Biomedical Engineering Reference
In-Depth Information
A
9.54
9.50
9.46
9.42
9.38
9.34
9.30
B
4
12
20
24
wt% CO 3
Figure 2.3. Effect of two types of substitution on the a -axis dimensions of synthetic carbon-
ate apatites. Type A (CO 3 -for-OH) causes an expansion and Type B (CO 3 -for-PO 4 ), a contraction
of the a -axis dimension compared to carbonate-free apatites [7,21,57,59,64,81,126].
substitution allows only partial substitution (maximum of 3 moles CO 3 for 3
moles PO 4 ) [57,57,64,126]. In addition to the effects on lattice parameters, substi-
tution of CO 3 (Type B) in the apatite structure causes changes in morphology, in
crystallite size (Figure 2.5A) and in dissolution properties [57,64,73,81,92]. In
synthetic apatites prepared at 60 ° C to 95 ° C
morphological changes were ob-
served: from needle-like or acicular crystals to rod-shaped to equi-axed crystals,
depending on the amount of carbonate incorporated in the apatite (Figure 2.5B)
[64,81,92]. When prepared at 37 °C, increasing the amount of carbonate incorpo-
ration decreases crystallite size (refl ected in the broadening of the diffraction
peaks) and eventually promotes the formation of ACP (Figure 2.6).
Crystal size of carbonate apatite also depends on the preparation method
and temperature of preparation: nano-crystals of CHA, similar to bone apatite
[Figures 2.7A and 2.7B], are obtained by precipitation method at 25 °C or 37 °C;
Search WWH ::




Custom Search