In the Laboratory edited by
Advanced Chemistry Classroom and Laboratory
Joseph J. BelBruno Dartmouth College Hanover, NH 03755
Applications of the Sol–Gel Process Using Well-Tested Recipes A. Celzard* and J. F. Marêché Laboratoire de Chimie du Solide Minéral, UMR 7555, Université Henri Poincaré – Nancy I, BP 239, 54506 Vandoeuvrelès-Nancy, France; *
[email protected] Generally speaking, glass is defined as an amorphous body, solid at room temperature, obtained by melting together siliceous sand and alkali carbonates. Even if such a definition is valid in most cases, recent synthetic routes allow the preparation of glassy materials without melting (1). These sol–gel processes constitute an important part of so-called “soft chemistry” (2). The sol–gel process has many technological applications (3, 4), such as production of coatings with specific optical properties, organic–inorganic hybrid materials, and supports for culture media in biology. A wide literature is devoted to this increasingly important chemistry. Unfortunately, the procedures described are often complex and require specific devices or use highly moisturesensitive chemicals (such as alkali alcoholates), making them difficult to perform outside of a research laboratory. Besides, the reactions are often very long, taking tens or even hundreds of hours. For these reasons, the practical educational use of the sol–gel process is not easy. In this paper, we present a few simple recipes that explore several major applications of the sol–gel process. These are not our entirely original work, but rather selections, improvements, modifications, and completions of published procedures. Most of the published accounts are incomplete in the sense that small but very important details are not given. The success of the experiments is highly sensitive to many parameters, and hence the rapid manufacture of “nice” materials is not at all straightforward. This is why we feel that the proposed recipes are of practical use in teaching soft chemistry; they were tested and modified until we were quite satisfied with the results. The reactions take place within a few hours, thus producing observable materials in a short time. General Considerations about the Sol–Gel Process
Basic Chemical Reactions The principle of the sol–gel process is rather simple (3–6 ): a network of an oxide is progressively built through inorganic polymerization reactions at room or moderate temperature. Depending on the regularity of the macro-
Figure 1. Schematic picture of the sol–gel transition. Molecular species grow by polycondensation (sol) until a giant cluster is formed (gel).
854
molecular structure obtained, crystalline (e.g., quartz) or amorphous (e.g., glass) materials may be prepared. The usual molecular precursors are metallo-organic compounds such as alkoxides M(OR)n, where M is a metal or a metalloid and R is an alkyl group (R = CH3, C2H5, …). For example, tetraethylorthosilicate (TEOS), Si(OC2H5)4, is commonly used in the sol–gel synthesis of silica and glasses. Such chemicals are dispersed in a solvent (usually organic, e.g., ethyl alcohol) and react according to the well-known steps in polymer chemistry: Initiation. Corresponds to the hydrolysis of the alkoxide: M(OR)n + xH2O
M(OR) n-x (OH)x + xROH
The reactive bond M–OH, which is necessary for the continuation of the reaction, is formed during this step. Propagation. Condensation of the hydrolyzed species, with formation of bridging oxygens, according to two possible mechanisms: oxolation, which corresponds to a dehydration (i.e., the leaving group is H2O)— OR
OR H RO
M
O
+
HO
M
OR OR
RO
OR
OR
M
OR O
OR
M
OR
+ H2O
OR
or alcoxolation, which corresponds to a dealcoholation (i.e., the leaving group is ROH)— OR H RO
M OR
O
OR
+
RO
M
OR OH
RO
OR
M OR
OR O
M
OH
+ ROH
OR
At the end, every oxygen is bridging and hence a pure and highly homogeneous oxide network is obtained. Depending on the chemical nature of the precursors, the final material contains one or several metal elements.
Sol–Gel Transition Between the starting solution and the final solid, several intermediate steps occur during which sols or gels are formed, thus giving the name of the sol–gel process. The polycondensation of the precursors leads to bigger and bigger molecular species. First, a sol (a colloidal suspension of solid particles in a liquid) is obtained. Since the polymerization reactions are going on, the particles grow and coalesce to form clusters continuously increasing in size (see Fig. 1). After a time, a giant cluster appears, a macromolecule as large as the vessel in which it was formed. This is a gel (an alcogel, if the reactions take place in an alcohol): that is to say a semisolid system comprising two phases, solid and fluid, embedded in each other in such a way that the pores of the solid (filled with solvent) are of colloidal dimensions.
Journal of Chemical Education • Vol. 79 No. 7 July 2002 • JChemEd.chem.wisc.edu
E (arb u)
η (arb u)
In the Laboratory
gel point
Time Figure 2. Divergence of the viscosity, η, of the sol below the gel point. At the transition, η tends toward infinity and a nonzero elastic modulus E appears. Aging of the gel makes E increase considerably.
The gel point, marking the sol–gel transition, may be identified by continuously recording the viscosity (η) during the synthesis (3). The appearance of the incipient alcogel is evidenced by the divergence of η; above this critical point, an elastic modulus E may be measured (Fig. 2). Next, the gel strengthens progressively as the residual isolated clusters make bonds with the developing network, a phenomenon usually called aging. The growing number of bonds and the occurrence of dissolving–reprecipitation reactions (such as Ostwald ripening) make the elastic modulus increase with time. The gel then reaches the favorable conditions for which it becomes possible to dry it with the lowest number of cracks.
Drying of the Gel The alcogel represents the penultimate stage of the process. Depending on its thermal treatment, various materials may be obtained. The solvent may be evacuated according to two methods (3). 1. Evaporation. Owing to the capillary forces exerted by the solvent, elimination of the solvent induces the shrinkage of the gel. A xerogel is thus obtained, which has a volume 5 to 10 times lower than that of the starting alcogel. A subsequent thermal treatment may convert it into a dense ceramic (Fig. 3).
Figure 3. Drying of a gel by simple evaporation of the solvent. The shrinkage is very important and the resulting material is called a xerogel. Thermal treatment of the xerogel leads to a more dense ceramic.
A
supercritical fluid
C
Pc
P
liquid
2. Supercritical evacuation. The alcogel is placed in an autoclave and brought to temperature and pressure higher than Tc and Pc, respectively. (Tc and Pc are the coordinates of the critical point of the solvent in the [P,T ] phase diagram.) In these conditions, in the presence of a supercritical fluid, liquid–gas menisci and hence capillary stresses vanish. Once the critical point has been passed through, the temperature is kept constant while the supercritical fluid is evacuated by decreasing the pressure (Fig. 4A). N OTE: for ethyl alcohol, Tc = 243 °C and Pc = 63 bar.
Thus the solvent is extracted in such a way that the solid phase is dried without undergoing any volume change. An extremely light material, called an aerogel, results; its porosity may be as high as 95%. Again, appropriate thermal treatment leads to more dense materials (Fig. 4B). Applications of the Sol–Gel Process
gas Tc
T B
Figure 4. A: Basic procedure for supercritical drying of a gel. The critical point C of the solvent is bypassed by appropriate changes of temperature and pressure. B: Drying of a gel by supercritical evacuation of the solvent. The shrinkage is almost zero and the resultant material is called an aerogel. Thermal treatment of the aerogel leads to more or less dense ceramics.
Owing to the high costs involved (a glass made by sol–gel is about 100 times more expensive than a classical one), the use of the sol–gel process should be justified (1). Since the ceramic is formed by reactions between molecular precursors, the homogeneity of the material is perfect at the atomic scale. Moreover, because the starting alkoxides are volatile, their purification is easy and yields materials of high purity. These two major advantages make the sol–gel process extremely interesting for the synthesis of materials for optical purposes. A few other applications are worth citing (7). 1. Coatings. Thin ceramic films can be produced by dipping substrates into alkoxide solutions. The main application of this process is the formation of coatings. 2. Controlling properties. Adjusting the viscosity of gels makes possible, for example, either drawing fibers directly (e.g., high-purity silica optical fibers) or obtaining materials
JChemEd.chem.wisc.edu • Vol. 79 No. 7 July 2002 • Journal of Chemical Education
855
In the Laboratory with controlled porosity (e.g., membranes for microfiltration, xerogels, aerogels). The porosity of the alcogel also allows the migration of ions and the encapsulation of organic molecules (progressive index of refraction glasses or photochromic glasses, solid lasers). 3. Synthetic chemistry. Since the reactions take place at moderate temperatures, it is possible to synthesize and process highly refractory ceramics that could not be obtained by classic melting and casting (e.g., zirconia fibers). 4. Materials formation. Since inorganic and organic polymerizations are compatible with each other, the use of organo-alkoxides leads to new hybrid materials (e.g., contact lenses).
Selected Syntheses
Borosilicate Glass Total elapsed time: from several days to several weeks, depending on how the alcogel is dried. Actual working hours: preparing the alcogel, 1–2 h aging the alcogel, 1 week drying the alcogel, 1 day–2 weeks firing the xerogel, 1 h
Set a 500-mL beaker containing a Teflon-coated magnetic rod on a magnetic stirrer. Pour the following solutions, in the order listed, into the beaker (this is the inverse of the order of the rate of hydrolysis of the alkoxides) (8). 1. 61 mL of ethyl alcohol 2. 40.4 mL of tetramethyl-orthosilicate, Si(OCH3)4 3. 4.9 mL of distilled water 4. 0.2 mL of 1M HCl 5. 9.2 mL of Al-sec-butoxide, Al(OC4H9)3, first dissolved in 9.1 mL of isopropyl alcohol
NOTE: It is very important that the Al-sec-butoxide be completely dissolved to form a clear solution before it is mixed with the other reagents. This solution must be poured into the beaker very slowly with vigorous stirring. Despite these precautions, some small white flocks of alumina gel may appear. It takes about 15–20 min of strong stirring before they completely dissolve, leaving a clear, colorless solution in the beaker.
is still very weak and needs to be aged about one week. During this time, the alcogel must be protected from drying by fitting a thin polymer film at the top of the beaker or pouring a little pure ethanol onto the surface of the gel. After aging, the alcogel may be dried either by simply keeping the beaker open at room temperature or by piercing a few small holes in the polymer film cover and putting the beaker into a drying oven kept at 50 °C. In the first case, complete drying requires about 2 weeks and “big” transparent colorless crumbs (up to 5 mm) are obtained. In the second case, small pieces (up to 3 mm) are recovered. In both cases, drying reduces the volume of the gel by a factor 5 to 10. (Note that such drying times stand for the whole quantity of the original alcogel; these times may be considerably reduced if only a small quantity of the aged alcogel is cut out and allowed to dry.) Rinsing the xerogel with water causes crackling and sputtering of the crumbs, owing to the intrusion of the (invisible) mesoporosity by water. A kind of coarse glass powder, with particles in the range 0.1–2 mm, is then obtained. X-ray diffraction proves that such material is completely amorphous. The xerogel is perfectly clear and transparent but still contains numerous hydroxo and unreacted organic groups. Conversion to a pure glass is achieved via a thermal treatment up to 500 °C, during which the temperature is rapidly increased (about 10 °C/min). Slow heating may close up the porosity before complete evacuation of the volatilizable matter. In such a case, carbon forms inside the xerogel, and dark and fractured crumbs may be obtained (9). Such problems are avoided by using a simple open cylindrical furnace (for easy oxidation or evacuation of organic volatile compounds) quickly heated.
Silica Gel and Aerogel Total elapsed time: at least 9 days for the xerogel and at least 15 days for the aerogel. Actual working hours: preparing the alcogel, 1–2 h aging the alcogel, 1 week getting the xerogel, a few days rinsing the alcogel, 1 week supercritical drying, 8–10 h
Set a 500-mL beaker containing a Teflon-coated magnetic rod on a magnetic stirrer. Two solutions are prepared. The first one contains:
6. 2.2 mL of distilled water
1. 40 mL of ethyl alcohol
7. 17.4 mL of trimethylborate, B(OCH3)3
2. 50 mL of tetraethyl-orthosilicate, Si(OC2H5)4
8. 12.9 mL of distilled water 9. 4 mL of pure acetic acid
In another beaker prepare the following mixture, which is required to catalyze the gelation of the previous solution:
10. 24.6 mL of an aqueous solution of 2 M sodium acetate
1. 35 mL of ethyl alcohol
11. 12.9 mL of distilled water
2. 70 mL of distilled water
12. 5.4 mL of an aqueous solution of 1 M barium acetate
3. 0.28 mL of 30% ammonia aqueous solution
After the addition of the barium acetate, the solution becomes increasingly opalescent because of the polycondensation of growing molecular species. Gelation usually occurs within one hour or a little more, depending on the temperature and the stirring. The viscosity increases rapidly and the solution sets abruptly, confining the magnetic rod. At this moment, the gel is colorless (sometimes milky), translucent, and opalescent. It has the consistency and the elasticity of baked custard, but 856
4. 1.2 mL of an aqueous solution of 0.5 M NH4F
With strong stirring, slowly pour the second (catalytic) solution into the silica (the first) (silica) solution. Complete homogenization requires as long as about 20 min despite the vigorous stirring. As soon as the solution is perfectly clear, remove the stirring rod. Gelation occurs within 1 to 2 hours, leaving a colorless, transparent, elastic alcogel. As in the case of the previous borosilicate gel, aging for one week is recom-
Journal of Chemical Education • Vol. 79 No. 7 July 2002 • JChemEd.chem.wisc.edu
In the Laboratory A
N2
200-250 bars
300 °C
B
350
T
T / °C, P / bar
300 250 200 150 100
P
50 0 0
1
2
3
4
5
6
7
8
9 10
Time / h Figure 5. A: Device used for aerogel synthesis. A block of alcogel is placed inside a glass tube full of alcohol, which is placed in an autoclave. The device is flushed with nitrogen and the autoclave is heated to 300 °C. B: Temperature and pressure conditions leading to good quality silica aerogels.
mended. Again, the alcogel should be protected by adding a small amount of ethanol or covering the beaker with a polymer film. Obviously, the time needed for the subsequent drying depends on the thickness of the starting alcogel. Obtaining a silica xerogel simply requires one to carefully cut some blocks from the alcogel and let them dry at room temperature. If these blocks have no cracks, the resulting xerogels will be monolithic and transparent. However, getting unbroken pieces larger than about 1 cm is rather difficult and happens rarely. Note that this kind of drying induces considerable shrinkage, and the final volume of the xerogel is only about 1/30 that of the starting alcogel. Obtaining an aerogel requires the supercritical evacuation of the solvent, which is achieved by bringing the material beyond the critical point of ethyl alcohol. However, this experiment should not be performed directly, because of some residual amount of water within the alcogel. Such water is not evacuated by the process, and it induces the crystallization of silica (10). In these conditions, a dense, brittle, white, and opaque aerogel is obtained. It is then of first importance to rinse the alcogel with pure ethanol until the last traces of water are completely removed. This is best achieved by soaking
alcogel blocks in alcohol for at least one week; the solvent is replaced daily with fresh pure ethyl alcohol. At last, the pieces of alcogel are transferred into an ordinary glass test tube, which is almost completely filled with alcohol. This test tube is then placed in an autoclave, which in turn is placed in a simple cylindrical furnace (Fig. 5A). For such an experiment, the simplest autoclave is recommended. This is basically a thick steel tube closed at one end. The other end has a screw cap with a flat gasket of flexible graphite to provide a seal. As shown in Figure 5A, the seal is outside the furnace, and the top of the vessel is connected to the pressure source (a nitrogen cylinder). The autoclave we used (Prolabo, high-pressure valves Prolabo Mecabar) had the following dimensions: i.d., 1.2 cm; o.d., 3.5 cm; length (tube plus cap), 30 cm. Such a device can withstand pressure and temperature as high as 500 bar and 800 °C, respectively. However, even if the actual working conditions described here are much less severe (the pressure is not greater than that of a classical gas cylinder), special protection is desirable. This may be a specially outfitted room or simply a heavy metallic shell or wall placed in front of the experiment. After flushing the device, a starting nitrogen pressure of about 120 bar is imposed. The temperature is increased at 1–2 °C per minute until 300 °C. Such a high initial pressure is chosen because it minimizes the consequences of the nonuniformity of temperature throughout the autoclave. Moreover, the higher the initial pressure is, the lower is the cracking of the sample after drying (11). If the autoclave is initially about 90% filled, the pressure reaches about 200 bar during the heating (Fig. 5B). The temperature is then maintained for 1 h. Next, alcohol is slowly evacuated by continuously passing nitrogen through the device at a constant pressure of 200 bar. For that purpose, the two valves shown in Figure 5A are simultaneously left open in such a way that the inner pressure does not vary and the gas flow is very low. The gas flow should be easily controlled by bubbling the outlet gas into a beaker full of water (no more than a few bubbles per second). Extraction of alcohol is assumed to be complete after 1 h. The flow of nitrogen is then progressively reduced, in such a way that the pressure is decreased to atmospheric pressure at a rate of 100 bar per hour. Finally, the furnace is switched off and the tube is removed from the autoclave as soon as the temperature allows it. The aerogels that are recovered have exactly the same morphology and the same volume as the starting alcogels. They are monolithic, colorless, and perfectly transparent. Seen by reflection, they are slightly opalescent.
Colored Coatings on Soda Lime Glass Substrate Total elapsed time: 2 days. Actual working hours: preparation of the sol, 1 h aging of the sol, 1 day getting a colored coating, 2 h
Set a 100-mL beaker containing a Teflon-coated magnetic rod on a magnetic stirrer. Introduce sequentially the following chemicals (12–14): 1. 5 mL of ethyl alcohol 2. 15 mL of tetraethyl-orthosilicate, Si(OC2H5)4 3. 8 mL of distilled water
JChemEd.chem.wisc.edu • Vol. 79 No. 7 July 2002 • Journal of Chemical Education
857
In the Laboratory 4. choice of Cu(NO3)2⭈6H2O, 5 g or Co(NO3)2⭈6H2O, 3 g or Cr(NO3)3⭈9H2O, 2 g or Fe(NO3)3⭈6H2O, 6 g or Mn(NO3)2⭈6H2O, 2 g or Ni(NO3)2⭈6H2O, 7 g 5. 1 mL of 3 M HNO3 (at this moment, the solution begins to become homogeneous) 6. 1 mL of 4 M acetic acid
Let the stirring complete the homogenization of the mixture. The resulting solution is left to stand for one day. Next, the following are added: 7. 30 mL of methyl alcohol 8. 10 drops of a wetting agent such as a commercial rinsing solution for dishwasher (unknown but nonionic wetting agents)
The resultant liquid is a stable sol, which may be used for several months, provided that evaporation of the solvent is avoided or compensated for by adding the required amount of methyl alcohol. Dip a perfectly clean glass microscope slide into the solution and extract it vertically and very slowly, about 2 to 3 mm per second. Withdrawing the glass substrate too rapidly leads to thick coatings, generally inhomogeneous, cracked, and nonadhesive. Immediately after withdrawal of the glass, check that the surface is completely wetted and that the film is very thin (iridescent). Put the slide horizontally into an open cylindrical furnace (so that the inner ceramic tube is only touched by the lateral edges of the slide) and heat it to 500 °C at 7 °C per minute. The temperature is then maintained for half an hour. Clear, homogeneous, and transparent coatings are thus obtained. The films are green with Cu, blue with Co, yellow with Cr, yellowish-brown with Fe, brown with purple shades with Mn, and smoky grey with Ni. Nitrates of other elements such as Cd, Pb, Bi, and Ag were tested without success (they yielded colorless or pale milky coatings). Since the films are very thin, 0.2–0.5 µm, depending on the composition of the initial sol and the withdrawal speed, they are rather clear. Nevertheless, intensification of the coloring is readily achieved by repeating the process (i.e., by a subsequent dip-coating of the glass substrate).
Silica Fibers Total elapsed time: a few hours. Actual working hours: getting fibers, 1 h and possibly getting a silica xerogel, a few days
Place a 100-mL beaker equipped with a Teflon-coated magnetic rod on a magnetic stirrer with heater, and introduce sequentially the following chemicals (15, 16 ): 1. 9 mL of ethyl alcohol 2. 35.5 mL of tetraethyl-orthosilicate, Si(OC2H5)4 3. 5 mL of distilled water 4. 0.8 mL of 2 M HCl
The resulting solution is heated with stirring to about 80 °C. During this operation, the beaker is left open and the solvent evaporates; gentle boiling of the liquid makes its 858
volume decrease until the viscosity is such that a sticky gel is obtained. This gel should be viscous but still fluid; it is rather difficult to estimate the moment when drawing fibers from it becomes possible. The best conditions are rapidly reached (within half an hour) if the evaporation of the solvent is rapid (e.g., in a wide beaker). In a narrow beaker, more than one hour is required. Hence, repeated trials may be required to obtain fibrous gel. Remove the beaker from the stirring–heating machine and cool it rapidly as soon as the gel appears viscous (a bowl full of ground ice may be used). Then immerse a glass rod into the solution and pull it up carefully. For the first trials, only a drop remains at the end of the rod; the gel is not viscous enough. Later, the gel is stretched without breaking if the glass rod is slowly withdrawn. Fibers as long as 15 cm are obtained. If these were continuously drawn and heattreated, they would strengthen and could be made as long as desired. However, owing to their fineness, the fibers are readily solidified in air. After about 10–15 min, the gel becomes jellylike and drawing fibers from it is no longer possible. A few days later, a perfectly colorless and transparent silica xerogel is recovered inside the beaker. Hazards Most alkoxides are volatile and flammable and strongly react with water. They should be handled in a fume hood far from any ignition and moisture sources. Bottles containing alkoxides should be always kept in a desiccator. Wearing suitable protective clothing, gloves, and eye protection should be compulsory. Any glass equipment should be rigorously dried before use with alkoxides. The installation of materials inside a furnace and their removal from it should always be done while wearing protective clothing and heat-insulating gloves and using suitable tools, such as long metal tongs. Be certain that the autoclave is able to withstand the temperature and pressure conditions required for the aerogel synthesis. A list of highpressure reactor manufacturers may found on the Web; see for example http://www.reactorvessels.demon.co.uk/, http:// www.haage.com/, or http://www.pdcmachines.com/. Also contact Autoclave Engineers Group, 2930 W. 22nd St., Box 5051, Erie, PA 16512-5051, USA. Tetraethyl-orthosilicate. Flammable, very toxic by inhalation, risk of serious damage to eyes. Tetramethyl-orthosilicate. Same as tetraethyl-orthosilicate, and irritating to skin. Al-sec-butoxide. Flammable, reacts violently with water, cause burns. Trimethylborate. Flammable, harmful in contact with skin and eyes, irritating to respiratory system. Isopropyl alcohol. Highly flammable, irritating to eyes and respiratory system. Methyl alcohol. Highly flammable, toxic. Ammonia and acids. Cause severe burns, do not breathe the vapors. Barium acetate. Harmful by inhalation and if swallowed. Sodium acetate. No special health warnings.
Journal of Chemical Education • Vol. 79 No. 7 July 2002 • JChemEd.chem.wisc.edu
In the Laboratory
Conclusions We have described simple synthetic procedures leading to various sol–gel products. In our opinion, these recipes are rapid, easy for students to follow, and illustrative of the numerous possibilities of the sol–gel process. The longest experiments may be begun one day, left to stand for a period of time, and finished during a following week. Owing to the growing importance of soft chemistry, demonstrative experiments like these could be advantageously integrated into undergraduate chemistry programs. Still further developments are possible, such as incorporation of various colored elements or molecules within the gels. Photochromic silica xerogels could be easily obtained by encapsulation of optically active organic compounds. Characterization of the materials by X-ray diffraction or electron microscopy would also be of interest. Literature Cited 1. Dislich, H. Angew. Chem., Int. Ed. Engl. 1971, 10, 363. 2. Soft Chemistry Routes to New Materials—Chimie Douce; Rouxel, J.; Tournoux, M.; Brec, R., Eds.; Proceedings of the International Symposium, Nantes, France, Sep 6–10, 1993; Materials Science Forum 1994, 152–153. 3. Brinker, C. J.; Scherer, G. W. Sol–Gel Science; Academic: New York, 1990. 4. Sol–Gel Technology for Thin Films, Fibers, Preforms, Electronics, and Specialty Shapes; Klein, L. C., Ed.; Noyes: Park Ridge, NJ, 1988. 5. Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18, 259. 6. Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. 7. Livage, J. Verre 2000, 6, 12. 8. Brinker, C. J.; Scherer, G. W. J. Non-Cryst. Solids 1985, 70, 301. 9. Yoldas, B. E. J. Mater. Sci. 1979, 14, 1843. 10. Zarzycki, J.; Prassas, M.; Phalippou, J. J. Mater. Sci. 1982, 17, 3371. 11. van Lierop, J. G.; Huizing, A.; Meerman, W. C. P. M.; Mulder, C. A. M. J. Non-Cryst. Solids 1986, 82, 265.
12. Orgaz, F.; Rawson, H. J. Non-Cryst. Solids 1986, 82, 378. 13. Geotti-Bianchini, F.; Gugliemi, M.; Polato, P.; Soraru, G. D. J. Non-Cryst. Solids 1984, 63, 251. 14. Sakka, S.; Kamiya, K.; Makita, K.; Yamamoto, Y. J. Non-Cryst. Solids 1984, 63, 223. 15. Sakka, S.; Kamiya, K. J. Non-Cryst. Solids 1982, 48, 31. 16. Kamiya, K.; Sakka, S.; Tatemichi, Y. J. Mater. Sci. 1980, 15, 1765.
Further Reading Buckley, A. M.; Greenblatt M. J. Chem. Educ. 1994, 71, 599. Dimarcello, F. V.; Wood, D.L.; Sigety, E.A. J. Non-Cryst. Solids 1984, 63, 155. Aerogels; Fricke, J., Ed.; Springer: New York, 1986. Higginbotham, C.; Pike, C. F.; Rice, J. K. J. Chem. Educ. 1998, 75, 461. Ilharco, L. M.; Martinho, J. M. G.; Martins C. I. J. Chem. Educ. 1998, 75, 1466. Kawaguchi, T.; Hishikura, H.; Iura, J.; Kokubu, Y. J. Non-Cryst. Solids 1984, 63, 61. Laughlin, J. B.; Sarquis, J. L.; Jones, V. M.; Cox, J. A. J. Chem. Educ. 2000, 77, 77. Mahler, W.; Bechtold, M. F. Nature 1980, 285, 27. Mukherjee, S. P.; Lowdermilk, W. H. J. Non-Cryst. Solids 1982, 48, 177. Rabinovich, E. M.; Macchesney, J. B.; Johnson, D. W.; Simpson, J. R.; Meagher, B. W.; Jabra, R.; Phalippou, J.; Zarzycki, J. J. NonCryst. Solids 1980, 42, 489. Sakka, S. J. Non-Cryst. Solids 1985, 73, 651. Woignier, T.; Phalippou, J.; Zarzycki, J. J. Non-Cryst. Solids 1984, 63, 117. Susa, K.; Matsuyama, I.; Satoh, S.; Suganuma, T. Electron. Lett. 1982, 18, 499. History, synthesis, processing, applications, and photographs of aerogels may be found on the Ernest Orlando Lawrence Berkeley National Laboratory Web site: http://eande.lbl.gov/ ECS/aerogels/satoc.htm (accessed Feb 2002).
JChemEd.chem.wisc.edu • Vol. 79 No. 7 July 2002 • Journal of Chemical Education
859
