Geoscience Reference
In-Depth Information
large single crystals
[229
238]
. Morey and Ingerson (1937)
[229]
first attempted
the hydrothermal synthesis of HAp crystals. Since then, several reports have
appeared in the literature. Roy (1971)
[234]
grew prismatic single crystals of HAp,
up to 4 mm length hydrothermally, from melts in the system 3CaO
P
2
O
5
Ca
(OH)
2
and 3CaO
H
2
O. Mengeot et al. (1973)
[235]
first suc-
ceeded preparing HAp single crystals of up to 7
P
2
O
5
Ca(OH)
2
3mm
3
in size by controlling
the temperature gradient. This work also revealed for the first time that HAp exhib-
ited retrograde solubility in hydrothermal fluids between 300
C and 670
C under
pressures of 3
3
3
3
4 kbar. With the appearance of this report, most of the researchers
placed nutrients at the cooler zone. Here, dissolution occurs and HAp crystals grow
at the hotter region where the seed crystals are placed. This arrangement facilitated
the growth of larger HAp single crystals. However, the preparation of large single
crystals of HAp has encountered several difficulties such as: the tendency to incor-
porate impurities, the stability of nonstoichiometric composition, and the restricted
field of crystallization in the phase diagram CaO
P
2
O
5
H
2
O, where HAp and
H
2
O only exist.
In the recent years, Ito and coworkers
[239,240]
have done excellent work on
the single crystal growth of HAp. Single crystals of carbonate-containing HAp
have been grown hydrothermally by gradually heating with a temperature gradient
applied to the vessel, using CaHPO
4
,H
2
O, and carbon dioxide as dry ice. The
hydrothermal reaction for the formation of HAp can be written as follows:
10CaHPO
4
1
2H
2
O
!
Ca
10
ð
PO
4
Þ
6
ð
OH
Þ
2
1
4H
3
PO
4
Crystal growth experiments were performed, slowly hydrolyzing CaHPO
4
by
gradually heating a pressure vessel with a temperature gradient applied.
Kikuchi et al. (1994)
[241]
have obtained Sr-substituted HAp crystals of size
5mm
3
under hydrothermal conditions. The starting materials such as
calcium and strontium hydrogen phosphate (SrHPO
4
) were gradually heated with a
temperature gradient. The crystal growth occurred at low pH.
In most of the works, the effects of CO
2
3
on the morphology of the crystals have
been reported: Needle-like crystals are predominant if the carbonate content is high-
er than 0.1%, whereas equiaxial crystals occur if the carbonate content is below
0.1%. Hata et al. (1983)
[242]
synthesized Cd HAp single crystals, partly substituted
with the first series transition elements for Cd ion, to investigate the substitution
effects on the crystal growth and structural changes. These crystals form a super-
structure of pure Cd HAp. Nagata et al. (1995)
[243]
have studied the synthesis of
HAp crystals in the presence of methanol and the morphological variations in HAp
crystals. It was found that the products obtained from the slurries without methanol
were rod-like or granular crystals. Addition of methanol to the slurries caused an
increase in the ratio of plate-like crystals to rod-like or granular crystals. When the
weight of methanol added was equal to the weight of the slurries, only plate-like
crystals were obtained.
Although several workers have reported a negative TC of soluble for HAp, there
is no systematic study either on the experimental or theoretical side. In this respect,
0.2
0.2
3
3
