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The densification of mass-fractal aerogels to fused silica: A Raman study of vibrational evolution
Physica A 157 (1989) 625-629 North-Holland, Amsterdam
THE DENSIFICATION OF MASS-FRACTAL AEROGELS TO FUSED SILICA: A RAMAN STUDY OF VIBRATIONAL EVOLUTION J. PELOUS, Laboratoire Languedoc, “Groupe de Languedoc,
J.L. SAUVAJOL”, T. WOIGNIER
and R. VACHER
de Science des Matkriaux Vitreux, Universitk des Sciences et Techniques F-34060 Montpellier Cedex, France Dynamique des Phases Condenskes, Universiti des Sciences et Techniques F-34060 Montpellier Cedex, France
du du
The low frequency Raman spectrum has been investigated for porous silica aerogels progressively densified, starting from a mass-fractal sample to obtain finally fused silica. The spectra show a broad peak due to modes characteristic of the constituent particles of mean dimension a. In the first stage of densification the peak shifts to lower frequencies corresponding to an increase of a. The disappearance of the peak below the instrumental resolution is accompanied with the appearance of the usual fused silica spectral features.
The vibrational properties of disordered media are a matter of high current interest. The low frequency modes, related to collective motions of the disordered atoms, appear very sensitive to the structure and the texture of the materials. Raman scattering is an efficient tool for investigation of these modes. For example, dense solids, as vitreous silica, exhibit in the very low frequency part of the spectrum highly anharmonic excitations called usually “light scattering excess” [l]; on the other hand, the Raman spectrum of porous media, as “vycor”, shows a peak related to the mean-average dimension of the pore [2]. Recently, silica aerogels have attracted much interest as they are porous and also fractal materials. The fractal character of silica aerogels is now well established [3]: Small angle neutron (SANS) and X-ray (SAXS) scattering experiments have demonstrated a fractal structure at length scales larger than a particle size a and smaller than a correlation length 5. So, these materials are good models for the study of the vibrational dynamics of fractal media. Evidence for fractons has been given by Brillouin spectroscopy [4] and an analysis of the spectra reveals the details of the phonon-fracton crossover [5]. Furthermore, localized vibrations over the full fracton frequency range were investigated by very low frequency Raman spectroscopy [6]. We present here an additional investigation of the low frequency spectrum - in the range 0378-4371/89/$03.50 @ Elsevier Science Publishers B .V. (North-Holland Physics Publishing Division)
J. Pelous ei ul. I Densificnlion of‘ uerogels: u Rarnurt study
626
- for aerogels lO-lOOcm_ sample to obtain finally frequency SANS The “Coderg
progressively dense
part of the spectrum
experiments Raman TSOO”
triple
silica.
is correlated
on similar
spectra
densified,
vitreous
starting The
from a mass-fractal
evolution
with the variation
of the
of a, observed
low
in
samples.
I(w)
were
recorded
monochromator.
at
The
room
temperature
instrumental
using
resolution
a was
1 cm- ‘. The incident light was emitted from an argon laser [5145 or 4880 A] with a mean power of 400 mW. The incident beam polarization was normal to the scattering plane and perpendicular to the scattered light polarization (VH configuration). The aerogels were prepared by hydrolysis and polycondensation of tetramethoxysilane (TMOS) followed by hypercritical drying [7]. TMOS was diluted with methanol and four moles of distilled water or amoniacal solution 0.05N were used per mole of TMOS. Two series of samples have been investigated: one base-catalyzed and another one prepared on neutral conditions (labelled B or N respectively in fig. 3). The samples were heat treated at 1050°C for various durations (5 to 4.5 min). Measurements of the bulk density give a quantitative information on the gel to glass transformation. The Raman spectrum from peak at very low frequency
base-catalyzed
is usually
aerogel
observed
/
Fig. 1. Typical
low frequency
Raman
in fig. 1: the
It has been assigned
I
50 Frequency
is shown
in aerogels.
I
100 ( cm-‘)
-
spectrum
for a base-catalyzed
aerogel
J. Pelous et al.
I Densification of aerogels: a Raman siudy
627
to surface vibrational modes of the constituent particles [8] which are connected to form the gel. The results are plotted as reduced intensity Z(o)l(n(w) + 1) where n(o) + 1 is the appropriate Bose-Einstein factor for Stokes scattering. Fig. 2 shows the spectra for five base-catalyzed partially densified samples. For the lowest densities the main feature of the spectra is a peak similar to that observed for the untreated sample. Following the previous interpretation [8] this peak can be assigned to the lowest energy torsional mode of silica particles, whose radius R is related to the peak frequency wMby o,/2n = O.gVJ2R; V, is the transverse sound velocity in particles. Assuming V, corresponds to the velocity for fused silica, we obtain for the initial sample a value R equal to 25 A. Further stages of densification induce a peak shift towards lower frequencies indicating an increase of the size of the particles. The behaviour of aerogels prepared under neutral conditions is qualitatively
p _ 240Kg.m-3
p = 420Kg.ni3
p _ 850Kg.mm3
p = 1600Kg.m-3
Frequency
Fig. 2. Low frequency Raman densities change corresponding
( cm-‘)
1
-
scattering from partially densified to different heat treatment.)
base-catalyzed
aerogels.
(The
J. Pelous et al. I Densification of aerogels:
628
a Raman study
N (p=86OKg.m
1
I
I
10
5
Frequency
-3
1
-i
20 (Cm-‘)
+
Fig. 3. log-log plot of the reduced intensities versus the frequency w of low frequency Raman scattering from a densified base-catalyzed and neutral-catalyzed aerogels. The duration of heat treatment at 1050°C has been adapted to obtain nearly the same density for the two samples.
similar.
This
evolution
agrees
with the variation
of a, deduced
from
SANS
measurements on the same set of samples [9]. In the last stage of densification the peak disappears below the instrumental resolution. When the density is higher than 2000 kg mm3 the usual fused silica spectral features appear. However, in the case of neutral samples, the peak occurs at higher frequencies and is broader. This is illustrated in fig. 3 where a comparison of the spectra due to neutral and base-catalyzed samples with nearly the same density is given. The broadening of the peak could be related to the distribution of the dimension of the clusters which builds the fractal network. This is also in agreement with SANS results and confirms the known result that the addition of a basic catalysor favors the formation of larger particles in the gel [3,9]. Moreover, condition
the differences are still observed
on the structure associated to the initial during the first stage of the sintering.
catalysis
References [l] J. Jackie, in: Amorphous Solids, W.A. Phillips, ed. (Springer, Berlin, 1981). p. 135. [2] A. Boukenter, Thesis University of Lyon, France (1988). [3] D.W. Schaefer and K.D. Keefer, Phys. Rev. Lett. 56 (1986) 2199; R. Vacher, T. Woignier, Pelous and E. Courtens, Phys. Rev. B 37 (1988) 6500.
J.
J. Pelous et al. I Demification
[4] E. Courtens, J. Pelous, J. Phalippou,
[5] [6] [7] [8] [9]
of aerogels: a Raman study
629
R. Vacher and T. Woignier, Phys. Rev. Lett. 58 (1987) 128. E. Courtens, R. Vacher, J. Pelous and T. Woignier, Europhysics Lett. 6 (1988) 245. Y. Tsujimi, E. Courtens, J. Pelous and R. Vacher, Phys. Rev. Lett. 60 (1988) 2757. T. Woignier, J. Phaiippou and J. Zarzycki, J. Non-Cryst. Solids 63 (1984) 117. A. Boukenter, B. Champagnon, E. Duval, J. Dumas, J.F. Quinson and J. Serughetti, Phys. Rev. Lett. 57 (1986) 2391. R. Vacher, T. Woignier, J. Phalippou, J. Pelous and E. Courtens, in: Fourth Int. Conf. on The Structure of Non-Crystalline Materials, C.N.J. Wagner and A.C. Wright, eds. (1988). J. Non-Cryst. Solids 106 (1988) 161.
THE DENSIFICATION OF MASS-FRACTAL AEROGELS TO FUSED SILICA: A RAMAN STUDY OF VIBRATIONAL EVOLUTION J. PELOUS, Laboratoire Languedoc, “Groupe de Languedoc,
J.L. SAUVAJOL”, T. WOIGNIER
and R. VACHER
de Science des Matkriaux Vitreux, Universitk des Sciences et Techniques F-34060 Montpellier Cedex, France Dynamique des Phases Condenskes, Universiti des Sciences et Techniques F-34060 Montpellier Cedex, France
du du
The low frequency Raman spectrum has been investigated for porous silica aerogels progressively densified, starting from a mass-fractal sample to obtain finally fused silica. The spectra show a broad peak due to modes characteristic of the constituent particles of mean dimension a. In the first stage of densification the peak shifts to lower frequencies corresponding to an increase of a. The disappearance of the peak below the instrumental resolution is accompanied with the appearance of the usual fused silica spectral features.
The vibrational properties of disordered media are a matter of high current interest. The low frequency modes, related to collective motions of the disordered atoms, appear very sensitive to the structure and the texture of the materials. Raman scattering is an efficient tool for investigation of these modes. For example, dense solids, as vitreous silica, exhibit in the very low frequency part of the spectrum highly anharmonic excitations called usually “light scattering excess” [l]; on the other hand, the Raman spectrum of porous media, as “vycor”, shows a peak related to the mean-average dimension of the pore [2]. Recently, silica aerogels have attracted much interest as they are porous and also fractal materials. The fractal character of silica aerogels is now well established [3]: Small angle neutron (SANS) and X-ray (SAXS) scattering experiments have demonstrated a fractal structure at length scales larger than a particle size a and smaller than a correlation length 5. So, these materials are good models for the study of the vibrational dynamics of fractal media. Evidence for fractons has been given by Brillouin spectroscopy [4] and an analysis of the spectra reveals the details of the phonon-fracton crossover [5]. Furthermore, localized vibrations over the full fracton frequency range were investigated by very low frequency Raman spectroscopy [6]. We present here an additional investigation of the low frequency spectrum - in the range 0378-4371/89/$03.50 @ Elsevier Science Publishers B .V. (North-Holland Physics Publishing Division)
J. Pelous ei ul. I Densificnlion of‘ uerogels: u Rarnurt study
626
- for aerogels lO-lOOcm_ sample to obtain finally frequency SANS The “Coderg
progressively dense
part of the spectrum
experiments Raman TSOO”
triple
silica.
is correlated
on similar
spectra
densified,
vitreous
starting The
from a mass-fractal
evolution
with the variation
of the
of a, observed
low
in
samples.
I(w)
were
recorded
monochromator.
at
The
room
temperature
instrumental
using
resolution
a was
1 cm- ‘. The incident light was emitted from an argon laser [5145 or 4880 A] with a mean power of 400 mW. The incident beam polarization was normal to the scattering plane and perpendicular to the scattered light polarization (VH configuration). The aerogels were prepared by hydrolysis and polycondensation of tetramethoxysilane (TMOS) followed by hypercritical drying [7]. TMOS was diluted with methanol and four moles of distilled water or amoniacal solution 0.05N were used per mole of TMOS. Two series of samples have been investigated: one base-catalyzed and another one prepared on neutral conditions (labelled B or N respectively in fig. 3). The samples were heat treated at 1050°C for various durations (5 to 4.5 min). Measurements of the bulk density give a quantitative information on the gel to glass transformation. The Raman spectrum from peak at very low frequency
base-catalyzed
is usually
aerogel
observed
/
Fig. 1. Typical
low frequency
Raman
in fig. 1: the
It has been assigned
I
50 Frequency
is shown
in aerogels.
I
100 ( cm-‘)
-
spectrum
for a base-catalyzed
aerogel
J. Pelous et al.
I Densification of aerogels: a Raman siudy
627
to surface vibrational modes of the constituent particles [8] which are connected to form the gel. The results are plotted as reduced intensity Z(o)l(n(w) + 1) where n(o) + 1 is the appropriate Bose-Einstein factor for Stokes scattering. Fig. 2 shows the spectra for five base-catalyzed partially densified samples. For the lowest densities the main feature of the spectra is a peak similar to that observed for the untreated sample. Following the previous interpretation [8] this peak can be assigned to the lowest energy torsional mode of silica particles, whose radius R is related to the peak frequency wMby o,/2n = O.gVJ2R; V, is the transverse sound velocity in particles. Assuming V, corresponds to the velocity for fused silica, we obtain for the initial sample a value R equal to 25 A. Further stages of densification induce a peak shift towards lower frequencies indicating an increase of the size of the particles. The behaviour of aerogels prepared under neutral conditions is qualitatively
p _ 240Kg.m-3
p = 420Kg.ni3
p _ 850Kg.mm3
p = 1600Kg.m-3
Frequency
Fig. 2. Low frequency Raman densities change corresponding
( cm-‘)
1
-
scattering from partially densified to different heat treatment.)
base-catalyzed
aerogels.
(The
J. Pelous et al. I Densification of aerogels:
628
a Raman study
N (p=86OKg.m
1
I
I
10
5
Frequency
-3
1
-i
20 (Cm-‘)
+
Fig. 3. log-log plot of the reduced intensities versus the frequency w of low frequency Raman scattering from a densified base-catalyzed and neutral-catalyzed aerogels. The duration of heat treatment at 1050°C has been adapted to obtain nearly the same density for the two samples.
similar.
This
evolution
agrees
with the variation
of a, deduced
from
SANS
measurements on the same set of samples [9]. In the last stage of densification the peak disappears below the instrumental resolution. When the density is higher than 2000 kg mm3 the usual fused silica spectral features appear. However, in the case of neutral samples, the peak occurs at higher frequencies and is broader. This is illustrated in fig. 3 where a comparison of the spectra due to neutral and base-catalyzed samples with nearly the same density is given. The broadening of the peak could be related to the distribution of the dimension of the clusters which builds the fractal network. This is also in agreement with SANS results and confirms the known result that the addition of a basic catalysor favors the formation of larger particles in the gel [3,9]. Moreover, condition
the differences are still observed
on the structure associated to the initial during the first stage of the sintering.
catalysis
References [l] J. Jackie, in: Amorphous Solids, W.A. Phillips, ed. (Springer, Berlin, 1981). p. 135. [2] A. Boukenter, Thesis University of Lyon, France (1988). [3] D.W. Schaefer and K.D. Keefer, Phys. Rev. Lett. 56 (1986) 2199; R. Vacher, T. Woignier, Pelous and E. Courtens, Phys. Rev. B 37 (1988) 6500.
J.
J. Pelous et al. I Demification
[4] E. Courtens, J. Pelous, J. Phalippou,
[5] [6] [7] [8] [9]
of aerogels: a Raman study
629
R. Vacher and T. Woignier, Phys. Rev. Lett. 58 (1987) 128. E. Courtens, R. Vacher, J. Pelous and T. Woignier, Europhysics Lett. 6 (1988) 245. Y. Tsujimi, E. Courtens, J. Pelous and R. Vacher, Phys. Rev. Lett. 60 (1988) 2757. T. Woignier, J. Phaiippou and J. Zarzycki, J. Non-Cryst. Solids 63 (1984) 117. A. Boukenter, B. Champagnon, E. Duval, J. Dumas, J.F. Quinson and J. Serughetti, Phys. Rev. Lett. 57 (1986) 2391. R. Vacher, T. Woignier, J. Phalippou, J. Pelous and E. Courtens, in: Fourth Int. Conf. on The Structure of Non-Crystalline Materials, C.N.J. Wagner and A.C. Wright, eds. (1988). J. Non-Cryst. Solids 106 (1988) 161.