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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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