Properties of silica gels prepared from high-acid hydrolysis of tetraethoxysilane

Ceramics hlternational 19 (1993) 315-325

Properties of Silica Gels Prepared from High-Acid Hydrolysis of Tetraethoxysilane Kuo T u n g C h o u & B u r t r a n d I. Lee* Department of Ceramic Engineering, Clemson University, Clemson, South Carolina 29634-0907, USA (Received 14 September 1992; accepted 22 October 1992)

A b s t r a c t : The gelation behavior and properties of silica gels prepared by hydrolytic polycondensation of tetraethoxysilane (TEOS) with large amounts of nitric acid as the catalyst have been studied. The molar compositions were TEOS: H20: HNO 3 = 1 : 10:x (x = 0' 1-1-0). The effects of the acid content, drying temperature and mixing rate were investigated. It was found that the gelation time decreased with increasing amount of acid and increasing gelling temperature. In the presence of a large amount of nitric acid, the gels had higher specific surface areas and higher pore volumes as the drying temperature increased from 45°C to 60°C. A higher drying temperature at 120°C led to an increase in pore volume, whereas it led to a decrease in specific surface area and density with an increase in acid content. Increasing the drying temperature is more effective than increasing the acid content in order to increase pore size.

1 INTRODUCTION

hypercritical drying m e t h o d in an autoclave. Orcel & Hench s made gels by using formamide as a drying control chemical additive (DCCA). Others used N, Nodimethylformamide 6 or glycol 7 to reduce the tendency of the drYing cracks. Another method of making crack-free gels was reported by Sakka et al., 8 who showed that o p a q u e gels with large pores and precipitates could be obtained by the hydrolysis of tetramethoxysilane (TMOS) with a small amount of water and a large a m o u n t (more than catalytic) of hydrochloric acid. Preparation o f the silica sol by direct hydrolysis o f T M O S 9 or T E O S 7' 1o- 12 with acidic water has been shown to be possible without addition o f an alcohol. The alcohol is a by-product of the hydrolysis and polymerization reactions. Hence, combination of solventless and high-acid-hydrolyzed TEOS appears to be a promising m e t h o d o f achieving a high-silicacontent sol and o f preparing crack-free gels. It is k n o w n that the structures of sol-gel polymers at gelation influence the final material properties, la

Polymeric silica gel may be produced by polycondensation of hydrolyzed tetraethoxysilane (TEOS) under acidic conditions through a sol-gel process. This method is particularly attractive to ceramic scientists since high-purity silicate glasses can thus be prepared at much lower temperatures than those required by the conventional melting method. 1"2 Although the sol-gel method has its advantages, a high tendency to crack during drying o f bulk silica gels is often encountered. Crack formation during the drying is caused primarily by the stress generated in the gel due to capillary forces. Therefore, it has been suggested that fracture can be avoided by making gels with larger pore sizes and by using solvents o f low surface tension and low volatility. 3 Zarzycki e t al. ~ obtained crack-free gels by the * To whom correspondence should be addressed. 315

316

K. T. Chou, B. L Lee

This study was conducted to investigate the effects of the amount of acid and the gelling temperature on gelation behavior and hence the properties of the dried silica gels. 2 EXPERIMENTAL PROCEDURE

2. 1 Gel preparation The starting materials included tetraethoxysilane (TEOS), nitric acid and distilled water. The alkoxide sol had the molar ratio of T E O S ' H E O ' H N O 3 = l ' 1 0 : x (x = 0.1-1-0). It was prepared by directly mixing TEOS with the acidified distilled water at room temperature under constant stirring by a magnetic stirrer. The temperature during the hydrolysis was monitored by using Corning's pH/°C 107 meter. The pH of the sol was measured and found to be below the limit of the meter (i.e. pH < 0). After hydrolysis for 20 min, the sol was kept in a sealed plastic mold of cylindrical shape, and allowed to gel at specified temperatures; this was followed by aging treatment in distilled water for one day. The gels were then dried in the presence of the mother aging liquor at the same temperature as the gelling temperature. However, the drying at 120°C was conducted in a container with a loose cover on it. In the course of gelation, the viscosity of the sol increased gradually to formation of a solid gel. The gelling time was therefore defined as the time period from the pouring of the sols into the molds to the moment when fluidity of the sol was no longer observed when the mold was tilted.

2.2 Gel characterization The specific surface areas of the gels were evaluated by the BET method using nitrogen gas at the temperature of liquid nitrogen. The pore volumes were calculated from the specific surface areas by assuming the pores to be cylindrical in shape. The gel samples were heated at 140°C and degassed under a nitrogen gas flow prior to all BET measurements. The density of the dried gel was obtained by measuring the diameter, length and weight of the dried, cylindrically shaped gels using a vernier caliper and a balance. The pore diameter was calculated from eqn (1): 14

d,)

where D is the average pore diameter, Sp is the specific surface area, d b is the bulk density and d t is the true density. The true density value of 2.05 g/cm 3 determined by Yamane t3 for an alkoxy-derived silica gel was used for the calculation of the pore diameter. The structural changes of gels were identified by IR diffuse reflectance spectroscopy using a Fourier T r a n s f o r m Infrared Spectrophotometer (FTIR, Perkin-Elmer Model 1600). The weight loss of the dried gel powder was measured as a function of temperature by thermogravimetric analysis (TGA) using a D u p o n t Instruments Thermal Analyst 2100. The gel sample was heated in an open platinum crucible in air from 20 to 800°C at 5°C/min for the TGA.

6O Ra=0.01 Ra=O.1 Ra=0.4 Ra=0.6 Ra=1.0

4o

30

20 0

5

10

(1)

15

20

25

Reaction Time (rain)

Fig. 1. Effectsof HNO3 content on heat evolution from the hydrolysis reaction for a

H20:TEOS

ratio of 10:1.

Properties ~/" silica gels

317

1000]

m 2soc ~ • •

L

45°C II 60°C

i=

.1

0.0

0.2

0.4 0.6 0.8 Molar Ratio of HNO 3/TEOS

1.0

1.2

Fig. 2. Effects of gelling temperature and HNO a content on gelling time for a H20:TEOS ratio of 10:1.

3 RESULTS

3. 1 Hydrolysis and polycondensation Since the acid-catalyzed hydrolysis reaction is exothermic, the increase in the rate o f reaction results in an increase in the temperature of the hydrolyzed sols. Typical curves for the heat evocontents with a H 2 0 : T E O S molar ratio of 10:1 are shown in Fig. 1. The sol temperatures for these compositions rise rapidly to maxima, then drop immediately. The maximum temperatures reached were higher for a greater proportion o f nitric acid, as represented by R a in the figure. The samples with H N O a "TEOS molar ratios greater than 0.4 reached

their m a x i m u m temperatures above 50°C within a b o u t 2 min after the acidic water was added to the TEOS. However, the sol with a H N O a : T E O S ratio o f 0.01 t o o k more than 10 min to reach its maximum temperature o f ~ 46°C. The sol with a H N O a :TEOS ratio of 0-1 was somewhere in between, as expected.

3.2 Variation of gelling time The rates o f hydrolysis and polycondensation are fast in the presence o f a high acid content. The dependence of the hydrolytic polycondensation reaction in terms o f gelling time on the amount of acid catalyst and the gelling temperature are illustrated in Figs 2 and 3. The amount of nitric acid

1000

100

[]

0.1Ra

•

0.4Ra

•

0.6Ra

•

1.0Ra

0 t" Q)

E

10

Ft" Q)

(.9

.1 2.8

2.9

3.0

3.1

3.2

3.3

3.4

1000IT (I/°K) Fig. 3. Arrhenius plot ofgelling time against inverse absolute gelling temperature with various HNO3 contents for a H 2 0 :TEOS ratio

of 10:1.

318

K. T. Chou, B. L Lee

Table 1. C a l c u l a t e d activation energy of gelling as a function of acid contents for T E O S : H 2 0 : H N O 3 = 1 : 1 0 : x at temperatures ranging from 25 t o 75~"C x (molar proportion HNO3)

Activation energy (kcal/mol) a

0.1 0.4 0.6 1.0

7.1 5.3 4.4 2.3

~1 kcal = 4 . 1 8 4 kJ.

affected the gelling time as indicated in Fig. 2: gelling time decreases dramatically with increasing H N O a :TEOS molar ratios, i.e. nitric acid content. The effect of the gelling temperature on the gelling time for the system is shown in Fig. 3, which shows that the gelling time decreases as the gelling temperature increases. As reported by Colby et al. 15 for the sols prepared from T M O S and TEOS under acidic condition, the plot of gelling time (t~) vs inverse temperature (l/T) is linear and follows the Arrhenius equation 1

-- = A exp(-

tg

(2)

E*/RT)

where A is a pre-exponential term, E* is an apparent activation energy and R is the gas constant. Values of the activation energy calculated in this study, ranging from 2"3 to 7"1 kcal/mol (depending on the nitric acid content), are listed in Table 1. The results suggest that the activation energy decreases with an increase in nitric acid content; this indicates the role of nitric acid as a catalyst.

3.3 Gel formation The effects o f the sol compositions on the visual appearance of the gels under a slow mixing rate (--,280rpm) are listed in Table 2. In the series o f solutions having a constant water content at a H 2 0 : T E O S ratio of 10: l, transparent sols and gels were obtained only at H N O a ' T E O S ratios <0-2:1. Translucent sols and gels were obtained at H N O a : T E O S ratios of 0.4-0.6: 1; and opaque sols and gels were obtained at a H N O 3 :TEOS ratio of l : l . Totally crack-free transparent/translucent gels were formed from sols with H N O 3 :TEOS ratios of 0.1-0.6: 1. However, gels with a H N O a : T E O S ratio of 1.0:l transformed into opaque fragments. In general, fast gelation offered a poor rate of success in producing crack-free dried gels. The variations in the appearance o f the sol, and hence the gel, are attributed to the formation of particles during stirring of the sol. In the course of gelation, slow sedimentation o f the precipitate occurred which was observed at the bottom of the dried gel. On the other hand, the sol and the gel for the same compositions remained transparent at a higher mixing rate ( ,-~600 rpm) or in the presence of a solvent, i.e. ethanol or acetone. For a constant acid content with a H N O 3 :TEOS ratio of 0"6: l, the transparency increased and the tendency for crack formation decreased when the a m o u n t o f water increased from H 2 0 " T E O S ratios of 4:1 to 30:1. At the lowest water content with a H20 :TEOS ratio of 4" 1 and a H N O a :TEOS ratio of 0"6: 1, the gels broke into opaque fragments.

3.4 Thermogravimetric analysis The

effect of acid

content

in terms

Table 2. Compositions and properties of sols and gels after aging and drying at 60C Composition H20 :TEOS

H NO=:TEOS

EtOH :TEOS

Appearance of dried gels"

10 10 10 10 10 10 10 10 10 4 10 20 30

0'1 0' 2 0.4 0-6 1.0 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6

0 0 0 0 0 0'3 0.75 1.53 3.05 0 0 0 0

TP TP TS TS OP TP TP TP TP OP TS TS TS

"TP, transparent; OP, opaque; TS, translucent.

Formation of monolith

Yes Yes Yes Yes No Yes Yes Yes Yes No Yes Yes Yes

of the

Properties qf silica gels

319 6.0

5.5

t/) O ..-I

.E .~

5.0

4.5'

4.0

.5

0.0

I

a

i

i

I

0.2

0.4

0.6

0.8

1.0

.2

Molar Ratio of HNO 3/TEOS

Fig. 4. Effect of HNO3 content on weight loss for gels with a H20:TEOS ratio of 10:1 for gels drying at 6ffC. pore size increased from 15 to 22,~ (1-5 to 2.2nm) with H N O a :TEOS ratios from 0.4:1 to 1.0: 1. On the other hand, Fig. 7 illustrates similar effects of the drying temperature on the density and the pore diameter o f gels with a HNO3 :TEOS ratio of 0.4: 1, i.e. the density decreased and pore diameter increased with increasing drying temperature. The pore diameter increased from 15 to 50 ~ (1.5 to 5 nm) when the drying temperature was raised from 60 to 120°C. It is also clear that from Figs 6 and 7 the gels with lower density contain larger pores. As shown in Figs 8 and 9, the plots o f the specific surface area and the total pore volume versus the H N O 3 : T E O S ratio, for the molar ratio of H 2 0 :TEOS of 10:1, illustrate the influences of acid content and drying temperature on the structure of silica gel. The effect o f acid content on the specific

HNO3 :TEOS ratio on weight loss is shown in Fig. 4. The weight loss is based on the total weight loss between 120 and 800°C. The results show that the weight loss in air decreased as the H N O 3 : T E O S ratio increased from 0" 1 : 1 to 0.6: 1, then increased at 1:1. In addition, a plot of the specific surface area against the weight loss is given in Fig. 5. A decreasing weight loss with an increasing specific surface area is observed. 3.5 Density, pore diameter, surface area and pore volume The dependence of density and pore diameter on the acid content for gels dried at 60°C is shown in Fig. 6. The density decreased and the pore diameter increased as the H N O a :TEOS ratio increased. The 6.0

5.5 5.0.

o, .E

4.5

4.0'

.5

650

e

i

|

!

700

750

800

850

900

Surface Area (m 2 /g)

Fig. $. Relationship of weight loss with specific surface area for a H20:TEOS ratio of 10:1 for silica gels drying at 60°C.

320

K. T. Chou, B. I. Lee 1.4

24

J

-

Density

J

~.

•

""

1.3

"22

E o "-"

E t~

20 1.2

.~ O

t~ tO

18

o.

16

o-~

1.1

1.0 0.0

. 0.2

.

.

. 0.6

0.4

.

14 0.8

1.0

1.2

M o l a r Ratio of H N O 3 / T E O S Fig, 6. Effects of HNO a content on density and calculated pore diameter for a H 2 0 : T E O S ratio of 10:1 after drying at 60°C.

surface area is shown in Fig. 8; the surface area increased with increasing nitric acid contents for the gels dried at 45 and 60°C. As the drying temperature was further increased to 120°C, a decrease in specific surface areas was observed with increasing nitric acid contents. Figure 8 also shows that, except for the 120°C-dried gels, the specific surface areas increased as the drying temperature increased. This suggests that the gel structure changes, as the drying temperature is higher than the boiling point of water. Increases in total pore volume o f the silica gels with both higher acid content and higher drying temperature are shown in Fig. 9. The coordination numbers (CN) estimated by the equation o f Meissner et al. 16 (eqn (3)) using the pore volume, CN = 2 exp (2"4~b)

(3)

where ~bis the volume fraction of solid, are presented in Table 3. The results show that coordination 1.6

I-

Table 3. Calculated coordination numbers (CN) for different acid contents and drying temperatures R."

~bc CN

Drying temp. (~C) b

0.1

0.4

0.6

1.0

45

60

120

0.55 7.5

0.42 5.5

0.37 4.9

0-38 4.9

0.45 5.9

0.42 5'5

0.41 5"3

a Drying temperature = 60°C. ~R~ = 0 ' 4 . cVolume fraction of solid.

numbers were reduced as the acid content and drying temperature increased. A relationship between gelling time and density for gels dried at 60°C is shown in Fig. 10, which indicates that an increase in gelling time results in an increase in gel density.

oon,,

60

I

.< v

5O

"-"

1.4

E o v

•40

E i:5 ==

•30

a.

1.2

O

¢/) C

7O

Q

1.0

20

0.8 40

. 60

.

. 80

. 100

Drying Temperature

120

'" _o o

10 140

(°C)

Fig. 7. Effectsof drying temperature on density and calculated pore diameter for a H20 :TEOS ratio of 10:1 and a HNO 3:TEOS ratio of 0.4:1.

Properties ~t/'silica gels

321

900"

800 o) Od

E

700

< 600 O0

500

_-_

45oc

1

60"C

=

120"C

400 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Molar Ratio of HNO3/TEOS Fig. 8. Effects of H N O 3 content and drying temperature on specific surface area of the dried gels for a H z O : T E O S ratio of 10:1

0.9 0.8

E

0.7

E

0.6

o >

0.5

o n

45"C

0.4

600C 120"C

0,3

0.0

i

i

i

i

i

0.2

0.4

0.6

0.8

1.0

.2

Molar Ratio of HNO 3/TEOS Fig. 9. Effects of HNO~ content and drying temperature on pore volume for gels prepared with a H20:TEOS ratio of 10: 1.

1.4

~-.

1.3

1.2

1.1

1.0

. 0

. 1

.

. 2 Gelling Time (hr)

3

4

Fig. 10. Relationship of gelling time and density for a H20:TEOS ratio of 10:1 after drying at 60°C.

K. T. Chou, B. I. Lee

322

o"(~

(b)

0

(c)

m

(e) Si-O-Si Si-OH

4000

I

I

I

3000

2000 wave nurnl~" (crrrI)

1000

Fig. I I. F T I R spectra of silica gels as a function of H N O 3 : T E O S ratios of (a) 0"I :l, (b) 0"2:1, (c) 0.4:1, (d) 0-6:1 and (e) I :I for a H20:TEOS ratio of I0: I after drying at 60°C.

3.6 FTIR analysis Infrared reflectance spectra of the gels with H N O a : T E O S ratios of 0.4:1, 0.6:1 and 1:1 are illustrated in Fig. ll. A broad OH band appeared between 2600 and 3 8 0 0 c m - t which is associated with stretching Si--OH groups (3650cm -1) and chemically/physically adsorbed water (3250 cm - 1).17 The bands at 1630 and 1500cm- ~ are attributed to H 2 0 and C - - H . ~7 The peak at 950cm-~ is usually attributed to the vibration of the Si--O bond in a glass containing nonbridging oxygens.~ 8 In a silica gel, it is preferable to assign this band to the stretching mode of the Si--OH.~9 There are three characteristic absorption bands of the silica gel that are usually observed between 400 and 1300 c m The peak at 1070cm -~ is due to the asymmetric stretching vibration of Si--O--Si. ~7.19 The peak at 800 c m - ~ is due to the symmetric stretching mode of S i - - O - - S i or the vibrational mode of the ring structure of the tetrahedral (SiO4). t9 The peaks at 1870 and 1960cm -1 which result from a combination of various vibrations of SIO2, 2° are also shown. 4 DISCUSSION

4.1 Rate of hydrolysis and polycondensation and acid content The temperature rise during the fast hydrolytic polycondensation reaction is seen in Fig. 1. The

diminishing difference in the a m o u n t of heat generated as a function of HNO3:TEOS ratios above 0.4 indicates that the catalytic effect of nitric acid peaks at a HNO3 :TEOS ratio of around 0"4. The increase in hydrolysis time to reach the maximum temperature for a HNO3 :TEOS ratio of l : l in comparison with 0.6:1 can be due to the reverse reaction. This reverse reaction becomes more significant at high-acid-catalyst concentrations, as observed by Assink & Kay. 21 In Fig. 1, the increase in the hydrolysis rate appears to be associated with the gelling time. The higher the molar ratio of HNO3 :TEOS, the sharper the rise and fall of the temperature, and the shorter the gelling time (Fig. 2). According to electrical double-layer theory, as ionic strength increases with increasing acid concentration, the thickness of the double layer tends to reduce, and so does the zeta potential. Hence the electrostatic repulsive forces and the gelling time are decreased. In Fig. 3 and Table 1, the activation energy decrease with increasing nitric acid content suggests that the hydrolysis becomes the rate-determining step in the high-acid-hydrolyzed gel. The polycondensation rate increases as acid content and gelling temperature increase. This can be explained by the fact that the sols with higher acid concentration have fewer hydroxyls available for further condensation than the other sols with lower acid concentration. This is because the extent of

323

Properties of silica gels

condensation for silicon species in the sol is greater at higher acid concentrations. Kelts & Armstrong 22 pointed out that the concentration of silicon in cyclic tetramer is higher than in linear dimeric silica; therefore one should be able, by using more acid, to maximize the number of rings in the oligomers and polymers to form bulk gels. 4.2 Gel formation Alcohol produced by the hydrolysis promotes the mutual dissolution of TEOS and water. In this study, when a mixture of TEOS and water is stirred at a slow mixing rate, an emulsion is formed. After about 4min of stirring, depending on the acid content, formation of silica precipitates is noticed, and the sol then turns cloudy. Formation of the more highly crosslinked polymers is expected, since a larger amount of water generates a larger number of silanol sites in the hydrolyzed TEOS. Besides, an increase in acid content also promotes the silanol intermediates. These intermediates then condense further to silica precipitates before the ethanol produced by the reaction can aid in dissolution of the reactants in the system. This is in agreement with the fact that the formation of precipitates disappears in the presence of ethanol. At low acid concentration, the silanol intermediates are provided at a lower rate and thus no precipitates are observed. 4.3 Thermogravimetric analysis As the sol-gel reactions proceed, polycondensation reaction reduces the a m o u n t of OH and OR groups with removal of the by-product, i.e. water or alcohol. This decreases the weight loss because fewer OH or OR groups remain on the gel surface. At temperatures greater than 150°C, all of the weight loss is attributable to either S i - - O R or Si--OH originally present in the gels. 23 As shown in Fig. 4, the decreasing T G A weight loss of the gel indicates that the surface coverage of OH and OR groups decreases with increasing acid content up to a HNO 3 :TEOS ratio of 0.6:1. This explains the fact that the rate of polycondensation dominates the hydrolysis when the acid content is below 0-6. However, the increase in weight loss for a gel at the HNO3:TEOS ratio of 1:1 can be caused by the occurrence in reverse reaction, which increases the S i - - O R concentration over Si--OH. Therefore, having fewer crosslinked units implies the increase in surface coverage of OH and OR groups. A higher surface coverage by OH and OR means a less

crosslinked gel due to the slower and smaller a m o u n t of condensation. As can be seen in Fig. 5, the higher surface coverage, due to a higher weight loss, corresponds to the lower surface area. This illustrates the fact that, at low acid contents, a longer gelling time allows the siloxane to rearrange and collapse into a denser structure. This produces a gel with higher density for lower acid content (Fig. 6). 4.4 Density, pore diameter, surface area and pore volume In this study the wet gels were aged in water which turned out to be an acidic liquid due to the excess acid in the pores of the wet gels. This was followed by drying the gels in the same acidic solution. As shown in Fig. 6, the increasing acid content led to an increase in pore diameter and pore volume with increasing specific surface area. Hence the gel density decreased (Figs 6, 8 and 9). This can be explained by the fact that the higher the condensation rate in the high-acid-content composition, the less likely is steric reorientation of the siloxane chain to occur, which leads to gels with larger pores. This is evident from the decrease in gelling time, leading to a reduction in density with increasing acid content (Fig. 10). Alternatively, this can be attributed to lower packing density in terms of coordination number, as shown in Table 3. In high acid concentration, the hydrolysis reaction occurs rapidly to produce monosilicic acids, acting as potential nuclei. Due to the larger number of nucleation centers, the particles formed are large in numbers but small in size. This results in large specific surface areas. The specific surface areas of dried gels became larger as the drying temperature increased from 45 to 60°C. This is attributed to the dependence of the coordination number of the primary particles on drying temperature. Drying at 60°C results in the formation of aggregates of a lower coordination number than drying at 45°C, as shown in Table 3. This is because the former has a shorter processing time to allow the aggregates for further rearrangement, and hence the lower coordination number is obtained. This in turn yields high specific surface areas, as explained by Yasumori etal. 24 However, as the gels are aged and dried at 120°C with covers, they become exposed to hot water vapor. During drying under the hot water vapor, the smaller particles, because of continuing hydrolytic polycondensation, grow to larger particles. 25 Hence, the decreased specific surface area of the gel can be attributed to the increased size of primary particles during the aging and drying.

K. T. Chou, B. L Lee

324

The effect of drying temperature on pore volume (Fig. 9) is probably associated with temperature dependence of the surface tension and vapor pressure of the pore liquid. The surface tension of the liquid decreases with increasing temperature. At a higher drying temperature, the lower surface tension and higher vapor pressure of water in the gel draw the gel network together less. In addition, the drying time, in which the compressive force is imposed on the gel, is shorter for a higher drying temperature. Therefore, higher pore volumes can be expected at higher drying temperatures. This is supported by less shrinkage of the gels dried at 120°C and a lower density than those gels dried at lower temperatures (Fig. 7). By comparing the effect of acid content and drying temperature on pore structure, it is shown that the effect of increasing pore size by increasing the drying temperature is more pronounced than that by increasing the acid content (Figs 6 and 7). 4.5 FTIR spectra

FTI R spectra show the possible formation of bonds and the relative concentration of each species by their spectral position and peak intensity. The absorption bands of the silica for HNO3:TEOS ratios up to 0"6:1 at 1870 and 1960cm-~ increased with an increase in acid content; this indicates an increasing amount of SiO2 (Fig. 11). This has been seen from the decreasing weight loss with increasing acid content in Fig. 4. For the HNO 3 :TEOS ratio of 1 : 1, the reduction in the increasing trend of the peak intensity at 1870 and 1960cm -~ for silica agrees with the low weight loss shown in Fig. 5.

5 CONCLUSIONS 1.

2. 3.

4.

5.

In the presence of high acid content, a slow mixing rate can lead to the formation of precipitates, changing the appearance of the resulting gels. The gelling time is determined by the amount of acid present and the gelling temperature. Larger amounts of nitric acid produce gels with a higher specific surface area, a higher pore volume and a lower density. A higher drying temperature leads to a decrease in specific surface area and density, but an increase in pore volume as the acid content increases. Increasing the drying temperature is more effective than increasing the acid content in increasing the pore size.

A C K N O W L E D G E M ENTS The authors thank Dr Michael J. Drews and Ms Kim Ivey in textile department in Clemson University for their assistance in conducting thermal analysis.

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