Biomedical Engineering Reference
In-Depth Information
experiments, where water interactions were minimized by allowing
for the escape of volatile components, crystallization of ACP was
found to begin at about 530°C [197]. Below this temperature, the
non-crystalline features of ACPs seemed thermally stable. The first
crystalline phase to appear was invariably β-TCP. However, between
600°C and 800°C, depending upon the preparation, α-TCP was found
to become generally the favored ignition product, even though β-TCP
is normally the stable phase up to ~ 1200°C. Neither washing + drying
procedures employed to isolate the amorphous material, nor the
choice of soluble orthophosphate salt used in its preparation, were
found to have any significant effect on the thermo-crystallization
properties of ACP [197]. However, in other studies, both α-TCP [147,
175, 176, 189] and carbonated CDHA [141] appeared to be the first
detectable crystalline phase at heating of various ACPs to 550-
600°C. Interestingly, but the presence of organic solvents (in that
case, polyethylene glycol) at the ACP preparation stage was found
to influence the products formed at elevated temperatures [150].
Namely, when the amount of polyethylene glycol was small, α-TCP
was formed at heating; when the amount of polyethylene glycol
was big, β-TCP was formed at heating; and biphasic (α-TCP + β-TCP)
formulations were formed when the amount of polyethylene glycol
was average (Fig. 2.5b). Similar effects of both aging time and the
solution pH were also detected [151].
ACPs with Ca/P ratios of 1.00 (“amorphous DCPA”) and 1.34
(“amorphous OCP”) were found to remain amorphous at heating
up to 600°C, while crystalline compounds (β-Ca
P
O
in the case
2
2
7
in the case of Ca/P = 1.34)
started to appear at 620°C [140]. In the same study, ACP with Ca/P
ratio of 1.51 (“amorphous TCP”) were found to remain amorphous at
heating up to ~ 550°C, while crystalline compounds (β-TCP) started
to appear at ~ 600°C. Interestingly, but heating of a crystalline DCPA
(monetite) leads to γ-Ca
of Ca/P = 1.00 and α-TCP + β-Ca
P
O
2
2
7
P
O
and then this phase is transformed
2
2
7
[238]. Thus, “amorphous DCPA” showed a
thermal behavior different from that of crystalline DCPA [140].
Furthermore, crystallization of ACPs is an exothermic process.
The heat produced was found to be ~21 kJ/mol, while the activation
energy was ~450 kJ/mol [168]. Other researchers reported the
activation energy values of 440 kJ/mol for crystallization of hydroxyl-
depleted areas of the amorphous phase to OA and 230 kJ/mol for
at ~ 750°C to β-Ca
P
O
2
2
7
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