High performance thermal insulation materials for buildings - Nanotechnology in Eco-efficient Construction - page 195

and the temperature is raised, it will be transformed at the critical tempera-
ture into a gas without two phases having been present at any time (Kistler,
1932). As such, high temperature supercritical drying and low temperature
supercritical drying from carbon dioxide is mostly used as drying process.
The unique structure of aerogels result in exceptional material proper-
ties: a bulk density typically of 70-150 kg/cm 3 due to a porosity of 85-95%,
a specifi c surface area of 600-1000 m 2 /g, a particle size below 5 nm, a f ( )

averaging 20 nm with maximum pore sizes of 100 nm close to the mean free
path of 70 nm for air molecules at standard temperature and pressure,
resulting in an effective thermal conductivity of 0.014 W/(mK) at atmo-
spheric conditions. Also translucent aerogels can be achieved depending on
water removal before the drying process, resulting in a transmittance
between 0.8 and 0.95 in the visible and (near-)infrared spectrum for a layer
of 1 cm and a low refraction index of around 1.0.

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9.3.2 Partial vacuum thermal insulators

Vacuum insulation panels (VIP) (see Fig. 9.3) are defi ned as an evacuated
foil-encapsulated open-porous material as thermal insulator. As such the
insulator is no typical thermal insulation material, but consists of a system
of three parts, each with their own specifi c purpose(s), i.e., the open-porous
core material, the foil envelope and the applied vacuum. A perfect vacuum
is the most effective reduction of the gas thermal conduction

cd,g , achieving
its limit value of 'zero'. This perfect vacuum is pure theoretically, but a low
pressure P g has a positive infl uence on the gaseous heat transfer. As such
the core has a dual purpose, i.e., to withstand the pressure of the applied
partial vacuum, and preferably to strengthen the thermal effect of this
vacuum by its pore size distribution f (

φ

). This core material is typically a
traditional open-porous thermal insulator, or a nanoporous high perfor-
mance thermal insulator. The foil envelope only serves to maintain the
applied partial vacuum in the core material and generally consists of an
aluminum layer. Due to the relatively high thermal conductivity of such an
envelope, the heat fl ux increases at the edges and corners. Furthermore, the
foil envelope is not able to keep the applied vacuum constant at the pristine
pressure due to gas and moisture mitigation through the foil and foil seams.
The envelope choice is generally a compromise between the allowed pres-
sure drop through time and the allowed thermal bridging at the panel edges.
Current state-of-the-art vacuum insulation panels consist of a core
material of fumed silica with a bulk density of 160-220 kg/m 3 , a specifi c
surface area of 100-400 m 2 /g and a f (

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￿ ￿ ￿ ￿ ￿ ￿

) with maximum pore sizes around
300 nm close to the mean free path of 70 nm for air molecules at standard
temperature and pressure, resulting in a effective thermal conductivity of
0.020 W/(mK) at atmospheric conditions. The applied partial vacuum is

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