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

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 degrees C to 900 degrees C for 1 h. The Sr profiles were determined by RBS (Rutherford backscattering spectrometry). They showed both diffusion and segregation between 400 degrees C to 900 degrees 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 degrees C to 800 degrees 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 degrees C the average drift velocity towards the surface was 39 pm/s and at 900 degrees C it was an order of maginitude higher at 0.34 nm/s.