Physicochemical transformation of silica gels during hypercritical drying

]OURNA L OF

Journal of Non-Crystalline Solids 145 (1992) 25-32 North-Holland

NON-CRYSTALNE SOLIDS

Physicochemical transformation of silica gels during hypercritical drying T. W o i g n i e r a, j. P h a l i p p o u a, J.F. Q u i n s o n b M. P a u t h e b a n d F. L a v e i s s i e r e a a Laboratoire de Science des Matdriaux Vitre~cr, UA CNRS 1119, Universitd de Montpellier II, 34094 Montpellier cedex 5, France b Laboratoire de ChimieAppliqude et Gdnie Chimique, UA CNRS 417, Universitd C. Bernard, Lyon 1, France

During hypercritical drying (HD), physicochemical transformations are induced which can be associated with an acceleration of the aging effect. The macroscopic signature of this evolution is an isotropic shrinkage characteristic of syneresis. During hypercritical drying, dangling bonds in the network can condense and form new links. T h e structural transformation of the silica gel is followed by small angle neutron scattering. T h e pore size evolution is deduced from thermoporometry. T h e increase of the connectivity in the solid network during H D is shown in the e n h a n c e m e n t of the elastic and mechanical features of the materials

1. Introduction Aerogels have attracted interest with respect to fundamental research and applications [1-3]. Applications as insulation materials and catalysts are essentially related to their porous texture. Aerogels may also be considered as precursors. Via sintering, silica aerogels can be easily transformed into pure silica glass [4]. Appropriate heat treatments lead to partially densified aerogel (PDA) which can be used as a host matrix for the synthesis of doped glasses or composites [5]. Whatever is the goal of the aerogel synthesis, it is important to know how the H D modifies the physical and chemical properties of the alcogel. Knowledge of these transformations would allow one to optimize the synthesis parameters of the initial gellifying solution. The aim of this paper is to characterize transformations on H D in the case of silica gels. After a brief description of the different H D procedures which allow one to obtained monolithic material, we study the influence of the synthesis parameters, such as the concentrations in the gellifying compound, p H of the hydrolysis solution and aging, on the physicochemical transformations observed during the hypercritical fluid extraction.

2. Hypercritical drying

2.1. Definitions If the liquid in the pores of a gel is water, we speak of an aquagel; if it is alcohol, we term it an alcogel. In both cases, drying under normal conditions at room temperature or in an oven leads to the formation of a xerogel. Hypercritical fluid extraction was initially developed by Kistler [6] and is accomplished by increasing the temperature and the pressure of the liquid in the pores above their critical values and replacing the supercritical fluid by air. For the materials obtained, Kistler proposed the term aerogel. In this study, the alcogels are prepared by hydrolysis and p o l y c o n d e n s a t i o n of tetramethoxysilane (TMOS). The TMOS is dissolved in various amounts of methanol, thereby adjusting the concentration. Solutions are hydrolyzed under neutral (distilled water), basic (NH4OH) or acidic (HNO 3) conditions. The molar ratio of H 2 0 and TMOS is 4: 1. The polycondensation proceeds and gellification occurs. The alcogels are allowed to age in an oven at --60°C for different times. The materials are labelled N, B

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

26

T.

Woignier et al. / Transformation of silica gels

or A x , where x is the vol.% of TMOS in the entire solution.

\

2. 2. Hypercritical drying procedures

C

Hypercritical drying prevents the capillary forces between the vapor air-liquid interface and the solid part of the gel, which occur during subcritical drying. These forces collapse the skeletal structure, giving rise to a hard glassy product (xerogel) with significantly less volume than the alcogel. Moreover, normal drying may induce the formation of cracks. The magnitude of the capillary stress depends on the interfacial energy, T. If the pressure and the temperature are above the critical point, C, gas and liquid become indistinguishable and the interfacial energy vanishes. In many cases, some water is present in the liquid phase of the alcogel. Thus, the critical parameters of the liquid in the pores are different from those of pure alcohol. For a rough estimate of the critical parameters of the binary solution C H 3 O H - H 2 0 , linear interpolation can be used [7]. The composition of the binary liquid is in the molar range 2 5 - 5 0 % H 2 0 (15-35 wt%) depending on the completion of the reactions, and the

30

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Fig. 1. Waysto bypass the estimated value of the critical point of a liquid (cross-hatched area) in a pressure versus temperature plot. @, pre-pressurized autoclave; •, extra amount of solvent added.

Gas

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Fig. 2. Phase diagram of pressure, P, versus specific volume, v. estimated critical parameters are Tc = 260-285°C and Pc = 10-13 MPa. Figure 1 shows two different routes to bypass the critical point. In method 1, the gel and its liquid are heated together; heating of the liquid induces an increase in pressure inside the autoclave. In order to reach a pressure and a temperature above the estimated critical values, it is necessary to add an extra volume of liquid. In method 2, the autoclave is pre-pressurized with an inert gas (pressure P0) before heating [8]. In both cases, the pressure is then decreased isothermally to the atmospheric pressure by condensation of the superfluid outside of the autoclave and the system is cooled to room temperature. Although the two routes seem to be equivalent, the effect on the monolithicity of the aerogel could be different. Figure 2 shows the pressurespecific volume diagram. The specific volumes of liquid and gas are v~ and vg, respectively. When P = Pc, we have v~ = vg = v c. If the specific volume, v l, is lower than v c, the system evolves vertically towards A. The gas in the autoclave condenses and the liquid meniscus rises. Now let us consider the case with pressurization and without an extra volume of solvent. The specific volume here is u 2. Due to the increase of pressure (pre-pressure and effect of temperature), the system evolves vertically to point B. In that case, the

T. Woignier et al. / Transformation of silica gels

liquid is transformed into a gas and the gel network is not totally immersed in the liquid. The upper part of the gel undergoes capillary phenomena which can induce shrinkage and the cracking of the gel. To avoid this problem, a small amount of alcohol is added on top of the gel tube

27

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[81. Table 1 shows the influence of the specific volume on the monolithicity of the aerogel. Even for a specific volume slightly higher than v c = 3.67 cm 3 g - l , the monolithicity is preserved. This result can be explained by the fact that, in this case, capillary phenomena appear at high temperatures and thus with low y. The stress is small and the gel can resist without cracking. These results show that the specific volume of the liquid gas system is a very important parameter for a successful hypercritical fluid extraction. It is not sufficient just to overpass the critical pressure and temperature of the solvent. As a result of the pressure and temperature employed, the hypercritical drying in alcohol can induce reactions in the autoclave. The esterification that replaces O H groups by methoxy groups is well known [9,10]. This methoxylation leads to the hydrophocity of the aerogels. Another phenomenon which occurs during the alcohol H D is the dissolution/redeposition of silica, due to the difference in the solubility between surfaces having different radii of curvature [11]. The solubility of silica is lower in the concave parts (necks) than in the convex parts (particles). The result of this effect is a reduction of the net curvature of the solid phase. The kinetics depend on the temperature, the p H and the nature of the solvent. Figure Table 1 Monolithicity of the aerogel as a function of the experimental specific volume, u

u (cm3 g-l)

Remarks

3.2 3.4 3.67 3.7 3.9 4.1 5.5 6.5

monolithic monolithic uc monolithic monolithic monolithic cracked cracked

/""

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300 T (°C)

Fig. 3. Effect of the temperature and water concentration on the silica solubility, S. x = 0.8 corresponds to a 0.8 volumic fraction of CH3OH. (From ref. [11].)

3 shows that silica is poorly soluble in C H 3 O H at room temperature; at 300°C, the solubility in C H 3 O H is higher than in water. As mentioned above, an alcogel contains between 50 and 80 vol.% C H 3 O H which corresponds to a quite low silica solubility. The solubility of the silica decreases with the molecular weight of alcohol. Silica is totally insoluble in acetone. Depending on the type of solvent used and on the water concentration, dissolution phenonema will thus appear and the texture of the solid network will be modified. In the case of a multicomponent alcogel, the high temperature and pressure can induce crystallization (SiO2-TiO 2 [12]), or a loss of the stoichiometry (SiO2-BzO3, SiOz-P205 [13]). This is due to the corrosive nature of alcohol and water. In order to avoid these phenomena, other liquids can be used to realize a gentle hypercritical drying. Monolithic silica aerogels have been obtained [8,14] by replacing alcohol by liquid CO 2 (Pc = 7.3 MPa, Tc = 31.1°C). CO z aerogels are not hydrophobic because they bear a low amount of methoxy groups. The other principal advantages of CO 2 are the inertness against metal oxides, the low toxicity and the non-flammability. However, in this procedure it is necessary to exchange the gel solvent (alcohol + water) by the chosen liquid. To improve the solvent exchange, an intermediate solvent, such as acetone, diethylether or amylacetate, may be useful. For large

T. Woignier et al. / Transformation of silica gels

28

samples, the exchange is time consuming because of the poor permeability of the alcogel [15].

,

3. Effect of synthesis parameters

30

As indicated above, different hypercritical procedures can lead to aerogels with different properties. The effects of the H D on the product depend strongly on the parameters used to synthesize the alcogel.

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3.1. Shrinkage The concentration and the aging time play an important role in the shrinkage, AL/Lo(HD), in the hypercritical drying process (fig. 4). The total shrinkage, AL/Lo(T), of a gel is larger than AL/Lo(HD) because also a shrinkage during aging at the alcogel state, AL/Lo(alco), exists. Since shrinkage was observed to be isotropic within experimental accuracy, it was determined simply by measuring the length of the bar of a gel with a cathetometer. For material having a low TMOS content, the total shrinkage is small ( < 7%) even after a long aging time. On the other hand, for more concentrated materials, AL/Lo(HD) and AL/Lo(T) can be higher than 15 and 22%, respectively.

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T"OS ( 70 ) Fig. 4. Linear shrinkage, A L / L o, of aerogels versus the TMOS concentration and aging. A L / L o ( T ) , total shrinkage; A L / L 0 ( H D ) , shrinkage due to hypercritical drying (HD). The aging time is given in days.

0

t

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Fig. 5. Linear shrinkage of aerogels versus pH. (Notation is as given in fig. 4.)

AL/Lo(T) is higher for a long aging time than for unaged materials; the opposite effect is observed for AL/Lo(HD). These results can be explained if we consider that the autoclave treatments accelerate the aging, changing the rate but not the nature of the densification process. Neutral gels are expected to consist of relatively flexible chains whose surfaces are covered by SiOH groups. Shrinkage results from condensation between neighbouring groups. The condensation rate should increase with TMOS concentration since the chains are closer together when the silica concentration is high. The densification during the H D is dependent on the rate of condensation and on the flexibility of the chains. It is well known that aging induces considerable shrinkage in the alcogel state [16] which has two consequences for HD: a large amount of SiOH available for condensation has already reacted and the network is strengthened by the formation of new bonds. The further shrinkage occuring during the hypercritical drying is thus less important than without aging. Figure 5 shows the influence of the p H of the hydrolyzing solution on the shrinkage, AL/Lo(T) and AL/Lo(HD), for two concentrations (26 and 40% TMOS, respectively). For both sets of mate-

T. Woignier et al. / Transformation of silica gels

rials, at high pH there is almost no shrinkage even during HD, whereas at low p H there is extensive shrinkage particularly during HD. The small shrinkage measured at high pH can be explained by two factors: the structure of the basic gels, and the solubility of the silica in basic conditions. At high pH, the network consists of large particles [17] and the branched parts are more widely spaced than in acidic or neutral materials. Thus, formation of new bonds between branches is relatively unlikely and less contraction is expected. Moreover, the high solubility of silica at high pH [11] induces dissolution-reprecipitation and transfer of material to the necks between particles; this effect stiffens the network. As mentioned above, the solubility increases with temperature and pressure. Thus, during the hypercritical drying, further stiffening is expected at the expense of the flexibility of the branches and of shrinkage.

3.2. Structural study Small angle scattering is a valuable tool for studying the structure of materials in the 1-100 nm range. Moreover, this technique provides information on the density-density correlation function and thus is particularly well suited to study fractal structures. Figure 6 shows the effect of the concentration and p H on the typical scattering curve obtained by SANS. For each sample, three regions can be recognized. At large q (Porod region), the experiment senses the particle surface and gives information on the roughness. In the intermediate regime, S(q) is dependent on the fractal dimension of the network, while at very small q the material becomes homogeneous. The positions of the two crossovers are related to the size of the elementary particles, a (high q), and the fractal clusters, ~ (small q). The curves for A26 (pH 3.5) and N26 give evidence for an S(q) (x qD over more than one order of magnitude; the fractal dimension is 2.2 and 2.4, respectively. For these materials, the radius of the elementary particles is smaller than 10 A. In the case of the heaviest materials, the fractal range is reduced. The results obtained for o

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q (~-') Fig. 6. SANS curves obtained for neutral, acid and basic sets of aerogels. A, acidic; B, basic; N, neutral; the numbers give the TMOS %. The numbers at the slopes indicate the fractal dimension.

the basic samples (pH 8) show that a fractal behaviour is observed only for a very low concentration (B4). In that case, the fractal dimension is close to 1.8 and the size of the elementary particles is larger than 15 A. These results confirm that neutral and acid sets have a similar fractal structure consisting of small particles. On the other hand, for the basic aerogels the structure of the cluster is more open and made of larger primary particles. The evolution of the fractal features during H D leads to an increase of D which is consistent with a partial collapse of the network [18-20]. An identical conclusion can be drawn if we compare the fractal dimension of acid and neutral aerogels with alcogels for which the experimental data give values of D in the range 1.8-2.2 [18,21,22]. For basic materials, a light scattering study [23] carried out on alcogels has shown a value of D of 1.7 which is Close to that measured for aerogels. It has been shown [18] at the particle range scale (Porod region) that the variation of the slope after H D corresponds to a smoothening of the

T. Woignier et al. / Transformation of silica gels

30

surfaces. These results confirm that in basic catalysis the transformations occur at the particle scale with little densification of the network.

3.3. Transformations in the porous texture It is difficult to compare the alcogel texture (the pores are filled with liquid) with that of an aerogel (air in the pores). T h e r m o p o r o m e t r y is a thermal method for textural characterization based on the analysis of phase transitions of a liquid inside the porous material [24]. By definition, aerogels are dried and thus we are obliged to re-wet the solid network. In fact for the 'rewetted aerogels' the hypercritical heat treatment is not followed by the evacuation of the superfluid, so the solvent re-impregnates the gel during cooling. This procedure allows preparation of materials filled with solvent for which the solid network has undergone the same heat treatment as for classical aerogels [25]. It can be assumed that those materials and the aerogels have comparable structure and texture. T h e r m o p o r o m e t r y experiments have been made on alcogels and re-wetted aerogels after exchange of alcohol by water. The total porous volume in a gel consists of the macroporosity R > 30 nm, the mesoporosity 1-30 nm and the microporosity R < 1 nm. Water t h e r m o p o r o m e t r y is applicable mainly for the r

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(nm)

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mesoporous range. However, it is possible to evaluate the microporous and macroporous volume indirectly [26]. Figure 7 and table 2 give the mesopore size distribution and the porous volumes for alcogels and re-wetted aerogels. The heat treatment of the acid alcogels obviously strongly modifies their texture. We observe an increase of the mesoporous volume, a shift of Rmax toward a larger value and a broadening of the pore size distribution. Table 2 shows a large decrease of v . . . . . . while /)micro disappears. The neutral set exhibits an identical behaviour except that the Umicro of the neutral alcogel is smaller. For the basic gel, the textural evolution is less pronounced, but we can also observe a shift of the pore size distribution towards higher values, the elimination of microporosity, and a small decrease in the macroporous volume. The removal of the microporosity seems to be due to the p h e n o m e n a of dissolution/reprecipitation. On the other hand, the increase of the mesoporosity at the expense of the macroporosity, especially for the A and N materials, must be related to macroscopic shrinkage. The formation of bonds between the chains causes a transformation of the small macropores into large mesopores.

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Table 2 Textural features of alcogels and rewetted aerogels

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3.4. Mechanical properties

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Fig. 7. Mesapore distribution curve obtained by thermoporometry, showing the influence of the HD process. A33, a normal alcogel; A33R, a re-wetted aerogel.

The transformations occurring during the H D must be correlated with an increase of the mechanical moduli. The elastic modulus, G, and the modulus of rupture, ~r, were measured by the three-point bending technique [24]. In the case of

T. Woignier et al. / Transformation of silica gels

solvent-filled gels, Scherer [15] has shown that, due to the low permeability of the gels, the volume of the sample is constant during the experiment, providing a measurement of the shear modulus, G. For aerogels, the test yields the Young's modulus. In order to compare the mechanical properties of these two types of materials, measurements have been made on alcogels and re-wetted aerogels. It is obvious that the mechanical properties depend on the load-bearing fraction of the solid. We have shown that the H D induces a shrinkage of the network and thus an increase of the fraction of the solid volume. Instead of plotting G and cr as a function of the TMOS concentration, the data have been plotted as a function of their respective density which correesponds to the fraction of solid. Nevertheless, the shear modulus and the modulus of rupture increase by a factor of 5 during H D for the different acidic (fig. 8), basic (fig. 9) and neutral (not shown) materials. This result confirms that during H D the structures strengthened. In the case of acidic and neutral gels, the strengthening can be explained by the creation of siloxane bonds between 'dead ends' in the alcogel which induces the shrinkage and increases the reticulation. These 'dead ends'

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Fig. 8. Effect of the H D o n the s h e a r m o d u l u s , G, a n d the m o d u l u s of r u p t u r e , or, for a set of acid m a t e r i a l s .

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P (g/c" 3) Fig. 9. Effect of the H D on the s h e a r m o d u l u s , G, and the m o d u l u s of r u p t u r e , o-, for a set of basic m a t e r i a l s .

contribute to the relative density of the alcogel but not to its connectivity and mechanical properties. Because no important shrinkage has been measured for basic gels, the strengthening is more likely to be due to the growth of the necks between the particles.

4. Conclusions

The evolution of the structural, textural and mechanical features of the solid network has been investigated. The magnitude of the transformations is dependent on the synthesis parameters of the alcogels, such as the TMOS concentration, aging and especially the pH of the hydrolyzing solution. Besides the well known esterification reactions, other transformations, such as dissolution/reprecipitation of silica and an increase of the reticulation by formation of siloxane bonds, occur in the autoclave. Thus, attempts to draw conclusions from the aerogel structure on the aggregation process of the gelling solution fails because the aerogel structure is in part the result of the hypercritical drying.

32

T. Woignier et al. / Transformation of sifica gels

The authors are indebted to George for helpful discussions and comments.

Scherer

References [1] J. Fricke, ed., Aerogels (Springer, New York, 1986). [2] 2nd Int. Symp. on Aerogels, Rev. Phys. Appl. 24 (C4) (1989). [3] R.J. Ayen and P.A. Iacobucci, Rev. Chem. Eng. 5 (1988) 158. [4] T. Woignier, J. Phalippou and M. Prassas, J. Mater. Sci. 25 (1990) 3118. [5] D. Bourret, R. Sempere, J. Bouaziz and A. Sivade, Rev. Phys. Appl. 24 (4) (1989) 71. [6] S.S. Kistler, Nature (1931) 741; J. Phys. Chem. 36 (1932) 52. [7] B. Lefrancois and Y. Bourgeois, Chim. Ind. G6nie Phys. 105 (1972) 989. [8] T. Woignier, thesis, University of Montpellier (1984). [9] G. Maertens and J.J. Fripiat, J. Coll. Inter. Sic. 42 (1973) 169. [10] S. Kitahara, K. Takada, T. Sakata and H. Muraishi, J. Coll. Inter. Sci. 84 (1981) 519. [11] R.K. Iler, The Chemistry of Silica (Wiley, New York, 1979). [12] M. Prassas and L. Hench, in: Ultrastructure Processing of Ceramics, Glasses and Composites, ed. L.L. Hench and D.R. Ulrich (Wiley, New York, 1984) p. 100.

[13] T. Woignier and J. Phalippou, in: Proc. 1st Int. Workshop on Non-Crystalline Solids, San Feliu de Guixols, ed. M.D. Baro and N. Clavaguera (World Scientific, Singapore, 1986) p. 415. [14] P.H. Tewari, A.J. Hunt and K.D. Lofftus, Mater. Lett. 3 (1985) 363. [15] G.W. Scherer, J. Non-Cryst. Solids 109 (1989) 183. [16] G.W. Scherer, J. Non-Cryst. Solids 100 (1988) 77. [17] C.J. Brinker, K.D. Keefer, D.W. Schaeffer, R.A. Assink, B.D. Kay and C.S. Ashley, J. Non-Cryst. Solids 63 (1984) 45. [18] D.W. Schaeffer, Rev. Phys. Appl. 24 (4) (1989) 121. [19] A. Yasumori, H. Kawazoe and M. Yamane, J. Non-Cryst. Solids 100 (1988) 215. [20] H. Vesteghem, D. Fargeot and A. Dauger, Rev. Phys. Appl. 24 (4) (1989) 65. [21] B. Cabane, M. Dubois and R. Duplessix, J. Phys. (Paris) 48 (1987) 2131. [22] R. Winter, D. Hua, P. Thiyagarajan and J. Jonas, J. Non-Cryst. Solids 108 (1989) 137. [23] P. Lourdin, J. Appel, J. Pelous, T. Woignier and R. Vacher, Rev. Phys. Appl. 24 (4) (1989) 197. [24] M. Brun, A. Allemand, J.F. Quinson and C. Eyraud, Thermochim. Acta 21 (1977) 59. [25] T. Woignier, J. Phalippou, H. Hdach and G.W. Scherer, Better Ceramics Through Chemistry IV, Materials Research Society Proceedings, Vol. 180 (Materials Research Society, Pittsburgh, PA, 1990) p. 1087. [26] M. Pauthe, J.F. Quinson, H. Hdach, T. Woignier, J. Phalippou and G.W. Scherer, J. Non-Cryst. Solids 130 (1991) 1.