The thermal conductivity of thermally insulating powders like silica aerogel (opacified with carbon black), precipitated silica, perlite and diatomite was measured by a stationary method in an evacuated guarded flat plate apparatus. The mean temperature was varied between 10 and 275 K, while the uniaxial external pressure load was varied from 0.15 up to 1bar.

The measured thermal conductivities are in the range of 0.1 • 10⁻³ W/(m • K) at T = 20K up to 3 • 10⁻³ W/(m • K) at T = 275 K for silica aerogel powders, 0.2 • 10⁻³ W/(m-K) and 3.5.10⁻³ W/(m •K) for precipitated silica, 0.3 •10⁻³ W/(m •K) and 5 • 10⁻³ W/(m • K) for perlite and 0.1 • 10⁻³ W/(m • K) and 7 • 10⁻³ W/(m • K) for diatomite, respectively.

The results are interpreted using two models which describe the thermal conductivity of a powder fill in terms of density, external load, elastic properties and the thermal conductivity of the monolithic material.

For better comprehension geometric factors g’ = λ_mat/λ_pow (ratio of the thermal conductivity of monolithic material and powder) and g = λ_{vitreous silica}/λ_pow (ratio of the thermal conductivity of vitreous silica to the one of powder) are introduced. They describe the ‘reduction’ of thermal conductivity caused by the ‘dilution’ of the material and the intergranular contact resistances.

The experimentally derived values of g are in the range of 100 to more than 1000. The values of g’ are in the range of 4 to 20. We tried to reproduce these values by using suitable models and realistic parameters.