On the formation of voids in internal tin Nb3Sn superconductors

Abstract

The formation of voids during the reaction heat treatment of internal tin Nb3Sn strands degrades the physical superconductor properties. The authors describe three void growth mechanisms on the basis of combined synchrotron microtomography and x-ray diffraction results obtained during in situ heating cycles. Initially, void growth is driven by a reduction of void surface area. The main void volume increase is caused by density changes during the formation of Cu3Sn in the strand. Long duration temperature ramps and isothermal holding steps neither reduce the void volume nor improve the chemical strand homogeneity prior to the superconducting A15 phase nucleation and growth.