Thermomigration in solid metals

Abstract

Atom transport produced by large direct currents (electromigration) and thermal gradients (thermomigration) was measured in gamma-uranium by observation of local dilatations induced in high purity rods which were heated with a.c. or d.c. in a vacuum of 10−8 Torr. or better. Both radial and longitudinal dimensional changes, the latter obtained from displacements of surface markers, were measured in order to determine the atom flux. Rates of dimensional change were observed constant during the 8–12 days duration of the experiments. In the d.c. experiments, where the electromigration effect dominated, mass transport was observed toward the anode with Z∗ƒ equal to − 1·6 ± 0·1; here f is the tracer diffusion correlation factor. The small negative value for Z∗, the effective charge, indicates a dominance of the electron ‘wind’ force over the combined forces of the hole ‘wind’ and the electrostatic field. In the a.c. experiments mass transport was observed toward the cooler regions with Q∗f equal to + 4.7 ± 0.5 kcalmole. If one adopts the Wirtz model for a vacancy mechanism, the thermomigration results imply a relatively large migration enthalpy and a correspondingly small formation enthalpy; the latter leads to a vacancy fraction at melting of about 2·7 per cent. Semi-log plots of DZ∗ƒ and DQ∗ƒ vs. 1T were found linear with ‘activation energies’ of 27·2 ± 0.6 and 23·0 ± 1.6 kcalmole, respectively. The former agrees with the activation energy obtained from tracer experiments while the latter is slightly less. A possible explanation is that Q∗ has some temperature dependence, which is not too surprising in view of the large absolute thermoelectric power of gamma-uranium.