In uence of N,N-dimethylformamide additive on the physical properties of citric acid catalyzed TEOS silica aerogels

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 0254-0584(99)00089-9

A.V. Rao et al. / Materials Chemistry and Physics 60 (1999) 268±273 269 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 Fig. 1. Application tree of the aerogels. Silica alcogels were prepared by hydrolysis and polycondensation of ethanol (EtOH) diluted TEOS in the presence of citric acid (C 6 H 8 O 7 H 2 O) 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 C 6 H 8 O 7 H 2 O (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 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 N 2 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 : H 2 Oˆ1:5:7, 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 (Model- 783). 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 mm 3 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 m 2 /g. The SEM picture (Fig. 8(a)) of the TEOS silica aerogel prepared without the use of DMF (Aˆ0) show smaller size of SiO 2 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 SiO 2 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

270 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 (cm 3 /g) Volume fraction of solids, Coordination number, n 0 (without DMF) 3.249 0.1228 2.685 0.2 4.376 0.0941 2.506 0.4 5.497 0.0763 2.402 0.6 6.348 0.0668 2.347 0.8 6.898 0.0618 2.319 1.0 7.237 0.0591 2.304 Fig. 2. Surface area as a function of DMF/TEOS molar ratio. observed from the SEM picture (Fig. 8(c)) that the SiO 2 particles are still larger in size compared to the SiO 2 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. coordination number and 1 corresponds to the porosity. The average coordination numbers estimated by pore volume are presented in Table 1. The pore volume, V p, was calculated using V p ˆ 1= b 1= s ; (2) 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 cm 1. 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 SiO 2 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 Fig. 3. Bulk density as a function of DMF/TEOS molar ratio.

A.V. Rao et al. / Materials Chemistry and Physics 60 (1999) 268±273 271 where, R and R 0 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 SiO 2 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 100V p b ; (3) Fig. 5. Percent volume shrinkage as a function of DMF/TEOS molar ratio. wherev p correspondstotheporevolume, b tothebulkdensity of the silica aerogel and s is the skeletal density of the silica aerogel which is 2.20 g cm 3. Hence an increase in Avalue 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. 4. Percent porosity as a function of DMF/TEOS molar ratio. Fig. 6. Percent optical transmission as a function of DMF/TEOS molar ratio.

272 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 SiO 2 particles are smaller in size, due to a few irregular pores and some larger size SiO 2 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 SiO 2 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 SiO 2 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 SiO 2 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±273 273 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 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:H 2 Oˆ 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/m 3 ), large surface area (between 843 and 736 m 2 /g), high optical transparency (90%), high porosity (between 90% and 92%), less volume shrinkage (between 20% and 10%) and with uniform pore size distribution. 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. References [1] J. Zarzycki, in: L.L. Hench, D.R. Ulrich (Eds.), Ultrastructure Processing of Ceramics, Glasses and Composites, Wiley, New York, 1983, p. 43. [2] S. Sakka, in: M. Tomozawa, R.H. Doremus (Eds.), Treatise on Materials Science and Technology, vol. 22, Glass IV, Acad. Press, New York, 1982, p. 43. [3] A. Venkateswara Rao, P.B. Wagh, G.M. Pajonk, D. Haranath, J. Mater. Sci. and Tech. 14 (1998) 236. [4] T. Adachi, S. Sakka, J. Non-Cryst. Solids 99 (1998) 118. [5] T. Adachi, S. Sakka, J. Non-Cryst. Solids 22 (1987) 4407. [6] J.B. Chan, J. Jones, J. Non-Cryst. solids 125 (1980) 79. [7] A.H. Boonstra, T.N.M. Bernards, J.F.T. Smith, J. Non-Cryst. Solids 109 (1989) 141. [8] P.B. Wagh, A. Venkateswara Rao, D. Haranath, Mater. Chem. and Phys. 53 (1998) 41. [9] P.B. Wagh, D. Haranath, A. Venkateswara Rao, J. Porous Mater. 4 (1997) 295. [10] A. Venkateswara Rao, G.M. Pajonk, N.N. Parvathy, J. Mater. Sci. 29 (1994) 1807. [11] A. Venkateswara Rao, G.M. Pajonk, N.N. Parvathy, J. Mater. Sci. 28 (1993) 3021. [12] A. Venkateswara Rao, G.M. Pajonk, N.N. Parvathy, E. Elaloui, Y.A. Attia (Eds.), Sol-Gel Processing and Applications, Plenum Press, New York, 1994, p. 237. [13] J. Zarzycki, M. Prassas, J. Phalippau, J. Mater Sci. 17 (1982) 3371. [14] T. Katagiri, T. Maekawa, J. Non-Cryst. Solids 134 (1991) 183. [15] R.K. Iler, in: The Chemistry of Silica, Wiley, New York, 1979. [16] H.P. Meissner, A.S. Michaels, R. Kaiser, Ind. Eng. Chem. Process Des. Div. 3 (1964) 202. [17] G. Orcel, L. Hench, J. Non-Cryst. Solids 79 (1986) 177. [18] T. Adachi, S. Sakka, J. Non-Cryst. Solids 100 (1988) 250. [19] G.W. Scherer, J. Non-Cryst. Solids 33 (1992) 145. [20] A.E. Scheidegger, The Physics of Flow Through Porous Media, 3rd ed., University of Toronto Press, Toronto, 1974.