Geoscience Reference
In-Depth Information
Figure 10.69 HAp whiskers under
hydrothermal conditions
[335]
.
Source: By M. Yoshimura.
(1)
PO
3
−
PO
3
−
Ca-Cit.
complex
Initial solution
Hydrothermal treatment
(2)
PO
3
−
Ca
2
+
Decomposition
of complex
Ca
2+
undersaturated
PO
3
−
Hydrothermal treatment
(3)
Formation of
HAp whisker
PO
3
−
Ca
2
+
PO
3
−
Ca
2+
saturated
HAp
Suchanek and Yoshimura
[336]
have studied the thermal stability and densifica-
tion of HAp whiskers in great detail.
The HAp whiskers have already been used as a biocompatible reinforcement in the
HAp/HAp (whisker) composites,
Figure 10.73a
. Consequently, the fracture toughness
of pure HAp ceramics has been improved even to the value of 2.0 MPa—this is the
highest value in 25 years. In addition, a variety of microstructurally controlled HAp-
based composites with improved reliability and high biocompatibility can be prepared
using
-NaCaPO
4
), a newly discovered weak interphase material for
HAp ceramics (
Figure 10.74a
β
-rhenenite (
β
c
). The fibrous HAp can also be used to fabricate
porous HAp ceramics or porous HAp/
-TCP composites as shown in
Figure 10.73b
.
Moreover, the HAp fibrous skeleton should be an appropriate reinforcement for HAp/
polymer biodegradable bone substitutes. Thus, HAp whiskers are very important in
materials science and technology.
The preparation of TiO
2
-based composites with SiO
2
, activated carbon, Al
2
O
3
,
zeolites, etc. have been reported in the literature. Several authors have worked exten-
sively on the preparation of TiO
2
β
HAp composites. Recently, the authors
[337]
have
prepared anatase nanocrystals deposited on HAp under hydrothermal conditions. The
HAp was prepared initially using CaCO
3
,H
3
PO
4
,andHNO
3
. Ammonia solution
was used to adjust the pH. Later, to synthesize TiO
2
-deposited HAp, 0.2 g of the
above-synthesized HAp or commercial HAp particles were added into a pH-adjusted
neutral solution of 4 ml titanium amine complex and distilled water. The precursor
was treated in an autoclave at 180
Cor120
C for various reaction times. The anatase
crystals were nucleated by heterogeneous nucleation and grew on the HAp surface.
