Civil Engineering Reference
In-Depth Information
the performance of nanogel windows is underlined and compared with
conventional glazing solutions. Finally, the main future research trends are
discussed.
10.2 Aerogels for windows
Aerogels have a large number of applications and they differ from each
other if the raw material is considered. They can be classifi ed into:
inorganic (silica-based and non-silicate aerogels);
organic (natural and synthetic);
composite (polymer crosslinked);
exotic (based on metal chalcogenide).
For the purposes of the present chapter, inorganic silica-based aerogels
will be considered, which are the most common as transparent insulating
materials (TIM), both in the form monolithic and granular materials.
10.2.1 Synthesis and production of silica aerogels
Aerogels are light and transparent solid materials obtained from a gel by
replacing the pore liquids with air and maintaining the network structure
as it is in the gel state. Silica aerogels are manufactured by means of differ-
ent processes; all them involve three general steps (Pierre and Pajonk, 2002;
Dorcheh and Abbasi, 2008; Baetens et al. , 2011):
Gel preparation (sol-gel process)
The raw materials for the aerogel production are solid particles. In particu-
lar, silicon alkoxides are often used, such as tetramethoxysilane (TMOS,
Si(OCH 3 ) 4 ), tetraethoxysilane (TEOS, Si(OC 2 H 5 ) 4) , and polyethoxydisilox-
ane (PEDS-P x , SiO n (OC 2 H 5 ) 4−2n ). They are dispersed in a liquid where the
solid nanoparticles collide and form a solid three-dimensional network,
which can extend through the liquid (silica sols). Acidic or basic catalysts
are usually added in the process. The gels are usually classifi ed according
to the dispersion medium used, e.g., hydrogel, alcogel and aerogel (for
water, alcohol, and air, respectively). The following reaction may describe
the synthesis of silica aerogels for insulation purposes (when the precursor
is tetramethoxysilane, Si(OCH 3 ) 4 ):
￿ ￿ ￿ ￿ ￿ ￿
(
) +
Si OCH
2
H O
↔+
SIO
4
CH OH
[10.1]
3
2
2
3
4
The material resulting from the process is a cross-linked structure (Fig.
10.1) of silicon dioxide (SiO 2 ) chains (0.2-15% in volume, depending on the
manufacturing method) with a large number of air-fi lled pores.
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