Materials Chemistry and Physics 60 (1999) 268±273
In¯uence of N,N-dimethylformamide additive on the physical
properties of citric acid catalyzed TEOS silica aerogels
A. Venkateswara Rao*, H.M. Sakhare, A.K. Tamhankar, M.L. Shinde, D.B. Gadave, P.B. Wagh
Air Glass Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India
Received 12 March 1999; received in revised form 3 April 1999; accepted 16 April 1999
Abstract
The use of N,N-dimethylformamide (DMF) as a drying control chemical additive (DCCA) was found to be effective in order to obtain the
best quality tetraethoxysilane (TEOS) silica aerogels in terms of monolithicity, transparency and low density. Silica sols were prepared by
keeping the molar ratio of TEOS : ethanol (EtOH) : water (0.001 M citric acid as a catalyst) constant at 1:5:7 respectively, while the DMF/
TEOS molar ratio (A) was varied from 0 to 1. The DCCA modi®ed silica alcogels were dried supercritically. It has been found that the bulk
density, linear shrinkage and surface area decreased with an increase in A values. On the other hand, the percent porosity has been found to
increase with increase in A values. Moreover, it has been observed that percent optical transmission increases up to Aˆ0.3 and then remains
almost constant up to Aˆ0.7 and for A > 0:7, per cent optical transmission decreases. It has been found that for A values between 0.3 and
0.7, the pore size distribution is narrow and uniform which reduces the differential pressure during supercritical solvent extraction process
and leading to monolithic, transparent and low density silica aerogels. The results have been supported by the scanning electron
microscopic observations of the aerogels. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Aerogels; DCCA; Monolithicity; Pore size distribution; Supercritical
1. Introduction
The most serious problems encountered in the preparation
of monolithic silica aerogels are the cracks and fracture
formation which occur during the supercritical drying process. This cracking is mainly due to temperature and pressure gradients which appear during the heating of alcogels
[1] in the autoclave. During the initial stages of heating the
alcogels, the capillary forces caused by the evaporation of
solvent from the micropores in the gel creates an overall
drying stress and local differential stresses due to non-uniform pore size distribution [2]. The capillary force depends
on the rate of evaporation which is a function of solvent
vapour pressure and is inversely proportional to pore size.
During the ®nal stage of drying, cracking is the result of nonuniform shrinkage of the drying body [2]. This can be due to
temperature gradients, compositional inhomogenieties and
different local rates of reaction. The pore sizes can be
controlled by adding an organic solvent, called a drying
control chemical additive (DCCA), to the sol.
For many applications, silica aerogels with speci®c
refractive index, surface area and porosity are needed [3].
*Corresponding author.
Various applications of silica aerogels are reported in one of
our recent publications [3] and mentioned here in the form of
an application tree (Fig. 1). Most of the published results on
silica aerogels deal with the in¯uence of various DCCAs on
the physical properties of the aerogels made by tetramethoxysilane (TMOS) and a base catalyst [4±7]. However, there is
less information in the literature regarding the preparation of
monolithic silica aerogels using N,N-dimethylformamide
(DMF) as a DCCA and TEOS as a precursor. We refer
the concept of monolithicity to the degree of obtaining large
single pieces of the aerogels with a minimum number of
cracks. That is, the lower the number of cracks, the greater
the monolithicity of the aerogels. TEOS precursor has
certain advantages over the TMOS precursor. The fumes
from the TMOS are dangerous and can cause blindness.
Therefore, we have used other esters of orthosilic acid and
found that TEOS gave transparent and low density aerogels
by making use of DMF as a DCCA. TEOS is not only less
toxic when compared to the TMOS but it is cheaper too!
Hence, TEOS is a suitable precursor for commercial production of silica aerogels. The effect of molar ratios of
precursor, solvent and water on surface area and porosity of
TEOS silica aerogels have been explained in our recent
publications [3,8]. However, the in¯uence of catalyst (citric
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 0 8 9 - 9
A.V. Rao et al. / Materials Chemistry and Physics 60 (1999) 268±273
Fig. 1. Application tree of the aerogels.
acid) concentration on the physical properties of TEOS
silica aerogels is explained in one of our other recent
publications [9]. Hence, in continuation to our earlier work
and in order to obtain still better quality aerogels, the present
paper deals with the in¯uence of DMF additive on the
physical properties like surface area, bulk density, percent
porosity, percent optical transmission and pore size distribution of citric acid catalyzed TEOS silica aerogels.
2. Experimental
Silica alcogels were prepared by hydrolysis and polycondensation of ethanol (EtOH) diluted TEOS in the presence of citric acid (C6H8O7H2O) as a new catalyst together
with dimethylformamide (DMF) as DCCA. The chemicals
used in the present work were: TEOS and DMF (Fluka,
Purissimum grade, Switzerland), EtOH and C6H8O7H2O
(Prolabo, Analytical reagent grade, France). The TEOS had
been previously diluted in ethanol and to this mixture, water
and DMF were added dropwise while stirring at room
temperature. After stirring for 15 min, the resulting homogenous sol was transferred to Pyrex glass test tubes of
18 mm outer diameter and 180 mm height. The test tubes
were then made airtight using wooden corks in order to
inhibit the evaporation of EtOH from the sols. Gelation
occurred at a constant temperature of 258C. After gelation,
the alcogels were covered with EtOH to prevent shrinkage
269
and cracks of the gels. All the gels were aged at 258C for
24 h. The aged alcogels were then supercritically dried in an
autoclave to obtain silica aerogels. The details of autoclave
drying, pre-pressures of N2 gas, heating and evacuation
rates, etc. were given in our recent publications [10±12].
In order to obtain best quality silica aerogels in terms of
monolithicity, transparency and low density, ®ve sets of
experiments were performed in which DMF/TEOS molar
ratios (A) were systematically varied from 0 to 1 for constant
molar ratios of TEOS : EtOH : H2Oˆ1:5:7, respectively.
The bulk densities of all the aerogels were measured from
their weights whose dimensions were known. The weights
of aerogels were measured using Dhona microbalance
(Model Dhona 100 DS) having a least count of 0.01 mg
while the dimensions of the samples, such as the diameter
and length were measured accurately using a travelling
microscope. The optical transmission of the aerogels (sample thickness of 1 cm) were measured at a wavelength of
800 nm, using a Perkin-Elmer Spectrophotometer (Model783). With a view to understand the reasons for the opacity,
transparency, cracking and monolithicity of the aerogels,
prepared using various A values, the microstructural observations were made on the aerogel samples by scanning
electron microscope (SEM) (Model: 250 MK3, Cambridge).
Aerogel samples were cut into 332 mm3 at atmospheric
pressure in a dustproof clean chamber. The samples were
then coated with gold, containing 20% palladium, at a
pressure of 1.333 Pa, in order to prevent electric charge
during the SEM observations. In the present work, ®ve
samples prepared under identical conditions have been
examined and the results have been found to be very
similar.
3. Results and discussion
3.1. Surface area
It has been postulated that the dry gels are aggregates of
silica particles, according to the model proposed by Zarzycki et al. [13]. This model related the surface area as a
function of the particle size and the extent of the coalescence. Assuming the primary particle is a hard sphere
having a uniform size, the surface area is related to the
inverse of the particle radius. It has been found that an
increase in DMF/TEOS molar ratio (A) leads to a decrease
in surface area (Fig. 2). Silica aerogels prepared using lower
A values (A < 0:3) or without making use of DCCA (i.e.
Aˆ0) show larger surface area 875 m2/g. The SEM picture
(Fig. 8(a)) of the TEOS silica aerogel prepared without the
use of DMF (Aˆ0) show smaller size of SiO2 particles
which are compactly arranged. At medium A values (i.e.
0:3 < A < 0:7), the SEM picture (Fig. 8(b)) shows uniform
pores, while SiO2 particles are larger compared to the
aerogels prepared without making use of DMF (i.e.
Aˆ0). For further increase in A values (A > 0:7), it is clearly
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A.V. Rao et al. / Materials Chemistry and Physics 60 (1999) 268±273
Table 1
Porosity and coordination number
DMF/TEOS
molar ratios,
A
Pore
volume
(cm3/g)
Volume
fraction of
solids, Coordination
number, n
0 (without DMF)
0.2
0.4
0.6
0.8
1.0
3.249
4.376
5.497
6.348
6.898
7.237
0.1228
0.0941
0.0763
0.0668
0.0618
0.0591
2.685
2.506
2.402
2.347
2.319
2.304
coordination number and 1ÿ corresponds to the porosity.
The average coordination numbers estimated by pore
volume are presented in Table 1. The pore volume, Vp,
was calculated using
Vp ˆ 1=b ÿ 1=s ;
Fig. 2. Surface area as a function of DMF/TEOS molar ratio.
observed from the SEM picture (Fig. 8(c)) that the SiO2
particles are still larger in size compared to the SiO2
particles sizes observed in Fig. 8(a) and (b). The growth
of the primary particle should be controlled by the polymerisation reaction. Increase in A values accelerate the
polymerisation reaction, the primary particles become
greater, so that the speci®c surface area decreases [14].
The coalescence is brought by the condensation of the
surface Si±OH on silica particles during the aging of the
alcogels [14]. Thus, the rate of condensation and the number
of surface silonols of the particles are effective to the degree
of coalescence. Therefore, coalescence is also related to the
rate of hydrolysis. If the proportion of the unreacted alkoxy
groups on the particle surface is large, then the coalescence
will be prevented. DMF is effected to prevent the coalescence by considering that the speci®c surface area increased
with the addition of such additive. Thus, the two effects,
promotion of polymerization and inhibition of coalescence,
were in competition with each other.
where b is the bulk density of the aerogel samples and s the
skeletal density of silica aerogel which is found to be
2.20 g cmÿ1. Fig. 3 shows that an increase in DMF/TEOS
molar ratio (A) leads to a decrease in the bulk density (b) of
the silica aerogel. Table 1 indicates that if TEOS silica
aerogel is prepared without DMF as an additive (i.e.
Aˆ0) then SiO2 particles have coordination number
2.685, whereas the coordination number of the particle
decreases up to 2.304 for Aˆ1, which leads to a decrease
in the bulk density (b) (Fig. 3). Hence the DMF additive
plays an important role in aggregation of primary particles
and to decrease the number density of secondary particles.
DMF enhances the polymerization reaction only in the low
concentration [14]. The reaction mechanism of DMF can be
explained as shown:
3.2. Bulk density, percent porosity and volume shrinkage
It is a well known fact that the bulk density depends on the
particle packing or the degree of aggregation [15]. The
degree of aggregation is sensitive to the charge on the
particle surface. If the particle surface is covered with the
solvent molecules, the charge on the surface area varies and
then the degree of aggregation and the bulk density are
in¯uenced. The packing of particles can be expressed by an
apparent coordination number. Meissner et al. [16] derived
the coordination number by
n ˆ 2 exp…2:4†;
(1)
where is volume fraction of solid part, n the average
(2)
Fig. 3. Bulk density as a function of DMF/TEOS molar ratio.
A.V. Rao et al. / Materials Chemistry and Physics 60 (1999) 268±273
where, R and R0 are the alkyl groups. Orcel and Hench [17]
studied the in¯uence of DMF on the hydrolysis and polymerization reactions by NMR and IR spectra and concluded
that the DMF decreased the hydrolysis rate and increased
the polymerization rate [4,18]. The acceleration mechanism
of DMF was considered to be that the DMF assists the
deprotonation step as explained by the above said reaction
mechanism. Hence, our results are in good agreement with
Orcel's results [17]. Hence, DMF is not only useful as a
DCCA but catalyst too! As discussed earlier, and after
observing the SEM pictures (Fig. 8(a)±(c)) it is clear that
increase in A values leads to an increase in both the pore as
well as the SiO2 particle sizes. Fig. 4 shows that an increase
in A values leads to an increase in percent porosity. The
percent porosity P% was calculated using the relation
P% ˆ 100Vp b ;
Fig. 4. Percent porosity as a function of DMF/TEOS molar ratio.
(3)
271
Fig. 5. Percent volume shrinkage as a function of DMF/TEOS molar ratio.
where Vp corresponds to the porevolume, b to the bulk density
of the silica aerogel and s is the skeletal density of the silica
aerogel which is 2.20 g cmÿ3. Hence an increase in A value
leads to a decrease in bulk density as discussed earlier which
attributes to an increase in the percent porosity (Fig. 4) and
decrease in the percent volume shrinkage (Fig. 5).
3.3. Percent optical transmission
The variation of percent optical transmission (for a 1 cm
thick sample and at a wavelength of 800 nm) as a function of
DMF/TEOS molar ratio (A) is shown in Fig. 6. The optical
Fig. 6. Percent optical transmission as a function of DMF/TEOS molar
ratio.
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A.V. Rao et al. / Materials Chemistry and Physics 60 (1999) 268±273
transmission is explained on the basis of SEM microstructure of the aerogels prepared using different A values. On the
basis of optimal conditions for obtaining good quality TEOS
aerogels in terms of monolithicity, transparency and low
density, it is assumed that for A < 0:3 as the lower A values,
0:3 < A < 0:7 as the medium A values and A>0.7 as the
higher A values. Fig. 8 shows the SEM microstructure of the
aerogels prepared using various DMF/TEOS molar ratios
(A): (a) Aˆ0, (b) Aˆ0.5 and (c) Aˆ0.9. The TEOS aerogels
prepared using lower A values (Aˆ0) show transparency
60%. Though the SiO2 particles are smaller in size, due to
a few irregular pores and some larger size SiO2 particles,
the aerogels are semitransparent (percent transparency
460%) (Fig. 8(a)). The aerogels prepared using medium
A values (0:3 < A < 0:7) show high optical transparency
(90%) due to uniform pores even though a slight increase
in SiO2 particle sizes have been observed (Fig. 8(b)) compared to those prepared using lower A values (Aˆ0)
(Fig. 8(a)).
Fig. 8(c) shows the SEM picture of the aerogel prepared
using higher A values (Aˆ1.0). It is clearly seen from the
SEM picture (Fig. 8(c)) that irregular and larger size SiO2
particles were found with a few larger and non-uniform
pores. Hence the aerogels prepared using higher A values
(A > 0:7) were found to be translucent. Increase in both
pore and particle sizes with an increase in A values was
discussed earlier.
3.4. Pore size distribution
The in¯uence of A on the pore size distribution (PSD) of
the aerogels with DMF (Aˆ0.5) and without DMF (Aˆ0) is
shown in Fig. 7. PSDs are represented as dv/d(log r) plots so
Fig. 8. SEM microstructures of the aerogels prepared using various DMF/
TEOS molar ratios (A): (a) Aˆ0, (b) Aˆ0.5 and (c) Aˆ1.0.
Fig. 7. PSD of TEOS silica aerogels prepared with DMF [DMF/TEOS
molar ratio (A)ˆ0.5] and without DMF (Aˆ0).
that the integrated area under the plot directly corresponds
to the pore volume. It is clearly seen from Fig. 7 that DCCA
(DMF) modi®ed TEOS silica aerogel shows narrow and
uniform PSD. Fig. 8(b) (SEM picture) clearly shows that
almost all particles are spherical in shape and the pores are
uniform. Simultaneous occurrence of hydrolysis and polycondensation reactions may be attributed to the formation of
spherical SiO2 particles and uniform pores.
On the other hand, the PSD for TEOS silica aerogel
without DMF (Fig. 7) shows wide PSD shifted towards
A.V. Rao et al. / Materials Chemistry and Physics 60 (1999) 268±273
smaller pore radii which leads to a differential pressure
during supercritical drying of the aerogels which, in turn,
leads to the probability of the crack formation in the
aerogels. The irregularity of pores and large number of
smaller size pores are clearly observed in the SEM picture
(Fig. 8(a)). Our results are in accordance with Scherrer's
results. Scherrer [19] gave a detailed analysis of various
stresses that develop during supercritical drying, leading to
the cracks in the aerogels. He concluded that a large part of
the stress results from syneresis ("s). Moreover, the permeability (D) value depends on the pore size (r) according to
the Carman±Kozeny equation [20]:
D…1 ÿ †r 2 ;
(4)
where is the density of the network. For a ®xed , lower r
values decrease the permeability of the solvent, leading to
cracks in the aerogels. Therefore, to obtain monolithic
aerogels, larger D values are needed. To increase the D
values, we have used DMF as an additive for the TEOS
aerogels.
4. Conclusions
It has been found that the properties of silica aerogels
prepared at a constant molar ratio of TEOS:EtOH:H2Oˆ
1:5:7, respectively, are strongly in¯uenced by DMF/TEOS
molar ratio (A) between 0 and 1. It has been found that the
bulk density, volume shrinkage and surface area decreases
with an increases in A values. On the other hand, the percent
porosity has been found to increase with an increase in A
values. Moreover, it has been observed that the percent
optical transmission increases with an increase in A values
up to 0.3 and then remains constant up to Aˆ0.7 and for
A > 0:7, the percent optical transmission decreases. The
PSD for TEOS aerogel prepared without any use of DMF
(Aˆ0) shows wide PSD shifted towards smaller pore radii
which leads to the probability of cracking of the aerogels
even after adopting supercritical drying process of the
alcogels. On the other hand, DMF modi®ed alcogels for
medium (0:3 < A < 0:7) A values resulted in good quality
silica aerogels in terms of monolithicity, low density
(between 183 and 140 kg/m3), large surface area (between
843 and 736 m2/g), high optical transparency (90%), high
porosity (between 90% and 92%), less volume shrinkage
(between 20% and 10%) and with uniform pore size distribution.
273
Acknowledgements
The grant received from the University Grants Commission (UGC) [Project no. F. 10-32/93 (SR-I)], Government of
India, for the research work on ``Silica aerogels'' is gratefully acknowledged. We are grateful to Dr. Gupta, Director,
and Mr. S.V. Rao, in charge of SEM, Regional Sophisticated
Instrumentation Centre (RSIC), Nagpur University, India,
for help in taking SEM images. We are grateful to Shri P.V.
Ravan, Draughtsman, University Science and Instrumentation Centre (USIC), Shivaji University, Kolhapur, India, for
his help in the ®ne tracing of the graphs. One of the authors
(PBW) is grateful to Prof. A.M. Shekatkar, Head of the
Physics Department, Smt. Kasturbai Walchand College,
Sangli, India, for his kind help and encouragement.
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