Characteristics of Radiation-induced Defects in Fluorite Structure Oxides Formed by Electron Irradiation

Abstract

Oxide ceramics with fluorite structure attracts considerable attention as a host of inert matrix fuels and a transmutation target for minor actinides and long life fission products. We will report the characteristic features of defect clusters in oxide ceramics with fluorite structure, such as yttria stabilized zirconia (YSZ: 13 mol% Y₂O₃-ZrO₂), ceria (CeO₂), which were investigated in situ by transmission electron microscopy under electron irradiation. Results on calcium fluorite (CaF₂) will be also shown for a comparison [1]. Electron irradiation with 100 to 1000 keV was found to induce large defect clusters in YSZ, which accompany strong black/black lobes contrast and multiply dislocation networks when they grow to a critical diameter of about 1.0 ìm. The defect clusters were considered to be charged oxygen platelets formed by the selective displacement damage of oxygen sublattice. A large difference in mass of constituent ions (O and Zr/Y ions) attributes the selective displacement damage under electron irradiation [2]. Similar defect clusters with strong diffraction contrast were formed in CaF₂ under 200 keV electron irradiation, and they also multiplied dislocations at a critical diameter [1]. In the case of CeO₂, a higher density of defect clusters was formed compared with YSZ and CaF₂. Defect clusters formed under electron irradiation less than 1250 keV were found to be faulted-prismatic-interstitial loops lying on {111} planes with Burgers vector parallel to <111> directions [3], whereas electron irradiation with 1500, 2000 and 3000 keV induces perfect dislocation loops of 1/2<110>{110} in nature. Formation of radiation-induced defects is found to depend on electron energy for crystals with fluorite structure investigated in this study. Results will be discussed through selective displacement damage and charge accumulation. This work was financially supported by the Budget for Nuclear Research of MEXT, based on the screening and counseling by the Atomic Energy Commission, and by the Grant-in-Aid for Scientific Research (B) from JSPS. [1] M. Watanabe, T. Noma, K. Yasuda, K. Yasunaga, S. Matsumura and C. Kinoshita, Proc. of 16th Electron Mmicroscopy Congress, (2006) 1854. [2] K. Yasuda, C. Kinoshita, S. Matsumura, A.I. Ryazanov, J. Nucl. Mater., 319 (2003) 74. [3] K. Yasunaga, K. Yasuda, S. Matsumura and T. Sonoda, Nucl. Instr. and Meth. B, 250 (2006) 114.