Geoscience Reference
In-Depth Information
Figure 10.78 Transparent
HAp block obtained through
HHP.
Source: Photograph by M.
Yoshimura.
Figure 10.79 Relation between
pressure and grain size or relative
density hydrothermal reaction
sintering of magnetite
[362]
.
1.00
4.0
550
°
C
72 h
0.98
3.0
0.96
2.0
0
500
1000
Pressure (kg/cm
2
)
1500
2000
Hydrothermal Reaction Sintering of Processing Materials
Hydrothermal reaction sintering is one of the methods used to produce high-density
oxide ceramics. A large variety of oxide ceramics have been processed by this
method. Oxide powders were obtained by reactions between solutions or nonaqueous
liquid and metals. Hydrothermal reaction sintering is a kind of reaction with hot iso-
static processing. The hydrothermal reaction sintering of oxides is as follows:
a. low-temperature sintering—able to make sintered body even if materials have high vapor
pressure, decomposition, and/or transit,
b. able to make a very fine grain-sized body,
c. able to make high-purity body,
d. able to make high-density body,
e. able to make uniform microstructures,
f. able to save energy due to low-temperature sintering,
g. able to control valency.
Factors, such as purity, grain size and shape of starting materials, reaction tem-
perature, pressure, duration time, ratio of metal and water, and kinds of salt in
solution, have made hydrothermal reaction sintering unclear. Several ceramic
powders have been processed using this technique. The important ceramic materi-
als obtained by this method are: magnetite, w¨stite, monoclinic ZrO
2
, chromia,
monoclinic HfO
2
, alumina, and so on
[361
366]
.
Figure 10.79
shows the rela-
tion between pressure and grain size or relative density hydrothermal reaction
sintering of magnetite
[362]
.
Figure 10.80
illustrates the SEM fracture surface
of high-density hydrothermal reaction sintered Cr
2
O
3
(100 MPa, 1000
Cfor3h,
0.01 M HNO
3
solution) with a relative density of 99.1%
[363]
.
Table 10.7
gives selected results of hydrothermal reaction sintering of monoclinic ZrO
2
for
