Biomedical Engineering Reference
In-Depth Information
Figure 2.6
Effect of pressure on X-ray diffraction patterns of HA and
hydrated TCP at 1 bar and high pressures. Reprinted from Ref.
[161] with permission.
2.3.2.4 Thermal
Furthermore, ACPs might be prepared using high energy processing
at elevated temperatures. This method is based on a rapid quenching
of melted calcium orthophosphates occurring, e.g., during plasma
spraying of HA [162-174]. A plasma jet, which possesses very
high temperatures (5000-20000°C), partly decomposes HA.
Furthermore, there is an opinion, that thermal spraying produces
the amorphous phase, not only due to the high cooling rate but also
to the removal of hydroxyl ions which make it more difficult for the
crystalline phase to form [167]. This generally leads to a mixture of
calcium orthophosphate phases with variable compositions, often
containing impurities, which is not convenient for preparation of
pure ACPs. Interestingly, but the amorphization degree of the plasma
sprayed HA coatings appeared to correlate with the presence of
vacancies of hydroxyl ions in the structure of HA: the more vacancies
were present in the apatite structure due to missing hydroxyl sites,
the more amount of ACP was present in the resultant coatings [166].
This might be due to the fact that particles resident in plasma for a
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