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. The improvement of thermal resistance of the building envelope can be achieved by decreasing the thermal conductivity.
Fricke et al. observed that both the solid conductivity and the gas conductivity were proportional to the density as shown below:
Hümmer et al. using these relations derived the following relation for the radiative conductivity, which is a relative equation for the thermal conductivity of opacified silica aerogels:
where (kg/m3) is the density (W/m.K) are the total conductivity, the conductivity for gas conduction, the conductivity for solid conduction, and the radiative conductivity, respectively; is the temperature, and the index 0 means that parameters are related to a reference material from an aerogel .
Aerogel is made of more than 90% of air, having extremely low weight, transparency, and excellent thermal conductivity. Aerogel is an ideal material for thermal insulation due to all these properties. Also their high visible solar transmittance is desirable for application in windows. Further decrease in thermal conductivity of aerogel can be observed if evacuated below 50 hPa; thermal conductivity decreased because of elimination of pore gas. Superinsulations with extremely low thermal conductivities can be implemented with evacuated highly porous powder, fiber, or gel spacers. Due to the Knudsen effect, thermal conductivity can become lower than that for the still air, that is, even less than 25 mW/m.K .
For example, silica aerogel is a highly porous material with pore diameters in the range of 10–100 nm. The porosity is more than 90% with a thermal conductivity lower than that of air, which makes these aerogels a highly insulating material. 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 through the material. The final mode of thermal transport through silica aerogels involves infrared radiation . Soleimani Dorcheh and Abbasi reported the synthesis of nanostructured silicon based transparent aerogels with pore diameter 20–40 nm.
Water molecules do not interact strongly with the hydrophobic aerogel pore walls and therefore will not lose much energy in colliding with the wall and the progress of these molecules will not be significantly slowed. Accordingly, the aerogel possesses high breathability, that is, high permeation selectivity between water vapor and agent vapors. Titania aerogels demonstrated an excellent mesoporous structure for application as photoanodes of dye-sensitized solar cells with power conversion efficiency improvement of 16% . Sol-gel derived silica has found tremendous applications as a biocompatible scaffold for the immobilization of cells. A new method for rapid, reproducible, and sensitive detection of rhizobia using aerogels has been reported for the first time.
Thermal insulation properties of aerogels are closely related to their acoustic properties too. The acoustic propagation in aerogels depends on the interstitial gas nature and pressure, density, and more generally the texture . Different applications of aerogels are given in Figure 4.