Sono-Aerogels

The group from the Universidade Estadual Paulista (Brasil) prepared sono-aerogels from their sonosols obtained in an excess of water (see footnote 1). Prior to drying, the solvent was exchanged with liquid CO2 and subsequently extracted under supercritical conditions. These fresh sono-aerogels have densities (0.3-0.4 g/cm ), which are comparable to the gels prepared in alcoholic solution from a TEO.S /water mixture. 

The fundamental role of the increase of / w on the structure of resulting aerogels is to increase the pore mean size and the pore volume, without, however, substantially modifying the average size of the silica particles forming the solid network of the aerogels [80]. 

Their isotherms are classified as type IV with most of the pores in the mesopore region. Table 20.7 shows the structural parameters of these sono-aerogels as a function of temperature as determined from nitrogen adsorption data. The sample densities p were evaluated from the total pore volume per mass unit from Vp measured from the N2 physisorption experiments. The mean pore size is evaluated as 1 = AV ISbet- Table 20.7 shows that the 

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WAXS studies on the structure of sono-aerogels established that the atomic sonogel stmcture is very cross-linked. Figure 20.10 represents the sonogel and silica glass reduced RDF,... 

The atomic arrangements around Ti atoms in titania-doped (5 mol%) silica sono-aerogel was deduced from Ti K-edge XAS [74]. The calculated PRDF is shown in Figure 20.11. The sonogel presents two well-defined peaks, corresponding to Ti-O and Ti-cation distances (Table 20.4). The network is formed up to the second neighboring level. 

Figure 20.12, MAS-NMR spectra of sono-aerogels prepared with different amounts of hydrolysis water. Reprinted with permission from [25],...

Figure 20.16. physisorption isotherm for sono-aerogel (triangles) and classic gel (squares). Solid symbols correspond to the adsorption branch and open symbols to the desorption one. The inset shows the pore size distribution of the same samples using the BJH model. The continuous line is for the sono and dotted line for the classic gel. 

The sono-aerogel has a high surface-to-volume ratio. 

Vollet et al. studied light sono-aerogels (p 0.3-0.4 gcm ) prepared from different proportions of TEOS and TMOS [79], showing the fundamental role of the Rtmos = [TMOS l[(JMOS + TEOS) on the structure of the aerogels the porosity and the pore mean size increases as Rtmos increases. 

Dense sono-aerogels have a closed porosity and very fine micropores (<1 nm). 

Table 20.7. Structural parameters of sono-aerogels as a function of temperature as determined...

Donatti and VoUet s group reported a complete structural evolution of their light sono-aerogels as a functirai of the heat treatment up to 1,100°C (Figure 20.19) [68]. These aerogels exhibit pronounced structural differences with those formerly described. They present a slope lailmass fractal, having neat pore-solid boundaries due to weak electronic fluctuations. 

Figure 20.19. Skeletal and bulk density for light sono-aerogels held for 10 h at temperatures up to 1,100°C. Reprinted with permission from [68],...

The SAKS structural parameters D, a, and the evaluated ratio a, are shown in Table 20.8 extracted from the curves I q) for sono-aerogel samples heat treated at 500°C and 900°C under atmospheric conditions for 10 h (Figure 20.20). 

The aerogel treated at 500°C practically abides by the condition of one order of magnitude for the ratio /a, corresponding to a D value obtained to standard preparation fotmd for acid-catalyzed classic silica aerogels [82]. Densification studies have shown that dense sono-aerogels can be readily converted into dense glasses, provided sufficient care is... 

Figure 20.20. SAXS intensity as a function of the modulus of scattering vector q for a light sono-aerogel. The small circle lines are the htting of the mass fractal approach to the experimental data at low- and medium- region. Reprinted from [67] with permission.

Figure 20.22. Left Change of the average size of pores (calculated from SAXS data) of mixed titania-silica aerogels upon first sintering stages. Right simplified model of the evolution of sono-aerogels upon first densification steps. Adapted with permission from [30],...

The elastic modulus of dense sono-aerogel is several orders of magnitude higher than for classic ones (Chap. 22). Table 20.9 summarizes results obtained for the bulk modulus and apparent density. 

Table 20.9. Bulk modulus and apparent density of sono-aerogels elaborated by hydrolysis (/f = 4, pH [HCl] = 1.5) and polycondensation of TMOS...

Esquivias L and Ramrrez-del-Solar (1997) Short-range order of titania doped silica sono-aerogel. J Non-Cryst SoUds 220 45-51... 

Sono-aerogel Aerogel obtained from sonogel... 

Figure 4.- XRD patterns of gels calcined at 773K. ASIO sono aerogel calcined at 1173 K. CCIO non calcined classic carbogel.

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