Diffusion mechanisms for the growth of Nb3Sn intermetallic layers

The effects of increasing the Sn concentration in niobium bronze were studied. It was found that simple grain boundary diffusion should lead to compound formation with a parabolic time dependence. However, when other factors enter in, such as grain growth, reduction in chemical potential or any other factor that reduces the flux to the growth interface, the time dependence can fall. Conversely, when factors such as interface kinetics become dominant either through crack formation or a suppression of the grain size, the growth rate may exceed a parabolic time dependence and actually approach a linear time dependence under optimal circumstances. The layer of Nb3Sn at a Nb-(Cu-Sn) interface has proven to be a versatile system in that it can be made to display a variety of growth mechanisms by minor modifications of the Sn concentration in the bronze or by doping the starting material.