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Aerogel Crystal Structure and Properties


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 conductivity, 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 a very small amount of solid silica. The solids that are present consist of very small particles linked in a three-dimensional crystal lattice 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. These gasses transport thermal energy through the AeroGel. The final mode of thermal transport through silica AeroGel involves infrared radiation. An 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, the radiative component of thermal transport is low. At higher temperatures, radiative transport becomes the dominant mode of thermal conduction. Attempting 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 conductivity. 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 measuring 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 with pressures 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 less dense, 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. This can 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 and actually 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 practical uses for AeroGel because of its thermal properties. Sometime in the near future AeroGel could find its way in to the insulation of every household.

 
References
  • www.aerogel.com
  • www.cae.wisc.edu/aerogel/aboutaerogel.html
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