The calibration of the absorption coefficient, a, of an OH band requires the determination of the absolute OH ... samples by another method than IR spectroscopy. .... wise, it will be shown that the IR spectroscopy can be ..... 30% [7, 14 and 15].
52>7
Original Paper Determination of the OH content of glasses ) 1
Heike Ebendorff-Heidepriem and Doris Ehrt Otto-Scnott-Institut für G i a s c h e m i e , Friedrich-Schiller-Universität Jena, J e n a (Germany)
The most widely used method for determination of the OH content of glasses is the IR spectroscopy. The absorption bands in the range of 2500 to 4000 c m * are due to the fundamental stretching vibrations of OH groups having different degrees of association. The calibration of the absorption coefficient, a, of an OH band requires the determination of the absolute OH content of some samples by another method than IR spectroscopy. Comparing water outgassing method with H N M R spectroscopy, a large differ ence in the OH content was observed. Therefore, it is more appropriate to use solely the absorption coefficient as a relative measure of the true OH content. If certain requirements are met, the quantitative analysis of the absorption coefficient of different glass samples is justified. 1
1
B E S T I M M U N G D E S O H - G E H A L T E S V O N GLÄSERN
Die am meisten verwendete Methode zur Bestimmung des OH-Gehaltes von Gläsern ist die IR-Spektroskopie. Die Absorptionsban den im Bereich von 2500 bis 4000 c m " beruhen auf den fundamentalen Valenzschwingungen der OH-Gruppen, die in unterschiedli chem Maße assoziiert im Glas vorliegen. Die Kalibrierung des Absorptionskoeffizienten, a. einer OH-Bande erfordert die Bestim mung des absoluten OH-Gehaltes von einigen Proben mittels einer anderen Methode als der IR-Spektroskopie. Beim Vergleich der Wasserfreisetzungsmethode mit der Ή-NMR-Spektroskopie wurde ein großer Unterschied im OH-Gehalt festgestellt. Aus diesem Grunde ist es zweckmäßiger, nur den Absorptionskoeffizienten als ein relatives Maß für den wahren OH-Gehalt zu verwenden. Wenn bestimmte Voraussetzungen erfüllt sind, ist die quantitative Auswertung des Absorptionskoeffizienten von unterschiedlichen Glasproben gerechtfertigt. 1
1. Introduction In general, glasses have low c o n t e n t s of hydroxyl g r o u p s ( < 1 w t % ) . However, these few O H g r o u p s strongly affect certain glass p r o p e r t i e s s u c h a s viscosity, p h a s e s e p a r a tion a n d t h e r m a l e x p a n s i o n [1 a n d 2]. F u r t h e r m o r e , O H g r o u p s serve as q u e n c h e r s of fluorescence r a d i a t i o n in the infrared spectral r e g i o n . T h u s , they have a great ef fect o n t h e laser p r o p e r t i e s of r a r e e a r t h ions in glasses [3 a n d 4]. T h e q u a n t i t a t i v e d e t e r m i n a t i o n o f t h e O H c o n t e n t is required in o r d e r t o investigate c o r r e l a t i o n s b e t w e e n glass p r o p e r t i e s a n d O H c o n t e n t . T h e k n o w l e d g e of s u c h c o r r e l a t i o n s is t h e p r e c o n d i t i o n o f designing glasses with definite p r o p e r t i e s . T h e O H c o n t e n t c a n be d e t e r m i n e d by t h e r m a l o u t gassing of water a n d s p e c t r o s c o p i c m e t h o d s [2]. In the first case, glass s a m p l e s a r e h e a t e d in different a t m o s pheres (air, a r g o n , n i t r o g e n , v a c u u m ) [5 to 13] o r dry gases (oxygen, n i t r o g e n ) a r e b u b b l e d t h r o u g h glass m e l t s [2, 14 a n d 15]. T h e s e t r e a t m e n t s lead t o a n o u t g a s s i n g of water from t h e glass s a m p l e s a n d melts, respectively. T h i s effect is d e s c r i b e d by t h e following r e a c t i o n : - O H + HO
> - O -
+ H O . 2
(1)
Received September 27, 1994. ') Presented in German at: 68th Annual Meeting of the German Society of Glass Technology ( D G G ) on May 31, 1994 in Bad Salzdetfurth (Germany). Glastech. Ber. Glass Sei. Technol. 68 (1995) No. 5
Several m e t h o d s a r e k n o w n for the d e t e r m i n a t i o n of the evolved water: -
m e a s u r e m e n t of the weight loss of t h e glass samples [5, 7 t o 9 a n d 1 1 ] , " c o u l o m e t r i c a l d e t e r m i n a t i o n of t h e water in an elec trolytic cell [6 a n d 10], m a s s s p e c t r o s c o p y [6], m e a s u r e m e n t of t h e c h a n g e of v a p o u r pressure [14], gravimetrical d e t e r m i n a t i o n of t h e water a b s o r b e d by a d r y a g e n t [15].
S p e c t r o s c o p i c m e t h o d s are infrared ( I R ) spec troscopy a n d H nuclear magnetic resonance ( N M R ) s p e c t r o s c o p y [2]. 1
In c o n t r a s t to t h e m e t h o d s of water o u t g a s s i n g , the s p e c t r o s c o p i c m e t h o d s offer a n in situ d e t e c t i o n of t h e O H g r o u p s , b u t t h e y r e q u i r e c a l i b r a t i o n s t a n d a r d s with k n o w n O H c o n t e n t s . I n c o m p a r i s o n t o t h e N M R spec troscopy, t h e r e q u i r e m e n t s of t h e I R s p e c t r o s c o p y o n t h e technical e q u i p m e n t is low. F u r t h e r m o r e , t h e m e a s u r e m e n t s of I R s p e c t r a t a k e place quickly, accurately a n d n o n - d e s t r u c t i v e . A c c o r d i n g t o these a d v a n t a g e s , the I R s p e c t r o s c o p y is t h e m o s t widely u s e d m e t h o d for deter m i n a t i o n of t h e O H c o n t e n t a n d for investigation of t h e i n c o r p o r a t i o n of O H g r o u p s in a glass m a t r i x . U p to now, m a n y w o r k h a s b e e n d o n e for c a l i b r a t i n g the I R s p e c t r o s c o p y in o r d e r t o d e t e r m i n e t h e a b s o l u t e O H c o n t e n t [2 a n d 5 t o 15]. F u r t h e r m o r e , t h e c o m p o s i t i o n a l d e p e n d e n c e of t h e O H v i b r a t i o n b a n d s h a s been studied [16 to 18]. 139
Heike E b e n d o r f f - H e i d e p r i e m ; Doris Ehrt;
Table 1. Compositions (in mol% I of the glasses investigated glass
SiO
Si Na—Ca—Si Na-B-Si Na-Al-B Na-Al-P Ca-P Sr-P Ba-P
100 70 70
---
2
B O 2
Al O
3
2
—
-
10 70
SrO
BaO
_
—
—
_
—
-
-15
20 20 15 15
10
-50
50
70 50 50 50
-
CaO
—
-
-
Na O 2
3
15
2
2
Figure 1. Transmission spectra of different glass types in the range of 2000 to 4500 c m " ' .
2
2
-
-
-—
-
—
Composition of a further glass: F - P : 10Sr(PO ) · 1 0 M g F • 3 0 C a F · 15SrF 3
50
35AlF .
-
-
3
Figure 3. Transmission spectra of different glasses in the range of 2000 to 4500 c m .
metaphosphate
- 1
wise, it will be s h o w n t h a t the I R s p e c t r o s c o p y can be used for accurate d e t e r m i n a t i o n of the relative O H c o n tent.
2. Experimental T h e c o m p o s i t i o n s of t h e glasses investigated a r e given in table 1. T h e S i O glass w a s p r o d u c e d by C h e m i c a l V a p o u r D e p o s i t i o n ( C V D ) from S i C l . T h e o t h e r glasses were melted in covered crucibles in air in electric fur naces at 1300 t o 1 8 0 0 K , d e p e n d i n g o n glass c o m p o sition. F o r t h e p h o s p h a t e glasses, vitreous silica crucibles were used. F o r t h e o t h e r glasses, p l a t i n u m crucibles were used. D u r i n g t h e r e m e l t i n g of the N a O - A l O - P O glass a n d the N a O - A l O - B O glass, d r y oxygen w a s bubbled t h r o u g h t h e melts. T h e m o l t e n glasses were p o u r e d in g r a p h i t e m o u l d s a n d cooled t o r o o m t e m p e r a ture at a rate of 3 K / m i n . T h e spectroscopic m e a s u r e m e n t s were carried o u t using polished giass s a m p l e s of different thicknesses. 2
4
Figure 2. Transmission spectra of different glass types in the range of 2000 to 4500 c m - ' .
2
2
In this paper, first a brief survey of t h e O H v i b r a t i o n b a n d s of different glass types is given. Subsequently, p r o b l e m s of c a l i b r a t i o n of I R s p e c t r o s c o p y a r e rep resented. Recently. N M R s p e c t r o s c o p y w a s used for q u a n t i t a t i v e O H d e t e r m i n a t i o n [19 a n d 20]. C o m p a r i n g the results of these N M R m e a s u r e m e n t s with those d e t e r m i n e d by water o u t g a s s i n g m e t h o d s m a n y years ago. large differences in t h e a b s o l u t e O H c o n t e n t s a n d . there fore, in t h e extinction coefficients are o b s e r v e d . O t h e r 140
2
3
2
2
3
2
5
3
T h e I R spectra in t h e r a n g e of 2000 to 4500 cm were recorded using the s p e c t r o m e t e r F T I R from M i d a c Polytec, W a l d b r o n n ( G e r m a n y ) . T h e I R spectra in the range of 3600 to 8000 c m " were r e c o r d e d using the 1
Glastech. Ber. Glass Sei. Technol. 6 8 (1995) N o . 5
Determination of the O H content of glasses
s p e c t r o m e t e r U V - 3 1 0 1 P C from S h i m a d z u ( E u r o p a ) G m b H . D u i s b u r g ( G e r m a n y ) . T h e e r r o r of t r a n s m i s s i o n measurements was ^ 2 % .
3. Results Figures 1 to 3 s h o w the I R t r a n s m i s s i o n spectra in the r a n g e of f u n d a m e n t a l O H stretching vibrations at 2000 to 4500 c m " for t h e glasses investigated. F i g u r e 4 shows the a b s o r p t i o n b a n d s c a u s e d by O H g r o u p s in the r a n g e of 3600 to 8000 c m - ' . 1
T h e effect of d r y oxygen b u b b l i n g t h r o u g h the glass melts o n the O H c o n t e n t is r e p r e s e n t e d in figure 5 for a p h o s p h a t e glass a n d a b o r a t e glass. T h e effect of glass c o m p o s i t i o n o n t h e t r a n s m i s s i o n in t h e r a n g e of funda m e n t a l O H v i b r a t i o n s is s h o w n in figure 6 for glass s a m p l e s melted in air.
Figure 4. Transmission spectra of different glass types in the range of 3600 to 8000 c m . - 1
T h e O H c o n t e n t of a B a ( P 0 ) glass s a m p l e was d e t e r m i n e d by I R , H N M R a n d P N M R spectroscopy. T h e results are given in table 2. 3
4.
2
3 1
1
Discussion
4 . 1 , IR spectra of glasses in the range of OH vibration bands T h e p o s i t i o n a n d w i d t h of t h e a b s o r p t i o n b a n d s caused by f u n d a m e n t a l O H stretching v i b r a t i o n s d e p e n d o n t h e degree of association to n e i g h b o u r i n g oxygens t h r o u g h hydrogen b o n d i n g . T h e b a n d s shift t o higher wave lengths a n d b e c o m e b r o a d e r w i t h increasing s t r e n g t h of hydrogen b o n d i n g , i.e. with increasing degree of associ ation [16]. - 1
For S i O glass, a s h a r p b a n d at 3670 c m is o b served (figure 1). T h i s b a n d is assigned to free O H g r o u p s w i t h o u t h y d r o g e n b o n d i n g d u e t o the rigid net work [16 a n d 17]. C o m p a r e d with t h e S i O glass, t h e presence of modifier ions in t h e N a O - C a O - S i O glass leads to the f o r m a t i o n of two b r o a d e r b a n d s at 3450 a n d 2800 c m ' (figure 1). T h e first o n e is a t t r i b u t e d t o O H g r o u p s associated t o b r i d g i n g oxygens t h r o u g h w e a k hy d r o g e n b o n d i n g (weakly associated O H g r o u p s ) . T h e s e c o n d o n e is c a u s e d by O H g r o u p s associated to n o n b r i d g i n g oxygens t h r o u g h s t r o n g h y d r o g e n b o n d ing (strongly associated O H g r o u p s ) [16]. T h e N a O - B O - S i O glass has two s h a r p b a n d s at 3590 a n d 2720 c m ' , as well as a b r o a d a b s o r p t i o n between these two b a n d s (figure 1). D u e t o t h e two n e t w o r k for m e r oxides B O a n d S i O in this glass, t h e O H g r o u p s are a t t a c h e d to b o r o n or silicon a t o m s a n d a r e associ ated to b r i d g i n g o r n o n b r i d g i n g oxygens a t t a c h e d t o b o ron o r silicon a t o m s . T h u s , m o r e t h a n two b a n d s like in the silicate glass a r e o b s e r v e d . T h e s h a r p b a n d at 3590 c m is consistent w i t h t h e b a n d of free O H g r o u p s in B O glass [16 a n d 18]. 2
2
2
2
Figure 5. Transmission spectra o f phosphate and borate glass samples melted in air without and with dry oxygen bubbling, respectively. The values in brackets are the absorption coef ficients, a, (in c m ) at 2850 c m ( N a - A l - P ) and at 3400 c m ( N a - A l - B ) , respectively. - 1
- 1
- 1
-
2
2
3
2
-
2
3
2
- 1
2
3
T h e N a O - A l O - B O glass h a s a c o m p a r a t i v e l y s h a r p b a n d at 3400 c m (figure 2). T h e p o s i t i o n a n d w i d t h of this b a n d suggest t h e o c c u r r e n c e of weakly as2
2
3
2
3
- 1
Glastech. Ber. Glass Sei. Technol. 6 8 (1995) N o . 5
Figure 6. Transmission spectra of different glass types melted in air in the range of 2000 to 4500 c m . The spectra d e m o n strate a qualitative comparison of the O H content. - 1
141
Heike E b e n d o r f f - H e i d e p r i e m ; Doris Ehrt:
Table 2. O H content o f a B a ( P O ) ; glass sample determined by different m e t h o d s (required for calculations: extinction coefficient related t o ( н at 2 9 0 0 c m : 65 1 m o l * c m " [22], density o f the glass sample: 3.668 g e m [21] 3
- 1
1
1
- 3
0
spectroscopic method
measured parameter
IR
absorption coefficient at 2 9 0 0 c m "
1
H NMR
3 1
2
P NMR
measured value
5.5 c m *
1
specific proton number
4.3 · 1 0
2 0
fraction o f * · ' groups
11 %
reference
2
O H concentration (in mol/!)
weight fraction o f O H (in wt%)
molar fraction ) o f H O (in mol%)
[21]
0.08
0.04
0.3
[20]
2.62
1.21
9.6
[27]
2.72
1.26
9.9
2
1
g"
) M o l a r fraction o f H O according to the c o m p o s i t i o n x H 0 : (1 — x ) B a ( P 0 ) . 2
2
3
2
sociated O H g r o u p s . T h e infrared edge of the b o r a t e glass is c a u s e d by t h e first o v e r t o n e of t h e B - O stretch ing v i b r a t i o n at a b o u t 1400 c m . D u e to t h e high inten sity of this n e t w o r k v i b r a t i o n , a n a s s e r t i o n a b o u t t h e oc c u r r e n c e of strongly associated O H g r o u p s a b s o r b i n g in t h e s a m e r a n g e is n o t possible.
4 . 2 . Problems of calibration of IR spectroscopy
T h e spectra of p h o s p h a t e glasses are d o m i n a t e d by a very b r o a d b a n d at 2800 t o 3000 c m * (figures 2 a n d 3) which is a t t r i b u t e d t o strongly associated O H g r o u p s [17]. Besides a s h o u l d e r at 3400 c m is observed. T h e a s s i g n m e n t of this s h o u l d e r is described later in this section.
OLi = EsJd=Ig(I II )Id
- 1
F o r a q u a n t i t a t i v e analysis, t h e a b s o r p t i o n coefficient, a. of a f u n d a m e n t a l O H v i b r a t i o n b a n d is calculated from t h e t r a n s m i s s i o n s p e c t r a a s follows:
0
(2)
x
1
- 1
T h e fluoride p h o s p h a t e glass h a s a very weak a n d b r o a d b a n d at a b o u t 3200 c m * (figure 2). T h e large w i d t h of t h e b a n d indicates different types of strongly associated O H g r o u p s . However, t h e p o s i t i o n of this b a n d is shifted t o h i g h e r wave n u m b e r s c o m p a r e d with t h e o n e of t h e b r o a d b a n d of p u r e p h o s p h a t e glasses. Therefore, t h e O H g r o u p s in t h e fluoride p h o s p h a t e glass m a y b e associated t o fluoride ions which form w e a k e r hydrogen b o n d s t h a n oxygen ions, leading t o a n O H v i b r a t i o n of h i g h e r energy.
w h e r e E ist t h e e x t i n c t i o n , I is t h e t r a n s m i s s i o n in the region where a b s o r p t i o n d o e s n o t t a k e place. I, is the t r a n s m i s s i o n of t h e O H b a n d at t h e wavelength /., a n d d is t h e thickness of t h e glass sample. A c c o r d i n g t o the L a m b e r t - B e e r law, 0
1
A s already n o t e d for t h e b o r a t e glass, t h e I R edge of all glasses, as well as t h e b a n d s at 2400 a n d 2 1 0 0 c m * of t h e p h o s p h a t e a n d fluoride p h o s p h a t e glasses, re spectively, a r e d u e t o t h e first o v e r t o n e s of t h e n e t w o r k v i b r a t i o n s having t h e highest frequencies. 1
E
= SxC d,
x
(3)
t h e extinction is p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n of the a b s o r b i n g species, c, a n d t o t h e s a m p l e thickness. T h e c o n s t a n t of t h e p r o p o r t i o n a l i t y is the extinction coef ficient, ε, labelled m o l a r e x t i n c t i o n coefficient o r m o l a r a b s o r p t i o n coefficient, t o o . T h u s , t h e a b s o r p t i o n coef ficient, a , of a n O H b a n d is p r o p o r t i o n a l t o the O H content, c H :
0
α = ε
Сон ·
(4)
- 1
T h e s h a r p b a n d s at a r o u n d 7000 c m (figure 4) are a t t r i b u t e d t o t h e first o v e r t o n e s of t h e O H stretching v i b r a t i o n s of free o r weakly associated O H g r o u p s [17 a n d 18]. F o r t h e p h o s p h a t e glass, t h e o v e r t o n e at 6800 c m c o r r e s p o n d s with t h e s h o u l d e r at 3400 c m * of the f u n d a m e n t a l O H v i b r a t i o n . T h u s , this s h o u l d e r is a t t r i b u t e d to weakly associated O H g r o u p s o b s c u r e d by the s t r o n g a b s o r p t i o n at 2900 c m [17]. T h e b a n d s in the r a n g e of 4000 t o 5500 c m a r e assigned t o c o m b i n a t i o n s of O H v i b r a t i o n s w i t h n e t w o r k v i b r a t i o n s [17 a n d 18]. T h e diffuse a b s o r p t i o n b a c k g r o u n d of the p h o s p h a t e glass is d u e t o t h e first o v e r t o n e of t h e b r o a d b a n d of strongly associated O H g r o u p s [17]. - 1
1
- 1
- 1
142
A c c o r d i n g t o e q u a t i o n (4), t h e value a n d unit of m e a s u r e of t h e extinction coefficient d e p e n d o n the t y p e of c o n c e n t r a t i o n d a t a . In t h e p a s t , ε w a s m o s t l y related to t h e H O c o n c e n t r a t i o n d e t e r m i n e d by t h e r m a l o u t g a s s i n g m e t h o d . Since H O is chemically dissolved a s O H g r o u p s in the glasses, ε is related t o the O H c o n c e n t r a t i o n . A c c o r d i n g t o e q u a t i o n (1), t h e O H c o n t e n t is twice higher t h a n t h e H O c o n t e n t . T h u s . ε Η calculated from O H c o n c e n t r a t i o n a n d ε , calculated from H O c o n c e n t r a t i o n a r e given by: 2
2
2
0
Η
г о н
= 0.5 ε
Η :
ο •
0
2
(5)
Glastech. Ber. Glass Sei. Technol. 6 8 (1995) N o . 5
Determination of the O H c o n t e n t of glasses
3
) ε Η calculated from ε , given in the references: е н 0.5ε ο· ) ε Η calculated from the results given in the references: е н α/сон, where a is the absorption coefficient at the corresponding wave number. =
0
4
Η
0
0
Η2
=
0
0
m
In o r d e r t o calculate the absolute O H c o n t e n t , с н > fr° the m e a s u r e d a b s o r p t i o n coefficient, t h e value of е н h a s t o be k n o w n (equation (4)). Otherwise, in o r d e r to d e t e r m i n e ε , ''он be m e a s u r e d by a n o t h e r m e t h o d t h a n I R spectroscopy. 0
1
Table 4. Samples investigated by H N M R spectroscopy [19]
0
n
a
s
sample
proton number in the sample
t 0
ΟΗ
- 1
F o r the O H b a n d of S i O glass at 3670 c m , t h e extinction coefficient, s , is given in table 3 from vari ous a u t h o r s [5, 6 a n d 11]. T h e lowest a n d highest values differ from e a c h o t h e r by t h e factor 2, a l t h o u g h all m e t h ods for d e t e r m i n a t i o n of the a b s o l u t e O H c o n t e n t b a s e d o n t h e r m a l o u t g a s s i n g of water. By c o n t r a s t , if different m e t h o d s are used such as t h e r m a l o u t g a s s i n g a n d H N M R spectroscopy, the difference is m u c h larger. T h i s effect is s h o w n in table 3 for ε of p h o s p h a t e glasses [7, 21 a n d 22]. T h e г н value of B a ( P O J glass from A b e [22] bases o n the works of Scholze [1 a n d 14], i.e. o n t h e r m a l outgassing m e t h o d . T h e £ н value of N a P O glass was calculated from m e a s u r e m e n t s of D a y [7] by m e a n s of t h e r m a l outgassing m e t h o d . T h e a b s o r p t i o n coefficient at 2910 c m was only d e t e r m i n e d before t e m p e r a t u r e t r e a t m e n t of the samples. However, t h e r m a l o u t g a s s i n g d o e s n o t lead to a c o m p l e t e removal of all O H g r o u p s in a sample as s h o w n in table 3. Therefore, the m e a s u r e d H O a n d O H c o n t e n t s , respectively, of D a y m a y be lower t h a n t h e true O H c o n t e n t s in the samples. By c o n t r a s t , H N M R s p e c t r o s c o p y m e a s u r e s the O H c o n t e n t in situ. T h u s , the O H c o n t e n t of a cer tain s a m p l e d e t e r m i n e d by H N M R m i g h t be larger t h a n t h e calculated one from the e x t i n c t i o n coefficient d e t e r m i n e d by t h e r m a l o u t g a s s i n g m e t h o d . T h i s as s u m p t i o n was confirmed for a B a ( P O I glass sample. By m e a n s of H N M R , a hydrogen c o n t e n t of 4.3 · 1 0 p r o t o n s / g s a m p l e was m e a s u r e d [19 a n d 20]. T h i s leads to a n O H c o n t e n t of 2.62 mol/l a n d 1.21 w t % , respec tively. F u r t h e r m o r e , the O H c o n t e n t w a s calculated from the a b s o r p t i o n coefficient m e a s u r e d by I R s p e c t r o s c o p y [21] using t h e extinction coefficient from A b e for B a ( P 0 ) glass (table 3). T h e ε Η value from A b e is simi lar t o t h e one from D a y m e a s u r e d by t h e r m a l o u t g a s s i n g 2
O H
( N H J M o O crystal powder N H N O crystal powder
1.4· 1 0 1.1 • 10
22
KjH PO
4.5 • 10
21
5.2 · 1 0
20
4
7
4
4
3
2
4
crystal
Ba(PO ), glass 3
22
1
Ο Η
0
3
2
0
3
- 1
2
1
1
3
2
1
3
2
2 0
0
m e t h o d (table 3). T h e O H c o n t e n t c a l c u l a t e d from a a n d ε Η the c o r r e s p o n d i n g wave n u m b e r of 2900 c m is only 0.08 mol/l a n d 0.04 w t % . respectively. a t
- 1
0
C o m p a r i n g the two m e t h o d s , t h e r m a l o u t g a s s i n g m e t h o d d o e s n o t directly m e a s u r e the O H c o n t e n t in a glass sample, b u t only t h e water o r i g i n a t i n g from t h e O H g r o u p s is detected. Besides, t h e relative e r r o r of this m e t h o d is e s t i m a t e d t o be relatively high, a b o u t 10 to 3 0 % [7, 14 a n d 15]. By c o n t r a s t , H N M R s p e c t r o s c o p y directly detects all O H g r o u p s in a glass s a m p l e . T h e relative e r r o r is also high, a b o u t 3 0 % [19 a n d 20]. T h e m a i n d i s a d v a n t a g e of H N M R s p e c t r o s c o p y is the low d e t e c t i o n sensibility, leading t o p r o b l e m s by c a l i b r a t i o n . C r y s t a l s a m p l e s having high h y d r o g e n c o n t e n t ( 1 0 t o 1 0 p r o t o n s in the s a m p l e s [19]) were used for cali b r a t i o n (table 4). However, the glass s a m p l e s m e a s u r e d h a d lower h y d r o g e n c o n t e n t s ( S l O p r o t o n s in t h e s a m p l e s [19]) (table 4). T h u s , t h e c a l i b r a t i o n c u r v e h a d to be e x t r a p o l a t e d t o low h y d r o g e n c o n t e n t s . N e v e r theless, t h e h i g h O H c o n t e n t of t h e B a ( P 0 ) glass s a m p l e n o t e d in table 2 was c o n f i r m e d by P N M R spectroscopy. 1
1
2 1
2 2
2 0
3
2
3 1
If a P O e v a p o r a t i o n d u r i n g t h e m e l t i n g of m e t a p h o s p h a t e glasses d o e s n o t occur, P N M R spec t r o s c o p y c a n also be used for O H d e t e r m i n a t i o n . By m e a n s of this m e t h o d , P O g r o u p s w i t h different n u m bers of b r i d g i n g oxygens a r e detected. A P O g r o u p w i t h 2
5
3 1
4
4
Glastech. Ber. Glass Sei. Technol. 6 8 (1995) N o . 5
143
Heike E b e n d o r f f - H e i d e p r i e m ; Doris Ehrt: O
O
1
fraction of Q
---O—P—O—P—O---
+
groups of 11 % w a s m e a s u r e d [27]. T h i s
leads t o a n O H c o n t e n t of 2 . 7 2 m o l / l a n d 1.26 w t % , re
HO 2
spectively. C o m p a r i n g the t w o values m e a s u r e d by О
О
N M R and
I
H
P N M R , they a r e in t h e r a n g e o f m e a s u r i n g 1
O H c o n t e n t of t h e B a ( P 0 ) glass s a m p l e d e t e r m i n e d by 3
2
the three m e t h o d s described are s u m m a r i z e d in table 2. О
+
1
error of a b o u t 3 0 % for H N M R . T h e v a l u e s for t h e
О ---О—P—ОН
3 1
Н О — Р — О —
О
О
•
·
Figure 7. Formation of Q} groups having OH groups due to the reaction of H O with phosphate chains composed of Q groups. 2
2
T h e differences in the O H c o n t e n t s a n d , therefore, in the extinction coefficients d e t e r m i n e d b y different m e t h ods p o i n t t o t h e p r o b l e m s by c a l i b r a t i n g t h e I R spec troscopy. T h u s , it is m o r e a p p r o p r i a t e t o u s e solely t h e m e a s u r e d a b s o r p t i o n coefficient a s a r e l a t i v e m e a s u r e of t h e O H c o n t e n t a n d t o r e n o u n c e t h e c a l c u l a t i o n of t h e absolute O H c o n t e n t by m e a n s of n o n - s u b s t a n t i a t e d ex tinction coefficients. Moreover, t h e a b s o r p t i o n coef ficients c a n b e m e a s u r e d with high a c c u r a c y a n d high reproducibility of lower t h a n 5 % relative e r r o r .
η b r i d g i n g oxygens is labelled Q" g r o u p . T h e area u n d e r a p e a k b e l o n g i n g t o a certain Q" g r o u p is p r o p o r t i o n a l t o t h e a m o u n t of t h i s Q" g r o u p in t h e glass sample. M e t a p h o s p h a t e glasses w i t h o u t traces of H O or o t h e r 2
2
oxides m a y consist of infinitely l o n g chains of Q 2
h e d r a . Besides, a fraction of t h e Q
tetra
4.3. Determination of OH content by IR spectroscopy
g r o u p s m a y form
The a b s o r p t i o n coefficient a n d t h e O H c o n t e n t calcu
ring s t r u c t u r e s [25 a n d 26]. However, ring structures d o
lated from it reflect only t h e t r u e O H c o n t e n t , if t h e
n o t have a n effect o n t h e Q" d i s t r i b u t i o n . T h e a d d i t i o n
following r e q u i r e m e n t s a r e given:
1
of H O t o t h e m e t a p h o s p h a t e c o m p o s i t i o n p r o d u c e s Q 2
2
g r o u p s . Finitely l o n g c h a i n s of Q
t e t r a h e d r a with
1
Q
g r o u p s at t h e e n d of t h e c h a i n s form (figure 7):
2Q
2
1
+ H O -* 2Q + 2OH .
(6)
2
a) T h e p r o p o r t i o n a l i t y between t h e a b s o r p t i o n coef ficient a n d t h e true O H c o n t e n t h a s t o b e valid in t h e whole c o n c e n t r a t i o n r a n g e of t h e m e a s u r e d s a m p l e s . I n other words, t h e extinction coefficient h a s t o b e i n d e p e n d e n t of t h e O H c o n t e n t . b) T h e extinction coefficient of a n O H b a n d h a s t o b e
T h e O H g r o u p s a r e located at t h e e n d of t h e p h o s p h a t e c h a i n s (figure 7). T h u s , t h e n u m b e r of O H g r o u p s is 1
e q u a l t o t h e n u m b e r of Q} g r o u p s . T h e fraction of Q groups, f ,
3 1
is d e t e r m i n e d by m e a n s of
P N M R from
1
1
the ratio of t h e Q p e a k a r e a t o t h e t o t a l a r e a of Q a n d 2
Q
i n d e p e n d e n t of t h e glass c o m p o s i t i o n . c) T h e r a t i o of t h e extinction coefficients o f different O H b a n d s h a s t o be c o n s t a n t . These r e q u i r e m e n t s are m e t in t h e f o l l o w i n g
manner.
U n d e r n o r m a l melting c o n d i t i o n s in a i r i n covered cru
peaks.
cibles, t h e O H c o n t e n t is lower o r a b o u t 1 w t % . T h u s , it 1
1
2
/ i = [Q VdQ ]
+ [Q )) •
(?)
In t h e case of B a ( P 0 ) , t h e a d d i t i o n of H O leads t o the p s e u d o b i n a r y s y s t e m x H 0 : ( 1 - х ) B a ( P 0 ) . A c c o r d i n g t o e q u a t i o n (6), t h e c o n t e n t of Q groups, [Q ], is given by: 3
2
2
2
3
2
1
1
[ρ ]
= 2[H O] = 2 x .
(8)
2
2
T h e c o n t e n t of Q} a n d Q
g r o u p s equals t h e c o n t e n t of
p h o s p h o r u s a t o m s in t h e B a ( P 0 ) 3
1
2
[Q ] + [Q ]
= И
2
composition:
= 2( 1 - х ) .
(9)
By c o n s i d e r i n g e q u a t i o n s (8 a n d 9),f / i = xl(l-x) Thus.
3 1
is given by:
.
(10)
P N M R yields t h e H O a n d O H c o n t e n t , re 2
spectively. In t h e case of t h e B a ( P O I 3
144
1
2
glass sample, a
is a s s u m e d that t h e first r e q u i r e m e n t is given u p t o this O H content. F o r s o d a — l i m e - s i l i c a glasses, S c h o l z e [14] p r o v e d that glasses with similar c o m p o s i t i o n s h a v e t h e s a m e positions of t h e O H b a n d s . T h e r a t i o of t h e a b s o r p t i o n coefficients of these O H b a n d s t o e a c h o t h e r is c o n s t a n t in these glasses. T h u s , t h e second a n d t h i r d r e q u i r e m e n t s are given in glasses with similar c o m p o s i t i o n s . I n this case, t h e a b s o r p t i o n coefficient of o n e b a n d of t h e fun d a m e n t a l O H vibrations reflects t h e w h o l e c o n t e n t of O H g r o u p s giving several O H b a n d s d u e t o different degrees of association. T h e spectra of p h o s p h a t e glasses w i t h different c o m positions are d o m i n a t e d by the b r o a d b a n d o f strongly associated O H groups. T h e shape of this b a n d d o e s n o t change with glass c o m p o s i t i o n , b u t only t h e p o s i t i o n of the b a n d is shifted (figures 2 a n d 3). T h u s , t h e second and third requirements may b e valid i n p h o s p h a t e glasses even in the case of different c o m p o s i t i o n s , p r o vided that t h e a b s o r p t i o n coefficients at t h e m a x i m u m of t h e O H b a n d s are regarded. Glastecn. Ber. Glass Sei. T e c h n o l . 6 8 (1995) N o . 5
Determination of the O H c o n t e n t of glasses
If different glass t y p e s such as silicate a n d p h o s p h a t e glasses a r e s t u d i e d , a q u a l i t a t i v e c o m p a r i s o n of the a b s o r p t i o n coefficients is o n l y justified. A q u a n t i t a t i v e analysis of O H s p e c t r a is s h o w n in figure 5. T h e two samples of each c o m p o s i t i o n were melted w i t h o u t a n d with dry oxygen b u b b l i n g , respectively. T h e spectra indi cate a d e c r e a s e of the i n t e n s i t y of the O H b a n d at 3400 a n d 2850 c m * . respectively, by using oxygen bubbling. F r o m the a b s o r p t i o n coefficient, a d e h y d r a t i o n effect of 6 0 % for t h e p h o s p h a t e glass was d e t e r m i n e d . T h i s ex a m p l e indicates t h a t a q u a n t i t a t i v e c o m p a r i s o n of t h e a b s o r p t i o n coefficients of a n O H b a n d represents a use ful tool for s t u d y i n g t h e effect of m e l t i n g c o n d i t i o n s o n the O H c o n t e n t in glass s a m p l e s with similar c o m p o sitions. F u r t h e r m o r e , this e x a m p l e shows that by m e a n s of t h e r m a l o u t g a s s i n g of w a t e r a c o m p l e t e removal of all O H g r o u p s c a n n o t b e achieved. 1
A q u a l i t a t i v e c o m p a r i s o n of t h e O H c o n t e n t is s h o w n in figure 6 for different glass types melted in air. F r o m t h e low t r a n s m i s s i o n o f t h e p h o s p h a t e a n d b o r a t e glasses in t h e r a n g e of t h e O H b a n d s , it is c o n c l u d e d that they have a c o m p a r a t i v e l y high O H c o n t e n t . T h e silicate a n d borosilicate glasses have a clearly higher t r a n s m i s s i o n in this r a n g e despite a higher s a m p l e thick ness. T h u s , they possess a clearly lower O H c o n t e n t . F o r the fluoride p h o s p h a t e glass, only a very w e a k O H b a n d is o b s e r v e d . A s a result, it h a s the lowest O H c o n t e n t a m o n g all glasses investigated.
5. S u m m a r y 1
T h e a b s o r p t i o n b a n d s in t h e r a n g e of 2000 to 4500 c m " are d u e to the f u n d a m e n t a l stretching v i b r a t i o n s of t h e O H g r o u p s in the glasses. T h e first o v e r t o n e s of the fun d a m e n t a l v i b r a t i o n s of free o r weakly associated O H g r o u p s a r e o b s e r v e d at a r o u n d 7000 c m " . T h e c o m b i nation b a n d s of O H a n d n e t w o r k v i b r a t i o n s occur in t h e r a n g e of 4000 t o 5500 c m " . 1
1
T h e d e t e r m i n a t i o n o f t h e extinction coefficient, ε, of an O H b a n d requires t h e m e a s u r e m e n t of t h e a b s o l u t e O H c o n t e n t of s o m e s a m p l e s by a n o t h e r m e t h o d t h a n IR spectroscopy. H o w e v e r , m u c h different values for t h e O H c o n t e n t of a B a ( P O J glass s a m p l e were d e t e r m i n e d using water o u t g a s s i n g m e t h o d a n d H N M R spec troscopy. T h e results of H N M R a n d P N M R spec t r o s c o p y are similar. B e c a u s e of these difficulties by cali brating the I R s p e c t r o s c o p y , it is m o r e a p p r o p r i a t e to use solely t h e a b s o r p t i o n coefficient, a, as a relative m e a s u r e of t h e t r u e O H c o n t e n t a n d t o r e n o u n c e the calculation of t h e a b s o l u t e O H c o n t e n t by m e a n s of n o n s u b s t a n t i a t e d e x t i n c t i o n coefficients, ε. 3
2
1
1
3 1
If glass s a m p l e s w i t h similar c o m p o s i t i o n s a r e inves tigated, t h e a b s o r p t i o n coefficient, a , c a n be used for a q u a n t i t a t i v e d e t e r m i n a t i o n of t h e relative O H c o n t e n t . Otherwise, if different glass types a r e studied, a q u a l i t a tive c o m p a r i s o n of t h e a b s o r p t i o n coefficients is only justified. Glastecn. Ber. Glass Sei. T e c h n o l . 6 8 (1995) No. 5
6.
References
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3 +
3
7
3
2
3
3
2
2
3
3
7
7
7
3
3
145
Heike Ebendorff-Heidepriem; Doris Ehrt: [23] Hosono. H.; Kamae, T.; Abe. Y.: Electrical conduction in magnesium phosphate glasses containing heavy water. J. Am. Ceram. Soc. 72 (1989) no. 2, p. 294-297. [24] Abe. Y.; Clark, D. E.: Determination of combined water in glasses bv infrared spectroscopy J. Mater. Sei. Lett. 9 (1990) p. 2 4 4 - 245. [25] Wazer, J. R. van: Phosphorus and its compounds. New York (et al.): Wiley 1958.
Determination of t h e O H content of glasses [26] Martin, S. W.: Review of the structures of phosphate glasses. Europ. J. Solid State Inorg. Chem. 28 (1991) p. 163-205. [27] Haubenreißer. U.: Experimentelle und theoretische Unter suchungen zur magnetischen P-Kernresonanz an polykri stallinen Phosphaten und Beiträge zur Aufklärung der Nahstruktur von Phosphatgläsern und Phosphatglaskera miken. Univ. Jena, thesis habil. 1985. 31
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