Experimental Investigations on the Thermal Conductivity of Silica Aerogels by a Guarded Thin-Film-Heater Method

Abstract

A thin-film-heater method is setup to measure the thermal conductivity of super insulating materials such as silica aerogels. The experimental setup is purposely designed for insulating materials and allows direct measurement of the thermal conductivity. Few experimental data are available in the literature concerning thermal conductivity of aerogels even though these materials are of major interest in thermal insulation. More data are necessary in order to understand thermal transport and to validate existing models. Monolithic and granular silica aerogels are investigated. Our experimental technique enables to quantify the influence of important parameters, such as air pressure and distribution of grain sizes, on the insulating performance of this material.

Analysis of Gas Molecule Mean Free Path and Gaseous Thermal Conductivity in Confined Nanoporous Structures

“…Use as a thermal insulation is one of most attractive applications of silica aerogel, and this potential use has been investigated comprehensively in the past two decades. The heat transfer mechanisms in silica aerogel include gaseous conduction, solid conduction, and thermal radiation [7][8][9]. Convective heat transfer can be ignored completely since the pores in aerogel are nanometer in size [10][11][12][13].…”

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

“…Thermal conductivity as a function of volumetric atomic number density of silicon and oxygen for the EISA, aerogel, and calcined-aerogel films along with bulk SiO 2 , 49 a sputtered SiO 2 thin film, 13 and bulk SiO 2 aerogels. 139,145 Predictions from the theoretical "porous minimum limit" to the thermal conductivity of SiO 2 (solid line -Eq. (35)) that is derived in this work shows good agreement with the thermal conductivity of the porous silica structures...................46 Figure 20.…”

“…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.…”

“…Our vacuum pumps the cryostat chamber down to less than 1.0 mTorr, which is sufficient pressure to remove any contribution we would have observed from conduction through the air in the aerogel samples. 139 Given this observation, we can now theoretically analyze the heat flow in the porous sample (i.e., aerogel film) during TDTR via the ``two-fluid'' model for heat transfer in porous media. 144 In the most general of experiments in which energy is absorbed in both the solid (silica) and fluid (air) phases, and the two phases are not in equilibrium, the heat conduction is governed by ( ) …”

“…19 along with the thermal conductivity of bulk SiO 2 , 49 a sputtered SiO 2 thin film, 13 other porous silica materials (XLK and FOx), 40 and bulk SiO 2 aerogels. 139,145 The error bars in our measurements represent the standard deviation about the mean value of κ determined from multiple data sets taken on each sample type (upwards of 10 different TDTR scans taken on each sample and two samples of each type of silica film). We measure the SiO 2 compositional percentage 146 and sample porosity in the EISA and aerogel films using ellipsometry and surface acoustic wave techniques.…”