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Impact of thermal coupling effects on the effective thermal conductivity of aerogels

  • Invited Paper: Nano- and macroporous materials (aerogels, xerogels, cryogels, etc.)
  • Published:
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Abstract

Nanoporous aerogels are excellent thermal insulation materials with thermal conductivities down to about 0.012 W m−1 K−1 at ambient conditions. So far, it was assumed that the total thermal conductivity of aerogels can be described by a simple superposition of the different individual heat transport contributions. However, recent investigations reveal that thermal coupling effects can result in a gas pressure dependent contribution that may be up to three times higher than expected from just a gas phase thermal conductivity, which is predicted by the Knudsen equation at given porosity and pore size. In this study, we use data from previous publications covering a gas pressure range from 10−5 to 10 MPa and analyze systematically the impact of pore size as well as solid phase and gas phase thermal conductivity on the coupling effect. The goal is to evaluate the data with respect to practical implications for aerogels in general. This means using the gas pressure dependence of the thermal conductivity of aerogels to determine their average pore size as well as allowing for a targeted optimization of aerogel-based insulations for applications at given gas pressure and temperature.

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Abbreviations

D :

mean pore size (wall distance)

F :

scaling factor

l g :

mean free path of free gas molecules

l g/D :

Knudsen number

p g :

gas pressure

p 0 :

reference gas pressure

T :

temperature

β :

gas-type dependent factor

Φ :

porosity

λ g :

gas-pressure dependent thermal conductivity

λ g,0 :

thermal conductivity of still free gas

λ Kn :

gaseous thermal conductivity according to Knudsen

λ s :

intrinsic thermal conductivity of the solid phase

λ tot :

total effective thermal conductivity

λ evac :

effective thermal conductivity in vacuum

λ parallel :

effective thermal conductivity according to a pure parallel thermal transport within two phases

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Acknowledgements

The authors would like to thank Mrs. D. Winkler and Mr. P. Ponath for support during the hot-wire measurements within the framework of their theses.

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Correspondence to Katrin Swimm.

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Swimm, K., Reichenauer, G., Vidi, S. et al. Impact of thermal coupling effects on the effective thermal conductivity of aerogels. J Sol-Gel Sci Technol 84, 466–474 (2017). https://doi.org/10.1007/s10971-017-4437-5

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  • DOI: https://doi.org/10.1007/s10971-017-4437-5

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