Structure and diffusional properties of nanocrystalline Pd

Abstract

Abstract Nanocrystalline (nc) solids are characterized by nanometre crystallite size and a high number of interfaces. In the present paper we report structural and diffusion studies on high purity nanocrystalline Pd (nc-Pd) that was prepared either by cluster compaction to full density or by severe plastic deformation. The size distribution of atomic free volumes in the interfaces studied by positron lifetime spectroscopy varies characteristically with the preparation technique. In pure high-density nanocrystalline metals, the crystallites grow rapidly upon annealing with an activation enthalpy of 1-1 eV determined for nc-Pd prepared by severe plastic deformation. The fast atomic diffusivity in the interfaces of nc-Pd is studied by sputter sectioning and Rutherford back scattering as a function of temperature, annealing state and type of diffusing metal atom. The diffusion process occurs at the interfaces, which migrate during diffusion annealing owing to rapid crystallite growth. Because of the high density of the specimens, the diffusivities are lower than reported earlier and similar to those in conventional grain boundaries.