Biomedical Engineering Reference
In-Depth Information
The central transition (CT) frequency of the spectrum of a quadrupolar nucleus of half
intereger spin, such as
27
Al (I = 5/2), depends on the orientation of each crystallite in the
static magnetic field to the second order in the perturbation theory. The quadrupolar
interaction between the nuclear electric quadrupole moment (eQ) and the electric field
gradient of the nucleus (eq), arising from any lack of symmetry in the local electron
distribution, is described by the quadrupolar coupling constant Cq (e
2
qQ/h) and the
symmetry parameter . It should be noted that disordered materials such as glasses have
a wide range of interatomic distances and, consequently, CT line broadening occurs due
to the distribution of
iso
and quadrupolar interactions [78]. After the material was heat-
treated at 1000°C, a single peak corresponding to Al
(VI)
predominated at 0.0 ppm,
indicating the structural change in the coordination state of aluminum. When Al atoms
are in tetrahedral coordination Al
(IV)
, their chemical shifts vary from 55 to 80 ppm.
Chemical shifts in the range of -10 to 10 ppm correspond to coordinated octahedral Al
(VI)
[79, 80, 47]. The spectra of the two samples prepared here presented three peaks at 10.4,
59.4, and 140.1 ppm, which are characteristic of Al
(VI)
, Al
(IV)
, and spinning side bands [81],
respectively. Although some authors have reported the presence of Al
(V)
atoms with
chemical shifts at 20 ppm [82], this peak was not detected. The dominant species in the
sample heat-treated at 50
o
C corresponded to Al
(IV)
. The chemical shifts between 50 and 60
ppm corresponding to Al
(IV)
depend on the Al/P molar ratio. Al
(IV)
has been found at 60
ppm in model glasses based on SiO
2
Al
2
O
3
CaOCaF
2
, and at about 50 ppm in glasses
containing phosphate where the molar Al:P ratio was 2:1 [81]. In this study an Al/P molar
ratio of approximately 10 was achieved, which is higher than that present in commercial
glasses. In our case, this chemical shift was very difficult to observe because of the lower
incidence of Al-O-P bonds. The
29
Si NMR results allow for analysis of the chemical
environment around silicon atoms in silicates, where Si is bound to four oxygen atoms.
The structure around Si can be represented by a tetrahedron whose corners link to other
tetrahedra. The Q
n
notation serves to describe the substitution pattern around a specific
silicon atom, with Q representing a silicon atom surrounded by four oxygen atoms and n
indicating the connectivity [83]. Figure 14 presents the NMR spectrum of the sample dried
at 50°C.
The material displayed a peak at -100 ppm and a shoulder at -110 ppm, and the values in
this range were attributed to Si atoms Q
4
and Q
4
or Q
3
, respectively. Figure 15 illustrates the
Q
3
and Q
4
structure.
As mentioned above, the chemical shift indicates the environment around the Si atoms in
the glass. The commercial calcium-fluoroaluminosilicate glass presents a broad peak
between -90 and -99 ppm [82]. On the basis of the results obtained here, our material
exhibits a vitreous lattice. The number of nearest neighbor aluminum atoms is given in
parentheses. Q
4
(3/4 Al) and Q
4
(1/2) [78] are the structures represented in Figure 16.
The chemical shift ranges overlap, so the resonances in Fuji II cement (commercial glass) at -
87, -92, -99, and -109 ppm may be due to Q
4
(3/4 Al), Q
4
(3 Al), Q
4
(1/2 Al), and Q
4
(0 Al),
respectively [78]. In our case, the chemical shifts at -110 and 100 ppm may be due to the Si
atoms Q
4
(1/2 Al) and Q
4
(0 Al), because of the molar ratio Al/Si < 1. Figure 17 illustrates
the
29
Si NMR of the sample heat-treated at 1000°C for 4 hours.
In the present case, only one peak at -88 ppm was detected, which can be attributed to the
presence of a Q
4
(3/4 Al) site in Si atoms due to the structural rearrangement of the
