Structure tailoring of alkoxide silica

]OURNA

Journal of Non-Crystalline Solids 147&148 (1992) 222-231 North-Holland

L OF

NON-CRYSTALLINE SOLIDS

Structure tailoring of alkoxide silica Jackie Y. Ying and Jay B. Benziger Department of Chemical Engineering, Princeton University, Princeton, NJ 08544-5263, USA

This study examines the influence of pH in the starting sol on the structure of alkoxide derived silica. The relative rates of hydrolysis and condensation of the alkoxide were altered by pH which yielded materials with different microstructure, stability and sintering behavior. Structural evolution of alkoxide silica during densification was followed in situ by photoacoustic Fourier-transform infrared spectroscopy (PAS). The acid-catalyzed silica gels contained a larger amount of adsorbed water and alcohol than the base-catalyzed gels/precipitates. Hydrogen-bonded surface species in acid-catalyzed gels were removed easily, allowing this material to be densified at 800°C. Although base-catalyzed gels had less hydrogenbonded silanol groups in the green state, it consisted of free silanol groups which maintained a high gel surface area and indicated little sintering below 1000°C.

1. Introduction

structural and energetics studies of other processing factors are presented elsewhere [18,25].

The synthesis of silica ceramics and glasses by the sol-gel technique has wide potential applications [1-10]. Silica gels may be produced either from colloidal dispersions, or by hydrolysis and polymerization of silicon alkoxides. In the alkoxide route [11-18], the sol is catalyzed with an acid catalyst a n d / o r a base catalyst. Gelation occurs irreversibly through the following chemical reactions [6,19,20]: - S i - O R + H 2 0 --+ - S i - O H + ROH,

(1)

-Si-OH + HO-Si- + -Si-O-Si- + H20 ,

(2)

- S i - O H + R O - S i - --+ - S i - O - S i - + ROH,

(3)

where R is an alkyl group (e.g., C2H 5 and CH3). Various factors affect the molecular structure and the extent of reaction, including the composition of the sol (water:alkoxide ratio and alcohol : alkoxide ratio), the catalyst (and pH) used in the reaction, and temperature and time of reaction [21-24]. The focus of this report is on the effect of catalyst. Our objective is to gain a better understanding of how pH of the starting sol affects subsequent processing, by relating the molecular structural development of the gels to their microstructural development. Detailed

2. Experimental Silicon alkoxide derived sols were produced by mixing 4 mol water and 1 mol tetraethoxysilane (TEOS). Four samples were prepared with the same sol composition but different catalysts at 50°C. Sample 1 was acid-catalyzed with 0.400 g of concentrated HC1 (12M). A clear sol of pH = 2.05 was obtained which gelled within 36 h. A two-step catalyzed gel, sample 2, was produced by adding 0.400 g of concentrated HC1 (12M) to the starting sol. After a one-phase solution was obtained upon stirring for 60 rain at 50°C, 27.0 g of 0.1N NH4OH was added drop by drop to bring the sol pH from 2.05 to 2.62. The clear sol gelled in 16 h, more quickly than sample 1 sol. Getation in sample 3 was catalyzed by the addition of 27.0 g of 0.1N NH4OH , resulting in a sol with pH = 7.73. The resulting sol was translucent and gelation occurred after 4 days. A stronger base of 0.500 g of concentrated NHgOH (28% NH 3) was used to precipitate sample 4 from a sol of pH ~ 12. Sols were poured into cylindrical polyethylene tubes to be gelled. After gelation occurred, gels

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

223

J.Y. Ying, J.B. Benziger / Structure tailoring of alkoxide silica

were dried at room conditions in the loosely capped tubes. This was done to allow removal of volatiles at room t e m p e r a t u r e to take place slowly so as to prevent cracking of the gels. Greenbodies of samples 1 - 4 were obtained after 1 month of drying. They were heated in oxygen to remove residual water and organics, and for densification. Thermal gravimetric analysis ( T G A ) was performed between 30 and 1000°C at 10°C/min in flowing oxygen on P e r k i n - E l m e r System 4. A Setaram DSC 111G twin microcalorimeter analyzed greenbodies between 30 and 800°C in flowing air at a scan rate of 5°C/min. Molecular structure of alkoxide silica gels was examined by photoacoustic Fourier-transform infrared spectroscopy (PAS). The h o m e - m a d e photoacoustic cell [26] provides for in situ sample degassing and heat treatments. PAS spectra were collected for the mid-infrared range of 450-4000 cm -a [18]. X-ray diffraction patterns were obtained with a Scintag Pad V Diffractometer for 2 ° < 20 < 98 ° at a scan rate of 1°/rain. Singlepoint B E T surface area of heat-treated samples

were obtained with Q u a n t a c h r o m e ' s Quantasorb Analyzer.

3. Results

The T G A curves are shown in fig. 1. Weight loss in all samples occurred rapidly in the first two stages of drying. The first stage (30-200°C) was attributed to removal of adsorbed and absorbed water. The second stage of weight loss (200-600°C) was associated with organic burnout. After the first two stages of substantial mass decrease, a much more gradual loss in weight was observed with removal of residual hydroxyl groups. Samples 1 and 2 had similar T G A curves. They had a total weight loss ( ~ 14%) greater than samples 3 and 4. T G A results of the latter were alike, involving an overall loss in weight of 7.7%. Figure 2 illustrates the DSC curves in air. The endotherm centered at 100°C was due to evaporation of a d s o r b e d / a b s o r b e d water. This peak corresponded to the first stage of weight loss in

(cO 95

(a)

t®

90

85 1 0

'

'

46o

'

66o

'

86o

Temperature [°C] Fig. 1. TGA curves of (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4.

1000

J.Y. Ying, J.B. Benziger / Structure tailoring of alkoxide silica

224

[0.2 _J

-'~.~_

(a)

£xo

........................................................................................... .........2.Z..~ ................... ~........... .t....................

O 0. .....

..............

..........

.t

....................

Exo . . . . .

0

. . . . . . . . .

'

. . . . . . . . . . . .

~

'

4()0

'

. . . . . . . . . .

6()0

. . . . . . . . . . . . . . . . .

'

. . . . . . . . . . . . . . . . . . . .

8d~)O

'

1000

Temperature [°C] Fig. 2. DSC curves of (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4.

T G A . The endotherms for samples 1 and 2 were similar and larger than those for samples 3 and 4. At higher temperatures, samples 1 and 2 had a large two-step exotherm centered at 300 and 410°C due to combustion of organic species. However, the heat releases in sample 2 were

smaller than sample 1. Sample 3 gave rise to a minor exotherm at 275°C and a large exotherm at 430°C. The latter exotherm has a shoulder tailing to 600°C. Sample 4 had a minor exotherm at 257°C and a broad exotherm at 490°C. Its main exotherm was smaller than that of sample 3, and

Table 1 Assignment of bands in photoacoustic spectra of samples 1-4 Wavenumber

Observed in sample

(cm-1)

1

2

3

4

470 800 960-1280 970 1490 1620 1640 1870 1960 2860 3200-3450 3400-3500 3540 3750

x x × x × × x x x x × × ×

× x × × × × x x x x x × ×

× × × × x × x x x x X x × ×

× × x × × x x x x x X × × ×

Assignment

Ref.

O - S i - O bending symmetric S i - O - S i stretching antisymmetric S i - O - S i stretching S i - O C 2 H 5 stretching C - H deformation vibration bending of molecular H 2° SiO 2 overtone SiO2 overtone SiO 2 overtone C - H stretching H-bonded O H vibration of alcohol H-bonded H 2 0 H-bonded - S i O H stretching free surface - S i O H stretching

[27] [25] [27] [28] [29] [30] [30] [30] [30] [29] [29] [31] [31] [31]

J.Y. Ying, J.B. Benziger / Structure tailoring of alkoxide silica

(a)

H-bonded O-H

225

Si-OC2H5 Si-O-Si

Green

200 °c sioa sio2 e-

.o_ 5 0 0 °C

H-bogded

-i

0 o 0 0 r13-

O-Si-O

~0

I 800 °c

H-bonded

0

z

-SiOH

~ooo °c

]

4000

I

i

2800

I

i

1600

400

Wavenumber [cm-~l Fig. 3. (a) Photoacoustic spectra of sample 1 through various heat treatments.

it occurred at a higher t e m p e r a t u r e with a lowt e m p e r a t u r e shoulder. This broad exotherm extends between 300 and 600°C. The assignments of the infrared absorption peaks are summarized in table 1. The greenbody of sample 1 (fig. 3(a)) showed an O - S i - O bending m o d e at 470 cm 1, and a symmetric S i - O - S i stretching at 760-880 cm -1. The feature at 970 cm 1 was due to Si-ethoxide stretching. The strong phonon band centered at 1090 and 1180 cm -1 was from antisymmetric S i - O - S i siloxane stretching. Deformation vibration and stretching of C - H gave rise to the features at 1490 and 2860 cm -1, respectively. The absorption at 1620 cm -1 came from bending of molecular H 2 0 . SiO 2

overtones were found at 1640, 1870 and 1960 cm 1. Hydrogen bonding between adsorbed species and surface species resulted in broad bands for the O H stretching features ranging from 3200 to 3550 cm-1. The alcoholic hydroxyl groups ranged from 3200 to 3450 cm -1. This range overlapped with molecularly adsorbed water which had strong adsorption features between 3400 and 3500 cm-1. The silica surface is terminated primarily with silanol ( S i - O H ) groups. Free silanol groups (those not hydrogen-bonded to adsorbed water or with neighboring silanols on the surface) give a sharp O H absorption feature at 3750 cm -1. Hydrogen bonding weakens the O H bond and shifts the feature due to the surface

J.Y. Ying, J.B. Benziger / Structure tailoring of alkoxide silica

226

(b)

H-bonded

Si-OC,~

O-__~H

Si-O-Si I ~[H20I f ~ ' ~ . . ~

11 Green

200 °c

Si02Si02 I~ - / 0O-S ¢-

(D o

0

.-OooOeO

0

H-bonded

0 c~ 0

z

O0°C

~

800 *C

_j ~ ~ " 4000

I

lOOO%

L

I

J

2800

Wavenumber

I

1600 [cm-ll

I

I

400

Fig. 3. (b) Photoacoustic spectra of sample 2 through various heat treatments.

silanols to 3540 cm -1. In sample 1, only hydrogen-bonded * silanol groups were found. The sharp peak due to free silanol groups was not detected in the greenbody. As the greenbody was heated to 200°C, Siethoxide groups were removed and siloxane backbone was consolidated. With the removal of water at this temperature, the water bending mode at 1620 cm -a was diminished and the O H stretching at 3400 cm-1 was also reduced. There was also evidence for removal of adsorbed * Hydrogen-bonded species referred to adsorbed and surface species which were bonded to another through hydrogen bonding.

ethanol, the C - H stretching feature at 2860 cm-1 was reduced and the C - H bending feature at 1490 cm-1 was also diminished. Upon heating to 500°C, PAS spectrum indicated less organic residue for the gel. The residual C - H features above 500°C were due to both remaining ethoxy groups and adsorption from the atmosphere during sample transfer from furnace to PAS cell. A relatively pure silica glass with little hydrogenbonded species was obtained at 800°C. The photoacoustic signal in the S i - O - S i phonon band became weaker due to densification of the gel network. When pores collapsed during sintering, signal from interstitial gas expansion was decreased [32]. Thus, the microstructural changes of

J,Y, ]ring, J.B. Benziger / Structure tailoring of alkoxide silica

(C)

227

Si-O-Si /~Si,..,OC2H s ~ 1

-

Green

I

c:.

/

l!

/

200 °C

o-s,;o

. _o

500 °C

0 (J

H-bonded

H~

0 en (11 N

800 °C

0

z

H- bonded -SiOH 1000 °(3

4000

]

I

J

2800

Wavenumber

[

1600 Icm -1]

i

I

400

Fig. 3. (c) Photoacoustic spectra of sample 3 through various heat treatments.

the gel were reflected in the silica PAS phonon band. As the material was heated to 1000°C, almost all impurities were eliminated. The greenbody PAS spectrum of sample 2 (see fig. 3(b)) was almost identical to that of sample 1, except that the former had less Si-ethoxide stretching at 970 cm-1. Heating to 200°C diminished the H-bonded broad band and the molecular H 2 0 bending mode. C - H deformation vibration disappeared, and C - H stretching was less for sample 2 than sample 1. With the removal of alkoxy groups at 200°C, the S i - O - S i phonon band and overtone became more intense, and greater consolidation of the silica backbone was detected in sample 2 than sample 1. Further heat treatment removed more hydrogen-bonded species.

Above 500°C, the spectrum of sample 2 was similar to that of sample 1, except that the former showed more silanol groups. Like sample 1, sample 2 was densified at 800°C. Sample 3 greenbody (fig. 3(c)) had less adsorbed water and organic impurities than samples 1 and 2. Some unreacted silicon alkoxy groups were present in sample 3. A sharp absorption at 3750 cm -1 not found in samples 1 and 2 indicated the presence of free - S i O H species in sample 3. The phonon band at 960-1280 cm 1 was much sharper and more intense in sample 3 than in samples 1 and 2. As sample 3 was heated to 200°C, hydrogenbonded species diminished while free silanol groups emerged. C - H stretching and deforma-

J.Y. Ying, J.B. Benziger / Structure tailoring of alkoxide silica

228

(d)

Si-O-Si S[-OC2H5 H-bonded

O-H

Green

~IH2OI Si-O-Si 200

Si02

¢E~

I;,

C-H

0 o cO 0

500 °C

H-bonded H20

O-Si-

"5 r-

Q_ "13

E

800 °c

free -SiOH

O

z

J H-bonded-SiOH 1000~

]

4000

=

I

=

2800

Wavenumber

I

1600 [cm-ll

J

I

400

Fig. 3. (d) Photoacoustic spectra of sample 4 through various heat treatments.

tion vibration decreased with the removal of organic residue. O H bending mode disappeared with the departure of adsorbed water. Si-ethoxide stretching decreased, while siloxane phonon band became more intense. Heating to 500°C removed more hydrogen-bonded groups. There were fewer unreacted silicon alkoxy groups, and the Si-ethoxide peak appeared as a shoulder on the growing phonon band. Upon heat treatment to 800°C, alkoxy groups were finally eliminated. Some hydrogen-bonded species were removed while the free silanol species remained unchanged. Unlike samples 1 and 2, sample 3 showed no signs of sintering at 800 or 1000°C. Sample 4 greenbody (fig. 3(d)) had more hydrogen-bonded species than sample 3; but less

than samples 1 and 2. It had an even more intense phonon band with less unreacted silicon alkoxy groups than sample 3. Heating to 200°C removed some hydrogen-bonded impurities and decreased the O H bending mode. C - H stretching decreased and C - H deformation vibration became undetectable. Little unreacted silicon alkoxy groups were left in the gel, and the silica phonon band and overtone were intensified. Heating to 500°C left sample 4 with fewer hydrogen-bonded species than sample 3, and no more alkoxy groups. At 800°C, hydrogen-bonded impurities were further reduced, while surface silanol groups appeared as a sharp peak at 3750 cm -1. Sample 4 had fewer hydrogen-bonded and free silanol groups than sample 3 at this temperature. When

ZY. Ying, ZB. Benziger / Structure tailoring of alkoxide silica Table 2 BET surface area of samples 1-4 (m2/g) Heating temperature

4. Discussion

Sample 1

Sample 2

Sample 3

Sample 4

517.0-t-2.2 110.3+3.8 0.4+0.1 0.4+0.1

657.4+9.6 327.6_+5.0 0.4+0.1 0.3+0.1

318.1+8.8 305.6+4.8 211.6+1.7 150.8+4.4

190.5+2.0 156.5+8.6 117.5+0.3 67.1+0.2

(°c) 200 500 800 1000

229

heated to 1000°C, some hydrogen-bonded and free silanol groups were removed, but the sample did not densify. The surface area results are summarized in table 2. Samples 1 and 2 heated to 200°C in 0 2 had surface areas of (517.0 _+ 2.2) and (657.4 ± 9.6) m2/g, respectively. However, the high surface areas decreased rapidly with heating such that samples 1 and 2 lost essentially all surface area by 800°C. After heating to 800°C, the bulk densities of the cylindrical samples 1 and 2 were found through volume and mass measurements to be (2.10 _+ 0.05) g / c m 3. Helium pycnometry experiments (Micromeritics Multivolume Pycnometer 1305) also indicated that skeletal (true) densities of samples 1 and 2 were (2.16 ± 0.02) g / c m 3 and (2.18 + 0.02) g / c m 3, respectively. When the two samples were heated from 800 to 1200°C, their bulk and skeletal densities were increased slightly to the theoretical density of fused silica glass (2.20 g/cm3). These physical characterizations confirmed the PAS findings that significant elimination of porosity ( > 95%) has occurred at 800°C. Base-catalyzed samples, especially sample 4, had smaller surface areas than samples 1 and 2 at 200°C. However, their surface areas were reduced very slowly with heat treatment. At 1000°C, there were still (150.8 + 4.4) and (67.1 ± 0.2) m 2 / g left in samples 3 and 4, respectively, indicating that sintering had not taken place. X-ray powder diffraction patterns of samples 1-4 indicated that all samples remained amorphous at 1000°C.

In this study, parameters other than catalyst (including water : alkoxide ratio, alcohol : alkoxide ratio, and temperature of reaction) were carefully held as constants for all four samples. This enabled us to determine the catalyst's unique contribution to the changes in the rate and the extent of hydrolysis reaction in eq. (1). Hydrolysis occurs by electrophilic substitution in the presence of an acid catalyst, and by nucleophilic substitution with a base catalyst [12]. Under acidic conditions, the first hydrolysis causes subsequent hydrolysis of the same unit to be more difficult. As a result, linear chain growth is favored relative to branching in the gel structure [19]. Consequently, the phonon bands for the silica are much different from fused silica glass, and less intense than the base catalyzed gels. In sample 2, although both acid and base catalysts were used, the acid catalyst was added to the sol first; therefore, it controlled the hydrolysis mechanism. When the base catalyst was subsequently added, it mainly catalyzed the condensation of the previously hydrolyzed species. Thus, TEOS was more completely reacted in sample 2. The fewer Si-ethoxide groups in sample 2 allowed its silica backbone to undergo initial consolidation at a lower temperature than sample 1. Also, more surface silanol groups were present in sample 2 from the reaction of base catalyst, giving it a higher surface area. In the base-catalyzed sols, subsequent hydrolysis of the same unit is easier. Therefore, samples 3 and 4 developed into three-dimensional crosslinked network gels or precipitates, with larger primary particles and longer range of S i - O - S i order. The larger particles were evident in the vibration spectra of the silica. The phonon bands were stronger and similar to fused silica glass and fumed silica particles [25,33,34]. Since sample 4 was catalyzed with a stronger base, its greenbody had a more intense phonon band and less unreacted silicon alkoxy groups than sample 3. The gel structure for samples 1 and 2 consisted of greater pore volume, providing a higher surface area for adsorption. Since volatiles were trapped in the small pores, samples 1 and 2 still had a large amount of hydrogen-bonded species

230

J.Y. Ying, J.B. Benziger / Structure tailoring of alkoxide silica

after drying. Volatiles present in the pores also permitted less interstitial gas expansion to take place, contributing to the weaker photoacoustic signal. The adsorbed volatiles were however easily removed by heat treatments to allow significant pore collapse by a low temperature of 800°C. The network gel of sample 3 had a smaller surface area and pore volume so that less volatiles were trapped in it. The emergence and preservation of free silanol groups in this sample illustrated that densification did not occur below 1000°C. The bigger pores or channels between the larger primary particles made sintering more difficult in sample 3 compared with samples 1 and 2. The large aggregated particles of sample 4 precipitated from the high pH sol did not have an extensive pore structure like samples 1-3. However, volatiles were adsorbed on the powder surface and in the pores between coagulated particles. This accounted for more hydrogen-bonded species in sample 4 than sample 3, although the latter had a greater surface area. The more open nature of a precipitate allowed removal of volatiles more readily from sample 4. While much hydrogen-bonded species were gone at 800°C, free silanol groups emerged and there was little sintering under 1000°C.

5. Conclusions

The molecular structure and microstructure of alkoxide gels were greatly affected by the catalysts used in sol-gel processing. Basic solution gave rise to more branching in the silica backbone, producing a longer persistence of S i - O - S i order. The larger primary particles in base-catalyzed gels could only be packed loosely to yield lower green surface areas. However, free silanol species were present in these samples, indicating that insignificant densification took place below 1000°C. Significant surface areas could be maintained at high temperatures so that thermally stable catalytic supports and adsorbents might be synthesized from these samples. The more porous gels obtained from acid catalysis trapped larger amounts of hydrogen-

bonded species in their greenbodies. However, the adsorbed species were easily removed to allow the small pores to collapse. Acid-catalyzed gels annealed at 784°C were shown to be as stable as fused silica glass [18,35]. The low-temperature sinterability of these samples suggests that acidcatalyzed and two-step catalyzed aIkoxide gels would be a less expensive alternative to making high-purity silica glass. Acknowledgement is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society for partial support of the research. J.Y.Y. acknowledges fellowships provided by the A T & T Foundation and the General Electric Foundation.

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