SILICA AEROGELS PREPARED BY HYPERCRITICAL ACETONE EVACUATION

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SILICA AEROGELS PREPARED BY

HYPERCRITICAL ACETONE EVACUATION

M. Pauthe, J. Phalippou

To cite this version:

M. Pauthe, J. Phalippou. SILICA AEROGELS PREPARED BY HYPERCRITICAL ACE- TONE EVACUATION. Journal de Physique Colloques, 1989, 50 (C4), pp.C4-215-C4-220.

�10.1051/jphyscol:1989435�. �jpa-00229511�

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REVUE DE PHYSIQUE APPLIQUEE

Colloque C4, Supplement au n04, Tome 24, avril 1989

SILICA AEROGELS PREPARED BY HYPERCRITICAL ACETONE EVACUATION

M. PAUTHE and J. PHALIPPOU

Laboratoire de Science des Materiaux Vitreux, Universite des Sciences et Techniques du Languedoc, Place E. Bataillon, F-34060 Montpellier Cedex, France

Resume - Le tetram6thoxysilane est le precurseur utilis6. La reaction d'hydrolyse, effectuee en milieu acetone, conduit a des gels monolithiques dans un grand domaine de compositions. Le sechage hypercritique est directement real is4 sur les gels tels qu'il s ont ete obtenus. Les conditions permettant d' el aborer des aerogels mono1 ithi

-

ques sont precisees. L'analyse structurale des aerogels montre qu'ils sont similaires

a ceux obtenus dans des conditions d'evacuation hypercritique de solvants alcooli- ques. Cependant, la distribution des groupements hydroxyles est fortement affectee par la nature du solvant employe. L'evolution de la structure des aerogels en fonction de la temperature est 6tudiee. Le frittage s'effectue a des temperatures superieures

i

1000°C et l'aerogel se-transforme en verre.

Abstract - Tetramethoxysilane is used as the silica source. The hydrolysis reaction, carried out in acetone, leads to gels which are monolithic in a large. domain of com- positions. The hypercritical drying is directly performed on the gels as obtained.

Conditions to prepare monolithic aerogels are reported. The structural analysis of the aerogels shows that they are similar to those obtained from hypercritical drying in alcoholic medium. However, the hydroxyl group distribution is strongly affected by the nature of the solvent. The evolution of the structure of the aerogels as a function of temperature is investigated. Sintering occurs above 1000°C and the aerogel transforms into glass.

1

- INTRODUCTION

Synthesis o f gels from organometallic compounds is a very flexible method allowing gels to be obtained under different experimental conditions. Particularly, various organic solvents are suitable to dilute metalal koxide compounds /I/. Among solvents, alcohols are widely used. Meanwhile, acetone presents many advantages with regard to alcohols. It is known as a dehydrating solvent for materials containing aqueous solutions. It is totally miscible with water and alcohols. Its critical temperature and pressure are Tc

=

235°C and PC

= 4.7

MPa respectively . These values are close to those of alcohols. On another hand, the solubility of silica into water-acetone mixture is very low even for temperatures higher than 200°C /2/. That last feature leads to the belief that textural modifications resulting from hypercritical drying will be decreased. Interest in that solvent also comes from its high ability to be diluted into liquid COz. When one wants to perform a silica aerogel by hypercritical COz evacuation, the exchange treatments will be shortened if the initial sol vent is acetone.

2

- PREPARATION CONDITIONS

The gels result from hydrolysis and condensation reactions of tetramethoxysilane (TMOS). Acetone plays the role of a mutual solvent for both TMOS and H20.

In the adopted nomenclature, V is the volume ratio (TMOSx102)/(TMOS+sol vent) and n, related to the hydrolysis conditions, is the molar ratio HzO/TMOS.

The samples will be labelled

X

V-n where

X

refers to the nature of the solvent.

The miscibility region of the ternary system CH~COCH~/HZO/TMOS is shown on figure la.

In this figure, the miscibility domain of CH30H/HzO/TMOS is also plotted. As observed, the miscibility region is lower when acetone is the solvent. A similar remark can be made for a higher molecular weight a1 koxide compound

:

the tetraethoxysi 1 ane (TEOS) [Fig. Ib] .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989435

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Figure 1 : Behaviour of ROH o r CH3COCH3/HzO/Si (OR)4 s o l u t i o n a t 25°C - (a) R = CH3

-

( b ) R ⁼C2Hs.

The g e l a t i o n r a t e of t h e s o l u t i o n s CH~COCH~/HZO/TMOS is q u i t e slow. The s o l u t i o n i s s y s t e m a t i c a l l y r a i s e d t o 37°C i n a thermostated bath t o a c c e l e r a t e t h e g e l a t i o n .

The g e l a t i o n time ( t g ) f o r s o l u t i o n s hydrolysed with n = 4 depends on t h e d i l u t i o n [Table 11.

Table 1 : Gel1 i n g t i m e s of s o l u t i o n s CH~COCH~/HZO/TMOS vs V.

For d i l u t i o n s lower than 40% t h e mixture i s no longer homogeneous and t h e r e s u l t s a r e not reported here.

The number of water molecules plays a very important r o l e on t h e g e l a t i o n time i r r e s p e c t i v e of t h e d i l u t i o n [Fig. 2 1 .

0 10 20

tg ( d a y ) 4

Figure 2 : Gelling times of s o l u t i o n s CH3COCH3/H20/TMOS vs n.

All t h e wet g e l s a r e monolithic. However, g e l s f o r which V l i e s between 30 and 40 when n i s l e s s o r equal t o two appear cracked a f t e r t h e autoclave t r e a t m e n t . The curves presented in f i g u r e 2 a r e l i m i t e d by t h e q u a n t i t y of water molecules which induces

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i m m i s c i b i l i t y when i t i s t o o high. This f e a t u r e occurs f o r n values above f i v e i n systems where V = 40. For V = 30 t h e s t u d i e s a r e done f o r n up t o nine.

I t c l e a r l y appears t h a t m o n o l i t h i c i t y i s always obtained when t h e hydrolysis i s c a r - r i e d out under s t o e c h i o m e t r i c c o n d i t i o n s (n = 4 ) . Thus we have chosen t o s y n t h e t i z e aerogels under t h o s e l a s t c o n d i t i o n s o f hydrolysis. This was done s o t h a t a simple comparison may be made with a e r o g e l s previously e l a b o r a t e d under h y p e r c r i t i c a l evacuation of methanol /3/.

The h y p e r c r i t i c a l drying i s performed in an autoclave of 1 l i t r e c a p a c i t y . The t o - t a l amount of acetone used i s 250 c c which corresponds t o 200 c c o f a d d i t i o n a l solvent. The autoclave i s heated a t a r a t e of 1.5"C/mn. The pressure a t 300°C i s 10 MPa. A t t h i s temperature t h e evacuation i s performed a t a r a t e of 5 MPa/hour.

3

-

CHARACTERIZATION OF THE AEROGELS

The a e r o g e l s produced from g e l s hydrolysed with f o u r molecules of water per mole of alkoxide a r e a l l monolithic. They a r e macroscopically homogeneous. The influence of the d i l u t i o n on t h e d e n s i t y and on t h e shrinkage during t h e autoclave process i s reported on t a b l e 2. Moreover, t h e d e n s i e s t g e l s a r e t h e most t r a n s p a r e n t .

Table 2 : Apparent d e n s i t y and l i n e a r shrinkage of a e r o g e l s vs t h e i r d i l u t i o n .

p(9/cm3

1

Linear shrinkage (%)

The s t r u c t u r a l evolution of an aerogel coming from a gel produced i n acetone (A) of 0.28 g/cm3 d e n s i t y i s studied a s a function of temperature using IR transmission spectroscopy. The sample t h i c k n e s s ( e ) i s 0.89 mm [Fig. 31. I t s thermal e v o l u t i o n may be compared t o t h a t of a c l a s s i c a l aerogel produced by hypercrit'ical drying in methanol ( M ) ( e = 0.82 mm;

p = 0.29 g/cm3) [Fig. 41. The thermal treatment i s performed a t t h e d e s i r e d temperature f o r t h i r t y minutes.

A t room temperature, t h e two a e r o g e l s A and M e x h i b i t some l i t t l e d i f f e r e n c e s in t h e i r s t r u c t u r e s . This i s shown by t h e presence of some a d d i t i o n a l o r d i f f e r e n t l y d i s t r i - buted bands i n t h e spectrum of aerogel A [Fig. 31 with regard t o t h e spectrum of aerogel M [Fig. 41.

F i r s t , t h e i n t e n s i t y of t h e band due t o f r e e s i l a n o l groups (3740 cm-l) i s higher than t h a t ascribed t o hydrogen bonded s i l a n o l s (3680 cm-')in t h e spectrum of aerogel A. Such a d i s t r i b u t i o n of hydroxyl groups i s unusual f o r c l a s s i c a l methanol i c a e r o g e l s .

Both s p e c t r a of a e r o g e l s A and M show several absorption bands l o c a t e d around 3000 cm-1. They a r e s i t u a t e d a t t h e same frequencies and a r e assigned t o t h e s t r e t c h i n g v i b r a t i o n s of C-H bonds. However, t h e r a t i o of t h e i n t e n s i t y of t h o s e bands t o t h e i n t e n s i t y of bands r e l a t i v e t o s i l a n o l groups i s l e s s i n aerogel A than i n aerogel M . Meanwhile, t h e e x i s t e n c e of such bands in t h e spectrum of aerogel A c l e a r l y demonstrates t h a t i t undergoes a methoxylation r e a c t i o n a s does aerogel M . As a consequence, a e r o g e l s elaborated i n acetone a r e a1 so hydrophobic.

Several e x p l a n a t i o n s may be proposed t o account f o r t h i s r e a c t i o n of e s t e r i f i c a t i o n in acetone medium. I t may r e s u l t from t h e methanol r e l e a s e d during t h e hydrolysis reac- t i o n . On an o t h e r hand, i t i s known t h a t acetone, submitted t o high temperature and pressure, can undergo decomposition and condensation r e a c t i o n s /4/. The by-products may r e a c t with t h e g e l . This l a s t assumption i s supported by t h e f a c t t h a t t h e spectrum of aerogel A shows two unusual absorption bands. Those bands a r e l o c a t e d a t 1710 cm-I and 1650 cm-l.

A gas chromatography study of t h e condensed l i q u i d , removed from autoclave during the h y p e r c r i t i c a l drying s t e p , shows t h a t i t contains, in a d d i t i o n t o acetone, several o t h e r heavier compounds.

Experiments a r e in progress t o b e t t e r understand t h e o r i g i n of t h e e s t e r i f i c a t i o n r e a c t i o n occuring i n t o t h e autoclave.

The s t r u c t u r a l evolution of aerogels A and M with temperature a r e very s i m i l a r . The appearence, in a small temperature range (around 300°C), of an absorption band l o c a t e d a t 1730 cm-1 i s due t o by-products of o x i d a t i o n , a s previously demonstrated /5,6/.

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C4-218 REVUE DE PHYSIQUE APPLIQUEE

Figure 3 : IR Spectra of aerogel A 40-4 Figure 4 : IR s p e c t r a of aerogel M 50-4 a s a f u n c t i o n of temperature. a s a function of temperature.

Measurements o f t h e weight l o s s e s of t h e aerogels a s a f u n c t i o n of temperature a r e performed with an heating r a t e of 3"C/mn. Between 200°C and 400°C t h e weight l o s s e s of aerogels A a r e lower than those of aerogels M of t h e same composition. They evolve in a s i m i l a r manner f o r temperatures above 400°C [Fig. 51. However, a s t h e d e n s i t y of aerogels A i n c r e a s e s , TGA curve e x h i b i t s pronounced weight l o s s e s s i t u a t e d a t 260°C-300°C and 450°C r e s p e c t i v e l y . This f e a t u r e i s probably r e l a t e d t o t h e desorption of chemical s p e c i e s produced by t h e oxidation r e a c t i o n . The t o t a l weight l o s s e s between 270°C and 600°C f o r g e l s A a r e i d e n t i c a l i r r e s p e c t i v e of t h e d i l u t i o n . However, t h e l o s s e s decrease a s t h e amount of water used t o c a r r y o u t t h e h y d r o l y s i s i n c r e a s e s .

T (-2) 4

Figure 5 : Weight l o s s e s of d i f f e r e n t a e r o g e l s a s a f u n c t i o n of temperature.

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The densificatian o f the aerogel

A

30-4 begins at 850°C and becomes very fast at 950°C as observed using dilatometric measurements carried out at a heating rate of 5"C/mn.

This sample was previously annealed at 500°C for ten hours in order to fully oxidize the organic residues. The total linear shrinkage of this sample, heated up to 110O0C, is about 45% [Fig.

61.

Figure 6

:

Linear shrinkage of aerogel

A

30-4 as a function of temperature.

The evolution of the density o f aerogel

A 40-4

under an isothermal treatment per- formed at 1040°C is shown on figure 7. To avoid fracture, the sample, annealed at 500°C for ten hours, is then heated at a rate of 10aC/mn to the above mentioned temperature. At this temperature the density of the aerogel increases first linearly with time. The density evolves slowly after six hours of treatment. More than ten hours of treatment are necessary to achieve the aerogel-glass conversion. The obtained silica glass has a refractive index similar to that of usual silica.

Figure 7

:

Density variation of aerogel

A 40-4

as a function of time.

The isothermal treatment is carried out a.t 1040°C.

t

^{2 -}

h 0

2 Fi

¹

V

a

0

The final water content is determined by IR spectroscopy on a massive sample using the intensity of the band situated at 2.73 pm. With an extinction coefficient equal to 77.5 l/mole/cm the measured water content of the glass is near 3500 ppm in weight.

T

-

^1040°C

/*

/O

-

<gkPO

I

2 4 6 8 10

Tlrne ( hours) --.-+

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4

-

CONCLUSION

S i l i c a g e l s may be e l a b o r a t e d f r o m o r g a n o s i 1 ic o n compounds by an h y d r o l y s i s r e a c t i o n performed w i t h acetone as s o l v e n t . The m o n o l i t h i c i t y o f t h e g e l s i s e a s i l y o b t a i n e d and t h e y can be t r a n s f o r m e d i n t o a e r o g e l s by h y p e r c r i t i c a l e v a c u a t i o n o f t h e s o l v e n t . Mono1 i t h i c i t y i s p r e s e r v e d d u r i n g t h i s process and p e r m i t s us t o t r a n s f o r m a e r o g e l s i n t o g l a s s by a s i m p l e thermal t r e a t m e n t . The d e n s i f i c a t i o n o c c u r s above 1000°C which i n d i c a t e s t h a t t h e s i n t e r i n g i s due t o a v i s c o u s f l o w phenomenon. The thermal e v o l u t i o n o f t h e s e a e r o g e l s i s analogous t o t h a t o f a e r o g e l s e l a b o r a t e d i n a l c o h o l i c medium. T h i s b e h a v i o u r can be r e l a t e d t o t h e i r s t r u c t u r e s w h i c h were demonstrated t o be s i m i l a r by I R spectroscopy.

REFERENCES

/ 1 / TEICHNER, S.J., NICOLAON, G.A., VICARINI, M.A., GARDES, M.E.E., Adv. C o l l o i d . I n t e r f a c e . S c i . 5 [ I 9 7 6 1 245.

/2/ KITAHARA, S., ASANO, T., B u l l . Fukuoka Univ. o f E d u c a t i o n 23 [ I 9 7 3 1 53.

/3/ WOIGNIER, T., PHALIPPOU, J., R i v . D e l l a Staz. Sper. V e t r o 5 [ I 9 8 4 1 47.

/4/ GRIGNARD, V., DUPONT, G., LOCQUIN, R., " T r a i t 6 de Chimie Organique", Tome V I I . Masson e t Cie, 1950.

/5/ ORCEL, G., PHALIPPOU, J., HENCH, L.L., J. Non C r y s t . S o l i d s 88 [I9861 114.

/6/ NICOLAON, G.A., TEICHIIER, S.J., B u l l . Soc. Chim. France 1 1 [1968] 4343.