Silica nanogel for energy-efficient windows - Nanotechnology in Eco-efficient Construction - page 210

of the material's properties, such as porosity ( 90% air) and pores size

(Aegerter et al. , 2011). A more detailed analysis of the synthesis process
and of the recent developments can be found in the literature (Dorcheh
and Abbasi, 2008; Aegerter et al. , 2011).

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10.2.2 Physical, mechanical, and thermal properties
of silica aerogels

The fi nal physical, optical, and thermal properties of silica aerogels depend
on both the starting silica source (TEOS, TMOS, PEDS-Px) and the process
methodology, in particular on the catalyst and solvent used (Tajiri and
Igarashi, 1998; Pajonk, 2003; Anderson et al. , 2009). Aerogels have unusual
properties as solid materials, due to their structure. Furthermore, the physi-
cal, mechanical, and thermal characteristics can vary over a wide range,
depending on the synthesis process.

Physical properties

Silica aerogels are amorphous materials; the skeleton density is about
2200 kg/m 3 , but the material is extremely light (the bulk density is in the
50-200 kg/m 3 range), due to the very high porosity (the pore volume is
above 90% of the total volume). The pore size is typically in the 5-100 nm
range. Current aerogels for building applications have an overall density of
70-150 kg/m 3 . The acoustic properties are very interesting: the acoustic
propagation through aerogels depends on the nature and pressure of the
interstitial gas, on the aerogel density, and more on the texture (Forest
et al. , 1998), but the speed of sound in silica aerogels is lower than in air
(down to about 40 m/s through monolithic aerogels and to about 100 m/s
through granular ones). They can also improve the sound insulation in
windows (see Section 10.4). The optical and scattering properties are dis-
cussed in the next paragraphs.

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Mechanical properties

Aerogels are very fragile materials: the tensile strength is negligible, the
compressive strength and the elastic modulus are very low, and they depend
on the network connectivity and density. The compressive strength is in the
1-2 MPa range (Parmenter and Milstein, 1998; Luo et al. , 2006). The Young's
modulus ( E ) is a function of the apparent density. It varies in the 10 −3 -10 GPa
range, when the apparent density varies in the 10 2 -2 10 3 kg/m 3 (Woignier

et al. , 1988; Hegde and Venkateswara Rao, 2007; Parmenter and Milstein,
1998); in Aegerter et al. (2011) an E value in the 2.7-8.6 MPa range was
found, when the density is about 150-200 kg/m 3 . The shear modulus G is in

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