A technique to study the diffusion of rhenium in graphite was developed based on the technique of Rutherford back-scattering spectroscopy. Graphite sample coated with rhenium were given diffusion anneals at temperatures between 1373K and 2273K. The diffusion profiles were determined by examining the energy distribution of the backscattered helium ions. It was therefore necessary to convert the Rutherford back-scattering spectra into concentration profiles. RUMP (Rutherford Universal Manipulation Program) was a commercial computer program which was limited to thin films with thicknesses of approximately 300 nm and was not suitable for this diffusion study, which utilized thicker rhenium films. A new algorithm for interpreting Rutherford back-scattering spectra was developed by comparing the calculated spectra with the experimental data, and better fits for the RBS data were obtained. The new algorithm was based on a more precise computation of the energy of the scattered helium ions and provides a more realistic modelling of the data. The intrinsic diffusion coefficients were determined using the Matano method. Rhenium was found to diffuse slower that carbon, as expected. The Arrhenius plots of both diffusion coefficients showed a change in the slope with temperature, indicating that there were two diffusion mechanisms involved. At lower temperatures, rhenium atoms diffuse along the open pores in the Poco graphite, whereas diffusion at high temperatures was through the graphite lattice. Carbon atoms diffuse along the grain boundaries of rhenium at lower temperatures, while diffusion of carbon atoms through the rhenium lattice became important at high temperatures.

Diffusion Analysis of Rhenium in Graphite Using Rutherford Backscattering Spectroscopy. Zee, R.H., Li, J.: Defect and Diffusion Forum, 2001, 194-199[1], 85-90