Diffusion and segregation of Sr in glassy carbon: Model and analysis

Abstract

Some diffusing systems segregation towards the surface also occurs. Such a system can be described by adding a force-induced velocity term to the Fick diffusion equation. The profile of an implanted material is usually Gaussian or at least very near a Gaussian function. In this paper this Fick partial differential equation in a force field was solved for an initially Gaussian profile. The simulated profiles exhibit the expected behavior. 200 keV implanted strontium into glassy carbon and isochronically annealed in vacuum from 300 °C to 900 °C for 1 h. The Sr profiles were determined by RBS (Rutherford backscattering spectrometry). They showed both diffusion and segregation between 400 °C to 900 °C. These profiles were fitted to the abovementioned solution to extract the diffusion coefficient and segregation drift velocity at each annealing temperature. In the temperature range 400 °C to 800 °C the diffusion coefficient had an activation energy for diffusion of 0.20 eV and 1.2 nm2/s for the pre-exponential factor D0. The segregation velocity, which is directly proportional to the segregating force, increased with increasing temperature. At annealing temperature of 400 °C the average drift velocity towards the surface was 39 pm/s and at 900 °C it was an order of maginitude higher at 0.34 nm/s.