Optical Materials 26 (2004) 167–172 www.elsevier.com/locate/optmat
Comparison between sintered and compressed aerogels Jean Phalippou *, Florence Despetis, Sylvie Calas, Annelise Faivre, Philippe Dieudonne, Robert Sempere, Thierry Woignier Laboratoire des Verres, UMR 5587, Universite de Montpellier, 2 Place E. Bataillon, 34095 Montpellier Cedex 5, France
Abstract Aerogels can be densified either by thermal sintering, or at room temperature by isostatic compression. We report here a comparative analysis of silica aerogel densified by these two methods. To better follow their structural evolutions we use SAXS measurements performed on aerogels exhibiting a fractal geometry. This fractal geometry specially gives information about the way the solid network is firstly established and how it evolves with densification. The structural features such as particle and cluster sizes are observed to change differently according to the densification method. While the specific surface area of sintered aerogels decreases with densification, it does not change when densification is performed under isostatic compression. Furthermore the pore size distribution analysis evidences that the pressure induces the collapse of the largest pores while sintering acts on all pores. A microscopic model is proposed. It allows to explain the structural changes observed both by isostatic compression and by thermal sintering. Moreover, it agrees well with the evolution of elastic constant and internal friction with densification. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Sintering; Isostatic compression; Aerogel
1. Introduction Silica gels are obtained by a hydrolysis–polycondensation reaction of tetraethoxysilane diluted in alcohol. A solid network appears (with time) and leads to a twophase material consisting in a porous solid and a liquid located within pores. A silica aerogel is obtained by a supercritical drying treatment [1] which allows to evacuate the pore liquid. Under those conditions capillary forces no more act on the solid network which consequently does not crack. Thus the material is now a solid network whose porosity is filled with atmospheric air. These materials are very light. They can be regarded as models both to verify theories describing aggregation processes [2] or to follow the physical properties of materials whose porosity can be varied between 99% and 0%. A simple firing allows to convert silica aerogel to full dense silica glass [3]. The low permeability of gels is the main reason of cracking during conventional drying methods [4]. *
Corresponding author. Tel.: +33-4-6752-5903; fax: +33-4-67544801/6714-3414. E-mail address:
[email protected] (J. Phalippou). 0925-3467/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2003.11.017
Obviously this low permeability is also a characteristic property of aerogels. It is mainly related to the very small size of the connected pores. According to the small pore sizes, mercury which is an oxide non-wetting liquid cannot enter the pores even under high-pressure levels. Nevertheless, the compression stress leads to an aerogel shrinkage. After pressure removal the aerogel exhibits a densification related to a partially irreversible shrinkage [5]. Porous partially densified aerogels can be used as a host matrix in which guest functional molecules can be incorporated. The potential applications concern photonic material, sensors, luminescent materials, etc. This paper deals with the comparison of the structural and textural evolution of silica aerogels densified by a thermal sintering or a compressive isostatic pressure.
2. Experimental The silica gels were obtained by hydrolysis conducted with slightly basic (0.01 N NH4 OH) aqueous solutions. All aerogel samples were submitted to an oxidation treatment to remove the organic residues located at the
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3. Results
10
6
0.07g/cm3 0.09g/cm3 0.16g/cm3 0.2g/cm3
S
I (q) (u.a.)
solid surface. These organic residues result of an esterification reaction occurring inside the autoclave. The thermal treatment was performed at 300 °C during 15 h. The sintering densification was performed at 1000 °C for different times. The compression densification was obtained, at room temperature, thanks to a mercury porosimeter. A first series of samples concerns light aerogels with a density of 0.07 g/cm3 . They exhibit a fractal geometry. The second series concerns aerogels of higher densities (0.19 g/cm3 ).The density is easily obtained by weighing samples of well defined dimensions. SAXS experiments are performed on the series of light aerogel. Details of the SAXS experiments have been previously reported [7]. The beam collimation is semi-linear. Deconvolution is done according to the Str€ obl procedure for an isotropic medium [8]. In this paper SAXS experiments are mainly used to investigate the structural evolution associated with the density increase resulting from the two different methods. Nitrogen adsorption–desorption experiments performed at 95 K provide an estimate of textural properties. The specific surface area is calculated using BET theory [9] and the pore distribution is obtained from BJH theory [10] applied to desorption curve. Low-density silica aerogels are not strong enough and their dimension evolves during nitrogen adsorption measurement. Consequently The whole porous volume measured at a relative pressure p=p0 , equal to 0.99, is smaller than that calculated from the bulk and skeletal densities [6]. As soon as the density of this second series of aerogels is higher than 0.3 g/cm3 the nitrogen adsorption measurements become meaningful. Moreover measurements of the mechanical properties are more easily performed with stiff samples. The Young’s modulus, E, of thermally sintered aerogels is obtained by a static 3 point flexural technique. For compressed aerogels a 3 point flexural vibrating setup is used. In addition, this method allows to measure the internal friction. The internal friction (or damping), tg d, is a value corresponding to the ratio (loss modulus)/(storage modulus) of the complex Young’s modulus. For these samples the internal friction does not depend on frequency in the range 1–100 Hz.
10
4
1 10
2
Fractal domain
0 0.001
0.01
1 b
˚ -1) q (A
0.1
1
Fig. 1. Small angle X-ray scattering intensity as a function of wave vector, Q, for partially sintered fractal aerogels labelled using their density in g/cm3 . The curves are shifted on IðQÞ axis for clarity. The slope )2.2 is drawn to guide the eyes.
log IðQÞ ¼ f ðlog QÞ;
ð1Þ
where Q is the wave vector. In the fractal range [11] the slope is equal to D. At the onset of densification the fractal dimension remains unchanged or increases slowly. On the other hand the fractal domain (between 1=b and 1=‘ in Fig. 1) narrows. The size of elementary particles, b, increases with densification while the correlation length, ‘, above which the aerogel is no more fractal, decreases. Obviously with densification these two dimensions which define the fractal range become closer and the fractal domain is difficult to observe. In the case of compression–densification carried out at room temperature the main change observed on SAXS curves concerns the correlation length (Fig. 2). The correlation length decreases while the particle size remains constant. Specific surface values obtained from BET measurements are compared in Fig. 3. For sintered samples, as soon as density increases, the specific surface area decreases. On the other hand, no changes in the 0.07g/cm3 0.09g/cm3 0.11g/cm3 0.23g/cm3
C
8
10 I (q) (u.a.)
168
6
10
4
SAXS measurements performed on four densified aerogels obtained by a sintering thermal treatment are shown in Fig. 1. The main features of the structure can easily be analysed in terms of fractal geometry and are shown to evolve with densification. The fractal dimension, D, is directly determined in the domain between the Guinier’s and Porod’s regimes from the slope of the curve:
10
0 0.001
1 1 b 0.1
0.01
1
˚ -1) q (A Fig. 2. Small angle X-ray scattering intensity as a function of wave vector, Q, for four partially compressed fractal aerogels (labelled using their density in g/cm3 ).
J. Phalippou et al. / Optical Materials 26 (2004) 167–172
dV/dΦ (cm3.g-1/nm)
450
Ssp (m2/g)
400 350
S
1.6
169
ρa ~ 0.37 g/cm3 S C
1.2 0.8 0.4
300
0 250
0.2
0.3
0.4
0.5
ρa (g/cm3)
(a)
15
20 25 Φ (nm)
30
Fig. 4. Pore size distribution dV =dU as a function of pore size, U, obtained from BJH theory using nitrogen desorption sintered (S) and compressed (C) aerogels having a density of 0.37 g/cm3 .
450 C
200
E (MPa)
350 300 250
(b)
0.2
0.3
0.4
0.5
Fig. 3. Specific surface area as a function of bulk density: (a) for partially sintered aerogels (S) and (b) for partially compressed aerogels (C).
specific surface area are observed in compressed–densified aerogels. Two aerogel samples have been densified up to almost the same density (0.37 g/cm3 ), one by sintering and the other by room temperature isostatic compression. It is interesting to remark that they display very different pore size distribution as measured by nitrogen adsorption (Fig. 4). The pore size distribution of the compressed sample is narrower and the mean pore size is smaller than that of sintered sample. The density increase obtained by sintering treatment leads to a material strengthening. The sintering improves the mechanical properties of the material as can be seen in Fig. 5, exhibiting Young’s modulus evolution as a function density. The Young’s modulus, E, varies as [12] E / q3:3 a :
100
ρa (g/cm3)
ð2Þ
For aerogels densified by isostatic pressure, at the onset of densification, the elastic constant evolution is not so straightforward. The Young’s modulus firstly decreases, then reaches a minimum before increasing with densification (Fig. 6). The evolution of internal friction, tg d has been analysed using a Dynamic Mechanical Analyser, in the compressed aerogel series. First, tg d increases with
0.2
0.3
Fig. 5. Evolution of the Young’s modulus for thermally sintered aerogels.
20
E (MPa)
Ssp (m2/g)
400
15
10
0.22
0.23
0.24
Fig. 6. Evolution of the Young’s modulus at the onset of compression densification.
density, then passes through a maximum for the density at which the Young’s modulus shows the lowest value (Fig. 7).
170
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particles. Correlatively, matter transport leads to a cluster volume shrinkage. Thus the correlation length which can be associated to the mean clusters size decreases with densification. The net effect is that the fractal range narrows while there is a small increase in the fractal dimension. Obviously the fractal dimension must reach 3 as the porous aerogel approaches the fully dense glass. For sample densified by isostatic pressure, the structural evolution is easier to follow. At the onset of densification only the correlation length decreases. The particle size and volume fractal dimension remain constant. The behaviour of structural units forming aerogel clearly depends on the way the densification is carried out. To describe the evolution of particles and clusters with densification, a schematic arrangement of particles (assumed to be fractal) is proposed in Fig. 8. During sintering the particles, here assumed to be spherical, increase in size. Because of matter flow from the finest solid arms of the cluster to the locations where the solid density is the highest, the cluster shrinks. Consequently pore size decreases. The net effect of the matter transfer and pores collapse of the pores is the sample shrinkage. On the other hand, regarding densification arising from an applied external pressure, there is no mass transfer. Thus the size of the particles cannot vary. According to the correlation length decrease, the densification likely occurs as a result of a better entanglement of clusters. Consequently the fractal range spans over a smaller scale range. The interpenetration of clusters requires a structural rearrangement. The cluster
0.08 1Hz
tgδ
0.06
0.04
0.02
0 0.215
0.225
0.235 ρa
0.245
0.255
0.265
(g/cm3)
Fig. 7. Evolution of the internal friction, tg d, of compressed aerogels as a function of density.
4. Discussion The gel structure is built up from silica monomers which polycondense to give rise to elementary silica particles. These particles stick together to form clusters which then aggregate to form the solid backbone. The volume fractal dimension, D, of base catalysed aerogels is equal to 2.2. Such a value indicates that the structure likely is the result of a cluster–cluster diffusion limited aggregation process (D ¼ 1:8). During supercritical drying a small shrinkage appears. The associated rearrangement of clusters induces an increase of the fractal dimension. That fractal dimension is indicated by the slope of the curve in Fig. 1. Thermal sintering of aerogels is due to viscous flow. With sintering the particle size increases since the surface energy acts to increase the curvature radius of
C
b
S
Fig. 8. Schematic model showing the solid transport induced by sintering (S) and the solid network motion induced by compression (C).
J. Phalippou et al. / Optical Materials 26 (2004) 167–172
Ssp ¼
A bqS
2.25 2.00 1.75 1.50 0.2
0.3 0.4 ρa (g/cm3)
0.5
Fig. 10. Log–log plot of the specific surface area as a function of the bulk density.
ð3Þ
where A is a shape factor and qS the skeletal density. The log–log plot of b as a function of qa (Fig. 9) gives: b / q0:3 a ;
ð4Þ
and consequently, Ssp / qa0:3 :
ð5Þ
Experimental plot of Ssp as a function of qa (Fig. 10) indicates a slope of )0.4. Regarding the fact that b is determined from SAXS measurements for light aerogel, and that Ssp measured for denser aerogels, this value )0.4 is not so different than the one expected ()0.3). Isostatically compressed aerogels do not display a surface change with densification. This behaviour is easily understood from the proposed model since compression mainly acts on arrangements and entanglements of clusters. The surface area does not evolve because the size and the number of elementary particles remain constant with densification. Under isostatic pressure only the elementary particles forming the links between clusters are moved.
400
Ssp (m2/g)
2.50
b (nm)
motion has been associated to buckling or cracking of the weakest arms between clusters. A direct consequence should be a drastic decrease of the size of pores located between clusters and which are likely the widest. The proposed model qualitatively accounts for the specific surface area decrease as sintering proceeds. The surface is mainly related to the area developed by elementary particles which in the case of base catalysed aerogels can be assumed to have a spherical shape. A rough estimate of the specific surface area can be determined at the onset of sintering. During the early stages, the neck diameter between particles is less significant than the particle size. So we can expect that:
171
350
300
250
0.2
0.4 0.3 ρa (g/cm3)
0.5
Fig. 9. Variation of the mean particle size, b, (see Fig. 1) versus the density for sintered aerogels.
Pore size distribution curves are also explained from the proposed structural model. Fig. 4 clearly indicates that for aerogels differently densified up to the same density the pore network is different. The pore size distribution of sintered aerogel is wide and the mean pore size is located at about 24 nm. The pore size distribution of compressed aerogel is narrow and the mean pore size is located at 16 nm. The pore size distribution curve of sintered samples indicates that the thermal treatment induces the closure of mesopores having the smallest size. For compressed samples the pressure likely induces the collapse of the mesopores having the largest dimensions. These last ones are transformed into pores having a smaller size. As a consequence there is an increase of the number of small mesopores. With sintering the smallest and the largest cut offs, b and ‘, vary in opposite directions. The smallest cut off, b, increases as the result of the smoothing of internal surface of cluster. There is local transfer of the matter from the surface locations with a large curvature radius to those with a small one. The net effect of this transfer is to strengthen the thinner arms. This strengthening is revealed by the enhancement of the Young’s modulus with the course of sintering (Fig. 5). It has been previously demonstrated that the width at half height of Brillouin diffusion curve decreases with sintering [13]. This physical characteristic is related to attenuation or internal friction. Its evolution with sintering indicates that the aerogel reinforces and that sintering does not induce a structure damage. On the opposite, at the onset of densification induced by compression, the Young’s modulus decreases. We can say that the load transfer is less efficient. Because arm size remains constant, the number of connections between clusters is likely reduced. A few arms establishing the bridges between clusters are assumed to be damaged as a result of the external pressure. This has been verified by internal friction measurements. This quantity is known to evidence damage. The internal friction indicates that at the beginning of compression a
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damage occurs. Obviously as densification proceeds, arms begin to contact each other and reactive species (Si–OH groups) covering the arm surface react together. Two Si–OH groups can polycondense to give rise to siloxane, Si–O–Si, bonds which form again bridges between clusters. The structure healing is well evidenced by both Young’s modulus increase and internal friction decrease.
smaller ones. Consequently the pore size distribution curve is shifted towards the smallest pores while the mean pore size is drastically reduced. The cluster rearrangement is proposed to arise from a cracking of thinnest arms connecting clusters. Such an assumption is well supported by the elastic modulus decrease and by the enhancement of the internal friction.
5. Conclusion
References
The analysis of light silica aerogels exhibiting fractal features allows to better understand their structural evolution with densification. Aerogels densification is obtained by a sintering thermal treatment or by a room temperature isostatic compression. The structure evolution with densification is well evidenced using small angle X-ray scattering experiments. According to fractal characteristic evolution, a structural model is proposed to account for these different densification processes. Textural properties evolution of usual aerogels partially densified are demonstrated to agree well with proposed model. Surface area of sintered aerogel decreases while it remains constant for compressed ones. For compressed aerogels at the onset of densification a simple rearrangement of clusters occurs. This rearrangement acts on the largest pores, located between clusters, which collapse. The pore size distribution indicates that these wide pores are transformed into
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