Instability of interstitial clusters under ion and electron irradiations in ceramic materials

Abstract

One type of candidate materials for future fusion reactors is ceramics, which can be applied as radio frequency windows, toroidal insulating breaks and diagnostic probes. The degradation of physical and mechanical properties of these materials under neutron irradiation is determined by the kinetics of radiation defects including a point defect cluster formation (dislocation loops, voids and so on). The physical mechanisms of defect structure development in ceramic materials, where point defects and their clusters can have an effective charge, are completely different from those in metals. We have investigated the physical mechanisms of instability of extended interstitial defect clusters (charged dislocation loops), which were formed in stabilized cubic zirconia under electron irradiation with 100–1000 keV due to the selective displacement damage in oxygen sublattice. A new theoretical model is suggested for the explanation of the growth process and instability of the interstitial clusters. The suggested model takes into account an accumulation of effective charge on growing dislocation loops due to the trapping of electrons in dislocation cores. Our calculations show that the elastic stress and strain fields are much intense around charged dislocation loops than non-charged dislocation loops, due to an additional stress and strain fields driven by an electric field of accumulated charge. The stress induced by the charged dislocation loops with the density of trapped electrons per atom n=0.4 is found to be comparable with the theoretical yield stress of zirconia, which explains the multiplication of dislocation network at a critical size of the defect clusters observed by experiments.