Tracer Diffusion in a Chemical Concentration Gradient in Silver-Cadmium

Abstract

Measurements have been made on seven Ag-$\ensuremath{\alpha}\mathrm{AgCd}$ diffusion couples of the diffusion of a layer of radioactive tracer atoms (either ${\mathrm{Ag}}^{110}$ or ${\mathrm{Cd}}^{109}$) located originally at the Ag-AgCd interface. It was found that the center of gravity of the layer of tracer atoms shifted from its original position, marked by tungsten wires. The shapes of the tracer profiles for ${\mathrm{Ag}}^{110}$ were quite different from those for ${\mathrm{Cd}}^{109}$. However, for both silver and cadmium tracer, the center of gravity moved toward the cadmium-rich region.A theoretically predicted shift in center of gravity is obtained, taking into consideration the fact that the diffusion coefficient, correlation factor, and chemical potential are functions of chemical composition. The predicted shift in center of gravity arising from the flow of imperfections and the lattice distortion associated with the Kirkendall shift also are considered. Good agreement is found between theory and experiment.Chemical interdiffusion and Kirkendall shift measurements were consistent, within errors, with those predicted by Darken.