Kirkendall voids--A detriment to Nb3Sn superconductors

Multifilamentary composites with constant Nb/bronze ratio were heat treated for a range of reaction times. With increasing reaction time, the size of diffusion (Kirkendall) voids in the bronze matrix increases, as does the Nb
3
Sn layer thickness. Because of the resultant changes in mechanical and superconducting properties, the strain at onset of degradation of critical current density, J
c
, decreases with increasing reaction time. Thus there is a compromise between strain sensitivity and current capacity at zero strain as reaction time is increased. For strains on the order of 0.2 - 1.0 %, the increased strain sensitivity of J
c
in specimens which were reacted for longer than ∼24 hrs offsets the gain in J
c
resulting from increased Nb
3
Sn layer thickness. The diffusion voids are also related to thermal instabilities and crack initiation.