Geoscience Reference
In-Depth Information
Table 5.17 Preparation Techniques for HAp
[204]
Techniques
Starting Materials
Synthesis
Conditions
Comments
900
1300
C,
usually with
water vapor
Solid-state
reaction
Ca
3
(PO
4
)
2
1
CaCO
3
Ca/P: 1.67, large grain size,
irregular forms,
inhomogeneous flowing
Ca
2
P
2
O
7
1
CaCO
3
Wet chemical
method
Ca(NO
3
)
2
1
(NH
4
)
2
RT: 100
C, pH:
7
Ca/P
,
1.67 fine irregular
crystals with low
crystallinity
inhomogeneous
HPO
4
Ca(OH)
2
1
12
H
3
PO
4
100
200
C
(1
2 MPa),
300
600
C
(1
2 kbar)
Hydrothermal
method
Wet chemically prepared
HAp, other calcium
phosphates, seeding
Ca/P
5
l.67 homogeneous,
fine single crystals or
large crystals
Gel
1
Ca
2
1
1
PO
4
3
2
RT: 60
C, pH:
7
10
Gel growth
method
Large Monetite, Brushite,
OCP, but small HAp
1325
C (FAp,
ClAp) HAp
Flux growth
method
CaF
2
, CaCl
2
as flux Ca
(OH)
2
as flux
Large crystals with little
lattice strain
used. CVD, plasma spray method and other (electro) chemical methods have been
mainly used to form HAp layers on substrates.
The hydrothermal method enables us to prepare well-crystallized, composition-
ally homogenous, uniform, and easily sinterable powders due to the great effects of
the high-temperature
high-pressure aqueous solutions.
The growth of HAp crystals has been carried out by several other methods
like the gel-growth method
[205]
. There are several reports on the melt growth
and flux growth of apatites of different compositions. Johnson (1961)
[206]
has
used the melt growth for the preparation of single crystals of apatite from the
stoichiometric melt. These crystals grown from the melt at high temperatures,
however, are usually severely strained due to the large temperature gradients
existing during the growth. Prener (1967)
[207]
used the flux growth method,
which is an excellent method since the fluxes like CaF
2
,CaCl
2
, and Ca(OH)
2
mixed with the starting apatite powders make lower liquidus temperature, far
below those needed for the melt growth method, resulting in the production of
less strained apatite crystals. Oishi and coworkers
[208
210]
have obtained
chlorapatite and fluorapatite (FAp) crystals by the flux method using NaCl, KF,
and Ca
2
ClPO
4
fluxes. Masuda et al. (1990)
[211]
have used the sol
gel method
to obtain HAp from metal alkoxides. Brendel et al. (1992)
[212]
have used the
polymerized method to achieve HAp coatings. Likewise, many other methods are
utilized in the processing of apatites. Thus, for the growth of HAp, the volatility
of water at
the necessary high temperatures makes hydrothermal conditions
unavoidable.
