Geoscience Reference
In-Depth Information
Suchanek et al.
[335]
have obtained several batches of HAp crystals under
hydrothermal conditions. Starting chemicals like H
3
PO
4
, Ca(OH)
2
, and lactic acid
were taken in a Teflon beaker, inserted into an autoclave and hydrothermally trea-
ted at 200
C for 5 h under a pressure of 2 MPa. The molar ratios of lactic acid/Ca
and Ca/P were 2.0, 4.0, 6.0, and 1.43, 1.80, 3.00, respectively.
Figure 10.68
shows
all the steps involved in the experimental procedure concerning the preparation of
HAp whiskers
[334]
. Yoshimura has proposed the mechanism of formation of HAp
whiskers under hydrothermal conditions (
Figure 10.69
)
[335]
.
The morphology of HAp crystals was controlled by both the lactic acid/Ca and
Ca/P molar ratios in the starting solution. The SEM studies revealed that the crystals
were not aggregated and had a shape of hexagonal rods or whiskers, elongated along
the c-axis (
Figure 10.70a
b
, the diameter
increases with increasing Ca/P molar ratio in the starting solution and is generally
larger for high lactic acid/Ca molar ratios. The aspect ratio of HAp crystals is in the
range of 5
d
)
[335]
. As shown in
Figure 10.70a
20. It decreases with increasing Ca/P ratio and is lower in the case of
high lactic acid/Ca ratios
Figure 10.70c and d
. When the lactic acid/Ca and Ca/P
starting ratios are low, crystals have the shapes of whiskers, while in other cases
large elongated grains are formed.
Distributions of the length and aspect ratio of selected whiskers are shown in
Figure 10.71a and b
. In this case, the aspect ratio for the majority of the crystals is
in the range of 10
25. The length of the whiskers shows a big scattering, suggest-
ing that some of them might be broken during and/or after the hydrothermal
treatments.
The Ca/P molar ratios of HAp crystals are in the range of 1.59
1.62. Deviation
from the stoichiometric value of 1.67 decreases slightly with the increasing lactic
acid/Ca ratio. It is difficult to explain why whiskers are nonstoichiometric. Several
theories have been proposed to explain the phenomenon of nonstoichiometry in
HAp. They include adsorbed phosphate species on the surface, the presence of addi-
tional phases as amorphous calcium phosphate or octacalcium phosphate, and vari-
ous combinations of lattice defects. This is in agreement with the model involving
Ca and OH vacancies and the substitution of some phosphate groups by HPO
2
4
:
To check the presence of various species of phosphates, carbonates, hydroxides,
etc., the IR-spectra are very useful. Absence of any distinct bands in the range of
1400
1500 cm
2
1
indicates that the HAp whiskers do not contain large quantities
of carbonate ions
[336]
.
Figure 10.72
shows the IR-spectra of the whiskers with
improved stoichiometry (Ca/P molar ratios 1.64 and 1.67).
HAp whiskers have the following lattice parameters: a
9.4317
˚
0.0007
˚
5
6
0.0012
˚
. These values correspond to HAp prepared in aqueous
systems. Thus, after a critical evaluation of the experimental conditions, Yoshimura
and coworkers have proposed that in the system Ca(OH)
2
and c
5
6.8822
6
H
3
PO
4
citric acid,
μ
μ
HAp whisker (30
m length, 0.1
1
m width) could be synthesized preferentially
under the conditions of:
lower concentration of Ca
2
1
(0.2 M),
i.
lower Ca/P molar ratio (1.67 for [Ca
2
1
]
5
0.2 M),
ii.
