Diffusion in Metals, Intermetallics, Silicides, Quasicrystals and Metallic Glasses
Diffusion in Molbedenum Dislilcide
Molybdenum disilicide (MoSi2) is a material with high technological potential for functional and
structural applications at extremely high temperatures. The diffusion behaviour of this material was completely
unknown when we started this project. We performed a study of Si, Ge, and Mo diffusion employing
sophisticated tracer techniques. The short-lived tracer 31Si (half-life only 2.6 hours) was produced
the IGISOL accelerator of the University of Jyväskyläe (Finland), implanted into the
samples, and diffusion was studied in situ. To this end we had to transfer a complete diffusion equipment to
Jyväskyläe. The isotopes 99Mo and 71Ge were produced by neutron
activation at the reactor of GKSS Geesthacht and then used in Münster.
Diffusion of Mo is a
very slow process. In the entire temperature region investigated (1437 to 2173 K), the Mo diffusivities in
both principal directions of the tetragonal MoSi2 single crystals obey Arrhenius laws. Diffusion
perpendicular to the tetragonal axis is faster by two to three orders of magnitude than parallel to it. Diffusion of
Si and its homologous element Ge is fast and is mediated by thermal vacancies of the Si sublattice. Diffusion of
Mo is several orders of magnitude slower than the diffusion of Si and Ge. This large difference suggests that Si
and Mo diffusion are decoupled and that the diffusion of Si and Mo likely takes place via vacancies on the
respective sublattice.
Correlation factors for Si diffusion by a vacancy mechanism in the Si sublattice of the
tetragonal MoSi2 structure have been calculated by combining an analytical and a Monte-Carlo
approach. The ratio between the Si diffusivities perpendicular and parallel to the tetragonal axis is also deduced.
An effect of forward correlation of tracer atom jumps in the Si sublattice with the corresponding partial
correlation factor of 1.5 appears at small rates of Si atom jumps along the tetragonal axis with respect to
the jump frequencies in the Si layer perpendicular to the tetragonal axis of the MoSi2 structure.
The measured anisotropy of Si diffusion in MoSi2 is explained in terms of correlation
effects of Si diffusion on its own sublattice.
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