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On the basis of considerations about classic diffusion and migration in crystalline solids with prevailing ionic bonding a microscopic degradation equation for ZnO varistors is developed, where different possibilities of Coulomb potential deflection at phase boundaries are included. The model is compared with experimental results of degradation and recovery of ZnO varistors under dc loading and without loading.

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This paper is focused on clarifying behaviour of concrete at elevated temperature with employing new test set-up constructed at Institute of Building Construction and Technology, Vienna University of Technology. This unique test set-up allows measuring gas permeability of different building materials such as concrete or ceramic at both high temperature (up to 400°C) and pressure (up to 6 bars). Present paper illustrates a new set up for permeability measurement during the heating and cooling and different testing procedures and evaluation of their influence on results.

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