Geoscience Reference
In-Depth Information
Vereecke and Lemaitre (1990)
[244]
reported the calculation of the solubility dia-
grams and the effects of some parameters such as T (
100
C), pH; partial CO
2
pressure, and ionic strength. According to this report, the solubility of HAp is low-
ered with temperature indicating the retrograde solubility of HAp below 100
C.
Onuma et al. (1998)
[245]
and Kanzaki et al. (1998)
[246]
have carried out the
direct growth rate measurement of HAp single crystals by Moire phase shift inter-
ferometry and AFM. They carried out direct investigations of the growth kinetics
for each crystal face of HAp. It was found that the a-face grew by step flow com-
bined with two-dimensional nucleation, and the rate-determining process of the
growth was found to be the incorporation of a growth unit at the step front, from
the measurement of step velocities. The growth rate of the a-face was estimated to
be of the order of 10
2
4
nm/s from the step advancing rate. This value is about 3
,
4
orders of magnitude smaller than those for soluble inorganic crystals
[245]
. In con-
trast, the growth kinetics of HAp (0001), c-face, remain unclear. These authors
have concluded that the volume diffusion process for growth is strong during the
initial stages. It is believed that a strong bond site exists in the [0001] direction and
there is no strong bond site in the
direction, which makes the rate-
determining process between c- and a-faces different.
Figure 5.64
shows the
carbonate-containing HAp single crystals grown on the polycrystalline HAp disk
and
Figure 5.65
shows carbonate-containing HAp single crystals.
Like zeolites, HAp is also an important technological material owing to its wide-
spread applications attracting the attention of scientists from interdisciplinary
branches of science—including physicists, chemists, ceramists, medical doctors,
and engineers. In fact, the growth of nanosize HAp crystals has progressed so
much that the nanocrystals of desired size, morphology, and surface chemistry
could be obtained at ease. Also, several International Symposia on Apatite have
½
1010
Figure 5.64 Carbonate-containing HAp single
crystals grown on the polycrystalline HAp disk.
Source: Photo courtesy of A. Ito.
