Structural characterization of silica during sintering

NanoSTRUCTURED MATERIALS VOL. 1, PP. 149-154, 1992 COPYRIGHT@1992 PERGAMONPRESS Ltd. ALL RIGHTSRESERVED

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STRUCTURAL CHARACTERIZATION OF SILICA DURING SINTERING Jackie Y. Ying and Jay B. Benziger Department of Chemical Engineering Princeton University, Princeton, NJ 08544-5263 Introduction Colloidal sol-gel synthesis involves the dispersion, gelation and sintering of particulates [1-8]. A colloidal silica suspension (called a sol) is formed by blending fumed silica powder and water. The sol is cast at room temperature for gelation. Removal of volatiles and densification of the gel are accomplished through drying and heat treatments. Shrinkage occurs in this stage; bulk crack-free monoliths may be obtained with proper control. Sintering of the gel may be achieved between 1000°C and 1300°C, considerably lower than the melting point of fused silica glass. In this study the use of a binder and the choice of firing atmosphere on the sintering of colloidal silica gels were examined with thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and photoacoustic Fourier-transform infrared spectroscopy (PAS). Physical characterization of the shrinkage and surface area of the gel during sintering was also followed. To examine the molecular structure of colloidal sol-gels we have employed photoaconstic infrared spectroscopy. Conventional transmission infrared spectroscopy has limited utility in characterizing opaque powdered samples. PAS is capable of examining these materials without elaborate sample preparation. Rosencwalg and Gersho first demonstrated the spectroscopic application of the photoaconstic effect to solids [9]. They recognized that the absorption of modulated light would cause a periodic heating of a solid sample, which would cause a periodic pressure variation of the ambient gas in a confined volume. The photoaconstic signal generated is proportional to the absorption coefficient provided the product of the absorption coefficient and the thermal diffusion length in the solid is less than unity. Porous materials, such as silica and alumina powders, have thermal diffusion lengths less than 10-Sm. The small thermal diffusion length gives PAS a larger dynamic range than transmission methods when applied to powdered samples, as shown by Benzigcr and co-workers [10-12]. For this reason PAS is capable of examining strong lattice vibrations and absorption bands due to adsorbed species in the same experiment. Another advantage of PAS is the ease of sample preparation. Direct use of powder samples avoids contamination encountered with formation of mulls, or elaborate preparation of thin discs. PAS is also not sensitive to light scattering and reflection that deteriorate the signal to noise ratio or cause signal distortion, as in the case of transmission spectroscopy. Thus PAS is a unique and novel technique that allows ceramics or non-transparent sols to be studied far more conveniently and with greater sensitivity than conventional infrared spectroscopy. Exverimental Colloidal silica was made from dispersing 10 g fumed silica (Sigma, 255 mZ/g) in 90 g deionized water. No binder was added to sample 1; 0.4 g polyvinyl alcohol (PVA) (Sigma, average MW = 10,000) was added as a binder to sample 2. The slurried silica was blended with added grinding media on a roller mill for a day. The resulting sol was cast in open-ended polypropylene syringes sealed with parylene films. Gelation occurred in approximately 2 days for sample 1 and in 3 hours for sample 2. The gels were allowed to sit until rigid and then were pushed out of the syringe. The cylindrical gel was dried on a matting enclosed in a drying box, where drying was slow and uniform. This casting technique allowed a sample to be removed from its mold as soon as gelation took place, so that shrinkage during drying could be achieved uniformly outside the east, greatly reducing the risk of cracking due to non-uniform shrinkage. 149

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After two weeks of drying at room temperature the gel was heated in oxygen or in nitrogen to remove residual water and burn out the binder and to be densified to the final ceramic. Heat treatment included heating to 200°C, 500°C, 800°C, 1000°C, and 1150°C at a ramp rate of 20°C/hr, soaking for 6 hours before ramping to the next temperature. Thermal gravimetric analysis was done on a Perldn Elmer System 4 Thermogravimetric Analyzer. Differential Scanning Calorimetry was clone on a Sctram DSC l l l G twin microcalorimeter. PAS-FTIR spectra of heat treated powdered samples were obtained with a Bomem DA 3.002 FTIR spectrometer using a specially built photoacoustic cell [13]. The interferometer assembly has been modified to provide acoustic isolation from both building and airborne noise [10]. Spectra were obtained over frequencies of 450-4000 era-1 at 4 cm-1 resolution. The raw spectra were normalized with reference to the blackbody spectrum of graphite powder. Surface areas of the treated samples were obtained with a Quantasorb Surface Area Analyzer using a single point BET method at P/Po = 0.3. Results Thermal gravimetric analyses of the two sample are shown in Figure 1, which show distinct differences. For sample 1 the major weight loss occurred in the range 50-200°C due to the vaporization of water. This is further illustrated in Figure 2, which shows the DSC results where an endotherm occurs at 100°C due to water evaporatio~ After the initial weight loss the TGA shows minimal additional weight loss and the DSC results show a sloping endotherm due to the heating of the sample. For sample 1 the TGA and DSC results show negligible dependence on the firing atmosphere. The silica gel prepared with a PVA binder, sample 2, is affected by the choice of firing atmosphere. First in the TGA results one sees a two step weight loss occurring at 10(FC and 350°C. In an inert (Nz or Ar) atmosphere the first weight loss at 100°C is not affected by the atmosphere. However, one sees less weight loss at 350°C in an inert atmosphere than in an oxidizing atmosphere. After the second step change in weight loss there continues to be a slow weight loss occurring up to higher temperatures. Here the weight loss is similar to that seen for the sample without binder. The total weight loss for the gel with the PVA binder is only about half the weight loss observed for samples without binder. This indicates that much less water is incorporated into the gel that has a PVA binder. The DSC results for the sample with PVA binder show the first weight loss at 100°C is an endothermic process, similar to the result for a sample without binder. The weight loss at 350°C in an oxidizing atmosphere occurs with a large exotherm, indicative of the combustion of the organic binder. The exotherm is a two step process with maxima at 300 and 500°C. When the gel with the PVA binder is fired in an inert atmosphere one sees two small endotherms, presumably from pyrolysis of the organic binder. 100

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Figure 2. Differential scanning calorimetry of colloidal silica gels (a) without binder (b) with polyvinylalcohol binder. To elucidate the molecular changes associated with the drying and sintering processes photoacoustic spectra of the gels were obtained as green bodies and after heating to 200°C, 500°C, 800°C, 1000°C, and 1150°C, In Figure 3 the sequence of PAS spectra for the gel with and without binder are shown. Assignraents of vibrational modes are shown in Table 1. Table 1 Vibrational Modes for Silica Gels Wavenumber (cm -z) 625 760-880 960-1280 1630 1860 1970 2800-2960 3200-3550 3300-3500 3730

Assignment cristobalite ring deformation SiO2 stretching SiO2 phonon bands water bending and SiOz overtone SiO2 overtone SiOz overtone SiO2 overtone H-bonded OH stretch of PVA H-bonded OH stretch of HzO free SiOH stretch

The PAS spectra of the silica gel without binder fired in oxygen are shown in Fig. 3. The greenbody shows a broad band between 3200-3600 cm-1 resulting from the adsorbed water. Heating to 200°C is seen to result in this peak being diminished in size due to removal of water, confirming the interpretation of the TGA and DSC results. Further heating to 500°C, 800°C and 1000°C resulted in the further diminution of the absorption peak due to water. As the water is seen to diminish the absorption band due to isolated OH increases in intensity for heat treatments up to 800°C; at 1000°C the OH absorption has decreased and at 1150°C the OH absorption is totally gone. The total PAS intensity was seen to decrease after firing to 115~C, indicative of densification. [McGovern et al showed that the photoacoustic signal for solids is dominated by interstitial gas expansion in porous materials [14], such that densification results in a decrease of the PAS signal.] The PAS spectra for the silica gel without binder fired in an inert atmosphere was identical to that fired in oxygen for temperatures up to 1000°C. However, the sample fired in an inert atmosphere to 1150°C did not show any evidence of densification. The band associated with isolated OH groups was not significantly diminished and the total PAS signal was also not diminished. This suggests that oxygen facilitated the sintering process of the silica. Also shown in Figure 3 are the PAS spectra for the silica gel with a PVA binder as a green body and after firing to 1150°C in air. The greenbody with a binder is distinguished from that without the binder in that there is little or no evidence in the infrared spectra at 3730 cm "1 of isolated hydroxyls in the green body with PVA binder. The binder in the gel gives rise to CH stretching features at 2900 cm -z. Even after heating to 200°C which removed the water there is no evidence for isolated hydroxyls in the PAS spectrum. After heating

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to 500°C the absorption band at 2900 cm"l.d~e to the PVA disappears and the absorption band associated with the isolated hydroxyls app~L, at 3730 cm" . The spectrum for the silica with PVA fired to 1150°C shows a distinct feature at 625 cm" that was not seen for the silica without the PVA. This feature is characteristic of six membered rings in crystalline silica materials such as cristobalite. The formation of cristobalite was verified with x-ray diffraction. The total PAS intensity for the sample with the PVA binder and fired to 1150°C was greatly diminished indicating the densification of the sample.

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4000 Wavenumber (cm) Figure 3. Photoacoustic spectra of silica gels (a) gzcenbody of silica gel without binder, (b) heated to 200°C in 02; (c) heated to 800oc in 02; heated to 1150oc in 02; (e) gmenbody of silica gel with PVA binder, (f) heated to 200°C; (g) heated to 1150oc in 0 2 .

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The effect of the fring atmosphere did not have a large effect on the photoacoustic spectra of the silica gels with the PVA binder. The spectra were nearly identical at all firing conditions. Even the sample fired in an inert atmosphere to 1150"C showed a diminished PAS intensity and the formation of cristobalite. In contrast to the silica without binder the sample with the PVA binder was found to densify in an inert atmosphere. The decreased photoacoustic signal from the sintered samples indicated densification. Physical characterization of this densification was done by volume shrinkage and surface area measurements. The results of those measurements are summarized in Table 2. Table 2 Sample Volumes and Surface Areas Volume (em 3)

Sample 1 BET area (m2/g)

Sample 2 Volume BET area (crn3) (m2/g)

Gel Greenbody

1.21 0.46

255

1.21 0.39

235

@2000C in O2 @500°C in O2 @800°C in O2 @1000°C in Oz @ll5&C in O2

0.43 0.39 0.38 0.38 0.17

254 254 247 199 37

0.23 0.23 0.23 0.14 0.06

234 240 211 108 0.4

@200°C in Nz @500°C in N2 @8000C in Nz @1000*C in N2 @1150°C in N2

0.41 0.43 0.39 0.37 0.28

261 254 243 220 156

0.23 0.22 0.21 0.16 0.07

243 235 220 145 0.3

The physical data given in Table 2 corroborate the photoacoustic results. Both the volume change and decrease in surface area show the densifcation occurred to a greater extent with the addition of a PVA binder. Also, the results show that densification of gels without binder occurred to a much greater extent in an oxygen atmosphere than in a nitrogen atmosphere. Discussion This study shows that photoacoustic spectroscopy can be combined with standard thermal analysis techniques and physical measurements to provide insight into the molecular processes occurring during sintering. In particular, the results presented here suggest that surface species are intimately related to the gel structure and the sintering processes. The thermal analysis results demonstrate that a silica gel with PVA binder differed from a silica gel with no binder by showing two processes occurring during drying and sintering. These two steps were the water removal and the subsequent removal of the organic material. The PAS infrared spectra showed that the PVA binder resulted in the disappearance of the isolated surface hydroxyl groups. This suggests that the PVA interacts strongly with the hydroxyl groups on the silica surface. Such an interaction could be expected based on the hydrogen bonding interactions between surface hydroxyls and water or alcohols. The polymer aided in gelation by tethering particles together, with the polymer anchored onto the silica particle by hydrogen bonding to the surface hydroxyls. As the surface hydroxyl groups were associated with the PVA molecules they were precluded from hydrogen bonding with water. This resulted in much less water being adsorbed on the silica gels with the PVA than the gels without a binder. Once the binder was removed during firing to 500°C the photoacoustic spectra indicated that the number of hydroxyl groups was nearly the same on gels with and without PVA.

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The photoacoustic spectra of the materials fired in different atmospheres also suggested that reactions of surface species are critical in the sintering process. The colloidal silica without binder showed that the densificaton (indicated by both decrease in volume and decrease in surface area), was correlated with the concentration of surface hydroxyl groups. In an oxidizing environment the hydroxyl groups were reactive with the oxygen and sintering was promoted. We believe that this results from the oxygen reacting with surface liberating surface hydroxyls that promotes mobility of surface species such as Si-OH that cause sintering. The thermodynamic driving force for the reaction between SiOz and N2 is not favorable so nitrogen does not promote sintering, Some recent results with the addition of water vapor to the sintering atmosphere have shown that increasing the hydroxyl concentration on the surface promotes sintering of colloidal silica gels [15]. Conclusions The drying and sintering of colloidal silica gels were studied with thermal analysis, physical characterization and photoacoustic infrared spectroscopy. It was found that water is removed from a colloidal silica without any binder near 100°C and sintering occurs at 1150°C in an oxidizing atmosphere. Minimal sintering of the gel occurred in an inert atmosphere at 1150°C. When polyvinyl alcohol is used as a binder the resulting gel is densified in nitrogen atmospheres as well as oxygen atmospheres at 1150°C. Photoacoustic spectra shows that the gel surface is covered with binder so that isolated OH groups are not observed until the binder is burnt out. By reinforcing the gel with hydrogen bonding, polyvinyl alcohol holds the structure more tightly together, enabling the silica gel to dry faster and sinter more easily and more completely. However, the PVA binder also promoted crystallization of the silica gels resulting in cristobalite formation. These studies illustrate the application of photoacoustic spectroscopy to elucidate the molecular changes associated with sintering. Acknowledgements Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of this research. One of us (JYY) acknowledges fellowships provided by the AT&T Foundation and the General Electric Foundation. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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