Bulk Diffusion of Carbon-14 through Polycrystalline Nickel Foil between 350 and 700°C

Abstract

The bulk diffusion of C-14 through polycrystalline Ni foil was investigated in the temperature range of 350 to 700°C. A thin Ni foil was bombarded by a molecular beam of acetylene tagged with C-14. A masking device situated very close to the ribbon surface shielded part of the surface from this beam, producing a well-defined pattern of high and low concentration of absorbed acetylene on the top surface of the foil. After diffusion annealing, this pattern would appear on the underside of the ribbon. Autoradiographic techniques were used to obtain radiation intensities from both surfaces. The ratio of these intensities determined bulk diffusion coefficients after the method of Kryukov. Most previous data had been obtained at temperatures of 700°C or higher. There is acceptable agreement between these data and the higher-temperature experiments of the present work. However, the present results show an activation energy of approximately 20 kcal/mole from 350 to 700°C, while there is substantial evidence that above 700°C the activation energy for this process is of the order of 40 kcal/mole.