Aerogel Crystal Structure
When Stephen Kistler first discovered Silica AeroGel he found that
one of the most extraordinary properties of Silica AeroGel was the
extremely low thermal conductivity. He also found that the thermal
conductivity decreased even more under a vacuum. During the 1930’s,
when Kistler first discovered AeroGel, thermal insulation was a low
priority and the use of AeroGel in insulation was not pursued. Around
1980 AeroGel technology coincided with an increased concern for energy
efficiency. It was obvious that Silica Aerogels were a good alternative
to traditional insulation due to their high insulating value and
environment-friendly production methods. Unfortunately, the production
costs of the material were too expensive for cost-sensitive industries
like residential housing.
The passage of thermal energy through an insulating material
occurs through three mechanisms; solid conductivity, gaseous
and radiative transmission. The sum of these three components gives
the total thermal conductivity of a material. Solid conductivity
is a fundamental property of a specific material. For dense silica,
solid conductivity is relatively high. A window pane transmits
a large amount of thermal energy, however silica AeroGel possess
very small amount of solid silica. The solids that are present
consist of very small particles linked in a three-dimensional
with many dead ends. That makes thermal transport through the solid
portion of silica AeroGel a very indirect path and not particularly
effective. The space that is not occupied with solid silica is
normally filled with air unless it is sealed under a vacuum.
transport thermal energy through the AeroGel. The final mode of
thermal transport through silica AeroGel involves infrared
advantage of AeroGel for insulation purposes is their transparency
which would allow their use in windows and skylights, but they
are reasonably transparent in infrared. At low temperatures,
component of thermal transport is low. At higher temperatures,
radiative transport becomes the dominant mode of thermal conduction.
to calculate the total thermal conductivity from these three components
can be difficult because they are coupled with each other. For
example changing the infrared absorbency changes the solid
It is easier to measure the total thermal conductivity rather than
trying to predict the effect of changing one component. The lab
at Berkley University designed and built an instrument for
thermal conductivity for large panels of AeroGel. The Vacuum Conductivity
Tester On Rollers or VICTOR is a thin-film heater based device
that can measure the thermal conductivity of AeroGel panels
of various gasses.
There is not much that can be done to reduce thermal transport
through the solid part of silica AeroGel, except by making it
which reduces the amount of solid present. The problem with this
is it makes the AeroGel mechanically weaker. The radiative component
of thermal conductivity becomes more important as the temperature
increases. If AeroGel is to be used at a temperature above 200
degrees Celsius, this mode of energy transport must be suppressed.
be accomplished by adding an additional component to the AeroGel.
The component must either absorb or scatter infrared radiation.
Elemental Carbon is an effective absorber of infrared radiation
increases the mechanical strength of the AeroGel.
Silica AeroGel is the lightest solid on the planet and has the
greatest thermal conductivity of any material. There are many
for AeroGel because of its thermal properties. Sometime in
the near future AeroGel could find its way in to the insulation