43Vol. XXVIII, N. 1-2 - Gennaio-Giugno 1999

Characterization and Optimization of Filler Materialsfor Vacuum Super Insulations

Ulrich Heinemann *, Roland Caps, Jochen FrickeBavarian Center for Applied Energy Research (ZAE Bayern), Am Hubland, D-97074 Wuerzburg, Germany

Low emissivity foils are excellent to minimize heat transfer in highly evacuated insulations, like dewars for cryogenics, where

the vacuum containment sustains the external load of the ambient pressure. In flat vacuum super insulations to be used for

example in refrigerators or building facades several types of foams, fibres, powders and gels may be used also as load bearing

fillers. Experimental methods to analyse the contributions to the total heat transfer, i.e. radiative heat transfer, solid conduction,

gaseous conduction and coupling terms are described. Possible means to reduce each heat transfer mode are given. The achiev-

able minimal thermal conductivity depends on the conditions of use, especially the applied temperature. For applications not only

the thermal performance is important but also the compression during the evacuation process, the time needed for evacuation and

the pressure level within the panel, which has to be maintained during the life time. Different types of materials require different

leak thightness of the envelope. The coarser the structure of the fill, the lower the required residual gas pressure and thus the lower

the permeation rate through the containment have to be. Typical values of thermal and mechanical properties will be given for the

above mentioned types of filler materials.

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