Optimization of monolithic silica aerogel insulants

Low-cost and fast synthesis of nanoporous silica cryogels for thermal insulation applications

“…However, there are only a few reports on the thermal conductivity of aerogels/xerogels [8,[31][32][33][34]. Figure 4 compares the thermal conductivities of typical silica cryogel powders (K C , X = 11) with those of pure nitrogen and a standard reference material (SRM, Glass fiberboard, NIST-certified 1450c, 300 × 300 × 25 mm) at 25…”

“…Firstly, the solid conductivity (λ s ) strongly depends on the density (ρ), which can be expressed as λ s < ρ α for ρ = 70-300 kg m −3 [31]. The loose silica cryogel networks obtained using TBA as a solvent have low densities in the range of 0.08-0.18 g cm −3 (table 1), which can effectively reduce the heat transfer from the contacted silica particles [32].…”

“…However, K N is much higher than K C and K S at pressures below 100 Pa, at which the heat conduction takes place directly via the nitrogen molecular vibrations. Secondly, the radiative conduction was prevented partially via the multiple absorption, re-emission and scattering processes [35] owing to the nanoporous structures of the silica cryogels at low temperature (<100 • C), whereas infrared opacifiers must be doped to resist the radiative heat conduction for the high-temperature application due to the absorption of silica at 3-8 µm [8,31]. Thirdly, the gaseous conduction in the porous structure relating to the pressure of the system is more complicated.…”

“…Thirdly, the gaseous conduction in the porous structure relating to the pressure of the system is more complicated. The porous structure of silica cryogels with small homogeneous pores as shown in figure 2(b) can significantly hinder the gaseous conduction according to Knudsen's equation [31]. At low pressures, the mean free path (λ) is larger than the average pore size (l) of the samples according to the Maxwell average molecular free path equation (1) where k is the Boltzmann constant, T is the temperature, d is the effective particle diameter and P is the pressure of the system.…”

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.…”