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Shrinkage and swelling of alkoxide silica gels on heating part II
Journal of Non-Crystalline Solids 104 (1988) 107-111 North-Holland, A m s t e r d a m
107
SHRINKAGE AND S W E L L I N G O F ALKOXIDE SILICA GELS ON H E A T I N G Part II
E.M. RABINOVICH, K. NASSAU, A.E. M I L L E R and P.K. G A L L A G H E R AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 23 December 1987 Revised manuscript received 26 February 1988
The shrinkage and swelling of two series of alkoxide-derived gels were studied. It was confirmed that fluoride ions universally reduce or eliminate swelling by substitution of F for O H - on the gel surface. Gels with higher low-temperature shrinkage are less vulnerable to swelling than gels with lower low-temperature shrinkage.
1. Introduction
The problems of bubble formation during heating or reheating of gel-derived silica glass have been discussed in our previous publications [1,2]. Two different modes of bubble formation can be observed in these glasses: (a) bubbling or even foaming upon reheating to -- 1700 ° C and higher of glass prepared from particulate (colloidal) gels. (b) swelling of alkoxide-derived gels during sintering or slightly above the sintering temperature. Rabinovich et al. [1] showed that the first type of bubbling was caused by chlorine retained in the glass structure during the dehydration step of sintering. Incorporation of F - ions into the silicon oxygen network during firing effectively prevented retention of C1 in Si-C1 bonds and subsequent release of C12 gas during reheating; thus the presence of F - ions eliminated this kind of bubbling. Water and O H - groups have nothing to do with this phenomenon, since is was even worse in "dry", C12-treated specimens than in " w e t " specimens fired without C12 and sometimes containing > 200 ppm OH. Another kind of swelling is observed in alkoxide-derived monolithic silica during sintering. Here O H - is apparently the main source. In Part I [2] we described the effect of F - ions upon the shrinkage and swelling of a series of silica gels 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
prepared using a tetraethyl orthosilicate (TEOS): ethanol (EtOH): water molar ratio equal to 1 : 4 : 50 (7F series). This paper describes a similar study on sols having a significantly lower water content, using a 1 : 4 : 4 ratio (1F series). Spectroscopic studies of both series have also been pub-
.10 7
106
---0---
'IF SERIES
----o--
7F SERIES
0 404
"
o
402
tot| 0
[]
"~
L
I
I
I
I
I
J
J
2
4
6
8
'10
'12
'14
16
,~
I 32
AMOUNT OF F-ADDED ( g / l O O g SiO 2)
Fig. 1. Times of gelation of 1F and 7F sols as a function of the fluoride content in the sols (from [5]).
E.M. Rabinovich / Shrinkage and swelling of gels I I
108
Table 1 Properties of 1F and 7F gels Sample
1F-0 1F-1 1F-2 1F-4 1F-8 1F-16 1F-32 7F-0 7F-1 7F-2 7F-4 7F-8 7F-16 7F-32
Additive, wt%, to H 2 0
BET Surface
F Content, g / 1 0 0 g SiO 2
H 2 0 (wt%)
HC1
HF
area (m2/g)
Added
Analyzed a
in dry gel
1.07 0.40 0 0 0 0 0 1.07 1.02 0.96 0.85 0.64 0 0
0 0.83 1.67 3.33 6.67 13.34 26.67 0 0.07 0.14 0.28 0.56 1.12 2.25
549 408 912 757 219 154 95 802 641 292 110 73 70 50
0 1 2 4 8 16 32 0 1 2 4 8 16 32
0 0.9 1.7 3.1 5.0 6.7 n.d. 0 1.0 1.3 2.5 3.0 3.6 3.4
5.0 n.d. b) 4.6 4.0 0.2 0.095 0.09 6.8 6.6 2.5 0.22 < 0.13 -0 -0
After drying at 150 o C. b n.d. = not determined. a
lished [3,4], as well as a general discussion of a fluoride behavior in silica gels [5].
200
TEMPERATURE (°C) 4 0 0 6 0 0 8 0 0 4000 1200
20
2. Experimental procedure Sols of 11= compositions presented in table 1 were prepared as described in Part I [2]. Gelation of all silica sols is strongly accelerated in the presence of F - ions, as shown in fig. 1 reproduced from [5]. Therefore some sols remained fluid after a 30 min mixing, while those containing high levels of F gelled during this time. All mixtures, gelled or not gelled, were aged at 60 ° C for 24 h in closed containers, dried at 1 5 0 ° C for 2 - 3 days and stored in ambient conditions. The thermomechanical analysis (TMA), thermogravimetry analysis (TGA) and mass spectrometric evolved gas analysis (EGA) were performed as described in Part I [2]. The specimens were held in the thermal analyzer's furnace for 3 h at 2 0 0 ° C intervals from 200 ° to 1 2 0 0 ° C with an intermediate heating rate of 10 ° C min-~. 3. Results and discussion
I0
"--- '&2 IF-2 ~ -I0 ,,~ - 2 0 z
IF-4
O3 Z
IF-8 -t -20
-3O -40
~-~ I
0
The T M A results for dry gels of the 1F series are shown in fig. 2. While they are qualitatively
\
-20 E
I
I
I
1
I
200 400 600 800 tO00 12OO TIME (vain)
Fig. 2. Thermomechanical analysis of dried 1F gels.
109
E.M. Rabinovich / Shrinkage and swelling of gels II
~ tO(-k"x9 ~. o 0
8
~'.,,
-----o---
tF SERIES
+
7F SERIES
'~ ,% ~.
o F0
6[
E 2: co
""o,,
I 0 0.0
k
I 0 5
i
I t.0
L
I t.5
J
I 2.0
[] ~ I I I 2.5 30
Q'-i
I 3.5
~ I 4.0
I
I 4.5
I 5.0
F-(g/IOOg SiO2) Fig. 3. Dependence of shrinkage of IF and 7F gels at 200-600 °C on the fluoride content of the 150 *C dried gels. A M O U N T OF
similar to the results for the 7F series [2], there are important quantitative differences. O n the one hand, the low temperature shrinkage of the 1F series is significantly larger than that of the 7F series, as shown in fig. 3, which is derived from fig. 2 and from similar data for the 7F series [2]. On the other hand, a significantly lower fluoride content is sufficient to suppress swelling completely in the 1F series; while in the 7F series swelling was still present when 2.5 g F / 1 0 0 g SiO 2 are retained in the dry gel (7F-4), in the ] F series no swelling was observed in 1F-2 gel containing only 1.7 g F / 1 0 0 g SiO 2. o 0 0
Fig. 4 shows the a m o u n t of volatiles in 1F and 7F gels calculated from the T G A and adjusted to the dry weight at 1 5 0 0 ° C . The volatiles are here presented as a function of the analyzed a m o u n t of fluoride present in the dried gel rather than as the a m o u n t of fluoride added, as was done in Part I [2]. The comparison between the two series shows that the a m o u n t of volatiles is somewhat higher in the ] F series, but a much larger part of material volatilizes below 200 ° C in 7F than in 1 F, and this applies not only for fluoride-free and low fluoride materials but for higher fluoride gels as well. To clarify this difference, E G A curves were determined. These curves showed significant evolution of carbon-containing c o m p o u n d s between 450 ° and 7 0 0 ° C which was not significantly affected by presence of fluoride. The a m o u n t of these c o m p o u n d s , however, was two orders of magnitude higher in the 1F series than in 7F. Apparently even 4 mol of water per 1 mol T E O S is not sufficient in practice for complete hydrolysis which proceeds better in the presence of a large water excess, such as in the 7F series. The E G A curves for water are shown in fig. 5. N o t e that surface-absorbed water does not show in these curves since it is lost below 200 ° C under the v a c u u m conditions used. As seen in this figure, significant water evolves at temperatures above 300 ° C in all tested gels except 7F-8. As expected, F-containing gels evolved much less water than
~ 40 O
40
O if)
I.<~ IF SERIES
~ 3c C5
7F SERIES
~- 3 0 ::E 0 bJ
(9
>-
g 20
~ 20, c~ '~
tO'
_l
J
p
I
o O 0
I
2
3
4
A M O U N T OF F - ( g / 1 O O g
5
SiO 2)
1
2
AMOUNT OF F - ( g / t O O g
3 S i O 2)
Fig. 4. Volatile contents of gels of the (a) 1F and (b) 7F series fired in air as a function of the amount of analyzed fluoride.
E.M. Rabinovich / Shrinkage and swelling of gels II
110
'1000 2 L,.,,,.~"'~ 7 F - 0 o F 7 F -
900
.,/.o...
800[ / ~ /
8
700
% ~6
<
600
~ <
50 o~
/~
--.-c--- 'IF SERIES '~,~
\
~
,
\\ \
400 z ht
\
,
n- 300
\
co 200
0 '~'~,,-0
~4
t00
Ld
I
>_
0
I
i
t
I
t
I
t
2 3 4 5 6 7 8 AMOUNT OF F-(g/'lOOg SiO 2) Fig. 7. Dependence of the surface area on the fluoride content of the 150 ° C dried gels in the 1F and 7F series, derived from data in [5] and table 1.
I-.J t,i
0~ 2
0 200
I ] I i 400 600 800 TEMPERATURE (°C) Fig. 5. Evolved gas analysis for H 2 0 in 1F-0, 1F-8, 7F-0 and 7F-8.
i
F-free gels and this is in agreement with parallel I R results [4]. What is more surprising is the much higher evolution of water from 1F gels than from 7F. Fig. 6 shows that the evolution peaks of H F and SiF3 (primary fragment of SiF4) occur in 1F-8 near 600 ° C and in 7F-8 near 750 ° C; the amounts of these species evolved from 1F-8 are 3 to 5 times higher than those from 7F-8.
8
7F-8
7[ ~~
Co x
---o--
'IF SERIES
~
7F SERIES
•,.: 6 3 "IF-8
v >,-
4
These results confirm that fluoride effectively helps to eliminate swelling in alkoxide-derived silica gels during and after sintering. It seems, however, that F - ions are more effective in the 1F series prepared from sols hydrolyzed with a smaller amount of water, than in the 7F series. Rabinovich and Wood [5] discussed the effects of fluoride on gelation, surface area, and water absorption in the 1F and 7F gels. They showed, but did not explain the significant differences, which occurred in the surface area and water content of the two series. The surface area was there plotted not against the actual F content but against the amount added to the solutions, and this created the ira-
4
(,3 Z Ld I--
7F SERIES
F- 5( z L~J 4 z
3
0
3
z 2
~z I I i t I I t I IIN~"~OJ [
'1 0 2oo
2
I ~
3
I I
It
4
I
I
I(~]
5
t T'("t-"r'"~
6
l If,,
7
8
AMOUNT OF F-(g/4OOg SiO 2) 400
600 TEMPERATURE (°C)
800
Fig. 6. Evolved gas analysis for HF and SiF3 in 1F-8 and 7F-8.
Fig. 8. Dependence of the water content in the 1F and 7F dried gels equilibrated with ambient as a function of the analyzed fluoride content derived from data in [5] and table 1.
E.M. Rabinovich / Shrinkage and swelling of gels 11
pression that these series were not significantly different, at least at higher fluoride contents. In fig. 7 these same data are used to illustrate the dependence of the surface area on the actual analyzed F content. It is seen that the 7F series exhibits a significantly lower surface area than the 1F series at equivalent F contents. The water contents of the 1F gels were also higher than those of the 7F series as seen in the similarly replotted fig. 8. Therefore, the higher effectiveness of F ions in elimination of swelling in the 1F series cannot be explained by a lower surface area retaining lesser quantities of water and OH. The only explanation, as we see it now, is in the structural difference between the two sets of gels. The degree of bloating in 1F-0 is about one third of that in 7F-0 (compare fig. 2 here with fig. 1 in Part I [2]). On the other hand, much higher amounts of F compounds are evolved from 1F-8 than from 7F-8, but this is readily explained by the higher original F content (table 1). Note, however, that this evolution occurs at a significantly lower temperature in 1F-8 than in 7F-8. The greater presintering shrinkage which is observed in the 1F series at temperatures below 800 ° C as compared with the 7F series (as seen in fig. 3) supports the idea of Brinker and Scherer [6] that a lower water-to-alkoxide ratio produces a reduced cross-linking in unsintered gels.
111
4. Conclusions The study of the two series of alkoxide-derived gels containing F ions confirmed that fluoride effectively reduces the H 2 0 and O H - content in these gels and in this way helps to eliminate swelling during sintering. Those gels studied here which exhibit large shrinkage at low temperatures were found to be less vulnerable to this swelling than the gels which exhibit low shrinkage at low temperatures. We wish to thank D.L. Wood for helpful discussions.
References [1] E.M. Rabinovich, D.L. Wood, D.W. Johnson Jr, D.A. Fleming, S.M. Vincent and J.B. MacChesney, J. Non-Cryst. Solids 82 (1986) 42. [2] K. Nassau, E.M. Rabinovich, A.E. Miller and P.K. Gallagher, ibid., 78. [3] D.M. Krol and E.M. Rabinovich, ibid., 143. [4] D.L. Wood and E.M. Rabinovich, ibid., 171. [5] E.M. Rabinovich and D.L. Wood, in: Better Ceramics Through Chemistry II, Mater. Res. Soc. Symp. Proc., Vol. 73, MRS, Pittsburgh (1986) p. 251. [6] C.J. Brinker and G.W. Scherer, J. Non-Cryst. Solids 70 (1985) 301.
107
SHRINKAGE AND S W E L L I N G O F ALKOXIDE SILICA GELS ON H E A T I N G Part II
E.M. RABINOVICH, K. NASSAU, A.E. M I L L E R and P.K. G A L L A G H E R AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 23 December 1987 Revised manuscript received 26 February 1988
The shrinkage and swelling of two series of alkoxide-derived gels were studied. It was confirmed that fluoride ions universally reduce or eliminate swelling by substitution of F for O H - on the gel surface. Gels with higher low-temperature shrinkage are less vulnerable to swelling than gels with lower low-temperature shrinkage.
1. Introduction
The problems of bubble formation during heating or reheating of gel-derived silica glass have been discussed in our previous publications [1,2]. Two different modes of bubble formation can be observed in these glasses: (a) bubbling or even foaming upon reheating to -- 1700 ° C and higher of glass prepared from particulate (colloidal) gels. (b) swelling of alkoxide-derived gels during sintering or slightly above the sintering temperature. Rabinovich et al. [1] showed that the first type of bubbling was caused by chlorine retained in the glass structure during the dehydration step of sintering. Incorporation of F - ions into the silicon oxygen network during firing effectively prevented retention of C1 in Si-C1 bonds and subsequent release of C12 gas during reheating; thus the presence of F - ions eliminated this kind of bubbling. Water and O H - groups have nothing to do with this phenomenon, since is was even worse in "dry", C12-treated specimens than in " w e t " specimens fired without C12 and sometimes containing > 200 ppm OH. Another kind of swelling is observed in alkoxide-derived monolithic silica during sintering. Here O H - is apparently the main source. In Part I [2] we described the effect of F - ions upon the shrinkage and swelling of a series of silica gels 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
prepared using a tetraethyl orthosilicate (TEOS): ethanol (EtOH): water molar ratio equal to 1 : 4 : 50 (7F series). This paper describes a similar study on sols having a significantly lower water content, using a 1 : 4 : 4 ratio (1F series). Spectroscopic studies of both series have also been pub-
.10 7
106
---0---
'IF SERIES
----o--
7F SERIES
0 404
"
o
402
tot| 0
[]
"~
L
I
I
I
I
I
J
J
2
4
6
8
'10
'12
'14
16
,~
I 32
AMOUNT OF F-ADDED ( g / l O O g SiO 2)
Fig. 1. Times of gelation of 1F and 7F sols as a function of the fluoride content in the sols (from [5]).
E.M. Rabinovich / Shrinkage and swelling of gels I I
108
Table 1 Properties of 1F and 7F gels Sample
1F-0 1F-1 1F-2 1F-4 1F-8 1F-16 1F-32 7F-0 7F-1 7F-2 7F-4 7F-8 7F-16 7F-32
Additive, wt%, to H 2 0
BET Surface
F Content, g / 1 0 0 g SiO 2
H 2 0 (wt%)
HC1
HF
area (m2/g)
Added
Analyzed a
in dry gel
1.07 0.40 0 0 0 0 0 1.07 1.02 0.96 0.85 0.64 0 0
0 0.83 1.67 3.33 6.67 13.34 26.67 0 0.07 0.14 0.28 0.56 1.12 2.25
549 408 912 757 219 154 95 802 641 292 110 73 70 50
0 1 2 4 8 16 32 0 1 2 4 8 16 32
0 0.9 1.7 3.1 5.0 6.7 n.d. 0 1.0 1.3 2.5 3.0 3.6 3.4
5.0 n.d. b) 4.6 4.0 0.2 0.095 0.09 6.8 6.6 2.5 0.22 < 0.13 -0 -0
After drying at 150 o C. b n.d. = not determined. a
lished [3,4], as well as a general discussion of a fluoride behavior in silica gels [5].
200
TEMPERATURE (°C) 4 0 0 6 0 0 8 0 0 4000 1200
20
2. Experimental procedure Sols of 11= compositions presented in table 1 were prepared as described in Part I [2]. Gelation of all silica sols is strongly accelerated in the presence of F - ions, as shown in fig. 1 reproduced from [5]. Therefore some sols remained fluid after a 30 min mixing, while those containing high levels of F gelled during this time. All mixtures, gelled or not gelled, were aged at 60 ° C for 24 h in closed containers, dried at 1 5 0 ° C for 2 - 3 days and stored in ambient conditions. The thermomechanical analysis (TMA), thermogravimetry analysis (TGA) and mass spectrometric evolved gas analysis (EGA) were performed as described in Part I [2]. The specimens were held in the thermal analyzer's furnace for 3 h at 2 0 0 ° C intervals from 200 ° to 1 2 0 0 ° C with an intermediate heating rate of 10 ° C min-~. 3. Results and discussion
I0
"--- '&2 IF-2 ~ -I0 ,,~ - 2 0 z
IF-4
O3 Z
IF-8 -t -20
-3O -40
~-~ I
0
The T M A results for dry gels of the 1F series are shown in fig. 2. While they are qualitatively
\
-20 E
I
I
I
1
I
200 400 600 800 tO00 12OO TIME (vain)
Fig. 2. Thermomechanical analysis of dried 1F gels.
109
E.M. Rabinovich / Shrinkage and swelling of gels II
~ tO(-k"x9 ~. o 0
8
~'.,,
-----o---
tF SERIES
+
7F SERIES
'~ ,% ~.
o F0
6[
E 2: co
""o,,
I 0 0.0
k
I 0 5
i
I t.0
L
I t.5
J
I 2.0
[] ~ I I I 2.5 30
Q'-i
I 3.5
~ I 4.0
I
I 4.5
I 5.0
F-(g/IOOg SiO2) Fig. 3. Dependence of shrinkage of IF and 7F gels at 200-600 °C on the fluoride content of the 150 *C dried gels. A M O U N T OF
similar to the results for the 7F series [2], there are important quantitative differences. O n the one hand, the low temperature shrinkage of the 1F series is significantly larger than that of the 7F series, as shown in fig. 3, which is derived from fig. 2 and from similar data for the 7F series [2]. On the other hand, a significantly lower fluoride content is sufficient to suppress swelling completely in the 1F series; while in the 7F series swelling was still present when 2.5 g F / 1 0 0 g SiO 2 are retained in the dry gel (7F-4), in the ] F series no swelling was observed in 1F-2 gel containing only 1.7 g F / 1 0 0 g SiO 2. o 0 0
Fig. 4 shows the a m o u n t of volatiles in 1F and 7F gels calculated from the T G A and adjusted to the dry weight at 1 5 0 0 ° C . The volatiles are here presented as a function of the analyzed a m o u n t of fluoride present in the dried gel rather than as the a m o u n t of fluoride added, as was done in Part I [2]. The comparison between the two series shows that the a m o u n t of volatiles is somewhat higher in the ] F series, but a much larger part of material volatilizes below 200 ° C in 7F than in 1 F, and this applies not only for fluoride-free and low fluoride materials but for higher fluoride gels as well. To clarify this difference, E G A curves were determined. These curves showed significant evolution of carbon-containing c o m p o u n d s between 450 ° and 7 0 0 ° C which was not significantly affected by presence of fluoride. The a m o u n t of these c o m p o u n d s , however, was two orders of magnitude higher in the 1F series than in 7F. Apparently even 4 mol of water per 1 mol T E O S is not sufficient in practice for complete hydrolysis which proceeds better in the presence of a large water excess, such as in the 7F series. The E G A curves for water are shown in fig. 5. N o t e that surface-absorbed water does not show in these curves since it is lost below 200 ° C under the v a c u u m conditions used. As seen in this figure, significant water evolves at temperatures above 300 ° C in all tested gels except 7F-8. As expected, F-containing gels evolved much less water than
~ 40 O
40
O if)
I.<~ IF SERIES
~ 3c C5
7F SERIES
~- 3 0 ::E 0 bJ
(9
>-
g 20
~ 20, c~ '~
tO'
_l
J
p
I
o O 0
I
2
3
4
A M O U N T OF F - ( g / 1 O O g
5
SiO 2)
1
2
AMOUNT OF F - ( g / t O O g
3 S i O 2)
Fig. 4. Volatile contents of gels of the (a) 1F and (b) 7F series fired in air as a function of the amount of analyzed fluoride.
E.M. Rabinovich / Shrinkage and swelling of gels II
110
'1000 2 L,.,,,.~"'~ 7 F - 0 o F 7 F -
900
.,/.o...
800[ / ~ /
8
700
% ~6
<
600
~ <
50 o~
/~
--.-c--- 'IF SERIES '~,~
\
~
,
\\ \
400 z ht
\
,
n- 300
\
co 200
0 '~'~,,-0
~4
t00
Ld
I
>_
0
I
i
t
I
t
I
t
2 3 4 5 6 7 8 AMOUNT OF F-(g/'lOOg SiO 2) Fig. 7. Dependence of the surface area on the fluoride content of the 150 ° C dried gels in the 1F and 7F series, derived from data in [5] and table 1.
I-.J t,i
0~ 2
0 200
I ] I i 400 600 800 TEMPERATURE (°C) Fig. 5. Evolved gas analysis for H 2 0 in 1F-0, 1F-8, 7F-0 and 7F-8.
i
F-free gels and this is in agreement with parallel I R results [4]. What is more surprising is the much higher evolution of water from 1F gels than from 7F. Fig. 6 shows that the evolution peaks of H F and SiF3 (primary fragment of SiF4) occur in 1F-8 near 600 ° C and in 7F-8 near 750 ° C; the amounts of these species evolved from 1F-8 are 3 to 5 times higher than those from 7F-8.
8
7F-8
7[ ~~
Co x
---o--
'IF SERIES
~
7F SERIES
•,.: 6 3 "IF-8
v >,-
4
These results confirm that fluoride effectively helps to eliminate swelling in alkoxide-derived silica gels during and after sintering. It seems, however, that F - ions are more effective in the 1F series prepared from sols hydrolyzed with a smaller amount of water, than in the 7F series. Rabinovich and Wood [5] discussed the effects of fluoride on gelation, surface area, and water absorption in the 1F and 7F gels. They showed, but did not explain the significant differences, which occurred in the surface area and water content of the two series. The surface area was there plotted not against the actual F content but against the amount added to the solutions, and this created the ira-
4
(,3 Z Ld I--
7F SERIES
F- 5( z L~J 4 z
3
0
3
z 2
~z I I i t I I t I IIN~"~OJ [
'1 0 2oo
2
I ~
3
I I
It
4
I
I
I(~]
5
t T'("t-"r'"~
6
l If,,
7
8
AMOUNT OF F-(g/4OOg SiO 2) 400
600 TEMPERATURE (°C)
800
Fig. 6. Evolved gas analysis for HF and SiF3 in 1F-8 and 7F-8.
Fig. 8. Dependence of the water content in the 1F and 7F dried gels equilibrated with ambient as a function of the analyzed fluoride content derived from data in [5] and table 1.
E.M. Rabinovich / Shrinkage and swelling of gels 11
pression that these series were not significantly different, at least at higher fluoride contents. In fig. 7 these same data are used to illustrate the dependence of the surface area on the actual analyzed F content. It is seen that the 7F series exhibits a significantly lower surface area than the 1F series at equivalent F contents. The water contents of the 1F gels were also higher than those of the 7F series as seen in the similarly replotted fig. 8. Therefore, the higher effectiveness of F ions in elimination of swelling in the 1F series cannot be explained by a lower surface area retaining lesser quantities of water and OH. The only explanation, as we see it now, is in the structural difference between the two sets of gels. The degree of bloating in 1F-0 is about one third of that in 7F-0 (compare fig. 2 here with fig. 1 in Part I [2]). On the other hand, much higher amounts of F compounds are evolved from 1F-8 than from 7F-8, but this is readily explained by the higher original F content (table 1). Note, however, that this evolution occurs at a significantly lower temperature in 1F-8 than in 7F-8. The greater presintering shrinkage which is observed in the 1F series at temperatures below 800 ° C as compared with the 7F series (as seen in fig. 3) supports the idea of Brinker and Scherer [6] that a lower water-to-alkoxide ratio produces a reduced cross-linking in unsintered gels.
111
4. Conclusions The study of the two series of alkoxide-derived gels containing F ions confirmed that fluoride effectively reduces the H 2 0 and O H - content in these gels and in this way helps to eliminate swelling during sintering. Those gels studied here which exhibit large shrinkage at low temperatures were found to be less vulnerable to this swelling than the gels which exhibit low shrinkage at low temperatures. We wish to thank D.L. Wood for helpful discussions.
References [1] E.M. Rabinovich, D.L. Wood, D.W. Johnson Jr, D.A. Fleming, S.M. Vincent and J.B. MacChesney, J. Non-Cryst. Solids 82 (1986) 42. [2] K. Nassau, E.M. Rabinovich, A.E. Miller and P.K. Gallagher, ibid., 78. [3] D.M. Krol and E.M. Rabinovich, ibid., 143. [4] D.L. Wood and E.M. Rabinovich, ibid., 171. [5] E.M. Rabinovich and D.L. Wood, in: Better Ceramics Through Chemistry II, Mater. Res. Soc. Symp. Proc., Vol. 73, MRS, Pittsburgh (1986) p. 251. [6] C.J. Brinker and G.W. Scherer, J. Non-Cryst. Solids 70 (1985) 301.