Geoscience Reference
In-Depth Information
20 MPa during HHP. The temperature was kept constant at 300
C for 1 h and then
the autoclave was cooled down to room temperature. The solidified compact was
removed from the autoclave and dried at 110
C.
Yanagisawa et al.
[355]
have studied the formation of anatase porous cera-
mics by HHP of amorphous titania spheres prepared by hydrolysis of titanium
tetraethoxide. After fine anatase crystals were formed in the original amorphous
spheres by HHP, the spherical particles were deformed and fine anatase crys-
tals were flowed into the interstices among the original spheres by compression
from outside the autoclave, to form a compact with homogeneous distribution
of fine pores. The fine anatase crystals in the compacts were bonded together
by dissolution and deposition to form a compact with high mechanical
strength. The presence of water accelerated the crystallization of the starting
amorphous titania to anatase, and anatase compacts were produced, even at
100
C
[355]
.
Yamasaki et al.
[348]
have worked out a method to solidify sewage sludge
incinerated ash by HHP and investigated superior conditions to solidify ash for pro-
ducing recycled products. The solidified sample had a high compressive strength of
98 MPa by HHP at T
300
C, P
49 MPa, NaOH dosage approximately 6%. The
product could be used as building materials, such as tiles and bricks
[356]
. These
authors have also worked out a method of immobilizing and a high-volume reduc-
ing technique for low-level radioactive waste from nuclear power plants or repro-
cessing plants, through the hydrothermal solidification method
[357]
. tis
confirmed through evaluation tests and a full-scale mock-up test that this technique
can be adapted to an actual system for spent solvent treatment in commercial repro-
cessing plants.
HIP allows rapid densification with mineral grain growth and is one of the most
effective ceramic densification processes. This method is being popularly employed
to process HAp-based bioceramics
[358
5
5
360]
. Implant materials require not only
biocompatibility, but also mechanical strength and porosity to promote the connec-
tion with tissues. Therefore, microstructure designing, i.e., grain size, pore size,
and porosity, are necessarily tailored in their application to bioceramics. HAp sin-
gle crystals of about 255 nm
90 nm in size synthesized hydrothermally at 200
C
under 2 MPa for 10 h were normally sintered in air for 3 h. The ceramics obtained
were hot-isostatically pressed at temperatures of 900
3
1100
C under 200 MPa of
Ar for 1 h, without any capsules. This postsintering brought about densification
up to
100% for the samples. The fully dense ceramics, with a grain size of about
B
0.54
m, showed transparency. Furthermore, dense/porous layered HAp ceramics
could be prepared by the same technique, from the fine crystals and coarse powders
with relatively low sinterability. Uematsu et al. [350a] have synthesized HAp pow-
der and have formed it into a compact in an aqueous medium using a filter-cake
method
[360]
. The compact was hot
μ
1000
C and
100 MPa for 2 h. Fully dense, transparent materials were obtained above 800
C.
Both forming and densification methods were found to be important in obtaining
transparent materials.
Figure 10.78
shows a transparent HAp block obtained
through HHP
[358]
.
isostatically pressed at 700
