Derivatives Aerogels

For some specific applications (for example, thermal insulation), making mesoporous cellulosic aerogels is of high interest. One of the approaches is cross-linking of cellulosic species. Because cross-linking of cellulose is necessary to be performed in heterogeneous media (mainly with formaldehyde [56], carboxylic acid [57] or epichlorohydrin [58]), one 

the crucial issue is identification and use of a solvent system compatible with the reactants, the gel and the extraction media (e.g., CO2). Substituting the hydroxyl groups of cellulose with some other specific chemical groups is known to increase solubility in organic 

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Though the MTMS -derived aerogels are very flexible and elastic, it does not take much force to compress them. For example, Rao [28] reports a Young s modulus of only 0.03-0.06 MPa for the flexible MTMS-derived aerogels ranging in density from 0.04 to 0.1 g/cm. Kanamori [29] does not report Young s modulus, but stress-strain curves indicate that stresses of less than 1 MPa are sufficient to compress samples with bulk densities around 0.2 g/cra to 25% strain. 

The process and the derived aerogels present the following features ... 

Powders from commercial sources, different phases of iron oxides, as well as sol-gel-derived aerogels and xerogels were evaluated by the TGA method. According to the results the iron (III) oxide aerogel reduced to metallic iron the most rapidly under constant conditions (1 1 v/v, H2/Ar at 450°C). This is possibly related to the extremely high surface... 

Pekala R.W., Alviso C.T., Kong F.M., Hulsey S.S. Aerogels derived from multifunctional organic monomers. J. Nn-Cryst Solids 1992 145 90-8. 

Aero-derivative systems, 10 142 Aerodynamic diameter, of a particle, 26 694 Aerodynamic web formation, 17 502-504 Aerogels, 1 748-769 23 56... 

Sol—gel, alumina derived from, 23 76—78 Sol-gel bioactive glasses, 23 82-83 Sol-gel chemistry aerogels, 1 749-753 ceramics processing, 5 642 Sol-gel coatings, 5 665... 

Figure 16. The pore-size distribution for sol—gel-derived birnessite Na(5Mn02 20 as processed into three pore-solid nanoarchitectures xerogel, ambigel, and aerogel. Distributions are derived from N2 physisorption measurements and calculated on the basis of a cylindrical pore model. (Reprinted with permission from ref 175. Copyright 2001 American Chemical Society.)...

Fig. 7.15. Angular correlation spectra (left) and corresponding derived positronium momentum distributions F(p) (see the text for details) for silica aerogel under the following conditions (a) vacuum (b) 1 atmosphere of N2 gas (c) 0.1 atmospheres of O2 gas (d) 0.2 atmospheres of 02 gas (e) 0.4 atmospheres of O2 gas (f) 0.8 atmospheres of O2 gas. The arrows on the right-hand diagrams indicate the momenta corresponding to the excitation energies of the a1Ag and the b1Eg states of O2. A discussion of the components marked I and II can be found in the text.

Fig. 7.18. Derived momentum distributions for the perturbed m = 0 state of ortho-positronium, for various gases (Nagashima et al., 1995) in an applied static magnetic field of 0.29 T. Reprinted from Physical Review A52, Nagashima et al., Thermalization of free positronium atoms by collisions with silica-powder grains, aerogel grains and gas molecules, 258-265, copyright 1995 by the American Physical Society.

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