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Aerogel materials are open cell, nanoporous materials that have a very high proportion of free
void volume (typically >90%) compared to conventional solid materials. Their high pore volume,
low solid content, and torturous path amorphous structure give rise to low values of thermal conductivity. Silica aerogels prepared via sol-gel processing have some of the best thermal
properties of any solid insulation material known, as elucidated long ago by Kistler.
Excellent thermal insulation properties have also been reported in organic and carbon based
aerogels as well as other inorganic metal oxides produced in situ sol-gel processing.
The primary modes of heat transfer through insulating materials can be considered cumulative
so that the total effective conductivity can be written as:
 Eq.1
where kg, ks, kr, and kc are the conductivities due to gas conduction, solid conduction, radiation
contribution and convection, respectively. In conventional insulation materials with pore sizes or
void volumes greater than about 1 mm, the convective thermal heat transfer component, kc, can
be significant at ambient pressure and temperature. However, aerogel materials have pore sizes
that average in the tens of nanometers or less and the convective component is completely
diminished. Thus for aerogel materials, the general effective thermal conductivity relationship reduces to:
 Eq.2
Gas conduction in nanoporous media such as aerogels is quite different from ordinary gas
conduction in free space. The mean free path (average distance traveled between collisions)
of gas molecules inside aerogels is significantly reduced by the small dimension of the nanoscale pores, inducing the interstitial gas molecules to collide with the pore walls more frequently than they collide with each other. The highly confined movement of atoms or molecules of gas within
an aerogel pore structure is typically defined to fall into a special category known as the
“Knudsen diffusion” regime. This has a dramatic impact on the overall thermal transport
properties of the aerogel. As a result, the gas conduction term (kg) in equation 1 expands to
the formula in equation 3:
Eq.3
where k 0g is the thermal conductivity of quiescent air (frequently cited to be around 26 mW/m-K
at ambient temperature and 1 atmosphere of pressure) and α is a constant specific to the gas in
the pores. The value for α is usually considered to be about 2 for air. The morphology of Aspen
Aerogels silica based materials (assuming a typical density of around 0.1 g/cm3) gives rise to
Knudsen numbers (Kn) of approximately 1-2 at ambient pressure, thus giving a kg term about
1/3 to 1/5 that of the gas conductivity value of quiescent air at ambient pressure and temperature.
This is a major factor explaining why aerogels are better insulators than still air and therefore,
conventional air filled insulations that are microporous or porous.
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Fig. 1 Thermal conductivity comparison between different insulation materials at ambient temperature and pressure. Values represent general averages for various product forms. |
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Fig. 2 Thermal resistance (expressed as R-value per inch) for different
insulation materials at ambient temperature and pressure. |
Thermal resistance, a measure of the ability of different materials to resist heat flow, is often
used to compare insulation performance. The comparison of R-value per inch (thermal
resistance) for the same materials shown in Figure 1 above to Aspen Aerogels’ Spaceloft™, Pyrogel®, and Cryogel™ products is shown in Figure 2.
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Aerogel materials are some of the most effective insulation fillers for vacuum applications (e.g. vacuum insulated panels or VIP’s) because of the rapid drop of the thermal conductivity as the ambient gas pressure inside of the pores is lowered (Figure 3).
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Fig. 3 Pressure dependence of Aspen Aerogels’ Spaceloft™ 6250 at 23°C after conditioning in a dry nitrogen atmosphere (data measured with Step Heating technique). |
The rapid drop of k values as the pressure decreases from one atmosphere is balanced by an
increase in thermal conductivity as the ambient pressure is increased. At pressures above one
atmosphere, the aerogel thermal conductivity continues to rise until approaching the value of
quiescent air (about 26 mWm-K). |
The amount of solid per unit volume of aerogel strongly affects the ability of radiation to
penetrate from hot side to cold side in thermal management applications. As more solid is
added to the insulation volume, the amount of interstitial gas is decreased, thus decreasing
the gas conductivity contribution to the overall thermal conductivity. Naturally, the solid thermal
conductivity is likewise dependent on the density of the solid and its structure. The three
opposing individual dependences of the conductivity contributions combine to create a minimum
thermal conductivity as a function of solid density. |
| The thermal conductivity of silica aerogel materials is also dependent on density. The various contributions to the thermal conductivity in aerogel materials are themselves dependent on various parameters (temperature, density, type of aerogel structure, type of interstitial gas, pore structure, and many others). This creates a rather complicated set of circumstances that makes it difficult to make broad generalizations of aerogel thermal properties. Aspen Aerogels scientists are continuing to refine their understanding of the fundamental drivers of aerogel thermal performance in order to support the development of the very best thermal insulation for a wide variety of applications and environments. |
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