Sintering of foamed silica gels

] O U R N A L OF

Journal of Non-Crystalline Solids 143 (1992) 133-139 North-Holland

NON-CRYSTALLINE SOLIDS

Sintering of foamed silica gels T a k a m i t s u Fujiu 1 a n d G a r y L. M e s s i n g Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA Received 9 July 1991 Revised manuscript received 6 December 1991

Foamed silica gels prepared from a colloidal silica containing sodium were leached to avoid c~-cristobalite crystallization during sintering. The sodium content in the foamed gels was decreased from 1.5 to 0.1 wt% by first drying at 80°C and then leaching in 5M nitric acid for 2 days at 40°C. When sintered at 1000°C for 4 h or ll00°C for 8 min, the leached foams did not crystallize, had a matrix density of 92%, and a foam bulk density of about 30%. Prior to densification, the foam consists of spherical cells of 100-150 txm diameter uniformly distributed in a matrix of 12 nm silica particles. The colloidal silica matrix densities according to Scherer's viscous sintering model. Due to their low surface curvature, cellular voids do not contribute to the sintering driving force and thus, bulk foam densification and cell size shrinkage can be calculated by applying Scherer's viscous sintering model.

1. Introduction

2. Experimental procedure

Low-density silica is finding increasing application because of its unique properties such as low thermal conductivity, low dielectric permittivity and low thermal expansion. Earlier we reported [1] a foaming process to fabricate lightweight silica and other ceramics [2,3] consisting of a bimodal pore distribution of macropores or ceils of 100-1000 ixm diameter distributed in a matrix of colloidal particles (fig. 1). For some applications removal of the fine or matrix porosity between the colloids is essential. The objectives of this paper are (i) to report on how sodium was leached from the bulk silica foams to avoid silica crystallization upon sintering, (ii) to characterize the shrinkage of the bulk foamed gel and large cells during sintering, and (iii) to model the shrinkage of the bulk foam and large cells as a function of matrix densification.

2.1. Leaching

x Present address: Nikon Corp., Sagamihara, Japan. * Ludox HS-40, E.I. duPont de Nemours, Wilmington, DE, USA.

As reported earlier [1], silica foams were produced from a commercial SiO 2 sol *, stabilized with 0.4% Na20, because of its rapid gelation characteristics. Unfortunately, alkalis promote asilica crystallization to cristobalite [4] and the subsequent loss of the desirable properties of the amorphous silica. To determine the best procedure for removing sodium, the foams were leached after three different stages of gel processing: (1) 12 h after completion of the foaming process (i.e., the wet gel in table 1); (2) after drying the SiO 2 gel foams for 1 week at 80°C; and (3) after heating the dried foam at 400°C. All gels were leached for 2 days by soaking a bulk foam of approximately 40 mm height and 35 mm diameter in 200 cm 3 of 5M nitric acid at 40°C. Since the foamed gels could float, they were weighted to keep them submerged in the leaching solution. After this leaching period, the gels were washed with running water for 10 min, rinsed in 200 cm 3 of deionized water at room

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T. Fujiu, G.L. Messing / Sintering of foamed silica gels

134

temperature for 10 h and then dried at 70°C for 2 days. The sodium content of the leached and unleached gels was analyzed by atomic absorption after dissolving the entire gel sample in hydrofluoric acid.

,~

•

2.2. Sintering Prior to sintering, all gels were leached (see below), dried and then heated at 400°C for 24 h. For the sintering studies, samples of approxi-

(a)

-+ J

- ~:,

<,o,;"

II

Fig. 1. Low and high magnification views of the foamed silica gel illustrating the macrovoids introduced by foaming (a) and the ultrafine structure of the gel matrix (b).

T. Fujiu, G.L. Messing / Sinteringof foamed silicagels Table 1 Effect of gel treatment on sodium concentration and crystallization at llO0°C after a standard leaching process Gel treatment before leaching Unleached gel Wet gel Dried gel (80°C) Heated gel (400°C)

Na concentration (wt%) 1.5 0.1 0.1 1.1

Crystallizationat 1100°C, 5 rain Yes No No Yes

mately one gram, cut from a bulk foamed gel, were preheated first at 500°C for about 10 min, and then immediately transferred to a horizontal tube furnace heated to the sintering temperature. The foam samples were isothermally sintered in air for periods of 2 min to 5 h at either 1000 or 1100°C. Typical loading and unloading times were less than 10 s. For this condition, the temperature fell about 25°C just above the sample but recovered to within 5°C of the set temperature in less than 30 s. Thus, the sintering time is not accurate for times less than 2 min.

2.3. Characterization The relative density and pore size distribution of the silica gel matrix were determined by mercury porosimetry. Foam samples were crushed into a coarse granular powder to avoid the inkbottle effect of the large cells. Approximately 0.4 g of the crushed powder was measured by scanning mercury porosimetry at 68 M P a / m i n to 410 MPa which corresponds to a minimum measurable pore size of 1.8 nm. From the apparent gel volume, including fine matrix porosity obtained by pycnometry and the volume of fine porosity as determined by porosimetry, the density of the silica glass phase was verified to be 2.2 g / c m 3. To determine the bulk density of the foamed gel, the sintered foamed gels were cut into ~ 2 mm thick disks with a diamond-coated wafering saw. The thickness and diameter of each disk was measured at four different positions and the sample bulk density was calculated from the average dimensions and weight. The average cell size was determined from SEM micrographs of sawed surfaces of sintered

135

foams. Three pictures were taken of each foam and more than three individually prepared foams were used for each foaming condition. The average diameter was calculated from ~ 100 cell measurements and multiplied by 1.5, assuming a random distribution of spherical cells [5]. Crystalline phases in the sintered silica gel were detected by X-ray diffraction at a scanning rate of 1°/rain and a time constant of 0.5. a-cristobalite was identified by the presence of its (101) peak. From X-ray diffraction of mixtures of 1 to 10 wt% cx-cristobalite and silica glass, the detection limit was determined to be ~ 3 wt% a-cristobalite. Usually ¢x-cristobalite was observed when samples crystallized but tridymite was also observed after heating for 24 h at 1000°C.

3. Results

3.1. Crystallization of foamed gels The silica gel matrix (silica particles plus ultrafine porosity) of the unleached foam was about 60% dense after sintering for 30 rain at 1000°C. One hour sintering at the same temperature led to extensive crystallization to cx-cristobalite. When the sintering temperature was increased to 1100°C, the (101) peak of a-cristobalite was observed after 5 rain and most of the major peaks of o~-cristobalite appeared after heating for 30 rain. Prior to crystallization, the silica matrix had a relative density of ~ 70% or less at either sintering temperature. Once the silica started to crystallize, the matrix showed little open porosity. The sodium concentrations after leaching are listed in table 1 together with the data for an unleached sample. Table 1 also lists the incidence of crystallization after heating these samples for 5 min at ll00°C. In the unleached sample, the sodium concentration of 1.5 wt% was higher than the 1.0 wt% calculated from the sodium content of the silica sol [6]. Leaching of the wet gel and the gel dried at 80°C reduced the sodium content by an order of magnitude to 0.1 wt%. These leached silica gels showed no crystallization after heating for 5 rain at 1100°C. The samples leached after 400°C heating had a sodium concentration

T. Fujiu, G.L. Messing / Sintering of foamed silica gels

136

100

&

,

A

Matrix D e n s i t y at 1100°C

/

80

co c

f

MaSx ty

preforms. Subsequently, the model was demonstrated to accurately predict gel densification [8,9] for a variety of silica gels. In this model the relative density, Ps/PO, of a sintered gel was related to the reduced time t r as follows:

60

tr = ._>

Foam Density . at 1100°C

40

rr 20

Z

0

i

0

FoamDensity at 1000°C I

1

,

I

2

-P0

~

i

I

i

3

Sintering time

I

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4

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i

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Fig. 2. Densification kinetics of the bulk silica foam an gel matrix at 1000 and ll00°C.

of 1.1 wt% which resulted in a-cristobalite formation when heated at ll00°C for 5 min. On the basis of these results, only the sintering behaviour of foams leached after 80°C drying was studied.

3.2. Sintering of leached foams The densification kinetics of the bulk foam and the silica gel matrix are shown in fig. 2 for 1000 and 1100°C sintering. After 24 h at 400°C, the foams had a bulk density of 15% and a matrix density of 42%. After 4 h at 1000°C, the foam bulk density increased to 32% while the matrix density increased to 92% without any detectable crystallization. After 8 min at ll00°C, the bulk density increased to 31% while the matrix density reached 92% without crystallization. Cristobalite formation was first detected by X-ray diffraction in samples sintered for 5 h at 1000°C or after 10 min at 1100°C.

4. Densification modeling

4.1. Matrix densification Scherer proposed a viscous sintering model [7] for the densification of low-density porous materials such as silica gels and flame-pyrolyzed SiO 2

('t - to) ,

(1)

where l 0 is the original length of the side of the cubic cell, Ps is the theoretical density, and P0 is the initial density of the gel. Values such as Ps, Po, Y and ~7 are either measurable or available from the literature. The value of l 0, is calculated from 1 2 urrd = (l 0 - 2 a ) 2,

(2)

where d is the pore diameter and a is the cylinder radius of a cubic cell. Given the relative density, P/Ps, the ratio a / l o can be calculated from the geometry-based relation

With this ratio and a measured value d for the gel, the original length of the side of the cell, 10, can be estimated. The fictitious time, to, can be determined if one set of experimental data for relative density and sintering time is known. The reduced time can be calculated from the relation x £ tr=

2dx

(3,rr_Sv~x)l/3X2/3'

\

(4)

where x is defined as a / l o. The relative density of the initial silica gel matrix (i.e., Ps/PO) in the kinetic study was 42% as shown in fig. 2. The cylinder-radius/cell-length ratio, a / l o, was calculated according to eq. (3) using the relative density data. An initial cell length, 10, of 25 nm was calculated from eq. (2) using the calculated value of a / l o and the pore diameter of 14 nm of the initial gel matrix. A surface tension of 280 d y n / c m was assumed for the silica gel [7,9,10]. Figure 3 shows the relation between the sintering time and the reduced time for sintering at 1000 and ll00°C, based on the matrix densifica-

T. Fujiu, G.L. Messing / Sintering of foamed silica gels

tion data in fig. 2. Since the reduced time is linearly proportional to the sintering time, it is concluded that the silica matrix of the foamed gel follows Scherer's gel densification model. It should be noted that the densification study was not extended to the region where a-cristobalite was first observed. In many of the previous studies, the reduced time versus sintering time relation has been used to estimate the viscosity of the silica at the sintering temperature as an additional test of its applicability [7,9,10]. Equation (1) shows that the slope in the reduced time versus sintering time plot is given by

From the slope in fig. 3 and eq. (5), it was determined that the viscosity of the silica gel was 4.1 × 1011 Pas at 1000°C and 1.2 × 10 l° Pas at 1100°C. Figure 4 compares the temperature dependence of the calculated viscosity relative to other

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Temperature " 1 (K- 1) Fig. 4. Viscosity of silica gel as a function of temperature calculated from Scherer's sintering model and compared with other data: (1) Johnson et al. [10], (2) Scherer and Bachman [8], (3: alkali free, 5: with sodium impurities) Zarzycki [11], and (4) Sacks and Tseng [9].

literature reports for silica gels. Zarzycki [11] used a high-temperature pressing method to obtain the viscosity of colloidal silica. The silica studied by Johnson et al. [10] was made from fumed silica, whereas Sacks and Tseng [9] and Zarzycki [11] prepared their silica gels from either Si(OCzHs) 4 or Si(OCH3) 4. Zarzycki [11] also used a colloidal silica which contained sodium impurities. Since the log ~/ versus 1/T plot for the foamed and leached gel silica matrix is greater than Zarzycki's gel containing sodium and similar to the gels presumably without alkali impurities, it is clear that the reduction in the sodium concentration due to the leaching significantly increased the viscosity of the silica gel matrix at these temperatures. The excellent agreement of our calculated viscosity data with previous viscosity reports further supports the characterization of silica matrix densification by Scherer's viscous densification model.

4.2. Bulk foam densification

~ e.o 1.8

,, ~ , r . , • , • 0

2 4 6 Sintering time

8 10 (min)

Fig. 3. Reduced time for gel matrix densification as a function of sintering time at 1000 and I100°C.

As the foamed gel consists of a silica matrix of < 10 nm pores and intentionally introduced large cells or macrovoids of > 100 p,m, the shrinkage behavior of the cells alone was examined to model the total shrinkage of the foamed gel. Because

T. Fujiu, G.L. Messing / Sintering of foamed silica gels

138

the cells are uniformly distributed in the porous silica matrix, they should shrink concurrent with the matrix and thus in accordance with Scherer's model. In this case, the foam density pf at time t is given by p p f = ~00Pfo , (6) where pro is the initial foam density, p is the matrix density as predicted from Scherer's model at time t and P0 is the initial matrix density. The predicted and measured shrinkage of the cellular voids are compared in figs. 5(a) and (b) for 1000 and 1100°C sintering, respectively. The theoretical line does not intersect the vertical axis at the initial foam density of 15% for the 1100°C data because of the uncertainty in the experimental data for sintering times less than 2 min. However, from the otherwise good agreement between experiment and calculation at longer sintering times, it is concluded that the cells in the silica matrix of the foamed gel shrink at a rate following Scherer's densification model. This result indicates that for the sintering conditions examined (i.e., high viscosity), the curvature of the cells is too low to induce any shrinkage. This result is not unexpected when one considers that the relative curvature difference between the macrovoids and the ultrafine pores between the colloids differs by at least four orders of magnitude.

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time

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From the above result, the change in the average cell size and the relative volume of the cells of the foamed gel during sintering can be calculated by simply assuming that all densification is a result of gel densification. If the cell shrinks at the same rate as the gel matrix, the cell size during sintering can be predicted from

D=Do

( P ° ) 1/3, p

(7)

where D is the cell size at sintering time t and D O is the initial cell size.

T = 1100°C

y

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-a 20

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,

Fig. 6. A comparison of calculated and observed cell size as a function of sintering time at 1000°C.

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T = 1000°C

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T = 1000°C

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Sintering time

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Fig. 5. A comparison of the calculated (solid line) and experimental foam sintering kinetics at 1000°C (a) and ll00°C (b).

T. Fujiu, G.L. Messing / Sintering of foamed silica gels

The maximum cell size reduction for the foamed silica in this study corresponds to only ~ 75% of the initial cell size as the initial matrix density is 0.924 g / c m 3 and the theoretical density is 2.2 g / c m 3. Thus, to avoid differences from foam-to-foam variation, the accuracy of cell size measurement was increased by cutting all samples from a single foamed gel for this part of the study. The average cell size was measured for the initial foamed silica and after sintering at 1000°C for either 3 or 5 h. As seen in fig. 6, there is good agreement between the predicted cell shrinkage and the experimental cell size changes during sintering, thus confirming the validity of the above conclusion concerning microporosity dominated shrinkage.

5. Summary Dense matrix silica foams were obtained by sintering foamed gels after leaching sodium from the initial colloidal silica. After sintering the leached gel at 1000°C for 4 h, no cristobalite formed, the matrix density reached 92% of the theoretical value and the overall foam density was 32% of the theoretical value. Similar results were obtained by sintering at 1100°C for 8 min. Because of the low curvature of the large cellular voids relative to the micropores in the gel matrix,

139

the driving force for shrinkage of the bulk foam is solely due to densification of the silica gel matrix which densified according to Scherer's viscous sintering model. Consequently, the cell size changes during sintering can be calculated by assuming the Scherer model.

References [1] T. Fujiu, G.L. Messing and W. Huebner, J. Am. Ceram. Soc. 73 (1990) 85. [2] R.D. Shoup, in: Ultrastructure Processing of Advanced Ceramics, ed. J.D. Mackenzie and D. Ulrich (Wiley, New York, 1988). [3] M. Wu, T. Fujiu and G.L. Messing, J. Non-Cryst. Solids 121 (1990) 407. [4] M. Wu and G.L. Messing, J. Am. Ceram. Soc. 73 (1990) 3497. [5] R.L. Fullman, J. Met. 197 (1953) 447. [6] Ludox Colloidal Silica: Properties, Uses, Storage and Handling, E.I. duPont de Nemours, Wilmington, DE, USA. [7] G.W. Scherer, J. Am. Ceram. Soc. 60 (1977) 236. [8] G.W. Scherer and D.L. Bachman, J. Am. Ceram. Soc. 60 (1977) 239. [9] M.D Sacks and T.Y. Tseng, J. Am. Ceram. Soc. 67 (1984) 532. [10] D.W. Johnson Jr., E.M. Rabinovich, J.B. MacChesney and E.M. Vogel, J. Am. Ceram. Soc. 66 (1983) 688. [11] J. Zarzycki, in: Nucleation and Crystallization in Glasses, ed. J.H. Simmons, D.R. Uhlmann and G.H. Beall (The American Ceramic Society, Columbus, OH, 1982).