Poor intergrain connectivity of PbMo6S8 in sintered Mo-sheathed wires and the beneficial effect of hot-isostatic-pressing treatments on the transport critical current density

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Abstract
The microstructure of Mo‐sheathed Chevrel‐phase superconducting wires was investigated by high‐resolution scanning electron microscopy (SEM). Excess Pb forms small nodules (10–30 nm) on the Chevrel‐phase grains or wetting layers between the Chevrel grains. The critical current density Jc is strongly reduced when wetting layers are present. However, second‐phase Mo exists as islands as large as 2–10 μm and has little harmful effect on Jc. Although some Mo‐sheathed wires sintered at 700 °C show relatively high Jc ( ≥ 2 × 108 A/m2 at 8 T), the Chevrel phase in these wires has a very porous microstructure, and the Jc values measured with a magnetic field parallel to the current, Jc(∥), were only about 10% higher than the Jc values measured with a field perpendicular to the current, Jc(⊥). This clearly suggests that the transport current flows percolatively in these wires. Much denser microstructures were obtained by hot‐isostatic‐pressing (HIP) treatments at 1200 °C, and very high Jc values, ≳ 5 × 108 A/m2 at 8 T and 9.3 × 107 A/m2 at 23 T, which are of the order of the highest Jc values so far reported, were observed. High‐resolution SEM observations of HIP’ed wires demonstrate that the interconnectivity between the Chevrel grains was much improved.