Formation process of interfaces and microdefects in nanostructured Ag studied by positron lifetime spectroscopy

Nanostructured Ag (polycrystalline Ag with nanometre-sized grains), synthesized by inert-gas condensation plus in situ vacuum compaction, has been investigated by positron lifetime spectroscopy (PLS). The results indicate that there is a common character, i.e. only three lifetime components (tau(1), tau(2) and tau(3)) are resolvable from the lifetime spectrum on each of the specimens synthesized under the whole compacting pressure range investigated (from 0.15 to 1.50 GPa). Corresponding to the three lifetime components, there are three types of defect (traps) in nanostructured Ag: (a) vacancy-like (VL) defects, (b) vacancy-cluster (VC) defects and (c) larger voids. Compacting pressure and annealing treatment has great influences on the positron annihilating behaviour. 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 mechanically and thermally unstable, and so it is inappropriate to consider them as structural elements in nanostructured Ag. Moreover, the interfaces 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 stay in various metastable states, which implies that its interfacial structures may range from total random states (gaslike) to complete ordered structures, depending on the number and size of the VL defects contained. The forming process of bulk nanostructured Ag revealed by PLS can be roughly divided into three stages: (1) formation stage of interfaces (compacting pressure p less than or equal to 0.6 GPa); (2) rapid 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 results obtained on nanostructured Ag, a density criterion D-c approximate to 96% that of the polycrystalline counterpart is proposed for the formation of bulk nanostructured materials.