Two fractal structures in aerogel

Transcription

1 Journal of Non-Crystalline Solids ) 175±180 Two fractal structures in aerogel C. Marliere, T. Woignier *, P. Dieudonne, J. Primera, M. Lamy, J. Phalippou Laboratoire des Verres, UMR CNRS 5587, Universite Montpellier II, Place E. Bataillon, Montpellier cedex, France Abstract Composite aerogels have been prepared by the addition of silica soot aerosil) in the solution before gelation. The fractal network previously described which results from the aggregation mechanism of the organosiloxane is strongly a ected by the in uence of the aerosil soot. Ultra small angle X-ray scattering measurements at ESRF show that besides the fractal network built up by the organosiloxane, the silica soot is forming another fractal structure at a higher scale. The fractal dimension characterizing the inorganic network could be interpreted as the signature of the linkage between the polymeric clusters. From a mechanical point of view, for aerosil content lower than 40 wt%, the elastic modulus is not dependent on the aerosil fractal network and the mechanical features are those of the polymeric gel. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: Eq; )j; Gt; )x 1. Introduction Gel and aerogel structures have been extensively studied during the past decade by di erent scattering techniques SAXS [1,2], SANS [3], light scattering [4]). Generally, the main conclusion of the studies is that the microstructure can be described as a fractal network in the length scale 10± 1000 A. The fractal structure is the result of an aggregation mechanism. The silica beads 10 A) build the clusters with a compactness characterized by the fractal dimension D f. The spatial extent of the clusters and their fractal dimension are strongly dependent on the synthesis conditions, i.e. the ph of the gelifying solution [5,6]. Such materials are thus very porous but not really permeable * Corresponding author. Tel.: ; fax: address: woignier@crit.univ-montp2.fr T. Woignier). because of the small pore size. Recently, silica aerogel has been used to con ne nuclear wastes [7] and for this purpose, it was necessary to increase the permeability by tailoring the pore size distribution in the macropore range. Composite gels and aerogels [8,9] have been synthesized by adding in the gelifying solution silica soot `aerosil' which tailors the pore size. For example, composite aerogel prepared with 70 wt% of aerosil exhibits a pore size distribution centered around 60 nm [9]. This kind of network is not fractal at all. Thus, the addition of the silica soot modi es strongly the aggregation mechanism. A relevant question is how the presence of silica particles disturbs the aggregation process and destroys the fractal microstructure generally expected. In this work, composite aerogels with increasing silica soot content from 0 to 70 wt% are prepared in order to understand the transition from a fractal to a non-fractal aerogel. For that, the network /01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S )

2 176 C. Marliere et al. / Journal of Non-Crystalline Solids ) 175±180 structure will be characterized in the length scale ranging from 10 to A corresponding to micro, meso and macroporosity. The connectivity of the network is dependent on its structure. To further investigates this connectivity the Young's modulus of the composite aerogels will be measured. 2. Experimental Typically, silica aerogel is made from an alcogel issued from hydrolysis and polycondensation reactions of organosilicate compounds dissolved in alcohol [10, p. 352]. After gelation the solid network gel) is lled with alcohol and, according to a procedure previously reported [11], the alcogels are transformed into aerogels by supercritical drying. Fig. 1. Schematic ow chart of the process for a composite aerogel. Such porous materials exhibit a broad pore size distribution. In the literature [8,9], it has been shown that the addition of pyrogenic silica such as aerosil in the gelifying solution favors the formation of macropores. Macropores are de ned as pores larger than 500 A [13]. This feature has been used to prepare the `composite aerogel'. The ow chart of the process is displayed in Fig. 1. The starting material was silicon tetraethoxide TEOS). TEOS was rst mixed with HCl 0.01 N molar ratio H 2 O/ TEOS ˆ 15). Fumed silica aerosil OX50 from Degussa) was added to the hydrolyzed solution of TEOS under stirring. A vigorous agitation is necessary because of the high viscosity of the solution. The aerosil weight percentage reported to the total silica weight) ranges between 0% and 70%. The ph of the sol was then adjusted to 4.5 with NH 4 OH 0.1 N) which leads to gelation in a few minutes. Supercritical drying of gels was carried out in an autoclave. The gels were transformed into aerogels by supercritical drying at 305 C and 13 MPa. The aerogels are labeled as CA y, where y is the aerosil content in wt%. These di erent samples covered bulk density within the range 0.25±0.4 g/cm 3. It is obvious that the elastic and mechanical properties are strongly dependent on the load bearing fraction of solid and on the way the solid phase is connected. To characterize the mechanical properties, Young's modulus E) and the rupture modulus r) were measured by a three-point bending technique using an Instron testing machine previously described [12]. Five experiments are done on each samples and the error bars in Fig. 3 characterize the data dispersion. USAXS spectra have been obtained from the Bonse±Hart optics on line ID2 at ESRF, the European Synchrotron Radiation Facilities. All the measured intensities have been normalized to a constant value of the incident X-ray ux. Then, in order to obtain the real di usion spectrum, DI, of the sample, we systematically measured the background spectrum without the samples). This spurious signal was subtracted from the original one and the resulting signal named `intensity' in the gures) was plotted versus the scattering vector in log±log scales. Experiments were done at a

3 C. Marliere et al. / Journal of Non-Crystalline Solids ) 175± wavelength of 1 A. The lateral area of the sample probed by X-ray was approximately of a few nanometers. The smallest scattering wave vector for which relevant data were measured was about A Results USAXS experiments generally give information on three main features of the aerogel structure: the mean size of the clusters n) which are connected to form the network, the mean size of the primary particles a) which stick together to build the cluster and the fractal dimension D f which expresses the clusters compactness [14]. Fig. 2 a) shows the evolution of the scattering intensity I (a) (b) Fig. 2. a) Scattered intensities versus the wave vector for the set of composite aerogels CA0±CA15). b) Scattered intensities versus the wave vector for the set of composite aerogels CA20± CA65). versus the wave vector q) for the aerogel composite set. The position of the two crossovers q n and q a ) for the sample CA0 are related, respectively, to the inverse of the cluster size n) and the inverse of the particle size a). The slope between the two crossovers corresponds to D f. In Fig. 2 a), the scattered intensities have been arbitrarily shifted for clarity. From the rst curve CA0), corresponding to the polymeric network issued of the TEOS gelation, we can deduce that a is close to 15 A, n close to 50 A and D f close to 2.3 Table 1). These results are in a good agreement with that previously published on classical aerogels issued from organometallic compounds [3,5,6]. We also notice a remarkable point, the intensity increases as q-value decreases below 10 3 A 1 ). This observation may be interpreted as the signature of macroporosity, the typical length scale of which is around a few hundred nanometers. The results reveal the strong e ect of the addition of silica soot on the aerogel structure. With only 5% of aerosil, the scattered intensity is strongly affected. The curves CA5, CA10 and CA15 show the same features. Besides the high q >10 2 A 1 ) part of the curve, characteristic of the polymeric gel previously described, there is a broad linear behavior between almost 10 4 and A 1. The crossover at q a ˆ A 1 is due to the aerosil particle size 200±500 A). The roughness of the aerosil particle is pointed out by a power-law dependence in q 3:2. The linear behavior at low q can be interpreted in terms of a fractal structure issued from the soot particles aggregation. The fractal dimension D f of this structure is close to 1.6 in close agreement with a DLCA aggregation process [15]. Fig. 2 b) con rms the results provided by spectra reported in Fig. 2 a). The crossover corresponding to the aerosil particles is more pronounced with the aerosil content increase. The slope corresponding to the fractal dimension of the network 1.6) and that of the surface fractal dimension 3) are not modi ed. The di erences observed are: rst, the extent of the fractal range of the aerosil cluster decreases with the increase of the aerosil content, secondly, the polymeric network geometry seems to be progressively destroyed by the presence of the aerosil particles. It is

4 178 C. Marliere et al. / Journal of Non-Crystalline Solids ) 175±180 Table 1 Sample D f a A) n A) D F A A) N A) CA ±60 ± ± ± CA ± > CA ± CA ± ± CA20 ± ± ± ± CA40 ± ± ± ± CA50 ± ± ± ± CA55 ± ± ± CA65 ± ± ± ± ± ± Fig. 3. Young's modulus and rupture modulus for the set of composite aerogels. not possible to observe the typical curve of the polymeric gel in the 10 2 ±10 1 range. Even for a low aerosil content 20%) the fractal geometry of the polymeric network seems to be destroyed. Fig. 3 shows the behavior of the Young's modulus and rupture modulus for the composite aerogel set. It is worth noticing that the mechanical features are constant and close to that of the polymeric gel CA0) in the whole range of concentration 0±40%). For higher concentration, the elastic constant increases strongly. The elastic modulus of the composite aerogel with 70% soot is almost 10 times higher than that of the polymeric gel. 4. Discussion From the curves of Fig. 2 a), several remarks can be drawn. The high q range of the CA0 curve con rms results already published in the literature, the structure of polymeric classical) aerogel is fractal in the range 10±50 A, with a fractal dimension close to 2.3. This fractal dimension has been interpreted as the signature of a cluster± cluster aggregation process followed by a possible restructuring [3,5,6]. The important syneresis e ect observed on such aerogels [16] may be responsible for the restructuring. It is not possible to infer the aggregation mechanism from the geometry of the clusters because restructuring raises the fractal dimension of a DLCA cluster to that characteristic of an RLCA cluster [15]. A new result on this polymeric classical aerogel is the low q region, the domain of long ranges of scale length, above 10 3 A) where only few and contradictory information has been available. The remarkable point is the increasing intensity for decreasing q values which can be associated to macropores with a typical length around 10 3 A. Such results are generally not observed by the usual scattering techniques SAXS, SANS and light scattering) because of limitations on accessible values of q. However, a previous paper using USAXS [5,16] mentions a similar result. Moreover a macroporous volume has been deduced from the thermoporometry results on analogous polymeric aerogels [17]. With the addition of aerosil the curves are strongly modi ed, indicating the important e ect of aerosil on the aerogel structure. For concentrations CA5±CA15, the two q ranges, ± A 1 and ± A 1, exhibit two di erent fractal structures corresponding to length scales in the range of 10±50 A which is the usual fractal range for polymeric aerogel) and length scales in the range of 150± 2000 A which can be attributed to the structure

5 C. Marliere et al. / Journal of Non-Crystalline Solids ) 175± issued from the aggregation of the aerosil grains). The fractal dimension is close to 1.6, in agreement with the DLCA model [15]. The fractal range is broad 2 orders of magnitude for the lowest aerosil content, CA5). The cluster size decreases with the aerosil content because the growth of the clusters is limited by the neighboring ones. The number of clusters per unit volume increases with the aerosil content and thus for the highest concentration CA65) the fractal range is no longer observed, the description of this material in terms of fractals is meaningless. From curves CA20±CA65 it seems that the fractal polymeric structure usually associated with TEOS gelation is absent. In the high q range, the typical curve exhibited by the CA0 sample) is not observed, likely because the comparatively strong scattered intensity of the aerosil structure overlaps that coming from the polymeric network. The other explanation is that the aggregation process of the TEOS is strongly perturbed by the aerosil in the solution and the structure characteristic of the classical aerogel is not present. However, the mechanical features measured on the sample set are quite constant between CA0 up to CA40. These composite aerogels show an elastic behavior analogous to that of the polymeric classical aerogel which proves that the polymeric network is still present. For samples CA0±CA40, the aerosil particles may be assumed as playing no role in establishing the elastic properties and may be considered as behaving as pores. They do not participate in load transfer, the elastic waves propagate only through the polymeric structure. Consequently the role of the aerosil particles is not di erent from that of hypothetic pores which would have the same size. For concentration higher than 40%, the elastic modulus rapidly increases with aerosil%, the aerosil particles play the expected role and strengthen the structure. These results can be explained by considering that the composite aerogel structure is the sum of the classical gelation of organosiloxane polymeric gel) and a network issued from the aerosil particles. In the literature, previous work has already discussed the fractal structure of vapor-phase aggregates. SANS experiments [18,19] lead to the conclusion that the fractal dimension of vaporphase aggregates is in the range of 2.4±2.6. Light and X-ray scattering measurements on fumed silica dispersed in water give fractal dimension close to 1.9 [20,21]. These fractal structures are interpreted as due to the aggregation of the primary particles in the gas-phase environment. In this study, the fractal structure described in the low q range could be the memory of the gas-phase environment, but the fractal dimension D F 1.6) is clearly lower than those of the literature. In this hypothesis, it is not clear why the cluster size N) decreases while the aerosil content increases. Moreover the fractal structure vanishes for high aerosil content curve CA65, Fig. 2 b)). Another explanation could be that the ephemeral fractal structure [21], issued from the gas-phase is destroyed in the solution because of the very strong stirring necessary to homogenize the high viscous solution and the fractal structure is that of a gelation process. Then, during the TEOS gelation, the aerosil particles covered with hydroxyl groups could be suitable sites to the hydrolysis of the TEOS. Therefore, the polymeric network will grow at the surface of the aerosil particles. For low aerosil content the gel forms as usually observed giving rise to the fractal structure previously described [3,5,6]. This structure is preserved in spite of the presence of the aerosil. It is supposed that the aerosil plays the role of a seed for the polymeric cluster formation. On the other hand, the network of aerosil particles is formed when the polymeric clusters surrounding the aerosil particles touch and link. The soot particles surrounded by the clusters move in the solution with a diffusion coe cient inversely proportional to the cluster size. When the clusters link, the gel point is reached and the spatial arrangement of the particles, characterized by its fractal dimension, should be the signature of the linkage process between the polymeric clusters. It has been proposed in the literature [10, p. 352] that such a cluster linkage could be percolation, but, because of the fractal dimension measured 1.6) it seems that the DLCA is more likely appropriated to describe the gelation.

6 180 C. Marliere et al. / Journal of Non-Crystalline Solids ) 175± Conclusion The composite aerogels show a structure clearly di erent from that usually described for classical aerogels. A fractal network of aerosil is embedded in a fractal network of a polymeric gel. These two fractal structures are identi ed because of their di erent length scale domains and di erent fractal dimensions. For low aerosil content the aggregation mechanism of the organosiloxane is preserved, but because of the strong reactivity of TEOS with the aerosil particles for high soot content a large part of the TEOS is consumed and the polymeric clusters cannot grow. Fractality completely disappears and the mechanical properties of the composite material are directly related to the soot content. Because of its enhanced mechanical features and its more homogeneous porous structure [7,8] the composite aerogel can be more easily impregnated by a liquid and can be used as a porous and sinterizable host matrix for nuclear wastes [9]. Acknowledgements We thank O. Diat, P. Bosecke and T. Narayanan, contact locals at the ESRF, Grenoble, France for very e cient technical help and for fruitful discussions. References [2] A. Emmerling, J. Fricke, J. Non-Cryst. Solids ) 113. [3] T. Woignier, J. Phalippou, R. Vacher, J. Pelous, E. Courtens, J. Non-Cryst. Solids ) 198. [4] D.H. Tewari, A.J. Hunt, K.D. Lo tus in Aerogels, Springer, New York, 1986, p. 31. [5] D.W. Scha er, B.J. Olivier, C.S. Ashley, D. Richter, B. Farago, B. Frick, L. Hrubesh, M.J. Van Bommel, G. Long, S. Krueger, J. Non-Cryst. Solids ) 105. [6] R. Vacher, T. Woignier, J. Pelous, E. Courtens, Phys. Rev. B 37 1) 1988) [7] T. Woignier, J. Reynes, J. Phalippou, J.L. Dussossoy, N. Jacquet-Francillon, J. Non-Cryst. Solids ) 353. [8] M. Toki, S. Miyashita, T. Takeuchi, S. Kanabe, J. Non- Cryst. Solids ) 479. [9] T. Woignier, J. Reynes, J. Phalippou, J.L. Dussossoy, J. Sol±Gel Sci. Technol ) 835. [10] J.F. Brinker, G.W. Scherer, Sol±Gel Science, Academic Press, New York, [11] S.S. Kistler, Nature ) 747. [12] P. Etienne, J. Phalippou, T. Woignier, A. Ha di Alaoui, J. Non-Cryst. Solids ) 19. [13] IUPAC, Manuals of symbol and terminology, Appendix 2. Part 1, Colloid and surface chemistry. Pure Appl. Chem ) 578. [14] S.H. Chen, J. Texeira, Phys. Rev. Lett ) [15] P. Meakin, R. Jullien, J. Chem. Phys. 89 1) 1988) 246. [16] H. Hdach, T. Woignier, J. Phalippou, G.W. Scherer, J. Non-Cryst. Solids ) 202. [17] M. Pauthe, J.F. Quinson, G.W. Scherer, H. Hdach, T. Woignier, J. Phalippou, J. Non-Cryst. Solids ) 1. [18] S.K. Sinha, T. Freltoft, J. Kjems, in: F. Family, D.P. Landau Eds.), Kinetics of Aggregation and Gelation, North-Holland, Amsterdam, 1984, p. 87. [19] T. Freltoft, J.K. Kjems, S.K. Sinha, Phys. Rev. B ) 269. [20] A. Hurd, D.W. Scha er, J. Martin, Phys. Rev. A 35 5) 1987) [21] J. Martin, A.J. Hurd, J. Appl. Crystallogr ) 61. [1] D.W. Schaefer, K.D. Keefer, Phys. Rev. Lett ) 2199.