Feature article atomic transport: A review with perspectives

Abstract

This article aims to cover the highlights of more than thirty years of research carried out by Hehenkamp and his group into atomic transport and related fields. In Hume-Rothery alloys, which served as model systems, the effective charge numbers of electromigration and the heats of transport in a thermal gradient were systematically investigated, particularly by means of the stationary state technique, which proved to be very reproducible. The effective charges were found to originate from momentum transfer of the charge carriers, which also gives rise to a pronounced electronic contribution to the impurity heat of transport and determines the concentration dependence of this quantity. The investigation of transport experiments in alloys requires knowledge of the thermodynamic factor Φ of diffusion. Even in dilute noble and transition metal alloys the latter turned out to be by no means negligible. The variation of Φ upon alloying cannot be described well in the framework of regular solution models based on pairwise interaction. The strong nonlinear enhancement of solvent and solute diffusion in these alloys is interpreted in terms of the complex model of Hehenkamp et al., as being due to the formation of higher-order vacancy-impurity complexes. Non-equilibrium vacancies, recently evidenced in thermomigration and chemical diffusion might seriously affect particularly non-stationary transport experiments. Consequences for the theoretical description of atomic transport will be outlined.