Biomedical Engineering Reference
In-Depth Information
rare single crystals of natural HA always exhibit the hexagonal space
group. The crystal structure of HA is well described elsewhere [28,
114-116], the detailed analysis of the electronic structure, bonding,
charge transfer, optical and elastic properties are also available
[346-350], while the readers interested in Posner's clusters
are referred to other papers [345, 351-353]. A shell model was
developed to study the lattice dynamics of HA [354], while a cluster
growth model was created to illustrate its growth [291]. Polarization
characteristics [355, 356] and pyroelectrical properties [357] of HA
bioceramics have been investigated. First-principles calculations for
the elastic properties of doped HA [358] and vacancy formation in
HA [359] were performed. Computer simulations of the structures
and properties of HA are well described in recent feature articles
[360, 361].
Several techniques might be utilized for HA preparation; they can
be divided into solid-state reactions and wet methods [362], which
include precipitation, hydrothermal synthesis and hydrolysis of
other calcium orthophosphates. Even under the ideal stoichiometric
conditions, the precipitates are generally non-stoichiometric,
suggesting intermediate formation of precursor phases, such as
ACP and CDHA. HA can be prepared in aqueous solutions by mixing
exactly stoichiometric quantities of Ca- and PO
-containing solutions
at pH > 9, followed by boiling for several days in CO
4
-free atmosphere
(the ageing or maturation stage), filtration, drying and, usually,
sintering at about 1000°C [363]. As the first precipitates are rich in
non-apatitic environments (see ACP and CDHA), the ageing stage
appears to be very important: the Ca/P molar ratio of 1.67 was found
to attain in as little as 5 hours after the completion of the reaction
at 90°C [364]. The surface of freshly precipitated HA is composed of
a structured hydrated layer containing easily exchangeable mobile
ionic species [365]. usually unsintered HA is poorly crystalline and
often non-stoichiometric, resembling the aforementioned CDHA.
However, well crystalline HA can be prepared from an aqueous
solution [366]. microcrystalline samples of HA can also be prepared
by solid-state reaction of other calcium phosphates (e.g., mCPm,
DCPA, DCPD, OCP) with CaO, Ca(OH)
2
at temperatures
above ~1200°C in an atmosphere of equal volumes of water and
nitrogen. HA can be prepared by hydrothermal synthesis [28, 277,
367, 368]. A water-free synthesis can be performed in ethanol from
Ca(OEt)
, or CaCO
2
3
(Et = ethyl) and H
PO
[369, 370]. In addition, HA might be
2
3
4
Search WWH ::




Custom Search