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ALUMINO-SILICATE AEROGELS
F. Chaput, André Lecomte, A. Dauger, J. Boilot
To cite this version:
F. Chaput, André Lecomte, A. Dauger, J. Boilot. ALUMINO-SILICATE AEROGELS. Journal de
Physique Colloques, 1989, 50 (C4), pp.C4-137-C4-142. �10.1051/jphyscol:1989421�. �jpa-00229497�
R E W E DE PHYSIQUE APPLIQu~E
Colloque C4, Supplement au n04, Tome 24, avril 1989
ALUMINO-SILICATE AEXOGELS
F. CHAPUT, A. LECOMTE*, A. DAUGER* and J.P. BOILOT
Groupe de Chimie du Sofide, Laboratoire de Physique de La matiere Condensee, Ecole Polytechnique, F-91128 Palaiseau cedex, France
" ~ c o l e Nationale Superieure de Ceramique Industrielle, F-87065 Limoges Cedex, France
RESUME
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Des aerogels dlalumino-silicates, de densite comprise entre 50 et 250 kg/m3, ont BtB Blabores B partir du prBcurseur mixte(Bu0) 2Al-O-Si (OEt) 3.
La diffusion centrale des rayons X montre que les structures des aerogels et des alcogels d'alumino-silicates sont form6es d'unitBs dlementaires dont l'assemblage par un mecanisme d'agregation conduit B des particules fractales en masse. L1auto-similarit6 est mise en 6vidence dans les aerogels pour un large domaine de densite.
ABSTRACT
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Alumino-silicate aerogels have been prepared from the (BuO) 2Al-O-Si (OEt) double precursor with different densities (from 50 to 250 kg/m3).SAXS study indicates that both alumino-silicate alcogel and aerogel structures can be represented by primary units which stick together into mass-fractal clusters (D = 1.8-1.9) by a mechanism of cluster-cluster aggregation. Self similarity is displayed in the alumino-silicate aerogels in a large range of densities.
We have previously shown /1/ that alumino-silicate alcogels can be prepared in the H20, H O ~ P ~ , (BuO) 2Al-O-Si (OEt) ternary phase diagram within a large range of oxide concentrations. The gelation time is governed by the initial water concentration, showing that the hydrolysis of Si-OR groups is the rate-determining process for the gelation.
The alcogel structure typically contains more than 90% by volume fine pores confining alcohol. To prevent damage to the gel structure caused by interfacial forces, the alcohol removal must be done under supercritical conditions. The resulting low density solid material is called aerogel and is now well known in the pure silica system /2-4/.
In this paper, we report the preparation of alumino-silicate aerogels with different densities and structural results obtained from alcogels and aerogels by small angle X-ray scattering (SAXS).
2
- EXPERIMENTAL
i-Gelation and drying
Alcogels have been prepared in the H20
-
H O ~ P ~-
( B u O ) ~ A ~ - O - S ~ (OEt) ternary phase diagram. A mixture of isopropanol and water is slowly poured into a mixture of precursor and isopropanol.The initial composition, noted 1.4.2 corresponds to one volume of water, four of solvent and two of precursor. Other compositions are obtained by dilution with 2-propanol.Samples were kept in sealed glass containers at 2S°C and the gel time was defined by observing the stiffness after tilting the container. For all the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989421
s a m p l e s , no s i g n i f i c a n t change o f volume i s n o t e d a t t h e g e l p o i n t .
S u p e r c r i t i c a l d r y i n g o f a l u m i n o - s i l i c a t e a l c o g e l s , i n an a u t o c l a v e system ( 6 5 b a r s
-
245OC) l e a d s t o a l u m i n o - s i l i c a t e a e r o g e l s which e x h i b i t d i f f e r e n t d e n s i t i e s . The a p p a r e n t d e n s i t y (p) measured by Hg v o l u m e t r y v a r i e s from 50 t o 250 kg/m3.ii-SAXS method
S m a l l a n g l e X-ray S c a t t e r i n g (SAXS) d a t a w e r e r e c o r d e d w i t h a s l i t t y p e camera (CuKa w a v e l e n g t h ) . The sample t o d e t e c t o r d i s t a n c e was 500mm and t h e s c a t t e r e d i n t e n s i t y was c o u n t e d w i t h a l i n e a r p o s i t i o n s e n s i t i v e c o u n t e r . The s c a t t e r i n g v e c t o r ~ = 4 l X s i n e / h where 8 i s t h e Bragg a n g l e , r a n g e s from 6 . 1 0 - ~ t o 2 . 1 0 - I A-l. E x p e r i m e n t a l r e s u l t s a r e c o r r e c t e d f o r p a r a s i t e s c a t t e r i n g and n o r m a l i s e d t o e q u i v a l e n t s a m p l e t h i k n e s s , i n c i d e n t i n t e n s i t y a n d c o u n t e r e f f i c i e n c y . To s t u d y t h e growth p r o c e s s , a sample c e l l i s u s e d w i t h 0,025 mm
"Mylar" windows and 0 , s t o l m m p a t h l e n g t h . Concerning SAXS measurements from a e r o g e l s a m p l e s , s l i c e s of a b o u t 2mm i n t h i k n e s s a n d l O m m i n d i a m e t e r were u s e d .
S c a t t e r i n g i n t h e G u i n i e r r e g i o n ( l o w a n g l e s ) d e p e n d s on t h e c h a r a c t e r i s t i c d i m e n s i o n R o f t h e s c a t t e r i n g e n t i t i e s a n d from t h e i n i t i a l d e c a y o f t h e s c a t t e r e d i n t e n s i t y I ( K ) , o n e c a n d e d u c e t h e e l e c t r o n i c r a d i u s o f g y r a t i o n R u s i n q t h e f o l l o w i n g f o r m u l a / 5 / :
3
(K)=n N e x p . ( - K ~ R ~ ~ / ~ ) (1)( R = ( 3 / 5 ) 'I2 x f? f o r s p h e r i c a l p a r t i c l e s )
where 'K i s t h e s c a t t e r i n g wave v e c t o r , N i s t h e number of s c a t t e r i n g p a r t i c l e s a n d n t h e number o f e l e c t r o n s i n e a c h p a r t i c l e ( n =p V where p i s t h e electronic P d e n s i t y o f p a r t i c l e s of volume V )
.
P P P PS c a t t e r i n g i n t h e P o r o d r e g i o n d e f i n e d a s R R >> 1 >> Ka, where t h e monomer s i z e i s a , d e p e n d s on t h e g e o m e t r i c s t r u c t u r e o f t h e s c a t t e r i n g e n t i t i e s / 5 / . The s c a t t e r e d i n t e n s i t y d e c a y s a s power l a w I ( K ) - K - ~ a n d t h e r e f o r e , i n a Log-Log p l o t of I ( K ) v e r s u s t h e s c a t t e r i n g v e c t o r , t h e s l o p e i s s e n s i t i v e t o t h e geometry o f t h e s c a t t e r e r s . F o r p o l y m e r - l i k e s t r u c t u r e s / 6 / ( m a s s - f r a c t a l o b j e c t s ) t h e c l a s s i c power-law e x p o n e n t x o b s e r v e d i n t h e P o r o d r e g i m e i s t h e f r a c t a l d i m e n s i o n D which r e l a t e s t h e s i z e R of t h e o b j e c t t o i t s mass M - R ~ .
i-Growth o f a l u m i n o - s i l i c a t e a l c o g e l s
N u c l e a r m a g n e t i c r e s o n a n c e / 7 / , s m a l l a n g l e X-ray s c a t t e r i n g / 8 / and s m a l l a n g l e n e u t r o n s c a t t e r i n g / 9 / s t u d i e s o f t h e g e l a t i o n f r o m t h e d o u b l e p r e c u r s o r (Bu0) 2Al-O-Si (OEt) have shown t h a t , i n t h e f i r s t s t e p s , a r a p i d f o r m a t i o n o f p a r t i c l e s t a k e s p l a c e by c o n d e n s a t i o n b e t w e e n A1-OH g r o u p s . T h e s e u n i t s formed o f A1-0-A1 l i n k a g e s a r e t h e n a g g l o m e r a t e d by a c l u s t e r a g g r e g a t i o n p r o c e s s c h e m i c a l l y l i m i t e d by t h e h y d r o l y s i s o f t h e Si-OR g r o u p s .
C o n c e r n i n g SAXS c u r v e s , t w o regimes a r e o b s e r v e d on t h e Porod p l o t s : -a power law regime w i t h a s l o p e o f 1 . 8 i n t h e (Rg/2)-10
A
l e n g t h r a n g e . -a d e v i a t i o n from t h i s power law ( s l o p e o f 2 . 6 ) i n t h e domain from 4 t o 8A.T h i s b e h a v i o r i s i n d e p e n d e n t of t h e p r e c u r s o r c o n c e n t r a t i o n and, f o r a g i v e n c o n c e n t r a t i o n and a g i v e n p o l y m e r i z a t i o n s t a t e , it d o e s n o t d e p e n a on f u r t h e r d i l u t i o n . I n f a c t , t h e s l o p e o f 1 . 8 i s o b s e r v e d , a t l e a s t n e a r t h e g e l p o i n t , o v e r a d e c a d e o f r e c i p r o c a l s p a c e ( A v e r y c l o s e v a l u e o f 1 . 9 h a s b e e n o b t a i n e d from S.A.N.S s t u d y / 9 / ) . T h e r e f o r e , t h e expo.nent, a t h i g h l e n g t h
s c a l e ( a b o v e 10A). c a n b e r e a s o n a b l y e q u a t e d t o t h e f r a c t a l dimension D . D u r i n g t h e g e l a t i o n , a n o t h e r i n d e p e n d e n t d e t e r m i n a t i o n o f t h e f r a c t a l
dimension can be realized £om variations of the extrapolated quantity I(K=O) by using the Guinier ap roximation:
I (K=O) = n N'
P (2)
Assuming that n .N is constant, i.e.the total mass in solution remains constant, then ?(K=o) = np and I(K-0) is proportional to the 2-averaged number of atoms in the particles. For a mass-fractal object:
I (K=O) a R D (3)
A plot of Log I(K=O? versus Log R gives for D a value very close to 1.8 consistent with a cluster-cluster growth mechanism /lo/. 9
In agreement with other observations of the gelation on silica and alumino-silicate sol-gel systems and in spite of the small range size explored, the SAXS study indicates that inorganic polymers in alcogels consist in primary polymeric particles, whose density depends on the hydrolitic conditions and on the reactivity of the precursor. These units stick together and form mass fractal clusters by a mechanism of cluster-cluster aggregation.
ii-Structure of alumino-silicate aerogels
Figure 1 contains SAXS data for six samples of different densities. For all the samples, two power-law regimes are seen in the Porod range with slopes of -1.85 and -4 (figure 2)
.
The crossover is ap roximately the same for all the samples and occurs at about K= 7.5.10- corresponding to a length of 13A.Using a fractal model to interpret the scattering data, the picture that emerges is a scattering from a smooth surface of homogeneous particles at short length scales (<13A) and scattering from a mass fractal of dimension 1.85 at larger scales.
Therefore, as observed for alcogel samples, the SAXS study indicates that the aerogel structure can be represented by a two level structure: It consists in primary dense particles (density pa and size a) attached into mass fractal clusters by a mechanism of cluster-cluster aggregation.
Self similarity previously displayed in the silica system /11/ can be easily shown in the alumino-silicate system:
-
Firstly, each sample has the same fractal dimension and the figure 3 clearly indicates that, in the fractal range, the different samples cannot be distinguished. Moreover, for high density samples a third regime exists at small K consistent with uniform nonfractal long range structure, corresponding to a decrease of the fractal range (R(A) in size) with increasing density.-
Secondly, from the initial decay of the intensity in the Guinier region, we deduce the electronic radius of gyration Rg. From the Log-Log plot of the radius of gyration versus the apparent density p (figure 4), one finds that the function relating the extension of the fractal domain to the density is the same for all the samples: Rg= 18.7* - O e e 4Using the mass-f ractal law: p /p = (R/a)g'3, ta;ing the same desciption for P a
primary units and considering as equal the density of the fractal particles and the average density of the sample (pp= p), one finds D=1.81 from the power-law exponent of figure 4, a value close to that determined from figure 2 establishing the mutual self-similarity within a large range of densities.
Moreover, it is important to note that, assuming the existence of spherical mass-fractal particles, the prefactor value (18.7) is consistent with the existence of primary units (13A in size) of density 2.1g/cm3
,
whichis relatively close to the one (2.5 g/cm3) of the usual alumino-silicate glass with the same composition (Al/Si molar ratio=l.) /12/.
-Finally, using equation 3, a plot of Log I(K=O) versus Log R leads for g
Fig.1
-
Scattered intensities for six alumjno-silicate aerogel samples. The curves are labelled with p in ~ g / r n ~ . Starting from the top, each intensity was divided by two compared to the previous one.Fig.2
-
Porod plots for alumino-silicate aerogels showing two power-law regimes with slopes of -1.85 and -4. The crossover approximately occurs at~ = 7 . 5 . a'-A
Fig.3
-
Scattered intensities at low K and in the intermediary K range for different alumino-silicate aerogel samples (the curves are labelled with p in Kg/m 3 ) showing that each sample has the same fractal dimension and that the different samples cannot be distinguished in the fractal range. Moreover, the fractal range decreases in size with increasing density.Fig.4
-
Function relating the extension of the fractal domain to the density for alumino-silicate aerogels, revealing that all samples are mutually self-similar.D to a value of 1-84, which is another verification of the self-similarity in the alumino-silicate aerogels.
Alumino-silicate alcogels can be prepared from the (Bu0) 2Al-O-Si (OEt) 3 complex precursor in a large range of oxide concentrations. Supercritical drying of alumino-silicate alcogels, in an autoclave system (65bars
-
24S°C)leads to alumino-silicate aerogels which exhibit different densities (from 50 to 250 kg/m3).
SAXS study indicates that both alumino-silicate alcogel and aerogel structures can be described by a two level structure: It consists in primary units, polymeric particles for alcogels (8A in size) and dense particles for aerogels (density 2.1 g/cm3 and 13A in size), which are attached together into mass fractal clusters by a mechanism of cluster-cluster aggregation. The fractal dimension (D=1.8-1.9) is the same in the two structures.
Self similarity previously displayed in the silica system is also observed in the alumino-silicate aerogels within a large range of densities.
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