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 Nb3Sn layer thickness. Because of the resultant changes in mechanical and superconducting properties, the strain at onset of degradation of critical current density, Jc, 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 Jcin specimens which were reacted for longer than ∼24 hrs offsets the gain in Jcresulting from increased Nb3Sn layer thickness. The diffusion voids are also related to thermal instabilities and crack initiation.