Thermal properties of organic and inorganic aerogels

Abstract

Aerogels are open-cell foams that have already been shown to be among the best thermal insulating solid materials known. This paper examines the three major contributions to thermal transport through porous materials, solid, gaseous, and radiative, to identify how to reduce the thermal conductivity of air-filled aerogels. We found that significant improvements in the thermal insulation property of aerogels are possible by (i) employing materials with a low intrinsic solid conductivity, (ii) reducing the average pore size within aerogels, and (iii) affecting an increase of the infrared extinction in aerogels. Theoretically, polystyrene is the best of the organic materials and zirconia is the best inorganic material to use for the lowest achievable conductivity. Significant reduction of the thermal conductivity for all aerogel varieties is predicted with only a modest decrease of the average pore size. This might be achieved by modifying the sol-gel chemistry leading to aerogels. For example, a thermal resistance value of R = 20 per inch would be possible for an air-filled resorcinol-formaldehyde aerogel at a density of 156 kg/m3, if the average pore size was less than 35 nm. An equation is included which facilitates the calculation of the optimum density for the minimum total thermal conductivity, for all varieties of aerogels.

Aerogel insulation for building applications: A state-of-the-art review

“…Furthermore, the inner skeleton structure has many 'dead-ends' resulting in an ineffective and long tortuous path of thermal transport. A typical minimum does exist in the thermal conductivity's dependence on density for aerogels: With a density below this optimum, the pore diameter increases followed by an increase of λ g (see Eqns.2-3) [27], whereas (ideally) a low mass density may also be envisioned with small pores.…”

Interfacial electron and phonon scattering processes in high-powered nanoscale applications.

“…For example, measuring heat transport in highly thermally insulating materials such as aerogels, let alone aerogel thin films, has proven to be particularly challenging using standard approaches due to convective and radiative losses. [137][138][139][140] Understanding heat transport in thin porous films is critical for low-k dielectric applications in microelectronics as well as optical coatings for solar panels. 44 In this work, we overcome these challenges and demonstrate the use time domain thermoreflectance (TDTR) to measure the thermal conductivity of aerogel thin-films from 80 -294 K. We adopt the "probe through the glass" the test geometry, as discussed in Section 4.3.…”

Synthesis, Structural and Thermal Properties of Nano-porous SiO2-based Aerogels

“…Carbon aerogels or porous carbons synthesized via the sol-gel route, are considered as very interesting materials for high-temperature thermal insulations in non-oxidizing atmospheres or vacuum (Pekala, 1989;Hrubesh & Pekala, 1994). Carbon aerogels are open porous solids consisting of a three-dimensional network of spherical interconnected primary particles.…”