The Structure of Superconducting Layers in Wire Composites with Internal Tin Sources Depending on the Wire Diameter

The structure of superconducting layers in composites with internal tin sources and distributed Nb barriers has been studied using transmission and scanning electron microscopy. It has been shown that the outer diameter of the composite (1, 0.7, 0.5 mm) affects the morphology, grain size, and composition of the superconducting Nb3Sn-phase layers formed upon reaction heat treatment in the regime 370°C for 100 h + 665°C for 40 h. A residual content of niobium has been identified in 10% of the subelements within the ∅1‑mm sample, 4% within the ∅0.7-mm sample, and 0.8% within the ∅0.5-mm sample. The minimum average grain size of Nb3Sn grains is observed in the composite with a diameter of 0.7 mm.