Biomedical Engineering Reference
In-Depth Information
strong chemical bonds are formed among the particles and loosely
compacted green bodies are hardened to denser materials [194-
197].
Figure 4.3
A schematic diagram representing the changes occurring with
particles under sintering.
In the case of calcium orthophosphates, several specific
processes occur during sintering. Firstly, moisture, carbonates and
all other volatile chemicals remaining from the synthesis stage, such
as ammonia, nitrates and any organic compounds, are removed
as gaseous products. Secondly, unless powders are sintered, the
removal of these gases facilitates production of denser ceramics
with subsequent shrinkage of the samples (Fig. 4.4). Thirdly, all
chemical changes are accompanied by a concurrent increase in
crystal size and a decrease in the specific surface area. Fourthly, a
chemical decomposition of all acidic orthophosphates and their
transformation into other phosphates (e.g
4
2−
Æ
7
4−
.
,
2HPO
P
O
+
2
H
) takes place. Besides, sintering causes toughening [247],
densification [248, 249], partial dehydroxylation (in the case of HA)
[249], as well as it increases the mechanical strength [250, 251]. The
latter events are due to presence of air and other gases filling gaps
among the particles of un-sintered powders. At sintering, the gases
move towards the outside of powders and green bodies shrink owing
to decrease of distances among the particles. For example, sintering
of a biologically formed apatites was investigated [252, 253] and the
obtained products were characterized [254, 255]. In all cases, the
numerical value of Ca/P ratio in sintered apatites of biological origin
was higher than that of the stoichiometric HA. One should mention
that in the vast majority cases, calcium orthophosphates with Ca/P
ratio < 1.5 (Table 1.1) are not sintered, since these compounds are
thermally unstable, while sintering of CDHA and ACP leads to their
transformation into various types of BCP (HA + β-TCP [121], HA +
α-TCP [136], α-TCP + β-TCP [139-141]).
O
≠
2
Search WWH ::
Custom Search