Geoscience Reference
In-Depth Information
pH range successively in the more acid direction. The stability of the calcium phos-
phates at higher temperatures can be seen in
Figure 5.63
. The equation numbers indi-
cated in
Figure 5.63
refer to the following reactions:
1. 3CaHPO
4
(c)
5
Ca
3
(PO
4
)
2
(c)
1
H
3
PO
4
(c, l)
2. 3CaH
2
PO
4
(c)
5
Ca
3
(PO
4
)
2
(c)
1
4H
3
PO
4
(c, l)
3. Ca
10
H
2
P
6
O
26
(c)
5
3Ca
3
(PO
4
)
2
(c)
1
CaO (c)
1
H
2
O (g)
4. CaH
5
PO
6
(c)
5
CaHPO
4
(c)
1
2H
2
O (g)
5. CaH
6
P
2
O
9
(c)
5
Ca(H
2
PO
4
)
2
(c)
1
H
2
O (g)
The Eh
pH diagrams have shown that HAp may be obtained at
both atmospheric and hydrothermal conditions.
Section 4.6 discusses the intelligent engineering of crystal growth processes
using thermochemical computation, and Figure 4.17 represents the stability field
diagram generated for HAp crystal growth under hydrothermal conditions.
pH and Pa
Ca
5.8.3 Crystal Growth
The crystal growth of HAp is a highly challenging area. The growth of large size
crystals and the control of crystal morphology have evaded the crystal growers for
decades. There are a large number of publications on the preparation of large HAp
single crystals. Today a large variety of organics are employed in the HAp crystal
growth for this purpose; to alter the crystal morphology to suit various applications.
Table 5.18
gives the representative works on the hydrothermal synthesis of HAp
Table 5.18 Hydrothermal Synthesis of HAp Large Single Crystals
[204]
Starting Materials
Synthetic Conditions
Comments
300
C, 1250 1b/in
2
, for 10 days;
500
C, 354 bar, for 10 days
CaHPO
4
wet chemically
prepared HAp
0.3 mm in length,
2.35 mm in length,
0.2 mm in diameter
350
C, l00 MPa
CaHPO
4
2 mm
15,000 psi, held at 850
900
C,
then cooled at 20
C/h to
B
750
C, quenched
Ca
3
(PO
4
)
2
Ca(OH)
2
or
Ca
3
(PO
4
)
2
Ca(OH)
2
H
2
O
system
4 mm in length
300
670
C, 45,000
60,000 psi 7
3
7
3
3mm
3
, identify
the retrograde
solubility of HAp
synthesized HAp as nutrient
and HAp seed
8
3
0.5 mm
2
Ca
3
(PO
4
)
2
Ca
(OH)
2
CaCO
3
H
2
O system,
Ca(OH)
2
as flux
Oscillating temperature
technique, 750
800
C,
14,500
17,500 psi
CaHPO
4
2H
2
O
100
300
C, 2 kbar
0.1 mm
Wet chemically, followed by
Soxhlet extracted HAp
430
500
C, 2 kbar
3.5 mm, check the
retrograde solubility
of HAp
2
