Classic Aerogel Papers

As in many fields, there have been numerous seminal papers in the development of aerogels–key milestones that paved the way for important future work. Here are references and abstracts for some of the all-time most important developments in aerogel technology. Note the early date of many of these discoveries!

Discovery of Aerogels

Coherent expanded aerogels and jellies

By Kistler, S. S.

Nature (London, United Kingdom) (1931), 127, 741.

The liquid in a gel can be replaced by a gas with little or no shrinkage. The liquid is displaced successively by liquids that are completely miscible with the preceding and succeeding one (H2O, EtOH, Et2O for instance), the last one having a low critical temperature. The jelly is placed in an autoclave with an excess of liquid, and the temperature is raised above the critical point. Upon allowing the gas to escape, an aerogel is obtained. Silica gel of apparent density as low as 0.02 was preared in this manner as a slightly opalescent although quite transparent glassy solid.

First Synthesis of Organic and Metal Oxide Aerogels

Coherent expanded aerogels

By Kistler, S. S.

Journal of Physical Chemistry (1932), 36, 52-64.

cf. C. A. 25, 3901. Aerogels were made by successive displacements of the liquid in a gel by other liquids, each of which must be completely miscible with the preceding one and the last of which has a low critical temperature so that it may be displaced by a gas. Aerogels of SiO2, Al2O2, WO3, Fe2O2, SnO2, Ni tartrate, cellulose, nitrocellulose, gelatin, agar and egg aluminum were made by removal of water from the normal gels. Rubber offered some difficulties but it was believed that these might be surmounted. The preparation and properties of these aerogels are briefly described and some discussion of the structure is included.

Identification of the Ultralow Thermal Conductivity of Aerogels

Thermal conductivity of silica aerogel

By Kistler, S. S.; Caldwell, A. G.

Journal of Industrial and Engineering Chemistry (Washington, D. C.) (1934), 26, 658-62.

An application is described for the measurement of heat conductivities of materials under variable mechanical and air pressures. Silica aërogel powders of different sieve analyses possess the lowest heat conductivity at atmospheric pressure of any insulator so far reported. The average value for the aërogel powder is 10% less than that for still air. A 30% decrease in conductivity is obtained when the gel is measured in the presence of CCl2F2, while the conductivity in vacuum is only 15% of normal. Mechanical pressure up to 15 lbs. per sq. in. does not greatly increase the conductivity in vacuum. An explanation of the very low conductivity is given, and several suggestions for commercial utilization are offered.

Introduction of Metal Alkoxides for Production of Aerogels

Inorganic oxide aerogels

By Teichner, S. J.; Nicolaon, G. A.; Vicarini, M. A.; Gardes, G. E. E.

Advances in Colloid and Interface Science (1976), 5(3), 245-73.

A method of preparation of aerogels of SiO2, Al2O3, TiO2, ZrO2, MgO, and mixed oxides was developed. The corresponding alcoholate, dissolved in an organic solvent, such as alcohol or C6H6, is hydrolyzed at room temperature and the solvent is evacuated under hypercritical conditions in an autoclave. This method does not require the purification of the precipitated metal oxide nor the substitution of an organic solvent for water which is necessary when the initial gel is prepared in H2O. SiO2 aerogels, having a surface area of 1000 m2 g-1, a pore volume of 18 cm3 g-1, and an apparent density of 0.05 g cm-3 were obtained. These aerogels are hydrophobic, but can be converted to hydrophilic aerogels and may also be made transparent. The transparent SiO2 aerogels are used as Cherenkov radiators for ionizing radiations. Aerogels of other oxides also exhibit high values of their textural characteristics in comparison with those of oxide gels prepared in a conventional way (xerogels). Mixed oxide aerogels (ZrO2-MgO, Al2O3-MgO, TiO2-MgO) exhibit higher surface areas than corresponding pure oxide aerogels. The method is also used to prepared oxide-supported metal or metal oxide aerogels for catalysts.

Introduction of Helium Pycnometry for Measuring the Skeletal Density of Aerogels

Skeletal density of silica aerogels

By Woignier, T.; Phalippou, J.

Journal of Non-Crystalline Solids (1987), 93(1), 17-21.

The skeletal density of silica aerogels, produced by hypercritical drying of gels, is studied by He pycnometry. The bulk and the skeletal densities vary as functions of parameters such as TMOS concentration, pH, and densifying heat treatment. Skeletal densities were slightly lower than that of vitreous silica. The results are compared to values obtained on xerogels.

Development of Resorcinol-Formaldehyde and Carbon Aerogels

Resorcinol-formaldehyde aerogels and their carbonized derivatives

By Pekala, R. W.; Kong, F. M.

Polymer Preprints (American Chemical Society, Division of Polymer Chemistry) (1989), 30(1), 221-3.

The morphology and mechanical properties of HCHO-resorcinol (I) copolymer aerogels and their carbonized derivatives were studied and compared. Both pristine and carbonized aerogels exhibited densities 30-300 mg/cm3, cell/pore sizes <1000 Å, and an open cell structure with continuous porosity. The density and surface area of the aerogels decreased with increasing [I]/[Na2CO3 polymn. catalyst] ratio. The carbonized aerogels had superior mechanical properties than the pristine aerogels.

Method for Producing Ultralow Density Silica Aerogels

Transparent ultralow-density silica aerogels prepared by a two-step sol-gel process

By Tillotson, T. M.; Hrubesh, L. W.

Journal of Non-Crystalline Solids (1992), 145(1-3), 44-50.

Interest in lowering aerogel densities for applications involving high-energy charged particle detection via the Cherenkov effect has led to the development of a 2-step sol-gel method for preparing ultralow-density aerogels. The method was used to produce uncracked, transparent aerogel tiles with densities 3-80 kg/m3. Comparative characterization of conventional single-step base-catalyzed aerogels to aerogels prepared by this 2-step approach is presented. Results indicate that the aerogel microstructure for the 2-step approach differs from the bead-like structure proposed for single-step base-catalyzed tetramethoxysilane (TMOS) aerogels. TEM micrographs of aerogels prepared by the 2-step method show an interlinked polymer-chain-like structure with an average chain diameter of 2-3 nm and an average chain length of ∼15 nm. UV-visible spectrophotometry shows the transmittance over the visible spectrum (400-800 nm) to be improved for the 2-step aerogels by ≤30%. Other measurements of the ultralow-density aerogels include BET surface area, and compressive modulus.

Development of Subcritical Drying

Preparation of low-density aerogels at ambient pressure for thermal insulation

By Smith, Douglas M.; Deshpande, Ravindra; Brinker, C. Jeffrey

Ceramic Transactions (1993), 31(Porous Materials), 71-80.

Low density ceramic aerogels have numerous properties which suggest a number of applications such as ultrahigh-efficiency thermal insulation. However, the commercial viability of these materials has been limited by their high cost associated with drying at supercritical pressures, low stability to water vapor, and low mechanical strength. Normally, critical point drying is employed to lower the surface tension and hence, capillary pressure, of the pore fluid to essentially zero before drying. A process is presented to control capillary pressure and gel matrix strength by employing a series of aging and pore chemical modification steps such that the gel shrinkage is minimal during rapid drying at ambient pressure. The total processing time from gelation to the final dried product is less than 48 h. The properties (density, surface area, pore size, SAXS) of aerogel monoliths prepared from base-catalyzed silica gels using this technique, CO2 critical point drying, and supercritical ethanol drying are compared. An additional advantage of this approach is that the final gels are hydrophobic. Densities in the range of 0.15 to 0.3 g/cm3 with pore sizes less than 100 nm are routinely made. Thermal conductivity is on the order of 0.02 W/mK at room temperature and atmospheric pressure.

Reintroduction of Epoxide-Assisted Gelation for Preparing Metal Oxide Aerogels

Synthesis of lanthanide and lanthanide-silicate aerogels

By Tillotson, T.M.; Sunderland, W.E.; Thomas, I.M.; Hrubesh, L.W.

Journal of Sol-Gel Science and Technology (1994), 1(3), 241-9.

The preparation of lanthanide oxide and mixed lanthanide-silicate aerogels from the chlorides of erbium, praseodymium, and neodymium was investigated. A two-step sol-gel method is described for preparing the mixed aerogels by using a sub-stoichiometric amount of water in the first step to prepare a partially condensed silica-lanthanide precursor. The lanthanide oxide aerogels were prepared directly from the chlorides by using propylene oxide as a scavenger for reaction generated hydrochloric acid. The aerogel microstructures vary from colloidal for the lanthanide oxide and high weight percent lanthanide-silicate aerogels to polymeric for the low weight percent lanthanide-silicate aerogels. This change in microstructure is also indicated by BET analyses, which show that the surface area decreases with increasing lanthanide concentration. In general, a decrease in lanthanide content occurred during the supercritical drying step due to insufficient linking and subsequent washing out of the lanthanide from the gels. Also, the retention efficiency for the lanthanide increases with increasing silica concentration and makes quantitative doping by this method practical only for the lower lanthanide concentrations.

First Demonstration of a Supercapacitor Using Carbon Aerogel

The Aerocapacitor: An electrochemical double-layer energy-storage device

By Mayer, S. T.; Pekala, R. W.; Kaschmitter, J. L.

Journal of the Electrochemical Society (1993), 140(2), 446-51.

Unique types of carbon foams, developed at Lawrence Livermore National Laboratory, were used to make an “Aerocapacitor.”. The aerocapacitor is a high power-density, high energy-density, electrochemical double-layer capacitor which uses carbon aerogels as electrodes. These electrodes possess very high surface area per unit volume and are electrically continuous in both the carbon and electrolyte phase on a 10 nm scale. Aerogel surface areas range from 100 to 700 m2/cm3 (as measured by BET analysis), with bulk densities of 0.3-1.0 g/cm3. This morphology permits stored energy to be released rapidly, resulting in high power densities (7.5 kW/kg). Materials parameterization was performed, and device capacitances of several tens of Farads per g and per cm3 of aerogel were achieved.

Refinement of Epoxide-Assisted Gelation for Preparing Metal Oxide Aerogels

New sol-gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors

By Gash, A. E.; Tillotson, T. M.; Satcher, J. H., Jr.; Hrubesh, L. W.; Simpson, R. L.

Journal of Non-Crystalline Solids (2001), 285(1-3), 22-28.

We have developed a new sol-gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol-gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its rate was studied by varying the solvent used, the Cr(III) precursor salt, the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution TEM (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe3+, Al3+, In3+, Ga3+, Zr4+, Hf4+, Ta5+, Nb5+, and W6+), provided the formal oxidation state of the metal ion is ≥ +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.

Invention of Mechanically Robust X-Aerogels

Nanoengineering strong silica aerogels

By Leventis, Nicholas; Sotiriou-Leventis, Chariklia; Zhang, Guohui; Rawashdeh, Abdel-Monem M.

Nano Letters (2002), 2(9), 957-960.

In the quest for strong lightwt. materials, silica aerogels would be very attractive, if they were not so fragile. The strength of silica aerogel monoliths has been improved by a factor of >100 through crosslinking the nanoparticle building blocks of preformed silica hydrogels with poly(hexamethylene diisocyanate). Composite monoliths are much less hygroscopic than native silica, and they do not collapse when in contact with liqs.

Synthetic Pathway for Producing Semiconducting Metal Chalcogenide Aerogels

Porous Semiconductor Chalcogenide Aerogels

By Mohanan, Jaya L.; Arachchige, Indika U.; Brock, Stephanie L.

Science (2005), 307(5708), 397-400.

Chalcogenide aerogels based entirely on semiconducting II-VI or IV-VI frameworks have been prepd. from a general strategy that involves oxidative aggregation of metal chalcogenide nanoparticle building blocks followed by supercrit. solvent removal. The resultant materials are mesoporous, exhibit high surface areas, can be prepd. as monoliths, and demonstrate the characteristic quantum-confined optical properties of their nanoparticle components. These materials can be synthesized from a variety of building blocks by chem. or photochem. oxidn., and the properties can be further tuned by heat treatment. Aerogel formation represents a powerful yet facile method for metal chalcogenide nanoparticle assembly and the creation of mesoporous semiconductors.

Discovery of Metal Nanofoams

Ultralow-Density Nanostructured Metal Foams: Combustion Synthesis, Morphology, and Composition

By B.C. Tappan, M.H. Huynh, M.A. Hiskey, D.E. Chavez, E.P. Luther, J.T. Mang, S.F. Son,

Journal of the American Chemical Society (2006), 128(20), 6589-6594.

The synthesis of low-density, nanoporous materials has been an active area of study in chemistry and materials science dating back to the initial synthesis of aerogels. These materials, however, are most often limited to metal oxides, e.g., silica and alumina, and organic aerogels, e.g., resorcinol/formaldehyde, or carbon aerogels, produced from the pyrolysis of organic aerogels. The ability to form monolithic metallic nanocellular porous materials is difficult and sometimes elusive using conventional methodology. Here we report a relatively simple method to access unprecedented ultralow-density, nanostructured, monolithic, transition-metal foams, utilizing self-propagating combustion synthesis of novel transition-metal complexes containing high nitrogen energetic ligands. During the investigation of the decomposition behavior of the high-nitrogen transition metal complexes, it was discovered that nanostructured metal monolithic foams were formed in a post flame-front dynamic assembly having remarkably low densities down to 0.011 g cm3 and extremely high surface areas as high as 270 m2 g1. We have produced monolithic nanoporous metal foams via this method of iron, cobalt, copper, and silver metals. We expect to be able to apply this to many other metals and to be able to tailor the resulting structure significantly.

Synthetic Pathway for Producing Carbon Nanotube Aerogels

Carbon nanotube aerogels

By Bryning, Mateusz B.; Milkie, Daniel E.; Islam, Mohammad F.; Hough, Lawrence A.; Kikkawa, James M.; Yodh, Arjun G.

Advanced Materials (2007), 19(5), 661-664.

The creation of carbon nanotube aerogels from aq.-gel precursors by crit.-point-drying and freeze-drying is reported. The aerogels are strong and elec. conducting and are a potential improvement over current technologies for applications such as sensors, electrodes, and thermoelec. devices. The aerogels can be reinforced by small amts. of polyvinyl alc. and can support 8000 times their own wt.

Synthetic Pathway for Producing Metal Aerogels

Smelting in the age of nano: Iron aerogels

By Leventis, Nicholas; Chandrasekaran, Naveen; Sotiriou-Leventis, Chariklia; Mumtaz, Arif

Journal of Materials Chemistry (2009), 19(1), 63-65.

Porous pig iron was produced by smelting interpenetrating resorcinol-formaldehyde and iron oxide xerogels. Crosslinking alters the thermolytic behavior leading to macropores, but most importantly by melting it mixes intimately the skeletal resorcinol-formaldehyde and iron oxide nanoparticles and depresses their reaction temp. by ≤400°. This is explored further with other interpenetrating networks of nanoparticles.

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