Biomedical Engineering Reference
In-Depth Information
Kinetics of Hydrothermal Crystallization under a Saturated Steam Pressure
Through XRD, XPS analysis results, and microscopical observations presented in the pre-
vious section, significant microstructural homogeneity and regeneration are achieved via
autoclaving hydrothermal treatments. This is a result of the replenishment of missing OH
−
groups with surrounding H
2
O molecules [59,63,181]. It also demonstrates that hydrother-
mal treatment can actually promote significant crystallization to improve the phase purity
and better crystallinity of low-crystalline plasma-sprayed HACs only at relatively lower
temperatures.
Referring to the crystallization mechanism reported before, these studies have indi-
cated that the kinetics of crystallization and chemical reactions during heat treatments
are significantly related to heating temperatures, which is recognized as a main factor for
promoting HA crystallization [120,167-169]. In addition, a higher crystallization rate and
lower activation energy of crystallization are demonstrated for the postheat treatment in
an ambient atmosphere with a partial water vapor pressure than in a vacuum environ-
ment [120]. Chen et al. [59] also reported that the crystallization of plasma-sprayed HACs
can be achieved by performing postheat treatments in the nitrogen (N
2
) or oxygen (O
2
)
atmosphere. The increase of crystallinity and the reduction of impurity phases (TCP and
TP) for heat-treated HACs within these controlled atmospheres are better than that of in
vacuum. There was no obvious difference in crystallinity between the HA coatings heated
in dry N
2
and dry O
2
. However, HACs heat-treated in humid N
2
(or humid O
2
) atmosphere
showed a higher degree of crystallinity than those only heat-treated in a dry atmosphere
[59]. It can be seen that incorporation of water vapor can significantly promote the crystal-
lization of HA whereas the intrinsic properties of dry atmosphere, such as N
2
and O
2
, have
less effect on the crystallization. Therefore, the ambient heating atmosphere with water
molecules is thought to be another significant factor that should be considered to affect the
reaction rate and the activation energy for the crystallization of HA [120,181]. The influence
of saturated steam pressure is more notable while the reactions occurred in a hermetical
atmosphere of autoclaving hydrothermal treatment. However, some previous studies did
not show the steam pressure factor for the kinetics of hydrothermal HA crystallization
[170,172]. Thus, this is discussed in detail with the following experimental results and the
derivation to clarify the effect of saturated steam pressure on the hydrothermal crystal-
lization kinetics.
The dehydroxylation effect is a result of OH
−
groups easily breaking away from the
HA crystal structure under the high temperature and high enthalpy of the plasma spray-
ing process, while the formation of amorphous calcium phosphate with a significantly
decreased crystallinity of HA occurred in the as-sprayed coating layers. When the hydro-
thermal treatment is applied to promote the crystallization of plasma-sprayed HACs, the
water is vaporized and the ionized water vapor molecules contain H
+
and OH
−
groups
within the hermetical autoclave as displayed in reaction 6.17. The content of H
+
and OH
−
groups increases with increasing the temperature [136]. The resultant OH
−
groups within
the water vapor atmosphere would be expected to react with amorphous and other low-
crystalline calcium phosphate components, and convert them into crystalline HA phase
through the replenishment of OH
−
groups. The hydroxylation process proceeds with the
replenishment of OH
−
groups as represented in reaction 6.18 [59,71].
H
2
O → H
+
+ OH
−
(6.17)
Ca
10
(PO
4
)
6
(OH)
2-2
x
O
x
⋅V
x
+
x
H
2
O → Ca
10
(PO
4
)
6
(OH)
2
(6.18)
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