Analysis of the tin diffusion step in Nb3Sn-Cu superconducting wire produced by the external tin process

Assuming volume diffusion is dominant, an analysis has been presented which allows one to analytically describe the three stages of the solid-state diffusion process for adding Sn in the external tin method for preparing Nb3Sn/Cu superconducting material. It is shown that one obtains a maximum intermetallic compound layer thickness in stage III which then decreases to zero thickness. Important practical questions in utilizing this process are the times required to complete stages I and II of the process, and this analysis predicts that the times required to complete stage I are on the order of days while stage II is completed in a matter of hours. The roles of grain-boundary diffusion and sample geometry are discussed and it is concluded that the analysis may be regarded as an upper bound for determining the times required to complete stages I and II of the solid state Sn diffusion process.