Aerogel applications thermal conductivity

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). 

All applications of aerogels make use of their high porosity, which is responsible for the low index of refraction, the small Young s modulus, the low acoustic impedance, the low thermal conductivity, and the excellent accessibility of the inner surface. In addition, in some applications the high optical transparency is of importance. 

Aerogels are particularly well suited for insulation applications because of their exceptionally low density, thermal stability, and high transparency. In fact, they can have a thermal conductivity only one-third that of polyurethane or polystyrene foam, and with recent process improvements that reduce the cost of manufacmre by an order of magnitude their practical use in certain construction applications is now feasible [31]. The insulating properties can be enhanced through the addition of IR opacifiers [32]. The high transparency of aerogels makes them suitable as insulation in windows or translucent panels. 

In fact, aerogels consist of a thin amorphous solid matrix network surrounded by nanoscale-sized pores, and therefore, they are reasonably described as transparent, highly porous, open-cell, extremely low-density materials. This extraordinary structure provides silica aerogels a variety of unique properties. For example, low thermal and electrical conductivity are a result of the low thermal conductivity of silica and nanometer pores sizes. The low thermal conductivity and other optical properties make them desirable for many applications. For example, they can be an attractive alternative in insulating applications, due to their high insulating value and environment-friendly production methods. They also possess low refractive index, low sound velocity, and low dielectric constant. 

Like in any porous insulation material, the total heat losses are the sum of the skeletal conduction (phonons), gas conduction (collisions between gas molecules), and radiation contributions. The particular mesoscopic structure of aerogels leads to a considerable reduction of the gas-phase conduction contribution due to a trapping effect of the pore gas. For most terrestrial applications, the thermal conductivity at ambient temperature and pressure is relevant, which although well studied, is still a challenge to determine accurately [203], primarily due to the lack of large-area homogeneous aerogel specimens. 

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