Effect of Pressure on the Intermetallic Diffusion of Silver in Lead

The diffusion of silver-110 into lead has been investigated using radioactive-tracer techniques in a temperature range within 200°C of the melting point of lead for six pressures between 0 and 40 kbar. The activation energy was found to increase from 15.2 to 21.9±0.3 kcal/mole as the pressure increased from atmospheric to 39.2 kbar. The activation volume for pressures below 11.9 kbar ranged from 0.54±0.06 to 0.48±0.05 atomic volumes as the temperature decreased from 769 to 556°K. Above 11.9 kbar the activation volume was nearly constant at 0.38±0.03 atomic volumes over the same temperature interval. As a result of the large decrease in the activation volume that occurs between 0 and 11.9 kbar it is suggested that the diffusion process for silver into lead changes from a composite of interstitial plus vacancy to an interstitial mechanism. Therefore the activation volume of 0.38±0.03 atomic volumes as measured for pressures above 11.9 kbar represents the activation volume of motion $\Delta V_m$ characterizing the interstitial mechanism. Within the accuracy of the experimental data for $\ln D$ versus $\frac{T_m}{T}$ the results suggest that $\frac{\Delta H\left(P\right)\mathrm{}}{T_m\mathrm{}\left(P\right)\mathrm{}}$ is independent of pressure to 40 kbar.