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Strengthening of silica gels and aerogels by washing and aging processes
Journal of Non-Crystalline Solids 285 (2001) 1±7
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Section 1. Inorganic/composite aerogels
Strengthening of silica gels and aerogels by washing and aging processes M.-A. Einarsrud a,*, E. Nilsen a, A. Rigacci b, G.M. Pajonk c, S. Buathier c, D. Valette d, M. Durant d, B. Chevalier e, P. Nitz f, F. Ehrburger-Dolle g a
Department of Chemistry, Norwegian University of Science and Technology, 7491 Trondheim, Norway Ecole des Mines de Paris CENERG, Sophia Antipolis, B.P. 207, 06 904 Sophia-Antipolis cedex, France UMR 5634 CNRS Universit e Claude Bernard Lyon1, 43 boulevard du 11 Novembre 1918, 69622 Villeurbanne, France d PCAS, B.P. 111, 91161 Longjumeau, France e CSTB 24 rue Joseph Fourier, 38400 Saint Martin d'H eres, France f Fraunhofer Insitute for Solar Energy Systems, Oltmannsstr. 5, D-79100 Freiburg, Germany g ICSI, CNRS, 15 rue Jean Stacky, F-68057 Mulhouse cedex, France b
c
Abstract Gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. During washing in water solution a signi®cant increase in the permeability of the gels was observed, showing that dissolution-reprecipitation occurs. After washing, the gels were further soaked in a solution of polyethoxydisiloxane precursor to strengthen and stien the gel. As expected, a signi®cant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability does not decrease below the value for the as-prepared gels. Hence, a promising process has been developed where both the stiness and the strength have been increased as well as the permeability. The increase in permeability is of importance to facilitate the supercritical drying process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Optical properties have been measured on laboratory scale aerogels. The corresponding results have been correlated with structural characteristics measured by small-angle X-ray scattering (SAXS). Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 82.33.Ln
1. Introduction This work is a part of the EU HILIT (Highly Insulating and Light Transmitting Aerogel Glazing for Window) project, the main aim of which is * Corresponding author. Tel.: +47-73 594 002; fax: +47-73 590 860. E-mail address: [email protected] (M.-A. Einarsrud).
to develop and investigate the pre-industrial production and application of monolithic silica aerogel in highly insulating and light transmitting glazings for windows. The production process is based on a direct supercritical drying route using CO2 and the use of commercially available polyethoxydisiloxane silica precursors. For this application, aerogel tiles of minimum size 60 60 cm2 are necessary and hence the mechanical properties of both the wet and dried gel are of importance.
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 4 2 3 - 9
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M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
Indeed, even if direct supercritical CO2 drying reduces capillary stresses [1], it appears interesting at large scale to increase both the modulus of rupture (MOR) and permeability (D) in order to diminish the impact of depressurization stresses [2]. Such a strengthening treatment may ensure success in obtaining large monolithic volumes through an acceptably fast process. This work is based on previous developments by Einarsrud et al. [3±7] where the mechanical properties of the wet gels are improved by an aging procedure. Fresh silica species from an aging solution is precipitated onto the already formed gel network, increasing both the stiness and strength of the gel. The motivation for this paper is twofold; ®rst, to discuss the eect of aging polyethoxydisiloxane-based gels in a solution also of the polyethoxydisiloxane precursors and second, to discuss the eect of scaling up these gels to laboratory scale samples (diameter 7 cm or 14 14 cm2 ). The eects on structure and optical properties will also be discussed. 2. Experimental Wet gels were prepared from the commercial polyethoxydisiloxane precursor P750 (PCAS, Longjumeau, France) by using P750 [8,9] and ethyl acetoacetate (Etac) in a volume ratio 50:50 and 21 N HF (2 vol.%) as a catalyst. The gels were prepared by adding Etac to the catalyst and then adding this mixture slowly to P750 . The solution was cast into Te¯on tubes with an inner diameter of 8.6 mm or Te¯on moulds (diameter 7 cm or 14 14 cm2 ), and kept at room temperature for 3 h for gelation. To study the eect of aging, the gels were ®rst soaked in a washing solution (20 vol.% H2 O/ethanol) for various lengths of time (0±24 h) at room temperature, 40°C or 60°C and then in an aging solution (35 or 70 vol.% P750/ethanol or 70 vol.% TEOS/ethanol). The aging was performed at room temperature or 70°C. As a reference sample for 0 h aging time, gels were washed only in ethanol. Generally the volume ratio of gel:washing or aging solution was 1:3. The gel rods were further washed in ethanol four times within 24 h at 50°C before measuring the wet
gel shear modulus (G) and the modulus of rupture, MOR, by standard 3-point beam bending or the G modulus and permeability, D, by relaxation experiments as described elsewhere [3,4,10,11]. The larger gels were supercritically dried with CO2 according to the procedure described by Rigacci et al. [12]. The density of the aerogels was determined by measuring the weight and dimensions, when possible, or it was estimated using the refractive index [13]. The initial density of the gels (which is the bulk density of the dried gels if there is no shrinkage during drying) was calculated from the volume of the wet gels and gravimetric measurement of heat treated gels (300°C). Surface area was measured by N2 adsorption (Micromeritics, ASAP 2000 or laboratory made equipment) using the BET method. The characteristic pore size of the wet gel, rw , was calculated from Eq. (1). D
1
q=qs rw2 ; 4j
1
j is the Kozeny `constant' and is given in Eq. (2) 0:5 qb q j 1:0 6:05 8:60 b qs qs 1:5 q 6:56 b ; 2 qs where q=qs is the relative density (volume fraction of solids) of the gel network. Eq. (2) is based on calculations by Happel and Brenner [14] for the permeability of an array of randomly arranged cylinders. Spectral direct and hemispherical transmittance measurements at normal incidence were performed using a commercial double beam spectrophotometer (Perkin Elmer k19) with an integrating sphere (Labsphere, diameter 15 cm). From the spectral data, the percentage of transn;hem mission in the visible range %TR : sn;dir vis =svis (normal direct transmittance/normal hemispherical transmittance) and eective extinction coecients, E, are calculated. SAXS measurements were performed at the French CRG BM2 beamline at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The experimental conditions and the associated theoretical treatments are described by Berthon et al. [15].
M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
3. Results and discussion Fig. 1 compares the eect on initial density of aging in ethanol solutions of P750 and TEOS. Aging the wet gels in solutions of P750 or TEOS clearly causes silica to be transported from the aging solution onto the silica network as the initial density is increasing. An increasing initial density with increasing aging time is observed for aging in P750 ; however aging in TEOS for more than 3 h (or an initial density above approximately 0.26 g/cm3 ) has no eect. Such a leveling o of the initial density has not previously been observed for other types of gels [3,4]. The decrease in eect of aging in a TEOS solution after 3 h might be caused by a high reactivity of TEOS resulting in plugging of the pore openings. This is supported by the fact that the TEOS aged gels do not show a further increase in G modulus after reaching the maximum initial density of 0.26 g/cm3 . From Fig. 1 it can be seen that the initial density is strongly in¯uenced by relatively short aging times, hence aging of the larger-sized gels was in addition to 70°C performed at room temperature. Shear modulus, permeability, characteristic pore radius and surface area (after drying) of the wet gels aged in P750 solution is given as a function of initial density in Fig. 2. Included in Fig. 2 are also the data after washing only in ethanol
Fig. 1. Initial density (0.005 g/cm3 ), minimum 10 measurements per sample) versus aging time in 70 vol.% P750 /ethanol (d) or TEOS/ethanol ( ) at 70°C after washing in 20 vol.% H2 O/ethanol at 60°C for 24 h. Zero hour aging means gels washed in ethanol () or in 20 vol.% H2 O/ethanol (j).
3
3
(qinitial 0:168 g=cm ) and after washing in the H2 O/ethanol solution (qinitial 0.175 g/cm3 ). During the washing there is a small but signi®cant decrease in G modulus. The most interesting feature however is the corresponding large increase in
Fig. 2. Shear modulus, G (1.68 MPa), permeability, D (0.55 and surface area nm2 ), characteristic pore radius, rw 5 A) (20 m2 /g) of wet gels versus initial density. The gels were aged in 70 vol.% P750 /ethanol at 70°C for 0±24 h after washing in 20 vol.% H2 O/ethanol for 24 h at 60°C. The lines are guides for the eye.
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M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
permeability and in characteristic pore size. We propose that the eect of the washing in the water solution is a dissolution of the smaller particles and deposition onto the larger ones as can also be seen from the large decrease in surface area (350 m2 /g) during the washing (Fig. 2). This also shows that the particle size distribution of the P750 -based gels is broad. Other eects of the washing in water are to hydroxylate the surface of the original wet gel and to increase the water content in the pores for hydrolysis of the P750 from the aging solution. During aging in the P750 solution, silica precipitates from the aging solution ®lling up the necks between the particles, the smallest pores, and ®nally increasing the particle size. This process is expected to give an increase in G modulus and a corresponding decrease in permeability and characteristic pore size as was observed in Fig. 2. However, it is interesting to observe that the permeability does not decrease below the value of the as-prepared gel until an initial density of 0.3 g=cm3 is obtained. This value is anyway too high compared to normal values for aerogel density. Such a combined increase in G modulus and permeability (compared to the as-prepared gels) has not previously been observed, but it might be due to a broad particle size distribution in the P750 based gels. The surface area during the aging shows an increase, which is quite unexpected. This increase might be explained by the fact that the structure of the silica precipitated from the aging solution is dierent from the original gel network. The eect of washing time in the water solution was further studied to optimize the washing process. MOR, shear modulus, and the permeability of wet gels are given as a function of washing time in 20 vol.% H2 O/ethanol at 60°C in Fig. 3. Both the results after washing in the water solution as well as after aging in 35 vol.% P750 /ethanol for 7 h at room temperature are shown. The vol.% P750 in the aging solution was decreased as compared to the results given in Figs. 1 and 2 to obtain a lower initial density. In addition room temperature aging was performed to reduce the reaction rate. The initial density for each sample is included in Fig. 3 as numbers on each data point. It can be seen that the washed only samples have an initial density of about 0.18 g/cm3 while the aged samples have an
Fig. 3. Modulus of rupture, MOR (0.04 MPa), shear modulus, G (0.07 MPa), and permeability, D (0.51 nm2 ) of wet gels as a function of washing time in 20 vol.% H2 O/ethanol at 60°C. Data for washed only gels are given (open symbols) as well as data after aging in 35 vol.% P750 for 7 h at room temperature (closed symbols). The initial density of the gels (0.007 g/cm3 ) is indicated by the numbers on the data points. The lines drawn are guides for the eye.
initial density of 0.22±0.23 g/cm3 ; hence the differences in MOR, G modulus and permeability with washing time cannot be explained by dierent initial densities. The dierences are, therefore, caused by dierences in the structure of the gels. As seen in Fig. 3, the decrease in both MOR and G modulus, with the increase in permeability con®rm that washing causes coarsening of the network by dissolution-reprecipitation. Aging in P750 solution results in an increase in both MOR and G modu-
M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
5
Table 1 Optical and structural characterization of selected aerogels Code
20 vol.% H2 O/ EtOH
35 vol.% P750/ EtOH
qaerogel g=cm3 0:005
SA (m2 =g 20
%TR1
Primary particle diameter (nm)0.1
Cluster diameter (nm)1
As-prepared w60 w60a70 WRTaRT w40a70
± 8 h 60°C 8 h 60°C 24 h RT 8 h 40°C
± ± 2 h 70°C 7 h RT 2 h 70°C
0.170 0.177 ± ± ±
979 613 570 613
84 81 82 84 84
2.8 3.7 3.8 3.3 ±
11 16 15 12 ±
lus; however the increase is most signi®cant for washing times less than 10 h. This shows that the most eective aging is obtained when coarsening of the primary gel network has occurred only to a small degree. Fig. 3 also shows that the permeability decreases during aging but the permeability is still higher than the value of untreated gels (6.7 nm2 ). The data show that a washing time of 5±8 h at 60°C would be bene®cial to obtain increased stiness and strength while retaining relatively high values of permeability. From these results, selected recipes were developed to prepare aerogels by supercritical drying. Short washing times at elevated temperatures were found to give the highest increase in both MOR and G modulus. The aerogels were therefore prepared by washing for 8 h at 40°C and 60°C or 24 h at room temperature. Aging was performed at short time intervals not to increase the initial density too much but to get the desired increase in MOR and G modulus. Properties of selected aerogels are given in Table 1. For comparison, gels without any treatment and washed only gels were studied. The eects of washing alone and washing plus aging treatment on internal structure of the aerogels were studied by comparing SAXS results from dierent samples. The low and intermediate q-value regions of the scattering curves given in Fig. 4 show that the structures of as-prepared, washed and/or aged samples are very similar. Indeed, dried samples are formed by fractal clusters of particles, whose sizes are determined from the Guinier model, and exhibit more or less the same mass fractal behavior, with an average mass fractal exponent close to 1.9 0.1. This may indicate that
the aggregation process might be governed by a DLCA-like model [16]. The washing and aging treatments seem to have a larger impact in the large q-value region (Fig. 4). Indeed, washing and aging treatments performed at elevated temperature lead to signi®cant bending of the curves above approximately 2 nm 1 . In contrast to as-prepared samples and samples treated at room temperature, the intensity does not decrease according to a q 4 power law, but as q 2:7 and q 3 , respectively. Because the q-exponent in this region can be interpreted as the fractal dimension of the primary particle surface minus 6 (qDSurface 6 ) [17], we propose that this is due to a
Fig. 4. Log±log plot of the SAXS intensity versus the scattering vector q of as-prepared, washed and washed/aged silica aerogels dried under direct supercritical CO2 conditions.
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M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
process and the strengthening may ensure success in obtaining large monolithic volumes through an acceptably fast process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Acknowledgements
Fig. 5. Plot of the SAXS I qq4 versus the scattering vector q of as-prepared, washed and washed/aged silica aerogels dried under direct supercritical CO2 conditions.
change in interparticle curvature or rougher particle surfaces caused by dissolution or precipitation of silica at elevated temperatures [18]. By plotting I q q4 as a function of q (Fig. 5), an estimation of the average particle size for the dierent samples was obtained. The primary particle size, cluster size and mass fractal dimension extracted from the SAXS data are included in Table 1. It is observed that washing and aging treatments increase both the cluster and the particle dimensions, however, treatments at elevated temperatures are more eective in doing so. Regarding optical transmittance measurements, Table 1 shows that the washing and aging of the wet gels lead to a slight degradation of the optical properties. This result may be explained by a shift of Rayleigh scattering from the UV to the visible region (enhanced scattering) caused by an increase of cluster and particle size [19±21]. 4. Conclusion A promising process has been developed where the stiness, strength and permeability of polyethoxydisiloxane-based wet gels have been increased. The increase in permeability is of importance to facilitate the supercritical drying
This work is a part of the Hilit EU Joule III Program under the contract JOR3-CT97-0187. Financial support from the European Commission is highly acknowledged. We are grateful to the ESRF, Grenoble, for access to the French CRG beamline BM2. Our warmest thanks are extended to F. Bley, E. Geissler, F. Livet and C. Rochas for their technical advice and for enlightening discussions. Thanks to Airglass AB for density measurement. References [1] R.C. Reid, J.M. Praustnitz, B.E. Poling, The Properties of Gas and Liquids, 4th Ed., MacGraw-Hill, New York, 1987. [2] T. Woignier, G.W. Scherer, A. Alaoui, J. Sol±Gel Sci. Technol. 3 (1994) 141. [3] M.-A. Einarsrud, E. Nilsen, J. Non-Cryst. Solids 226 (1998) 122. [4] S. Hñreid, E. Nilsen, M.-A. Einarsrud, J. Porous Mater. 2 (1996) 315. [5] M.-A. Einarsrud, J. Non-Cryst. Solids 225 (1998) 1. [6] S. Hñreid, E. Nilsen, M.-A. Einarsrud, J. Non-Cryst. Solids 204 (1996) 228. [7] M.-A. Einarsrud, M.B. Kirkedelen, E. Nilsen, K. Mortensen, J. Samseth, J. Non-Cryst. Solids 231 (1998) 10. [8] G.M. Pajonk, E. Elaloui, P. Achard, B. Chevalier, J.L. Chevalier, M. Durant, J. Non-Cryst. Solids 186 (1995) 1. [9] R. Begag, G.M. Pajonk, E. Elaloui, B. Chevalier, Mater. Chem. Phys. 58 (1999) 256. [10] G.W. Scherer, J. Non-Cryst. Solids 142 (1992) 18. [11] G.W. Scherer, J. Sol±Gel Sci. Technol. 1 (1994) 285. [12] A. Rigacci, G. Petermann, L. Gullberg, K.I. Jensen, J.M. Schultz, B. Chevalier, P. Nitz, D. Valette, P. Achard, G.M. Pajonk, M. Durant, M. Ryden, S. Buathier, M.-A. Einarsrud, E. Nilsen, in: proceedings of the Seventh Meeting on Supercritical Fluids, Antibes/Juan-Les-Pins, France, December 6±8, 2000. [13] G. Poelz, in: J. Fricke (Ed.), Aerogels, Springer Proc. Phys. 6, Springer, Berlin, Heidelberg, p. 176. [14] J. Happel, H. Brenner, in: Low Reynolds Number Hydrodynamics, Martinus Nijho, Dordrecht, 1986.
M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7 [15] S. Berthon, O. Barbieri, F. Ehrburger-Dolle, E. Geissler, P. Achard, F. Bley, A.M. Hecht, F. Livet, G.M. Pajonk, N. Pinto, A. Rigacci, C. Rochas, J. Non-Cryst. Solids, accepted. [16] P. Meakin, R. Jullien, J. Chem. Phys. 89 (1988) 246. [17] A. Emmerling, J. Fricke, J. Non-Cryst. Solids 145 (1992) 113. [18] S. Hñreid, J. Anderson, M.-A. Einarsrud, D.W. Hua, D.M. Smith, J. Non-Cryst. Solids 185 (1995) 221.
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[19] A. Emmerling, R. Petricevic, A. Beck, P. Wang, H. Scheller, J. Fricke, J. Non-Cryst. Solids 185 (1995) 240. [20] A. Borne, B. Chevalier, J.L. Chevalier, D. Quenard, E. Elaloui, J. Lambard, J. Non-Cryst. Solids 188 (1995) 235. [21] A. Rigacci, F. Ehrburger-Dolle, E. Geissler, B. Chevalier, H. Sallee, P. Achard, O. Barbieri, S. Berthon, F. Bley, F. Livet, G.-M. Pajonk, N. Pinto, C. Rochas, these Proceedings, p. 187.
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Section 1. Inorganic/composite aerogels
Strengthening of silica gels and aerogels by washing and aging processes M.-A. Einarsrud a,*, E. Nilsen a, A. Rigacci b, G.M. Pajonk c, S. Buathier c, D. Valette d, M. Durant d, B. Chevalier e, P. Nitz f, F. Ehrburger-Dolle g a
Department of Chemistry, Norwegian University of Science and Technology, 7491 Trondheim, Norway Ecole des Mines de Paris CENERG, Sophia Antipolis, B.P. 207, 06 904 Sophia-Antipolis cedex, France UMR 5634 CNRS Universit e Claude Bernard Lyon1, 43 boulevard du 11 Novembre 1918, 69622 Villeurbanne, France d PCAS, B.P. 111, 91161 Longjumeau, France e CSTB 24 rue Joseph Fourier, 38400 Saint Martin d'H eres, France f Fraunhofer Insitute for Solar Energy Systems, Oltmannsstr. 5, D-79100 Freiburg, Germany g ICSI, CNRS, 15 rue Jean Stacky, F-68057 Mulhouse cedex, France b
c
Abstract Gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. During washing in water solution a signi®cant increase in the permeability of the gels was observed, showing that dissolution-reprecipitation occurs. After washing, the gels were further soaked in a solution of polyethoxydisiloxane precursor to strengthen and stien the gel. As expected, a signi®cant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability does not decrease below the value for the as-prepared gels. Hence, a promising process has been developed where both the stiness and the strength have been increased as well as the permeability. The increase in permeability is of importance to facilitate the supercritical drying process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Optical properties have been measured on laboratory scale aerogels. The corresponding results have been correlated with structural characteristics measured by small-angle X-ray scattering (SAXS). Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 82.33.Ln
1. Introduction This work is a part of the EU HILIT (Highly Insulating and Light Transmitting Aerogel Glazing for Window) project, the main aim of which is * Corresponding author. Tel.: +47-73 594 002; fax: +47-73 590 860. E-mail address: [email protected] (M.-A. Einarsrud).
to develop and investigate the pre-industrial production and application of monolithic silica aerogel in highly insulating and light transmitting glazings for windows. The production process is based on a direct supercritical drying route using CO2 and the use of commercially available polyethoxydisiloxane silica precursors. For this application, aerogel tiles of minimum size 60 60 cm2 are necessary and hence the mechanical properties of both the wet and dried gel are of importance.
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 4 2 3 - 9
2
M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
Indeed, even if direct supercritical CO2 drying reduces capillary stresses [1], it appears interesting at large scale to increase both the modulus of rupture (MOR) and permeability (D) in order to diminish the impact of depressurization stresses [2]. Such a strengthening treatment may ensure success in obtaining large monolithic volumes through an acceptably fast process. This work is based on previous developments by Einarsrud et al. [3±7] where the mechanical properties of the wet gels are improved by an aging procedure. Fresh silica species from an aging solution is precipitated onto the already formed gel network, increasing both the stiness and strength of the gel. The motivation for this paper is twofold; ®rst, to discuss the eect of aging polyethoxydisiloxane-based gels in a solution also of the polyethoxydisiloxane precursors and second, to discuss the eect of scaling up these gels to laboratory scale samples (diameter 7 cm or 14 14 cm2 ). The eects on structure and optical properties will also be discussed. 2. Experimental Wet gels were prepared from the commercial polyethoxydisiloxane precursor P750 (PCAS, Longjumeau, France) by using P750 [8,9] and ethyl acetoacetate (Etac) in a volume ratio 50:50 and 21 N HF (2 vol.%) as a catalyst. The gels were prepared by adding Etac to the catalyst and then adding this mixture slowly to P750 . The solution was cast into Te¯on tubes with an inner diameter of 8.6 mm or Te¯on moulds (diameter 7 cm or 14 14 cm2 ), and kept at room temperature for 3 h for gelation. To study the eect of aging, the gels were ®rst soaked in a washing solution (20 vol.% H2 O/ethanol) for various lengths of time (0±24 h) at room temperature, 40°C or 60°C and then in an aging solution (35 or 70 vol.% P750/ethanol or 70 vol.% TEOS/ethanol). The aging was performed at room temperature or 70°C. As a reference sample for 0 h aging time, gels were washed only in ethanol. Generally the volume ratio of gel:washing or aging solution was 1:3. The gel rods were further washed in ethanol four times within 24 h at 50°C before measuring the wet
gel shear modulus (G) and the modulus of rupture, MOR, by standard 3-point beam bending or the G modulus and permeability, D, by relaxation experiments as described elsewhere [3,4,10,11]. The larger gels were supercritically dried with CO2 according to the procedure described by Rigacci et al. [12]. The density of the aerogels was determined by measuring the weight and dimensions, when possible, or it was estimated using the refractive index [13]. The initial density of the gels (which is the bulk density of the dried gels if there is no shrinkage during drying) was calculated from the volume of the wet gels and gravimetric measurement of heat treated gels (300°C). Surface area was measured by N2 adsorption (Micromeritics, ASAP 2000 or laboratory made equipment) using the BET method. The characteristic pore size of the wet gel, rw , was calculated from Eq. (1). D
1
q=qs rw2 ; 4j
1
j is the Kozeny `constant' and is given in Eq. (2) 0:5 qb q j 1:0 6:05 8:60 b qs qs 1:5 q 6:56 b ; 2 qs where q=qs is the relative density (volume fraction of solids) of the gel network. Eq. (2) is based on calculations by Happel and Brenner [14] for the permeability of an array of randomly arranged cylinders. Spectral direct and hemispherical transmittance measurements at normal incidence were performed using a commercial double beam spectrophotometer (Perkin Elmer k19) with an integrating sphere (Labsphere, diameter 15 cm). From the spectral data, the percentage of transn;hem mission in the visible range %TR : sn;dir vis =svis (normal direct transmittance/normal hemispherical transmittance) and eective extinction coecients, E, are calculated. SAXS measurements were performed at the French CRG BM2 beamline at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The experimental conditions and the associated theoretical treatments are described by Berthon et al. [15].
M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
3. Results and discussion Fig. 1 compares the eect on initial density of aging in ethanol solutions of P750 and TEOS. Aging the wet gels in solutions of P750 or TEOS clearly causes silica to be transported from the aging solution onto the silica network as the initial density is increasing. An increasing initial density with increasing aging time is observed for aging in P750 ; however aging in TEOS for more than 3 h (or an initial density above approximately 0.26 g/cm3 ) has no eect. Such a leveling o of the initial density has not previously been observed for other types of gels [3,4]. The decrease in eect of aging in a TEOS solution after 3 h might be caused by a high reactivity of TEOS resulting in plugging of the pore openings. This is supported by the fact that the TEOS aged gels do not show a further increase in G modulus after reaching the maximum initial density of 0.26 g/cm3 . From Fig. 1 it can be seen that the initial density is strongly in¯uenced by relatively short aging times, hence aging of the larger-sized gels was in addition to 70°C performed at room temperature. Shear modulus, permeability, characteristic pore radius and surface area (after drying) of the wet gels aged in P750 solution is given as a function of initial density in Fig. 2. Included in Fig. 2 are also the data after washing only in ethanol
Fig. 1. Initial density (0.005 g/cm3 ), minimum 10 measurements per sample) versus aging time in 70 vol.% P750 /ethanol (d) or TEOS/ethanol ( ) at 70°C after washing in 20 vol.% H2 O/ethanol at 60°C for 24 h. Zero hour aging means gels washed in ethanol () or in 20 vol.% H2 O/ethanol (j).
3
3
(qinitial 0:168 g=cm ) and after washing in the H2 O/ethanol solution (qinitial 0.175 g/cm3 ). During the washing there is a small but signi®cant decrease in G modulus. The most interesting feature however is the corresponding large increase in
Fig. 2. Shear modulus, G (1.68 MPa), permeability, D (0.55 and surface area nm2 ), characteristic pore radius, rw 5 A) (20 m2 /g) of wet gels versus initial density. The gels were aged in 70 vol.% P750 /ethanol at 70°C for 0±24 h after washing in 20 vol.% H2 O/ethanol for 24 h at 60°C. The lines are guides for the eye.
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M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
permeability and in characteristic pore size. We propose that the eect of the washing in the water solution is a dissolution of the smaller particles and deposition onto the larger ones as can also be seen from the large decrease in surface area (350 m2 /g) during the washing (Fig. 2). This also shows that the particle size distribution of the P750 -based gels is broad. Other eects of the washing in water are to hydroxylate the surface of the original wet gel and to increase the water content in the pores for hydrolysis of the P750 from the aging solution. During aging in the P750 solution, silica precipitates from the aging solution ®lling up the necks between the particles, the smallest pores, and ®nally increasing the particle size. This process is expected to give an increase in G modulus and a corresponding decrease in permeability and characteristic pore size as was observed in Fig. 2. However, it is interesting to observe that the permeability does not decrease below the value of the as-prepared gel until an initial density of 0.3 g=cm3 is obtained. This value is anyway too high compared to normal values for aerogel density. Such a combined increase in G modulus and permeability (compared to the as-prepared gels) has not previously been observed, but it might be due to a broad particle size distribution in the P750 based gels. The surface area during the aging shows an increase, which is quite unexpected. This increase might be explained by the fact that the structure of the silica precipitated from the aging solution is dierent from the original gel network. The eect of washing time in the water solution was further studied to optimize the washing process. MOR, shear modulus, and the permeability of wet gels are given as a function of washing time in 20 vol.% H2 O/ethanol at 60°C in Fig. 3. Both the results after washing in the water solution as well as after aging in 35 vol.% P750 /ethanol for 7 h at room temperature are shown. The vol.% P750 in the aging solution was decreased as compared to the results given in Figs. 1 and 2 to obtain a lower initial density. In addition room temperature aging was performed to reduce the reaction rate. The initial density for each sample is included in Fig. 3 as numbers on each data point. It can be seen that the washed only samples have an initial density of about 0.18 g/cm3 while the aged samples have an
Fig. 3. Modulus of rupture, MOR (0.04 MPa), shear modulus, G (0.07 MPa), and permeability, D (0.51 nm2 ) of wet gels as a function of washing time in 20 vol.% H2 O/ethanol at 60°C. Data for washed only gels are given (open symbols) as well as data after aging in 35 vol.% P750 for 7 h at room temperature (closed symbols). The initial density of the gels (0.007 g/cm3 ) is indicated by the numbers on the data points. The lines drawn are guides for the eye.
initial density of 0.22±0.23 g/cm3 ; hence the differences in MOR, G modulus and permeability with washing time cannot be explained by dierent initial densities. The dierences are, therefore, caused by dierences in the structure of the gels. As seen in Fig. 3, the decrease in both MOR and G modulus, with the increase in permeability con®rm that washing causes coarsening of the network by dissolution-reprecipitation. Aging in P750 solution results in an increase in both MOR and G modu-
M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7
5
Table 1 Optical and structural characterization of selected aerogels Code
20 vol.% H2 O/ EtOH
35 vol.% P750/ EtOH
qaerogel g=cm3 0:005
SA (m2 =g 20
%TR1
Primary particle diameter (nm)0.1
Cluster diameter (nm)1
As-prepared w60 w60a70 WRTaRT w40a70
± 8 h 60°C 8 h 60°C 24 h RT 8 h 40°C
± ± 2 h 70°C 7 h RT 2 h 70°C
0.170 0.177 ± ± ±
979 613 570 613
84 81 82 84 84
2.8 3.7 3.8 3.3 ±
11 16 15 12 ±
lus; however the increase is most signi®cant for washing times less than 10 h. This shows that the most eective aging is obtained when coarsening of the primary gel network has occurred only to a small degree. Fig. 3 also shows that the permeability decreases during aging but the permeability is still higher than the value of untreated gels (6.7 nm2 ). The data show that a washing time of 5±8 h at 60°C would be bene®cial to obtain increased stiness and strength while retaining relatively high values of permeability. From these results, selected recipes were developed to prepare aerogels by supercritical drying. Short washing times at elevated temperatures were found to give the highest increase in both MOR and G modulus. The aerogels were therefore prepared by washing for 8 h at 40°C and 60°C or 24 h at room temperature. Aging was performed at short time intervals not to increase the initial density too much but to get the desired increase in MOR and G modulus. Properties of selected aerogels are given in Table 1. For comparison, gels without any treatment and washed only gels were studied. The eects of washing alone and washing plus aging treatment on internal structure of the aerogels were studied by comparing SAXS results from dierent samples. The low and intermediate q-value regions of the scattering curves given in Fig. 4 show that the structures of as-prepared, washed and/or aged samples are very similar. Indeed, dried samples are formed by fractal clusters of particles, whose sizes are determined from the Guinier model, and exhibit more or less the same mass fractal behavior, with an average mass fractal exponent close to 1.9 0.1. This may indicate that
the aggregation process might be governed by a DLCA-like model [16]. The washing and aging treatments seem to have a larger impact in the large q-value region (Fig. 4). Indeed, washing and aging treatments performed at elevated temperature lead to signi®cant bending of the curves above approximately 2 nm 1 . In contrast to as-prepared samples and samples treated at room temperature, the intensity does not decrease according to a q 4 power law, but as q 2:7 and q 3 , respectively. Because the q-exponent in this region can be interpreted as the fractal dimension of the primary particle surface minus 6 (qDSurface 6 ) [17], we propose that this is due to a
Fig. 4. Log±log plot of the SAXS intensity versus the scattering vector q of as-prepared, washed and washed/aged silica aerogels dried under direct supercritical CO2 conditions.
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process and the strengthening may ensure success in obtaining large monolithic volumes through an acceptably fast process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Acknowledgements
Fig. 5. Plot of the SAXS I qq4 versus the scattering vector q of as-prepared, washed and washed/aged silica aerogels dried under direct supercritical CO2 conditions.
change in interparticle curvature or rougher particle surfaces caused by dissolution or precipitation of silica at elevated temperatures [18]. By plotting I q q4 as a function of q (Fig. 5), an estimation of the average particle size for the dierent samples was obtained. The primary particle size, cluster size and mass fractal dimension extracted from the SAXS data are included in Table 1. It is observed that washing and aging treatments increase both the cluster and the particle dimensions, however, treatments at elevated temperatures are more eective in doing so. Regarding optical transmittance measurements, Table 1 shows that the washing and aging of the wet gels lead to a slight degradation of the optical properties. This result may be explained by a shift of Rayleigh scattering from the UV to the visible region (enhanced scattering) caused by an increase of cluster and particle size [19±21]. 4. Conclusion A promising process has been developed where the stiness, strength and permeability of polyethoxydisiloxane-based wet gels have been increased. The increase in permeability is of importance to facilitate the supercritical drying
This work is a part of the Hilit EU Joule III Program under the contract JOR3-CT97-0187. Financial support from the European Commission is highly acknowledged. We are grateful to the ESRF, Grenoble, for access to the French CRG beamline BM2. Our warmest thanks are extended to F. Bley, E. Geissler, F. Livet and C. Rochas for their technical advice and for enlightening discussions. Thanks to Airglass AB for density measurement. References [1] R.C. Reid, J.M. Praustnitz, B.E. Poling, The Properties of Gas and Liquids, 4th Ed., MacGraw-Hill, New York, 1987. [2] T. Woignier, G.W. Scherer, A. Alaoui, J. Sol±Gel Sci. Technol. 3 (1994) 141. [3] M.-A. Einarsrud, E. Nilsen, J. Non-Cryst. Solids 226 (1998) 122. [4] S. Hñreid, E. Nilsen, M.-A. Einarsrud, J. Porous Mater. 2 (1996) 315. [5] M.-A. Einarsrud, J. Non-Cryst. Solids 225 (1998) 1. [6] S. Hñreid, E. Nilsen, M.-A. Einarsrud, J. Non-Cryst. Solids 204 (1996) 228. [7] M.-A. Einarsrud, M.B. Kirkedelen, E. Nilsen, K. Mortensen, J. Samseth, J. Non-Cryst. Solids 231 (1998) 10. [8] G.M. Pajonk, E. Elaloui, P. Achard, B. Chevalier, J.L. Chevalier, M. Durant, J. Non-Cryst. Solids 186 (1995) 1. [9] R. Begag, G.M. Pajonk, E. Elaloui, B. Chevalier, Mater. Chem. Phys. 58 (1999) 256. [10] G.W. Scherer, J. Non-Cryst. Solids 142 (1992) 18. [11] G.W. Scherer, J. Sol±Gel Sci. Technol. 1 (1994) 285. [12] A. Rigacci, G. Petermann, L. Gullberg, K.I. Jensen, J.M. Schultz, B. Chevalier, P. Nitz, D. Valette, P. Achard, G.M. Pajonk, M. Durant, M. Ryden, S. Buathier, M.-A. Einarsrud, E. Nilsen, in: proceedings of the Seventh Meeting on Supercritical Fluids, Antibes/Juan-Les-Pins, France, December 6±8, 2000. [13] G. Poelz, in: J. Fricke (Ed.), Aerogels, Springer Proc. Phys. 6, Springer, Berlin, Heidelberg, p. 176. [14] J. Happel, H. Brenner, in: Low Reynolds Number Hydrodynamics, Martinus Nijho, Dordrecht, 1986.
M.-A. Einarsrud et al. / Journal of Non-Crystalline Solids 285 (2001) 1±7 [15] S. Berthon, O. Barbieri, F. Ehrburger-Dolle, E. Geissler, P. Achard, F. Bley, A.M. Hecht, F. Livet, G.M. Pajonk, N. Pinto, A. Rigacci, C. Rochas, J. Non-Cryst. Solids, accepted. [16] P. Meakin, R. Jullien, J. Chem. Phys. 89 (1988) 246. [17] A. Emmerling, J. Fricke, J. Non-Cryst. Solids 145 (1992) 113. [18] S. Hñreid, J. Anderson, M.-A. Einarsrud, D.W. Hua, D.M. Smith, J. Non-Cryst. Solids 185 (1995) 221.
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[19] A. Emmerling, R. Petricevic, A. Beck, P. Wang, H. Scheller, J. Fricke, J. Non-Cryst. Solids 185 (1995) 240. [20] A. Borne, B. Chevalier, J.L. Chevalier, D. Quenard, E. Elaloui, J. Lambard, J. Non-Cryst. Solids 188 (1995) 235. [21] A. Rigacci, F. Ehrburger-Dolle, E. Geissler, B. Chevalier, H. Sallee, P. Achard, O. Barbieri, S. Berthon, F. Bley, F. Livet, G.-M. Pajonk, N. Pinto, C. Rochas, these Proceedings, p. 187.