- Email: [email protected]
Inorganic and Inorganic–Organic Aerogels
Inorganic and Inorganic–Organic Aerogels Aerogels are highly porous solid materials with very low densities and high specific surface areas. Their structure consists of a filigrane solid network with open, cylindrical, branched mesopores. This results in interesting physical properties, such as extremely low thermal conductivity and low sound velocity, combined with high optical transparency. Aerogels can be obtained as monoliths, granulates, films, or powders. There are several review articles (Ayen and Iacobucci 1988, Fricke and Emmerling 1992, Gesser and Goswami 1989, Heinrich et al. 1995, Hu$ sing and Schubert 1998) and symposium proceedings on aerogels (Fricke 1986, 1992, Pekala and Hrubesch 1995, Phalippou and Vacher 1998).
1.
Network Formation
A gel consists of a sponge-like solid network, the pores of which are filled with a gas or a liquid. When gels are prepared from solutions of molecular precursors (through the sol stage), the pores are filled with the solvent (‘‘wet’’ gels). Aerogels are obtained when the pore liquid in the wet gel is replaced by air without decisively altering the highly porous network structure or the volume of the gel body. In contrast, xerogels (see Xerogels) are formed by conventional drying of the wet gels with concomitant large shrinkage (and mostly destruction) of the gel body. The procedures for the preparation of xerogels and aerogels are the same up to the wet gel stage, but the drying process is different. However, because the properties of aerogels very critically depending on the structure of the gel network, the chemical parameters governing the network formation have to be controlled more deliberately than required for most xerogels. Inorganic or inorganic–organic gels are prepared by sol–gel processing (Brinker and Scherer 1990) from purely inorganic precursors (hydrolyzable metal salts or oxide sols) or from metal or semi-metal alkoxides and their organically substituted derivatives (see Sol–Gel Reactions : Theory). The network and thus the pore structure of gels is formed by the aggregation of small primary colloidal or polymeric (sol) particles with typical diameters of 1–3 nm. In colloidal gels, dense colloidal primary particles are interconnected like a string of pearls. In polymeric gels, linear or branched polymer chains are formed by condensation of small clusters (the oligomeric species formed during sol–gel processing). There are several theoretical models for the aggregation of the primary particles to three-dimensional networks (Brinker and Scherer 1990, Schaefer 1989). Aerogel networks are usually formed from colloidal particles. However, each aerogel has its own structural
characteristics because the microstructure strongly depends on the preparation conditions. 1.1 Silica-based Aerogels The term ‘‘SiO gel’’ (correspondingly for other gels) is # used to characterize the type of inorganic skeleton. ‘‘SiO gels’’ and the corresponding aerogels often have # the composition SiOx(OH)y or, if they are prepared from alkoxides, SiOx(OH)y(OR)z, where the values of y and z can be rather high. Most laboratory preparations of silica aerogels use tetraalkoxysilanes (Si(OR) ) as the silica source. The % silicate solutions (water less expensive aqueous sodium glass) are used for larger-scale technical applications. The sodium silicate solution is ion-exchanged and the resulting silicic acid solution gelled by lowering the pH. The kind of inorganic network formed by sol–gel processing depends to a large extent on the relative rates of the hydrolysis and condensation reactions. Hydrolysis is the rate-determining step under basic conditions, where reaction at central silicon atoms of an oligomeric unit is favored. The network of the resulting colloidal gels consists of big particles and large pores. The clusters mainly grow by the condensation of monomers, because condensation of clusters is relatively unfavorable. Acidic conditions (pH l 2–5) favor hydrolysis, and condensation is the rate-determining step. A great number of monomers or small oligomers with reactive Si–OH groups is simultaneously formed. Reactions at terminal silicon atoms are favored. This results in polymer-like networks with small pores. Complete removal of the pore liquid from the smaller pores in polymeric gels is more difficult, resulting in a larger shrinkage during drying. For this reason, silica aerogels are usually prepared by basecatalyzed reaction of Si(OR) . % A modification of this procedure is to prehydrolyze Si(OR) with a small amount of water under acidic % conditions. In a second step, a defined amount of aqueous acid or base is added. Base catalysis in the second step results in a stiffening of the network that stabilizes the gels. This two-step procedure allows a more deliberate control of the microstructure of the SiO gels. # macroscopic properties of SiO aerogels differ The widely, due to structural differences.# Silica aerogels with bulk densities as low as 0.003 g cm−$ (99.8% porosity) have been prepared. Typical values are in the range 0.1–0.2 g cm−$ (0.85–0.90% porosity). Skeletal densities are in the range 1.7–2.1 g cm−$, mean pore diameters are in the range 20–150 nm, and specific surface areas are in the range 100–1600 m# g−". Silica aerogels are always amorphous. The spectrum of properties can be widened by modification of silica aerogels with organic entities. For example, the hydrophobicity and the elastic 1
Inorganic and Inorganic–Organic Aerogels properties of SiO aerogels are improved by incor# groups, and new applications can poration of organic be envisioned by the integration of functional organic moieties. Any modification should retain the gel network and the pore structure, because the interesting physical properties of aerogels result from these. Possibilities of postsynthesis doping or modification of aerogels with organic compounds are limited. Therefore, the organic groups have to be incorporated during sol–gel processing. Embedding molecules or polymers in gels is achieved by dissolving them in the precursor solution. The gel matrix is formed around them and traps them. The doped wet gels can be converted to aerogels. However, the probability is very high that the organic groups are leached out during the drying process. A more general route for the organic modification is to use hydrolyzable and condensable precursors for sol–gel processing in which the organic group is covalently attached to the silicon atom and will therefore be bonded to the inorganic network. Silica aerogels modified by non-functional or functional organic groups are prepared by sol–gel processing of RhSi(OR) \Si(OR) mixtures followed by supercritical $ wet gels. % The process can be controlled in drying of the such a way that the organic groups cover the inner surface of the gel network without influencing its basic structure.
1.2 Metal Oxide Aerogels The principles for network formation of non-silicate inorganic gels are the same as for SiO gels. Aqueous salt solutions or metal alkoxides are# employed for sol–gel processing. Metal alkoxides are much more reactive towards water than alkoxysilanes. This is due to the lower electronegativity and higher Lewis acidity as well as the possibility of increasing the coordination number. Whereas the reactivity of alkoxysilanes has to be promoted by catalysts, the reaction rates of metal alkoxides must be moderated to obtain gels instead of precipitates. The most common method is the substitution of part of the alkoxy groups by bidentate anionic ligands, such as carboxylates, β-diketonates, etc. Unlike silica aerogels, their non-silicate counterparts can also form crystalline primary particles. As a general rule, the crystalline portion is favored by a large excess of water in the hydrolysis reaction. For example, TiO aerogels can be prepared totally amor# a network of mesoparticles (about phous or with 50 nm in diameter) formed by aggregation of crystalline anatase nanoparticles of about 5 nm diameter. Amorphous and partially crystalline ZrO aerogels # fractal are found to have a branched, polymer-like structure consisting of small primary particles with average diameters of 5.2 nm (partially crystalline) or 2.5 nm (amorphous). 2
One-component oxide aerogels include titania, alumina, and zirconia (mainly prepared from the propoxides and butoxides), V O (prepared from # &CrX , X l NO , Cl, VO(OR) ), Cr O (prepared from $ # $ $ ), or OAc), Fe O (prepared from FeCl$ or Fe(acac) # $ $ $ MoO ( prepared from MoO (acac) ), and Nb O $ # # # & (prepared from Nb(OEt) ). & Mixed-metal, binary, or ternary aerogels can be prepared from the same types of precursors. Typical examples are TiO \SiO , mullite, Fe O \SiO , # # PbO\Al O , PbO\ZrO # $ #, Fe O \Al O , V O \MgO, # $ # $ # & # $ # BaO\Al O , Li O\B O , PbTiO , NiO\SiO \MgO, # $ # Many # $of these $were prepared # and cordierite. for catalytic applications (Schneider and Baiker 1995, 1997; Pajonk 1991, 1997). When mixtures of two or more precursors with very dissimilar hydrolysis and condensation rates are reacted, the microstructure of the product may become rather heterogeneous, and phase separation may even occur. The faster reacting compound may form sol particles, which are coated by the slower reacting component. This effect can be avoided by moderating the reaction rate of the faster reacting component by the already discussed bidentate ligands. Alternatively, the slower reacting precursor can be prehydrolyzed and the faster reacting component added afterwards. In mixed-oxide aerogels, the SiO and Al O # crystalline # $ portions are always amorphous. However, aluminates are observed (e.g., NiAl O ). The struc# % tural data and properties of non-silicate aerogels depend very strongly on the particular system and on the preparation conditions. Typical densities are in the range 0.1–0.3 g cm−$, and typical porosities in the range 80–95%. Pore radii range from 1 to 25 nm, specific surface areas from 80 to 800 m# g−", and primary particle diameters from 2 to 50 nm.
1.3 Metal–Aerogel Composites A second metal component in the gels may not necessarily yield a binary oxide aerogel. The use of alcohols for supercritical drying can, under the applied conditions, result in the reduction of oxides to metals. Metal particles can also be generated by subsequent reduction of the aerogels with hydrogen at high temperatures or by mixing hydrogen into the autoclave during supercritical drying. Postsynthesis doping of aerogels with metals is achieved by impregnation with the alcoholic solution of a suitable metal salt. The solvent has then to be removed again by another supercritical drying step in order to prevent destruction of the aerogel network (two-step process). Composites prepared by this approach include Cu\SiO , Cu\MgO, Pt\MoO , # $ Pd\Al O , and Pd\CeO . Alternatively, suitable metal # $ # compounds can be mixed into the precursor solution, which results in the incorporation of the metal compounds in the gel during sol–gel processing.
Inorganic and Inorganic–Organic Aerogels Examples of metal-containing aerogels prepared by this approach include Pt\TiO , Pt\Al O , Pd\Al O , # # $ # $ V\SiO , and Cu\SiO . # # 2. Drying Methods The evaporation of the liquid from a wet gel is a very complex process. When wet gels are conventionally dried, the slower shrinkage of the network in the interior of the gel body results in a pressure gradient. Furthermore, the meniscus of the liquid drops faster in larger pores. The walls between pores of different size are therefore subjected to uneven stress and cracking (Brinker and Scherer 1990). The capillary forces exerted by the meniscus of the pore liquid and the pressure gradient exerted by the large shrinkage of the network are the main reasons for the collapse of the filigrane structure upon conventional drying. For the production of aerogels, methods must be applied which conserve the pore structure of the wet gels.
2.1 Supercritical Drying In this procedure, the pore liquid is put into the supercritical state, where the liquid\gas interface disappears. The wet gel is placed in an autoclave and both the temperature and the pressure are adjusted to values above the critical point of the corresponding solvent ( pc, Tc). The fluid is then slowly vented at constant temperature, while the pressure drops. When ambient pressure is reached, the vessel is cooled to room temperature. Often the vessel is pre-pressurized by nitrogen to avoid evaporation of the solvent. The gas\liquid phase boundary must not be crossed during the drying process. The pore liquid itself, which is usually an alcohol or acetone from the wet gel preparation, can be put into the supercritical state; about 250 mC and 7000 kPa are required. Problems arise from the combination of high temperatures and high pressures, and the flammability of these solvents. Furthermore, rearrangement reactions in the gel network are highly probable because of the high temperatures. As a consequence, the resulting aerogels have a lower specific surface area, a more narrow pore radii distribution (elimination of microporosity), and a stiffer network. Organic groups may be destroyed during drying. Phase separation and loss of stoichiometry or crystallization may occur in multicomponent gels. An alternative is the use of liquid CO . This has the advantage of a low critical temperature #(Tc l 31.1 mC) at a moderate critical pressure ( pc l 7392 kPa). However, a time-consuming solvent exchange is necessary before the actual drying step. Another requirement is the miscibility of the pore liquid with carbon dioxide. Structural changes only take place to a minor extent with CO as the supercritical fluid. #
2.2 Freeze Drying Another possible way to avoid the gas\liquid interface during drying is by freeze drying. The pore liquid is frozen and then sublimed in acuo. However, the aging period has to be extended to stabilize the gel network, the solvent must be exchanged by another with a low thermal expansion coefficient and a high pressure of sublimation, and low freezing temperatures have to be achieved by addition of salts. Another disadvantage is that the network may be destroyed by crystallization of the solvent in the pores. Therefore, only powders are obtained.
2.3 Ambient Pressure Drying Drying of the gels at ambient pressure requires strengthening of the network by exact control of the aging conditions to avoid its collapse. One approach involves a series of solvent exchange processes and a modification of the inner surface. The water\alcohol mixture in the pores of the gel is first exchanged for a water-free solvent, and the Si–OH groups at the inner surface are silylated. The gel surface is thus hydrophobized and its reactivity reduced. In the first stage of the drying process the gel shrinks strongly, as expected. However, no irreversible narrowing of the pores by formation of Si–O–Si bonds is possible because of the silylation. Therefore, the gel expands to nearly its original size in the final stage of the drying process (‘‘spring-back’’ effect). The network of the modified gels must be stable enough to tolerate a reversible shrinkage of up to 28% of its original volume. In another strategy, the strength and stiffness of the network is increased by aging the wet gels in solutions of tetraalkoxysilanes in aqueous alcohols. The monomers condense in the smallest pores and at the particle necks, the microporosity of the gels is lost, and shrinkage during drying can be completely avoided. The gels can be dried at ambient pressure at temperatures of 20–180 mC due to the stiffer network. With the ambient pressure drying technique for the preparation of aerogels, a simpler and therefore less expensive preparation method is available, which makes the industrial application of aerogels economically more interesting. Ambient pressure drying also facilitates the possibility of preparing aerogel films.
3. Postsynthesis Modification of Aerogels Postsynthesis treatment with a liquid is difficult because wet gels are again formed. Doping with metal salts by impregnation has already been mentioned in Sect. 1.3. Another, more general possibility for the post3
Inorganic and Inorganic–Organic Aerogels synthesis modification of aerogels is their reaction with gaseous compounds. Organic compounds can be sublimed into the network as shown for a porphyrin derivative. Carbon was deposited on the inner surface of aerogels by decomposition of gaseous organic compounds to improve the thermal properties of aerogels. The chemical vapor infiltration method has been employed to prepare Si\SiO nanocomposites by reaction of SiH or HSiCl , and# for the deposition of $ iron or tungsten% particles from ferrocene, Fe(CO) or & W(CO) . In an oxidizing atmosphere, amorphous iron ' oxide particles in SiO aerogels were obtained which in subsequent reactions# were transformed to Fe O or $ % in Fe S -doped SiO aerogels. WN or WS particles * "! # # aerogels were formed by post-treatment of the tungsten-doped aerogels with ammonia or sulfur. The coating of the inner aerogel surface with carbon structures can also be achieved by pyrolysis of organic groups covalently bonded to the gel network.
4. Properties A big problem with unmodified SiO aerogels, par# ticularly for technical uses, is their long-term stability in humid atmospheres. Owing to the large number of silanol groups on their inner surface, adsorption and capillary condensation of water and eventually cracking of the gel body by the resulting capillary forces results. Hydrophobation of the aerogels is achieved by silylation reactions via the gas phase, or by treatment of the wet gel with a silylating reagent or by using organically modified precursors during sol–gel processing. The optical properties of aerogels strongly depend on their chemical composition. SiO aerogels and the derived inorganic–organic hybrid# aerogels vary between transparent and translucent. They appear yellowish under light and bluish against a dark background due to Rayleigh scattering. Objects viewed through a silica aerogel tile appear slightly blurred. For most applications a high optical transmission is desirable. Improvements were, inter alia, achieved by optimizing the drying procedures and the synthesis conditions. Although the rigidity of aerogels is low due to their structure and the brittleness of the SiO particles, it is sufficient for most applications. The# compressive strength of monolithic SiO aerogels is between 0.15 # on the density, with an and 0.30 N mm−#, depending elastic compression of about 2–4%. The rigidity in vacuum is distinctly higher. The tensile strength is about 0.020 N mm−# and the modulus of elasticity 0.002–100 MPa. One of the decisive parameters for mechanical properties is the connection of the primary particles of the gel network. Silica aerogels with covalently bonded 4
organic groups have improved elastic properties because the organic groups at the surface of the particles impair stiffening of the particle necks by Ostwald ripening. The sound velocities in SiO aerogels of 100– 300 m s−" are among the lowest for#inorganic solids due the particular network structure. SiO aerogels have the lowest acoustic impedance of all #solid materials (Z l 104–105 kg m−# s−"). Aerogels have the lowest thermal conductivities of all solids (0.017–0.021 W m−" K−" at 300 K in air). The solid conductivity portion increases with increasing bulk density, whereas the reverse is true for gas and radiative transport. Heat transport via the gas phase can be avoided by evacuation of the system. For example, an evacuated SiO aerogel at about 300 K shows a thermal conductivity# of only 0.010 W m−" K−". Reduction of the radiative heat transport is important for high-temperature applications of aerogels, because the radiation maximum is then in a wavelength range in which SiO does not sufficiently absorb (3–5 µm). # by doping the aerogels with infrared This is achieved opacifiers, such as carbon soot or TiO . #
5. Applications SiO aerogels are the best investigated aerogels with # to their preparation, structure, and properties, regard and they can also be prepared at lower cost than other aerogels. There are already commercial applications. Non-silicate aerogels increasingly play an important role, particularly as novel catalysts. SiO aerogels are installed in several Cherenkov # worldwide. Their refraction indices (1.007– detectors 1.024) are in a range not covered by the previously used compressed gases or liquids. Aerogels as solid materials additionally allow easier construction of the detectors. Aerogels belong to the best thermal insulation materials currently available. Additional advantages are their nonflammability and their transparency. However, since the production of large aerogel panels is still a big problem, conventional window systems cannot be equipped. The commonly prepared granulate, which is not transparent due to the roughness of its surface, can only be used for ‘‘daylight windows’’ (e.g., bathroom, staircase, or ceiling windows). In addition to transparent thermal insulation, applications for a variety of other insulation problems are conceivable, for example in cooling or heating systems, high-temperature batteries, insulation for piping, heat and cold storage appliances, automotive exhaust pipes, transport vehicles, and vessels. Aerogels can also be utilized for passive use of solar energy, for example for paneling of house walls or for coating solar-energy collectors. A layer of the transparent aerogel allows the penetration of the sun’s radiation to
Inorganic and Inorganic–Organic Aerogels the blackened wall, but not the escape of the heat generated there. All the advantages of sol–gel processing for preparing catalysts or catalyst supports—in particular, the controllable dispersion of the active component and the possibility of deliberately influencing the microstructure—also apply for aerogels. Additionally, the very high and accessible inner surface, the large pore volumes, an excellent temperature stability, and the high selectivities in catalytic processes predestines them for these applications. Nearly all oxides relevant for catalytic applications can be prepared as aerogels, including binary and ternary compositions, mixtures of metal oxides, and metal particles on oxide carriers. The investigated systems are summarized in special reviews (Schneider and Baiker 1995, 1997, Pajonk 1991, 1997). The high porosity and very large surface area of silica aerogels can be utilized for applications as absorbing and encapsulation media, and hydrogen fuel storage. Partially sintered aerogels that show a strengthened texture can be used for the storage, thickening, or transport of liquids, for example rocket fuels. Aerogels can be sintered at low temperatures and can therefore be processed to extremely pure and totally homogeneous glasses. The inner surface area and the porosity decrease during sintering and the pore structure can be modified in a controlled way by partial sintering. This was used, for example, for producing gas filters with pores in the range of 20–100 nm. Doping of aerogels with dyes or rare earth elements allows the fabrication of glass for various applications including lasers and radioluminescent light sources. Aerogel films can be exploited as optical coatings for solar cells because of their low refractive index, and can thus replace the currently used glass layers with high refractive indices. More light reaches the active surface because of the lower Fresnel scattering losses. These films have also been applied as coatings for fiber optics to improve the light collection and propagation efficiency. Aerogels are excellent materials for low dielectric constant applications. While bulk aerogels can be used for microwave electronics and high-voltage insulators, thin aerogel films can compete with existing technologies in microelectronics, where the low dielectric constant of 2.0 plays a particularly important role. In addition, the dielectric constant is easily controlled because it is directly correlated with the porosity and thus the density of the materials. Some metal oxide aerogels exhibit superconducting, thermoelectric, or piezoelectric behavior. Aerogels are efficient sound insulation materials. They have very low acoustic impedances and can therefore be used as λ\4 layers for matching the high acoustic impedance of piezoceramics to the low impedance of air.
SiO aerogels are currently one of the standard media#for collecting cosmic dust. They are fixed on the outside of spacecraft. Aerogels have the advantage that they are extremely light. The unique mesostructure enables a ‘‘soft landing’’ of very fast extraterrestrial particles hitting with a velocity of more than 3 km s−", and also allows their later investigation by optical methods.
6. Future Perspectives The unique optical, thermal, acoustic, and mechanical properties of aerogels originate from the combination of a solid matrix (the chemical composition of which can be modified) and nanometer-sized pores filled with air. There is a direct connection between the chemistry of the sol–gel process and the structure of the gels on one hand, and between the structure and the properties of the aerogels on the other. The most important area for the application of aerogels is thermal insulation, while important acoustic insulation applications are emerging. There is no doubt about the physical and ecological advantages (nontoxic, nonflammable, easily disposed) of SiO aerogels compared with most other materials on the# market. Apart from special applications in which material costs only play a minor role, the rather high price of supercritically dried aerogels has prevented a broader range of applications. The new ambient pressure drying techniques will probably make the technical preparation much cheaper and will thus make aerogels more competitive. They will also allow the preparation of aerogels with standard laboratory equipment.
Bibliography Ayen R J, Iacobucci P A 1988 Metal oxide aerogel preparation by supercritical extraction. Re. Chem. Eng. 5, 157–98 Brinker C J, Scherer G W 1990 Sol–Gel Science. Academic Press, New York Fricke J 1986 (ed.) Proc. 1st Int. Symp. Aerogels. Springer Proc. Phys. 6. Springer, Berlin Fricke J 1992 Proc. 3rd Int. Symp. Aerogels. J. Non-Cryst. Solids 145 Fricke J, Emmerling A 1992 Aerogels—preparation, properties, applications. Struct. Bonding 77, 37–87 Gesser, H D, Goswami P C 1989 Aerogels and related porous materials. Chem. Re. 89, 765–88 Heinrich T, Klett U, Fricke J 1995 Aerogels—nanoporous materials part I: sol–gel process and drying of gels. J. Porous Mater. 1, 7–17 Hu$ sing N, Schubert U 1998 Aerogels—airy materials: chemistry, structure, and properties. Angew. Chem. Int. Ed. Engl. 37, 23–45 Pajonk G M 1991 Aerogel catalysts. Appl. Catal. 72, 217–66 Pajonk G M 1997 Catalytic aerogels. Catal. Today 35, 319–37 Pekala R W, Hrubesch L W 1995 Proc. 4th Int. Symp. Aerogels. J. Non-Cryst. Solids 186
5
Inorganic and Inorganic–Organic Aerogels Phalippou J, Vacher R 1998 Proc. 5th Int. Symp. Aerogels. J. Non-Cryst. Solids 225 Schaefer D W 1989 Polymers, fractals, and ceramic materials. Science 243, 1023–7 Schneider M, Baiker A 1995 Aerogels in catalysis. Catal. Re. Sci. Eng. 37, 515–56
Schneider M, Baiker A 1997 Titania-based aerogels. Catal. Today 35, 339–65 Vacher R, Phalippou J, Pelous J Woignier T 1989 Proc. 2nd Int. Symp. Aerogels. Re. Phys. Appl. C4(Suppl. 4)
U. Schubert and N. Hu$ sing
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 4096–4101 6
1.
Network Formation
A gel consists of a sponge-like solid network, the pores of which are filled with a gas or a liquid. When gels are prepared from solutions of molecular precursors (through the sol stage), the pores are filled with the solvent (‘‘wet’’ gels). Aerogels are obtained when the pore liquid in the wet gel is replaced by air without decisively altering the highly porous network structure or the volume of the gel body. In contrast, xerogels (see Xerogels) are formed by conventional drying of the wet gels with concomitant large shrinkage (and mostly destruction) of the gel body. The procedures for the preparation of xerogels and aerogels are the same up to the wet gel stage, but the drying process is different. However, because the properties of aerogels very critically depending on the structure of the gel network, the chemical parameters governing the network formation have to be controlled more deliberately than required for most xerogels. Inorganic or inorganic–organic gels are prepared by sol–gel processing (Brinker and Scherer 1990) from purely inorganic precursors (hydrolyzable metal salts or oxide sols) or from metal or semi-metal alkoxides and their organically substituted derivatives (see Sol–Gel Reactions : Theory). The network and thus the pore structure of gels is formed by the aggregation of small primary colloidal or polymeric (sol) particles with typical diameters of 1–3 nm. In colloidal gels, dense colloidal primary particles are interconnected like a string of pearls. In polymeric gels, linear or branched polymer chains are formed by condensation of small clusters (the oligomeric species formed during sol–gel processing). There are several theoretical models for the aggregation of the primary particles to three-dimensional networks (Brinker and Scherer 1990, Schaefer 1989). Aerogel networks are usually formed from colloidal particles. However, each aerogel has its own structural
characteristics because the microstructure strongly depends on the preparation conditions. 1.1 Silica-based Aerogels The term ‘‘SiO gel’’ (correspondingly for other gels) is # used to characterize the type of inorganic skeleton. ‘‘SiO gels’’ and the corresponding aerogels often have # the composition SiOx(OH)y or, if they are prepared from alkoxides, SiOx(OH)y(OR)z, where the values of y and z can be rather high. Most laboratory preparations of silica aerogels use tetraalkoxysilanes (Si(OR) ) as the silica source. The % silicate solutions (water less expensive aqueous sodium glass) are used for larger-scale technical applications. The sodium silicate solution is ion-exchanged and the resulting silicic acid solution gelled by lowering the pH. The kind of inorganic network formed by sol–gel processing depends to a large extent on the relative rates of the hydrolysis and condensation reactions. Hydrolysis is the rate-determining step under basic conditions, where reaction at central silicon atoms of an oligomeric unit is favored. The network of the resulting colloidal gels consists of big particles and large pores. The clusters mainly grow by the condensation of monomers, because condensation of clusters is relatively unfavorable. Acidic conditions (pH l 2–5) favor hydrolysis, and condensation is the rate-determining step. A great number of monomers or small oligomers with reactive Si–OH groups is simultaneously formed. Reactions at terminal silicon atoms are favored. This results in polymer-like networks with small pores. Complete removal of the pore liquid from the smaller pores in polymeric gels is more difficult, resulting in a larger shrinkage during drying. For this reason, silica aerogels are usually prepared by basecatalyzed reaction of Si(OR) . % A modification of this procedure is to prehydrolyze Si(OR) with a small amount of water under acidic % conditions. In a second step, a defined amount of aqueous acid or base is added. Base catalysis in the second step results in a stiffening of the network that stabilizes the gels. This two-step procedure allows a more deliberate control of the microstructure of the SiO gels. # macroscopic properties of SiO aerogels differ The widely, due to structural differences.# Silica aerogels with bulk densities as low as 0.003 g cm−$ (99.8% porosity) have been prepared. Typical values are in the range 0.1–0.2 g cm−$ (0.85–0.90% porosity). Skeletal densities are in the range 1.7–2.1 g cm−$, mean pore diameters are in the range 20–150 nm, and specific surface areas are in the range 100–1600 m# g−". Silica aerogels are always amorphous. The spectrum of properties can be widened by modification of silica aerogels with organic entities. For example, the hydrophobicity and the elastic 1
Inorganic and Inorganic–Organic Aerogels properties of SiO aerogels are improved by incor# groups, and new applications can poration of organic be envisioned by the integration of functional organic moieties. Any modification should retain the gel network and the pore structure, because the interesting physical properties of aerogels result from these. Possibilities of postsynthesis doping or modification of aerogels with organic compounds are limited. Therefore, the organic groups have to be incorporated during sol–gel processing. Embedding molecules or polymers in gels is achieved by dissolving them in the precursor solution. The gel matrix is formed around them and traps them. The doped wet gels can be converted to aerogels. However, the probability is very high that the organic groups are leached out during the drying process. A more general route for the organic modification is to use hydrolyzable and condensable precursors for sol–gel processing in which the organic group is covalently attached to the silicon atom and will therefore be bonded to the inorganic network. Silica aerogels modified by non-functional or functional organic groups are prepared by sol–gel processing of RhSi(OR) \Si(OR) mixtures followed by supercritical $ wet gels. % The process can be controlled in drying of the such a way that the organic groups cover the inner surface of the gel network without influencing its basic structure.
1.2 Metal Oxide Aerogels The principles for network formation of non-silicate inorganic gels are the same as for SiO gels. Aqueous salt solutions or metal alkoxides are# employed for sol–gel processing. Metal alkoxides are much more reactive towards water than alkoxysilanes. This is due to the lower electronegativity and higher Lewis acidity as well as the possibility of increasing the coordination number. Whereas the reactivity of alkoxysilanes has to be promoted by catalysts, the reaction rates of metal alkoxides must be moderated to obtain gels instead of precipitates. The most common method is the substitution of part of the alkoxy groups by bidentate anionic ligands, such as carboxylates, β-diketonates, etc. Unlike silica aerogels, their non-silicate counterparts can also form crystalline primary particles. As a general rule, the crystalline portion is favored by a large excess of water in the hydrolysis reaction. For example, TiO aerogels can be prepared totally amor# a network of mesoparticles (about phous or with 50 nm in diameter) formed by aggregation of crystalline anatase nanoparticles of about 5 nm diameter. Amorphous and partially crystalline ZrO aerogels # fractal are found to have a branched, polymer-like structure consisting of small primary particles with average diameters of 5.2 nm (partially crystalline) or 2.5 nm (amorphous). 2
One-component oxide aerogels include titania, alumina, and zirconia (mainly prepared from the propoxides and butoxides), V O (prepared from # &CrX , X l NO , Cl, VO(OR) ), Cr O (prepared from $ # $ $ ), or OAc), Fe O (prepared from FeCl$ or Fe(acac) # $ $ $ MoO ( prepared from MoO (acac) ), and Nb O $ # # # & (prepared from Nb(OEt) ). & Mixed-metal, binary, or ternary aerogels can be prepared from the same types of precursors. Typical examples are TiO \SiO , mullite, Fe O \SiO , # # PbO\Al O , PbO\ZrO # $ #, Fe O \Al O , V O \MgO, # $ # $ # & # $ # BaO\Al O , Li O\B O , PbTiO , NiO\SiO \MgO, # $ # Many # $of these $were prepared # and cordierite. for catalytic applications (Schneider and Baiker 1995, 1997; Pajonk 1991, 1997). When mixtures of two or more precursors with very dissimilar hydrolysis and condensation rates are reacted, the microstructure of the product may become rather heterogeneous, and phase separation may even occur. The faster reacting compound may form sol particles, which are coated by the slower reacting component. This effect can be avoided by moderating the reaction rate of the faster reacting component by the already discussed bidentate ligands. Alternatively, the slower reacting precursor can be prehydrolyzed and the faster reacting component added afterwards. In mixed-oxide aerogels, the SiO and Al O # crystalline # $ portions are always amorphous. However, aluminates are observed (e.g., NiAl O ). The struc# % tural data and properties of non-silicate aerogels depend very strongly on the particular system and on the preparation conditions. Typical densities are in the range 0.1–0.3 g cm−$, and typical porosities in the range 80–95%. Pore radii range from 1 to 25 nm, specific surface areas from 80 to 800 m# g−", and primary particle diameters from 2 to 50 nm.
1.3 Metal–Aerogel Composites A second metal component in the gels may not necessarily yield a binary oxide aerogel. The use of alcohols for supercritical drying can, under the applied conditions, result in the reduction of oxides to metals. Metal particles can also be generated by subsequent reduction of the aerogels with hydrogen at high temperatures or by mixing hydrogen into the autoclave during supercritical drying. Postsynthesis doping of aerogels with metals is achieved by impregnation with the alcoholic solution of a suitable metal salt. The solvent has then to be removed again by another supercritical drying step in order to prevent destruction of the aerogel network (two-step process). Composites prepared by this approach include Cu\SiO , Cu\MgO, Pt\MoO , # $ Pd\Al O , and Pd\CeO . Alternatively, suitable metal # $ # compounds can be mixed into the precursor solution, which results in the incorporation of the metal compounds in the gel during sol–gel processing.
Inorganic and Inorganic–Organic Aerogels Examples of metal-containing aerogels prepared by this approach include Pt\TiO , Pt\Al O , Pd\Al O , # # $ # $ V\SiO , and Cu\SiO . # # 2. Drying Methods The evaporation of the liquid from a wet gel is a very complex process. When wet gels are conventionally dried, the slower shrinkage of the network in the interior of the gel body results in a pressure gradient. Furthermore, the meniscus of the liquid drops faster in larger pores. The walls between pores of different size are therefore subjected to uneven stress and cracking (Brinker and Scherer 1990). The capillary forces exerted by the meniscus of the pore liquid and the pressure gradient exerted by the large shrinkage of the network are the main reasons for the collapse of the filigrane structure upon conventional drying. For the production of aerogels, methods must be applied which conserve the pore structure of the wet gels.
2.1 Supercritical Drying In this procedure, the pore liquid is put into the supercritical state, where the liquid\gas interface disappears. The wet gel is placed in an autoclave and both the temperature and the pressure are adjusted to values above the critical point of the corresponding solvent ( pc, Tc). The fluid is then slowly vented at constant temperature, while the pressure drops. When ambient pressure is reached, the vessel is cooled to room temperature. Often the vessel is pre-pressurized by nitrogen to avoid evaporation of the solvent. The gas\liquid phase boundary must not be crossed during the drying process. The pore liquid itself, which is usually an alcohol or acetone from the wet gel preparation, can be put into the supercritical state; about 250 mC and 7000 kPa are required. Problems arise from the combination of high temperatures and high pressures, and the flammability of these solvents. Furthermore, rearrangement reactions in the gel network are highly probable because of the high temperatures. As a consequence, the resulting aerogels have a lower specific surface area, a more narrow pore radii distribution (elimination of microporosity), and a stiffer network. Organic groups may be destroyed during drying. Phase separation and loss of stoichiometry or crystallization may occur in multicomponent gels. An alternative is the use of liquid CO . This has the advantage of a low critical temperature #(Tc l 31.1 mC) at a moderate critical pressure ( pc l 7392 kPa). However, a time-consuming solvent exchange is necessary before the actual drying step. Another requirement is the miscibility of the pore liquid with carbon dioxide. Structural changes only take place to a minor extent with CO as the supercritical fluid. #
2.2 Freeze Drying Another possible way to avoid the gas\liquid interface during drying is by freeze drying. The pore liquid is frozen and then sublimed in acuo. However, the aging period has to be extended to stabilize the gel network, the solvent must be exchanged by another with a low thermal expansion coefficient and a high pressure of sublimation, and low freezing temperatures have to be achieved by addition of salts. Another disadvantage is that the network may be destroyed by crystallization of the solvent in the pores. Therefore, only powders are obtained.
2.3 Ambient Pressure Drying Drying of the gels at ambient pressure requires strengthening of the network by exact control of the aging conditions to avoid its collapse. One approach involves a series of solvent exchange processes and a modification of the inner surface. The water\alcohol mixture in the pores of the gel is first exchanged for a water-free solvent, and the Si–OH groups at the inner surface are silylated. The gel surface is thus hydrophobized and its reactivity reduced. In the first stage of the drying process the gel shrinks strongly, as expected. However, no irreversible narrowing of the pores by formation of Si–O–Si bonds is possible because of the silylation. Therefore, the gel expands to nearly its original size in the final stage of the drying process (‘‘spring-back’’ effect). The network of the modified gels must be stable enough to tolerate a reversible shrinkage of up to 28% of its original volume. In another strategy, the strength and stiffness of the network is increased by aging the wet gels in solutions of tetraalkoxysilanes in aqueous alcohols. The monomers condense in the smallest pores and at the particle necks, the microporosity of the gels is lost, and shrinkage during drying can be completely avoided. The gels can be dried at ambient pressure at temperatures of 20–180 mC due to the stiffer network. With the ambient pressure drying technique for the preparation of aerogels, a simpler and therefore less expensive preparation method is available, which makes the industrial application of aerogels economically more interesting. Ambient pressure drying also facilitates the possibility of preparing aerogel films.
3. Postsynthesis Modification of Aerogels Postsynthesis treatment with a liquid is difficult because wet gels are again formed. Doping with metal salts by impregnation has already been mentioned in Sect. 1.3. Another, more general possibility for the post3
Inorganic and Inorganic–Organic Aerogels synthesis modification of aerogels is their reaction with gaseous compounds. Organic compounds can be sublimed into the network as shown for a porphyrin derivative. Carbon was deposited on the inner surface of aerogels by decomposition of gaseous organic compounds to improve the thermal properties of aerogels. The chemical vapor infiltration method has been employed to prepare Si\SiO nanocomposites by reaction of SiH or HSiCl , and# for the deposition of $ iron or tungsten% particles from ferrocene, Fe(CO) or & W(CO) . In an oxidizing atmosphere, amorphous iron ' oxide particles in SiO aerogels were obtained which in subsequent reactions# were transformed to Fe O or $ % in Fe S -doped SiO aerogels. WN or WS particles * "! # # aerogels were formed by post-treatment of the tungsten-doped aerogels with ammonia or sulfur. The coating of the inner aerogel surface with carbon structures can also be achieved by pyrolysis of organic groups covalently bonded to the gel network.
4. Properties A big problem with unmodified SiO aerogels, par# ticularly for technical uses, is their long-term stability in humid atmospheres. Owing to the large number of silanol groups on their inner surface, adsorption and capillary condensation of water and eventually cracking of the gel body by the resulting capillary forces results. Hydrophobation of the aerogels is achieved by silylation reactions via the gas phase, or by treatment of the wet gel with a silylating reagent or by using organically modified precursors during sol–gel processing. The optical properties of aerogels strongly depend on their chemical composition. SiO aerogels and the derived inorganic–organic hybrid# aerogels vary between transparent and translucent. They appear yellowish under light and bluish against a dark background due to Rayleigh scattering. Objects viewed through a silica aerogel tile appear slightly blurred. For most applications a high optical transmission is desirable. Improvements were, inter alia, achieved by optimizing the drying procedures and the synthesis conditions. Although the rigidity of aerogels is low due to their structure and the brittleness of the SiO particles, it is sufficient for most applications. The# compressive strength of monolithic SiO aerogels is between 0.15 # on the density, with an and 0.30 N mm−#, depending elastic compression of about 2–4%. The rigidity in vacuum is distinctly higher. The tensile strength is about 0.020 N mm−# and the modulus of elasticity 0.002–100 MPa. One of the decisive parameters for mechanical properties is the connection of the primary particles of the gel network. Silica aerogels with covalently bonded 4
organic groups have improved elastic properties because the organic groups at the surface of the particles impair stiffening of the particle necks by Ostwald ripening. The sound velocities in SiO aerogels of 100– 300 m s−" are among the lowest for#inorganic solids due the particular network structure. SiO aerogels have the lowest acoustic impedance of all #solid materials (Z l 104–105 kg m−# s−"). Aerogels have the lowest thermal conductivities of all solids (0.017–0.021 W m−" K−" at 300 K in air). The solid conductivity portion increases with increasing bulk density, whereas the reverse is true for gas and radiative transport. Heat transport via the gas phase can be avoided by evacuation of the system. For example, an evacuated SiO aerogel at about 300 K shows a thermal conductivity# of only 0.010 W m−" K−". Reduction of the radiative heat transport is important for high-temperature applications of aerogels, because the radiation maximum is then in a wavelength range in which SiO does not sufficiently absorb (3–5 µm). # by doping the aerogels with infrared This is achieved opacifiers, such as carbon soot or TiO . #
5. Applications SiO aerogels are the best investigated aerogels with # to their preparation, structure, and properties, regard and they can also be prepared at lower cost than other aerogels. There are already commercial applications. Non-silicate aerogels increasingly play an important role, particularly as novel catalysts. SiO aerogels are installed in several Cherenkov # worldwide. Their refraction indices (1.007– detectors 1.024) are in a range not covered by the previously used compressed gases or liquids. Aerogels as solid materials additionally allow easier construction of the detectors. Aerogels belong to the best thermal insulation materials currently available. Additional advantages are their nonflammability and their transparency. However, since the production of large aerogel panels is still a big problem, conventional window systems cannot be equipped. The commonly prepared granulate, which is not transparent due to the roughness of its surface, can only be used for ‘‘daylight windows’’ (e.g., bathroom, staircase, or ceiling windows). In addition to transparent thermal insulation, applications for a variety of other insulation problems are conceivable, for example in cooling or heating systems, high-temperature batteries, insulation for piping, heat and cold storage appliances, automotive exhaust pipes, transport vehicles, and vessels. Aerogels can also be utilized for passive use of solar energy, for example for paneling of house walls or for coating solar-energy collectors. A layer of the transparent aerogel allows the penetration of the sun’s radiation to
Inorganic and Inorganic–Organic Aerogels the blackened wall, but not the escape of the heat generated there. All the advantages of sol–gel processing for preparing catalysts or catalyst supports—in particular, the controllable dispersion of the active component and the possibility of deliberately influencing the microstructure—also apply for aerogels. Additionally, the very high and accessible inner surface, the large pore volumes, an excellent temperature stability, and the high selectivities in catalytic processes predestines them for these applications. Nearly all oxides relevant for catalytic applications can be prepared as aerogels, including binary and ternary compositions, mixtures of metal oxides, and metal particles on oxide carriers. The investigated systems are summarized in special reviews (Schneider and Baiker 1995, 1997, Pajonk 1991, 1997). The high porosity and very large surface area of silica aerogels can be utilized for applications as absorbing and encapsulation media, and hydrogen fuel storage. Partially sintered aerogels that show a strengthened texture can be used for the storage, thickening, or transport of liquids, for example rocket fuels. Aerogels can be sintered at low temperatures and can therefore be processed to extremely pure and totally homogeneous glasses. The inner surface area and the porosity decrease during sintering and the pore structure can be modified in a controlled way by partial sintering. This was used, for example, for producing gas filters with pores in the range of 20–100 nm. Doping of aerogels with dyes or rare earth elements allows the fabrication of glass for various applications including lasers and radioluminescent light sources. Aerogel films can be exploited as optical coatings for solar cells because of their low refractive index, and can thus replace the currently used glass layers with high refractive indices. More light reaches the active surface because of the lower Fresnel scattering losses. These films have also been applied as coatings for fiber optics to improve the light collection and propagation efficiency. Aerogels are excellent materials for low dielectric constant applications. While bulk aerogels can be used for microwave electronics and high-voltage insulators, thin aerogel films can compete with existing technologies in microelectronics, where the low dielectric constant of 2.0 plays a particularly important role. In addition, the dielectric constant is easily controlled because it is directly correlated with the porosity and thus the density of the materials. Some metal oxide aerogels exhibit superconducting, thermoelectric, or piezoelectric behavior. Aerogels are efficient sound insulation materials. They have very low acoustic impedances and can therefore be used as λ\4 layers for matching the high acoustic impedance of piezoceramics to the low impedance of air.
SiO aerogels are currently one of the standard media#for collecting cosmic dust. They are fixed on the outside of spacecraft. Aerogels have the advantage that they are extremely light. The unique mesostructure enables a ‘‘soft landing’’ of very fast extraterrestrial particles hitting with a velocity of more than 3 km s−", and also allows their later investigation by optical methods.
6. Future Perspectives The unique optical, thermal, acoustic, and mechanical properties of aerogels originate from the combination of a solid matrix (the chemical composition of which can be modified) and nanometer-sized pores filled with air. There is a direct connection between the chemistry of the sol–gel process and the structure of the gels on one hand, and between the structure and the properties of the aerogels on the other. The most important area for the application of aerogels is thermal insulation, while important acoustic insulation applications are emerging. There is no doubt about the physical and ecological advantages (nontoxic, nonflammable, easily disposed) of SiO aerogels compared with most other materials on the# market. Apart from special applications in which material costs only play a minor role, the rather high price of supercritically dried aerogels has prevented a broader range of applications. The new ambient pressure drying techniques will probably make the technical preparation much cheaper and will thus make aerogels more competitive. They will also allow the preparation of aerogels with standard laboratory equipment.
Bibliography Ayen R J, Iacobucci P A 1988 Metal oxide aerogel preparation by supercritical extraction. Re. Chem. Eng. 5, 157–98 Brinker C J, Scherer G W 1990 Sol–Gel Science. Academic Press, New York Fricke J 1986 (ed.) Proc. 1st Int. Symp. Aerogels. Springer Proc. Phys. 6. Springer, Berlin Fricke J 1992 Proc. 3rd Int. Symp. Aerogels. J. Non-Cryst. Solids 145 Fricke J, Emmerling A 1992 Aerogels—preparation, properties, applications. Struct. Bonding 77, 37–87 Gesser, H D, Goswami P C 1989 Aerogels and related porous materials. Chem. Re. 89, 765–88 Heinrich T, Klett U, Fricke J 1995 Aerogels—nanoporous materials part I: sol–gel process and drying of gels. J. Porous Mater. 1, 7–17 Hu$ sing N, Schubert U 1998 Aerogels—airy materials: chemistry, structure, and properties. Angew. Chem. Int. Ed. Engl. 37, 23–45 Pajonk G M 1991 Aerogel catalysts. Appl. Catal. 72, 217–66 Pajonk G M 1997 Catalytic aerogels. Catal. Today 35, 319–37 Pekala R W, Hrubesch L W 1995 Proc. 4th Int. Symp. Aerogels. J. Non-Cryst. Solids 186
5
Inorganic and Inorganic–Organic Aerogels Phalippou J, Vacher R 1998 Proc. 5th Int. Symp. Aerogels. J. Non-Cryst. Solids 225 Schaefer D W 1989 Polymers, fractals, and ceramic materials. Science 243, 1023–7 Schneider M, Baiker A 1995 Aerogels in catalysis. Catal. Re. Sci. Eng. 37, 515–56
Schneider M, Baiker A 1997 Titania-based aerogels. Catal. Today 35, 339–65 Vacher R, Phalippou J, Pelous J Woignier T 1989 Proc. 2nd Int. Symp. Aerogels. Re. Phys. Appl. C4(Suppl. 4)
U. Schubert and N. Hu$ sing
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 4096–4101 6