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 the diffusion of molten inclusions have been studied by transmission electron microscopy. At room temperature the inclusions are faceted and crystalline with a BCC structure and they are aligned topotactically with the Pt (FCC) matrix.

The diffusion of inclusions in the liquid state was investigated by annealing at temperatures of 1227 K, 1432 K and 1534 K. The diffusion coefficient was found to have a dependence on the inclusion radius and an activation energy of 2.8+or-0.2 eV, this being consistent with a mechanism by which volume self-diffusion of Pt controls the diffusion rate of the inclusions.

Diffusion of Pt at the interfaces between the Pt matrix and the liquid Na inclusions was found to be strongly inhibited compared with the free surface self-diffusion of Pt. The results are used to propose a method to produce sources for positron annihilation spectroscopy at high temperatures.