Positron annihilation in nanoparticle materials was considered in terms of the thermal diffusion of positrons at grain surfaces, and trapping at grain interfaces and interfacial defects. The diffusion trapping model was used to obtain the mean lifetime of positrons as a function of grain size and temperature, and the S-parameter of the annihilation radiation as a function of grain size in the various metallic nanocrystallites. The calculated mean positron lifetimes were compared with available experimental results. It was found that the value of the mean lifetime decreased with increasing size of the grains. This suggested that the density of grain boundaries gradually decreased when the grain grew. A rapid decrease in the mean lifetime was observed in alloys but not in pure metals. This implied that, in the case of alloys, grains of larger size provided trapping centers. The calculations showed that the characteristics of the nanocrystalline materials underwent appreciable changes at lower grain sizes. For larger grain sizes, the materials exhibited a behavior that was similar to that of bulk samples. The lifetime data also suggested that the grain boundaries might be composed of vacancy clusters. The diffusion coefficient was found to be described by the relationship: D  T-½.

Trapping of Positrons at Grain Boundaries in Nanoparticle Systems. V.Thakur, S.B.Shrivastava, M.K.Rathore: Nanotechnology, 2004, 15[5], 467-72