Electromigration and Thermal Transport in Sodium Metal

In an effort to study mass transport under the influence of an electric current and a temperature gradient in sodium, marker-motion experiments have been carried out with both direct and alternating current. The motion of scratches on the surface of samples heated in a vacuum by currents of about 5000 A/${\mathrm{cm}}^2$ has been observed as a function of time and temperature. In the direct-current measurements, maximum marker motion (at 5 to 10°C below the melting point) was toward the cathode at 100 to 200 μ/day. The resulting mass transport was toward the anode; the effective charge of the sodium ions was found to be $-(2.4\mathrm{}\pm 0.5)\left|e\right|\mathrm{}$. This is the anticipated sign, but the magnitude is considerably smaller than expected. The activation energy, on the other hand, was 0.42±0.03 eV, in agreement with existing self-diffusion data. From the alternating-current measurements, a Soret heat of transport of -0.07±0.02 eV has been deduced, with an activation energy of 0.38±0.04 eV. The heat of transport, or the energy carried per atom moved by a temperature gradient, shows the expected sign, but is lower in magnitude by a factor of 6 than that predicted by existing theories. A Soret heat of transport of -0.11±0.07 eV has also been determined indirectly from the direct-current measurements; this is in reasonable agreement with the alternating-current result.