Shrinkage and swelling of alkoxide silica gels on heating

78

Journal of Non-Crystalline Solids 82 (1986) 78-85 North-Holland, Amsterdam

SHRINKAGE AND SWELLING OF ALKOXIDE SILICA GELS ON HEATING K. NASSAU, E.M. RABINOVICH, A.E. MILLER and P.K. GALLAGHER A T & T Bell Laboratories, Murray Hill, NJ 07974, USA

Alkoxide gels were prepared, dried and studied by thermal methods. Parameters investigated include shrinkage, swelling (bloating), weight loss, and gas evolution. Shrinkage, studied up to 1200°C, was found to occur at all temperatures, but swelling only set in at 1000°C in the absence of fluoride. The addition of fluoride modified the content of volatiles, the temperature of the onset of swelling, and the degree of swelling. The swelling can be attributed to release of residual OH.

1. Introduction The preparation of oxide glasses from gels made by hydrolysis of metal alkoxides has been reviewed by Sakka [1]. Recent conferences have also dealt with this subject [2,3]. The gelation process is strongly affected by the nature of the catalyst [1] and by the presence of fluoride additions [4]. Removal of volatile species from pores during heating can result in abnormal expansion (swelling, bloating, foaming etc.) in limited temperature intervals superimposed on the normal shrinkage and sintering. Parallel infrared [5] and Raman [6] investigations are reported elsewhere in these proceedings.

2. Experimental procedure The compositions of the gels are presented in tables 1 and 2. All these compositions had the water: ethyl alcohol:tetraethyl orthosilicate (TEOS) molar ratio 50:4:1; additives (acids or ammonia) were counted as water in this calculation. The gels were prepared by pouring the alcohol-TEOS solution into the water-additive solution during about 30 s, while mixing with a magnetic bar, The mixing was continued for 30 rain. Fluoride-free mixtures formed a transparent or slightly opalescent sol after the mixing, while mixtures with fluoride formed a gel either in the process of mixing (which, in this case, was then stopped), or soon during the following aging. The sols and gels were aged in stoppered glass containers at 60°C for 24 h and then opened for drying at 150°C for 2-3 days. The dried samples were then stored in ambient. Two series of the gels were prepared. The series of table 1 contained increasing amounts of fluoride introduced as HF (49%) which replaced (on the 0022-3093/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

K. Nassau et al. / Shrinkage and swelling of alkoxide silica gels

79

Table 1 Gels with varied fluoride additives Designation

Additive as wt% of H 2° HCI HF

F as g/100g SiO 2 Added Analyzed after 150°C

pH of

7F-0 7F-1 7F-2 7F-4 7F-8 7F-16 7F-32

1.07 1.02 0.96 0.85 0.64 0 0

0 1 2 4 8 16 32

1 ~0 1,0 1.0 1.0 1.1 1.2 1.2

0 0.07 0.14 0.28 0.56 1.12 2.25

0 1.0 1.3 2.5 3.0 3.6 3.4

H 2°

Appearance of dried gel

solu. ~ clear clear slightly opalescent strongly opalescent opaque opaque opaque

a~ Of aqueous solution before adding TEOS and alcohol.

weight basis) HC1 (38%) until the amount of HF exceeded the original amount of HC1. The second series of table 2 contained different acids or ammonia as additives to water. The pH of gels and solutions was determined on a 1500 p H / i o n meter with a Fisher combination electrode Cat. No. 13-639-90. Analysis of the fluorine remaining in the dried gels was done by C. McGroryJoy [7]. The thermomechanical analysis (TMA) was performed on a DuPont 1090 Thermal Analyzer in air, using approximately 2 mm cubes of dried gel. The heating at 10°C min-1 included 3 h holding stages every 200°C from 200 to 1200°C. The weight loss was determined by weighing about 50 mg specimens after heating for 2 h at 200, 400, 1000 and 1500°C in a platinum crucible in air and then cooling in a dessicator. Results were calculated on the basis of the dry weight at 1500°C, since it can be assumed that volatiles remaining at this temperature will be negligible compared to the total volatiles initially present. Mass spectrometric evolved gas analysis, EGA, was performed using apparatus described in detail elsewhere [8]. Samples, 20-30 mg, were contained in a Pt crucible and heated in vacuum at 20°C rain-~ from room temperature

Table 2 Gels with various additives Designation

Additive

% of H 2 0

pH a~

Appearance of dried gel

7F-0 7F-16 7N 7B 7S 7C 8

HCI HF HNO 3 HBr H 2SO4 CH3COOH N H 4 OH

1.07 1.12 1.0 0.71 0.75 27.8 1.60

1.0 1.2 0.9 1.0 1.0 2.0 11.0

clear opaque clear clear clear clear opalescent

~) Of aqueous solution before adding TEOS and alcohol.

80

K. Nassau et al. / Shrinkage and swelling of alkoxide silica gels

to about ll00°C. Temperature, total pressure, and the intensity of selected mass peaks were monitored and the digital data stored for subsequent plotting and analysis. The system pressure in the absence of any sample outgassing was about 5 × 10 -8 Torr.

3. Results and discussion The 7F-0 to 7F-32 series of table 1 and fig. 1 shows the effect of fluoride additions on shrinkage and swelling for the temperature cycle used. In 7F-0, in the absence of fluoride, shrinkage is already present at 200°C, is 2.2% total at 400°C and continues up to the end of the 800°C holding period, at which point it is 8.5% total. At 1000°C after a delay of 14 rain from the start of the heating step, a drastic swelling of 62% occurs, with additional 24% swelling on heating to 1200°C. Shrinkage sets in again 14 rain after the beginning of heating to 1200°C; at the end of this step the specimen length is 34% smaller than initially. As the fluorine content is increased, the shrinkage in the 200 to 800°C region decreases drastically with a minimum at 7F-4 and then increases again but slowly. At 7F-4 the total shrinkage at the end of the 400°C step is only 0.07% (7F-0 is 2.2% and 7F-32 is 0.13%) while at 800°C the figure is 0.75% (8.5% and 3.4%). This behavior is not merely a function of the content of volatiles in the air dried gels, since fig. 2 shows a continuing decrease of volatiles to 7F-4, with an approximately constant plateau beyond this. The BET surface area of the dried gel decreases monotonously from 802 m2/g for 7F-0 to 73 m2/g for 7F-8 and thus cannot by itself explain this behavior. As little as 1% added fluorine prevents the occurence of swelling at 1000°C and displaces it up to the 1200°C heating step. The rate of swelling decreases steadily from 7F-0 to 7F-4 (maximum 4.0% min-1 swelling at 1200°C for 7F-0 and 0.086% min -~ for 7F-4), and does not occur at all for 7F-8, 7F-16 and 7F-32 as seen in fig. 1. For 7F-32, the total shrinkage by the end of the 1200°C step is 55% of the original size. The volatiles content was determined by weight loss and indicates a large decrease for the dried gel from 30.0% (based on the dry weight) for 7F-0 to 4.9% for 7F-32 as shown in fig. 2. There are only small changes in increasing the fluoride from 7F-4 to 7F-32. EGA analysis was used to identify volatile species emitted between 200 ° and 1100°C, as illustrated for H 2 0 in fig. 3. A large amount of volatiles is also emitted below 200°C, merely on applying the vacuum, but this region was not further investigated. It has been previously shown that evacuation removes the bulk of the hydrogen-bonded water [5], so that the H 2 0 emissions shown in fig. 3 correspond predominantly to water released from S i - O H silanol groups. The effect of the extra fluoride in reducing both the hydrogen bonded water [5,6] (fig. 2) as well as the incorporated silanol group becomes evident. It should also be noted that the temperature distribution of evolution is different

K. Nassau et a L / Shrinkage and swelling of alkoxide sifica gels

81

TEMPERATURE (°C)

8O

400

200

600

800

I000

40

1200

/

60

20

0,.

7F-O ~ .

. ~

0

~7F-I ~

~_

7F-2 UJ

z

<

o

7F-4

o

-20

j_-

z ,,,

:E

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-

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-60

i 0

, 200

~ 400

~

600 800 TIME ( m i n)

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1200

Fig. 1. Thermomechanical analysis of the specimens of table 1 with 0% to 32% added fluoride.

for the specimens of fig. 3. Fluoride drastically reduces the amount of easily lost S i - O H derived water (near the surface?) emitted 200 to 700°C range but only slightly affects that less readily lost (deep bulk?) emitted in the 800 to l l 0 0 ° C range. Since the surface is reduced more than this reduction in OH, other factors must be involved. The emission of fluoride was studied as two species, HF and SiF3+ (derived by the cracking within the detection system from SiF 4) as shown in fig. 4. The former begins to appear at 200°C, peaks at about 700°C, and then falls

1(, Nassau et al. / Shrinkage and swelling of alkoxide silica gels

82

~: ~ ,, o~

IW

/ - - DRIEDAT 15O°C AND THEN EQUILIBRATEDINA E M N TB I

o2o~~

o o

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SERIES

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0 I

4

i

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6 % FLUORINE

I

i

8

,, 32

ADDED

Fig. 2. Volatiles content of the samples of table 1 varying with added fluoride.

steadily up to the limit of 1100°C. This is undoubtedly derived predominantly from hydrogen-bonded H F attached to S i - O H or S i - O - S i or Si-F. This H F presumably is not removed by the vacuum as is the H 2 0 because it is expected to be more strongly hydrogen bonded as discussed below. The SiF3 emission does not begin until about 500°C, peaks near 780°C, goes through a minimum near 950°C, and is increasingly steadily at the limit of 1100°C. This species is clearly derived from F attached to Si (again perhaps near the surfaces vs deep bulk), although some reaction with S i O 2 of H F liberated from hydrogen bonding could be expected at intermediate temperatures. An examination of the emission of carbon-containing compounds indicated no noteworthy effects derived from fluoride additions. In an attempt to investigate whether these effects are unique for fluoride, a series of gels was prepared and analyzed using different additive as shown in table 2 and fig. 5. The results fell into three groups. The first group included hydrochloric, nitric, and acetic acids as well as ammonia, all showing swelling occurring after a delay ranging from 34 to 80 min at 1000°C. The second group included hydrobromic and sulfuric acids, which showed swelling after a delay ranging from 30 to 40 min at 1200°C. The total shrinkage by the end of the 800°C for the first group ranged from 5.8 to 8.5%, while for the second group it ranged from 2.1 to 4.4%. Only fluoride produced in no swelling in the range studied. This study confirms the results of a parallel infrared study [5] which shows that the fluoride ion in alkoxide gels effectively drives out O H - groups, making the silica gel surface less "hydrophilic". It is known [9] that in the glass structure the F - ion can replace O 2- in the [SiO4 ] tetrahedron due to the almost equal size of both ions, although their different polarizabilities can limit this substitution. The O H - ion on the silica gel surface has practically the same size as both 0 2- and F - ions, and it appears that F - can substitute for

K. Nassau et al. / Shrinkage and swelling of alkoxide silica gels

83

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K. Nassau et aL / Shrinkage and swelling of alkoxide silica gels

84

(°C} 800 IO00 1200

TEMPERATURE

8O

200

400

600

60

/

4O

0 7F-O

HCL

"-'-- ~

~ O- 7 N - " - ' "

~

~/

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, 200

, 400

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,

, I000

F i g . 5. T h e r m o m e c h a n i c a l 1200

the specimens additives.

analysis of

of table 2 with various

O H - even m o r e easily than for 0 2 - . This reduces not o n l y the a m o u n t of silanol b o n d s but the a m o u n t of h y d r o g e n b o n d e d H 2 0 as well, because the strength of the h y d r o g e n b o n d S i - O H . - - O H 2 can be e s t i m a t e d to be a b o u t twice that of the S i - F • • • H O H b o n d b y a n a l o g y with the H O H • • - O H 2 a n d H - F . . . H O H systems [10]. This explains w h y the presence of the F - ions in the gels reduces the H 2 0 c o n t e n t a n d reduces or c o m p l e t e l y eliminates swelling d u e to release of water d u r i n g sintering.

K. Nassau et al. / Shrinkage and swelling of alkoxide silica gels

85

4. Conclusions Alkoxide-derived dry gels when heated to 1200°C exhibit swelling due to release of water from the silanol bonds. I n t r o d u c t i o n of the fluoride ion (as H F ) results in significant reduction or complete e l i m i n a t i o n of the swelling, because the F - ion substitutes for O H - in the silanol b o n d s a n d strongly reduces the O H a n d H 2 0 contents in the gels. Other acids or a m m o n i a do not produce an equivalent effect. W e wish to t h a n k C. M c G r o r y - J o y for the F analyses and J.W. Fleming, D.W. J o h n s o n Jr, D.M. Krol, J.B. M a c C h e s n e y and D.L. W o o d for helpful discussions.

References [1] S. Sakka, in: Treatise on Materials Science and Technology, Vol. 22, Glass III, eds., M. Tomozawa, R.H. Doremus (Academic Press, New York, 1982) p. 129. [21 Glasses and Glass Ceramics from Gels, Proc. 2nd Int. Workshop, J. Non-Cryst. Solids 63, Nos. 1-2 (1984). [3] C.J. Brinker, D.E. Clark and D.R. Ulrich, eds., Better Ceramics Through Chemistry (NorthHolland, New York, 1984) [4] R.K. ller, The Chemistry of Silica (Wiley, New York, 1979) p. 211. [5] D.L. Wood and E.M. Rabinovich, these Proceedings(Gels '85) J. Non-Cryst. Solids 82 (1986) 171. [6] D.M. Krol and E.M. Rabinovich, these Proceedings(Gels '85) J. Non-Cryst. Solids 82 (1986) 143. [7] C. McGrory-Joy, private communication. [8] P.K. Gallagher, Thermochim. Acta 82 (1984) 325; 26 (1978) 175. [9] E.M. Rabinovich, Phys. Chem. Glasses 24 (2) (1983) 54. [10] A. Beyer, A. Karpfen and P. Schuster, in: Hydrogen Bonds, ed.. P. Schuster (Springer, New York, 1984) p. 13.