Diffusion Investigations with Synchrotron Radiation

Diffusion phenomena are of primary importance for high-temperature properties of solid materials. General understanding of the kinetics of bulk systems and films which is responsible for their stability at increased temperatures is of great importance. In contrast to the macroscopical methods, deducing atomic events for instance from the concentration gradient of tracer atoms, the microscopic or atomistic methods "follow" the atoms directly on their diffusive jumps. Taking advantage of the coming-up of third generation synchrotron x-ray sources, we recently added a new technique to the spectrum of microscopic methods for studying diffusion. The aim of our research program was twofold: - Since the newly developed method was insufficiently explored, an important intention was to check those theoretical predictions which had not yet been proven experimentally. The diffusion effect which manifested itself as an accelerated decay of the intensity in the time spectra was observed in the scattering instead of the transmission geometry and the diffusion parameters corresponding to the tracer results were obtained. This technique will extend the applicability of the method to measurements of brittle samples which cannot be prepared as thin foils as required for transmission experiments. - The advantages of x-rays in diffusion studies are clearly visible in investigations of diffusion in intermetallic alloys: The very small cross section of the synchrotron-beam combined with the small beam divergence (i.e. the high "brilliance" of the synchrotron radiation) enabled studies of the microscopic diffusion mechanism of iron atom in intermetallic alloys which are good candidates for high-temperature applications. The iron-aluminium alloy, where the nearest neighbours of each iron atom are aluminium atoms, revealed a non-obvious jump model, namely priority of effective third- over second-nearest neighbour jumps. Moreover, a number of experiments have been performed with the aim to enhance the effectivity of the use of synchrotron radiation for future dynamic studies: - The diffusion mechanism was investigated via non-resonantly scattered X-rays. This technique allows studies of samples independent from their atomic constitution instead of samples consisting of atoms selected appropriately to the investigation method. - Metallic films are currently a subject of extensive studies due to their diverse applications in nanotechnology and electronics. Preliminary investigations of x-ray grazing incidence reflectometry coupled with nuclear resonance scattering provided information on strongly enhanced diffusivity in the near-surface region of the iron sample. In conclusion, the results of the project P12492 proved and extended the capabilities of synchrotron radiation in diffusion studies by clearly demonstrating the advantages of time-resolved investigations with the highly brilliant synchrotron beam.