THERMAL CONDUCTIVITY OF EVACUATED LOAD-BEARING POWDER AND FIBRE INSULATIONS UNDER VARIABLE EXTERNAL LOAD.

New measurements of the thermal conductivity of several evacuted load-bearing porous insulations at temperatures between 300 and 650 K are described. In an earlier set of measurements the external load was fixed at 1 bar. The large guarded hot plate device has now been altered to provide variable mechanical boundary conditions, and measurements have also been taken at smaller external loads (10 mbar-1 bar). Data are analyzed in terms of radiative and the solid conduction contributions to the measured total heat flow. The dependence of the solid conductivity on the external load is compared with a theoretical model.