DIFFUSION OF NANOSIZED SODIUM INCLUSIONS IN PLATINUM

Na inclusions with diameters in the range from 2 nm to 15 nm have been made by ion implantation of Na into 70 nm thick single-crystalline Pt foils followed by annealing. The structure of solid inclusions and th e diffusion of molten inclusions have been studied by transmission ele ctron microscopy. At room temperature the inclusions are faceted and c rystalline with a BCC structure and they are aligned topotactically wi th the Pt (FCC) matrix. The diffusion of inclusions in the liquid stat e was investigated by annealing at temperatures of 1227 K, 1432 K and 1534 K. The diffusion coefficient was found to have a dependence on th e inclusion radius and an activation energy of 2.8 +/- 0.2 eV, this be ing consistent with a mechanism by which volume self-diffusion of Pt c ontrols the diffusion rate of the inclusions. Diffusion of Pt at the i nterfaces between the Pt matrix and the liquid Na inclusions was found to be strongly inhibited compared with the free surface self-diffusio n of Pt. The results are used to propose a method to produce sources f or positron annihilation spectroscopy at high temperatures.