Mechanism on the formation of characteristic defect clusters and dislocation multiplication in fluorite structure ceramics

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Oxides with fluorite structure, such as stabilized zirconia and ceria, have potential applications for inert matrix in the nuclear industry. Previously we have found in stabilized zirconia that electron irradiation induces characteristic defect clusters whose nature and growth process is quite different from perfect dislocation loops. The aim of this project is to gain insights into t he formation mechanism of such characteristic defect clusters, and to understand the microstructure stability of fluorite structure ceramics. The followings are the main results and conclusions obtained in this project.(1) Platelets (or dislocation loops) are formed under electron irradiation on {111} planes in stabilized zirconia, ceria, and calcium fluorite. They accompany strong strain contrast and convert into perfect dislocations when they grow up to a critical size.(2) Electron energy dependent analysis on the defect clusters in ceria have clarified the formation of oxygen-platelet less than 1250 keV and perfect dislocation loops above 1500 keV.(3) MD simulations for the relaxation process of oxygen Frenkel pairs have shown that oxygen interstitials aggregate to form two dimensional defects on {111} planes between two Ce-Ce ion planes.(4) TEM analysis and MD simulations in the present study conclude that the characteristic defect clusters are oxygen (anion)-platelets which are formed under selective displacement damage on relatively lighter element of oxygen (anion).(5) An important role of low PKAs are suggested on the radiation damage process in fluorite structure ceramics, in which lighter elements of anions aggregate to form defect clusters and multiple dislocations.