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Proceedings Paper

New methods of characterizing the thermal properties of silica aerogel/xerogel
Author(s): R. Stangl; Werner J. Platzer; Volker Wittwer
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Paper Abstract

Integral as well as spectral methods to characterize the thermal properties of Silica Aerogel and Xerogel samples are presented. A new method to determine the effective thermal conductivity with the help of an infrared camera was developed, which allows for measuring small samples. Keeping the lower surface of the samples at a constant temperature, the IR camera monitors the upper surface temperature. With the help of a reference sample of known thermal conductivity, the total heat flux through the samples can be calculated. This is done under different pressure conditions in order to observe the Knudsen effect. As there is a complex coupling between the different modes of heat transport, the spectrum of the emitted radiation is of interest. Emission spectroscopy on Silica Aerogel has not been previously investigated in detail. An emission chamber was constructed, which permits measuring of the emitted spectra due to an applied temperature gradient using a Perkin Elmer Infrared Grating spectrometer. Different boundary emissivities and varying pressure conditions can be applied. For a correct interpretation of the measured spectra, a numerical treatment of the underlying heat transport mechanism is necessary. Measurements as well as computer simulations are presented. Computer simulations show that a variation of the total thermal conductivity in the range between 1 mW/mK and 30 mW/mK leads to a characteristic damping of the emitted radiation between 500 cm-1 and 2000 cm-1. This characteristic damping of the emitted radiation can be observed by measuring the emitted spectra of Silica Aerogel/Xerogel at varying pressure conditions. Therefore, in principle, it should be possible to extract the thermal conductivity through the gas pores and through the silica skeleton. Boundary effects, however, prevent this data from being extracted. Hopefully these problems will be solved in the near future.

Paper Details

Date Published: 9 September 1994
PDF: 12 pages
Proc. SPIE 2255, Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XIII, (9 September 1994); doi: 10.1117/12.185409
Show Author Affiliations
R. Stangl, Fraunhofer-Institut fuer Solare Energiesysteme (Germany)
Werner J. Platzer, Fraunhofer-Institut fuer Solare Energiesysteme (Germany)
Volker Wittwer, Fraunhofer-Institut fuer Solare Energiesysteme (Germany)

Published in SPIE Proceedings Vol. 2255:
Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XIII
Volker Wittwer; Claes G. Granqvist; Carl M. Lampert, Editor(s)

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