Biomedical Engineering Reference
In-Depth Information
TABLE 6.2
Preparation Methods for Crystalline HA
StartingMaterials
SyntheticConditions
Comments
Solid state reaction
Ca
3
(PO
4
)
2
, CaO,
Ca
2
P
2
O
7
, CaCO
3
900-1300°C
Ca/P ~ 1.67
Large, irregular grain
Inhomogeneous
Wet chemical method
Ca(OH)
2
,
H
3
PO
4
RT ~100°C
pH: 7-12
Ca/P < 1.67
Irregular crystals with low
crystallinity
Inhomogeneous
Hydrothermal method
Wet chemically prepared
HA
100-200°C
(1-2 MPa)
Ca/P = 1.67
Fine single crystals
Homogeneous
Flux growth method
CaF
2
, CaCl
2
as flux
Ca(OH)
2
as flux
1325°C
FA, HA
Large crystals with lattice
strain
main ways for the preparation of HA powders are solid-state reactions and wet chemical
methods. In the case of HA fabrication, the wet methods can be divided into precipitation,
hydrothermal technique, and hydrolysis of the calcium phosphates. Depending on these
techniques, materials with nanocrystalline, various morphologies of blades, rods, needles
or equiaxed crystals, stoichiometry, and various levels of crystallinity can be obtained.
Solid-state reactions can usually obtain stoichiometric and well-crystallized HA prod-
ucts. However, they require relatively high heating temperatures and long heat-treatment
time. In addition, the HA powder prepared by solid state reactions usually have irreg-
ular forms with a large grain size, and often have heterogeneity in composition due to
incomplete reactions. The wet chemical method is a relatively easier way of obtaining
HA powder. The reactions may occur in aqueous solutions at lower temperatures. But
the lack of wet chemical methods in atmosphere means that the products are usually less
crystallized, heterogeneous in composition, and irregularly formed crystals with lattice
defects. The flux growth method, which is a better method since the fluxes such as CaF
2
,
CaCl
2
, NaCl, KF, and Ca(OH)
2
mixed with starting apatite powders make lower liquid tem-
perature, results in the production of less strained apatite crystals [129-131]. Among these
methods mentioned in Table 6.2, the hydrothermal technique is a very important method
TABLE 6.3
Chemical Reactions for Various Synthesis Methods of HA
1. Solid state reaction
Ca
2
P
2
O
7
+ CaCO
3
→
Ca
10
(PO
4
)
6
(OH)
2
, with H
2
O vapor
Ca
3
(PO
4
)
2
+ CaO
→
Ca
10
(PO
4
)
6
(OH)
2
(at 1000°C)
6CaHPO⋅2H
2
O + 4CaCO
3
→
Ca
10
(PO
4
)
6
(OH)
2
+ 4CO
2
+ 14H
2
O (at 1000°C)
2. Wet chemical method
10Ca(OH)
2
+ 6H
3
PO
4
→
Ca
10
(PO
4
)
6
(OH)
2
+ 18H
2
O (at pH = 8)
10CaCl
2
+ 6Na
2
HPO
4
+ 2H
2
O
→
Ca
10
(PO
4
)
6
(OH)
2
+ 12NaCl + 8HCl (at pH = 8)
10Ca(NO
3
)
2
+ 6(NH
4
)
2
HPO
4
+ 2H
2
O
→
Ca
10
(PO
4
)
6
(OH)
2
+ 12NH
4
NO
3
+ 8HNO
3
(at pH = 8-10)
3. Hydrothermal method
10CaHPO
4
+ 2H
2
O
→
Ca
10
(PO
4
)
6
(OH)
2
+ 4H
3
PO
4
(at 200°C)
4. Flux method
CaO + P
2
O
5
+ B
2
O
3
→
Ca
10
(PO
4
)
6
(OH)
2
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