Biomedical Engineering Reference
In-Depth Information
56
55
54
53
52
51
50
49
48
-2
Al (VI)
-2.5
-3
-3.5
Al (IV)
-4
-4.5
-5
0
1
2
3
Fluoride content (X)
Figure 12.8. Al(IV) and Al(VI) peak positions plotted as a function of fl uoride content in the
glass [35].
but there was no effect from the presence of fl uoride on the peak position of
Al(IV) species at low fl uoride contents.
At high fl uoride contents ( x
0.5), there was signifi cant infl uence of fl uoride
on the peak position and a consistent reduction in the peak values corresponding
to Al(VI). Similar reduction was observed in 2SiO 2 - Al 2 O 3 - (2
>
x)CaO - xCaF 2
glasses providing evidence to support the existence of Al-F bonds being present
in both Al(IV) and Al(VI) species. It was suggested by Stamboulis et al. [35] that
the aluminium species in a series of glasses based on the composition 4.5SiO 2 -
3Al 2 O 3 - 1.5P 2 O 5 - (5 - x)CaO - xCaF 2 can be represented by [AlO x F y ] n− where x = 3 - 6,
y = 6 - x and n = charge of the total complex with Al(IV), Al(V) and Al(VI) present.
These species had previously been postulated by Youngman et al. [43] by 19 F - 27 Al
cross-polarisation. However, since Al(V) and Al(VI) were present in small quan-
tities within the glass in study, the dominant species should be [AlO 3 F] . On the
other hand, the formation of Al(V) and Al(VI) at high fl uorine contents might
not be due only to the presence of Al-F complexes but also due to the fact that the
presence of fl uoride limited the ability of calcium to perform its charge balancing
functions within the glass compositions as a result of forming F-Ca(n) type species.
Therefore, there might be insuffi cient Ca 2+ and F - Ca + to charge balance alumin-
ium and maintain it in four-fold coordination and consequently aluminium will
take higher coordination states. However, there was no evidence that calcium was
coordinated entirely by fl uorine but in the contrary calcium might be still also
involved with NBOs and other oxygen species. In addition, it is worth noticing
that the glass preparation involved quenching, which may had resulted in struc-
tures present in the melt to “freeze.” Often, in the melt aluminium takes higher
than four coordination states and it is possible that Al(V) and Al(VI) were
“frozen” during glass quenching. Another explanation for the presence of higher
than four-fold coordinated aluminium species in the glasses might be the observa-
tion by Kirkpatrick and Brow [44] that in Na-alumino-silicate glasses with Al/P
ratio
1, the aluminium speciation was mostly IV and VI. The Al/P ratio might
play a role on the aluminium speciation, however, in the glass compositions
studied by Stamboulis and Hill the Al/P ratio was
<
>
1 .
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