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

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.

Role of c-axis field components in making the transport E(J) characteristics of (Bi,Pb)2Sr2Ca2Cu3Ox tapes insensitive to the direction of field within the tape plane

“…Any local segments for which the field H and the current density J are not fully orthogonal may have a reduced Lorenz force and a higher J c . In both Chevrel-phase wires 10 and YBa 2 Cu 3 O 7Ϫ␦ tapes, 11 it has been shown that the ratio J c (H ʈ J)/J c (HЌJ) increases as the connectivity of the sample improves, consistent with the view that strongly percolative current flow always has a significant (HЌJ) component, whatever the macroscopic orientation of H and J.…”

Preparation and Thermoelectric Properties of Chevrel-Phase Cu<I><SUB>x</SUB></I>Mo<SUB>6</SUB>S<SUB>8</SUB> (2.0&le;<I>x</I>&le;4.0)

“…The fracture surface of Chevrel-phase sulfides has been conventionally used for the grain size measurement. 28,29) From Fig. 3, the grain size of the sintered samples is estimated to be of the order of micrometers.…”

Critical current distribution of hot isostatically pressed PbMo6S8 wires

“…With respect to a conventional annealing without pressure, nearly the same Jc have been obtained. Yamasaki et al [6,7] were the first to apply HIP to PMS wires with a molybdenum barrier. It has been observed that conventionally annealed PMS wires show an important porosity and the Jc is limited to about 2× l0 s A m -2 at 8 T and 4.2 K. In addition, the ratio of J¢(HII)/ J¢(H± ) is near unity, suggesting a non-unidirectional, percolation-type current flow through the wire.…”