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TEXTURAL AND STRUCTURAL STUDIES OF AEROGELS BY RAMAN SCATTERING AND
THERMOPOROMETRY
J. Rousset, A. Boukenter, Bernard Champagnon, E. Duval, J. Quinson, M.
Chatelut, J. Dumas, J. Serughetti
To cite this version:
J. Rousset, A. Boukenter, Bernard Champagnon, E. Duval, J. Quinson, et al.. TEXTURAL AND STRUCTURAL STUDIES OF AEROGELS BY RAMAN SCATTERING AND THERMOPOROM- ETRY. Journal de Physique Colloques, 1989, 50 (C4), pp.C4-163-C4-166. �10.1051/jphyscol:1989426�.
�jpa-00229502�
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Colloque C4, Supplement au n04, Tome 24, avril 1989
TEXTURAL AND STRUCTURAL STUDIES OF AEROGELS BY RAMAN SCATTERING AND THERMOPOROMETRY
J . L .
ROUSSET,
A. BOUKENTER, B.CHAMPAGNON, E.
DWAL, J.F.QUINSON*,
M.CHATELUT* ,
J.DUMAS* * and
J.SERUGHETTI* *
Laboratoire de Physicochimie des MatBriaux Luminescents, CNRS
UA-442.UniversitB C. Bernard, Lyon I ,
F - 6 9 6 2 2V i l l e u r b a m e Cedex, France ' ~ a b o r a t o i r e d e Chirnie Appliquee et de G6nie Chimique, CNRS
UA-417,Universite C. Bernard, Lyon I , . DBpartement de Physique des Materiaux, CNRS
F - 6 9 6 2 2V i l l e u r b a m e Cedex, France
UA-172,~ n i v e r s i t e C.
Bernard, Lyon I , F-
6 9 6 2 2V i l l e u r b a m e Cedex, France
Les r 6 s u l t a t s exp6rimentaux obtenus par d i f f u s i o n Raman b a s s e frgquenee e t thermo- porom6trie s u r l a s t r u c t u r e d 1 a 6 r o g e l s pr6par6s en m i l i e u b a s i q u e s o n t p r 6 s e n t b s . Les dimensions cks u n i t & t e x t u r a l e s -ou b i l l e s - e t d e s mesopores s o n t dzterminbs. Les r6- s u l t a t s s o n t en accord avec une s t r u c t u r e f r a c t a l e uniquement dans l e s u n i t b s t e x t u r a - l e s .
A b s t r a c t
Experimental r e s u l t s on t h e s t r u c t u r e of base-catalysed a e r o g e l s o b t a i n e d from low frequency Raman s c a t t e r i n g and thermoporometry a r e p r e s e n t e d . The s i z e of t e x t u r a l u n i t s o r beads and of mesopores a r e determined. These r e s u l t s a r e i n agreement with a f r a c t a l s t r u c t u r e only i n t h e t e x t u r a l u n i t s .
From very low frequency Raman s c a t t e r i n g two p r i n c i p a l informations on heterogeneous m a t e r i a l s can be e x t r a c t e d : (1) t h e s i z e of t h e h e t e r o g e n e i t i e s , (2) t h e d e n s i t y of v i b r a -
t i o n a l s t a t e s which depends on t h e m a t e r i a l s t r u c t u r e . The l i g h t i s s c a t t e r e d from t h e v i - b r a t i o n s l o c a l i z e d on t h e s u r f a c e of t h e h e t e r o g e n e i t i e s [ 13. The frequency of t h e s u r f a c e v i b r a t i o n s depends on t h e s i z e and on t h e shape of t h e h e t e r o g e n e i t i e s . I n t h e c a s e of sphe- r i c a l h e t e r o g e n e i t i e s a maximum of Raman s c a t t e r i n g r e l a t e d t o t h e s u r f a c e v i b r a t i o n s occurs a t t h e frequency urn, [l] :
wmax= 0.8 vo/ 2a (1)
v i s t h e sound v e l o c i t y i n t h e h e t e r o g e n e i t y and a t h e r a d i u s of t h e h e t e r o g e n e i t i e s I t has been shownr2 t h a t t h e Stokes Raman i n t e n s i t y I ( @ ) s c a t t e r e d from a f r a c t a l a t t h e frequency w i s given
d
y t h e following e q u a t i o n :Ilw) /ln(w) +
g d
w 2 d+;ID
a-i (o)(2)
D .
When t h e s c a t t e r i n g from t h e d i f f e r e n t p o i n t s i n a f r a c t a l volume R 1 s i n c o h e r e n t R i s t h e l o c a l i z a t i o n l e n g t h of t h e f r a c t a l v i b r a t i o n of frequency w
g(w) i s d e n s i t y of v i b r a t i o n a l s t a t e s , D t h e f r a c t a l dimension, T t h e f r a c t o n dimensionality, d+ t h e s u p e r l o c a l i z a t i o n exponent, which i s such t h a t t h e f r a c t o n wave f u n c t i o n decays a s p ] :
r ) N exp
-
ard* ( 3 )2
t h e c a s e of c o h e r e n t s c a t t e r i n g i t would be found L4.51 :-
4I(w)/&l(w) +
d6<
a 2d+d /D w-d w- 1g (w) (4)
Defining t h e reduced i n t e n s i t y a s
I r e d (w) = I(W) .wIE(w) + 1
1
t h e f o l l o w k g power-law i s o b t a i n e d :
Using t h e r e l a t i o n of g(w) v e r s u s @ f o r a f r a c t a l [ 6 ] :
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989426
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t h e Raman e x p o n e n t v i s :
i n t h e c a s e of i n c o h e r e n t s c a t t e r i n g
.
And :f o r coherent s c a t t e r i n g
Thermoporometry i s a thermal method [7,8] f o r determining t h e p o r e r a d i u s , t h e porous volume and t h e s p e c i f i c s u r f a c e . T h i s method a n a l y s e s t h e c o n d i t i o n s o f t h e s o l i d i f i c a t i o n of a condensate s a t u r a t i n g t h e porous m a t e r i a l . I t has been shown t o be a powerful method t o c h a r a c t e r i s e t h e porous t e x t u r e of wet gels[9]
.
However t h i s technique can be extended t o t h e s t u d y of a e r o g e l s i f t h e p r e p a r a t i o n conditions of t h e samples a r e c a r e f u l l y chosen.Low frequency Raman s c a t t e r i n g and thermoporometry a r e two complementary techniques f o r t e x t u r a l and s t r u c t u r a l s t u d i e s of porous m a t e r i a l s l i k e s i l i c a - a e r o g e l s .
The s i l i c a - a e r o g e l s samples were obtained from A i r g l a s s ( S t a f f e n s t o r p , Sweden). They were prepared by base c a t a l y z e d h y d r o l y s i s and condensation of t e t r a m e t h o x y s i l a n e i n methanol i n presence of ammoniac. The s o l v e n t was removed under h y p e r c r i t i c a l c o n d i t i o n s . The d e n s i t y of t h e samples v a r i e d from 0.075 t o 0.25. The experimental c o n f i g u r a t i o n t o observe t h e Raman s c a t t e r i n g was conventional. The i n c i d e n t l i g h t was e m i t t e d from an argon l a s e r . Various l i n e s were used 4545, 4880 and 5145 A'. The power of t h e l a s e r v a r i e d between 100 and 300
mw.
A Jobin-Yvon UlOOO s p e c t r o g r a p h and a photon c o u n t i n g system were used t o a n a l y s e t h e s c a t t e r e d l i g h t .For thermoporometry t h e f i l l i n g of a e r o g e l s pores by t h e condensate l i q u i d induces a c a p i l l a r y s t r a i n p r o p o r t i o n n a l t o t h e s u r f a c e t e n s i o n between t h e l i q u i d and t h e s k e l e t o n of t h e a e r o g e l , and a p t t o d e s t r o y t h e f r a g i l e t e x t u r e of t h e sample. I n our c a s e t h e condensate used f o r thermoporometry i s water, l i q u i d w i t h a s t r o n g s u r f a c e t e n s i o n . To avoid t h e t e x t u r e of a e r o g e l s t o c o l l a p s e t h e following procedure h a s been r e t a i n e d :
a ) The a e r o g e l sample i s k e p t i n vaporous e t h a n o l d u r i n g 20 hours, and t h e n f i l l e d w i t h l i q u i d e t h a n o l . Ethanol has a low s u r f a c e t e n s i o n .
b) The e t h a n o l i s slowly exchanged w i t h w a t e r , by u s i n g ethanol-water m i x t u r e of i n c r e a s i n g water c o n c e n t r a t i o n i n o r d e r t o r a i s e slowly t h e s u r f a c e t e n s i o n u n t i l t o completely f i l l t h e sample w i t h pure water.
I n f i g u r e 1 t h e Raman s c a t t e r i n g I(U)$(U) + ] ] a t t h e f o o t of t h e Rayleigh l i n e i s shown f o r t h e f o u r a e r o g e l s samples. The maximum r e l a t e d t o t h e s i z e of t h e building-blocks o r t h e diameter 2a of t h e beads s h i f t s towardsthe Rayleigh l i n e when t h e d e n s i t y i n c r e a s e s . Using t h e e q u a t i o n ( l g and choosing a sound v e l o c i t y VO = 4000 m/s, t h e diameters of t h e beads a r e de- duced and g i v e n i n table-1. The chosen sound v e l o c i t y i n t h e beads i s j u s t i f i e d by t h e den- s i t y i n t h e beads which i s c l o s e t o 1.8 a s determined by Walrafen e t al.[10].
The f i g u r e 2 shows t h e logarithm of t h e reduced i n t e n s i t y v e r s u s u
.
The p l o t s a r e l i n e - a r between approximately 30 cm and 200 -1.
The s l o p e s a r e c l o s e t o 1 f o r t h e d i f f e r e n t a e r o g e l s .From t h e o r e t i c a l c o n s i d e r a t i o n s [11]
,
t h e p o s s i b l e v a l u e s of d4 a r e l i m i t e d : 1< cl+< 1.3On t h e o t h e r hand SAXS measurements i n d i c a t e t h a t t h e s t u d i e d a e r o g e l s a r e f r a c t a l on l e n g t h s s m a l l e r t h a n 100 A' with a f r a c t a l dimension D z I . 9
.
Comparispn w i t h t h e Raman~ c a t t e r i n g measurements ( t a b l e 1) and a l s o w i t h t h e e l e c t r o n microscopy L12,13]leads t o a f r a c t a l s t r u c t u r e i n s i d e t h e beads o r b u i l d i n g blocks.
From e q u a t i o n s ( 6 ) , i t i s o b t a i n e d 0.85 <
2
< 1i f t h e Raman s c a t t e r i n g i s i n c o h e r e n t 1131 and 1.45<
d<
2i f t h e Raman s c a t t e r i n g i s coherent.
Our measurements do n o t a l l o w t o d e c i d e i f t h e Raman s c a t t e r i n g i s c o h e r e n t o r incohe- r e n t . However we can assume t h a t i n t h e a e r o g e l s t h e d i s o r d e r d e s t r o y s t h e coherence of t h e f r a c t a l v i b r a t i o n s . I n e l a s t i c neutron s c a t t e r i n g would b r i n g t h e s o l u t i o n of t h i s problem. I n t h e c a s e of epoxy-resins and DGEBA comparison beetween Raman s c a t t e r i n g and i n e l a s t i c n e u t r o n s c a t t e r i n g showed t h a t ' t h e Raman s c a t t e r i n g i s i n c o h e r e n t 1141
.
F i g u r e 3 shows t h e p o r e s i z e d i s t r i b u t i o n c u r v e s f o r t h e f o u r a e r o g e l samples, obtained from thermoporometry measurements. The r e s u l t a r e c o l l e c t e d i n t a b l e 1. The pore d i a m e t e r s correspond t o t h e maximum of t h e pore s i z e d i s t r i b u t i o n curve.
T a b l e 1
s
10 13 20 215 R a m o n s h i f t(cm- )
Density of a e r o g e l
0.075 0.1 0.2 0.25
F i g u r e 1
-
Low frequency Raman s p e c t r a f o r F i g u r e 2-
Log-log p l o t of t h e reduced base c a t a l y z e d a e r o g e l s of d i f f e r e n t i n t e n s i t y v e r s u s wd e n s i t i e s p
The thermoporometry measurements allowed u s t o determine t h e c h a r a c t e r i s t i c s of t h e mesopores. Comparison between t h e a e r o g e l d e n s i t y and t h e mesoporous volume i n d i c a t e s t h a t t h e macroporous volume (diameter of p o r e s l a r g e r than 1000 A') i s dominant.
Comparison between Raman s c a t t e r i n g and thermoporometry ( t a b l e I ) i s i n t e r e s t i n g . The r a t i o of t h e s i z e of mesopore over t h e s i z e of beads v a r i e s from 2 t o 3 when t h e d e n s i t y of t h e a e r o g e l i n c r e a s e s . The r a t i o of t h e number of beads over t h e number of p o r e s i n t h e meso- s t r u c t u r e goes from 3 t o 6. The d e n s i t y of t h e mesopores s t r u c t u r e (without macropores) i s e q u a l tow0.3. The a p p a r e n t d e n s i t y of t h e a e r o g e l s i s mainly dependent only on t h e macropo- r o u s volume.
The meso-structure would be s i m i l a r t o a non f r a c t a l d i s o r d e r e d cubic network i n which a u n i t edge would be b u i l t with t h r e e o r f o u r beads. The f r a c t a l s t r u c t u r e e x i s t s only i n t h e beads o r b u i l d i n g blocks.
Diameter of beads 2 a
As 53 ? 5 61 ? 5 8 5
"
82
'
5Diameter of mesopores
A 100
+
5140
"
246 9 2 2 5 4 2 12
Mesoporous Volume p e r lg,, of matter(cm2) 1.45
1.64 2.23 2.52
A
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Figure 3
-
Pore size distribution as determinedfrom thermoporometry A: p = 0.075, B,ip = 0.1, C: p = 0.2, D: p = 0.25REFERENCES
/I/ DWAL,E., BOUKENTER, A., and CHAMPAGNON, B., Phys.Rev.lett.
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(1986) 2052/2/ BOUKENTER, A., CHAMPAGNON, B., DUVAL, E., DUMAS, J., QUINSON, J.F., and SERUGHETT1,J.
Phys.Rev.1ett. 57 (1986) 2381
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(1985) 6447/4/ BOUKENTER, A., CHAMPAGNON, B., DUVAL, E., ROUSSET, J.L., and ROSENBERG, H.M., Phil.Mag.
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