanical, electrical,
thermal, and optical properties [89,90] . Ceria- and zirconia-based oxide nanoparticles
are being studied extensively for a variety of applications like catalysis, UV adsorbers,
gas sensors, abrasive, electrochemistry, optics, and also biological applications.
Structural, optical, electronic, and catalytic properties of ceria-based oxide nanocrystals
are strongly dependent on their crucial geometrical parameters like size and shape.
Therefore, the preparation of high quality of ceria nanocrystals of desired morphology
is of great fundamental and technological interest. Physical and chemical properties
associated with these characteristics are closely related to structures and phase changes.
Therefore, numerous studies have been undertaken on phase diagrams and structural
changes in the zirconia systems [91
93] . There are many contradictions and discrepan-
cies, especially around 1000 C. One of the most important steps in solving this problem
is the study of metastable phase diagram [94 96] . Yashima et al. [92] have studied the
problem of diffusionless cubic
tetragonal (c t 0 ) phase transition, where t 0 emphasizes
a metastable tetragonal phase. Thus, it can be distinguished from the stable t,whichis
formed diffusionally. Figure 10.17 shows a metastable
stable phase diagram of the
ZrO 2
CeO 2 system that is depicted in a composition temperature field. On cooling
from 1650 C, the high-temperature cubic phase (B) transforms without cationic diffu-
sions into a metastable tetragonal phase below the tetragonal cubic transformation. Such
a metastable phase boundary is drawn by dashed and dotted lines [97] . There are several
groups in the world working on the hydrothermal processing of ceria-based oxide nano-
particles [98
100] .
3000
Liquid
2500
Cubic
2000
B
Tetragonal
1500
1000
Monoclinic
500
A
t'
C
m'
0
0
20
40
60
80
100
t''
ZrO 2
mol% CeO 2 in ZrO 2
CeO 2
Figure 10.17 Metastable stable phase diagram of the ZrO 2 CeO 2 system [97] .
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