Very large-scale structures in sintered silica aerogels as evidenced by atomic force microscopy and ultra-small angle X-ray scattering experiments

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1 Journal of Non-Crystalline Solids ) 148±153 Very large-scale structures in sintered silica aerogels as evidenced by atomic force microscopy and ultra-small angle X-ray scattering experiments C. Marliere *, F. Despetis, P. Etienne, T. Woignier, P. Dieudonne, J. Phalippou Laboratoire des Verres, UMR CNRS-UM2 5587, Universite Montpellier 2, C.C. 69-Place E. Bataillon, F Montpellier cedex 5, France Abstract During the last few years the bulk structure of silica aerogels has been extensively studied mainly by scattering techniques neutrons, X-rays, light). It has been shown that small silica particles aggregate to constitute a fractal network. Its spatial extension and fractal dimension are strongly dependent on the synthesis conditions e.g., ph of gelifying solutions). These typical lengths range from 1 to 10 nm. Ultra-small angle X-ray scattering USAXS) and atomic force microscopy AFM) experiments have been carried out on aerogels at di erent steps of densi cation. The results presented in this paper reveal the existence of a spatial arrangement of the solid part at a very large length scale. The evolution of this very large-scale structure during the densi cation process has been studied and reveals a contraction of this macro-structure made of aggregates of clusters. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: Eq; )d; Gt; Hv 1. Introduction * Corresponding author. Tel.: ; fax: address: marliere@crit1.univ-montp2.fr C. Marliere). Silica aerogels have been extensively studied in the last few years [1,2]. This great interest has been motivated by practical challenges such as the synthesis and optimization of thermal insulating materials [3] and ultra low dielectric constants components [4]. A wider use of these materials is often limited by intrinsic problems such as their poor mechanical properties. As these properties are very dependent on structure [5] it is of great importance to understand, in detail, the spatial organization of aerogels at all relevant length scales. Recently, much work has been devoted to structural characterization of aerogels at `small' length scales 0.1±10 nm) through the use of small angle X-ray scattering SAXS) techniques. At these scales, aerogels are described as self-similar fractal) materials, the structure of which is characterized by di erent parameters such as the fractal dimension, D, the particle size, a typically in the range of 1 nm) and the mean size of the fractal clusters i.e., its coherence length), n in the order of a few tens of nanometers). The relevant physical processes responsible for these fractal structure are now well understood: they are reaction limited /01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S )00446-X

2 C. Marliere et al. / Journal of Non-Crystalline Solids ) 148± cluster aggregation [6] and di usion limited cluster aggregation [7]. Until now, little information about the spatial arrangement of aerogels, at larger length scale than n, has been available. However some experimental data indicate the need for a far better knowledge at larger scales. For instance the mass density, q, of aerogels is much lower than that deduced from a simple model assuming a `compact' packing of fractal clusters where the density of the constitutive particles typical dimension ˆ a) is equal to that of bulk silica. This suggests the presence of structural features solid part and voids) with typical length scales larger than that reachable by previously used experimental techniques. The presence of such a very large length scale structure was never clearly revealed in studies involving SAXS [8,9] or atomic force microscopy AFM) [10]. In order to detect the presence of this sub-micrometric structure, two complementary techniques, one in reciprocal space ultra-small angle X-ray scattering USAXS)) and the other one in real space AFM), have been used to characterize several sets of silica aerogels at di erent stages of densi cation. 2. Experimental Silica gels were prepared from tetraethoxysilane TEOS) and hydrolyzed with pure water under neutral conditions ph ˆ 7) according to a wellknown method [11]. The solvent used was ethanol. Samples were aged during eight days at room temperature before being supercritically dried 305 C ± 13 MPa). This step was followed by an oxidation treatment at 350 C in air for 15 h in order to remove organic residues. The density of these as-made samples was equal to 0:20 0:02 g cm 3. These `native' aerogels were then cut in several parts. These pieces were sintered at a temperature of 1000 C for variable times. The nal density was then calculated from the sample's weight and dimensions. For X-ray experiments, 1 mm thick aerogels slices were cut using a diamond saw. Samples for AFM measurements were prepared by brittle fracturation of the aerogel blocks. Using this approach, the clean surfaces needed for AFM experiments were obtained. USAXS experiments were done on line ID2 at the European Synchrotron Research Facility ESRF), Grenoble, France, by using a Bonse±Hart optics [12]. All the measured intensities have been normalized to a constant value of incident X-ray ux. For each sample, the background spectrum, i.e., the di usion curve as obtained without the studied sample, was systematically measured. Then this spurious signal was subtracted from the original one and the resulting signal, DI q, was plotted versus the scattering vector q on log±log scales. Experiments were done at a wavelength of 0.1 nm. The lateral section of the X-ray beam was approximately a few mm 2. The smallest scattering wave vector for which relevant data were measured is about nm 1. AFM experiments were done on a commercial apparatus [13] in a high amplitude resonant mode `tapping' mode). As already mentioned [10] this mode is very suitable to surface investigation of brittle samples at the nanometer scale. Experiments were performed in ambient conditions at a relative humidity of approximately 50%. Silicon cantilevers with a pyramidal shape typical cone half-angle 20 ; tip curvature radius >10 nm) were used. The typical scan rate was around 1 Hz. The vibrating frequency of the cantilever was typically in the range of 350 khz. The working vibration amplitude was chosen to be around 75% of the free oscillation amplitude. Squared images were acquired by using a digitalization rate of at least 256 points per line mostly 512). The samples were xed on the substrate by means of a thermoplastic glue. Special attention was paid to drastically reduce contamination of the AFM tip by small aerogel debris in order to avoid any spurious e ects. 3. Results The log DI vs. log q plot of USAXS scattering Fig. 1) reveals several similar features for all the studied aerogels. For gels with the lowest values of density Fig. 1 full symbols), the linear regime due to the well-known fractal structure of aerogels [11] is observed at high values of q

3 150 C. Marliere et al. / Journal of Non-Crystalline Solids ) 148± AFM measurements were performed on the same set of sintered samples with increasing densities. The aim of that study was to probe real space in order to con rm the presence of that large-scale structure. The fracture surface of asmade aerogels density of 0:20 g cm 3 ) was rst studied. One striking feature Fig. 3: facing white Fig. 1. USAXS normalized intensities as a function of wave vector for sintered silica aerogels with increasing values of density. The background spectrum has been systematically subtracted see text for more details). The curves are arbitrarily shifted along y-axis to improve clarity. Errors are smaller than the size of the symbols ±1nm 1 ). This regime is delimited by two transition q values, q a and q n, respectively related to the inverses of the mean size of the primary particle, a, and of the mean fractal cluster size, n. The slope of the scattering curves in this linear regime corresponds to D f ), where D f is the fractal dimension. It is veri ed that D f 2:2 for the lowest values of density Fig. 1 full symbols), as is expected when sintering processes are only partially complete. Typical of neutral aerogels [11], q a and q n values are separated by less than one order of magnitude. Thus the so-called fractal domain, evidenced by the linear regime on the log DI vs. log q plots, is rather narrow. As q a decreases and q n increases with increasing values of density, that fractal domain vanishes for the highest values of density Fig. 1 open symbols). At lower values of q below approximately 10 1 nm 1 ) log DI is almost constant and a plateau is observed on the curve. For q values smaller than 10 2 nm 1, an important and linear ± on the log±log scale ± increase of intensity occurs. The transition between the plateau regime and this linear regime occurs at q ˆ q X. Furthermore, these experiments reveal another important feature: q X increases with increasing values of aerogel density Fig. 2 empty circles) meaning that the related length scale, X, in real space, is decreasing. Fig. 2. Variations of the typical length scale, X, of aggregates of clusters versus the sintered silica aerogel density as measured by USAXS empty circles) and by AFM full squares) measurements. The error bars apply to all the data points. The length of the error bar for the USAXS curve comes from the accuracy in determination of the linear approximation of the I q curves Fig. 1). The error bar for AFM measurements was estimated from a statistical study of the size of the superstructure. Fig. 3. AFM data of the fracture surface of as-made aerogel with a density of 0.20 g cm 3, lateral dimensions: 400 nm 2 ; the gray scale corresponds to a vertical amplitude of 100 nm. The facing black and white arrows indicate the typical size of the aggregates of clusters, the facing white ones that of the clusters, as deduced from the AFM measurements. 3

4 C. Marliere et al. / Journal of Non-Crystalline Solids ) 148± and black arrows) is the presence of large-scale features the typical length of which is around 100 nm. The value of this characteristic length is very near that of X, the value deduced from the USAXS experiments. This observation proves that the large increase in log DI for decreasing values of q q < 10 2 nm 1 is due to large-scale solid diffusion centers separated by voids. Furthermore, the AFM measurements reveal that this solid large-scale structure is built of smaller size objects, the typical length of which is in the range of 10 nm Fig. 3: facing white arrows). Thus it can be assumed that: a) this large-scale structure results from the aggregation of fractal clusters, the presence of which was also revealed by X-ray experiments at high q values 10 1 ±1nm 1 ); b) The quantity, X, introduced in USAXS experiments is the typical length of these aggregates of clusters. Similar AFM experiments were done on samples with increasing densities. Figs. 4 a)± d) show that the size, X, of these aggregates of clusters decreases with densi cation. As shown in Fig. 2 full squares) the variation of X versus q as detected by AFM is identical to that deduced from USAXS experiments. 4. Discussion Thus these results, obtained by two di erent experimental methods one in real space the other one in Fourier space), prove: (a) (b) (c) (d) Fig. 4. AFM data of the fracture surface of sintered aerogels with increasing values of the density q. Lateral dimensions: 1 lm 2. The arrows indicate the typical size of the aggregates as deduced from the AFM measurements. The gray scales correspond to the following vertical amplitudes VA): a) q ˆ 0:33 g cm 3,VAˆ 200 nm; b) q ˆ 0:49 g cm 3,VAˆ 150 nm; c) q ˆ 0:65 g cm 3,VAˆ 200 nm; d) q ˆ 1:13 g cm 3,VAˆ 80 nm.

5 152 C. Marliere et al. / Journal of Non-Crystalline Solids ) 148± the existence of a very large-scale super-structure made of aggregates of clusters of silica particles; 2. that the increase of USAXS intensity for very low q values is likely caused by scattering from this super-structure. The present experiments can explain the discrepancy between the experimental values of the macroscopic density ± as measured by weight and volume determination ± and those deduced from a simple model where fractal clusters of silica particles would be contiguous. In that latter case, the aerogel density would be given by that of the fractal clusters and calculated [14,15] according to the formula: q n ˆq a n=a D f 3 ; where q a is the density at the particle scale i.e., the vitreous silica density 2:2 gcm 3 ). For instance, in the case of the as-made aerogels for which the ratio n=a is almost equal to 5 Fig. 1, full circles), q n is close to 0.6 g cm 3, a value well higher than the macroscopic density 0.2 g cm 3 ). The di erence results from the existence of a porous volume located between the aggregates of clusters typical length higher than n) revealed by the present experiments. It is well known that the viscous ow process drives the silica aerogels sintering and causes the evolution of the whole structure at di erent length scales. As already reported [16], a minute study of the portion of the log DI vs. log q plot corresponding to the highest values of q shows that D f slightly increases when the density of sintered aerogels goes up. This reveals the structural shrinking of the clusters at short length scales. In addition, the experiments described in this paper reveal Fig. 2) that sintering causes a decrease in size of the solid structures i.e., the aggregates of clusters) at a much higher length scale than ever observed. Finally, in the last stages of densi cation, AFM measurements Fig. 5), performed on samples with densities higher than 1.2 g cm 3, show that the aggregates of clusters become very close to each other and tend to coalesce because of a likely increase of connectivity. Fig. 5. AFM data of the fracture surface of sintered aerogel with a density of 1:30 g cm 3. Lateral dimensions: 220 nm 2. The gray scale corresponds to a vertical amplitude of 50 nm. The arrows indicate the zone of coalescence between aggregates due to the viscous ow. 5. Conclusion The combined use of two complementary methods USAXS and AFM) on aerogel samples at di erent stages of densi cation reveal a superstructure at a large scale 100 nm for q ˆ 0:2 gcm 3 ) made of aggregates of clusters the last ones being fractal at the rst stages of sintering). Using these techniques, the evolution of this superstructure versus the sintering process was investigated. During this process the silica particles increase in size, provoking the contraction of the clusters. The present experiments showed that this last e ect induces a similar collapse of the aggregates of clusters. Acknowledgements We thank O. Diat, P. Bosecke and T. Narayanan, contact locals at the ESRF, Grenoble, France, D. Abriou and H. Arribart, UMR CNRS/Saint-Gobain, Aubervilliers, France, V. Vie and C. Le Grimellec, CNRS UMR 5048 ± INSERM U414, Montpellier, France for very e cient technical help and/or fruitful discussions. 3

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