THERMAL-CONDUCTIVITY OF IR-OPACIFIED SILICA AEROGEL POWDERS BETWEEN 10 K AND 275 K

The thermal conductivity of evacuated pure and opacified silica aeroge l powders was measured in a guarded hot plate apparatus for temperatur es from 10 K up to 275 K and external loads from 1.5 x 10(4) Pa to 10( 5) Pa. Infrared optical measurements of the extinction give the possib ility to separate contributions from solid thermal conduction and radi ative thermal transport. Measured thermal conductivities range from 29 to 80 x 10(-6) W m(-1) K-1 at a temperature of 15 K and from 0.9 to 3 .1 x 10(-3) W m(-1) K-1 at a temperature of 250 K. The solid thermal c onductivities lambda(powder) Of the powders were compared with those o f the monolithic silica aerogel (lambda(monolith)) the powder grains c onsist of. The ratio g' = lambda(monolith)/lambda(powder) was found to be temperature independent in the investigated temperature range. Thu s g' can be interpreted as a geometrically induced reduction factor. T he mean thermal conductivities averaged over the application temperatu res range from 0.7 to 2.2 x 10(-3) W m(-1) K-1 for boundary temperatur es of 77.6 and 290 K and from 0.5 to 1.8 x 10(-3) W m(-1) K-1 for boun dary temperatures of 20.4 and 290 K. The investigated silica aerogel p owders show a large improvement potential for cryogenic thermal powder insulations. They are well suited to substitute expanded Perlite as c ryo insulation.