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Thermal conductivity of IR-opacified silica aerogel powders between 10 K and 275 K

The thermal conductivity of evacuated pure and opacified silica aerogel powders was measured in a guarded hot plate apparatus for temperatures from 10 K up to 275 K and external loads from 1.5*104 Pa to 105 Pa. Infrared optical measurements of the extinction give the possibility to separate contributions from solid thermal conduction and radiative thermal transport. Measured thermal conductivities range from 29 to 80*10-6 W m-1 K-1 at a temperature of 15 K and from 0.9 to 3.1*10-3 W m-1 K-1 at a temperature of 250 K. The solid thermal conductivities lambda powder of the powders were compared with those of the monolithic silica aerogel ( lambda monolith) the powder grains consist of. The ratio g'= lambda monolith/ lambda powder was found to be temperature independent in the investigated temperature range. Thus g' can be interpreted as a geometrically induced reduction factor. The mean thermal conductivities averaged over the application temperatures range from 0.7 to 2.2*10-3 W m-1 K-1 for boundary temperatures of 77.6 and 290 K and from 0.5 to 1.8*10-3 W m-1 K-1 for boundary temperatures of 20.4 and 290 K. The investigated silica aerogel powders show a large improvement potential for cryogenic thermal powder insulations. They are well suited to substitute expanded Perlite as cryo insulation.