Aerogels are gels that are dried without significant shrinkage. They were first produced in 1931 by Kistler, who exchanged silica gels into alcohol, then heated the gel in an autoclave above the critical point of the alcohol; the vapor was then slowly released while holding the temperature above the critical point. The point of this procedure is that drying shrinkage is caused by capillary pressure, but there is no surface tension (because there is only one phase present) above the critical point. It later became more popular to perform a solvent exchange into liquid CO2, and then perform supercritical drying (SCD), since the critical temperature of CO2 is only 31˚C.
In spite of the absence of surface tension, there are other mechanisms that create stresses during SCD, including dilatation during solvent exchange and during depressurization. some of the following papers analyze those situations.
Characterizing aerogels is challenging, because the network is so compliant that it is compressed during nitrogen sorption, and is crushed by mercury porosimetry. We have analyzed the deformation during characterization by various techniques and provided criteria for judging the amount of error.
[More recently, similar materials have been made by reacting a silica gel with chlorosilane, so that the surface is covered with methyl groups, and then drying. There is large contraction during drying, but the passivated surfaces of the gel do not react when the silica chains are brought into contact during drying, so the gel springs back. It is difficult to make bodies larger than a few mm this way, because of the large contraction, but it is cheaper and easier than SCD for making granules and other small objects. We have not done any work in this area, which was pioneered by Jeff Brinker, Doug Smith, and their students.]
Relevant papers:
Structure and Properties
“Effect of Aging and pH on the Modulus of Aerogels”, H. Hdach, T. Woignier, J. Phalippou, and G.W. Scherer, J. Non-Cryst. Solids 121 (1990) 202-205
“Mechanical Properties of Silica Alcogels and Aerogels”, T. Woignier, J. Phalippou, H. Hdach, and G.W. Scherer, pp. 1087-1099 in Better Ceramics Through Chemistry IV, eds. B.J.J. Zelinski, C.J. Brinker, D.E. Clark, and D.R. Ulrich (Mat. Res. Soc., Pittsburgh, PA, 1990)
“Evolution of Mechanical Properties during the Alcogel-Aerogel-Glass Process”, T. Woignier, J. Phalippou, G. Larnac, F. Pernot, G.W. Scherer, J. Non-Cryst. Solids 147&148 (1992) 672-680
“Ultraporous materials with low permeability”, J. Phalippou, G.W. Scherer, T. Woignier, D. Bourret, and R. Sempéré, J. Non-Cryst. Solids 186 (1995) 64-72
“Elastic properties of crosslinked Resorcinol-Formaldehyde gels and aerogels”, J. Gross, G.W. Scherer, C. Alviso, and R. Pekala, J. Non-Cryst. Solids 211 [1,2] (1997) 132-142
“Structural efficiency and microstructural modeling of wet gels and aerogels”, J. Gross and G.W. Scherer, J. Sol-Gel
Sci. Techn. 13 (1998) 957-960
“Room temperature densification of aerogel by isostatic compression”, A.H. Alaoui, T. Woignier, J. Phalippou, and G.W. Scherer, J. Sol-Gel Sci. Techn. 13 (1998) 365-369
“Mechanical structure-property relationship of aerogels”, H.-S. Ma, A.P. Roberts, J.-H. Prévost, R. Jullien, and G.W. Scherer, J. Non-Cryst. Solids 277 (2000) 127-141
Drying Stress
“Stress in Aerogel during Depressurization of Autoclave: I. Theory”, G.W. Scherer, J. Sol-Gel Sci. Tech. 3 (1994) 127-139
“Stress in Aerogel during Depressurization of Autoclave: II. Silica Gel”, T. Woignier, G.W. Scherer, and A. Alaoui, J. Sol-Gel Sci. Tech. 3 (1994) 141-150
“Optimization of the rapid supercritical extraction process for aerogels”, G. W. Scherer, J. Gross, L.W. Hrubesh, and P.R. Coronado, J. Non-Cryst. Solids 311 (2002) 259-272
Sintering
“The Sintering of Silica Aerogels Studied by Thermoporometry”, T. Woignier, J.F. Quinson, M. Pauthe, M. Repellin Lacroix, J. Phalippou, and G.W. Scherer, J. Sol-Gel Sci. Tech. 2 [1/2/3] (1994) 277-282
“Densification kinetics and structural evolution during sintering of silica aerogel”, G.W. Scherer, S. Calas, and R. Sempéré, J. Non-Cryst. Solids 240 (1998) 118-130
“Sintering of aerogels”, G.W. Scherer, S. Calas, and R. Sempéré, J. Sol-Gel Sci. Tech. 13 (1998) 937-943
Characterization
“Deformation of aerogels during characterization”, G.W. Scherer, D.M. Smith, and D. Stein, J. Non-Cryst. Solids 186 (1995) 309-315
“Compression of aerogels”, G.W. Scherer, D.M. Smith, X. Qiu, and J.M. Anderson, J. Non-Cryst. Solids 186 (1995) 316-320
“Adsorption in sparse networks: I. Cylinder model”, G.W. Scherer, J. Colloid Interface Sci. 202 (1998) 399-410
We originally thought that this work explained the erroneous results obtained from nitrogen sorption measurements on aerogels, but we later realized that the real problem is the contraction described in the following papers by Reichenauer:
“Adsorption in sparse networks: II. Application to silica aerogels”, G.W. Scherer, S. Calas, and R. Sempéré, J. Colloid Interface Sci. 202 (1998) 411-416
“Adsorption in aerogel networks”, G.W. Scherer, J. Non-Cryst. Solids 225 (1998) 192-199
“Characterization of aerogels”, G.W. Scherer, Advances in Colloid and Interface Science 76-77 (1998) 321-339
“Effects upon nitrogen sorption analysis in aerogels”, G. Reichenauer and G.W. Scherer, J. Colloid Interface Sci. 236 (2001) 385-386
“Extracting the pore size distribution of compliant materials from nitrogen adsorption”, G. Reichenauer and G.W. Scherer, Colloids and Surfaces A 187-188 (2001) 41-50
“Nitrogen sorption in aerogels”, G. Reichenauer and G.W. Scherer, J. Non-Cryst. Solids 285 (2001) 167-174
“Dynamic Pressurization: Novel method for measuring fluid permeability”, J. Gross and G.W. Scherer, J. Non-Cryst. Solids 325 (2003) 34-47
“Comparison between flexural and uniaxial compression tests to measure the elastic modulus of silica aerogel”, A. Hafidi Alaoui, T. Woignier, G.W. Scherer, J. Phalippou, J. Non-Cryst. Solids 354 (2008) 4556–4561
Computer Simulation
“Mechanical structure-property relationship of aerogels”, H.-S. Ma, A.P. Roberts, J.-H. Prévost, R. Jullien, and G.W. Scherer, J. Non-Cryst. Solids 277 (2000) 127-141
“Computer simulation of mechanical structure-property relationship of aerogels”, H.-S. Ma, J.-H. Prévost, R. Jullien, and G.W. Scherer, J. Non-Cryst. Solids 285 (2001) 216-221
“Modeling of Sol-Gel Transition with Loop Network Formation and its Implications on Mechanical Properties”, H.-S. Ma, J.-P. Prévost, R. Jullien and G. W. Scherer, in Advances in Materials Theory and Modeling - Bridging Over Multiple-Length and Time Scales, edited by V. Bulatov, F. Cleri, L. Colombo, L. Lewis and N. Mousseau, Mater. Res. Soc. Symp. Proc. No. 677 (Materials Research Society, Pittsburgh, 2001), AA7.10
“Dangling bond deflection model: Growth of gel network with loop structure”, H.-S. Ma, R. Jullien, and G.W. Scherer, Phys. Rev. E 65 (2002) 401-403
“Elasticity of DLCA model gels with loops”, H.-S. Ma, J. Prévost, R. Jullien, G.W. Scherer, Int. J. Solids Struct. 39 [18] (2002) 4605-4614