Science of Silica Aerogels
After preparing the first silica aerogels, Kistler proceeded to characterize them as thoroughly as possible. One of the extraordinary properties that he discovered was their very low thermal conductivity. Kistler also found that the thermal conductivity decreased even further under vacuum. However, in the 1930's thermal insulation was a low priority and applications of aerogels in insulation systems was not pursued. The renaissance of aerogel technology around 1980 coincided with an increased concern for energy efficiency and the environmental effects of chlorofluorocarbons (CFC's). It was then readily apparent that silica aerogels were an attractive alternative to traditional insulation due to their high insulating value and environment-friendly production methods. Unfortunately, the production costs of the material were prohibitive to cost-sensitive industries such as housing. A significant research effort was undertaken, and is continuing, at several institutions worldwide (including Berkeley Lab) to circumvent this problem by increasing the insulative performance and lowering the production costs of silica aerogels.
The passage of thermal energy through an insulating material occurs through three mechanisms; solid conductivity, gaseous conductivity, and radiative (infrared) transmission. The sum of these three components gives the total thermal conductivity of the material. Solid conductivity is an intrinsic property of a specific material. For dense silica, solid conductivity is relatively high (a single-pane window transmits a large amount of thermal energy). However, silica aerogels possess a very small (~1-10%) fraction of solid silica. Additionally, the solids that are present consist of very small particles linked in a three-dimensional network with many "dead-ends". Therefore, thermal transport through the solid portion of silica aerogel occurs through a very tortuous path and is not particularly effective. The space not occupied by solids in an aerogel is normally filled with air (or another gas) unless the material is sealed under vacuum. These gases can also transport thermal energy through the aerogel. The pores of silica aerogel are open and allow the passage of gas (albeit with difficulty) through the material (see the section on the Pore Structure of aerogels). The final mode of thermal transport through silica aerogels involves infrared radiation. An advantage of silica aerogels for insulation applications is their visible transparency which will allow their use in windows and skylights. However, they are also reasonably transparent in the infrared (especially between 3-5 microns). At low temperatures, the radiative component of thermal transport is low, and not a significant problem. At higher temperatures, radiative transport becomes the dominant mode of thermal conduction, and must be dealt with. The infrared spectrum of silica aerogel can be found in the section on Optical Properties.
Attempting to calculate the total thermal conductivity arising from the sum of these three modes can be difficult, as they modes are coupled (changing the infrared absorbency of the aerogel also changes the solid conductivity, etc.). It is generally easier to measure the total thermal conductivity directly rather than predict the effect of changing one component. To achieve this, the Microstructured Materials Group at Berkeley Lab designed and built an economical, but accurate, instrument for measuring the thermal conductivity of large aerogel panels. The Vacuum Insulation Conductivity Tester (On Rollers)—VICTOR, is a thin-film heater based device that can measure the thermal conductivity of panels up to 26 cm on edge, with pressures of various gases down to 0.01 Torr.
Minimizing the Solid Component of Thermal Conductivity
There is little that can be done to reduce thermal transport through the solid structure of silica aerogels. Lower density aerogels can be prepared (as low as 0.003 g/cm3), which reduces the amount of solid present, but this leads to aerogels that are mechanically weaker. Additionally, as the amount of solids decreases the mean pore diameter increases (with an increase in the gaseous component of the conductivity). These are, therefore, generally not suitable for insulation applications. However, as noted above, the tortuous solid structure of silica aerogels leads to an intrinsically low thermal transport. Granular aerogels have an extremely low solid conductivity component. This is due to the small point of contact between granules in an aerogel bed. However, in granular aerogel, the inter-granule voids increase the overall porosity of the material thereby requiring a higher vacuum to achieve the maximum performance (see below).
Minimizing the Gaseous Component of Thermal Conductivity
A typical silica aerogel has a total thermal conductivity of ~0.017 W/mK (~R10/inch). A major portion of this energy transport results from the gases contained within the aerogel. This is the transport mode that is most easily controllable. As a consequence of their fine pore structure, the mean pore diameter of an aerogel is similar in magnitude to the mean free path of nitrogen (and oxygen) molecules at standard temperatures and pressures. If the mean free path of a particular gas were longer than the pore diameter of an aerogel, the gas molecules would collide more frequently with the pore walls than with each other. If this were the case, the thermal energy of the gas would be transferred to the solid portion of the aerogel (with its low intrinsic conductivity). Lengthening the mean free path relative to the mean pore diameter can be accomplished in three ways: by filling the aerogel with a gas with a lower molecular mass (and a longer mean free path) than air, by reducing the pore diameter of the aerogel, and by lowering the gas pressure within the aerogel.
The first of these methods is generally not practical, as light gases are relatively expensive and would eventually escape the system. The mean pore diameter can be reduced by increasing the density of the aerogel. However, any benefit from a lower gaseous conductivity component is counteracted by an increase in the solid conductivity component. The pore diameter can be reduced somewhat (while keeping the aerogel's density constant) by using the two-step process to prepare the aerogel (see the section on Making Aerogel). The greatest improvement is found by reducing the gas pressure. Vacuum insulations are commonplace in various products (such as Thermos bottles). These systems generally require a high vacuum to be maintained indefinitely to achieve the desired performance. In the case of aerogels, however, it is only necessary to reduce the pressure enough to lengthen the mean free path of the gas relative to the mean pore diameter. This occurs for most aerogels at a pressure of about 50 Torr. This is a very modest vacuum that can be easily obtained and maintained (by sealing the aerogel in a light plastic bag).
Minimizing the Radiative Component of Thermal Conductivity
As noted above, the radiative component of thermal conductivity becomes more important as temperatures increase. If silica aerogels are to be used at temperatures above 200 degree C, this mode of energy transport must be suppressed. This can be accomplished by adding an additional component to the aerogel, either before or after supercritical drying. (See the section on Nanocomposites). The second component must either absorb or scatter infrared radiation. A major challenge for this process is to add a component that does not interfere with the mechanical integrity of the aerogel or increase its solid conductivity. One of the most promising additives is elemental carbon. Carbon is an effective absorber of infrared radiation and, in some cases, actually increases the mechanical strength of the aerogel.