Civil Engineering Reference
In-Depth Information
2.2 Silica Aerogels Production and Properties
Inorganic Silica-Based Aerogels are obtained from a gel by replacing the pore
liquids with air and maintaining the network structure as it is in the gel state.
They are manufactured by means of different processes, involving three general
steps (Pierre and Pajonk
2002
; Soleimani Dorcheh and Abbasi
2008
). In the sol-
gel process (phase 1), silicon alkoxides are dispersed in a liquid, where the solid
nanoparticles collide and form a solid three-dimensional network through the
liquid (silica sols). Acidic or basic catalysts are usually added in the process. The
gels are usually classified according to the dispersion medium used, e.g. hydrogel,
alcogel, and aerogel (for water, alcohol, and air, respectively). Then the gels are
usually aged before drying (gel ageing phase), in order to increase stiffness and
strength and to mechanically reinforce the tenuous solid skeleton generated during
the sol-gel process. Finally, during the gel drying phase, the solid framework of
the sol-gel is isolated from liquid; it is the most critical step of the process: the
structure can collapse or be fractured, due to the capillary pressure. Two different
methods are usually carried out: ambient pressure drying (APD) and supercritical
drying (SCD), where the capillary tension can be avoided by removing the liquid
above the critical temperature and pressure. A more detailed analysis of the
synthesis process and the recent developments could be found in the literature
(Soleimani Dorcheh and Abbasi
2008
; Aegerter et al.
2011
).
The physical, mechanical, optical, and thermal properties of silica aerogels can
vary in a wide range and depend on both the starting silica source (such as
tetramathoxysilane (TMOS, Si(OCH
3
)
4
and tetraethoxysilane (TEOS, Si(OC
2
H
5
)
4
)
and the process methodology, in particular on the used catalyst and solvent (Tajiri
and Igarashi
1998
; Anderson et al.
2009
; Pajonk
2003
).
Silica aerogels are extremely light amorphous materials: even if the skeleton
density is about 2,200 kg/m
3
; the bulk density is in the 50-200 kg/m
3
range, because
of the very high porosity; and the pore size is typically in the 5-100 nm range.
Because of this structure, aerogels are very fragile: the tensile strength is
negligible and the compressive strength and the elastic modulus are very low
(Beatens et al.
2011
; Parmenter and Milstein
1998
). In general, the contact with
water must be avoided for monolithic aerogels: in commercial applications,
aerogel may be used in vacuum conditions, with evident advantages in terms of
thermal
insulation.
Nevertheless,
the
commercial
granular
aerogels
are
hydrophobic.
The optical and thermal properties are widely discussed in the next paragraphs.
Finally, considering safety, the material is not carcinogenic, non-flammable,
and non-reactive (Baetens et al.
2011
). The main physical properties are sum-
marized in Table
1
.
According to the Kistler's procedure, the first commercial aerogels were pro-
duced by Monsanto Chemical Corporation (USA) after 1940. Then, the production
was stopped between 1960s and 1980s. Nowadays, especially in the last two dec-
ades, the production of aerogels is located in the USA, Europe (Sweden, Germany),
