Sandra Moll

Structural modification of nanocrystalline ceria by ion beams

Exceptional size-dependent electronic-ionic conductivity of nanostructured ceria can significantly alter materials properties in chemical, physical, electronic and optical applications. Using energetic ions, we have demonstrated effective modification of interface volume and grain size in nanocrystalline ceria from a few nm up to ∼25 nm, which is the critical region for controlling size-dependent material property. The grain size increases and follows an exponential law as a function of ion fluence that increases with temperature, while the cubic phase is stable under the irradiation. The unique self-healing response of radiation damage at grain boundaries is utilized to control the grain size at the nanoscale. Structural modification by energetic ions is proposed to achieve desirable electronic-ionic conductivity.

The effect of electronic energy loss on irradiation-induced grain growth in nanocrystalline oxides

Grain growth of nanocrystalline materials is generally thermally activated, but can also be driven by irradiation at much lower temperature. In nanocrystalline ceria and zirconia, energetic ions deposit their energy to both atomic nuclei and electrons. Our experimental results have shown that irradiation-induced grain growth is dependent on the total energy deposited, where electronic energy loss and elastic collisions between atomic nuclei both contribute to the production of disorder and grain growth. Our atomistic simulations reveal that a high density of disorder near grain boundaries leads to locally rapid grain movement. The additive effect from both electronic excitation and atomic collision cascades on grain growth demonstrated in this work opens up new possibilities for controlling grain sizes to improve functionality of nanocrystalline materials.

Comprehensive study of the effect of the irradiation temperature on the behavior of cubic zirconia

Cubic zirconia single-crystals (yttria-stabilized zirconia (YSZ)) have been irradiated with 4 MeV Au2+ ions in a broad fluence range (namely from 5 × 1012 to 2 × 1016 cm−2) and at five temperatures: 80, 300, 573, 773, and 1073 K. Irradiated samples have been characterized by Rutherford backscattering spectroscopy in channeling mode, X-ray diffraction and transmission electron microscopy techniques in order to determine the disordering kinetics. All experimental results show that, whatever is the irradiation temperature, the damage build-up follows a multi-step process. In addition, the disorder level at high fluence is very similar for all temperatures. Thus, no enhanced dynamic annealing process is observed. On the other hand, transitions in the damage accumulation process occur earlier in fluence with increasing temperature. It is shown that temperature as low as 573 K is sufficient to accelerate the disordering process in ion-irradiated YSZ. ACKNOWLEDGMENTS

Irradiation damage in Gd{sub 2}Ti{sub 2}O{sub 7} single crystals: Ballistic versus ionization processes

The structural transformations induced in Gd{sub 2}Ti{sub 2}O{sub 7} single crystals irradiated at high energies (870-MeV Xe), where ionization processes (electronic stopping) dominate, and at low energies (4-MeV Au), where ballistic processes (nuclear stopping) dominate, have been studied via the combination of Rutherford backscattering spectrometry and channeling (RBS/C), Raman spectroscopy, and transmission electron microscopy (TEM) experiments. At high energy, amorphization occurs directly in individual ion tracks from the extreme electronic-energy deposition, and full amorphization results from the overlapping of these tracks as described by a direct impact model. The track diameters lie in the range 6-9 nm. At low energy, amorphization occurs via indirect processes, driven by ballistic nuclear energy deposition from the ions, that is accounted for in the framework of both direct-impact/defect-stimulated and multi-step damage accumulation models. The ion fluence for total amorphization of the irradiated layer is much higher at low energy (0.5 ion nm{sup -2}) than at high energy (0.05 ion nm{sup -2}), consistent with the nuclear stopping at low energy (5.2 keV/nm) compared to the electronic stopping at high energy (29 keV/nm).