STRUCTURE AND DIFFUSIONAL PROPERTIES OF NANOCRYSTALLINE PD

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 v olumes in the interfaces studied by positron lifetime spectroscopy var ies characteristically with the preparation technique. In pure high-de nsity nanocrystalline metals, the crystallites grow rapidly upon annea ling with an activation enthalpy of 1.1 eV determined for nc-Pd prepar ed by severe plastic deformation. The fast atomic diffusivity in the i nterfaces of nc-Pd is studied by sputter sectioning and Rutherford bac k 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 g rowth. Because of the high density of the specimens, the diffusivities are lower than reported earlier and similar to those in conventional grain boundaries.