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Structural characterization of SiO2Al2O3 aerogels
J O U R N A L OF
ELSEVIER
Journal of Non-Crystalline Solids 186 (1995) 149-158
Structural characterization of
SiO2-A1203 aerogels
B. Himmel a,*, Th. Gerber a, H. Biirger b, G. Holzhfiter a, A. Olbertz b ~ FB Physik, Unit'ersitiit Rostock, Unicersitiitsplatz 3, D-18051 Rostock, Germany b Otto-Schott-lnstitut, Friedrich-Schiller-Unit,ersithtJena, Fraunhoferstrasse 6, D-O7743.1ena, Germany
Abstract Low-density x S i O 2 - ( l - x ) A l 2 0 3 aerogels with x = 0, 0.3, 0.7, 1.0 (mole fractions) were prepared by CO 2 supercritical drying of alkogels formed from Si(OMe) 4 and AI(OBu~)3 by hydrolytic polycondensation under different chemical conditions. The resulting aerogels were structurally characterized by X-ray scattering at small and large angles as well as by electron diffraction and transmission electron microscopy. By investigating the dependence of linear shrinkage on the heat treatment it was found that the sintering of the aerogel is influenced by the presence of A I 2 0 3 in the network. Crystallization experiments were also performed up to I200°C. The reduced height of the first sharp diffraction peak indicates less order in the SiO 2 network of the aerogel in comparison with amorphous SiO 2 prepared by other methods.
1. Introduction
Development of inorganic porous materials, which are resistent to sintering for temperatures higher than 800°C, offers many technological applications, for instance, in the field of catalytic combustion. The sol-gel method for preparing glasses, glass-ceramics and ceramics provides both the possibility of tailoring multi-component systems on the molecular scale and coating of materials by dipping or spraying at ambient conditions. The consolidation of the surface coating without loss of porosity can be achieved by supercritical drying. In the following, we discuss structural investigation of aerogels in the SiO 2AI20 3 system. The differences in structure and bond chemistry between Si- and Al-alkoxides requires a sophisticated procedure of hydrolysis and condensation of
* Corresponding author: Tel: +49-38l 492 3212. Telefax: +49-381 498 1626. E-mail: [email protected].
mixed (A1,Si) gels for the tailoring on the molecular scale. The tri-alkoxides of aluminum are more complex than the tetra-alkoxides of silicon, which are known to be monomeric [1]. As concluded from the 27A1 nuclear magnetic resonance (NMR) investigation by Kriz, the [AI(OBu~)3],, complex is trimeric (n = 3), composed of one fivefold- and two fourfold-coordinated aluminum atoms depending on parameters such as solvent and temperature [2,3]. As a rule, the metal prefers maximal coordination but that is hindered by steric effects [1]. Structural changes of alkoxides during hydrolysis and condensation reactions are not completely understood. Several routes are available for preparing chemically homogeneous xSiO2-(1 - x ) A I 2 0 3 gels via the alkoxide gel method. Heinrich and co-workers [4-6] used chelated Al-alkoxides in order to reduce the reaction rate of [AI(OBu~)3],, in comparison with Si(OEt) 4. They found, by in situ Small-angle X-ray scattering (SAXS) of solutions having the stoichiometric composition of mullite, that the primary particle size decreases with the number of blocked con-
01122-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 0 4 5 - 3
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B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
densation sites. A fractal dimension of df = 2.19 was found prior to gelation. Pouxviel et al. [7,8] used the double alkoxide ( B u O ) 2 A I - O - S i ( O E t ) 3. They concluded from their small-angle neutron scattering (SANS) studies that the hydrolysis of the S i - O R groups is the rate-determining step. After a rapid formation of primary particles via A 1 - O - A I linkages the further condensation and aggregation reactions lead to the formation of fractal clusters with df = 1.9. A third route could be the adjustment of the reaction rates by prehydrolysis of the slower reacting Si-alkoxide. Yoldas [9] developed the process of hot water hydrolysis of AI(OBu) 3 with a large excess of water followed by the peptization of the resulting fibrillar boehmite in an acidic environment for the preparation of transparent monoliths. Conditions during the hydrolysis and gelation influence the crystallization of mullite [10-13]. Also, for pure A1203 gels, the chemistry is determinative for the structure formation. Pierre and Uhlmann [14] distinguished between boehmite-like gels and 'super-amorphous' gels depending on the temperature. Mizushima et al. prepared alumina aerogels of p = 180 k g / m 3 (CO 2 supercritical drying) and of p = 130 k g / m 3 ( C 2 H s O H supercritical drying)from chelated AI(OBu~)3 diluted with ethanol and obtained more or less pronounced crystalline peaks of boehmite depending on the drying regime [15]. It is well known that SiO 2 aerogels sinter at temperatures above 700°C leading to shrinkage and densification. The present study is focused upon synthesis and characterization of low-density x S i O 2 - ( l - x ) A 1 2 0 3 aerogels that are resistant to sintering.
The x S i O 2 - ( 1 - x ) A l 2 0 3 samples were prepared as 10 wt% solutions of TMOS in methanol and of aluminum sec-butylate (Al(OBuS)3) in iso-butanol and water without use of a catalyst (r w = 4). The mixture of AI(OBu~)3 and i-BuOH was added dropwise to the mixture of TMOS, 1 mol H 2 0 and half of the quantity CH3OH under vigorous stirring. After some minutes, the residual H 2 0 , together with the second half of CH3OH, was added dropwise. The TMOS was not prehydrolyzed. The time to gelation was less than 30 min for both compositions. The preparation of homogeneous amorphous A1203 alkogels is comparatively difficult, because of the occurrence of dimeric and oligomeric units in aluminum alkoxides and their sensitivity to hydrolysis. Partially hydrolyzed products precipitate easily by use of a normal hydrolysis procedure, whereby the alkoxide is first desolved in half of the solvent followed by water together with the other half under vigorous stirring. An indirect hydrolysis of the alkoxide via ambient humidity saturated with alcohol leads to the formation of white surface layers of partially crystallized products, that sink and redevelop after some time. Depending on the water content, A1 concentration and aging time, crystallization can occur even in the solution. According to Teichner et al. [16] completely amorphous solids can be obtained for r w < 4 and for concentrations < 10 wt% AI(OBuS)3 in butanol. Therefore the aluminum secbutylate was diluted in the first case with ethanol (AOE) and in the second case with i-butanol (AOB) to 5 wt%. The quantity of water necessary for r w = 3 was generated in situ through the equilibrium reaction of alcohol and acetic acid: C2HsOH + CH3COOH
2. Experimental 2.1. Sample preparation Preparation of the SiO 2 samples was performed by dropwise addition of the required quantity of TMOS (Si(OMe) 4) to methanol (rl. = 15, where r L is the molar ratio of solvent and alkoxide). After vigorous stirring for 2 min, a mixture of H 2 0 and 0.5 normal NH 3 was added dropwise, to realize r w = 2 and pH ~ 8 (r,~, is the molar ratio of water and alkoxide). The gelation time was 55 rain.
O ) H2O + C H 3 - - C H 2 - - O - - C - - C H
3.
(1) The reaction equilibrium is shifted in favour of the formation of the ester, because the water is consumed immediately by hydrolysis. After a few days compact white alkogels were formed in closed tubes. Drying of the alkogels was performed above the critical point of CO 2 (Tc = 304.2 K, Pc = 74 × 105 Pa) after an exchange of water and alcohol to carbon dioxide by use of the critical point dryer CPD2
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158 Table 1 Compositions, preparation and densities xSiO2-(l - x)Al203 aerogels
p ( g / c m 3) of the
Sample
x
rw
Conc.
pH
Aging ~
p
SiO 2 AS70 AS30 AOE AOB
1.0 0.7 0.3 0.0 0.0
2 4 4 3 3
r L = 15 10 wt% 10 wt% 5 wt% 5 wt%
basic neutral neutral acid acid
7 20 20 12 12
0.131 0.063 0.080 0.168 0.158
with 2
Aging (days) means the time between gelation and supercritical drying.
(Pelco). The liquid-fluid transition could be observed clearly through a window. The solvent exchange of the cylindrical samples took 8 h inside the pressure vessel of the autoclave cooled to about 285 K. The samples were heated slowly and continuously up to (315 + 1) K, where the maximum pressure of (90_+ 4 ) × 105 Pa was reached. The composition and the density of the aerogels are listed in Table 1. For the temperature-dependent investigation, the aerogels were heated at a rate of 5 K / m i n to the final temperature and held for 10 min at this temperature. Cooling to room temperature was performed slowly in the oven.
2.2. Characterization methods The (SAXS) experiments were performed on a Kratky compact camera (Anton Paar, Graz) mounted on a 2 kW X-ray generator with Ni-filtered Cu Ko~ radiation by use of a proportional counter. The SAXS experiment is characterized by a visual field rmax = nV/Smio of 80 nm with s = 4rr s i n ( 0 ) / a (20 is the scattering angle and h is the X-ray wavelength). During the measurements the aerogels were stored in sealed capillaries. We discuss the infinite-width slitsmeared scattering curves as well as the corresponding distribution functions. Details of data treatment have been given previously [17]. The correlation function was calculated from the equispaced points of the scattering curve l(k As) by discrete Fourier transformation using the expression [18] 1
C(r) -
151
k max
,~2
~ sin kv:yZ~U77Tx 2
'n" "o
V'Y: + x2
and
K Cre(r)-
~
Jo (sr)
4Tr2 , ,,x~+±~/2.
S2
dr,
(4)
where the remainder term, Cre(r), takes into account the termination ( K = Porod constant with l ( s ) = K~ s3, J0 = Bessel function). The distance distribution function, 4 " n r 2 C ( r ) , gives the probability for finding electron density in a spherical, infinitesimally thin shell of radius, r. However, the finer features at smaller values of r are hidden in this function as a result of the stronger weighting of larger distances. This information can be displayed as the chord length
distribution, A(I) [19]: A(l) = l(daC(r)/dr
x),= ,.
(5)
A(l)dl is a measure of the probability that a line between two surface points lies in the interval (l, / + d/). The X-ray diffraction system XRD3000 (Seifert) equipped with a curved position-sensitive detector (PSD INEL, 120 ° angular aperture) was used to determine crystalline phases with Cu K~ radiation after heat treatment. The channels of the PSD were calibrated with polycrystalline silicon powder. The instrument used for our electron microscopic electron diffraction investigation is a Zeiss EM912 with an integrated electron filter (omega filter). Images are recorded on Scienta 23D56 films (Agfa Gevaert). The diffraction patterns were recorded via the photomultiplier system of the EM912 with camera lengths between 3600 and 7200 mm. All measurements were carried out with zero-loss electrons at 100 kV. The reduced intensity function, which was extracted from the experimental data, is defined as
i(s)
=
(lexp(S)
-
(f(s)2>)/(f(s)>2,
(6)
with
~_~ (k As)21(k A s ) Z ( k , A s r ) 4"rr2 k= 1
( f ( s ) 2} = ~ x i f , ( s ) 2 q- Cre ( r ) ,
(2)
i-1
(7)
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
152
Table 2 Structure parameters of the investigated aerogels
and (f(s)
~ xifi(s), i-I
2) =
(8)
where f, is the atomic form amplitude of the atom i according to Doyle and Turner [20], x i is the concentration with £ x i = 1 and n is the number of different types of atom.
3. Results Fig. l(a) shows the dependence of the S A X S curves of the S i O 2 - A 1 2 0 3 aerogels on the composition. With higher A I 2 0 3 content, we observe the formation of a weak maximum in the region of 3 nm ~ < s < 4 nm 1 (small inhomogeneities), a shift of the shoulder in the central region in direction to
i(s)
. . . . . . . .
10'
i
I
611
1'.0
3~ 2
1¢
1--
10z
(a) 0.01
.......
s into'i]'
c(o 1.0 0.8 0.6 0.4 0.2 0.0
Fig. 1. Scattcring curves (a) and correlation functions (b) of aerogels having different compositions (1 - SIO2; 2 - AS70; 3 AS30; 4 - AOB; 5 - AOE).
Sample
R~c I (nm)
SiO 2 * AS70 AS30 AOB * AOE *
-
0.4 0.4 0.4
Rsc 2 (rim) <
< < < <
8
25 30 12 12
R~c3 (rim)
b
d (nm)
9.8 23.7 29.5 > 40 > 40
2.96 2.72 3.08 3.03 3.00
5-10 10-20 10-25 (22-25) (22-25)
R~c, electronic radius of gyration calculated with the Guinier approximation (R~c I and R~c2) and as momentum of the correlation function (Rsc3); b, negative slope of the scattering curve between Smi,, = 0.5 nm I and Sm~X = 2.5 nm I approximated with l ( s ) ~ s b + c using the damped Newton method [21] (* Sm~,x = 2.0 nm ~); d is the particle diameter estimated from TEM.
lower s-values (growth of intermediate-sized inhomogeneities) and a steeper rise for s < 0.1 nm 1. The scattering of the alumina for s < 0.07 n m - 1 can be attributed to a new generation of inhomogeneities on a larger length scale. The correlation functions calculated according to Eq. (2) can be seen in Fig. l(b). The dependence of intermediate-sized inhomogeneities on composition is manifested by the peak shift in the function rC(r). The shoulder in the correlation function of the aerogels with higher A1203 content ( x = 0.0 and x = 0.3) at lower values of r is determined by the smallest inhomogeneities. The calculated structure parameters for all aerogels investigated are summarized in Table 2. Typical electron micrographs are shown in Fig. 2. The structure of the mixed aerogels ( x = 0.3 and x = 0.7) can be characterized by small agglomerated particles having nearly a spherical shape. These particles are connected through necks and form branched chain-like clusters. These clusters contain mesopores and macropores of variable size. Most of them are larger then the visual field of the S A X S experiment. The microstructure of the SiO 2 aerogel is similar to that of the both mixed aerogels. However the particles are smaller and the clusters seems to be denser. A rough estimation of the average particle diameters deduced from the transmission electron microscopy (TEM) micrographs is given in Table 2. By contrast with the SiO2-containing aerogels, the microstrueture of the A l e O 3 aerogels can be described by very large inhomogeneous particles ( > 150 nm in diameter) which are composed of distorted area-like regions de-
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
tectable by TEM only near the surface of the particles. The lateral extension of some flat regions amounts to approximately 20 × 25 nm 2. By analyzing the weak contrast differences near the surface of the particles (not resolved by the photocopy shown in Fig. 2(d), we conclude that these flat regions have a thickness of approximately 2 nm. To investigate the linear shrinkage, S L, the SiO 2 aerogel and the 0.7SIO2-0.3A120 3 aerogel were heat-treated at 300, 500, 700 and 900°C for 10 min. The temperature dependence of the linear shrinkage is nearly the same for both aerogels in the temperature range to about 700°C (Fig. 3). However, at higher temperatures densification of the SiO 2 aerogel increases drastically caused by enhanced processes
153
of viscous sintering. In contrast to the SiO 2 aerogel showing a linear shrinkage of approximately 48% after a heat treatment at 900°C the linear shrinkage observed for the 0.7SIO2-0.3A120 3 aerogel is only 25%. This behaviour indicates a retardation of viscous sintering due to the presence of A I 2 0 3 . The structural changes and crystallization of the aerogel AS70 (0.7SIO2-0.3A1203) was investigated after heating at 900, 1000, 1100 and 1200°C for 10 min. The aerogel remains amorphous until heating above 1000°C (Fig. 4). First traces of poorly crystallized mullite were observed after heating at 1100°C, although the composition of the aerogel deviates from the ideal mullite of AI:Si = 3:1 (3 A I 2 0 3 2SIO2). There is no hint of weakly crystallized ",/-
50 nm I
I
d
n Fig. 2. Transmissionelectron microscopicpictures of the xSiO2-(1 - x)AI203 aerogels:(a) SiO2, (b) AS70, (c) AS30, (d) AOB.
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
154 g
I(s)
4. . . . . . . . %
106
3
104
i
t
........
b
.......
5o ......................................................................
gc
.~ 20 .........
i ........
i............. i .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
lO
.
.
.
.
.
.........
0
400
200
600 800 Temperature
A1OOH boehmite in the untreated aerogel at low temperatures or of an intermediate "y-A1203-1ike AISi spinel phase in the aerogel heat-treated to 1200°C. All the peaks could be assigned to the mullite phase
'
'
'
1 flO00°C ' 2- 1100°(3 3 - 1200°C
\
15 * 1 / ~ ~ / 2 ~ . . . 10 /2
*
5
*
*
10
20
30
I l/ J
II
50 s [rum"I]
40
Jl,tf
i A
I , II I
0.0
.......
6'.1
.......
i'.o
'
' '~L~-'I]
Fig. 5. Dependence of the scattering curves of the aerogcl AS70 on the heat treatment in the representations l o g ( I ) versus log(s).: 1 - room temperature, 2 - 900°C, 3 - 1000°C, 4 - l l 0 0 ° C , 5 1200°C (time: 10 min).
for T > 1000°C as marked in Fig. 4 by stars. The intensity decrease for s < 2 nm -t is caused by a primary beam stop close to the detector. The results of the SAXS investigations are presented in Figs. 5 and 6. Two clearly separated Guinier regions can be seen in the scattering curves of the aerogel that was treated at 900°C, indicative of the formation of two types of inhomogeneity. By contrast with the inhomogeneity of the untreated aerogel, these inhomogeneities (particle or pores) can be described by smooth phase boundaries, and accordingly the Porod plot I(s) versus s 3 shows two plateaus. The maxima of the chord length distribution (Fig. 6) is determined by the size of these
1.2
J,
t
I0z
[°C]
Fig. 3. Linear shrinkage, S L = (1 - L ~ / L o ) X 100, on the temperature, Lo and L~ being the size of the sample before and after the heat treatment ( l I , SiO2; 0 , 0.7SIO2-0.3A1203). The lines are drawn as a guide to the eye.
I(s) ~V ~ 20 ~
5
A(1)
-2Si02
l,l
0.8
I
4-
A1203 I
I
,
0.4
i
Corundum I
.,
1. ,
, ,, rJ
Fig. 4. Crystallization bchaviour of the aerogel AS70 after a heat treatment at 1000°C (curve 1), l l 0 0 ° C (curve 2) and 1200°C (curve 3) for 10 rain. The peak positions of selected crystalline modifications according to Refs. [22,23] are shown for comparison.
0.0 0
10
20
30
4o
50
t[nm] Fig. 6. Chord length distributions calculated from the scattering curves shown in Fig. 5 : 1 - room temperature, 2 - 900°C, 3 1000°C, 4 - l l 0 0 ° C (time: 10 rain).
155
B. Himmel et al. /Journal of Non-Crystalline Solids" 186 (1995) 149-158
inhomogeneities. They become gradually larger with further heat treatment to 1100°C. The scattering curve of the aerogel heated to 1200°C is dominated by scattering from large mullite crystallites. A powerlaw decay l ( s ) c x s -3 is observed in the low-s region. Both A1203 aerogels show similar structural features inside the size regime probed by SAXS (Fig. l(a)). The smallest inhomogeneity (peak at s =-3 nm 1) corresponds to R~c = 0.4 nm which is a measure for the average thickness of the disc-like regions observed by TEM. The peak at s = 3 nm i may indicate interference of close-packed inhomogenities. The lateral dimension of these distorted area-like regions estimated from TEM micrographs is confirmed by the SAXS investigations (Rsc 2 in Table 2). The most probable chord lengths of 3.6 nm for the gel AOB and 3.4 nm for the gel A O E are derived from the chord length distribution. The tail of the curves shows deviations from the power-law decay l ( s ) oc s 3 valid for smooth phase boundaries as for all other CO2-dried aerogels. Assuming an additive constant the relation l ( s ) c z s 3 + c is true in the region from s = 0.5-2.0 nm ~. This background is probably a superposition of surface scattering of
1 - AOB 15
tt-- 2
2 - AOE
lO
10
15
20
s [ran"z]
Boehmite I
I
,
,.
Bayerite Fig. 7. WAXS curves of the AI203 aerogels AOB and AOE by comparison with Al-ethylat. The peak positions of 'y-AIOOH boehmite and o-AI(OH)3 bayerite according to Ref. [23] are shown for comparison.
0iau:00 i -10
20
40
6~
J [nm-
20
]~,' " ~
10 0 -10
20
40
60
80
Fig. 8. Reduced intensity functions, si(s), of a physical vapour deposited SiO 2 thin film (freestanding film, 20 nm thick) and of the supercritical dried SiO 2 sample obtained by electron diffraction (a.u. = atomic units).
intermediate-sized particles as well as of the volume scattering of smallest inhomogenities. The existence of micropores can not excluded. The most obvious differences between both A1203 aerogels are visible in the large-angle scattering region especially around s ~ 7 nm 1, where a clear maximum can be seen for the aerogel AOE prepared by use of ethanol as a solvent for Al(OBuS)3 (Fig. 7). In general, a maximum like this is typical for many alkoxides. It should be pointed out that this maximum as well as the broad crystalline peaks found at higher s-values do not correspond to the well-known AI hydroxide phases, e.g. boehmite or bayerite. However, a surprising similarity is revealed by comparison with the diffraction pattern of the crystalline [AI(OEt)3] .. Fig. 8 shows the reduced scattering curves obtained by transmission electron diffraction for the SiO 2 aerogel. The scattering curve of physical vapour deposited (PVD) SiO 2 measured under identical experimental conditions is shown for comparison, because such SiO 2 is known for a higher structural disorder than vitreous silica. The attention is focused toward the first sharp diffraction peak (FSDP) which is determined by the intermediate-range order in the network. By comparison with the PVD-SiO~ showing still a well-pronounced FSDP, this first scattering
156
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
maximum of the aerogel is much broader and significantly lower as a consequence of a higher network disorder. The maximal correlation determined by Fourier transformation of the FSDP according to Ref. [24] is 1 nm for the aerogel and 1.5 nm for PVD-SiO?.
4. Discussion
One- and two-component aerogels x S i O 2 - ( 1 x)Al:O 3 with densities down to 0.063 g / c m 3 were prepared by hydrolytic polycondensation of the corresponding alkoxides Si(OMe) 4 and Al(OBuS)3 by use of different routes of hydrolysis and subsequent supercritical CO 2 drying. After drying all aerogels are amorphous. The microstructure of the aerogels can be described by a broad distribution of intermediate-sized spherical inhomogenities for SiO 2 and flat disc-like regions in the case of pure A1203. The existence of micropores down to molecular sizes could not be excluded. It should be emphasized that increasing A1203 content leads to increased scattering from the smallest inhomogeneities at s ~-- 3 n m Apart from these inhomogeneities (probably small alumina oligomers or micropores), only one class of particles was detected for all untreated aerogels by TEM and SAXS, suggesting that Si and A1 species are distributed in these particles according to the molar ratio. Phase separation of AI- and Si-rich regions is, of course, conceivable because of the different reactivities of the alkoxides. It is improbable, however, that even divergent compositions would yield Si- and Al-rich condensates of equal size. It should be pointed out that this is an indirect conclusion, because element-specific statements about the distribution of the Si- and the Al-containing components in the aerogels can not be derived from a single scattering experiment at only one energy or from TEM micrographs. The incorporation of aluminum atoms into the SiO 2 network has a significant influence on the macroscopic properties of the aerogels, as shown by the retarded linear shrinkage of the 0.7SiO 20.3A1203 aerogei with temperature. We believe that the aluminum sites are the shielding of adjacent reactive S i - O H - - . H O - S i groups as well as the reducing of the number of hydrogen bonds.
The preparation of xSiO2-(1 - x ) A 1 2 0 3 compositions can lead either to monophasic or to diphasic gels. According to Hoffmann et al. [10], monophasic gels are homogeneous at nearly the atomic scale. Diphasic gels, however, consist of two discrete phases (10 and 100 nm size) which can react independently up to typically 1200°C for minutes [10]. The X-ray diffraction data of the 0.7SIO2-0.3A1203 aerogel do not reveal the presence of phases other than mullite (in particular no ",/-A1203) after a heat treatment between 900 and 1200°C. The absence of the 3,-A1203-1ike spinel phase or cristobalite, which should be highly probable for diphasic gels, strongly suggests a single phase. This observation means that Si-O-A1 bonds are more likely than S i - O - S i or A1-O-A1 bonds. During heat treatment evolution of particles takes place through a network formation on the molecular level. It can be assumed that a phase separation takes place prior to the mullite crystallization. The structure formed after heating at 1200°C for 10 min is characterized by large mullite crystallites that are homogeneous, at least for distances smaller than --- 80 nm. It should be pointed out that the structure of the alumina aerogels is sensitive to the alcohol used as solvent. The dissolution of AI(OBuS)3 in butanol leads to a completely amorphous structure, whereas the reaction of AI(OBuS)3 with ethanol as solvent results in well-pronounced crystalline peaks found in the diffraction pattern of the final aerogel. One possible explanation for these crystalline peaks can be deduced from the comparison with the diffraction patterns of the corresponding AI alkoxides. The amazing similarity to the diffraction pattern of the white crystalline solid AI(OEt) 3 rather than to the one of the colourless liquid AI(OBuS)3 implies alcohol exchange reactions prior to the gelation: Al(OBuS)3 + (3 - x ) E t O H , AI(OEt)3 ~(OBu s) + (3 - x)BusOH,
(9)
where the value of x is close to zero. However, according to Bradley et al. [1], the transition mechanism is not well established, because it is susceptible to a number of factors (e.g., steric hindrance). Nevertheless, structural characteristics of the AI ethoxide may originate in the wet chemical process. Te-
B. Himmel et al. /Journal of Non-Crystalline Solids" 186 (1995) 149-158
trameric alumina groups with a central sixfold aluminum atom surrounded by three fourfold-coordinated aluminum atoms via oxygen bridges similar to those in [AI(OEt)3] 4 [2] should be the key element of structure. Additional investigation with spectroscopic methods or with diffraction, focused on the shortrange order, is desirable to expose the simultaneous processes of hydrolysis and alcoholysis in more detail. It was shown previously that the intermediaterange order of differently prepared amorphous SiO 2 (silicas [24,25], xerogels [26], vapour deposited SiO 2 [27] can be described as well-defined polyhedra having four six-membered S i - O rings, the origin of the pronounced F S D P found in the scattering curves for all of these samples. The F S D P of the CO2-dried SiO 2 aerogel, however, is reduced to a small shoulder. This points to a disturbed intermediate-range order. The bulk density of particles in SiO 2 aerogels was estimated by Mulder [28] from the Brunauer, Emmett and Teller (BET) surface area (520 m 2 / g ) , the macroscopic density (0.15 g / c m 3) and the average particle diameter derived from TEM (20 nm) to about 0.38 g / c m 3. This is much lower than the bulk density, p = 2.2 g / c m 3, of silica. That is why it is justified to assume a network of the SiO 2 condensates in the aerogel which includes a remarkable portion of microvoids down to the molecular scale. These microvoids prevent the formation of larger correlations than over two or three adjacent SiO4/2 tetrahedra. A decreasing FSDP is the result of this rather random arrangement of SiO4/2 tetrahedra.
5. Summary Low-density x S i O 2 - ( 1 - x ) A l 2 0 3 aerogels with x = 0, 0.3, 0.7, 1.0 (mole fractions) were prepared by CO 2 supercritical drying of alcogels formed from Si(OMet) 4 and Al(OBuS)3 by hydrolytic polycondensation under different chemical conditions. The aerogels are characterized by a distribution of interconnected spherical particles whose size increases with the AI203 content ( 1 0 - 6 0 nm in diameter). The particles form branched clusters. By investigating the linear shrinkage on the heat treatment, it was found that the sintering of the A1203 containing aerogels is strongly retarded compared with the pure SiO 2 aero-
157
gels. In particular, the SiO 2 aerogel and the 0.7SIO2-0.3A120 3 aerogel show a linear shrinkage of approximately 48% and of only 25% after a heat treatment at 900°C, respectively. The crystallization starts at l l 0 0 ° C and results in the formation of large particles ( > 80 nm) of well-crystallized mullite. Other phases (in particular the ",/-A1203-1ike A 1 - S i spinel phase or cristobalite) could not be detected. The A120 3 aerogels are composed of flat disc-like regions of approximately 20 × 25 nm 2 lateral extension having an average layer thickness of 2 nm. These platelets form particles larger than 150 nm. The structure of the aerogels (amorphous or crystalline) can be influenced through the choice of the solvent, as well. The existence of micropores down to the molecular level could not be excluded. For the structure of the SiO 2 aerogel it is concluded from the transmission electron diffraction data that the internal particle structure must be highly disturbed probably by microvoids. The authors grateful acknowledge the support of this research by the Deutsche Forschungsgemeinschaft D F G (grants Hi 522 and Ge 6 6 7 / 2 - 2 ) .
References [1] D.C. Bradley, R.C. Mehrotra and D.P. Gaur, Metal Alkoxides (Academic Press, London, 1978). [2] O. Kriz, B. Casensky, A. Lycka, J. Fusek and S. Hermanek, J. Magn. Reson. 60 (1984) 375. [3] C.J. Brinker and G.W. Scherer, Sol-Gel Science (Academic Press, Boston, 1990). [4] T. Heinrich, R. Raether, O. Spormann and J. Fricke, J. Appl. Crystallogr. 24 (1991) 788. [5] T. Heinrich and F. Raether, J. Non-Cryst. Solids 147&148 (1992) 152. [6] T. Heinrich, J. Non-Cryst. Solids 168 (1994) 14. [7] J.C. Pouxviel, J.P. Boilot, A. Dauger and A. Wright, J. Non-Cryst. Solids 103 (1988) 331. [8] J.P. Boilot, J.C. Pouxviel, A. Dauger and A. Wright, in: Better Ceramics Through Chemistry III, MRS Symp. Vol. 121, ed. C.J. Brinker, D.E. Clark and D.R. Ulrich (Materials Research Society, Pittsburgh, PA, 1988) p. 121. [9] B.E. Yoldas, J. Mater. Sci. 10 (1975) 1856. [10] D.W. Hoffman, R. Roy and S. Komarneni, J. Am. Ceram. Soc. 67 (1984) 468. [11] C.S. Hsi, F.S. Yen and Y.H. Chang, J. Mater. Sci. 24 (1989) 2041. [12] J.C. Huling and G.L. Messing, J. Non-Cryst. Solids 147&148 (1992) 213.
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[13] A.K. Chakravorty, J. Mater. Sci. 29 (1994) 1558. [14] A.C. Pierre and D.R. Uhlmann, J. Non-Cryst. Solids 82 (1986) 271. [15] Y. Mizushima and M. Hori, J. Non-Cryst. Solids 167 (1994) 1. [16] S.J. Teichner, G.A. Nicolaon, M.A. Vicarini and G.E.E. Gardes, Adv. Colloid lnterf. Sci. 5 (1976) 245. [17] B. Himmel, H. Biirger, Th. Gerber and A. Olbertz, J. NonCryst. Solids 185 (1995) 56. [18] Th. Gerber and B. Himmel. J. Phys. (Paris) C 85 3 (1993) 523. [19] J. Mering and D. Tchoubar, J. Appl. Crystallogr. 1 (1968) 153. [20] P.A. Doyle and P.S. Turner, Acta Crystallogr. A24 (1968) 390. [21] G. Engeln-Miillges and F. Reutter, Formelsammlung zur numerischen Mathematik mit C-Programmen (B1-Wiss.-Verl., Mannheim, 1990).
[22] L.I. Mirkin, Handbuch zur R6ntgenstrukturanalyse der Polykristalle (Staatlicher Verlag f'fir Physikalisch-mathematische Literatur, Moscow, 1962). [23] K. Wafers and C. Misra, Oxides and Hydroxides of Aluminum, Alcoa Tech. Paper 19 (1987). [24] Th. Gerber and B. Himmel, J. Non-Cryst. Solids 83 (1986) 324. [25] Th. Gerber and B. Himmel, J. Non-Cryst. Solids 92 (1987) 407. [26] B. Himmel, Th. Gerber and H. Biirger, J. Non-Cryst. Solids 91 (1987) 122. [27] B. Himmel, Th. Gerber and H.-G. Neumann, Phys. Status Solidi (a)88 (1985) K127. [28] C.A.M. Mulder, G. van Leeuwen-Stienstra, J.G. van Lierop and J.P. Woerdman, J. Non-Cryst. Solids 82 (1986) 148
ELSEVIER
Journal of Non-Crystalline Solids 186 (1995) 149-158
Structural characterization of
SiO2-A1203 aerogels
B. Himmel a,*, Th. Gerber a, H. Biirger b, G. Holzhfiter a, A. Olbertz b ~ FB Physik, Unit'ersitiit Rostock, Unicersitiitsplatz 3, D-18051 Rostock, Germany b Otto-Schott-lnstitut, Friedrich-Schiller-Unit,ersithtJena, Fraunhoferstrasse 6, D-O7743.1ena, Germany
Abstract Low-density x S i O 2 - ( l - x ) A l 2 0 3 aerogels with x = 0, 0.3, 0.7, 1.0 (mole fractions) were prepared by CO 2 supercritical drying of alkogels formed from Si(OMe) 4 and AI(OBu~)3 by hydrolytic polycondensation under different chemical conditions. The resulting aerogels were structurally characterized by X-ray scattering at small and large angles as well as by electron diffraction and transmission electron microscopy. By investigating the dependence of linear shrinkage on the heat treatment it was found that the sintering of the aerogel is influenced by the presence of A I 2 0 3 in the network. Crystallization experiments were also performed up to I200°C. The reduced height of the first sharp diffraction peak indicates less order in the SiO 2 network of the aerogel in comparison with amorphous SiO 2 prepared by other methods.
1. Introduction
Development of inorganic porous materials, which are resistent to sintering for temperatures higher than 800°C, offers many technological applications, for instance, in the field of catalytic combustion. The sol-gel method for preparing glasses, glass-ceramics and ceramics provides both the possibility of tailoring multi-component systems on the molecular scale and coating of materials by dipping or spraying at ambient conditions. The consolidation of the surface coating without loss of porosity can be achieved by supercritical drying. In the following, we discuss structural investigation of aerogels in the SiO 2AI20 3 system. The differences in structure and bond chemistry between Si- and Al-alkoxides requires a sophisticated procedure of hydrolysis and condensation of
* Corresponding author: Tel: +49-38l 492 3212. Telefax: +49-381 498 1626. E-mail: [email protected].
mixed (A1,Si) gels for the tailoring on the molecular scale. The tri-alkoxides of aluminum are more complex than the tetra-alkoxides of silicon, which are known to be monomeric [1]. As concluded from the 27A1 nuclear magnetic resonance (NMR) investigation by Kriz, the [AI(OBu~)3],, complex is trimeric (n = 3), composed of one fivefold- and two fourfold-coordinated aluminum atoms depending on parameters such as solvent and temperature [2,3]. As a rule, the metal prefers maximal coordination but that is hindered by steric effects [1]. Structural changes of alkoxides during hydrolysis and condensation reactions are not completely understood. Several routes are available for preparing chemically homogeneous xSiO2-(1 - x ) A I 2 0 3 gels via the alkoxide gel method. Heinrich and co-workers [4-6] used chelated Al-alkoxides in order to reduce the reaction rate of [AI(OBu~)3],, in comparison with Si(OEt) 4. They found, by in situ Small-angle X-ray scattering (SAXS) of solutions having the stoichiometric composition of mullite, that the primary particle size decreases with the number of blocked con-
01122-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 0 4 5 - 3
150
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
densation sites. A fractal dimension of df = 2.19 was found prior to gelation. Pouxviel et al. [7,8] used the double alkoxide ( B u O ) 2 A I - O - S i ( O E t ) 3. They concluded from their small-angle neutron scattering (SANS) studies that the hydrolysis of the S i - O R groups is the rate-determining step. After a rapid formation of primary particles via A 1 - O - A I linkages the further condensation and aggregation reactions lead to the formation of fractal clusters with df = 1.9. A third route could be the adjustment of the reaction rates by prehydrolysis of the slower reacting Si-alkoxide. Yoldas [9] developed the process of hot water hydrolysis of AI(OBu) 3 with a large excess of water followed by the peptization of the resulting fibrillar boehmite in an acidic environment for the preparation of transparent monoliths. Conditions during the hydrolysis and gelation influence the crystallization of mullite [10-13]. Also, for pure A1203 gels, the chemistry is determinative for the structure formation. Pierre and Uhlmann [14] distinguished between boehmite-like gels and 'super-amorphous' gels depending on the temperature. Mizushima et al. prepared alumina aerogels of p = 180 k g / m 3 (CO 2 supercritical drying) and of p = 130 k g / m 3 ( C 2 H s O H supercritical drying)from chelated AI(OBu~)3 diluted with ethanol and obtained more or less pronounced crystalline peaks of boehmite depending on the drying regime [15]. It is well known that SiO 2 aerogels sinter at temperatures above 700°C leading to shrinkage and densification. The present study is focused upon synthesis and characterization of low-density x S i O 2 - ( l - x ) A 1 2 0 3 aerogels that are resistant to sintering.
The x S i O 2 - ( 1 - x ) A l 2 0 3 samples were prepared as 10 wt% solutions of TMOS in methanol and of aluminum sec-butylate (Al(OBuS)3) in iso-butanol and water without use of a catalyst (r w = 4). The mixture of AI(OBu~)3 and i-BuOH was added dropwise to the mixture of TMOS, 1 mol H 2 0 and half of the quantity CH3OH under vigorous stirring. After some minutes, the residual H 2 0 , together with the second half of CH3OH, was added dropwise. The TMOS was not prehydrolyzed. The time to gelation was less than 30 min for both compositions. The preparation of homogeneous amorphous A1203 alkogels is comparatively difficult, because of the occurrence of dimeric and oligomeric units in aluminum alkoxides and their sensitivity to hydrolysis. Partially hydrolyzed products precipitate easily by use of a normal hydrolysis procedure, whereby the alkoxide is first desolved in half of the solvent followed by water together with the other half under vigorous stirring. An indirect hydrolysis of the alkoxide via ambient humidity saturated with alcohol leads to the formation of white surface layers of partially crystallized products, that sink and redevelop after some time. Depending on the water content, A1 concentration and aging time, crystallization can occur even in the solution. According to Teichner et al. [16] completely amorphous solids can be obtained for r w < 4 and for concentrations < 10 wt% AI(OBuS)3 in butanol. Therefore the aluminum secbutylate was diluted in the first case with ethanol (AOE) and in the second case with i-butanol (AOB) to 5 wt%. The quantity of water necessary for r w = 3 was generated in situ through the equilibrium reaction of alcohol and acetic acid: C2HsOH + CH3COOH
2. Experimental 2.1. Sample preparation Preparation of the SiO 2 samples was performed by dropwise addition of the required quantity of TMOS (Si(OMe) 4) to methanol (rl. = 15, where r L is the molar ratio of solvent and alkoxide). After vigorous stirring for 2 min, a mixture of H 2 0 and 0.5 normal NH 3 was added dropwise, to realize r w = 2 and pH ~ 8 (r,~, is the molar ratio of water and alkoxide). The gelation time was 55 rain.
O ) H2O + C H 3 - - C H 2 - - O - - C - - C H
3.
(1) The reaction equilibrium is shifted in favour of the formation of the ester, because the water is consumed immediately by hydrolysis. After a few days compact white alkogels were formed in closed tubes. Drying of the alkogels was performed above the critical point of CO 2 (Tc = 304.2 K, Pc = 74 × 105 Pa) after an exchange of water and alcohol to carbon dioxide by use of the critical point dryer CPD2
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158 Table 1 Compositions, preparation and densities xSiO2-(l - x)Al203 aerogels
p ( g / c m 3) of the
Sample
x
rw
Conc.
pH
Aging ~
p
SiO 2 AS70 AS30 AOE AOB
1.0 0.7 0.3 0.0 0.0
2 4 4 3 3
r L = 15 10 wt% 10 wt% 5 wt% 5 wt%
basic neutral neutral acid acid
7 20 20 12 12
0.131 0.063 0.080 0.168 0.158
with 2
Aging (days) means the time between gelation and supercritical drying.
(Pelco). The liquid-fluid transition could be observed clearly through a window. The solvent exchange of the cylindrical samples took 8 h inside the pressure vessel of the autoclave cooled to about 285 K. The samples were heated slowly and continuously up to (315 + 1) K, where the maximum pressure of (90_+ 4 ) × 105 Pa was reached. The composition and the density of the aerogels are listed in Table 1. For the temperature-dependent investigation, the aerogels were heated at a rate of 5 K / m i n to the final temperature and held for 10 min at this temperature. Cooling to room temperature was performed slowly in the oven.
2.2. Characterization methods The (SAXS) experiments were performed on a Kratky compact camera (Anton Paar, Graz) mounted on a 2 kW X-ray generator with Ni-filtered Cu Ko~ radiation by use of a proportional counter. The SAXS experiment is characterized by a visual field rmax = nV/Smio of 80 nm with s = 4rr s i n ( 0 ) / a (20 is the scattering angle and h is the X-ray wavelength). During the measurements the aerogels were stored in sealed capillaries. We discuss the infinite-width slitsmeared scattering curves as well as the corresponding distribution functions. Details of data treatment have been given previously [17]. The correlation function was calculated from the equispaced points of the scattering curve l(k As) by discrete Fourier transformation using the expression [18] 1
C(r) -
151
k max
,~2
~ sin kv:yZ~U77Tx 2
'n" "o
V'Y: + x2
and
K Cre(r)-
~
Jo (sr)
4Tr2 , ,,x~+±~/2.
S2
dr,
(4)
where the remainder term, Cre(r), takes into account the termination ( K = Porod constant with l ( s ) = K~ s3, J0 = Bessel function). The distance distribution function, 4 " n r 2 C ( r ) , gives the probability for finding electron density in a spherical, infinitesimally thin shell of radius, r. However, the finer features at smaller values of r are hidden in this function as a result of the stronger weighting of larger distances. This information can be displayed as the chord length
distribution, A(I) [19]: A(l) = l(daC(r)/dr
x),= ,.
(5)
A(l)dl is a measure of the probability that a line between two surface points lies in the interval (l, / + d/). The X-ray diffraction system XRD3000 (Seifert) equipped with a curved position-sensitive detector (PSD INEL, 120 ° angular aperture) was used to determine crystalline phases with Cu K~ radiation after heat treatment. The channels of the PSD were calibrated with polycrystalline silicon powder. The instrument used for our electron microscopic electron diffraction investigation is a Zeiss EM912 with an integrated electron filter (omega filter). Images are recorded on Scienta 23D56 films (Agfa Gevaert). The diffraction patterns were recorded via the photomultiplier system of the EM912 with camera lengths between 3600 and 7200 mm. All measurements were carried out with zero-loss electrons at 100 kV. The reduced intensity function, which was extracted from the experimental data, is defined as
i(s)
=
(lexp(S)
-
(f(s)2>)/(f(s)>2,
(6)
with
~_~ (k As)21(k A s ) Z ( k , A s r ) 4"rr2 k= 1
( f ( s ) 2} = ~ x i f , ( s ) 2 q- Cre ( r ) ,
(2)
i-1
(7)
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
152
Table 2 Structure parameters of the investigated aerogels
and (f(s)
~ xifi(s), i-I
2) =
(8)
where f, is the atomic form amplitude of the atom i according to Doyle and Turner [20], x i is the concentration with £ x i = 1 and n is the number of different types of atom.
3. Results Fig. l(a) shows the dependence of the S A X S curves of the S i O 2 - A 1 2 0 3 aerogels on the composition. With higher A I 2 0 3 content, we observe the formation of a weak maximum in the region of 3 nm ~ < s < 4 nm 1 (small inhomogeneities), a shift of the shoulder in the central region in direction to
i(s)
. . . . . . . .
10'
i
I
611
1'.0
3~ 2
1¢
1--
10z
(a) 0.01
.......
s into'i]'
c(o 1.0 0.8 0.6 0.4 0.2 0.0
Fig. 1. Scattcring curves (a) and correlation functions (b) of aerogels having different compositions (1 - SIO2; 2 - AS70; 3 AS30; 4 - AOB; 5 - AOE).
Sample
R~c I (nm)
SiO 2 * AS70 AS30 AOB * AOE *
-
0.4 0.4 0.4
Rsc 2 (rim) <
< < < <
8
25 30 12 12
R~c3 (rim)
b
d (nm)
9.8 23.7 29.5 > 40 > 40
2.96 2.72 3.08 3.03 3.00
5-10 10-20 10-25 (22-25) (22-25)
R~c, electronic radius of gyration calculated with the Guinier approximation (R~c I and R~c2) and as momentum of the correlation function (Rsc3); b, negative slope of the scattering curve between Smi,, = 0.5 nm I and Sm~X = 2.5 nm I approximated with l ( s ) ~ s b + c using the damped Newton method [21] (* Sm~,x = 2.0 nm ~); d is the particle diameter estimated from TEM.
lower s-values (growth of intermediate-sized inhomogeneities) and a steeper rise for s < 0.1 nm 1. The scattering of the alumina for s < 0.07 n m - 1 can be attributed to a new generation of inhomogeneities on a larger length scale. The correlation functions calculated according to Eq. (2) can be seen in Fig. l(b). The dependence of intermediate-sized inhomogeneities on composition is manifested by the peak shift in the function rC(r). The shoulder in the correlation function of the aerogels with higher A1203 content ( x = 0.0 and x = 0.3) at lower values of r is determined by the smallest inhomogeneities. The calculated structure parameters for all aerogels investigated are summarized in Table 2. Typical electron micrographs are shown in Fig. 2. The structure of the mixed aerogels ( x = 0.3 and x = 0.7) can be characterized by small agglomerated particles having nearly a spherical shape. These particles are connected through necks and form branched chain-like clusters. These clusters contain mesopores and macropores of variable size. Most of them are larger then the visual field of the S A X S experiment. The microstructure of the SiO 2 aerogel is similar to that of the both mixed aerogels. However the particles are smaller and the clusters seems to be denser. A rough estimation of the average particle diameters deduced from the transmission electron microscopy (TEM) micrographs is given in Table 2. By contrast with the SiO2-containing aerogels, the microstrueture of the A l e O 3 aerogels can be described by very large inhomogeneous particles ( > 150 nm in diameter) which are composed of distorted area-like regions de-
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
tectable by TEM only near the surface of the particles. The lateral extension of some flat regions amounts to approximately 20 × 25 nm 2. By analyzing the weak contrast differences near the surface of the particles (not resolved by the photocopy shown in Fig. 2(d), we conclude that these flat regions have a thickness of approximately 2 nm. To investigate the linear shrinkage, S L, the SiO 2 aerogel and the 0.7SIO2-0.3A120 3 aerogel were heat-treated at 300, 500, 700 and 900°C for 10 min. The temperature dependence of the linear shrinkage is nearly the same for both aerogels in the temperature range to about 700°C (Fig. 3). However, at higher temperatures densification of the SiO 2 aerogel increases drastically caused by enhanced processes
153
of viscous sintering. In contrast to the SiO 2 aerogel showing a linear shrinkage of approximately 48% after a heat treatment at 900°C the linear shrinkage observed for the 0.7SIO2-0.3A120 3 aerogel is only 25%. This behaviour indicates a retardation of viscous sintering due to the presence of A I 2 0 3 . The structural changes and crystallization of the aerogel AS70 (0.7SIO2-0.3A1203) was investigated after heating at 900, 1000, 1100 and 1200°C for 10 min. The aerogel remains amorphous until heating above 1000°C (Fig. 4). First traces of poorly crystallized mullite were observed after heating at 1100°C, although the composition of the aerogel deviates from the ideal mullite of AI:Si = 3:1 (3 A I 2 0 3 2SIO2). There is no hint of weakly crystallized ",/-
50 nm I
I
d
n Fig. 2. Transmissionelectron microscopicpictures of the xSiO2-(1 - x)AI203 aerogels:(a) SiO2, (b) AS70, (c) AS30, (d) AOB.
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
154 g
I(s)
4. . . . . . . . %
106
3
104
i
t
........
b
.......
5o ......................................................................
gc
.~ 20 .........
i ........
i............. i .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
lO
.
.
.
.
.
.........
0
400
200
600 800 Temperature
A1OOH boehmite in the untreated aerogel at low temperatures or of an intermediate "y-A1203-1ike AISi spinel phase in the aerogel heat-treated to 1200°C. All the peaks could be assigned to the mullite phase
'
'
'
1 flO00°C ' 2- 1100°(3 3 - 1200°C
\
15 * 1 / ~ ~ / 2 ~ . . . 10 /2
*
5
*
*
10
20
30
I l/ J
II
50 s [rum"I]
40
Jl,tf
i A
I , II I
0.0
.......
6'.1
.......
i'.o
'
' '~L~-'I]
Fig. 5. Dependence of the scattering curves of the aerogcl AS70 on the heat treatment in the representations l o g ( I ) versus log(s).: 1 - room temperature, 2 - 900°C, 3 - 1000°C, 4 - l l 0 0 ° C , 5 1200°C (time: 10 min).
for T > 1000°C as marked in Fig. 4 by stars. The intensity decrease for s < 2 nm -t is caused by a primary beam stop close to the detector. The results of the SAXS investigations are presented in Figs. 5 and 6. Two clearly separated Guinier regions can be seen in the scattering curves of the aerogel that was treated at 900°C, indicative of the formation of two types of inhomogeneity. By contrast with the inhomogeneity of the untreated aerogel, these inhomogeneities (particle or pores) can be described by smooth phase boundaries, and accordingly the Porod plot I(s) versus s 3 shows two plateaus. The maxima of the chord length distribution (Fig. 6) is determined by the size of these
1.2
J,
t
I0z
[°C]
Fig. 3. Linear shrinkage, S L = (1 - L ~ / L o ) X 100, on the temperature, Lo and L~ being the size of the sample before and after the heat treatment ( l I , SiO2; 0 , 0.7SIO2-0.3A1203). The lines are drawn as a guide to the eye.
I(s) ~V ~ 20 ~
5
A(1)
-2Si02
l,l
0.8
I
4-
A1203 I
I
,
0.4
i
Corundum I
.,
1. ,
, ,, rJ
Fig. 4. Crystallization bchaviour of the aerogel AS70 after a heat treatment at 1000°C (curve 1), l l 0 0 ° C (curve 2) and 1200°C (curve 3) for 10 rain. The peak positions of selected crystalline modifications according to Refs. [22,23] are shown for comparison.
0.0 0
10
20
30
4o
50
t[nm] Fig. 6. Chord length distributions calculated from the scattering curves shown in Fig. 5 : 1 - room temperature, 2 - 900°C, 3 1000°C, 4 - l l 0 0 ° C (time: 10 rain).
155
B. Himmel et al. /Journal of Non-Crystalline Solids" 186 (1995) 149-158
inhomogeneities. They become gradually larger with further heat treatment to 1100°C. The scattering curve of the aerogel heated to 1200°C is dominated by scattering from large mullite crystallites. A powerlaw decay l ( s ) c x s -3 is observed in the low-s region. Both A1203 aerogels show similar structural features inside the size regime probed by SAXS (Fig. l(a)). The smallest inhomogeneity (peak at s =-3 nm 1) corresponds to R~c = 0.4 nm which is a measure for the average thickness of the disc-like regions observed by TEM. The peak at s = 3 nm i may indicate interference of close-packed inhomogenities. The lateral dimension of these distorted area-like regions estimated from TEM micrographs is confirmed by the SAXS investigations (Rsc 2 in Table 2). The most probable chord lengths of 3.6 nm for the gel AOB and 3.4 nm for the gel A O E are derived from the chord length distribution. The tail of the curves shows deviations from the power-law decay l ( s ) oc s 3 valid for smooth phase boundaries as for all other CO2-dried aerogels. Assuming an additive constant the relation l ( s ) c z s 3 + c is true in the region from s = 0.5-2.0 nm ~. This background is probably a superposition of surface scattering of
1 - AOB 15
tt-- 2
2 - AOE
lO
10
15
20
s [ran"z]
Boehmite I
I
,
,.
Bayerite Fig. 7. WAXS curves of the AI203 aerogels AOB and AOE by comparison with Al-ethylat. The peak positions of 'y-AIOOH boehmite and o-AI(OH)3 bayerite according to Ref. [23] are shown for comparison.
0iau:00 i -10
20
40
6~
J [nm-
20
]~,' " ~
10 0 -10
20
40
60
80
Fig. 8. Reduced intensity functions, si(s), of a physical vapour deposited SiO 2 thin film (freestanding film, 20 nm thick) and of the supercritical dried SiO 2 sample obtained by electron diffraction (a.u. = atomic units).
intermediate-sized particles as well as of the volume scattering of smallest inhomogenities. The existence of micropores can not excluded. The most obvious differences between both A1203 aerogels are visible in the large-angle scattering region especially around s ~ 7 nm 1, where a clear maximum can be seen for the aerogel AOE prepared by use of ethanol as a solvent for Al(OBuS)3 (Fig. 7). In general, a maximum like this is typical for many alkoxides. It should be pointed out that this maximum as well as the broad crystalline peaks found at higher s-values do not correspond to the well-known AI hydroxide phases, e.g. boehmite or bayerite. However, a surprising similarity is revealed by comparison with the diffraction pattern of the crystalline [AI(OEt)3] .. Fig. 8 shows the reduced scattering curves obtained by transmission electron diffraction for the SiO 2 aerogel. The scattering curve of physical vapour deposited (PVD) SiO 2 measured under identical experimental conditions is shown for comparison, because such SiO 2 is known for a higher structural disorder than vitreous silica. The attention is focused toward the first sharp diffraction peak (FSDP) which is determined by the intermediate-range order in the network. By comparison with the PVD-SiO~ showing still a well-pronounced FSDP, this first scattering
156
B. Himmel et al. /Journal of Non-Crystalline Solids 186 (1995) 149-158
maximum of the aerogel is much broader and significantly lower as a consequence of a higher network disorder. The maximal correlation determined by Fourier transformation of the FSDP according to Ref. [24] is 1 nm for the aerogel and 1.5 nm for PVD-SiO?.
4. Discussion
One- and two-component aerogels x S i O 2 - ( 1 x)Al:O 3 with densities down to 0.063 g / c m 3 were prepared by hydrolytic polycondensation of the corresponding alkoxides Si(OMe) 4 and Al(OBuS)3 by use of different routes of hydrolysis and subsequent supercritical CO 2 drying. After drying all aerogels are amorphous. The microstructure of the aerogels can be described by a broad distribution of intermediate-sized spherical inhomogenities for SiO 2 and flat disc-like regions in the case of pure A1203. The existence of micropores down to molecular sizes could not be excluded. It should be emphasized that increasing A1203 content leads to increased scattering from the smallest inhomogeneities at s ~-- 3 n m Apart from these inhomogeneities (probably small alumina oligomers or micropores), only one class of particles was detected for all untreated aerogels by TEM and SAXS, suggesting that Si and A1 species are distributed in these particles according to the molar ratio. Phase separation of AI- and Si-rich regions is, of course, conceivable because of the different reactivities of the alkoxides. It is improbable, however, that even divergent compositions would yield Si- and Al-rich condensates of equal size. It should be pointed out that this is an indirect conclusion, because element-specific statements about the distribution of the Si- and the Al-containing components in the aerogels can not be derived from a single scattering experiment at only one energy or from TEM micrographs. The incorporation of aluminum atoms into the SiO 2 network has a significant influence on the macroscopic properties of the aerogels, as shown by the retarded linear shrinkage of the 0.7SiO 20.3A1203 aerogei with temperature. We believe that the aluminum sites are the shielding of adjacent reactive S i - O H - - . H O - S i groups as well as the reducing of the number of hydrogen bonds.
The preparation of xSiO2-(1 - x ) A 1 2 0 3 compositions can lead either to monophasic or to diphasic gels. According to Hoffmann et al. [10], monophasic gels are homogeneous at nearly the atomic scale. Diphasic gels, however, consist of two discrete phases (10 and 100 nm size) which can react independently up to typically 1200°C for minutes [10]. The X-ray diffraction data of the 0.7SIO2-0.3A1203 aerogel do not reveal the presence of phases other than mullite (in particular no ",/-A1203) after a heat treatment between 900 and 1200°C. The absence of the 3,-A1203-1ike spinel phase or cristobalite, which should be highly probable for diphasic gels, strongly suggests a single phase. This observation means that Si-O-A1 bonds are more likely than S i - O - S i or A1-O-A1 bonds. During heat treatment evolution of particles takes place through a network formation on the molecular level. It can be assumed that a phase separation takes place prior to the mullite crystallization. The structure formed after heating at 1200°C for 10 min is characterized by large mullite crystallites that are homogeneous, at least for distances smaller than --- 80 nm. It should be pointed out that the structure of the alumina aerogels is sensitive to the alcohol used as solvent. The dissolution of AI(OBuS)3 in butanol leads to a completely amorphous structure, whereas the reaction of AI(OBuS)3 with ethanol as solvent results in well-pronounced crystalline peaks found in the diffraction pattern of the final aerogel. One possible explanation for these crystalline peaks can be deduced from the comparison with the diffraction patterns of the corresponding AI alkoxides. The amazing similarity to the diffraction pattern of the white crystalline solid AI(OEt) 3 rather than to the one of the colourless liquid AI(OBuS)3 implies alcohol exchange reactions prior to the gelation: Al(OBuS)3 + (3 - x ) E t O H , AI(OEt)3 ~(OBu s) + (3 - x)BusOH,
(9)
where the value of x is close to zero. However, according to Bradley et al. [1], the transition mechanism is not well established, because it is susceptible to a number of factors (e.g., steric hindrance). Nevertheless, structural characteristics of the AI ethoxide may originate in the wet chemical process. Te-
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trameric alumina groups with a central sixfold aluminum atom surrounded by three fourfold-coordinated aluminum atoms via oxygen bridges similar to those in [AI(OEt)3] 4 [2] should be the key element of structure. Additional investigation with spectroscopic methods or with diffraction, focused on the shortrange order, is desirable to expose the simultaneous processes of hydrolysis and alcoholysis in more detail. It was shown previously that the intermediaterange order of differently prepared amorphous SiO 2 (silicas [24,25], xerogels [26], vapour deposited SiO 2 [27] can be described as well-defined polyhedra having four six-membered S i - O rings, the origin of the pronounced F S D P found in the scattering curves for all of these samples. The F S D P of the CO2-dried SiO 2 aerogel, however, is reduced to a small shoulder. This points to a disturbed intermediate-range order. The bulk density of particles in SiO 2 aerogels was estimated by Mulder [28] from the Brunauer, Emmett and Teller (BET) surface area (520 m 2 / g ) , the macroscopic density (0.15 g / c m 3) and the average particle diameter derived from TEM (20 nm) to about 0.38 g / c m 3. This is much lower than the bulk density, p = 2.2 g / c m 3, of silica. That is why it is justified to assume a network of the SiO 2 condensates in the aerogel which includes a remarkable portion of microvoids down to the molecular scale. These microvoids prevent the formation of larger correlations than over two or three adjacent SiO4/2 tetrahedra. A decreasing FSDP is the result of this rather random arrangement of SiO4/2 tetrahedra.
5. Summary Low-density x S i O 2 - ( 1 - x ) A l 2 0 3 aerogels with x = 0, 0.3, 0.7, 1.0 (mole fractions) were prepared by CO 2 supercritical drying of alcogels formed from Si(OMet) 4 and Al(OBuS)3 by hydrolytic polycondensation under different chemical conditions. The aerogels are characterized by a distribution of interconnected spherical particles whose size increases with the AI203 content ( 1 0 - 6 0 nm in diameter). The particles form branched clusters. By investigating the linear shrinkage on the heat treatment, it was found that the sintering of the A1203 containing aerogels is strongly retarded compared with the pure SiO 2 aero-
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gels. In particular, the SiO 2 aerogel and the 0.7SIO2-0.3A120 3 aerogel show a linear shrinkage of approximately 48% and of only 25% after a heat treatment at 900°C, respectively. The crystallization starts at l l 0 0 ° C and results in the formation of large particles ( > 80 nm) of well-crystallized mullite. Other phases (in particular the ",/-A1203-1ike A 1 - S i spinel phase or cristobalite) could not be detected. The A120 3 aerogels are composed of flat disc-like regions of approximately 20 × 25 nm 2 lateral extension having an average layer thickness of 2 nm. These platelets form particles larger than 150 nm. The structure of the aerogels (amorphous or crystalline) can be influenced through the choice of the solvent, as well. The existence of micropores down to the molecular level could not be excluded. For the structure of the SiO 2 aerogel it is concluded from the transmission electron diffraction data that the internal particle structure must be highly disturbed probably by microvoids. The authors grateful acknowledge the support of this research by the Deutsche Forschungsgemeinschaft D F G (grants Hi 522 and Ge 6 6 7 / 2 - 2 ) .
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