Regensburg 2004 – wissenschaftliches Programm

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M: Metallphysik

M 18: Postersitzung

M 18.19: Poster

Dienstag, 9. März 2004, 14:30–16:30, Saal C

Thermal-Spike-Induced Crystal Growth in Nanocrystalline Nickel — •Th. Zumkley, G. Schumacher, and S. Klaumünzer — Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109 Berlin

In nanocrystalline materials many physical properties are altered compared to their coarser-grained counterparts because 30% of the atoms belong to or are affected by the presence of interfaces. Due to their distorted structure these interfaces are expected to affect also the behavior of radiation-induced defects and the time evolution of thermal spikes. It is argued that a small grain size leads to a reduced thermal conductivity and, consequently, to an enhancement of the spike lifetime.

Nanocrystalline Ni foils of 100 µm thickness were irradiated uniformly with 200 MeV Kr, 230 MeV Xe or 350 MeV Au ions. For the evaluation of the x-ray diffraction data both the (111) and (200) peaks have been used.

Obviously, at the beginning ion bombardment induces a rapid crystal growth accompanied by a rapid decrease of microstrains. This process ends on a fluence scale of a few 10¹³ ions/cm² and is followed by a comparatively slow process. Therefore, the fast process is definitively caused by the electronic energy loss. We deduce from the data an increase of the mean diameter of the nanocrystals by (1±0.1)nm prior to and after irradiation. If we assume that crystal growth proceeds isotropically this finding implies that about 25% of the specimen atoms are involved in the fast process. Probably, a thermal spike released by the electronic excitations in a radius of about 2 nm around a projectile^′s path induces a rearrangement of all atoms to a final position in the grain boundaries.