Biomedical Engineering Reference
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dimensions (diameter of the hollow cores) would be 1.8-4.2 times
a
or 2.4-5.8 times
c
(for the screw dislocations), and even 3.8-9.1
times
(for the edge dislocations). Thus, each
dislocation outlet on the crystal surface looks like a large dissolution
nucleus equal to detachment of 3-19 unit cells, (if this is taken to be
the product
a
or 5.2-12.5 times
c
) for the screw dislocations and even of 15-90 unit
cells for the edge ones.
ac
Figure 7.5
Surface of a crystal according to Kossel [151]. The various
types of imperfections are shown. Reprinted from Ref. [30]
with permission.
The process of etch pits formation and growth on apatites
was described in details elsewhere [37-43, 61, 62, 97-112]. In
the case of single crystals of pure HA, an acid attack was found to
start formation of one etch pit in the basal plane which was the
preferred dissolution spot or active site. Immediately afterwards
the acid formed a longitudinal hole parallel to the
-axis [101]. On
the contrary, dissolution of natural FA was found to happen with
formation of a great number of etch pits (about 5 × 10
c
5
−2
) on the
surface (Fig. 7.6) [104-106]. The latter just points out to the great
differences in amount of crystal defects and impurities between
the chemically pure [37-43, 61, 62, 97-103, 109-112] and natural
[104-106] apatites.
cm
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