Geoscience Reference
In-Depth Information
Figure 10.18 Hydrothermal
homogeneous precipitation
method [99].
ZrOCl
2
8H
2
O
YCl
3
6H
2
O
Water
3 mol%-Y
2
O
3
0.25-1 M/l
150-220°C
3.0-7.0 Mpa
0-24 h
ZrOCl
2
YCl
3
mixed solution
CO(NH
2
)
2
Hydrothermal treatment
Centrifugation, wash to remove Cl
-
and NH
+
5000-10000 r.p.m.
Washing in ethanol
120°C, 10h, in air
Drying
Calcination
Ball milling
800-1100°C, 10h, in air
12
h, in water
Drying
120°C, 10h, in air
Tani et al. [97a] have studied in detail the effect of mineralizers on the crystalliza-
tion of solid solutions in the system ZrO
2
CeO
2
under hydrothermal conditions [98].
Samples prepared by the coprecipitation method were treated under 100 MPa at 600
C
for 2
72 h using distilled water or a solution of alkaline metal (Li, Na, K) fluorides,
chlorides, bromides, carbonates, nitrates, sulfates, or hydroxides as mineralizers.
Somiya et al. [121] have given the following flowchart for the hydrothermal
homogeneous precipitation method
Figure 10.18
[99]. The autoclaves used were
(i) Turtle type, (ii) 500 cl, (iii) 1000 ml, (iv) 5000 ml, and (v) 20,000 ml. The lining
metals were Pt, Zr, and Ti. By this means, the authors could obtain very fine pow-
ders of ZrO
2
and Y
2
O
3
ZrO
2
of size 20 nm, with a narrow size distribution, very
high purity, excellent sinterability, less defects, and controllable shapes. Hence,
hydrothermal processing provides an excellent mean fraction by obtaining con-
trolled shape and size from the ceramic particles.
In the hydrothermal oxidation of metals, pulverization and oxidation proceed
simultaneously during the reaction with high-temperature and high-purity water.
These phenomena have been applied in the preparation of fine oxide powders
(20
30 nm) of Fe
3
O
4
,Cr
2
O
3
, ZrO
2
, HfO
2
,Al
2
O
3
, and others at relatively lower
temperatures (400
600
C) [100
102].
Figure 10.19a
d
shows the fine crystals of
zirconia and ceria.
Figure 10.20a
c
shows the representative photographs of
hydrothermally processed ceramic powders [99].
In order to obtain surface modification and a desired size and shape, several
modifiers are being used by researchers. A variety of organic acids and amines are
being used for this purpose.
Figure 10.21a
shows the TEM image of CeO
2
nanoparti-
cles obtained through such an approach by Ahniyaz et al. (2005) [103]. This
figure shows the self-assembly of fine nanoparticles of several nanometers size. The
valency state of the Ce ions alter with the size of the nanoparticles, and a control
over such an alteration is quite important in nanomaterials processing. Similarly,
Ahniyaz et al. (2005) have obtained fine nanoparticles of 5
1 nm size particles of
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