Mössbauer Spectroscopic Studies on Atomic Diffusion in Materials

Abstract

Atomic jumps within the lifetime of a Mossbauer nuclear probe are known to cause a line broadening, which is directly connected to the diffusivity, as predicted theoretically by Singwi and Sjolander in 1960. Many research groups applied and further extended their theory for the studies of elementary jump processes of Fe atoms in single crystals: They analyzed Mossbauer spectrum in terms of line broadening and motional averaging as functions of temperature as well as crystal orientation. To observe such dynamical behaviours, we developed specially designed experimental set-ups such as a high-temperature Mossbauer furnace for laboratory, in-beam and on-line Mossbauer set-ups for accelerator facilities. First of all, we will introduce such measuring techniques suitable for homogeneous materials, explaining their unique features in this chapter. Secondary, we present an imaging technique in Mossbauer spectroscopy which is opening a new possibility to study diffusion processes with a diffusion-length from μm to mm after diffusion annealing at high temperature. This original method enables us to measure the diffusion profiles separately for the different spectral components in the material containing a complex microstructure. As examples, we will explain the studies on Fe diffusion in single and multi-crystalline Si materials using “Mossbauer spectroscopic microscope (MSM),” which enables us to measure the diffusion profiles separately for different chemical states with a spatial resolution of several micrometres. This new method provides a possibility to investigate a diffusion process by considering of the interactions and the correlations between Fe impurities and lattice defects such as dislocations, grain boundaries, and residual stresses in different grains of materials. Finally, we apply this technique to investigate carbon diffusion and segregation processes in Fe-steel, and the mapping images at different temperatures are obtained separately for the spectral components that originated from carbon impurities.