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The effect of formamide on silica sol-gel processes
Journal of Non-Crystalline Solids 109 (1989) 141-152 North-Holland, Amsterdam
141
THE EFFECT OF F O R M A M I D E O N SILICA S O L - G E L PROCESSES A.H. BOONSTRA, T.N.M. BERNARDS and J.J.T. SMITS Philips Research Laboratories, PO Box 80000, 5600 JA Eindhoven, The Netherlands Received 12 December 1988 Revised manuscript received 1 March 1989
The effect of formamide on the hydrolysis and condensation reactions at 50 ° C, both in the acid and in the basic step of a two-step sol-gel process, was investigated on mixtures of TEOS, ethanol and water, using 29Si N M R spectroscopy at - 75 ° C. In the acid step the hydrolysis rate and, especially, the dimerization rate are reduced as a function of the formamide concentration. As a consequence, the gelation time in the basic step is influenced. From density and specific surface area measurements on dried gels we determined at higher formamide concentrations an increase in particle size and an even greater increase in mean pore size. The results obtained are ascribed to a competitive reaction of formamide with the catalyzing protons, which explains the behavior of formamide as a so-called drying control chemical additive (DCCA).
1. Introduction
The alkoxide sol-gel process is a method for the preparation of silica glass products at a relatively low sinter temperature and, under appropriate conditions, of extremely low concentrations of impurities [1-7]. These properties are very important for the manufacturing of thin layers and especially for the preparation of the basic material for the drawing of optical fibers [8-11]. The most serious problem in the production of gel monoliths through a sol-gel process is fracture and cracks formation which may occur in the conversion of a wet gel to a dry gel. This cracking is probably caused by capillary forces which appear during the drying [12]. To minimize the effect of capillary forces, a number of methods is applied. The first method is based on slow and controlled evaporation of the pore liquid and gives good results. However, the processing time is extremely long [13]. The use of a hypercritical drying technique [14,15] overcomes the difficulties of capillary forces but increases the problems caused by the expulsion of liquid out of the pores of the gel [16]. Hypercritical drying is the fastest drying method to date, but for industrial applications it has the disadvantage of being a discontinuous process. 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Recently, for shortening the drying time while maintaining the monolithicity of the gels, drying control chemical additives (DCCA) have been successfully used [17-20]. The addition of formamide to a catalysed tetramethoxysilane (TMOS), methanol and water mixture was found to reduce the gelation and drying time but to slow the hydrolysis rate [21-23]. Other DCCAs such as N,N-dimethylformamide, 1,4-dioxane and acetonitrile were found to influence hydrolysis, which was tentatively explained by a change in the viscosity of the solution and by hydrogen bonding of the additives with the network [24,25]. The use of formamide was found to produce gels with a lower density, indicating a gel pore distribution shifted to larger mean sizes, which reduces the magnitude of capillary forces [7,26]. Adachi et al. [27,28], using N,N-dimethylformamide, showed that not only large pore sizes, but also the low surface tension of the liquid remaining in the pores of the gels at the final stage of drying, were favorable for the formation of dried gel monoliths without cracks or fracture. Recently Orcel et al. [29] concluded that the major role of formamide is to allow the removal of water and pore liquid at low temperature by forming a protective layer on the gel surface. The catalyst, more
142
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
than formamide, is the predominant factor in controlling the drying behavior. As was suggested by Artaki et al. [30], the use of DCCAs may possibly favor the condensation of incompletely hydrolysed species, resulting in a larger percentage of alkoxy groups in the gels. In particular when using the sol-gel process for the preparation of precursors for optical fibers, enclosures in the silica glass of the smallest amounts of carbon, caused by cracking of organic groups, lead to an unacceptable increase in the optical attenuation of the resulting fibers. For theoretical as well as practical reasons we stUdied the influence of formamide on the hydrolysis and condensation reactions in the wet phase. 29Si N M R spectroscopy [31-33] was selected for our investigations. Because of our experiences, a two-step sol-gel process [34-38] was employed with tetraethoxysilane (TEOS) as the alkoxysilane. We already found that with this method the separate steps of hydrolysis and dimerization, followed by prolonged hydrolysis and polycondensation, can be resolved qualitatively [39]. At the temperature normally used for the hydrolysis-condensation process, the rates of the hydrolysis of TMOS and of TEOS were found to be large and hard to follow, especially when the starting materials are acid-catalyzed [40,41]. As a consequence we carried out 29Si N M R measurements at - 7 5 ° C to investigate the influence of formamide on the hydrolysis and condensation reactions of different TEOS and water compositions. To obtain information concerning the course of the reaction processes, we drew samples from the reacting mixtures at each desired moment, immediately followed by a fast cooling process to below - 1 0 0 ° C to practically stop the reactions.
2. Experimental 2.1. Sample preparation The influence of formamide on the dependence of the gelation time on the hydrolysis time in a two-step sol-gel process was investigated. Therefore, TEOS, ethanol and water mixtures were used
with a final molar ratio of l : 4 : ( a + 0.5), where the water fraction in the acid step a varied from 1 to 3, and 0.5 was the basic water fraction. To each mixture formamide was added before the hydrolysis started. The experiments were repeated using different amounts of formamide. For all compositions, an HC1 concentration of 0.01M in water was used in the acid fraction. In the basic fraction an N H a O H concentration was used varying between 2.0M (a = 1) and 0.12M (a = 3), to compensate the HC1 amounts and to select an excess of N H 4 O H in order to obtain the desired gelation time [39]. Both water fractions were diluted with an equal weight of ethanol to avoid immiscibility during the addition [42,43]. The rest of the ethanol was mixed with TEOS and formamide and brought to 50°C. To this well-stirred mixture, the acid water-ethanol fraction was added in about 1 min. From the hydrolysing mixture samples were drawn every few rains. Each sample, maintained at 50 o C, was mixed with the specified amount of the basic water-ethanol fraction. The sample was then poured into a test tube and the gelation time at 50 ° C was determined with an inaccuracy of + 15 s. Each sample was subjected to the same gelation procedure. In this way the mutual comparability of the samples was good. During hydrolysis and gelation the system was closed to prevent evaporation and reactions with water vapor present in the atmosphere. The hydrolysis time (tH) is defined as the time interval between the addition of the acid and the basic fraction. The time interval between the addition of the basic fraction and the moment when, tilting the test tube, no fluidity is observed, is defined as the gelation time (tG) [39]. The influence of formamide on the hydrolysis and condensation reactions during the acid step was also investigated with 29Si N M R a t - 75 °C. Therefore a number of acid samples having different T E O S - w a t e r ratios, drawn at several hydrolysis times, were directly cooled to - 1 0 0 ° C to quench the hydrolysis-condensation reactions.
2.2. Characterization of gels To examine the influence of the formamide concentration in the mixture on the resulting
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
properties of the dried gels, samples were drawn at different hydrolysis times and gelated, as described in sect. 2.1, in test tubes whose inside walls were coated with an anti-adhesive layer. After gelation at 50 ° C the sol fraction of the gels was reduced by keeping the gels at that temperature for about 3 h. The samples were dried in an autoclave. Only series of wet gels prepared from mixtures with the same final molar compositions of TEOS, ethanol, water and formamide were treated simultaneously in one autoclave cycle [16]. To prevent temperature gradients during heating, the autoclave was filled with N 2 gas up to a pressure of 2 MPa. The presence of s o m e N 2 gas also prevents boiling during evaporation of the ethanol and water still present in the gels. The autoclave was subsequently heated to 3 0 0 ° C at a rate of 100 ° C / h and kept at this temperature for about 2 h. Next, the pressure was gradually reduced to about 10 2 Pa in 2 h. In this way physisorbed compounds were removed from the surface of the gel particles. The autoclave was then cooled to room temperature. Next the chemisorbed groups on the surface of the gel particles were removed by firing in air at 4 0 0 ° C for 16 h. After this treatment the specific surface area of each sample was determined with the BET method at 78 K using krypton (reproducibility better than 1%) [44,45]. The effect of prolonged gelation, to reduce the sol fraction in the gel, on the resulting specific surface area of the dried gels, was also investigated. Mixtures of TEOS, ethanol, water and formamide with a molar composition of 1 : 4 : ( 1 + 0.5): x, with x between 0 and 0.14 were hydrolysed for 30 min. Then the basic fraction was added. The resulting mixture was divided into 8 portions. After gelation at 50 ° C the samples were kept at that temperature for different lengths of time. After the selected time each sample was very rapidly cooled to room temperature, evacuated at that temperature to 102 Pa and maintained under these conditions for 20 h to remove the unhydrolysed compounds. Then the samples were heated very slowly to 4 0 0 ° C in air and maintained at that temperature for 20 h. After this treatment the specific surface area of each sample was measured.
2.3.
295i N M R
143
measurements
29Si N M R experiments at - 7 5 ° C were performed in a Nicolet NT-200 spectrometer equipped with a b r o a d b a n d probe. The observation frequency was 39.743 M H z using the quadrature detection mode. Spectra were collected of 16K data points at a spectral width of 6000 Hz, employing a 45 ° pulse of 10 #s. The negative nuclear Overhauser effect (NOE) was suppressed by gated decoupling, using approximately 2.5 W. The relevant signals were sufficiently far removed from the broad signals of the silicon present in the N M R sample tube that no spin-echo techniques had to be applied. Because of the long relaxation times of 29Si, small but constant amounts of chromium(III)acetylacetonate (Cr(acac)3), dissolved in ethanol, were added to the samples. A low concentration of the paramagnetic relaxation agent (0.01M) is sufficient to provide a path for dipolar relaxation, making it possible to collect spectra at intervals of 5 s. No differences in N M R spectra were observed with or without the Cr(acac)3 solution. Spectra were accumulated until a signal-to-noise ratio of 100 was achieved. Depending on the degree of hydrolysis of the mixture, 80 to 500 transients were acquired. All chemical shifts were reported relative to tetramethylsilane (TMS). However, because of its poor solubility in acetone at low temperatures, TMS could not be used directly as an integration standard. We therefore used phenyltrimethoxysilane with a chemical shift at - 7 5 ° C of - 5 3 . 6 p p m with respect to TMS. A 5 m m N M R tube with a solution of phenyltrimethoxysilane and Cr(acac)3 in acetone was inserted coaxially into the 12 m m sample tube to serve as the standard. Deuterated acetone was used to provide a lock signal. During the measurements the samples were kept at a temperature of - 7 5 _+ 1 ° C by a N 2 stream, previously led through a Dewar vessel filled with melting ethanol. At this temperature no change in spectra was found even after 6 h. Line widths of 2 Hz could be obtained, indicating that the samples were not too viscous.
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
144
3. Results
Table 1
In fig. 1 the influence of formamide on the relation between the hydrolysis time and the gelation time is given for the TEOS, ethanol and water mixture with a final molar ratio of 1 : 4 : (1 + 0.5). At increasing formamide concentrations up to 8 mol% a decrease in gelation time is found. Only at hydrolysis times smaller than 10 rain is the gelation rate lower. At formamide concentrations above 8 mol% the gelation time increases for all hydrolysis times. In fig. 2 similar results are given for the mixture with a final molar ratio of 1 : 4 : (3 + 0.5). An increase in the gelation rate is now found to be caused by the addition of formamide up to 15 mol%. Above this value a decrease in the gelation rate as a function of the formamide concentration is found. This cannot be ascribed to decreases in concentration, because even after addition of 35 mol% formamide the change in volume is less than 3%. The amounts of formamide needed for the maximum gelation rate are not constant. We found that these are linearly dependent on the amounts of HC1 added in the acid fraction. A molar ratio between the formamide and HC1 of
T M S . a: Species with no b r i d g i n g o x y g e n S i ( O E t ) 4 _ , ( O H ) n ; b: Species with one b r i d g i n g o x y g e n - • S i - O - S i ( O E t ) 3 _ , ( O H ) ,
29Si c h e m i c a l
T
8O
,s+'Jh'q~A~+'/++......._A...-.--A~A
with respect to
a
b
n = 0
- 81.5 + 0.1
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- 78.7
- 86.1
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n = 3
- 74.7
- 82.8
n = 4
- 73.1
-
450 was measured for the TEOS, ethanol and water mixture of 1 : 4 : (1 + 0.5) whereas a value of about 300 was found for the ratio of 1 : 4 : (3 + 0.5). The effect of formamide on the hydrolysiscondensation reactions in the acid step is clearly shown in the 29Si N M R spectra of TEOS, ethanol and water mixtures measured at - 7 5 ° C. In table 1 the 29Si chemical shifts at - 7 5 ° C are given in p p m with respect to the reference TMS for the different Si compounds. In figs. 3 and 4 the results are given for a number of hydrolysis times for the mixtures with a final molar ratio of 1 : 4 : 1 and 1 : 4 : 3, respectively. In both figures a decrease in
o
I 80
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shifts g i v e n in p p m at - 7 5 ° C
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_.....~x
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Fig. 1. T h e relation b e t w e e n the gelation time t G a n d the hydrolysis time t H of T E O S , ethanol, w a t e r a n d f o r m a m i d e m i x t u r e s with a m o l a r ratio of 1 : 4 : (1 + 0.5) : x, w h e r e x is 0 (1), 0.04 (2), 0.08 (3), 0.14 (4) a n d 0.15 (5).
20 0
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' t H (rnifl)
Fig. 2. T h e relation b e t w e e n the gelation time t G a n d the h y d r o l y s i s time t H of T E O S , ethanol, w a t e r a n d f o r m a m i d e m i x t u r e s with a m o l a r ratio of 1 : 4 : ( 3 + 0 . 5 ) : y, w h e r e y is 0 (1), 0.02 (2), 0.15 (3), 0.32 (4) a n d 0.35 (5).
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
TEOS=EtOH=H20
145
= 1;4:1
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Fig. 3. 29Si N M R spectra measured at - 75 ° C o f drawn samples of a hydrolysing mixture of T E O S , ethanol, water and formamide with a molar ratio of 1 : 4 : 1 : a, quenched after different hydrolysis times, for a = 0, 8 and 14 mol%.
A.H. Boonstra et al. / Effect of formamide on sifica sol-gel processes
146
TEOS:EtOH:H20
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F i g . 4. 29Si N M R
spectra measured
a t - 75 o C o f d r a w n s a m p l e s o f a h y d r o l y s i n g m i x t u r e o f T E O S , e t h a n o l , w a t e r a n d f o r m a m i d e
w i t h a m o l a r r a t i o o f 1 : 4 : 3 : b, q u e n c h e d a f t e r d i f f e r e n t h y d r o l y s i s t i m e s , f o r b = 0, 15 a n d 32 m o l % .
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
80
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Fig. 5. The relation between the gelation time t o a n d the h y d r o l y s i s time t H of TEOS, ethanol, w a t e r (10 2M HCI) a n d f o r m a m i d e m i x t u r e s w i t h a m o l a r ratio of 1 : 4 : ( 1 + 0 . 5 ) : 0 . 0 8 (A), after 45 m i n h y d r o l y s i s a n extra a d d i t i o n of a n e q u a l a m o u n t of HC1 in e t h a n o l (B) or 3 times the a m o u n t of HCI in e t h a n o l (C).
intensity of peaks corresponding to dimers is already found by adding small amounts of formamide. At higher formamide concentrations the
147
smaller increase of the intensities of peaks due to hydroxy monomers is clearly visible. With 295i N M R we do not find any evidence to support the possibility of chemical bonds between formamide and any Si compound [21,30]. A possible consumption of water by formamide during the hydrolysis was investigated with 1H N M R and no indication of a reaction was observed. In fig. 5 the effect of renewed addition of HC1 is shown for the TEOS, ethanol, water and formamide mixture with a final molar ratio of 1 : 4 : (1 + 0.5) : 0.08. This composition is selected because at that formamide concentration the fastest gelation time is determined and no increase in gelation time as a result of dimerization is observed (figs. 1 and 3). After a hydrolysis time of 45 min, once (B) or three times (C) the equivalent of HC1 is added as a solution of 1 N HC1 in absolute ethanol. From that moment, a great increase of the gelation time in the basic step is measured. This is caused by dimerization [39], as confirmed by 29Si N M R measurements at - 7 5 ° C shown in fig. 6. In table 2 the specific surface area and the density (A o < 0.002) are given for autoclave-dried gels. The samples drawn at different hydrolysis times are prepared from TEOS, ethanol, water and
Table 2 Specific surface area A ( m 2 / g ) and density p ( g / c m 3) of autoclave-dried silica gels from T E O S : ethanol : w a t e r : f o r m a m i d e = 1 : 4 : (1 + 0 . 5 ) : x, at different h y d r o l y s i s times tH (min) tH
7 12 17 42 62
x = 0
x = 0.06
x = 0.08
x = 0.14
A
p
A
p
A
p
A
p
770 901 937 929 892
0.159 0.198 0.229 0.288 0.336
631 730 785 815 833
0.138 0.160 0.176 0.202 0.214
617 715 775 802 788
0.135 0.146 0.147 0.176 0.172
543 569 570 565
0.124 0.136 0.133 0.134
Table 3 Specific surface area A (mZ/g) a n d density O ( g / c m 3) of autoclave-dried silica gels from T E O S : ethanol : w a t e r : f o r m a m i d e = 1 : 4 : ( 3 + 0.5): y, at different h y d r o l y s i s times t n (min) tn
7 12 22 62
y = 0
y = 0.02
y = 0.15
y = 0.32
A
p
A
p
A
p
A
p
425 452 448 457
0.197 0.301 0.333 0.335
423 454 460 464
0.187 0.259 0.300 0.311
315 375 390 385
0.138 0.184 0.220 0.241
229 264 260
0.118 0.127 0.135
148
/
A . H . Boonstra et aL
Effect o f f o r m a m i d e on sifica s o l - g e l processes
Table 4 The residual w e i g h t w4o0oc (%) a n d specific surface area A ( m 2 / g ) of gels w i t h a final m o l a r ratio T E O S : e t h a n o l : w a t e r : f o r m a m i d e = 1 : 4 : (1 + 0.5) : x after v a c u u m d r y i n g at 25 o C a n d firing in air at 400 o C as a function of p r o l o n g e d g e l a t i o n tp (h) tp
x = 0.00
x = 0.08
x = 0.10
W4oo . c
A
W4oo * c
A
W,~o . ¢
A
W4oo. C
A
W4ooOC
A
0 0.2 0.5 1.5 3.5 6 23
10.8 11.0 11.1 11.0 11.9 12.3 13.1
711 750 871 1010 1056 1089 1096
6.6 6.7 7.5 7.8 8.6 9.6 11.5
969 950 984 990 978 959 936
4.9 5.0 5.4 6.0 6.9 7.9 10.4
990 968 926 856 801 714 696
3.1 3.2 3.6 4.2 5.2 6.1 8.9
750 706 668 589 560 532 496
2.5 2.4 2.6 3.2 4.2 5.0 7.6
440 438 406 361 313 298 303
TEOS: EtOH: H20 = h 4:1 + B molZ ?ormamide
(k) : -
. . . .
I
. . . .
-75
]
. . . .
I
-80
. . . .
-85
I
. . . .
-90
I
-95
. . . .
PPM
(B): i e 9 HCI
. . . .
I
. . . .
-75
I
. . . .
I
-80
. . . .
-85
I
. . . .
I
-90
-95
. . . .
PPM
(C): 3 QcI HCI
'
'
'
'
]
-75
. . . .
I
-80
'
'
'
'
I
-85
. . . .
I
-90
'
'
'
1
-95
I
.... PPM
x = 0.12
x = 0.14
Table 5 Specific surface area A ( m 2 / g ) and density p ( g / c m 3) of autoclave-dried silica gels from T E O S : ethanol : water : form a m i d e = 1 : 4 : (3 + 0.5) : 0.15, at different hydrolysis times t H (rain); f o r m a m i d e is a d d e d in the basic fraction tH
A
p
7 12 22 42 62
423 459 456 462 476
0.162 0.248 0.276 0.299 0.299
formamide mixtures with a molar ratio of 1 : 4 : (1 + 0.5): x. The value of x varies between 0 and 0.14. The results of the mixtures with the ratio 1 : 4 : (3 + 0.5) : y, with y between 0 and 0.32, are shown in table 3. The influence of prolonged gelation on the residual weight of samples after evaporation at 2 5 ° C and heating at 4 0 0 ° C (w40ooc) is represented in table 4 for the TEOS, ethanol, water and formamide mixtures with a composition of 1 : 4 : (1 + 0.5) : x. The results of specific surface area measurements on these samples are also given. A reduction of the specific surface area at prolonged gelation is shown for compositions with x >_ 0.10. Fig. 6. 29Si N M R spectra m e a s u r e d at - 75 o C of s a m p l e s of a m i x t u r e of TEOS, ethanol, w a t e r ( 1 0 - 2 M HC1) a n d f o r m a m i d e w i t h a m o l a r r a t i o of 1 : 4 : 1 : 0.08, q u e n c h e d after a h y d r o l y s i s t i m e of 60 min, (A) w i t h o u t additions, (B) after 45 rain h y d r o l y s i s a n a d d i t i o n of an e q u i v a l e n t a m o u n t of H C I in e t h a n o l a n d (C) after 45 m i n h y d r o l y s i s 3 times the a m o u n t of HCI in ethanol.
149
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
The influence of formamide on the gelation behavior of hydrolysed mixtures is also investigated. Formamide is mixed with the basic water-ethanol fraction before its addition to the hydrolysed TEOS, ethanol and water mixtures. Only a small decrease in gelation time is found, which is ascribed to the very minute basic behavior of formamide. In table 5 the results of specific surface area and density measurements are given for autoclave-dried gels, prepared from mixtures of TEOS, ethanol, water and formamide with a molar ratio of 1 : 4 : ( 3 + 0.5) :0.15. Comparing these results with those of the addition of formamide in the acid step, as given in table 3, shows a much better agreement with a zero addition instead of the addition of 15 mol% formamide.
where R is H, C2H 5 or Si(OR)3. The dimerization results in a decrease in the silanol concentration and therefore in an increase of the gelation time in the basic step. The influence of formamide in the acid step on the relation between the hydrolysis time and the gelation time is dual. At low formamide concentrations, the dimerization reaction is suppressed, as shown in figs. 3 and 4. The resulting higher silanol concentration is accountable for the shorter gelation times given in figs. 1 and 2. At higher formamide concentrations no dimerization occurs in the acid step which, after complete hydrolysis, results in a constant gelation time in the basic step. The effect of formamide on the condensation can be ascribed to a decrease in the H ÷ concentration as given by H
4. D i s c u s s i o n
I The relation between the hydrolysis time in the acid step and the gelation time in the basic step, as shown in figs. 1 and 2, have recently been described [41]. At short hydrolysis times, the decrease in gelation time is caused by the formation of silanols. This hydrolysis reaction proceeds by an electrophilic reaction mechanism [46,47] RO~ H3 O+ +
H30++ H2N--C~O H
I , +H3N--C=O
+ H20,
which slackens the formation of (RO)3SiOH ~ . The concentration of (RO)3SiOH ~ already formed is reduced by H
/OR
I
Ro/SI~oR
(RO)3Si--OH ~ + H 2 N - - C = O H
H
R O ~ H + OR ' O" Si H / RO / ~'OR HO~
I , (RO)3Si--OH+ +H3N--C=O.
./OR
,
(4)
+ H O R + H +.
(1)
Ro/S~oR When in the acid step almost all the water added has been consumed, dimerization of silanols becomes the dominant reaction. This condensation [39,48] is given by the reactions (RO)3Si--OH + H3 O+ , (RO)3Si--OH ~- + H20
(2)
The presence of adequate amounts of formamide may restrain completely the dimerization reaction (3) and no water is released for prolonged hydrolysis. In figs. 3 and 4 it is also shown that formamide reduces the hydrolysis reactions. At low formamide concentration this effect is compensated by the suppression of the dimerization reaction. The equation H+
H (6)
(RO)3Si--OC2H 5 + H 2 N - - C ~ O
and
H
(RO) 3Si-- OHm- + H O - - Si(OR) 3 , (RO)3Si--O--Si(OR)3 + H30 +,
(5)
I
(3)
, (RO)3Si--OC2H 5+ + H 3 N - - C ~ O
,
A.H. Boonstra et a L / Effect of formamide on silica sol-gel processes
150
as well as eq. (4), are in competition with the hydrolysis reaction as given in eq. (1). The reaction of formamide with H ÷ is followed by H
I
O=C--NH~-
+ C1H
I , O=C--NH
2-HC1.
(7)
In our opinion the larger influence of formamide on the dimerization effect has to be ascribed to the fact that the isoelectric point of (RO)3SiOH is situated at a higher H + concentration than that of (RO)3SiOEt. The position of the isoelectric points of the compounds will depend on the number and nature of the - O R groups as well as on the solvent used. Determination of the isoelectric points may be a simple way to predict the course of the hydrolysis-condensation reactions. In fig. 5 it is shown that as a function of the hydrolysis time a constant gelation time can be obtained when in a TEOS, ethanol and water mixture with a final ratio of 1 : 4 : ( 1 + 0 . 5 ) a selected amount of formamide is also present. The increase in gelation time after renewed addition of HC1 can be explained by eqs. (2) and (3) as an increase in the dimer concentration. This explanation is confirmed by 29Si N M R measurements, as shown in fig. 6. In table 2 it is shown that the larger the quantity of formamide in the acid step, the smaller the specific surface area of dried gels. Under these circumstances the degree of hydrolysis is lower, resulting in a smaller concentration of silanols, as shown in figs. 3 and 4. The decrease in concentration may lead to a smaller amount of nuclei at the beginning of the gelation. Because of the prolonged hydrolysis and gelation after the addition of the basic fraction, the nuclei may grow to larger particles [49] or new particles may be formed. The growth of the gel particles depends on the ratio of - O H and - O C 2 H s groups at their surfaces, which is determined by the ratio of H 2 0 and C2HsOH in the liquid phase of the wet gel [38]. The larger H 2 0 / C 2 H s O H ratio, and therefore the larger - O H / - O C 2 H 5 ratio, is also the explanation for
the smaller values of specific surface area in table 3 in comparison with those in table 2. The smaller values of specific surface area after short hydrolysis times may also be ascribed to smaller amounts of nuclei caused by the lower concentration of silanols. Assuming the same gel density, the growth to larger particles in a wet gel results in larger pore sizes, filled with liquid. The larger the particles and the larger the mean distance between the chains of particles, the smaller the shrinkage during the drying process. Gels with an increased amount of formamide were found to exhibit less shrinkage, which results in lower values of density, as shown in tables 2 and 3. The lower the density of gels having the same specific surface area, the larger the mean size of the pores. The evaporation of the liquid during the drying process of these gels will therefore proceed more rapidly. Also, the larger the particles, the smaller the amount of gaseous compounds desorbed from the surface of the gel particles during drying, assuming the same density. As is known, the capillary force (z~p), exerted on the pore walls of the silica network, may be the main reason for crack formation. For a cylindrical capillary of radius r, Ap is expressed by Laplace's formula, zap -
2y cos
q,
(8)
r
where 3' is the surface tension at the liquid-air interface and q, is the wetting angle. Therefore, Ap decreases with decreasing surface tension and increasing radii of the pores. The resistance of the pore walls to the capillary force can be increased by strengthening the network of particles. This strengthening can be achieved by creating larger particles and by stimulating neck formation between particles [16]. Problems with capillary forces can be avoided when gels are dried hypercritically in an autoclave. However, following the decrease in density of liquid ethanol from P20oc = 0.79 to P243oc = 0.28 [50], the largest fraction of ethanol is expelled from the pores as liquid ethanol during heating to the critical temperature. For the same reason the gel has to be built up by a network of large,
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
c o m p a c t particles, f o r m i n g solid pore walls which occur when f o r m a m i d e is present in the acid step. T h e effect of f o r m a m i d e o n the hydrolysis a n d d i m e r i z a t i o n reactions as described will stimulate particle growth, b o t h in the basic step a n d d u r i n g the drying process. This behavior m a y be the e x p l a n a t i o n for the ultimate effect of f o r m a m i d e as a drying control chemical additive ( D C C A ) in the p r e p a r a t i o n of dried silica gels.
5. S u m m a r y F o r m a m i d e present i n a hydrolysing mixture of TEOS, e t h a n o l a n d water decreases the hydrolysis rate and, i n particular, d i m e r i z a t i o n reactions in the acid step which is ascribed to a reaction of f o r m a m i d e with the catalyzing protons. As a result of the restrained d i m e r i z a t i o n only slight a m o u n t s of water are formed, leaving unreacted T E O S as shown b y the 29Si N M R spectra o b t a i n e d at - 75 o C. W h e n f o r m a m i d e is added in the basic step hardly a n y influence o n the properties of the u l t i m a t e gels is f o u n d which is ascribed to the very m i n u t e basic behavior of f o r m a m i d e only. T h e silica particles formed m a y grow b y prolonged hydrolysis of the u n r e a c t e d T E O S in the basic step, followed by c o n d e n s a t i o n of the silanols o n the surface of the particles. D e p e n d i n g o n the a m o u n t of formamide, the n e t w o r k of the resulting gel is therefore built up of larger particles. A s s u m i n g the same density, gels with larger particles therefore have larger m e a n pore sizes. T h e higher the f o r m a m i d e c o n c e n t r a t i o n , the smaller the density after drying of the gels. This effect increases the differences in m e a n pore size. The larger the pores a n d the more solid the pore walls, the smaller the effect of capillary forces d u r i n g the drying process. These results explain the f u n c t i o n of f o r m a m i d e as a D C C A which m a k e possible the acceleration of the drying process of gels to monoliths. The authors would like to t h a n k J.M.E. Baken for m a n y valuable discussions, H.M. van d e n Bogaert for helpful advice c o n c e r n i n g the N M R m e a s u r e m e n t s a n d R.M. Salemink for his autoclave activities.
151
References [1] B.E. Yoldas, J. Mat. Sci. 14 (1979) 1843. [2] S. Sakka, in: Treatise on Materials Science and Technology, Vol. 22, Glass III, eds M. Tomozowa and R.H. Doremus (Academic Press, New York, 1982) p. 129. [3] B.E. Yoldas, in: Ultrastructure Processing of Ceramics, Glasses and Composites, eds. L.L. Hench and D.R. Ulrich (Wiley, New York, 1984) p. 60. [4] J. Zarzycki, in: Glass...Current Issues, eds A.F. Wright and J. Dupuy (Nijhoff, Dordrecht, 1985) p. 203. [5] C.J. Brinker, K.D. Keefer, D.W. Schaefer and C.S. Ashley, J. Non-Cryst. Solids 70 (1985) 301. [6] J.D. Mackenzie, J. Non-Cryst. Solids 73 (1985) 631. [7] L.L. Hench, in: Science of Ceramic Chemical Processing, eds L.L. Hench and D.R. Ulrich (Wiley, New York, 1986) p. 52. [8] P.C. Schultz, J. Am. Ceram. Soc. 57 (1974) 309. [9] 1. Matsuyama, K. Susa, S. Satoh and T. Suganuma, Cer. Bull. 63 (1984) 1408. [10] J.B. MacChesney, 13th ECOC, Helsinki (1987) 71. [11] S. Shibata, T. Kitagawa and M. Horiguchi, 13th ECOC, Helsinki (1987) 147. [12] J. Zarzycki, in: Ultrastructure Processing of Ceramics, Glasses and Composites, eds L.L. Hench and D.R. Ulrich (Wiley, New York, 1984) p. 43. [13] D.M. Krol and J.G. van Lierop, J. Non-Cryst. Solids 63 (1984) 131. [14] J. Zarzycki, M. Prassas and J. Phalippou, J. Mat. Sci. 17 (1982) 3371. [15] M. Prassas, J. Phalippou and J. Zarzycki, J. Mat. Sci. 19 (1984) 1656. [16] A.H. Boonstra and J.M.E. Baken, J. Non-Cryst. Solids 109 (1989) 1. [17] S. Wallace and L.L. Hench, Mat. Res. Soc. Syrup. Proc. 32 (1984) 47. [18] S.H. Wang and L.L. Hench, Mat. Res. Soc. Symp. Proc. 32 (1984) 71. [19] G. Orcel and L.L. Hench, Mat. Res. Soc. Syrup. Proc. 32 (1984) 79. [20] Y. Abe, N. Sugimoto, Y. Nagao and T. Misono, J. NonCryst. Solids 104 (1988) 164. [21] I. Artaki, M. Bradley, T.W. Zerda and J. Jonas, J. Phys. Chem. 89 (1985) 4399. [22] L.L. Hench and G. Orcel, J. Non-Cryst. Solids 82 (1986) 1. [23] G. Orcel and L.L. Hench, J. Non-Cryst. Solids 79 (1986) 177. [24] I. Artaki, T.W. Zerda and J. Jonas, Mat. Letters 3 (1985) 493. [25] I. Artaki, T.W. Zerda and J. Jonas, J. Non-Cryst. Solids 81 (1986) 381. [26] LL. Hench, G. Orcel and J.L. Nogues, Mat. Res. Soc. Symp. Proc. 73 (1986) 35. [27] T. Adachi and S. Sakka, J. Mat. Sci. 22 (1987) 4407. [28] T. Adachi and S. Sakka, J. Non-Cryst. Solids 99 (1988) 118. [29] G. Orcel, J. Phalippou and L.L. Hench, J. Non-Cryst. Solids 104 (1988) 170.
152
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
[30] I. Artaki, M. Bradley, T.W. Zerda, J. Jonas, G. Orcel and L.L. Hench, in: Science of Ceramic Chemical Processing, eds. L.L. Hench and D.R. Ulrich (Wiley, New York, 1986) p. 73. [31] L.W. Kelts, N.J. Effinger and S.M. Melpolder, J. NonCryst. Solids 83 (1986) 353. [32] C.W. Turner and K.J. Franklin, J. Non-Cryst. Solids 91 (1987) 402. [33] R.A. Assink and B.D. Kay, J. Non-Cryst. Solids 99 (1988) 359. [34] C.J. Brinker, K.D. Keefer, D.W. Schaefer and C.S. Ashley, J. Non-Cryst. Sohds 48 (1982) 47. [35] C.J. Brinker, K.D. Keefer, D.W. Schaefer, R.A. Assink, B.D. Kay and C.S. Ashley, J. Non-Cryst. Sohds 63 (1984) 45. [36] R.A. Assink and B.D. Kay, Mat. Res. Soc. Symp. Proc 32 (1984) 301. [37] J.G. van Lierop, A. Huizing, W.C.P. Meerman and C.A.M. Mulder, J. Non-Cryst. Sohds 82 (1986) 265. [38] A.H. Boonstra and C.A.M. Mulder, J. Non-Cryst. Solids 105 (1988) 201. [39] A.H. Boonstra and T.N.M. Bernards, J. Non-Cryst. Solids 105 (1988) 207.
[40] R.K. Harris and C.T.G. Knight, J. Chem. Soc. Chem. Commun. 421 (1980) 726. [41] A.H. Boonstra and T.N.M. Bernards, J. Non-Cryst. Solids 108 (1989) 249. [42] H. Pentinghaus, J. Non-Cryst. Solids 63 (1984) 193. [43] K. Kamiya and T. Yoko, J. Mat. Sci. 21 (1986) 842. [44] S. Brunauer, P.H. Emmett and E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [45] A.H. Boonstra, Philips Res. Reports Suppl. 3 (1968). [46] R. Aelion, A. Loebel and F. Eirich, J. Am. Chem. Soc. 72 (1950) 5705. [47] K.D. Keefer, Mat. Res. Soc. Symp. Proc. 32 (1984) 15. [48] C. Okkerse, in: Physical and Chemical Aspects of Adsorbents and Catalysts, ed. B.G. Linsen (Academic Press, New York, 1970) Ch. 5. [49] G. Orcel, L.L. Hench, I. Artaki, J. Jonas and T.W. Zerda, J. Non-Cryst. Solids 105 (1988) 223. [50] E.W. Washburn, ed., International Critical Tables of Numerical Data, Physics, Chemistry and Technology, Vol. llI (Mc Graw-Hill, New York, 1928).
141
THE EFFECT OF F O R M A M I D E O N SILICA S O L - G E L PROCESSES A.H. BOONSTRA, T.N.M. BERNARDS and J.J.T. SMITS Philips Research Laboratories, PO Box 80000, 5600 JA Eindhoven, The Netherlands Received 12 December 1988 Revised manuscript received 1 March 1989
The effect of formamide on the hydrolysis and condensation reactions at 50 ° C, both in the acid and in the basic step of a two-step sol-gel process, was investigated on mixtures of TEOS, ethanol and water, using 29Si N M R spectroscopy at - 75 ° C. In the acid step the hydrolysis rate and, especially, the dimerization rate are reduced as a function of the formamide concentration. As a consequence, the gelation time in the basic step is influenced. From density and specific surface area measurements on dried gels we determined at higher formamide concentrations an increase in particle size and an even greater increase in mean pore size. The results obtained are ascribed to a competitive reaction of formamide with the catalyzing protons, which explains the behavior of formamide as a so-called drying control chemical additive (DCCA).
1. Introduction
The alkoxide sol-gel process is a method for the preparation of silica glass products at a relatively low sinter temperature and, under appropriate conditions, of extremely low concentrations of impurities [1-7]. These properties are very important for the manufacturing of thin layers and especially for the preparation of the basic material for the drawing of optical fibers [8-11]. The most serious problem in the production of gel monoliths through a sol-gel process is fracture and cracks formation which may occur in the conversion of a wet gel to a dry gel. This cracking is probably caused by capillary forces which appear during the drying [12]. To minimize the effect of capillary forces, a number of methods is applied. The first method is based on slow and controlled evaporation of the pore liquid and gives good results. However, the processing time is extremely long [13]. The use of a hypercritical drying technique [14,15] overcomes the difficulties of capillary forces but increases the problems caused by the expulsion of liquid out of the pores of the gel [16]. Hypercritical drying is the fastest drying method to date, but for industrial applications it has the disadvantage of being a discontinuous process. 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Recently, for shortening the drying time while maintaining the monolithicity of the gels, drying control chemical additives (DCCA) have been successfully used [17-20]. The addition of formamide to a catalysed tetramethoxysilane (TMOS), methanol and water mixture was found to reduce the gelation and drying time but to slow the hydrolysis rate [21-23]. Other DCCAs such as N,N-dimethylformamide, 1,4-dioxane and acetonitrile were found to influence hydrolysis, which was tentatively explained by a change in the viscosity of the solution and by hydrogen bonding of the additives with the network [24,25]. The use of formamide was found to produce gels with a lower density, indicating a gel pore distribution shifted to larger mean sizes, which reduces the magnitude of capillary forces [7,26]. Adachi et al. [27,28], using N,N-dimethylformamide, showed that not only large pore sizes, but also the low surface tension of the liquid remaining in the pores of the gels at the final stage of drying, were favorable for the formation of dried gel monoliths without cracks or fracture. Recently Orcel et al. [29] concluded that the major role of formamide is to allow the removal of water and pore liquid at low temperature by forming a protective layer on the gel surface. The catalyst, more
142
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
than formamide, is the predominant factor in controlling the drying behavior. As was suggested by Artaki et al. [30], the use of DCCAs may possibly favor the condensation of incompletely hydrolysed species, resulting in a larger percentage of alkoxy groups in the gels. In particular when using the sol-gel process for the preparation of precursors for optical fibers, enclosures in the silica glass of the smallest amounts of carbon, caused by cracking of organic groups, lead to an unacceptable increase in the optical attenuation of the resulting fibers. For theoretical as well as practical reasons we stUdied the influence of formamide on the hydrolysis and condensation reactions in the wet phase. 29Si N M R spectroscopy [31-33] was selected for our investigations. Because of our experiences, a two-step sol-gel process [34-38] was employed with tetraethoxysilane (TEOS) as the alkoxysilane. We already found that with this method the separate steps of hydrolysis and dimerization, followed by prolonged hydrolysis and polycondensation, can be resolved qualitatively [39]. At the temperature normally used for the hydrolysis-condensation process, the rates of the hydrolysis of TMOS and of TEOS were found to be large and hard to follow, especially when the starting materials are acid-catalyzed [40,41]. As a consequence we carried out 29Si N M R measurements at - 7 5 ° C to investigate the influence of formamide on the hydrolysis and condensation reactions of different TEOS and water compositions. To obtain information concerning the course of the reaction processes, we drew samples from the reacting mixtures at each desired moment, immediately followed by a fast cooling process to below - 1 0 0 ° C to practically stop the reactions.
2. Experimental 2.1. Sample preparation The influence of formamide on the dependence of the gelation time on the hydrolysis time in a two-step sol-gel process was investigated. Therefore, TEOS, ethanol and water mixtures were used
with a final molar ratio of l : 4 : ( a + 0.5), where the water fraction in the acid step a varied from 1 to 3, and 0.5 was the basic water fraction. To each mixture formamide was added before the hydrolysis started. The experiments were repeated using different amounts of formamide. For all compositions, an HC1 concentration of 0.01M in water was used in the acid fraction. In the basic fraction an N H a O H concentration was used varying between 2.0M (a = 1) and 0.12M (a = 3), to compensate the HC1 amounts and to select an excess of N H 4 O H in order to obtain the desired gelation time [39]. Both water fractions were diluted with an equal weight of ethanol to avoid immiscibility during the addition [42,43]. The rest of the ethanol was mixed with TEOS and formamide and brought to 50°C. To this well-stirred mixture, the acid water-ethanol fraction was added in about 1 min. From the hydrolysing mixture samples were drawn every few rains. Each sample, maintained at 50 o C, was mixed with the specified amount of the basic water-ethanol fraction. The sample was then poured into a test tube and the gelation time at 50 ° C was determined with an inaccuracy of + 15 s. Each sample was subjected to the same gelation procedure. In this way the mutual comparability of the samples was good. During hydrolysis and gelation the system was closed to prevent evaporation and reactions with water vapor present in the atmosphere. The hydrolysis time (tH) is defined as the time interval between the addition of the acid and the basic fraction. The time interval between the addition of the basic fraction and the moment when, tilting the test tube, no fluidity is observed, is defined as the gelation time (tG) [39]. The influence of formamide on the hydrolysis and condensation reactions during the acid step was also investigated with 29Si N M R a t - 75 °C. Therefore a number of acid samples having different T E O S - w a t e r ratios, drawn at several hydrolysis times, were directly cooled to - 1 0 0 ° C to quench the hydrolysis-condensation reactions.
2.2. Characterization of gels To examine the influence of the formamide concentration in the mixture on the resulting
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
properties of the dried gels, samples were drawn at different hydrolysis times and gelated, as described in sect. 2.1, in test tubes whose inside walls were coated with an anti-adhesive layer. After gelation at 50 ° C the sol fraction of the gels was reduced by keeping the gels at that temperature for about 3 h. The samples were dried in an autoclave. Only series of wet gels prepared from mixtures with the same final molar compositions of TEOS, ethanol, water and formamide were treated simultaneously in one autoclave cycle [16]. To prevent temperature gradients during heating, the autoclave was filled with N 2 gas up to a pressure of 2 MPa. The presence of s o m e N 2 gas also prevents boiling during evaporation of the ethanol and water still present in the gels. The autoclave was subsequently heated to 3 0 0 ° C at a rate of 100 ° C / h and kept at this temperature for about 2 h. Next, the pressure was gradually reduced to about 10 2 Pa in 2 h. In this way physisorbed compounds were removed from the surface of the gel particles. The autoclave was then cooled to room temperature. Next the chemisorbed groups on the surface of the gel particles were removed by firing in air at 4 0 0 ° C for 16 h. After this treatment the specific surface area of each sample was determined with the BET method at 78 K using krypton (reproducibility better than 1%) [44,45]. The effect of prolonged gelation, to reduce the sol fraction in the gel, on the resulting specific surface area of the dried gels, was also investigated. Mixtures of TEOS, ethanol, water and formamide with a molar composition of 1 : 4 : ( 1 + 0.5): x, with x between 0 and 0.14 were hydrolysed for 30 min. Then the basic fraction was added. The resulting mixture was divided into 8 portions. After gelation at 50 ° C the samples were kept at that temperature for different lengths of time. After the selected time each sample was very rapidly cooled to room temperature, evacuated at that temperature to 102 Pa and maintained under these conditions for 20 h to remove the unhydrolysed compounds. Then the samples were heated very slowly to 4 0 0 ° C in air and maintained at that temperature for 20 h. After this treatment the specific surface area of each sample was measured.
2.3.
295i N M R
143
measurements
29Si N M R experiments at - 7 5 ° C were performed in a Nicolet NT-200 spectrometer equipped with a b r o a d b a n d probe. The observation frequency was 39.743 M H z using the quadrature detection mode. Spectra were collected of 16K data points at a spectral width of 6000 Hz, employing a 45 ° pulse of 10 #s. The negative nuclear Overhauser effect (NOE) was suppressed by gated decoupling, using approximately 2.5 W. The relevant signals were sufficiently far removed from the broad signals of the silicon present in the N M R sample tube that no spin-echo techniques had to be applied. Because of the long relaxation times of 29Si, small but constant amounts of chromium(III)acetylacetonate (Cr(acac)3), dissolved in ethanol, were added to the samples. A low concentration of the paramagnetic relaxation agent (0.01M) is sufficient to provide a path for dipolar relaxation, making it possible to collect spectra at intervals of 5 s. No differences in N M R spectra were observed with or without the Cr(acac)3 solution. Spectra were accumulated until a signal-to-noise ratio of 100 was achieved. Depending on the degree of hydrolysis of the mixture, 80 to 500 transients were acquired. All chemical shifts were reported relative to tetramethylsilane (TMS). However, because of its poor solubility in acetone at low temperatures, TMS could not be used directly as an integration standard. We therefore used phenyltrimethoxysilane with a chemical shift at - 7 5 ° C of - 5 3 . 6 p p m with respect to TMS. A 5 m m N M R tube with a solution of phenyltrimethoxysilane and Cr(acac)3 in acetone was inserted coaxially into the 12 m m sample tube to serve as the standard. Deuterated acetone was used to provide a lock signal. During the measurements the samples were kept at a temperature of - 7 5 _+ 1 ° C by a N 2 stream, previously led through a Dewar vessel filled with melting ethanol. At this temperature no change in spectra was found even after 6 h. Line widths of 2 Hz could be obtained, indicating that the samples were not too viscous.
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
144
3. Results
Table 1
In fig. 1 the influence of formamide on the relation between the hydrolysis time and the gelation time is given for the TEOS, ethanol and water mixture with a final molar ratio of 1 : 4 : (1 + 0.5). At increasing formamide concentrations up to 8 mol% a decrease in gelation time is found. Only at hydrolysis times smaller than 10 rain is the gelation rate lower. At formamide concentrations above 8 mol% the gelation time increases for all hydrolysis times. In fig. 2 similar results are given for the mixture with a final molar ratio of 1 : 4 : (3 + 0.5). An increase in the gelation rate is now found to be caused by the addition of formamide up to 15 mol%. Above this value a decrease in the gelation rate as a function of the formamide concentration is found. This cannot be ascribed to decreases in concentration, because even after addition of 35 mol% formamide the change in volume is less than 3%. The amounts of formamide needed for the maximum gelation rate are not constant. We found that these are linearly dependent on the amounts of HC1 added in the acid fraction. A molar ratio between the formamide and HC1 of
T M S . a: Species with no b r i d g i n g o x y g e n S i ( O E t ) 4 _ , ( O H ) n ; b: Species with one b r i d g i n g o x y g e n - • S i - O - S i ( O E t ) 3 _ , ( O H ) ,
29Si c h e m i c a l
T
8O
,s+'Jh'q~A~+'/++......._A...-.--A~A
with respect to
a
b
n = 0
- 81.5 + 0.1
- 88.4 + 0.1
n = 1
- 78.7
- 86.1
n = 2
- 76.5
- 84.3
n = 3
- 74.7
- 82.8
n = 4
- 73.1
-
450 was measured for the TEOS, ethanol and water mixture of 1 : 4 : (1 + 0.5) whereas a value of about 300 was found for the ratio of 1 : 4 : (3 + 0.5). The effect of formamide on the hydrolysiscondensation reactions in the acid step is clearly shown in the 29Si N M R spectra of TEOS, ethanol and water mixtures measured at - 7 5 ° C. In table 1 the 29Si chemical shifts at - 7 5 ° C are given in p p m with respect to the reference TMS for the different Si compounds. In figs. 3 and 4 the results are given for a number of hydrolysis times for the mixtures with a final molar ratio of 1 : 4 : 1 and 1 : 4 : 3, respectively. In both figures a decrease in
o
I 80
o
shifts g i v e n in p p m at - 7 5 ° C
1
o
o
6O
i
°~o..++ a+
_.....~x
/ ~'o~
x ~
\ ~ 4O
+/ •
•
20
0
O S
x/
i
+iiio~.o~~ %O~.o.~.....o....._._.o o ~
~ o =
7"++~+--+--+-+--+--+
"~o.e,~s-e-
i 10
i
o.++.<-_. . . . . x
/,,
/.
2
x
e-o
310
o 5
* 4
e--e
i
-
40
i 50
•
i
i 70
•
3
I
L
90
tH(rain)
Fig. 1. T h e relation b e t w e e n the gelation time t G a n d the hydrolysis time t H of T E O S , ethanol, w a t e r a n d f o r m a m i d e m i x t u r e s with a m o l a r ratio of 1 : 4 : (1 + 0.5) : x, w h e r e x is 0 (1), 0.04 (2), 0.08 (3), 0.14 (4) a n d 0.15 (5).
20 0
10. .
.
.
30. .
.
50
;0
' t H (rnifl)
Fig. 2. T h e relation b e t w e e n the gelation time t G a n d the h y d r o l y s i s time t H of T E O S , ethanol, w a t e r a n d f o r m a m i d e m i x t u r e s with a m o l a r ratio of 1 : 4 : ( 3 + 0 . 5 ) : y, w h e r e y is 0 (1), 0.02 (2), 0.15 (3), 0.32 (4) a n d 0.35 (5).
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
TEOS=EtOH=H20
145
= 1;4:1
60
0 mo]% ?ormomide // / tl
'
'
i"
11
'
'
'
'
I
'
•i i 1
It . . . .
I
-70
. . . .
-75
I
. . . .
-80
0
I
-85
-S
-100
PPM
'
B mo1% ?ormamide • Zl
'
'
I
. . . .
I
. . . .
I
. . . .
I
'
'
'
~ ri
z
j
z
'
i
' •
'
I
. . . .
1
~'
'
'
= ' ~I
: . . . .
" I
'
~'
/ ' /'/I
" 15
@
i I
/ /
/ f
/ /
-7'o'"
]G'"
'
'
'
I
'
'
'
'
-90
-100
PPH
60
14 mo1% (ormamide /p|
," '
,
,-',.,-,~1~
~
,
,
" 5
,-'3 !*,
II
jpz
""
s'
,,
/
i ¢ t -70
"¢"
~
/
. . . .
I -75
. . . .
I -80
. . . .
I
-85 PPM
. . . .
I
-90
. . . .
I
-95
. . . .
IJ
0
-100
Fig. 3. 29Si N M R spectra measured at - 75 ° C o f drawn samples of a hydrolysing mixture of T E O S , ethanol, water and formamide with a molar ratio of 1 : 4 : 1 : a, quenched after different hydrolysis times, for a = 0, 8 and 14 mol%.
A.H. Boonstra et al. / Effect of formamide on sifica sol-gel processes
146
TEOS:EtOH:H20
=
1:4:3
~
0
mol%
,
?ormomide
JS
4
II
. . . .
I
,
60
'
'
I
. . . .
I
. . . .
I
. . . .
I
. . . .
/
• /
, . . . .
, ....
-70
'
(
' '
, ....
-75
, . . . .
-80
, ....
-85
, . . . .
-90
I¢
~/
/
,
-95
-100
PPM
60
15 mol% ?ormomide
,"
• q ''
Z Z
/
• ,r-,,,. '
'
. . . .
I . . . .
,
i . . . .
. . . .
,
, . . . .
. . . .
. '
'
, , , ,
'
;
..,,," ~.~"
,
• , . . . .
I . . . .
, ....,
,f
5 /
s /
/
I
t' -70
i
. . . .
, , , , " ,
. . . .
-TS
-oo
,
. . . .
,
.
.
.
.
.
.
.
-ss
l" -loo
o
PPM
l " - - ~ - ~ - r - 1' " ~
H'
" 'I "
I
. . ". ' . ' ' ~ ' I~ ' . . .: - .~ - ' ~
I :. . . .~
I~ '
'
' •'fi
60
5 ~
I
/ .
.
.
-70
.
.
-75
.
.
.
.
.
-60
.
.
!
-85
. . . .
I
-90
. . . .
I
-95
. . . .
/ -100
PPM
F i g . 4. 29Si N M R
spectra measured
a t - 75 o C o f d r a w n s a m p l e s o f a h y d r o l y s i n g m i x t u r e o f T E O S , e t h a n o l , w a t e r a n d f o r m a m i d e
w i t h a m o l a r r a t i o o f 1 : 4 : 3 : b, q u e n c h e d a f t e r d i f f e r e n t h y d r o l y s i s t i m e s , f o r b = 0, 15 a n d 32 m o l % .
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
80
60
o* ' °
o
o
o
. ÷ ~ ÷--,t . . . .
+
t
,o, ,
•
I I
40
,+.
r I
+s t
•_•_...• l, °
,
,
• A
20
10
70
0
t H (rain)
Fig. 5. The relation between the gelation time t o a n d the h y d r o l y s i s time t H of TEOS, ethanol, w a t e r (10 2M HCI) a n d f o r m a m i d e m i x t u r e s w i t h a m o l a r ratio of 1 : 4 : ( 1 + 0 . 5 ) : 0 . 0 8 (A), after 45 m i n h y d r o l y s i s a n extra a d d i t i o n of a n e q u a l a m o u n t of HC1 in e t h a n o l (B) or 3 times the a m o u n t of HCI in e t h a n o l (C).
intensity of peaks corresponding to dimers is already found by adding small amounts of formamide. At higher formamide concentrations the
147
smaller increase of the intensities of peaks due to hydroxy monomers is clearly visible. With 295i N M R we do not find any evidence to support the possibility of chemical bonds between formamide and any Si compound [21,30]. A possible consumption of water by formamide during the hydrolysis was investigated with 1H N M R and no indication of a reaction was observed. In fig. 5 the effect of renewed addition of HC1 is shown for the TEOS, ethanol, water and formamide mixture with a final molar ratio of 1 : 4 : (1 + 0.5) : 0.08. This composition is selected because at that formamide concentration the fastest gelation time is determined and no increase in gelation time as a result of dimerization is observed (figs. 1 and 3). After a hydrolysis time of 45 min, once (B) or three times (C) the equivalent of HC1 is added as a solution of 1 N HC1 in absolute ethanol. From that moment, a great increase of the gelation time in the basic step is measured. This is caused by dimerization [39], as confirmed by 29Si N M R measurements at - 7 5 ° C shown in fig. 6. In table 2 the specific surface area and the density (A o < 0.002) are given for autoclave-dried gels. The samples drawn at different hydrolysis times are prepared from TEOS, ethanol, water and
Table 2 Specific surface area A ( m 2 / g ) and density p ( g / c m 3) of autoclave-dried silica gels from T E O S : ethanol : w a t e r : f o r m a m i d e = 1 : 4 : (1 + 0 . 5 ) : x, at different h y d r o l y s i s times tH (min) tH
7 12 17 42 62
x = 0
x = 0.06
x = 0.08
x = 0.14
A
p
A
p
A
p
A
p
770 901 937 929 892
0.159 0.198 0.229 0.288 0.336
631 730 785 815 833
0.138 0.160 0.176 0.202 0.214
617 715 775 802 788
0.135 0.146 0.147 0.176 0.172
543 569 570 565
0.124 0.136 0.133 0.134
Table 3 Specific surface area A (mZ/g) a n d density O ( g / c m 3) of autoclave-dried silica gels from T E O S : ethanol : w a t e r : f o r m a m i d e = 1 : 4 : ( 3 + 0.5): y, at different h y d r o l y s i s times t n (min) tn
7 12 22 62
y = 0
y = 0.02
y = 0.15
y = 0.32
A
p
A
p
A
p
A
p
425 452 448 457
0.197 0.301 0.333 0.335
423 454 460 464
0.187 0.259 0.300 0.311
315 375 390 385
0.138 0.184 0.220 0.241
229 264 260
0.118 0.127 0.135
148
/
A . H . Boonstra et aL
Effect o f f o r m a m i d e on sifica s o l - g e l processes
Table 4 The residual w e i g h t w4o0oc (%) a n d specific surface area A ( m 2 / g ) of gels w i t h a final m o l a r ratio T E O S : e t h a n o l : w a t e r : f o r m a m i d e = 1 : 4 : (1 + 0.5) : x after v a c u u m d r y i n g at 25 o C a n d firing in air at 400 o C as a function of p r o l o n g e d g e l a t i o n tp (h) tp
x = 0.00
x = 0.08
x = 0.10
W4oo . c
A
W4oo * c
A
W,~o . ¢
A
W4oo. C
A
W4ooOC
A
0 0.2 0.5 1.5 3.5 6 23
10.8 11.0 11.1 11.0 11.9 12.3 13.1
711 750 871 1010 1056 1089 1096
6.6 6.7 7.5 7.8 8.6 9.6 11.5
969 950 984 990 978 959 936
4.9 5.0 5.4 6.0 6.9 7.9 10.4
990 968 926 856 801 714 696
3.1 3.2 3.6 4.2 5.2 6.1 8.9
750 706 668 589 560 532 496
2.5 2.4 2.6 3.2 4.2 5.0 7.6
440 438 406 361 313 298 303
TEOS: EtOH: H20 = h 4:1 + B molZ ?ormamide
(k) : -
. . . .
I
. . . .
-75
]
. . . .
I
-80
. . . .
-85
I
. . . .
-90
I
-95
. . . .
PPM
(B): i e 9 HCI
. . . .
I
. . . .
-75
I
. . . .
I
-80
. . . .
-85
I
. . . .
I
-90
-95
. . . .
PPM
(C): 3 QcI HCI
'
'
'
'
]
-75
. . . .
I
-80
'
'
'
'
I
-85
. . . .
I
-90
'
'
'
1
-95
I
.... PPM
x = 0.12
x = 0.14
Table 5 Specific surface area A ( m 2 / g ) and density p ( g / c m 3) of autoclave-dried silica gels from T E O S : ethanol : water : form a m i d e = 1 : 4 : (3 + 0.5) : 0.15, at different hydrolysis times t H (rain); f o r m a m i d e is a d d e d in the basic fraction tH
A
p
7 12 22 42 62
423 459 456 462 476
0.162 0.248 0.276 0.299 0.299
formamide mixtures with a molar ratio of 1 : 4 : (1 + 0.5): x. The value of x varies between 0 and 0.14. The results of the mixtures with the ratio 1 : 4 : (3 + 0.5) : y, with y between 0 and 0.32, are shown in table 3. The influence of prolonged gelation on the residual weight of samples after evaporation at 2 5 ° C and heating at 4 0 0 ° C (w40ooc) is represented in table 4 for the TEOS, ethanol, water and formamide mixtures with a composition of 1 : 4 : (1 + 0.5) : x. The results of specific surface area measurements on these samples are also given. A reduction of the specific surface area at prolonged gelation is shown for compositions with x >_ 0.10. Fig. 6. 29Si N M R spectra m e a s u r e d at - 75 o C of s a m p l e s of a m i x t u r e of TEOS, ethanol, w a t e r ( 1 0 - 2 M HC1) a n d f o r m a m i d e w i t h a m o l a r r a t i o of 1 : 4 : 1 : 0.08, q u e n c h e d after a h y d r o l y s i s t i m e of 60 min, (A) w i t h o u t additions, (B) after 45 rain h y d r o l y s i s a n a d d i t i o n of an e q u i v a l e n t a m o u n t of H C I in e t h a n o l a n d (C) after 45 m i n h y d r o l y s i s 3 times the a m o u n t of HCI in ethanol.
149
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
The influence of formamide on the gelation behavior of hydrolysed mixtures is also investigated. Formamide is mixed with the basic water-ethanol fraction before its addition to the hydrolysed TEOS, ethanol and water mixtures. Only a small decrease in gelation time is found, which is ascribed to the very minute basic behavior of formamide. In table 5 the results of specific surface area and density measurements are given for autoclave-dried gels, prepared from mixtures of TEOS, ethanol, water and formamide with a molar ratio of 1 : 4 : ( 3 + 0.5) :0.15. Comparing these results with those of the addition of formamide in the acid step, as given in table 3, shows a much better agreement with a zero addition instead of the addition of 15 mol% formamide.
where R is H, C2H 5 or Si(OR)3. The dimerization results in a decrease in the silanol concentration and therefore in an increase of the gelation time in the basic step. The influence of formamide in the acid step on the relation between the hydrolysis time and the gelation time is dual. At low formamide concentrations, the dimerization reaction is suppressed, as shown in figs. 3 and 4. The resulting higher silanol concentration is accountable for the shorter gelation times given in figs. 1 and 2. At higher formamide concentrations no dimerization occurs in the acid step which, after complete hydrolysis, results in a constant gelation time in the basic step. The effect of formamide on the condensation can be ascribed to a decrease in the H ÷ concentration as given by H
4. D i s c u s s i o n
I The relation between the hydrolysis time in the acid step and the gelation time in the basic step, as shown in figs. 1 and 2, have recently been described [41]. At short hydrolysis times, the decrease in gelation time is caused by the formation of silanols. This hydrolysis reaction proceeds by an electrophilic reaction mechanism [46,47] RO~ H3 O+ +
H30++ H2N--C~O H
I , +H3N--C=O
+ H20,
which slackens the formation of (RO)3SiOH ~ . The concentration of (RO)3SiOH ~ already formed is reduced by H
/OR
I
Ro/SI~oR
(RO)3Si--OH ~ + H 2 N - - C = O H
H
R O ~ H + OR ' O" Si H / RO / ~'OR HO~
I , (RO)3Si--OH+ +H3N--C=O.
./OR
,
(4)
+ H O R + H +.
(1)
Ro/S~oR When in the acid step almost all the water added has been consumed, dimerization of silanols becomes the dominant reaction. This condensation [39,48] is given by the reactions (RO)3Si--OH + H3 O+ , (RO)3Si--OH ~- + H20
(2)
The presence of adequate amounts of formamide may restrain completely the dimerization reaction (3) and no water is released for prolonged hydrolysis. In figs. 3 and 4 it is also shown that formamide reduces the hydrolysis reactions. At low formamide concentration this effect is compensated by the suppression of the dimerization reaction. The equation H+
H (6)
(RO)3Si--OC2H 5 + H 2 N - - C ~ O
and
H
(RO) 3Si-- OHm- + H O - - Si(OR) 3 , (RO)3Si--O--Si(OR)3 + H30 +,
(5)
I
(3)
, (RO)3Si--OC2H 5+ + H 3 N - - C ~ O
,
A.H. Boonstra et a L / Effect of formamide on silica sol-gel processes
150
as well as eq. (4), are in competition with the hydrolysis reaction as given in eq. (1). The reaction of formamide with H ÷ is followed by H
I
O=C--NH~-
+ C1H
I , O=C--NH
2-HC1.
(7)
In our opinion the larger influence of formamide on the dimerization effect has to be ascribed to the fact that the isoelectric point of (RO)3SiOH is situated at a higher H + concentration than that of (RO)3SiOEt. The position of the isoelectric points of the compounds will depend on the number and nature of the - O R groups as well as on the solvent used. Determination of the isoelectric points may be a simple way to predict the course of the hydrolysis-condensation reactions. In fig. 5 it is shown that as a function of the hydrolysis time a constant gelation time can be obtained when in a TEOS, ethanol and water mixture with a final ratio of 1 : 4 : ( 1 + 0 . 5 ) a selected amount of formamide is also present. The increase in gelation time after renewed addition of HC1 can be explained by eqs. (2) and (3) as an increase in the dimer concentration. This explanation is confirmed by 29Si N M R measurements, as shown in fig. 6. In table 2 it is shown that the larger the quantity of formamide in the acid step, the smaller the specific surface area of dried gels. Under these circumstances the degree of hydrolysis is lower, resulting in a smaller concentration of silanols, as shown in figs. 3 and 4. The decrease in concentration may lead to a smaller amount of nuclei at the beginning of the gelation. Because of the prolonged hydrolysis and gelation after the addition of the basic fraction, the nuclei may grow to larger particles [49] or new particles may be formed. The growth of the gel particles depends on the ratio of - O H and - O C 2 H s groups at their surfaces, which is determined by the ratio of H 2 0 and C2HsOH in the liquid phase of the wet gel [38]. The larger H 2 0 / C 2 H s O H ratio, and therefore the larger - O H / - O C 2 H 5 ratio, is also the explanation for
the smaller values of specific surface area in table 3 in comparison with those in table 2. The smaller values of specific surface area after short hydrolysis times may also be ascribed to smaller amounts of nuclei caused by the lower concentration of silanols. Assuming the same gel density, the growth to larger particles in a wet gel results in larger pore sizes, filled with liquid. The larger the particles and the larger the mean distance between the chains of particles, the smaller the shrinkage during the drying process. Gels with an increased amount of formamide were found to exhibit less shrinkage, which results in lower values of density, as shown in tables 2 and 3. The lower the density of gels having the same specific surface area, the larger the mean size of the pores. The evaporation of the liquid during the drying process of these gels will therefore proceed more rapidly. Also, the larger the particles, the smaller the amount of gaseous compounds desorbed from the surface of the gel particles during drying, assuming the same density. As is known, the capillary force (z~p), exerted on the pore walls of the silica network, may be the main reason for crack formation. For a cylindrical capillary of radius r, Ap is expressed by Laplace's formula, zap -
2y cos
q,
(8)
r
where 3' is the surface tension at the liquid-air interface and q, is the wetting angle. Therefore, Ap decreases with decreasing surface tension and increasing radii of the pores. The resistance of the pore walls to the capillary force can be increased by strengthening the network of particles. This strengthening can be achieved by creating larger particles and by stimulating neck formation between particles [16]. Problems with capillary forces can be avoided when gels are dried hypercritically in an autoclave. However, following the decrease in density of liquid ethanol from P20oc = 0.79 to P243oc = 0.28 [50], the largest fraction of ethanol is expelled from the pores as liquid ethanol during heating to the critical temperature. For the same reason the gel has to be built up by a network of large,
A.H. Boonstra et al. / Effect of formamide on silica sol-gel processes
c o m p a c t particles, f o r m i n g solid pore walls which occur when f o r m a m i d e is present in the acid step. T h e effect of f o r m a m i d e o n the hydrolysis a n d d i m e r i z a t i o n reactions as described will stimulate particle growth, b o t h in the basic step a n d d u r i n g the drying process. This behavior m a y be the e x p l a n a t i o n for the ultimate effect of f o r m a m i d e as a drying control chemical additive ( D C C A ) in the p r e p a r a t i o n of dried silica gels.
5. S u m m a r y F o r m a m i d e present i n a hydrolysing mixture of TEOS, e t h a n o l a n d water decreases the hydrolysis rate and, i n particular, d i m e r i z a t i o n reactions in the acid step which is ascribed to a reaction of f o r m a m i d e with the catalyzing protons. As a result of the restrained d i m e r i z a t i o n only slight a m o u n t s of water are formed, leaving unreacted T E O S as shown b y the 29Si N M R spectra o b t a i n e d at - 75 o C. W h e n f o r m a m i d e is added in the basic step hardly a n y influence o n the properties of the u l t i m a t e gels is f o u n d which is ascribed to the very m i n u t e basic behavior of f o r m a m i d e only. T h e silica particles formed m a y grow b y prolonged hydrolysis of the u n r e a c t e d T E O S in the basic step, followed by c o n d e n s a t i o n of the silanols o n the surface of the particles. D e p e n d i n g o n the a m o u n t of formamide, the n e t w o r k of the resulting gel is therefore built up of larger particles. A s s u m i n g the same density, gels with larger particles therefore have larger m e a n pore sizes. T h e higher the f o r m a m i d e c o n c e n t r a t i o n , the smaller the density after drying of the gels. This effect increases the differences in m e a n pore size. The larger the pores a n d the more solid the pore walls, the smaller the effect of capillary forces d u r i n g the drying process. These results explain the f u n c t i o n of f o r m a m i d e as a D C C A which m a k e possible the acceleration of the drying process of gels to monoliths. The authors would like to t h a n k J.M.E. Baken for m a n y valuable discussions, H.M. van d e n Bogaert for helpful advice c o n c e r n i n g the N M R m e a s u r e m e n t s a n d R.M. Salemink for his autoclave activities.
151
References [1] B.E. Yoldas, J. Mat. Sci. 14 (1979) 1843. [2] S. Sakka, in: Treatise on Materials Science and Technology, Vol. 22, Glass III, eds M. Tomozowa and R.H. Doremus (Academic Press, New York, 1982) p. 129. [3] B.E. Yoldas, in: Ultrastructure Processing of Ceramics, Glasses and Composites, eds. L.L. Hench and D.R. Ulrich (Wiley, New York, 1984) p. 60. [4] J. Zarzycki, in: Glass...Current Issues, eds A.F. Wright and J. Dupuy (Nijhoff, Dordrecht, 1985) p. 203. [5] C.J. Brinker, K.D. Keefer, D.W. Schaefer and C.S. Ashley, J. Non-Cryst. Solids 70 (1985) 301. [6] J.D. Mackenzie, J. Non-Cryst. Solids 73 (1985) 631. [7] L.L. Hench, in: Science of Ceramic Chemical Processing, eds L.L. Hench and D.R. Ulrich (Wiley, New York, 1986) p. 52. [8] P.C. Schultz, J. Am. Ceram. Soc. 57 (1974) 309. [9] 1. Matsuyama, K. Susa, S. Satoh and T. Suganuma, Cer. Bull. 63 (1984) 1408. [10] J.B. MacChesney, 13th ECOC, Helsinki (1987) 71. [11] S. Shibata, T. Kitagawa and M. Horiguchi, 13th ECOC, Helsinki (1987) 147. [12] J. Zarzycki, in: Ultrastructure Processing of Ceramics, Glasses and Composites, eds L.L. Hench and D.R. Ulrich (Wiley, New York, 1984) p. 43. [13] D.M. Krol and J.G. van Lierop, J. Non-Cryst. Solids 63 (1984) 131. [14] J. Zarzycki, M. Prassas and J. Phalippou, J. Mat. Sci. 17 (1982) 3371. [15] M. Prassas, J. Phalippou and J. Zarzycki, J. Mat. Sci. 19 (1984) 1656. [16] A.H. Boonstra and J.M.E. Baken, J. Non-Cryst. Solids 109 (1989) 1. [17] S. Wallace and L.L. Hench, Mat. Res. Soc. Syrup. Proc. 32 (1984) 47. [18] S.H. Wang and L.L. Hench, Mat. Res. Soc. Symp. Proc. 32 (1984) 71. [19] G. Orcel and L.L. Hench, Mat. Res. Soc. Syrup. Proc. 32 (1984) 79. [20] Y. Abe, N. Sugimoto, Y. Nagao and T. Misono, J. NonCryst. Solids 104 (1988) 164. [21] I. Artaki, M. Bradley, T.W. Zerda and J. Jonas, J. Phys. Chem. 89 (1985) 4399. [22] L.L. Hench and G. Orcel, J. Non-Cryst. Solids 82 (1986) 1. [23] G. Orcel and L.L. Hench, J. Non-Cryst. Solids 79 (1986) 177. [24] I. Artaki, T.W. Zerda and J. Jonas, Mat. Letters 3 (1985) 493. [25] I. Artaki, T.W. Zerda and J. Jonas, J. Non-Cryst. Solids 81 (1986) 381. [26] LL. Hench, G. Orcel and J.L. Nogues, Mat. Res. Soc. Symp. Proc. 73 (1986) 35. [27] T. Adachi and S. Sakka, J. Mat. Sci. 22 (1987) 4407. [28] T. Adachi and S. Sakka, J. Non-Cryst. Solids 99 (1988) 118. [29] G. Orcel, J. Phalippou and L.L. Hench, J. Non-Cryst. Solids 104 (1988) 170.
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