FORMATION PROCESS OF INTERFACES AND MICRODEFECTS IN NANOSTRUCTURED AGSTUDIED BY POSITRON LIFETIME SPECTROSCOPY

Nanostructured Ag (polycrystalline Ag with nanometre-sized grains), sy nthesized by inert-gas condensation plus in situ vacuum compaction, ha s been investigated by positron lifetime spectroscopy (PLS). The resul ts indicate that there is a common character, i.e. only three lifetime components (tau(1), tau(2) and tau(3)) are resolvable from the lifeti me spectrum on each of the specimens synthesized under the whole compa cting pressure range investigated (from 0.15 to 1.50 GPa). Correspondi ng to the three lifetime components, there are three types of defect ( traps) in nanostructured Ag: (a) vacancy-like (VL) defects, (b) vacanc y-cluster (VC) defects and (c) larger voids. Compacting pressure and a nnealing treatment has great influences on the positron annihilating b ehaviour. The lifetimes tau(1), tau(2) and corresponding intensities I -1, I-2 decreased irreversibly with compacting pressure and annealing temperature, indicating that the VL defects and VC defects are both me chanically and thermally unstable, and so it is inappropriate to consi der them as structural elements in nanostructured Ag. Moreover, the in terfaces in n-Ag can be considered as superpositions of VL defects on the normal ordered boundaries, and since the number and size of the VL defects change with external conditions, the interfaces in n-Ag can s tay in various metastable states, which implies that its interfacial s tructures may range from total random states (gaslike) to complete ord ered structures, depending on the number and size of the VL defects co ntained. The forming process of bulk nanostructured Ag revealed by PLS can be roughly divided into three stages: (1) formation stage of inte rfaces (compacting pressure p less than or equal to 0.6 GPa); (2) rapi d elimination of the three types of defect (0.6 GPa < p < 1.1 GPa); (3 ) gradual elimination of those defects (p > 1.1 GPa). Based on these r esults obtained on nanostructured Ag, a density criterion D-c approxim ate to 96% that of the polycrystalline counterpart is proposed for the formation of bulk nanostructured materials.