Structure of Defects and Microstructure Evolution in Oxide Ceramics-Role of Electronic Excitation and Selective Displacement Damage

Fluoriteand spinel-type oxide ceramics, such as yttria-stabilized zirconia, ceria, magnesium aliminate spinel, are known to be exceptionally resistant to irradiation damage with energetic particles. Those oxide compounds, thereofore, have potential applications to advanced nuclear fuels and innert matrices of transmutaion targets. Fundamental understanding of radiation damage is apparently essential to assess the microstructure stability of fuel/target materials under the hostile environment. In this presentation, structure and stability of radiation-induced defects in the oxide ceramics is reported. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) is utilized to examine the following topics focucing on role of electronic excitation and selective displacement damage: (1) effects of selective displacement damage of oxygen sublattice in fluorite-type oxides, such as stabilided cubic ZrO2, CeO2 [1-3], (2) atomic structure of ion tracks in CeO2 and MgAl2O4 induced by swift heavy ions, and microstructure evolution at high fluences under overlapping irradiation of high density electronic excitation damage [4-6], and (3) production and stability of defects in MgAl2O4 and alumina (-Al2O3) under synergistic irradiation with dispacement damage and electronic excitation [7].

It is noted in fluorite-type oxides that there exists a significant difference in mass between cations and anions, and that the displacement energy of cation-sublattice is larger than the anion one.This leads to a higher displacement damage rate or selective displacement damage in oxygen sublattice in fluorite-type oxides.Figure 1 shows examples of TEM images in CeO 2 showing dislocation loops formed by electron irradiation with energies ranging from 200 to 1250 keV [1].From contrast analysis and atomic resolution observations with TEM and STEM, those defects are found lying on {111} planes and have been discussed to be composed of solely oxygen ions formed under the selective displacement damage [1].Similar defects were formed in yttria stabilized ZrO 2 , in which entirely different growth process were observed compared to the normal (perfect) dislocation loops [2].Molecular dynamic simulations in CeO 2 , which showed the recovery process of multiple oxygen Frenkel pairs, revealed a tendency of oxygen interstitials for clustering on {111} planes, supporting the interpretation from the TEM and STEM observations [3].
Atomic resolution TEM and STEM observations in MgAl 2 O 4 and CeO 2 irradiated with 200 MeV Xe ions have shown that the crystal structures are retained at the core region of ion tracks, showing their excellent resistance to high density electronic excitation [4,5].Figure 2 is an example of atomic resolution image of ion tracks in CeO 2 with annular bright-field (ABF) STEM technique, showing that the crystal structure of Ce-cation column is retained at the core region of ion tracks, whereas O-anion column is preferentially distorted at the core region of ion tracks [5].The atomic density inside the core damage region of ion tracks in both crystals was found to be decreased, which suggests the existence of high density of vacancies and/or small vacancy clusters inside the ion tracks [5].Overlap of ion tracks at high fluence was found to develop dislocation structure in both MgAl 2 O 4 and CeO 2 .The development of dislocation structure also results in the formation of small sub-grains in CeO 2 .Those microstructure evolution was discussed with the generation of interstitial atoms at the peripheral region of the core damage region of ion tracks [4,6].
It has been shown in MgAl 2 O 4 and -Al 2 O 3 that synergistic irradiation with displacement damage and electronic excitation retards the nucleation of dislocation loops and enhance their growth [7].Electronic excitation with 200 keV electrons makes small dislocation loops of interstitial-type unstable in MgAl 2 O 4 and-Al 2 O 4 .Dislocation loops were found to be disappeared during electron irradiation.Analysis of the temperature dependence of the elimination process leads to a conclusion that the [eV] [eV] ⇒Role of oxygen point defects is important for defect kinetics in fluorite-type oxides.

Fig. 1 :Fig. 2 : 2 •
Fig. 1: Dislocation loops formed by electron irradiation ranging from 200 to 1250 keV at room temperature.Dislocation loops are considered to be interstitial-type loops consist of oxygen ions on {111} planes.
Anomalous defects in YSZ: selective displacement of O-ions 300 keV O ions ⇒ 200 keV electrons • Strong stress and strain field around the defect • Entirely different growth process • Multiplication of dislocations during the growth.K. F ~ 3×10 26 e/m 2 )

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// <111>, on (111) planes K Yasunaga et al, NIMB (2008) A model for charged disl.loop consist of O-ions  accumulation of oxygen ions, preferentially at dislocations or invisible defect clusters  oxygen ions are considered to lose electrons during diffusion process  the defect clusters are considered to trap free electrons and grow as a charged dislocation loop et al, JNM 323 (2003) 372.Charge of O -n ions HAADF STEM image of Dislocation loops in CeO 2 200 keV electron irradiation at 300 K S. Takaki et al.Mater.Res.Soc.Symp.Proc.1514 (2013) 93.HAADF STEM image  Lattice planes are strongly distorted around the dislocation loop.No additional Ce-plane is inserted at the dislocation loop, indicating that this is not the perfect dislocation loop.⇒Theloop is suggested to be on a (111) plane consist of oxygen ions.

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Ce-signal intensity decreases at the center of ion track (~2-3 nm). The size, where the Ce signal intensity is decreased, is comparable to the size of Fresnel contrast in BF image .

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15 10 16 10 17 10 18 10 19 CeO 2 irradiated with 200 MeV Xe CeO 2 irradiated with 210 MeV Xe MgAl 2 O 4 irradiated with 200 MeV Xe Fluence (ions/m 2 ) Areal density of ion tracks (m The density is saturated at high fluence, although damage area does not covers the whole region.⇒ balance between the production and recovery Topics Selective displacement damage of oxygen sub-lattice  Structure of ion tracks  Stability of dislocation loops under electronic excitation .Zinkle, MRS Symp.Proc.439 (1997)  Loop formation is suppressed by electronic excitation. Spinel is most sensitive to electroic exciation.ions ＋ 200 keV electrons K. Yasuda, Philos Mag.78 (1998) Displacement damage and electronic excitation Elimination of dislocation loops under electronic excitation Electron flux：7.0×10 22e/m 2 s Electronic stopping of 200 keV ：~1 eV/nm 300 K MgAl 2 O 4 K. Yasuda NIMB 266 (2008) 2834. Areal density of dislocation loops vs electron fluence C L exp(-lf t) CL : loop density, f : electron flux, t : irradiation time , l : cross section ⇒ Evaluation of elimination cross section: l  Cross section(l) for elimination of loops Same temp.dependence⇒samemechanism： loops dissociate into isolated interstitials  Loops in spinel is more unstable than alumina

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Size variation vs. electron fluence  Schematic showing for the elimination of loops