Biomedical Engineering Reference
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ions adsorbed on ACP formed under varying conditions also revealed
that ACP is either a non-crystalline form of hydrated TCP or a solid
solution with the composition of Ca
[198]. Other
researchers reported a stoichiometry of the precipitated ACP more
akin to OCP [138, 140, 142, 199-204], DCPA [140], and DCPD [205];
however, with a few exceptions [140, 142] the subsequent terms “an
amorphous OCP” and “an amorphous DCPD” were not introduced.
Namely, although the Ca/P ratio of early formed ACP phases was
varied between 1.35 and 1.38, which was close to that of OCP (Table
1.1), the authors of Refs. [202-204] used a term ACP2 to explain
initial variations in solution pH during the transformation of ACP
into more crystalline phases of OCP and/or CDHA. Further, based
on the results of TEM analysis, ACP2 was identified as a separate
amorphous phase with a floccular morphology and no electron
diffraction pattern, if compared with a spherular morphology of the
initially precipitated amorphous phase ACP1 [202-204]. The same
terms ACP1 and ACP2 were used in another study [145], in which
the authors studied ageing of calcium orthophosphate precipitates
in methanol at room temperature, finally leading to formation of a
nano-sized β-TCP. Furthermore, at precipitation experiments from
aqueous solutions containing polyethylene glycol, two types of ACPs
were detected [150]: one with a broadened peak centered at ~31°
and another with a broadened peak centered at ~32° (Fig. 2.5a,
spectra 16:1 and 4:1, respectively). According to the authors, the
first one was similar to the basic structure of β-TCP, while the second
one was similar to the basic structure of HA [150]. Perhaps, one
can mention on amorphous β-TCP and amorphous HA, respectively.
A similar shift of the position of the amorphous maximum but
obtained at different aging time of ACP precipitates (Table 2.1) has
been detected in another study [132].
(PO
)
(HPO
)
3
4
l.87
4
0.2
Table 2.1
Position of the X-ray diffraction amorphous maximum at
different aging time of ACP precipitates [132].
Aging time 2 min 1 h 2 h 3 h 4 h 5 h 6 h
2θ, degree 29.5 30.1 30.4 30.3 30.5 30.8 29.0
Note: The 2θ values were derived from the profile analysis of the scattering curves.
Negligible changes were found for longer aged ACPs. The experimental error ± 0.5
degrees 2θ [132].
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