Civil Engineering Reference
In-Depth Information
is similar to the wavelength of the incident light. The presence of pores
within this range
f
(
) acts as scattering centers, and the effi ciency of scat-
tering will depend on the size of the scattering centers as well as the wave-
length
λ
. Heat treatment of aerogels can increase their transparency, and
the optical properties can be infl uenced further by selecting optimal syn-
thesis parameters in the sol-gel process.
The sound absorption of a material increases with an increasing surface
area facing the sound. As aerogels have a high porosity and a high specifi c
surface area, sound waves are strongly absorbed and attenuated: monolith
silica aerogels have a lower speed of sound than air. Sound velocities down
to 40 m/s have been measured, whereas non-monolith commercial products
claim to have a sound velocity of 100 m/s through the structure. Granular
aerogels, on the other hand, are exceptional refl ectors of audible sound,
making excellent barrier materials. By combining multiple layers with dif-
ferent granular sizes, average attenuations of -60 dB have been found for
a total thickness of only 0.07 m.
In contrast to combustible organic foam insulation that emits deadly
fumes and smoke when burning, silica aerogel materials are non-combus-
tible due to their non-organic SiO
2
structure and withstand heat up to
1400°C.
Due to their physical solid structure, aerogels show a low tensile strength
and a brittle nature, whereas its porous structures make it very sensible to
moisture due to high surface tensions.
Aerogel insulation sheets suffer from dust production. As most of the
commercial aerogel insulation products consist of complete amorphous
(and thus 0% crystalline) silica, exposure limits in the range of 5 mg/m
3
for
respirable dust are set in the US by the Occupational Safety and Health
Administration (OSHA). However, the International Agency for Research
on Cancer (IARC) considers synthetic amorphous silica to be not classifi -
able as to its carcinogenicity to humans (i.e., belongs in group 3). No evi-
dence of silicosis has been found from epidemiological studies of workers
with long-term exposure to synthetic silica, whereas studies of various
animal species show that amorphous silica can be completely cleared from
the lungs (Merget
et al.
, 2002; Warheit, 2001).
λ
Areas of application
Aerogel insulation panels have only recently been introduced to the market
in small-scale production. The main building applications may be divided
into two groups, i.e., as traditional thermal insulators by means of aerogel
blankets, and in translucent form for high performance glazing.
Commercial manufacture of aerogel blankets began around the year
2000 and has been developed to meet various demands. An aerogel blanket
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