Effect of Nb3Sn layer structure and morphology on critical current density of multifilamentary superconductors

The microstructure and morphology of superconducting Nb3Sn layers in multifilamentary composites differing in the fabrication route (bronze technology and internal-tin method), the shape of Nb filaments (continuous, coupled and tubular) and in the mode of Ti doping (doping of bronze matrix or Nb filaments) have been studied. Significant factors determining critical current density of these wires are the average grain sizes and the fraction of equiaxed grains in the superconducting layers. The minimal grain sizes are characteristic of the composites with tubular Nb filaments, whereas the maximal fraction of equiaxed grains is obtained in the internal-tin wires.