Preparation of High Porosity Xerogels by Chemical Surface Modification.

This invention is in the field of extremely porous materials such as aerogels and more particularly to aerogels and processes for making aerogels having ultra-fine microstructure and extraordinary properties such as very high insulation qualities. Xerogels are made by the removal of fluid from a fluid-containing gel when the solvent is removed by evaporation and normally exhibit a lower amount of porosity.

Aerogels and xerogels are extremely porous materials representing the lower end of the density spectrum of man-made materials. Densities as low as 0.003 g/cm.sup.3 have been reported for silica aerogels [L. W. Hrubesh, Chemistry and Industry, 24 (1990) 824]. These fascinating materials have numerous unique properties as a result of their ultra-fine microstructure. Aerogels with the porosities in the range of 0.85 to 0.98 may be transparent and translucent. The porosity is defined as the fraction of the sample volume which is pores. Aerogels exhibit strong Rayleigh scattering in blue region and very weak in red region. In the infrared (IF) region of electromagnetic spectrum, the radiation is strongly attenuated by absorption leading to application in radioactive diode systems, which effectively transmit solar radiation but prevent thermal IR leakage [J. Fricke and R. Caps in Ultrastructure Processing of Advanced Ceramics: ed. J. D. Mackenzie and D. R. Ulrich (Wiley, New York, 1988)]. Unusual acoustic properties (sound velocity as low as 100 m/s) suggests use in impedance matching and other acoustic applications. Aerogels with a refractive index of 1.015 to 1.06 cover the region not occupied by any gas or solid resulting in applications in high energy physics.

Silica aerogels with their thermal conductivities as low as 0.02 W/mK (0.01 W/mK when evacuated) find application in superinsulation systems [J. Fricke in Sol-Gel Technology for Thin Films, Fibers, Performs, Electronics and Speciality Shapes: Ed. L. C. Klein (Noyes Publications, Park Ridge N.J., 1988)]. Fricke has described several aerogel applications based on their insulating properties. They include reduction of heat losses through windows, energy-effective greenhouses, translucent insulation for solar collectors, and solar ponds for long-term energy storage. Fricke has also discussed the mechanism for thermal transport through aerogel tiles and the efforts of density and gas pressure on their thermal conductivity.

Since aerogels are made by sol-gel processing, their microstructure may be tailored to optimize properties desired for specific applications. Various precursors, including metal alkoxides, colloidal suspensions, and a combination of both under several mechanisms of gelation may be used to synthesize gels [C. J. Brinker and G. W. Scherer, Sol-Gel Science (Academic Press, San Diego, 1990)]. By varying aging conditions such as time, temperature, pH, and pore fluid, the parent wet gel microstructure may be altered [P. J. Davis, C. J. Brinker and D. M. Smith, Journal of Non-Crystalline Solids; P. J. Davis, C. J. Brinker, D. M. Smith and R. A. Assink, Journal of Non-Crystalline Solids; R. Deshpande, D. W. Hua, C. J. Brinker, and D. M. Smith, Journal of Non-Crystalline Solids]. In addition to metal oxide gels such as silica, aerogels may be made from wet precursor gels which contain both inorganic and organic components or from organic gels. For the composite gels, the organic and inorganic phases may be mixed on different length scales such that the organic component resides solely on the internal pore surface, is incorporated into the spanning gel structure, or forms a separate (from the inorganic phase) gel structure.

The conversion of the wet gel to a dried aerogel or xerogel requires the removal of large quantities of solvent. As solvent is removed, the gel tends to shrink as a result of capillary pressure. The capillary pressure generated during drying is related to the pore fluid surface tension, y.sub.LV, and contact angle, .theta., between the fluid meniscus and pore wall as follows,

where r is the pore radius. For submicron capillaries, such as in silica gels, very large stresses are developed during drying. Aerogel synthesis involves the reduction of capillary pressure by lowering the surface tension, y.sub.LV, to avoid shrinkage of the wet gel during drying.

Aerogels are not new materials. They were first reported by Kistler almost 60 years ago [S. S. Kistler, Nature, 127 (1931) 741]. However, recent advances in sol-gel processing technology along with increasing environmental concern have regenerated interest in energy conservation and alternate thermal insulation applications. The most common method of making aerogels involves directly removing the pore fluid above its critical point (for ethanol T.sub.c =243.degree. C., P.sub.c =63 bar). The critical point is a chemical specific point on the pressure-temperature phase diagram and at temperature and pressure above the critical point values, a liquid and gas cannot coexist but rather, a supercritical fluid will. This avoids liquid-vapor menisci and thus, capillary stresses during drying and essentially preserves the wet gel structure. An alternate low temperature method involves replacing the pore fluid with liquid CO.sub.2 and then removing CO.sub.2 above its critical point (T.sub.c =31.degree. C., P.sub.c =73 bar). All of these prior approaches require high pressure supercritical processing to reduce shrinkage.

Although aerogels exhibit very unique properties, they suffer from several drawbacks for commercial applications. An important disadvantage is their high processing cost because of the need for high pressures associated with supercritical drying of monoliths which require large autoclaves. For the high temperature process significant chemical and physical changes in gel structure can occur as a result of the greatly accelerated rates of aging and changes in the equilibrium behavior of various reactions whereas the low temperature carbon dioxide exchange process is limited to certain pore fluids, since they must be miscible in liquid CO.sub.2.

This invention involves the production of a material which has the properties of a supercritically dried aerogel but with a non-supercritical process. By treating either inorganic metal oxide gels of the general formula M.sub.x O.sub.y such as silica (SiO.sub.2) and alumina (Al.sub.2 O.sub.3), composite inorganic-organic gels of the general formula R.sub.x M.sub.y O.sub.z, or organic gels, in the wet state before drying with surface modification agents or compounds of the form R.sub.x MX.sub.y where R are organic groups such as CH.sub.3, C.sub.2 H.sub.5, etc. and X is a halogen, usually Cl, so that significant changes in the subsequent drying of the gels at non-supercritical pressures are obtained which leads to greatly reduced shrinkage during drying.

The presence of the organic groups on the internal surface of the pores and the proper selection of the final solvent from which the gel is dried results in a contact angle of the fluid in the pore of near 90.degree.. This reduces the capillary pressure to near zero (see Equation 1). Thus, in contrast to high pressure supercritical drying which manipulates the surface tension of the pore fluid to lower capillary pressure during drying, the arrangement of this invention changes the contact angle to lower capillary pressure. The process consists of a series of aging, washing, and/or surface modification steps which are undertaken upon a wet gel before drying. In one embodiment of the invention, water in the water/alcohol mixture initially contained in the wet gel is removed by solvent exchange with either a protic (i.e. alcohol) or aprotic (i.e., acetone) solvent to remove water. The wet get is then placed in a mixture of the surface modification compound (R.sub.x MX.sub.y) and a solvent in which it is soluble such as benzene or toluene. After reaction, the gel is again washed with a protic or aprotic solvent and the gel is dried at sub-critical pressure (e.g., vacuum to ambient to sub-critical pressure). Sub-critical is defined as any pressure less than the critical pressure for the fluid, P.sub.c. In this manner, high porosity (0.60<porosity) and low density (density<0.3 g/cm.sup.3) ambiently dried gels (xerogels) are obtained with properties essentially the same as supercritically-dried aerogels.

Reference is now made to the drawings in which

FIG. 1 is a schematic diagram of fluid contained in a pore. Depending upon the fluid and the surface modification of the pore wall, the degree of wetting changes which causes a significant change in the fluid meniscus shape and hence, magnitude of capillary pressure.

FIG. 2 is a graph depicting variation of pore volume with final pore fluid surface tension for conventional high temperature (ethanol) and low temperature (carbon dioxide) supercritically-dried aerogels, for conventional xerogels dried from pore fluids of different surface tensions, and the ambient pressure, surface modified gels processed in accordance with an embodiment of the invention;

FIG. 3 is a graph showing a comparison of the pore size distributions obtained for dried gels processed using the surface modification, ambient pressure processing in accordance with the invention and xerogels which were not surface modified and dried from different solvents;

FIG. 4 is a graph showing a change in sample weight and sample length during drying for surface modified, ambient pressure gel processed in accordance with the invention, illustrating the initial shrinkage followed by expansion of the gel during the final stages of drying;

FIG. 5 is a graph showing water intrusion illustrating the hydrophobic nature of surface modified gels, described in Example 1, in accordance with the invention;

FIG. 6 is a graph showing hexane imbibition into dried gels with the same pore size. The much slower rate of imbibition for the surface modified gel demonstrates the lower capillary pressure and contact angle closer to 90.degree..

In accordance with the invention inorganic metal oxide gels such as silica (SiO.sub.2) and alumina (Al.sub.2 O.sub.3) and of the general formula M.sub.x O.sub.y or inorganic-organic composite gels of the formula R.sub.x M.sub.y O.sub.z or organic gels are treated in the wet state before drying with a suitable surface modification compound. Such compound may take the form R.sub.x MX.sub.y where R are organic groups such as CH.sub.3, C.sub.2 H.sub.5, etc. and X is a halogen, usually Cl. In accordance with the invention, the surface modification compound greatly reduces shrinkage during drying. As described earlier, capillary pressure causes the gel to collapse during drying.

By decreasing the magnitude of the capillary pressure, less shrinkage is obtained during drying. Because the surface area of wet gels is very high (10-2000 m.sup.2 /g), a significant fraction of the atoms in the gel will be surface species. For a metal oxide gel, the surface will be terminated with either hydroxyl (OH) or alkoxy (OR) groups. In prior art drying processes, these surface groups react to produce MOM bonds via water producing condensation (MOH+MOH<->MOM+H.sub.2 O) or alcohol producing (MOH+MOR<->MOM+ROH) reactions. These MOM bonds thus retain the structure of the collapsed wet gel and, disadvantageously, do not allow the gel to expand after the capillary pressure is released, i.e., the gel is dried. In accordance with the invention by capping these surface MOH and MOR surface groups with surface modification compounds of the form R.sub.x MX.sub.y, the surface becomes covered with unreactive MR.sub.x groups. Advantageously, the unreactive MR.sub.x groups increase the contact angle of the fluid meniscus in the pores of the wet gel and prevent condensation reaction during drying. While surface modification groups have previously been employed to modify the surface properties of material to make them hydrophobic, such groups are employed in this invention for a much different purpose--they are used to modify the contact angle of the fluid meniscus in the pores during drying to minimize shrinkage of the gel.

The process used in accordance with the invention consists of a surface modification step which is undertaken upon a wet gel before drying. The wet gel may be produced via hydrolysis and condensation of metal alkoxides, gelling of particulate or colloidal metal oxides, gelation of organic precursors, or a composite of these approaches. A series of aging steps to increase wet gel strength and washing steps to remove water from the wet gel since water reacts with the surface modification compound, may be used before the surface modification step.

If water is contained in the wet gel, it may be removed by solvent exchange with either a protic (i.e. alcohol) or aprotic (i.e., acetone, hexane) solvent. The wet gel is then placed in a mixture of the surface modification compound (R.sub.x MX.sub.y) and a solvent in which the surface modification compound is soluble and which is miscible with the solvent in the gel. A wide range of solvents such as benzene, toluene, and hexane may be used. The surface modification compound reacts with hydroxyl groups on the surface as: (eg. R.sub.3 MCl+MOH.fwdarw.MOMR.sub.3 +HCl). After the reaction is completed, the gel is again washed with a protic or aprotic solvent and the gel is dried at pressures less than the critical point (vacuum to sub-critical pressure) and typically at ambient pressure. As a consequence of the surface modification, the wetting angle .theta. is much larger and the meniscus shape is flatter as shown in FIG. 1. This results in significantly lower capillary pressure during drying than would exist for a gel not modified in accordance with the invention. As a result of the lower capillary pressure, little linear shrinkage occurs during drying. The small degree of shrinkage (less than 5% of the sample length) results in low density, high porosity dried gels. For a gel not modified in accordance with the invention, the linear shrinkage under the same drying conditions would exceed 30%. In this manner, in accordance with the invention high porosity (0.60<porosity<0.95) low density (0.1<density<0.3 g/cm.sup.3) dried gels (xerogels) are obtained with properties essentially the same as supercritically-dried aerogels. Specific processes and the resultant dried gel properties in accordance with embodiments of the invention are illustrated in the following Examples.

The dried gels were characterized by conventional porous material analysis techniques including BET analysis of nitrogen adsorption isotherms at 77K (surface area), analysis of the desorption branch of the nitrogen isotherm (pore size distribution), the difference between the bulk and skeletal densities and/or the total volume of nitrogen condensed at high relative pressure (total pore volume), and the drying characteristics (weight loss and sample length change). The pore volume (V.sub.p, the total pore volume per mass of the dried gel) is related to the porosity (.epsilon., the fraction of the total material volume which is porous) by .epsilon.=V.sub.p /(V.sub.p +1/.rho..sub.t) where .rho..sub.t is the skeletal density of the solid matrix which is usually measured via helium displacement. For silica gels, .rho..sub.t is approximately 2 g/cm.sup.3 and thus, a pore volume of 2 cm.sup.3 /g corresponds to a solid which is 80% air and a pore volume of 4.5 cm.sup.3 /g corresponds to a solid which is 90% air.

The pore volume for a number of silica gels made using the B2 silica gel recipe described above are shown in FIG. 2 and were: 1) surface modified and dried from a variety of solvents at ambient pressure (Example #1), 2) dried from solvents without surface modification at ambient pressure (conventional xerogel process), 3) supercritically dried without surface modification at high temperature from ethanol (conventional aerogel process), and 4) supercritically dried without surface modification at low temperature from carbon dioxide (conventional aerogel process). The pore volumes for the surface modified gels are independent of surface tension and essentially equal to the two different aerogel samples. The pore size distribution for several of the surface modified gels in accordance with the invention is presented in FIG. 3 and shows the same lack of dependence on surface tension. For comparison, two unmodified gels are included to show the collapse (i.e., decrease in pore size) that would normally occur during conventional drying. Nomeally, as a gel is dried, it undergoes a monotonic decrease in size.

However, by capping the surface sites in accordance with the invention, our process actually results in a expansion during the final stages of drying as shown in FIG. 4. In carrying out processing in accordance with the invention, the degree of this shrinkage and subsequent recovery in sample size is a function of the surface tension of the fluid and fluid-pore wall contact angle. After surface rnodification, the gels are hydrophobic as compared to conventional gels which are hydrophillic. This is illustrated in FIG. 5. A significant external pressure is required to force water into the pores of the surface modified, ambient pressure gels. For a conventional unmodified aerogel or xerogel, water would rapidly wick into the gel at ambient pressure destroying its mechanical and insulating properties.

In order to demonstrate that the capillary 2pressure in surface modified gels made via this process have a lower capillary pressure during drying, imbibition experiments were performed on both modified and unmodified samples. In this experiment, a dried gel is brought into contact with the fluid. The fluid wicks into the dried gel due to capillary pressure. From the rate of uptake, the contact angle is determined. FIG. 6 is a plot of imbibition uptake curves for n-hexane. If the contact angle for both samples was the same, the slopes of the curves would match. The final height is different for the samples because different sample lengths were employed. Contact angles for four different fluids and both modified and not modified gels are shown in Table 1. The contact angle is consistently nearer to 90.degree. (lower capillary pressure) for the surface modified gels.

Accordingly, it is seen that the invention advantageously provides an extremely low density finely pored gel through an inexpensive process not requiring high pressure, having beneficial and useful insulating and other properties.