Biomedical Engineering Reference
In-Depth Information
the mechanism of coarsening of FAP involved dissolution of the small crystals
and re-crystallisation of FAP on the remaining coarse crystallites. The mecha-
nism, however, should have been controlled by the composition phase diagram
and not the thermodynamic drive to reduce the surface energy.
Multinuclear MAS-NMR analysis in the ionomer glass-ceramics [67] verifi ed
APS occurring in the glass during heat treatments at three temperatures; the fi rst
crystallisation temperature Tp1, the second crystallisation temperature Tp2 and
an intermediate temperature (Tp1 + Tp2)/2 half way between Tp1 and Tp2. As
mentioned above,
27
Al MAS-NMR spectra showed, that Al(V) and Al(VI) were
present in the glasses with increasing fl uoride content. By heat treating the glasses,
the above aluminium species disappeared and appeared again during crystallisa-
tion at Tp2. A typical example is shown in Figure 12.11 for a glass composition
based on 4.5SiO
2
- 3Al
2
O
3
- 1.5P
2
O
5
- (5 - x)CaO - xCaF
2
(
x
= 2) that was heat treated
at Tp1, Tp(1 + 2)/2 and Tp2. The small Al(VI) peak, that appeared in the original
glass decreased signifi cantly with heat treatments and increased again when the
glass was heat treated at Tp2.
A small but distinct peak appeared at about 39 ppm assigned by Dollase et al.
to crystalline AlPO
4
[68] .
19
F MAS-NMR analysis showed that the two peaks in
the original glass, that corresponded to F-Ca(n) species and Al-F-Ca(n) species
decreased signifi cantly with heat treatments (Figure 12.12).
Specifi cally, the F-Ca(n) species disappeared already during heat treatment
at Tp1, indicating a contribution of F-Ca(n) to the formation of F-Ca(3) species,
that correspond to crystalline FAP and appeared at ca -103 ppm. On the other
hand, the Al-F-Ca(n) species decreased with heat treatments but they were
present in the residual glass even at heat treatments to Tp2. Most of the Al-F-
Figure 12.11.
27
Al MAS NMR spectra of 4.5SiO
2
-3Al
2
O
3
-1.5P
2
O
5
-(5-x)CaO-xCaF
2
glass composi-
tion, where
x
= 2; LG26 and LG26 heated treated to Tp1, Tp(1 + 2)/2 and Tp2 [67].
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