Interface structure studies by atomic resolution electron microscopy, order–disorder phenomena and atomic diffusion in gas-phase synthesized nanocrystalline solids

The paper summarizes recent studies of the structure and atomic diffusion properties of gas-phase synthesized nanocrystalline solids. The atomic structure of interfaces in nanocrystalline solids with vacancy-like free volumes and nanovoids of triple junctions is specifically studied by positron lifetime spectroscopy. The recently studied temperature variation of the positron lifetime indicates a strong temperature dependence of the positron trapping rate of these free volumes. From the investigation of the orientation correlationship of pairs of two adjacent crystallites in n-Pd by atomic resolution microscopy it can be concluded that predominantly high-energy interfaces are present in nanocrystalline metals after gasphase synthesis. Tracer substitutional-diffusion and self-diffusion studied in highly dense nanocrystalline metals demonstrate that the atomic diffusion is similar to that in conventional grain boundaries. The 18O diffusion in the interfaces of n-ZrO2 is by 3 to 4 orders of magnitude faster than volume diffusion which gives prospects for an increase of oxygen conductivity in nanocrystalline ion conductors. Nanocrystalline ordered intermetallics as, e.g. n-FeAl and n-NiAl can be prepared by gasphase condensation in a partially disordered state. The ordering in n-FeAl occurs at lower temperatures than in n-NiAl which is correlated to the different vacancy migration enthalpies in the two intermetallic alloys.