Electromigration of cadmiun in lead

The electromigration of cadmium in lead up to a concentration of 510 at ppm Cd was measured at constant temperatures between 250 and 290°C by the steady state method and also by a somewhat novel transient technique; the latter required shorter running time and also gave additional information, namely the chemical diffusion coefficient, D_ch. From the steady state runs the observed effective charge number was Z¹_ss = + 2.5 ± 0.3 for the lowest concentration. The positive value was apparently the result of the counterflowing of the lead in a vacancy-interstitial pair mechanism. The temperature dependence of Z¹ is described by $\text{Z}{}^1\text{= (0.5 ± 2.4) + (0.9 ± 1.1)}$$\text{10}{}^{\mathrm{?4}}\text{ρ}$ and both the wind force and the electrostatic Z_el appear to be small. The Z¹_ss, appeared to increase slightly with concentration. The transient measurements were in general less precise, but were consistent with the steady state results within their experimental accuracy. The values of $\text{D}{}_{\mathrm{ch}}\text{D}{}^1$ appeared to be unity within the same accuracy for a range of composition up to 510 at ppm Cd. To a standard dissociation model for impurity mobility we have added a rather extended atomic exciton mechanism to help explain the diffusion kinetics.