Silica aerogels thermal conductivity

Figure 23.5 (a) Relationship between pore filled with nitrogen (denoted as "free space") gas pressure and thermal conductivity of MSQ is also shown for comparison, (b) Large-area xerogel and silica aerogel. Thermal conductiv- MSQ xerogel panels can be prepared by the ity change in the free measurement space optimized process. 

Silica aerogels, a newly developing type of material, also have been produced as thermal insulations with superinsulation characteristics. The nanometer-size cells limit the gas phase conduction that can take place. The aerogels are transparent to visible light, so they have potential as window insulation. The use of superinsulations at present is limited by cost and the need to have a design that protects the evacuated packets or aerogels from mechanical damage. 

Two more recent applications for amorphous silicas are expected to grow to large volumes. Precipitated silicas are used in the manufacture of separator sheets placed between cells in automotive batteries. Their function is to provide a controlled path for the migration of conductive ions as a result of the porosity of the silica particles. Additionally, both precipitated silicas and aerogels are being developed for use in low temperature insulation, where the low thermal conductivity of the dry silica powders makes them useful in consumer products such as refrigerators (83). 

Since the pores in an aerogel are comparable to, or smaller than, the mean free path of molecules at ambient conditions (about 70 nm), gaseous conduction of heat within them is inefficient. Coupled with the fact that solid conduction is suppressed due to the low density, a silica aerogel has a typical thermal conductivity of 0.015 W/ (m-K) without evacuation. This value is at least an order of magnitude lower than that of ordinary glass and considerably lower than that of CFC (chlorofluorocarbon)-blown polyurethane foams (54). 

Thermal Insulation. In addition to their low thermal conductivity, as discussed in the section above, silica aerogels can be prepared to be highly transparent in the visible spectrum region. Thus, they are promising materials as superinsulating window-spacer. To take further advantage of its... 

One of the major characteristics of silica aerogels is their very low thermal conductivity, typically of the order of 0.015 W m at ambient temperature, pressure, and relative humidity. These values are significantly lower than the conductivity of air under the same conditions, e.g., 0.025 W m K . Thus, silica aerogels are among the best-known thermal insulating materials [124]. Besides, silica is nonflammable and silica aerogels are amorphous. 

Wei, T, Chang, T, Lu, S, Chang, Y (2007) Preparation of monolithic silica aerogel of low thermal conductivity by ambient pressure drying. J Am Ceram Soc 90(7) 2003-2007. 

When done properly, systematic studies of sol-gel synthesis and processing can facilitate the search for new materials, particularly silica aerogels, with desired properties such as low thermal conductivity, high optical transparency, improved mechanical properties, or reduced dust release behavior. 

Kistler S S, Caldwell A G (1934) Thermal conductivity of silica aerogel. Ind Eng Chan 26 658-662 Kistler S S, Swann S Jr. Appel E G (1934) Aerogel catalysts - thoria preparation of catalyst and conversions of organic acids to ketones. Ind Eng Chem 26 388-391... 

Figure 23.2. Experimentally derived values symbols) of the ratio backbone thermal conductivity/solid thermal conductivity XoHs as a function of the solid volume fraction f for different silica aerogels by Lu and Heinemaim [8,11], Also the ratio is given for porous glass Vycor [12]. The data indicate that the soUd thermal conductivity for the investigated silica aerogel samples follows a scaling law according to (23.7) with the scaling exponent a = 2 solid line).

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