Journal of Non-Crystalline Solids 136 (1991) 111-118 North-Holland
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The effect of aging on acid-catalyzed aerogels B h a r a t h R a n g a r a j a n and Carl T. Lira 1 Michigan State University, East Lansing, MI, 48824, USA Received 10 April 1991 Revised manuscript received 16 July 1991
Acid-catalyzed silica aerogels have been prepared using supercritical CO 2. During aging of the wet gel, the density and modulus increases. Drying these gels results in additional shrinkage, with the youngest gels shrinking the most and providing the most dense aerogels with the highest modulus. The structural changes (dissolution and reprecipitation) which occur on aging result in reduced stresses during hypercritical drying and therefore a lower density aerogel.
1. Introduction
The basic principle of the supercritical drying process is the removal of liquid from a porous object by manipulating the temperature and pressure in a manner that precludes the formation of two phases. The liquid is converted into a supercritical fluid and subsequently a gas, resulting in an elimination of surface tension and preservation of the structure of the material being dried. The supercritical drying process dates back to Kistler [1]. It was used to dry alkoxide-derived gels by Teichner et al. [2], and to make monolithic gels [3-5]. In these cases, the fluid removed had a fairly high critical temperature and pressure (Tc ~ 240 ° C, Pc ~ 6 - 8 MPa). A low temperature version of the supercritical drying process, using CO 2 (Tc = 31.1 ° C, Pc = 7.4 MPa), has been used by biologists for preparing samples for electron microscopy [6,7]. Tewari et al. [8] have used CO 2 to dry silica gels. Both the high-temperature alcohol process as well as the low temperature CO 2 process do not, in general, eliminate drying shrinkage [7,9-11]. The notable exceptions are the processes of Mulder and van Lierop [9], where a nitrogen over
1 Author to whom correspondence should be addressed.
pressure is used to reduce or eliminate shrinkage, and the drying of low density base-catalyzed gels, which show little or no shrinkage [10]. Characterizing and understanding the shrinkage is important since it affects the final dimension and properties of aerogels. Shrinkage during drying (drying shrinkage) is preceded by shrinkage during aging (syneresis or aging shrinkage) of alcogels (wet gels). Throughout this paper, the term alcogel will be used to refer to wet gels, and aerogel will refer to the dried material. For the purposes of this paper, the age of a gel is the time between gelation and the initiation of the drying process, i.e., the age of the aerogel is the same as the age of the alcogel from which it was prepared. Recently, Hdach et al. [10] studied the systematic shrinkage behavior for alcogels and the shrinkage occurring on supercritical drying using the high temperature alcohol process for acid-, neutral- and base-catalyzed gels. They show drying causes increasing shrinkage with lower pH for gels of a given Si content and age. They studied aging of neutral gels, and show that older gels result in the most dense aerogels. The acid-catalyzed aerogels in this low-temperature drying study show the opposite trend, with the older gels resulting in less dense aerogels. Experiments were conducted to quantify the shrinkage, so that the behaviors could be corn-
0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
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B. Rangarajan, C.T. Lira / The effect of aging on acid-catalyzed aerogels
pared and a more complete understanding of the phenomena could be developed. A systematic approach is taken to differentiate between shrinkage during alcogel aging and shrinkage during drying. Various factors affecting the shrinkage during the supercritical process are discussed. All of the specimens in this study are monolithic aerogels. The choice of the system under study was arbitrary. An acid-catalyzed system was used since the pores are small and the gels shrink significantly during drying. Formamide is used to maintain a narrow pore size distribution [12]. It must be emphasized that the validity of this study is limited to the specific gel formulation used here, and that any extrapolations should be limited to similar systems.
2. Experimental procedure The gels were prepared by the acid-catalyzed hydrolysis of tetra-ethoxy silane (TEOS), with a molar ratio, R, of water to TEOS of 10. 15 ml of TEOS were mixed with 6 ml of absolute ethanol, and 12.5 ml of electrophoresis grade formamide. 12.1 ml of water and 2 ml of 70 wt% nitric acid were mixed together with 6.5 ml of ethanol. The two solutions were mixed in a Pyrex vessel for 10 min with a Teflon-coated spinbar. The mixture was poured into tetragonal acrylic cuvettes of 1.05 cm width and cylindrical syringes of 1.15 or 1.39 cm diameter, with an average length to width ratio of 1.9, which were sealed and stored at room temperature. The gels were allowed to age for different periods of time in the container. The free liquor (the liquid surrounding the gel) was decanted and weighed. The alcogel was transferred into absolute ethanol. The mass of the wet gel, which includes the mass of the skeleton as well as entrained liquid (pore liquor), was determined by subtracting the weight of the free liquor from the total weight. The dimensions of the specimens were measured by carefully using calipers, with an uncertainty of + 1%. For wet gels, this was done as soon as it was transferred into absolute ethanol.
A standard procedure was used for drying gels with C O 2 which is described in detail elsewhere [11]. The gels were soaked for a day in absolute ethanol, after which they are placed into a high pressure vessel filled with ethanol. The vessel is sealed and flushed with liquid CO z for 4 h at 10 + 0.4 MPa, 16-20 ° C. The vessel was heated to 40 °C (_+ 1 ° C). The system was flushed with supercritical CO 2 for 2 h at 10 _+ 0.4 MPa, 40 ° C (_+ 1 ° C), and depressurized over a period of 2 h.
3. Results Dimensional changes have been normalized to compare results from different geometries. The specimens were small enough to exhibit isotropic shrinkage, at least within the uncertainties of measurement. The dimension reported is the diameter or width of the specimen. No significant differences were found in the shrinkage behavior of the two geometries, so the results from the two shapes are used without distinction. No correlations were found between variations in the drying process within the ranges mentioned above and the shrinkage of the gel. It was also found that lengthening the time of any stage of the drying process did not have any noticeable effect. Combining the heating and the subsequent flushing with supercritical CO 2 also did not make any difference in the observed shrinkage. In all cases, the time is measured relative to the time of gelation. The macroscopic gel point was noted to occur 19.5 h after mixing. Figure 1 shows the dependence of the dimension of the alcogels and aerogels on age. The linear dimension is expressed as a percentage of the corresponding container dimension. The difference between the curves for the alcogels and aerogels represents the drying shrinkage. While young alcogels show significant drying shrinkage, less than 35% in linear dimension, older alcogels show low drying shrinkage, on the order of 1%. Figure 2 shows the evolution of the mass of the alcogels. The measured mass is expressed as a percentage of the original sol mass. The measured mass includes the entrained pore liquor and the solid skeleton. When the alcogels are dried,
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A g e (days) Fig. 2. The d e p e n d e n c e of alcogel mass on age. The solid line is calculated using w = (w o - wf) exp( - k j ) + wf, where w is the normalized mass of the wet gel, Wo = 100 (sol mass), wf = 65%, k w = 0.06 day 1.
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B. Rangarajan, C.T. Lira / The effect of aging on acid-catalyzed aerogels
the resulting aerogels have a mass of 10.7% with a standard deviation of 0.9% of the original sol, regardless of age. The evolution of microstructure in these gels is shown in fig. 3. These gels were of different ages (Gel A was aged 3 days, Gel B 80 days, and Gel C a year), but were dried under identical conditions. The fracture surface of the aerogel was coated with gold and viewed in an SEM. The bars
on the micrographs correspond to 100 nm. Figure 4 shows the dependence of the density of the aerogel on the extent of aging in the pore liquor. The young aerogels (aged less than 30 days) were clear and transparent, although with a faint blue color. Objects viewed through it appeared reddened, as reported by Hunt and Berdahl [13]. Aerogels of age more than 60 days were opaque.
4. Discussion
As the gel ages in the pore liquor, it exhibits syneresis. Since the alcogel shrinks and the mass of the solid skeleton remains the same, the apparent density of the structure increases with time. Because the skeletal mass remains constant, the only changes are textural and structural. Upon drying in the absence of stresses, the dimensions and density of the aerogel should be the same as that of the alcogel structure. However, this is not so, and there is shrinkage during the drying process, which affects the final density of the aerogel. The extent of this shrinkage (during drying) depends on the age of the gel. Figure 1 shows that gels aged over 100 days exhibit about 14% aging shrinkage (syneresis) and only about 1% shrinkage upon drying. However the younger gels show much higher drying-shrinkage, and virtually all of this shrinkage occurs during the final depressurization [11]. The changes in modulus and wet gel structure (pore sizes and shapes) affect the macroscopic shrinkage and final density of the aerogel. The relationship between the dimensional changes and mechanical properties of alcogels has been studied by Scherer [14], who presents a curve for shear modulus versus linear shrinkage for gels prepared by the acid-catalyzed hydrolysis of TEOS (R = 16). All datapoints fall on a single curve, indicating that the modulus depends on the shrinkage. Our approximate fit to this curve indicates that Fig. 3. Evolution of microstructure with aging in pore liquor. SEM micrographs of the fracture surface of three different aerogels: gel a aged 3 days, gel b 80 days, and gel c 1 year. The bar corresponds to 100 nm.
G = 1.05 exp(0.18Ed) MPa,
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B. Rangarajan, C.T. Lira / The effect of aging on acid-catalyzed aerogels
catalyzed system; Scherer has reported an empirical fit for shear modulus: G = 0.13 exp(0.17Ed) MPa,
calculate strain. To apply this relationship, we assume that the shrinkage phenomena occurring during drying have approximately the same effect on modulus as syneresis, and that the strength of the gel comes from the structure and not from the fluid in the pores. These calculations show that, while the modulus of alcogels increases with time, the modulus of the final aerogels decreases with the extent of aging. At first glance, these results may seem contradictory, but are reconciled if one considers the fact that the aerogel densities also decrease with a greater extent of aging in pore liquor. The elastic modulus of the aerogels is calculated from the shear modulus using a Poisson's ratio of 0.2 [14,17], and is shown in fig. 6. The estimated values of modulus are reasonable and agree with the values reported by Gronauer et al. [18] and Woigner et al. [19]. A log-log plot of the estimated modulus from eq. (4) of an aerogel versus density has a scaling exponent with the gel density (slope of the line) close to 5, as compared with 2.9-3.8 for aerogels. The modulus of aerogels, foams and other highly porous materials, when measured as a function of density, often obey a simple scaling relationship. This has been
(2)
which indicates a similar dependence of shear modulus on strain, although different values of modulus for the same strain [14-16]. Equation (1) is adapted to qualitatively show trends in the modulus of our gels, although the system we have studied used R = 10. The precision of these estimates depends on the Si content, the molar ratio R, the catalyst, temperature of gelation, etc., but the trends predicted should be correct. Using the data on syneresis (fig. 1) for our gels, we obtain the curve fit Ed% = 12.5(1 -- e x p ( - 0 . 0 4 t ) ) ,
(3)
where t is measured in days, which along with eq. (1) gives G(t)
~ 1.05 exp(2.25*(1 - e x p ( - 0 . 0 4 t ) ) )
MPa.
(4) Figure 5 shows the variation of predicted modulus of alcogels with time. Using eq. (4) we can also estimate the modulus of aerogels, as shown in fig. 5, if we use the dimensions after drying to 10000 - -
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B. Rangarajan, C. T. Lira / The effect of aging on acid-catalyzed aerogels
noted for both silica gels [10,17-20] as well as organic polymeric resorcinol-formaldehyde aerogels [20]. In a recent paper, Hdach et al. [10] report an exponent of 3.7 ( + 0.2), and also report that the Young's modulus of the gel depends on the pH of the gel. They show that this relation holds regardless of whether the density is achieved by aging the gel or by increasing its TMOS content. It is interesting to note that when the hightemperature process is used, younger neutral aerogels have lower densities than older neutral aerogels [10]. Hdach et al. show increased shrinkage for young acid-catalyzed aerogels, relative to neutral gels, but they do not present a study of shrinkage versus aging for acid-catalyzed gels, which prevents a direct comparison with their work. Aging the alcogels at room temperature results in syneresis. After a 15% linear shrinkage, the alcogels cease to shrink. At this point, as shown in fig. 5, the estimated shear modulus of the alcogels remains constant at about 10 MPa. Upon drying the aerogels show a slight increase in shear modulus, to about 15.6 MPa. On the other hand, very young alcogels, with a shear modulus of 1-3 MPa, on drying show a dramatic increase in shear modulus to 100-300 MPa. The fact that the young alcogels shrink more than 15% implies that the shrinkage on low temperature drying is not merely accelerated aging. Since the temperature does not go above 4 0 ° C during the drying process, accelerated aging is not expected. Viewing the gel as it dries has shown that most of the shrinkage during the drying process does not take place during the heating and purging stages (40 ° C, 4 h), but during depressurization [11]. The dramatic shrinkage and increase in modulus on the drying of young alcogels shows that the stresses during drying are not constant and depend on the age of the alcogel. The fact that the young aerogels shrink more, and have a higher density and modulus than older aerogels, implies that they must have experienced higher stresses during drying to compress them to this extent. The shrinkage behavior is related to the structural and textural changes which occur during aging. Dumas et al. [21] used thermoporometry to study the evolution of the texture of acid-cata-
117
lyzed gels, made with TEOS (R = 4). They show that gels aged in water at room temperature for 3.5 months show a disappearance of microporosity, an increase of mean pore radius, from 2.2-2.5 nm to 6-7.8 nm, an increase in volume of mesopores from an average of 405 m m 3 / g to 2180 m m 3 / g and a decrease of macroporous volume from an average of 4575 m m 3 / g to 1473 mm3/g. They also state that the decrease in macroporous volume corresponds to an increase in the Young's modulus of the gel. The micrographs presented in this work show the effect of aging on the structure of the aerogel. Qualitatively, the microstructure seems to have a significant effect on the drying stresses, and therefore the final aerogel density. The absence of small pores, in significantly aged gels [21], results in reduced drying stresses, and therefore lower density aerogels. In young alcogels, the presence of micropores resuits in significantly larger drying stresses, even with the use of a supercritical fluid. The phenomena of adsorption of supercritical fluids in porous solids is not new [22-24]. The adsorbed phase normally has liquid-like densities. It is not unreasonable to expect that in micropores, where even an adsorbed monolayer would fill the pores, stresses could arise. The situation would be even more complicated if residual liquor was present in these pores. In an earlier study [11], we show that the evolution of species from the gel during depressurization involves only extremely small quantities of organics. For example, the ethanol evolving from a 2 cm 3 aerogel dried in a 100 cm 3 vessel yielded an ethanol concentration of less than 10 ppm in the exiting gas [11]. This quantity of ethanol is less than 0.2% of the aerogel mass. Based on a BET surface area for these aerogels of 680 m2/g, significantly more ethanol is necessary to provide a monolayer.
5. Conclusions
During aging, the mass of solid in gels does not change significantly. The particles grow and the modulus of the wet gel increases. During low-temperature supercritical drying, the older
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gels shrink less than younger gels; the older acidcatalyzed gels result in a less dense aerogel. The higher modulus of older gels enable then to withstand stresses during drying, and the coarser structure may also lead to lower drying stresses since the magnitude of stresses is dependent on pore sizes and the presence of micropores. The dependence of stresses during supercritical drying on pore size is a consistent argument but not a conclusive one.
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[10] H. Hdach, T. Woigner, J. Phalippou and G.W. Scherer, J. Non-Cryst. Solids 121 (1990) 202. [11] B. Rangarajan and C.T. Lira, J. Supercritical Fluids 4 (1991) 1. [12] L.L. Hench, in: Science of Ceramic Chemical Processing, eds. L.L. Hench and D.R. Ulrich (Wiley, New York, 1986) p. 52. [13] A.J. Hunt and P. Berdahl, Mater. Res. Soc. Symp. Proc. 32 (1984) 275. [14] G.W. Scherer, J. Non-Cryst. Solids 109 (1989) 183. [15] G.W. Scherer, S.A. Pardenek and R.M. Swiatek, J. NonCryst. Solids 107 (1988) 14. [16] G.W. Scherer, in: Better Ceramics Through Chemistry III, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (Materials Research Society, Pittsburgh, PA, 1988) p. 179. [17] T. Woigner and J. Phalippou, in: Proc. 2nd Int. Symp. on Aerogels, eds. R.' Vacher, J. Phalippou, J. Pelous and T. Woigner, Coll. Phys. C4 (1989) 179. [18] M. Gronauer, A. Kadur and J. Fricke, in: Aerogels, ed. J. Fricke (Springer, Berlin 1986) 167. [19] T. Woigner, J. Phalippou and R. Vacher, in: Better Ceramics Through Chemistry III, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (Materials Research Society, Pittsburgh, PA, 1988) p. 697. [20] J.D. Lemay, T.M. Tillotson, L.W. Hrubesh and R.W. Pekala, MRS Soc. Symp. Proc. 180 (1990) 321. [21] J. Dumas, J.F. Quinson and J. Serughetti, J. Non-Cryst. Solids 125 (1990) 244. [22] P.G. Menon, Chem. Rev. 68 (1968) 277. [23] J. Specovius and G.H. Findenegg, Ber. Bunsenges. Phys. Chem. 82 (1978) 174. [24] S. Blumel, F. Koster and G.H. Findenegg, J. Chem Soc., Faraday Trans. 2 78 (1982) 1753.