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 p roperties of gas-phase synthesized nanocrystalline solids. The atomic struc ture of interfaces in nanocrystalline solids with vacancy-like free volumes and nanovoids of triple junctions is specifically studied by positron life time spectroscopy. The recently studied temperature variation of the positr on lifetime indicates a strong temperature dependence of the positron trapp ing rate of these free volumes. From the investigation of the orientation c orrelationship of pairs of two adjacent crystallites in n-Pd by atomic reso lution microscopy it can be concluded that predominantly high-energy interf aces are present in nanocrystalline metals after gasphase synthesis. Tracer substitutional-diffusion and self-diffusion studied in highly dense nanocr ystalline metals demonstrate that the atomic diffusion is similar to that i n conventional grain boundaries. The O-18 diffusion in the interfaces of n- ZrO2 is by 3 to 4 orders of magnitude faster than volume diffusion which gi ves prospects for an increase of oxygen conductivity in nanocrystalline ion conductors. Nanocrystalline ordered intermetallics as, e.g. n-FeAl and n-N iAl can be prepared by gasphase condensation in a partially disordered stat e. 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 int ermetallic alloys. (C) 2000 Elsevier Science S.A. All rights reserved.