Monolithic aerogels in the systems SiO2B2O3, SiO2P2O5, SiO2B2O3P2O5

Journal of Non-Crystalline Solids 63 (1984) 117-130 North-Holland, Amsterdam

117

MONOLITHIC AEROGELS IN THE SYSTEMS SIO2-B203, SIO2-P205, SiO2-B203-e20 s T. WOIGNIER, J. PHALIPPOU and J. ZARZYCKI Laboratory of Materials Science and CNRS Glass Laboratory, University of Montpellier II, France

The gels in the binary and ternary systems: SiO2-B203, SIO2-P205, SiO2-B203-P2Os; were prepared by hydrolysis and polydensation of metalorganic compounds. The gelling times vary with the molar percentage of SiOz. The solvent was evacuated under hypercritical conditions in an autoclave in order to obtain aerogels free of cracks. The monolithicity of the aerogels is influenced by the method of preparation of the alcogels. The crystallization of BPO4 was observed in the ternary system only. These materials can be converted into glasses by heat treatment. The structural evolution was followed by means of infrared spectroscopy and textural evolution by dilatometric measurements.

1. Introduction

In recent years, the sol-gel process for making glasses has gained scientific and technological importance [1-3]. In this process the required ingredients are reacted at low temperature to form a gel, which is then densified into a glass. The main advantage of this method is that it enables glasses to be obtained with a high degree of homogeneity which are difficult to prepare by conventional methods of melting. A lot of work has been clone on various multicomponent systems to obtain glassy materials at relatively low temperatures [4-6]. However, before the gel-glass transformation, a slow drying is necessary to avoid cracking the gel into small fragments. Such gels are called xerogels and generally have small dimensions. Another method consists of agglomerating the powder of gel by hot pressing; but actually, the main interest remains on the production of monolithic gels of sizeable dimensions. Recently, Prassas et al. [7] prepared pure silica aerogels, free of cracks, by evacuation of the solvent under hypercritical conditions in an autoclave. The purpose of this work is to prepare monolithic mixed aerogels in the binary systems: SIO2-B203, SiO2-P205 and in the ternary system: SiO2-B2Oa-P205.

2. Experimental It is noted from the literature that many authors have employed two different methods to make borosilicate gels. The first method involves the 0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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T. Woignier et al. / Monolithic aerogels

destabilization of a colloidal solution of amorphous silica to which boron is added as boric acid [8] a n d / o r ammonium pentaborate [4]. The second method is based on the ability of the organometallic compounds of silicon, to undergo hydrolysis and polycondensation reactions. In this case boron may be added either as boric acid [9] or as an organometallic compound such as B(OCH3) 3 [3,10,11]. As far as the binary SiO2-P205 system is concerned, there is very little information in the literature. Jabra et al. [12] prepared gels in the SiO2-P205 system by employing the first method, the gellifying ingredients being "Ludox A s " with ammonium pentaphosphate. Gels in the SiO2-P205 system can also be prepared by allowing the polymerization reaction of tetraethoxysilane to take place in the presence of phosphoric acid [13]. No work has been reported so far on the ternary system SiO2-B202-P205. In the present work, all the gels were synthesized by hydrolysis and polycondensation reactions of organometallic compounds. Tetramethoxysilane (Si(Ofn3)4) was used as a source of SiO2, trimethylborate (B(OCH3)3) and trimethylphosphite (PO(CH3)3) were used for B203 and P205 respectively. Gelation took place in the presence of methanol which is readily soluble with the organometallic compounds and the distilled water used for hydrolysis. For the SiO2-B203 system, methanol and tetramethoxysilane were poured into a reactor and hydr01yzed initially with half of the total amount of water at 45°C for 1 h. B(OCH3) 3 and the remainder of the water were then added and the mixture was stirred at the same temperature for 4 h. The resulting clear solution was poured into pyrex tubes which were then closed. The solutions were maintained at several temperatures (room temperature, 45°C and 60°C) for different periods of time required for gelation and ageing. It was necessary to promote a preferential hydrolysis of Si(OCH3) 4, because the rate of hydrolysis of this compound is lower than that of B(OCH 3)3 [14,15]. The trimethylborate reacts with the OH groups of the partially hydrolyzed tetramethoxysilane to form Si-O-B bonds: ( o f n 3 ) 4 _ n S i ( O n ) n + B(OCH3) 3 --~

(OCH3)4-n(OH)n-1Si-O-B(OCH3)2 +

CH3OH"

Many authors think that the Si-O-B bonds are susceptible to hydrolysis [16] and an equilibrium between the formation and the breaking of the S i - O - B bonds must exist in the presence of water. For the SIO2-P205 system, the gels were prepared in the same manner as for the SiO2-B203 system. Trimethylphosphite was allowed to react with partially hydrolyzed tetramethoxysilane. The rest of the preparation steps remained the same. 'For the ternary SiO2-B203-P205 system, methanol and Si(OCH3) 4 were hydrolyzed initially under stirring at 45°C. B(OCH 3)3, P(OCH3)3 and the rest of the water were added 1 h later. The stirring was continued for 4 h. Gels are denoted as MxS, where M stands for 13203 or P205, x is their molar

T. Wolgnier et al. / Monolithic aerogels

119

percentage and S for silica. For a given system gels were prepared by varying different parameters such as y and R, where R=

volume of CH3OH total volume of organometallic compounds

and y = number of moles of water per mole of Si(OCH3) 4. For the hypercritical evacuation of the solvent, the gels were introduced in an autoclave, into which a certain amount of methanol was added to attain a pressure higher than the critical pressure, at critical temperature. To prepare the pure silica aerogels the mixture was sometimes poured into the autoclave without waiting for gelation [17]. However, in the present work, the gels were aged for a few'days, at constant temperature before introducing them into the autoclave. Then, the temperature was increased to 270°C at a rate of 1.6°C/min. When this temperature was attained, the pressure corresponded approximately to 180 bar. For pure methanol, the temperature and the pressure corresponding to the critical point were respectively 242°C and 78.5 bar. The temperature of the autoclave was kept constant and a slow evacuation of the solvent was performed (3-5 h). Dry argon was used to eliminate the last traces of the solvent. The density was measured with a mercury volumeter on bulk samples. Porosity was calculated from the density value (2.12 g / c m 3) of a glass B2OS [4]. The specific surface area was measured by the BET method using nitrogen adsorption. Shrinkage measurements were carried out using an electronic dilatometer. The evolution of the structure was studied by transmission infrared spectroscopy. The IR spectra could be obtained for bulk samples due to the high porosity of the materials; the quantity of the matter traversed by the beam being small. The crystallization of the gels was studied by X-ray diffraction at room temperature with CuK~ radiation.

3. Results and discussion

3.1. SiO2-B203 system To obtain a monolithic aerogel, the alcogel must, first, be free of cracks. Moreover, the gelling time of the solution is an important factor in the efficiency of the method. The solution must gelify in a reasonable time. A solution is considered as gelified, when it no longer flows. The influence of several parameters on the gelling time and on the monolithicity was studied. The gelling time at room temperature is plotted against B203 content in fig. 1. The role of B(OCH3) 3 is to increase the gelling time. In

T. Woignier et al• / Monolithic aerogels

120

o

I.--

I 0

J

A

i

ob 0

z

e o

E

0

~ u

0

0

\

......

~ o

, o

~0

T. Woignier et al. / Monolithic aerogels

121

fact, the solution of trimethylborate in methanol contains a complex alkoxo acid [18] formed by the reaction: H

I

(CH30)3B + O - C H 3 ~ (CH30)4BH. The acidity must be due to the formation of the methyloxonium ion (CH3OH 2 + ). Then, the acidity and the gelling time of the solution increase with the B203 content. If the solution is gelified at 65°C, the gelling time varies between 5 and 50 h. The alcogels B5S, B20S, B30S and B50S remained monolithic when aged at room temperature or at 40°C. At 65°C, cracks sometimes appeared for B5S and B15S gels. The gelling time is plotted against the temperature in fig. 2. The increase in temperature accelerates the hydrolysis and polycondensation reactions. Above 65°C, the alcogels showed cracks induced by an inhomogeneous polymerization over the entire volume of the solution. Tables 1 and 2 show the gelling time as a function of y and R. Clearly, an increase in y accelerates the hydrolysis and the gelation process. Table 2 shows that if the gelifying species are more diluted in methanol, the gelling time is longer. Fig. 3 shows three gels prepared under the same conditions, but aged at various temperatures. After three days, the alcogel aged at 60°C is the only one which gives an aerogel free of cracks. Gelation was followed by a syneresis phenomenon. The increase in temperature accelerates the polycondensation reaction [19,20] and increases the rigidity of the alcogel. Hence, all the alcogels were aged for one week at temperatures between 55°C and 60°C. The aerogels B5S, B15S, B20S and B30S were monolithic (see fig. 4). The aerogels (B20S) prepared with different values of R(0.5-1.3) and y(2-8), were also free of cracks. It is thus, possible to prepare aerogels having different values of density and specific surface area (see tables 3 and 4). The textural characteristics of a gel (B15S) are shown in table 5 for different ageing temperatures. The gel is more dense when aged at higher temperature [19].

Table 1 Gelling time of a gel B20S (R = 0.65) at 60°C against y y Gelling time (h)

2 40

3 20

4 15

6 10

8 8

1 350

1.3 600

Table 2 Gelling time of a gel B20S ( y = 4) at room temperature against R R Gelling time (h)

0.5 150

0.65 180

0.8 220

122

T. Woignier et al. / Monolithic aerogels

d~

n~ I

.=.

0

0 Q

0

.o

8

.

T. Woignier et al. / Monolithic aerogels

123

Table 3 Textural characteristics of a gel B20S (R = 1) for different values of y y

Density ( g / c m 3)

Porosity (%)

Specific surface area (m2/g)

2 3 6 8

0.42 0.34 0.3 0.24

80 84 86 89

854 710 670 370

Table 4 Textural characteristics of a gel B20S ( y = 5) for different values of R R

Density ( g / c m 3 )

Porosity (,%)

Specific surface area (m2/g)

0.53 0.8 1 1.3

0.32 0.31 0.3 0.26

85 85.5 86 88

850 410 610 500

Table 5 Textural characteristics of a gel B15S at different ageing temperatures T (°C)

Density ( g / c m 3)

Porosity (%)

Specific surface area (m2/g)

25 45 60

0.3 0.31 0.32

86 85.5 85

770 710 660

It was noted that it is difficult to find a col,elation between the parameters used for the preparation of the gels and the specific surface area. The borosilicate aerogels shrink as the temperature increases. The rate of shrinkage varies with the temperature (see fig. 5). The temperature range where the shrinkage is maximum is related to the B203 content. This shrinkage is due to various phenomena, the most important being viscous flow [21]. The transformation of these aerogels into glasses could be made at temperatures between 800°C and 950°C. This temperature range corresponds to viscosities between 109.5 and 10115 P approximately [3]. These results are in good agreement with those found for pure silica aerogel [22]. The evolution of the structures of the borosilicate aerogels are shown in fig. 6. The aerogels were heat treated at 600°C for 5 h to eliminate the organic residues. Four important absorption bands can be observed at 3720, 3680, 3500 and 3300 cm-t, respectively. The band at 3720 cm-t is attributed to the vibration of the Si-OH isolated groups [2]. The bands at 3680 cm-t and 3500 cm-t corresponds respectively to the weakly and strongly hydrogen-bonded silanol groups [23]. The intensity of the band at 3300 cm-1 increases with the B 2 0 3 content. It is probably due to the B-OH groups, but the fundamental stretching frequency of B-OH in the glass is 3600 cm-t [24].

T. l'Voignier et a L / Monolithic aerogels

124

AL . I 0 0 Lo 5

'/ T (*C)

2o0

360

5;0

6;0

760

860

900

Fig. 5. Shrinkageof the borosilicateaerogelsagainst the temperature. On the right hand side of the spectra, two bands appear with the B203 content (2700 cm -1 and 2540 c m - t ) . These bands are attributed to the harmonics of the B - O bonds vibrations [25]. The intensities of these two bands increase when the gel is heat-treated at 1000°C. At this temperature, shrinkage is important and the gel begins to transform into glass (see fig. 7). The spectrum is similar to that of "Vycor" glass [26].

3.2. SiO2-P205 system In this case, the gelling time of the phosphosilicate solutions is smaller than that for borosilicate solutions; at room temperature, between 100 and 50 h for compositions between 5 and 50% of P205. At 65°C, all the solutions were gelified after 10 h and some of the alcogels showed cracks. If they gelified at room temperature, they are usually monolithic but no correlation could be found between the monolithicity and the P205 content. A monolithic aerogel was obtained if the monolithic alcogel was aged for three weeks at room temperature or one week at 60°C. These results are not always reproducible, and no gels of composition above P20S could be obtained in a monolithic form (fig. 8). Phosphosilicate aerogels were white, very brittle and difficult to cut without breaking them into small pieces. All the phosphosilicate gels shown in fig. 8 were prepared with y = 4 and R = 1. But it is possible to use different contents of water and methanol in the initial solution to obtain various textural characteristics. The shrinkage of the aerogel is plotted against the temperature as shown in fig. 9. Like the borosilicate gels, the shrinkage is affected by the I)205 content, but the range of temperatures where the densification begins to be significant is at higher temperature (1000-1080°C). In this temperature interval, the corresponding viscosities of the glasses are between 1011 and 10115 p [4].

T. Woignier et al. / Monolithic aerogels

125

o

t~

3:

.o ¢;

ca

m

ca

u

v

E •>

e~ .-

2

[-

126

T. Woignier et a L / Monolithic aerogels

Fig. 8. Monolithic aerogels in the SiO2-P205 binary system.

0

L--~o x 100

5

S

IC

T('C) 300

400

500

600

,

,

,

700

800

900

Fig. 9. Shrinkage of the phosphosilicate aerogels.

\

.

1000

T. Woignier et al. / Monolithic aerogels

127

e~

to

to

8 &

o.

.=_ e. 0

128

T. Woignier et al. / Monolithic aerogels

All the aerogels made in the autoclave show a small amount of a crystalline phase which is not yet identified, two small peaks ( d = 2.01 and 2.33) are present. The aerogels heat-treated at 1100°C are amorphous. The infra-red transmission spectra of phosphosilicate aerogels were recorded under the same conditions as before but the thickness of the samples was equal to 1 mm. The large band situated between 3500 cm -1 and 3300 cm -1 is widened and shifted towards the lower frequencies with the addition of P205 (fig. 10). This is induced by the presence of the P - O H band centered around 3300 cm-1 in the glass [25]. Two bands appear at 2640 cm-1 and 3300 cm-1 respectively, which are characteristic of" the P = O bond [27] and P - O H bond [251 (fig. 11). 3.3.

Si02-B203-e205system

The phenomena observed and the results obtained for the two binary systems show that the" conditions of preparation of monolithic alcogels and aerogels depend on the composition of the solution. For compositions rich in P205, the gels must be aged at low temperature ( < 50°C) for three weeks. For compositions rich in B203, one or two weeks at a temperature between 40°C and 60°C are necessary to obtain monolithic ternary aerogels (fig. 12). The X-ray diffraction spectra of the borophosphosilicate gels show the presence of a crystalline phase. The intensities of the peaks increase with the

Fig. 12. Monolithicaerogels in the SiO2-B203-P20 5 system.

T. Woignier et al. / Monolithic aerogels

129

2O I

,

I

3O

I

I

20

Fig. 13. Intensity of the BPO4 X-ray peak, as a function of B203 and P205 content.

B203 and P205 contents (see fig. 13). The crystalline phase is identified as the borophosphate (BPO 4), isoelectric to the silica. This phase is very stable in the glass containing simultaneously B203 and P205 [28]. Contrary to the phosphosilicate gels, the heat treatment at l l 0 0 ° C cannot eliminate the X-ray peaks due to BPO 4. 4. Conclusion

Monolithic aerogels in the binary systems 8 i O 2 - B 2 0 3 , S I O 2 - P 2 0 5 , and in the ternary system SiO2-B203-P205 have been prepared. The temperatures and ageing times are very important parameters in order to obtain aerogels free of cracks. The conditions of preparation of the alcogels influence the textural characteristics of the final aerogels obtained. The borosilicate aerogels can be obtained as monoliths with a good reproductibility. As far as the phosphosilicates are concerned, the ageing time must be longer and the results are not always reproducible. The densification of the aerogels begins to be significant in a temperature range lower than for pure silica (800-900°C for BXS and 1000-1080°C for PXS). After a heat-treatment at 1100°C these gels are amorphous and show infra-red spectra similar to those of glasses of the same composition. The transformation of these aerogels into glasses can be made at lower temperatures compared to those necessary for making the same glasses by conventional fusion techniques and no fining is necessary. The aerogels of the ternary system contain BPO4 crystal which cannot be avoided even using a high temperature heat treatment.

T. Wotgnier et al. / Monolithic aerogels

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R. Roy, J. Amer. Ceram. Soc. 52 (1969) 244. H. Dislisch, Angew. Chem. lnternat. Ed. Engt. 10 (1971) 363. M. Decottignies, J. Phalippou and J. Zarzycki, J. Mater. Sci. 13 (1978) 2605. R. Jabra, J. Phalippou and J. Zarzycki, J. Non-Crystalline Solids 42 (1980) 489. T. Woignier, J. Phalippou and J. Zarzycki, Int. Congress on Physics of Non Crystalline Solids, Montpellier, France (July 1982); J. de Phys. Coll. 43 (12) (1982) 261. K. Kamiya, S. Sakka and Y. Tatemichi, J. Mater. Sci. 15 (1980) 1765. M. Prassas, J. Phalippou and J. Zarzycki, Int. Congress on Physics of Non Crystalline Solids, Montpellier, France (July, 1982); J. de Phys. Coll. 43 (12) (1982) 257. W.L. Konijnendijk, M. Van Duuren and H. Groenendijk, Verres Refract. 27 (1973) 11. S.P. Mukherjee, Materials Processing in the Reduced Gravity Environment of Space, ed., G.E. Rindone (North-Holland, 1982) p. 321. C.J. Brinker and S.P. Mukherjee, J. Mater. Sci. 16 (1981) 1980. M. Nogami and Y. Moriya, J. Non-Crystalline Solids 48 (1982) 359. R. Jabra, J. Phalippou and J. Zarzycki, J. Chim. Phys. 78 (1981) 77. 1. Thomas, US Patent No. 767-432 (1973). S.P. Mukherjee, J. Non-Crystalline Solids 42 (1980) 477. B.E. Yoldas, J. Mater. Sci. 14 (1979) 1843. V. Bazant and V. Chvalousky, The Chemistry of Organosilicon Compounds, Vol. 1 (Academic Press, New York, 1965). M. Prassas et al., to be published. H. Copaux, Compt. Rend. 127 (1898) 719. G.A. Nicolaon and S.J. Teichner, Bull. Soc. Chim. France 8 (1968) 3107. L.C. Klein and G.J. Garvey, J. Non-Crystalline Solids 38-39 (1980) 45. M. Prassas et al., to be published. J. Phalippou, T. Woignier and J. Zarzycki, Proc. Cont. on Ultrastructure Processing of Ceramics and Glasses and Composites, Gainesville, Florida (February, 1983), to be published. S. Kondo, H. Fujiwara, E. Okazaki and T. Ichii, J. Coll. lnterf. Sci. 75 (1980) 328. H. Franz, J. Amer. Ceram. Soc. 49 (1966) 473. R. Jabra, Thesis, Montpellier, France (1979). M.E. Nordberg, J. Amer. Ceram. Soc. 27 (1944) 299. T. lzawa, N. Shibata and A. Takeda, Appl. Phys. Lett. 31 (1977) 33. G.W. Morey and E. Ingerson, Amer. Min. 22 (1937) 37.

Discussion

Q. c.J. Brinker: B e c a u s e h y p e r c r i t i c a l d r y i n g is a h i g h t e m p e r a t u r e p r o c e s s a n d I believe h y d r o t h e r m a l c o n d i t i o n s c a n b e a c h i e v e d , h o w d o e s this m e t h o d a f f e c t c h e m i c a l c o m p o s i t i o n a n d p o l y m e r s t r u c t u r e e s p e c i a l l y c o m p a r e d to x e r o g e l f o r m a t i o n u n d e r low t e m p e r a t u r e c o n d i t i o n s ? A. T h e h i g h t e m p e r a t u r e p r o c e s s i n c r e a s e s the rate of e s t e r i f i c a t i o n r e a c t i o n b e t w e e n silanol g r o u p s a n d r e s i d u a l o r a d d i t i o n a l a l c o h o l . F o r c o m p a r i s o n d r i e d x e r o g e l s a r e h y d r o p h i l i c m a t e r i a l s , d r i e d a e r o g e l s are h y d r o p h o b i c materials. Q. J.D. Mackenzie: T h e S i O 2 - P 2 0 5 crystal is a rare e x a m p l e w h e r e Si e x h i b i t s six c o o r d i n a t i o n . D o y o u see a n y e v i d e n c e o f a n y Si i o n h a v i n g six c o o r d i n a tion? A. N o , we h a v e n o t seen a n y s u c h e v i d e n c e .