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The potential of NMR techniques for studies of the effects of thermal history on glass structure
Journal of Non-Crystalline Solids 71 (1985) 411-428 North-Holland, Amsterdam
411
Section XIII. NMR, ESR and Raman spectra T H E P O T E N T I A L OF NMR T E C H N I Q U E S FOR STUDIES O F T H E EFFECTS OF T H E R M A L H I S T O R Y ON GLASS S T R U C T U R E * P.J. BRAY and E.J. H O L U P K A Department of Physics, Brown University, Providence, RI 02912, USA
Nuclear Magnetic Resonance is used to determine the boron coordination in CABAL and Si-CABAL glasses subjected to various thermal histories. In the Si-CABAL glass system the boron coordination was found to depend on the quench rate. Other N M R techniques are described for future studies of the thermal history of glass.
I. Introduction
Nuclear magnetic resonance (NMR) has been employed for some 25 years to study the structure of glasses [1-10]. Various characteristics of the N M R spectrum from nuclei such as HB, 29Si, 27A1 and many others are sensitive to the symmetry of the environment near the atom containing the resonating nucleus, the type and distance of neighboring atoms, and the character of the chemical bonds formed by the atom. More recently it has been found by I~B N M R that a difference of boron coordination can exist between rapidly cooled glasses and similar samples that have experienced a normal, slower rate of cooling [11]. This particular success of N M R in detecting thermal history effects in glasses has brought the realization that N M R has a large potential usefulness for a broad study of such effects. After a brief review of pertinent N M R theory, and particularly the nuclear interactions with the environment that should be sensitive to thermal history, initial N M R data are presented along with research suggested for the future.
2. Theory Nuclei possessing a spin of 1 or greater have a magnetic moment given by i~= ggol,
(1)
where I is the quantum mechanical spin vector, g is the nuclear g-factor, and go is the Bohr nuclear magneton. If the spin is placed in an external static magnetic field H o, the magnetic moment of the nucleus couples to the external * Research supported by the National Science Foundation through Grant No. DMR-8004488 and by the Materials Science Program of Brown University. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
412
magnetic field giving rise to a Zeeman interaction. The energy of this interaction is given by E = -I~.H o = -g#ol'Ho
= -g~toHom
(2)
where m is called the magnetic quantum number and can take on 2 I + 1 values ranging from - I to I in integer steps. The difference in energy between any two adjacent levels is given by (3)
A E = gt~oHo = ho o
where v0 is called the resonant frequency. The energy levels for a spin nucleus, such as 11B, are shown in fig. la. If the nucleus is now subjected to a radio-frequency electromagnetic field of frequency v0 the nucleus will absorb energy in units of hv o and make transitions to different energy levels. It is this absorption of energy that is measured in an N M R experiment. Fig. lb shows the power absorbed by the sample as a function of frequency. If the Zeeman interaction were the only interaction present, then the sample of spins would only absorb energy at the resonant frequency. This, however, is not the case. There are three important interactions that affect the N M R resonant lineshape or frequency. They are (i) the quadrupole interaction, (ii) the dipole interaction, and (iii) the chemical shift interaction. 2.1. The quadrupole interaction
Any nucleus possessing a spin of greater than ½ also possesses a quadrupole moment. This quadrupole moment couples to the local electric-field gradient (EFG) at the site of the nucleus. This leads to a quadrupole interaction of the form Ho=e2qO[312-I(I+l)+(1/2),l(I2+I2_)]/nhI(2I-1),
(4)
where Q is the quadrupole moment of the nucleus. I z is the z component of the quantum spin vector and
(5)
I+_ = I x + i!~.
m=-~ G
FFI=-~ FFI=.~ Z/o
o,
(a)
(b)
r43=÷BFig. 1. Energy levels and N M R spectrum for a spin ~ nucleus in a magnetic field (Zeeman interaction only).
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413
3 FF~=-~m = - ~ -I Fl-q_-+ 2~_
~--vo Z/c
FD--+~
(Q)
(b)
Fig. 2. Energy levels and NMR spectrum for a spin ~ nucleus in a magnetic field (Zeeman and quadrupolar interactions present).
In eq. (4) the spin operators are expressed in the laboratory frame where the static magnetic field defines the z-axis. The components of the electric-field gradient tensor Vu are expressed in the principal axis system, where
V~:> V~,v>v~, eq= v~,
7/= (V~x- K,~)/I/.-~,
(6)
and ~ is the asymmetry parameter. In most physical situations the quadrupole interaction is much smaller than the Zeeman interaction and can be treated as a perturbation. The energy shifts can than be calculated to any order in the perturbative expansion. Fig. 2a shows the first-order correction to the energy levels due to the quadrupole interaction for a spin -~ nucleus. The energy shifts are a function of O and @,where 0 and @are polar angles of the magnetic field in the principal axis system of the EFG. In a glassy sample all values of 0 and q, are possible. Hence, to obtain the resonant lineshape for a glass it is necessary to average over all angles. This results in a "powder pattern". The first-order powder pattern of a spin 3 nucleus is shown in fig. 3. If the quadrupole coupling constant Qcc = e2qQ/h is very large the struc-
m=J
-~
-~(I+~)-~(I-~)
0
2
~(I-~)
~(1+~)
. br_Vo
wo
2 2 2 2 Fig. 3. First-order powder pattern for a random ensemble of spin 23 nuclei. Here ve = 3Qcc/21(21 -1).
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
414
ture of the first-order powder pattern for the satellite transitions (i.e., rn = 2 to m = - ~ and r n = { to m = ½ ) will be spread out over a very large frequency range rendering the structure unobservable in an N M R experiment unless special efforts are made (i.e., high spectrometer sensitivity and a broad sweep range). However, the second-order effects of the quadrupole interaction on the central transition (m = 1 to m = - ½) are then observable. The second-order powder pattern for the central transition is shown in fig. 4.
2.2. The dipole interaction Consider now two identical spins possessing a magnetic moment. These two spins will interact via the dipole interaction given by H d = [1~1 .ix2 - 3(Ix]. r12)(p, 2 • r ] 2 ) / r ~ 2 ] r1-23
(7)
where i~] and ix2 are the magnetic moments of spins 1 and 2 (here considered identical), r]2 is the unit vector joining spin 1 and spin 2, and q2 is the distance between them. Once the dipolar interaction is averaged over all angles it leads to a broadening of the resonant lineshape. If we now consider the dipole interaction among neighboring spins along with the quadrupole interaction, the sharp features in the quadrupole powder pattern will be smeared out and broadened. Fig. 5 shows the theoretical spectrum for the case where e 2 q Q / h is small and yields a reasonably narrow width for the first-order quadrupole spectrum. The solid line represents the
•
42 i ~ ~3-'~) 2' (3+ "7)2
-160.~7) -16(I-~) frequency scale in units of
~ Z.,"-~
~
-
4~i (3--,/)2 | ~16(l+'r])
-16(I-~)
0& :,l 8(I-~ 2) j(3+'r])2
-v-~o
Fig. 4. Second-order powder pattern for the central transition of fig. 3.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
-2A I
-A I
0
AI
415
2A I
Fig. 5. R e s o n a n c e l i n e s h a p e (solid curve) for a n u c l e u s o f spin 1 = 2, ~ s u c h as N B, with a small q u a d r u p o l e i n t e r a c t i o n in a glass or p o l y c r y s t a l l i n e p o w d e r . A 1 = 4 Qcc, "q = O.
pure quadrupole spectrum while the dashed curve represents the quadrupole spectrum with dipolar broadening. Fig. 6 displays the case where e2qQ/h is large and hence shows the second-order quadrupole spectrum with (solid line) and without (dashed line) the dipolar interaction.
2.3. The chemical shift interaction The effect of the magnetic field on the electrons near the nucleus may cause changes in the value of the magnetic field experienced by the nucleus. This will cause the resonance frequency v to be shifted from the value %. This effect is sensitive to the chemical environment of the nucleus and is thus called the chemical shift interaction. The strength of the chemical shift is proportional to H 0 and, for a crystal, depends on the orientation of the magnetic field with respect to the crystal axes. For polycrystalline powders or glasses, this dependence can yield a powder pattern of some width and structure. Fig. 7 shows the II
iI II
-
16A
-~ 2
i
Av2
0
A2
Fig. 6. R e s o n a n c e l i n e s h a p e (solid curve) for a n u c l e u s w i t h a large q u a d r u p o l e i n t e r a c t i o n Qcc a n d 1 1 "q = 0 in a glass o r p o l y c r y s t a l l i n e p o w d e r . O n l y the rn = 2 to m = - 2 t r a n s i t i o n is p r e s e n t e d .
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
416
POWDER PATTERN
(I-(~.)Uo
( I- ~,, )Vo
""''""" .......... " ""'"'I"" ' "" ' "'"" "'"1 "'""" "'""'"'.
".
DERIVATIVE ".." Fig. 7. Powder pattern resulting from the presence of chemical shift interactions.
powder pattern and derivative lineshape found in the case of an axially symmetric chemical shift tensor. Under conditions of rapid molecular motion or magic angle spinning, the angular dependence is averaged out and the chemical shift is reduced to its isotropic value which is proportional to H 0. This isotropic value of the chemical shift is different for different chemical environments. Thus, by measuring the relative positions of resonance lines in an N M R spectrum, information about the different nuclear environments that exist in the sample can be obtained.
3. Effects of thermal history We now turn to the actual and possible uses of N M R in studying thermal history effects on glasses. Attention will be focused on the quadrupole and chemical shift interactions as sensitive indicators of thermal history. 3.1. Boron coordination
Within a borate glass, boron atoms may bond in three basic ways (i) 4-coordinated by four oxygens (ii) 3-coordinated with all three oxygens in the boron-oxygen bonds either bridging or non-bridging and (iii) 3-coordinated with one or two oxygens being non-bridging. Since the
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
417
electronic environments of these three configurations are different they will lead to different EFGs and hence different tlB NMR spectra. Fig. 8 shows the three boron units along with their corresponding 11B (derivative) NMR spectra. If a glass sample contains both 3-coordinated and 4-coordinated borons the
)
,c,Y
f
(a)
I 0
-o--B
o0
~0
~B
~ 0
0j
~
o~o
J
0
I Fig. 8. The tlB N M R spectrum of (a) a B4 unit, (b) a B 3 unit, (c) a B2 unit and (d) a sample containing all three units.
418
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
resultant lineshape will be that of fig. 9. This is a trace of the absorption spectrum. The ratio N 4 of 4-coordinated borons to the total boron content (N4) can be determined using N M R by calculating the area of the central peak and dividing by the total area of the resonance. Values of N4 have previously been determined for many borate glass systems as a function of composition [12-18]. In what follows, N4 will be compared in glasses subjected to different thermal histories. 3.2. Experimental
A number of glass samples were received from the Owens-Corning Fiberglas C o r p o r a t i o n over the past four years. Samples of composition SiO2-CaO-B203-A1203 and CaO-B203-A12 03 (CABAL glass) were received in two forms: bulk and drawn fiber. Glass samples of the same two compositions (although possessing different relative weight per cents due to our inability to obtain the high temperatures necessary to produce the glass samples received from Owens-Coming) were prepared in this laboratory in two forms: bulk and roller quenched. Subjecting the glass samples to fiber drawing and roller quenching will result in a higher fictive temperature for the processes with a higher quench rate [19]. Table 1 lists the measured N4 values for the CABAL glass system subject to the different thermal histories. It is noticed
INTEGRATE
A4 N4 --
A3 + A4
Fig. 9. Method for determining N4 in a l i B N M R spectrum.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
419
that the measured N 4 values for this glass system show no significant d e p e n dence on thermal history (the quoted N 4 values have an experimental uncertainty of at least +0.01). The glass system S i O 2 - C a O - B 2 0 3 - A 1 2 0 3 was also studied. T a b l e 1 lists the measured N 4 values for this glass system, again subjected to the different thermal histories. It is noticed that for this system there is a noticeable change in the b o r o n c o o r d i n a t i o n between the bulk a n d the fiberized samples, while the roller q u e n c h e d samples show no change in N4 c o m p a r e d to the bulk samples. The N M R spectra were acquired using c o n v e n t i o n a l C o n t i n u o u s Wave (CW) N M R . A Varian Associates model V2100-B regulated m a g n e t power supply together with a 12" diameter magnet supplied the magnetic field. A P r i n c e t o n - A p p l i e d Research model HR-8 lock-in amplifier, a n N M R Spectrometer model V-4210A variable frequency R F unit a n d a Nicolet model 1170 signal averager comprised the spectrometer. All experiments carried out o n these four glasses were done at 15.700 M H z a n d at room temperature. Fig. 10 shows a representative a b s o r p t i o n N M R spectrum of a bulk C A B A L glass sample of c o m p o s i t i o n 49.68 : 27.99 : 22.87 wt%. 3.3. Discussion
The C A B A L glass system was previously studied by Bishop a n d Bray [20]. It was established that the presence of a l u m i n u m alters the b o r o n c o o r d i n a t i o n in the glass a n d that the a l u m i n u m competes with the b o r o n for b o n d i n g with oxygens. It is possible that a glass sample with a high fictive temperature m a y have more a l u m i n u m atoms b o n d i n g in 6 - c o o r d i n a t i o n hence yielding a lower value for N 4.
Table 1 Resultant N4 values for the Si-Cabal and CABAL glass system subjected to different thermal histories wt% SiO3-CaO-B203-A1203 41.30 : 32.04 : 8.33 : 19.14 50.00 : 30.00 : 10.00 : 10.00 64.00 : 23.00 : 8.70 : 4.40 61.30 : 22.00 : 8.30 : 8.30 65.38 : 23.51 : 8.89 : 2.22 55.19:13.05 : 18.53 : 12.73 CaO-B203-A1203 25.00 : 25.00 : 50.00 45.00 : 10.00 : 45.00 49.68 : 27.99 : 21.87
N4 Bulk
Drawn fiber
Roller quench
0.26 0.18 0.12 0.16 0.09 0.16
0.076 0.095 0.08 0.10
0.28 0.16
0.09 0.23 0.17
0.15
0.08 0.24
420
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
Fig. 10. 11B N M R absorption spectrum for a CABAL glass of wt% 49.68 : 27.99 : 22.87.
Araujo has used a statistical mechanical model to calculate the fraction of 4-coordination in boron containing glasses [21,22]. It is predicted from this model that the fraction of borons in 4-coordination will decrease as the temperature increases. The roller quenched and fiberized glass samples have a higher fictive temperature than the bulk samples. In consideration of Araujo's theory, one may postulate that since the roller quenched and fiberized samples have a higher Tt, their N4 values would decrease. This is observed in the fiberized glass samples but not for the roller quenched glass samples. The roller quenching process may not produce a high enough quench rate for this effect to be observed. It should also be noted that the CABAL glass system contains no silicon whereas the Si-CABAL glass system does. It may be that the silicon content of the glass plays a role in altering the boron coordination at higher fictive temperatures.
4. Future Studies
There are many additional ways in which the quadrupolar and chemical shift interactions can be used to detect thermal history effects. Only a few are suggested here. 4.1. Additional use of quadrupolar effects When an atom containing a ilB nucleus is bonded to three bridging oxygens, as in vitreous B203, the EFG tensor has exactly (or nearly) trigonal symmetry and ,/--- 0. But when one or two of the oxygens are non-bridging, the trigonal symmetry is lost and ,/4= 0 (see eq. (6)). In that case the dipolar-broadened 11B N M R response (arising from the powder pattern of fig. 4) is that shown in fig. 8c. Computer simulation of the experimental spectrum (fig. 8d.) of a typical glass permits evaluation of N3s and N3A a s well a s N4, where N3s and N3A are the fractions of borons in symmetric and asymmetric BO 3 units.
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
421
Since these fractions should also be sensitive to temperature (i.e. the relative numbers of structural groupings containing B3 and B3- units will vary with T), careful measurements of N3s and N3A should be made as a function of the thermal history of the glass (e.g. the cooling rate). N 4, N3s, and N3A are all measured using the m = ½ to m = - ½ transition of 11B that is broadened by second-order quadrupole effects. But the "satellite" transitions (3 to ½ and - 3 to - ½) are affected in first-order and should be more sensitive to changes in Qcc and v (i.e. to changes in the components of the E F G tensor). The satellite transitions for 11B in polycrystalline boron phosphate are shown in fig. 11. Fig. 12 displays the derivative of the absorption curve for 7Li (another I = ~ nucleus) in a lithium borate glass. The spectrometer sensitivity and sweep range were adjusted to detect the satellite transitions. But the satellite transitions can also be detected for 3-coordinated borons (fig. 13) and for other nuclei such as 27A1 (fig. 14) which is a spin 1 =
O
7Li
in I
L i z O BzO3 Glass
- O I
O
25*/.
Li20
v
eq Q,,,187 Mcps
_N_ O
t,~l O
O
Fig. 11. l i B N M R derivative s p e c t r u m in polycrystalline b o r o n phosphate,
Fig. 12. 7Li N M R derivative s p e c t r u m in a l i t h i u m b o r a t e glass.
422
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
I 7./o
Fig. 13. The arrow indicates the first-order satellite 32 to ~1 transition for 3-coordinated boron.
nucleus having four satellite transitions ( + ~ to + 3 and + 3 to + ½). Future N M R studies of thermal history effects will include careful measurements of these satellite transitions which should be particularly sensitive to changes in the size and distribution of the EFG components.
4.2. Chemical shift effects Dipolar and other broadening effects in solids (e.g. distributions of Qcc and r/ values in glasses) yield lines whose width is usually measured in kilohertz. This breadth can prohibit resolution of N M R spectra from nuclei with relatively small chemical shifts (due to different bonding or environments) and
I O 0 Gauss i-----4
Fig. 14. 27A1 N M R absorption spectrum displaying the 4 satellite transitions.
423
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
wash out changes in chemical shifts due to thermal history effects. However, both the dipolar interaction and the chemical shift interactions with axial symmetry are proportional to (1-3cos 2 #), where # is the angle between the applied field and the internuclear vector (dipole interaction) or axis of symmetry (chemical shift). By orienting the sample at the angle # = cos-](~) which reduces (1-3cos 2 O) to zero, and rotating the sample at a rate greater than the static line width, resonance spectra can be greatly reduced in width [23]. This technique of magic angle sample spinning (MASS) permits determination of chemical shifts in solids in the manner employed by chemists for many years for liquids (where the rapid molecular motion averages out the interactions). Fig. 15 displays the 29Si r e s o n a n c e obtained by MASS for several silicate compounds [24]. The small width of the lines is apparent from the part-permillion of frequency (ppm) scale for the chemical shifts. Small sidebands may arise from anisotropy of the chemical shift or from the spinning technique. In the latter case, the separation of the sidebands is proportional to the spinning frequency. Fig. 16 displays the 295i spectrum for amelia albite glass, showing resolution of the spectra from silicons in SiO4 tetrahedra having different numbers (2 or 3) of adjacent tetrahedra with A1 rather than Si at the center [25]. There is even resolution of two different Q3 sites (i.e. 3 aluminate tetrahedra bonded to the silicate tetrahedra). Fig. 17 shows the separation of I
omelio
I
olbit
I
e
I
I
Q3
0.t[3 KYANT IE
B 10568
I lec
pulses delay
Q°Q3
SL ILM I AN ET i ~ ~ I
'
'
"
I
0
'
'
'
I
"
-200
PPM
'
'
I
-400
5'o
~) -~'o -,oo -,;o CHEMICAL SHIFT(ppm)
Fig. 15. 295i NMR absorption spectra using MASS of several silicate compounds. Fig. 16.29Si NMR absorption spectra of amelia albite glass using MASS.
424
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
29Si responses from tetrahedra bridged to different numbers (2 or 3) of neighboring silicate tetrahedra [24]. Early N M R studies of 23Na and 27A1 in glasses revealed asymmetric lines [20]. It was assumed that, for aluminum, this arises from an unresolved superposition of responses from 4- and 6-coordinated aluminum atoms. These two types of aluminum should be distinguished by different chemical shifts and quadrupole interactions. MASS at high magnetic fields reduces the quadrupolar and dipolar broadening and does, indeed, permit resolution of aluminums in different sites [23]. Fig. 18 displays the MASS 27A1 spectra for several CaO-AI203-P205 glasses. Three different sites are clearly resolved. The chemical shifts between 0 and - 2 2 ppm correspond to octahedrally coordinated aluminums (AIO4) and shifts between + 62 and + 80 ppm are assigned to tetrahedrally coordinated aluminums (A104). Muller et. al. also discovered a peak at - 2 1 ppm in a glass of composition CaO-A1203-P205. This peak is assigned to cationic, si-coordinated aluminum which is acting as a network modifier with phosphorus atoms in the second coordination sphere. 23Na MASS at high fields may also permit resolution of sodiums in different sites (e.g. at non-bridging oxygen or tetrahedral BO4 units. This sensitivity of MASS high-field N M R for 29Si, 27A1, 23Na and many other nuclei should provide a valuable tool for studying the effects of thermal history on chemical bonding, ion distribution, and other microstructural properties of glasses. 4.3. Other nuclei
Many nuclei, in addition to those discussed above, will prove useful in N M R studies of thermal history, l°B N M R is perhaps 30 times more sensitive
O2
Xonotlite
C°6(0H)2[Si6OI7]
I_Q3_I Q~
_Q3_ Q2_ Q2 n
JL . -60-80-100-I20 ppm (TMS) Fig. 17. 29SiNMR absorption spectrum of xonolite.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
425
to changes in quadrupolar effects than liB [10]. The powder pattern for this spin I = 3 nucleus in B203 is shown in fig. 19 (top) along with the observed derivative of this absorption pattern (bottom). When N a 2 0 is added to form a sodium borate glass, the BO3 units are converted to BO4 tetrahedra and feature A shown in fig. 20 emerges (the molar per cent of N a 2 0 is noted for each composition). This feature is extremely sensitive to the glass structure. In fig. 21, just this feature is shown on an expanded scale along with a computersimulated curve for each case. The simulation involves variation of only one parameter that adjusts the relative weighting of the spectra from crystalline compounds in the Na20-B203 glass system. It is clear that the observed spectra are reproduced to a high degree of accuracy using this concept (due to Krogh-Moe), that the borate glasses consist of mixtures of the structural groupings (e.g. boroxol, diborae, metaborate, etc.) from the compounds of the system [26]. It is probable that the relative amounts of each structural grouping in glasses will depend on the fictive temperature and other thermal history. The features of figs. 19 and 20 should be very sensitive indicators of thermal history effects. Oxygen bonding and environment can also be studied by NMR. Fig. 22 displays the quadrupolar-broadened central (½ to - ½) transition for this spin
I
- I00
I
!
0
I00
oom Fig. ]8.27AI N M R absorption spectra of ¢aO-AI203-P205 glasses.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
426
J" = ~ nucleus in vitreous SiO2 [27]. Qcc, )1, and distributions in these parameters can be obtained. The 170 spectrum for vitreous B203 is displayed in fig. 23. Two oxygen sites are evident: one for oxygens in the boroxol ring; one for oxygens that connect the rings. The distributions in the quadrupolar parameters for the latter are large, reflecting the distribution in the B - O - B bond angle. 170 is only 0.037% abundant, so glasses must be fabricated from t70 enriched materials (e.g. water) to obtain these resonances. However, it is clear that NMR yields 170 spectra that should be sensitive to variations in oxygen bonding and environment, and thus to thermal history. Many other nuclei can be employed for nMR studies of thermal history. These include isotopes of H, Li, Be, F, Mg, P, Sc, v, Cu, Ga, Zr, Cd, Sn, Te, TI and Pb in addition to the nuclei discussed in the text. The use of high field magnetic fields ( > 5 T) and MASS, and the development of fast quenching techniques (fiberization and splat cooling) should open a very broad and rewarding program of study to determine the effects of thermal history on the structure of glasses at the atomic level.
a
b
I 0
d
e
I f
-V-~o
Fig. 19. (a) l°B N M R powder pattern for 2 of the 6 transitions. The shoulders b and f and the divergence c correspond to the - 1 to 0 transition and the shoulders a and e and divergence d corresponds to the 1 to 0 transition (b) l°B derivative N M R spectrum for B203 glass.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
427
A B
s ~ :" ~ . < . 7 \'i
J
/f
2
)
° / o
oO
Uo Fig. 20. l°B N M R derivative spectra for eight N a 2 0 - B z O 3 glasses. Mol.% Na20 is indicated to the right of each trace.
Fig. 21. Feature A of fig. 20 on an expanded scale.
'70
NMR
in Amorphous
B2() s
5 0 kHz
50 kHz
~U
Q~c = 5.17 MHz
T/°= 0.2
~
o~= 0 2
= 0.7 MHz
Fig. 22. 170 N M R derivative spectra in amorphous SiO 2.
uo
Fig. 23. 170 NMR derivative spectra in amorphous B203.
428
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
A.H. Silver and P.J. Bray, J. Chem. Phys. 29 (1958) 984. P.J. Bray and J.G. O'Keefe, Phys. Chem. Glasses 4 (1963) 37. M. Leventhal, P.J. Bray, Phys. Chem. Glasses 6 (1965) 113. P.J. Bray, Proc. Seventh Int. Glass Congress, Brussels, Belgium (1965) p. 40.1. P.J. Bray, Interaction of Radiation with Solids, ed., A. Bishay (Plenum, New York, 1967) p. 25. J.F. Baugher and P.J. Bray, Phys. Chem. Glasses 10 (1969) 77. F.R. Landsberger and P.J. Bray, J. Chem. Phys. 53 (1970) 2757. H.M. Kriz and P.J. Bray, J. Mag. Res. 4 (1971) 69. J.R. Hendrickson and P.J. Bray, J. Chem. Phys. 61 (1974) 2754. G.E Jellison, L.W. Panek, F.J. Bray and G.B. Rouse, J. Chem. Phys. 66 (1977) 802. P.K. Gupta, M. Lui and P.J. Bray, Bull. Am. Ceram. Soc. 62 (3) (1983) 420. S.G. Bishop and P.J. Bray, J. Chem. Phys. 48 (1968) 1709. D. Kline, P.J. Bray, H.M. Kriz, J~ Chem. Phys. 48 (1968) 5277. H.M. Kriz, S.G. Bishop, P.J. Bray, J. Chem. Phys. 49 (1968) 557. J.F. Baugher and P.J. Bray, Phys. Chem. Glasses 10 (1969) 77. M.J. Park and P.J. Bray, Phys. Chem. Glasses 13 (1972) 50. K.S. Kim and P.J. Bray. Phys. Chem. Glasses 15 (1974) 47. K.S. Kim and P.J. Bray, J. Chem. Phys. 64 (1976) 4459. C.T. Moynihan, A.J. Easteal, M.A. DeBolt and J. Tucker, J. Am. Ceram. Soc. 59 (1976) 12. S.G. Bishop and P.J. Bray, Phys. Chem. Glasses 7 (1968) 73. R.J. Araujo, J. Non-Crystalline Solids, 42 (1980) 209. R.J. Araujo, J. Non-Crystalline Solids, 57 (1983) 237. D. Muller, G. Berger, I. Grunze, G. Ladwig, E. Hallas, U. Haubenreisser, Phys. Chem. Glasses 24 (1983) 37. K.A. Smith, R.J. Kirkpatrick, E. Oldfield and D.M. Henderson, Am. Miner. 68 (1983) 1206. B.H.W.S. and M. Schramm, Geochim. Cosmochim. Acta 45 (1983) 1223. J. Krogh-Moe, Phys. Chem. Glasses 3 (1961) 101. I.A. Harris and P.J. Bray, Phys. Chem. Glasses 21 (1980) 156.
411
Section XIII. NMR, ESR and Raman spectra T H E P O T E N T I A L OF NMR T E C H N I Q U E S FOR STUDIES O F T H E EFFECTS OF T H E R M A L H I S T O R Y ON GLASS S T R U C T U R E * P.J. BRAY and E.J. H O L U P K A Department of Physics, Brown University, Providence, RI 02912, USA
Nuclear Magnetic Resonance is used to determine the boron coordination in CABAL and Si-CABAL glasses subjected to various thermal histories. In the Si-CABAL glass system the boron coordination was found to depend on the quench rate. Other N M R techniques are described for future studies of the thermal history of glass.
I. Introduction
Nuclear magnetic resonance (NMR) has been employed for some 25 years to study the structure of glasses [1-10]. Various characteristics of the N M R spectrum from nuclei such as HB, 29Si, 27A1 and many others are sensitive to the symmetry of the environment near the atom containing the resonating nucleus, the type and distance of neighboring atoms, and the character of the chemical bonds formed by the atom. More recently it has been found by I~B N M R that a difference of boron coordination can exist between rapidly cooled glasses and similar samples that have experienced a normal, slower rate of cooling [11]. This particular success of N M R in detecting thermal history effects in glasses has brought the realization that N M R has a large potential usefulness for a broad study of such effects. After a brief review of pertinent N M R theory, and particularly the nuclear interactions with the environment that should be sensitive to thermal history, initial N M R data are presented along with research suggested for the future.
2. Theory Nuclei possessing a spin of 1 or greater have a magnetic moment given by i~= ggol,
(1)
where I is the quantum mechanical spin vector, g is the nuclear g-factor, and go is the Bohr nuclear magneton. If the spin is placed in an external static magnetic field H o, the magnetic moment of the nucleus couples to the external * Research supported by the National Science Foundation through Grant No. DMR-8004488 and by the Materials Science Program of Brown University. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
412
magnetic field giving rise to a Zeeman interaction. The energy of this interaction is given by E = -I~.H o = -g#ol'Ho
= -g~toHom
(2)
where m is called the magnetic quantum number and can take on 2 I + 1 values ranging from - I to I in integer steps. The difference in energy between any two adjacent levels is given by (3)
A E = gt~oHo = ho o
where v0 is called the resonant frequency. The energy levels for a spin nucleus, such as 11B, are shown in fig. la. If the nucleus is now subjected to a radio-frequency electromagnetic field of frequency v0 the nucleus will absorb energy in units of hv o and make transitions to different energy levels. It is this absorption of energy that is measured in an N M R experiment. Fig. lb shows the power absorbed by the sample as a function of frequency. If the Zeeman interaction were the only interaction present, then the sample of spins would only absorb energy at the resonant frequency. This, however, is not the case. There are three important interactions that affect the N M R resonant lineshape or frequency. They are (i) the quadrupole interaction, (ii) the dipole interaction, and (iii) the chemical shift interaction. 2.1. The quadrupole interaction
Any nucleus possessing a spin of greater than ½ also possesses a quadrupole moment. This quadrupole moment couples to the local electric-field gradient (EFG) at the site of the nucleus. This leads to a quadrupole interaction of the form Ho=e2qO[312-I(I+l)+(1/2),l(I2+I2_)]/nhI(2I-1),
(4)
where Q is the quadrupole moment of the nucleus. I z is the z component of the quantum spin vector and
(5)
I+_ = I x + i!~.
m=-~ G
FFI=-~ FFI=.~ Z/o
o,
(a)
(b)
r43=÷BFig. 1. Energy levels and N M R spectrum for a spin ~ nucleus in a magnetic field (Zeeman interaction only).
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
413
3 FF~=-~m = - ~ -I Fl-q_-+ 2~_
~--vo Z/c
FD--+~
(Q)
(b)
Fig. 2. Energy levels and NMR spectrum for a spin ~ nucleus in a magnetic field (Zeeman and quadrupolar interactions present).
In eq. (4) the spin operators are expressed in the laboratory frame where the static magnetic field defines the z-axis. The components of the electric-field gradient tensor Vu are expressed in the principal axis system, where
V~:> V~,v>v~, eq= v~,
7/= (V~x- K,~)/I/.-~,
(6)
and ~ is the asymmetry parameter. In most physical situations the quadrupole interaction is much smaller than the Zeeman interaction and can be treated as a perturbation. The energy shifts can than be calculated to any order in the perturbative expansion. Fig. 2a shows the first-order correction to the energy levels due to the quadrupole interaction for a spin -~ nucleus. The energy shifts are a function of O and @,where 0 and @are polar angles of the magnetic field in the principal axis system of the EFG. In a glassy sample all values of 0 and q, are possible. Hence, to obtain the resonant lineshape for a glass it is necessary to average over all angles. This results in a "powder pattern". The first-order powder pattern of a spin 3 nucleus is shown in fig. 3. If the quadrupole coupling constant Qcc = e2qQ/h is very large the struc-
m=J
-~
-~(I+~)-~(I-~)
0
2
~(I-~)
~(1+~)
. br_Vo
wo
2 2 2 2 Fig. 3. First-order powder pattern for a random ensemble of spin 23 nuclei. Here ve = 3Qcc/21(21 -1).
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
414
ture of the first-order powder pattern for the satellite transitions (i.e., rn = 2 to m = - ~ and r n = { to m = ½ ) will be spread out over a very large frequency range rendering the structure unobservable in an N M R experiment unless special efforts are made (i.e., high spectrometer sensitivity and a broad sweep range). However, the second-order effects of the quadrupole interaction on the central transition (m = 1 to m = - ½) are then observable. The second-order powder pattern for the central transition is shown in fig. 4.
2.2. The dipole interaction Consider now two identical spins possessing a magnetic moment. These two spins will interact via the dipole interaction given by H d = [1~1 .ix2 - 3(Ix]. r12)(p, 2 • r ] 2 ) / r ~ 2 ] r1-23
(7)
where i~] and ix2 are the magnetic moments of spins 1 and 2 (here considered identical), r]2 is the unit vector joining spin 1 and spin 2, and q2 is the distance between them. Once the dipolar interaction is averaged over all angles it leads to a broadening of the resonant lineshape. If we now consider the dipole interaction among neighboring spins along with the quadrupole interaction, the sharp features in the quadrupole powder pattern will be smeared out and broadened. Fig. 5 shows the theoretical spectrum for the case where e 2 q Q / h is small and yields a reasonably narrow width for the first-order quadrupole spectrum. The solid line represents the
•
42 i ~ ~3-'~) 2' (3+ "7)2
-160.~7) -16(I-~) frequency scale in units of
~ Z.,"-~
~
-
4~i (3--,/)2 | ~16(l+'r])
-16(I-~)
0& :,l 8(I-~ 2) j(3+'r])2
-v-~o
Fig. 4. Second-order powder pattern for the central transition of fig. 3.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
-2A I
-A I
0
AI
415
2A I
Fig. 5. R e s o n a n c e l i n e s h a p e (solid curve) for a n u c l e u s o f spin 1 = 2, ~ s u c h as N B, with a small q u a d r u p o l e i n t e r a c t i o n in a glass or p o l y c r y s t a l l i n e p o w d e r . A 1 = 4 Qcc, "q = O.
pure quadrupole spectrum while the dashed curve represents the quadrupole spectrum with dipolar broadening. Fig. 6 displays the case where e2qQ/h is large and hence shows the second-order quadrupole spectrum with (solid line) and without (dashed line) the dipolar interaction.
2.3. The chemical shift interaction The effect of the magnetic field on the electrons near the nucleus may cause changes in the value of the magnetic field experienced by the nucleus. This will cause the resonance frequency v to be shifted from the value %. This effect is sensitive to the chemical environment of the nucleus and is thus called the chemical shift interaction. The strength of the chemical shift is proportional to H 0 and, for a crystal, depends on the orientation of the magnetic field with respect to the crystal axes. For polycrystalline powders or glasses, this dependence can yield a powder pattern of some width and structure. Fig. 7 shows the II
iI II
-
16A
-~ 2
i
Av2
0
A2
Fig. 6. R e s o n a n c e l i n e s h a p e (solid curve) for a n u c l e u s w i t h a large q u a d r u p o l e i n t e r a c t i o n Qcc a n d 1 1 "q = 0 in a glass o r p o l y c r y s t a l l i n e p o w d e r . O n l y the rn = 2 to m = - 2 t r a n s i t i o n is p r e s e n t e d .
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
416
POWDER PATTERN
(I-(~.)Uo
( I- ~,, )Vo
""''""" .......... " ""'"'I"" ' "" ' "'"" "'"1 "'""" "'""'"'.
".
DERIVATIVE ".." Fig. 7. Powder pattern resulting from the presence of chemical shift interactions.
powder pattern and derivative lineshape found in the case of an axially symmetric chemical shift tensor. Under conditions of rapid molecular motion or magic angle spinning, the angular dependence is averaged out and the chemical shift is reduced to its isotropic value which is proportional to H 0. This isotropic value of the chemical shift is different for different chemical environments. Thus, by measuring the relative positions of resonance lines in an N M R spectrum, information about the different nuclear environments that exist in the sample can be obtained.
3. Effects of thermal history We now turn to the actual and possible uses of N M R in studying thermal history effects on glasses. Attention will be focused on the quadrupole and chemical shift interactions as sensitive indicators of thermal history. 3.1. Boron coordination
Within a borate glass, boron atoms may bond in three basic ways (i) 4-coordinated by four oxygens (ii) 3-coordinated with all three oxygens in the boron-oxygen bonds either bridging or non-bridging and (iii) 3-coordinated with one or two oxygens being non-bridging. Since the
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
417
electronic environments of these three configurations are different they will lead to different EFGs and hence different tlB NMR spectra. Fig. 8 shows the three boron units along with their corresponding 11B (derivative) NMR spectra. If a glass sample contains both 3-coordinated and 4-coordinated borons the
)
,c,Y
f
(a)
I 0
-o--B
o0
~0
~B
~ 0
0j
~
o~o
J
0
I Fig. 8. The tlB N M R spectrum of (a) a B4 unit, (b) a B 3 unit, (c) a B2 unit and (d) a sample containing all three units.
418
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
resultant lineshape will be that of fig. 9. This is a trace of the absorption spectrum. The ratio N 4 of 4-coordinated borons to the total boron content (N4) can be determined using N M R by calculating the area of the central peak and dividing by the total area of the resonance. Values of N4 have previously been determined for many borate glass systems as a function of composition [12-18]. In what follows, N4 will be compared in glasses subjected to different thermal histories. 3.2. Experimental
A number of glass samples were received from the Owens-Corning Fiberglas C o r p o r a t i o n over the past four years. Samples of composition SiO2-CaO-B203-A1203 and CaO-B203-A12 03 (CABAL glass) were received in two forms: bulk and drawn fiber. Glass samples of the same two compositions (although possessing different relative weight per cents due to our inability to obtain the high temperatures necessary to produce the glass samples received from Owens-Coming) were prepared in this laboratory in two forms: bulk and roller quenched. Subjecting the glass samples to fiber drawing and roller quenching will result in a higher fictive temperature for the processes with a higher quench rate [19]. Table 1 lists the measured N4 values for the CABAL glass system subject to the different thermal histories. It is noticed
INTEGRATE
A4 N4 --
A3 + A4
Fig. 9. Method for determining N4 in a l i B N M R spectrum.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
419
that the measured N 4 values for this glass system show no significant d e p e n dence on thermal history (the quoted N 4 values have an experimental uncertainty of at least +0.01). The glass system S i O 2 - C a O - B 2 0 3 - A 1 2 0 3 was also studied. T a b l e 1 lists the measured N 4 values for this glass system, again subjected to the different thermal histories. It is noticed that for this system there is a noticeable change in the b o r o n c o o r d i n a t i o n between the bulk a n d the fiberized samples, while the roller q u e n c h e d samples show no change in N4 c o m p a r e d to the bulk samples. The N M R spectra were acquired using c o n v e n t i o n a l C o n t i n u o u s Wave (CW) N M R . A Varian Associates model V2100-B regulated m a g n e t power supply together with a 12" diameter magnet supplied the magnetic field. A P r i n c e t o n - A p p l i e d Research model HR-8 lock-in amplifier, a n N M R Spectrometer model V-4210A variable frequency R F unit a n d a Nicolet model 1170 signal averager comprised the spectrometer. All experiments carried out o n these four glasses were done at 15.700 M H z a n d at room temperature. Fig. 10 shows a representative a b s o r p t i o n N M R spectrum of a bulk C A B A L glass sample of c o m p o s i t i o n 49.68 : 27.99 : 22.87 wt%. 3.3. Discussion
The C A B A L glass system was previously studied by Bishop a n d Bray [20]. It was established that the presence of a l u m i n u m alters the b o r o n c o o r d i n a t i o n in the glass a n d that the a l u m i n u m competes with the b o r o n for b o n d i n g with oxygens. It is possible that a glass sample with a high fictive temperature m a y have more a l u m i n u m atoms b o n d i n g in 6 - c o o r d i n a t i o n hence yielding a lower value for N 4.
Table 1 Resultant N4 values for the Si-Cabal and CABAL glass system subjected to different thermal histories wt% SiO3-CaO-B203-A1203 41.30 : 32.04 : 8.33 : 19.14 50.00 : 30.00 : 10.00 : 10.00 64.00 : 23.00 : 8.70 : 4.40 61.30 : 22.00 : 8.30 : 8.30 65.38 : 23.51 : 8.89 : 2.22 55.19:13.05 : 18.53 : 12.73 CaO-B203-A1203 25.00 : 25.00 : 50.00 45.00 : 10.00 : 45.00 49.68 : 27.99 : 21.87
N4 Bulk
Drawn fiber
Roller quench
0.26 0.18 0.12 0.16 0.09 0.16
0.076 0.095 0.08 0.10
0.28 0.16
0.09 0.23 0.17
0.15
0.08 0.24
420
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
Fig. 10. 11B N M R absorption spectrum for a CABAL glass of wt% 49.68 : 27.99 : 22.87.
Araujo has used a statistical mechanical model to calculate the fraction of 4-coordination in boron containing glasses [21,22]. It is predicted from this model that the fraction of borons in 4-coordination will decrease as the temperature increases. The roller quenched and fiberized glass samples have a higher fictive temperature than the bulk samples. In consideration of Araujo's theory, one may postulate that since the roller quenched and fiberized samples have a higher Tt, their N4 values would decrease. This is observed in the fiberized glass samples but not for the roller quenched glass samples. The roller quenching process may not produce a high enough quench rate for this effect to be observed. It should also be noted that the CABAL glass system contains no silicon whereas the Si-CABAL glass system does. It may be that the silicon content of the glass plays a role in altering the boron coordination at higher fictive temperatures.
4. Future Studies
There are many additional ways in which the quadrupolar and chemical shift interactions can be used to detect thermal history effects. Only a few are suggested here. 4.1. Additional use of quadrupolar effects When an atom containing a ilB nucleus is bonded to three bridging oxygens, as in vitreous B203, the EFG tensor has exactly (or nearly) trigonal symmetry and ,/--- 0. But when one or two of the oxygens are non-bridging, the trigonal symmetry is lost and ,/4= 0 (see eq. (6)). In that case the dipolar-broadened 11B N M R response (arising from the powder pattern of fig. 4) is that shown in fig. 8c. Computer simulation of the experimental spectrum (fig. 8d.) of a typical glass permits evaluation of N3s and N3A a s well a s N4, where N3s and N3A are the fractions of borons in symmetric and asymmetric BO 3 units.
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
421
Since these fractions should also be sensitive to temperature (i.e. the relative numbers of structural groupings containing B3 and B3- units will vary with T), careful measurements of N3s and N3A should be made as a function of the thermal history of the glass (e.g. the cooling rate). N 4, N3s, and N3A are all measured using the m = ½ to m = - ½ transition of 11B that is broadened by second-order quadrupole effects. But the "satellite" transitions (3 to ½ and - 3 to - ½) are affected in first-order and should be more sensitive to changes in Qcc and v (i.e. to changes in the components of the E F G tensor). The satellite transitions for 11B in polycrystalline boron phosphate are shown in fig. 11. Fig. 12 displays the derivative of the absorption curve for 7Li (another I = ~ nucleus) in a lithium borate glass. The spectrometer sensitivity and sweep range were adjusted to detect the satellite transitions. But the satellite transitions can also be detected for 3-coordinated borons (fig. 13) and for other nuclei such as 27A1 (fig. 14) which is a spin 1 =
O
7Li
in I
L i z O BzO3 Glass
- O I
O
25*/.
Li20
v
eq Q,,,187 Mcps
_N_ O
t,~l O
O
Fig. 11. l i B N M R derivative s p e c t r u m in polycrystalline b o r o n phosphate,
Fig. 12. 7Li N M R derivative s p e c t r u m in a l i t h i u m b o r a t e glass.
422
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
I 7./o
Fig. 13. The arrow indicates the first-order satellite 32 to ~1 transition for 3-coordinated boron.
nucleus having four satellite transitions ( + ~ to + 3 and + 3 to + ½). Future N M R studies of thermal history effects will include careful measurements of these satellite transitions which should be particularly sensitive to changes in the size and distribution of the EFG components.
4.2. Chemical shift effects Dipolar and other broadening effects in solids (e.g. distributions of Qcc and r/ values in glasses) yield lines whose width is usually measured in kilohertz. This breadth can prohibit resolution of N M R spectra from nuclei with relatively small chemical shifts (due to different bonding or environments) and
I O 0 Gauss i-----4
Fig. 14. 27A1 N M R absorption spectrum displaying the 4 satellite transitions.
423
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
wash out changes in chemical shifts due to thermal history effects. However, both the dipolar interaction and the chemical shift interactions with axial symmetry are proportional to (1-3cos 2 #), where # is the angle between the applied field and the internuclear vector (dipole interaction) or axis of symmetry (chemical shift). By orienting the sample at the angle # = cos-](~) which reduces (1-3cos 2 O) to zero, and rotating the sample at a rate greater than the static line width, resonance spectra can be greatly reduced in width [23]. This technique of magic angle sample spinning (MASS) permits determination of chemical shifts in solids in the manner employed by chemists for many years for liquids (where the rapid molecular motion averages out the interactions). Fig. 15 displays the 29Si r e s o n a n c e obtained by MASS for several silicate compounds [24]. The small width of the lines is apparent from the part-permillion of frequency (ppm) scale for the chemical shifts. Small sidebands may arise from anisotropy of the chemical shift or from the spinning technique. In the latter case, the separation of the sidebands is proportional to the spinning frequency. Fig. 16 displays the 295i spectrum for amelia albite glass, showing resolution of the spectra from silicons in SiO4 tetrahedra having different numbers (2 or 3) of adjacent tetrahedra with A1 rather than Si at the center [25]. There is even resolution of two different Q3 sites (i.e. 3 aluminate tetrahedra bonded to the silicate tetrahedra). Fig. 17 shows the separation of I
omelio
I
olbit
I
e
I
I
Q3
0.t[3 KYANT IE
B 10568
I lec
pulses delay
Q°Q3
SL ILM I AN ET i ~ ~ I
'
'
"
I
0
'
'
'
I
"
-200
PPM
'
'
I
-400
5'o
~) -~'o -,oo -,;o CHEMICAL SHIFT(ppm)
Fig. 15. 295i NMR absorption spectra using MASS of several silicate compounds. Fig. 16.29Si NMR absorption spectra of amelia albite glass using MASS.
424
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
29Si responses from tetrahedra bridged to different numbers (2 or 3) of neighboring silicate tetrahedra [24]. Early N M R studies of 23Na and 27A1 in glasses revealed asymmetric lines [20]. It was assumed that, for aluminum, this arises from an unresolved superposition of responses from 4- and 6-coordinated aluminum atoms. These two types of aluminum should be distinguished by different chemical shifts and quadrupole interactions. MASS at high magnetic fields reduces the quadrupolar and dipolar broadening and does, indeed, permit resolution of aluminums in different sites [23]. Fig. 18 displays the MASS 27A1 spectra for several CaO-AI203-P205 glasses. Three different sites are clearly resolved. The chemical shifts between 0 and - 2 2 ppm correspond to octahedrally coordinated aluminums (AIO4) and shifts between + 62 and + 80 ppm are assigned to tetrahedrally coordinated aluminums (A104). Muller et. al. also discovered a peak at - 2 1 ppm in a glass of composition CaO-A1203-P205. This peak is assigned to cationic, si-coordinated aluminum which is acting as a network modifier with phosphorus atoms in the second coordination sphere. 23Na MASS at high fields may also permit resolution of sodiums in different sites (e.g. at non-bridging oxygen or tetrahedral BO4 units. This sensitivity of MASS high-field N M R for 29Si, 27A1, 23Na and many other nuclei should provide a valuable tool for studying the effects of thermal history on chemical bonding, ion distribution, and other microstructural properties of glasses. 4.3. Other nuclei
Many nuclei, in addition to those discussed above, will prove useful in N M R studies of thermal history, l°B N M R is perhaps 30 times more sensitive
O2
Xonotlite
C°6(0H)2[Si6OI7]
I_Q3_I Q~
_Q3_ Q2_ Q2 n
JL . -60-80-100-I20 ppm (TMS) Fig. 17. 29SiNMR absorption spectrum of xonolite.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
425
to changes in quadrupolar effects than liB [10]. The powder pattern for this spin I = 3 nucleus in B203 is shown in fig. 19 (top) along with the observed derivative of this absorption pattern (bottom). When N a 2 0 is added to form a sodium borate glass, the BO3 units are converted to BO4 tetrahedra and feature A shown in fig. 20 emerges (the molar per cent of N a 2 0 is noted for each composition). This feature is extremely sensitive to the glass structure. In fig. 21, just this feature is shown on an expanded scale along with a computersimulated curve for each case. The simulation involves variation of only one parameter that adjusts the relative weighting of the spectra from crystalline compounds in the Na20-B203 glass system. It is clear that the observed spectra are reproduced to a high degree of accuracy using this concept (due to Krogh-Moe), that the borate glasses consist of mixtures of the structural groupings (e.g. boroxol, diborae, metaborate, etc.) from the compounds of the system [26]. It is probable that the relative amounts of each structural grouping in glasses will depend on the fictive temperature and other thermal history. The features of figs. 19 and 20 should be very sensitive indicators of thermal history effects. Oxygen bonding and environment can also be studied by NMR. Fig. 22 displays the quadrupolar-broadened central (½ to - ½) transition for this spin
I
- I00
I
!
0
I00
oom Fig. ]8.27AI N M R absorption spectra of ¢aO-AI203-P205 glasses.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
426
J" = ~ nucleus in vitreous SiO2 [27]. Qcc, )1, and distributions in these parameters can be obtained. The 170 spectrum for vitreous B203 is displayed in fig. 23. Two oxygen sites are evident: one for oxygens in the boroxol ring; one for oxygens that connect the rings. The distributions in the quadrupolar parameters for the latter are large, reflecting the distribution in the B - O - B bond angle. 170 is only 0.037% abundant, so glasses must be fabricated from t70 enriched materials (e.g. water) to obtain these resonances. However, it is clear that NMR yields 170 spectra that should be sensitive to variations in oxygen bonding and environment, and thus to thermal history. Many other nuclei can be employed for nMR studies of thermal history. These include isotopes of H, Li, Be, F, Mg, P, Sc, v, Cu, Ga, Zr, Cd, Sn, Te, TI and Pb in addition to the nuclei discussed in the text. The use of high field magnetic fields ( > 5 T) and MASS, and the development of fast quenching techniques (fiberization and splat cooling) should open a very broad and rewarding program of study to determine the effects of thermal history on the structure of glasses at the atomic level.
a
b
I 0
d
e
I f
-V-~o
Fig. 19. (a) l°B N M R powder pattern for 2 of the 6 transitions. The shoulders b and f and the divergence c correspond to the - 1 to 0 transition and the shoulders a and e and divergence d corresponds to the 1 to 0 transition (b) l°B derivative N M R spectrum for B203 glass.
P.J. Bray, E.J. Holupka / Effects of thermal history on glass structure
427
A B
s ~ :" ~ . < . 7 \'i
J
/f
2
)
° / o
oO
Uo Fig. 20. l°B N M R derivative spectra for eight N a 2 0 - B z O 3 glasses. Mol.% Na20 is indicated to the right of each trace.
Fig. 21. Feature A of fig. 20 on an expanded scale.
'70
NMR
in Amorphous
B2() s
5 0 kHz
50 kHz
~U
Q~c = 5.17 MHz
T/°= 0.2
~
o~= 0 2
= 0.7 MHz
Fig. 22. 170 N M R derivative spectra in amorphous SiO 2.
uo
Fig. 23. 170 NMR derivative spectra in amorphous B203.
428
P.J. Bray, E.J. Holupka /Effects of thermal history on glass structure
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