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Microstructure and superconducting properties of hot isostatic pressed SnMo6S8 were systematically studied. Magnetization was measured in fields up to 30 T at 4.2–14 K. SnMo6S8 exhibits critical current densities limited by flux pinning, not by granularity. Samples exhibited Birr(T)∼Bc2(T), indicative of strong flux pinning. Pinning force curves contained two distinct peaks, indicating that at least two pinning mechanisms are active. The microstructure contained two types of defects which could pin flux: grain boundaries and planar intragranular defects. Lower-field pinning (B/Bc2≤0.3) was directly correlated to the grain boundary density. Higher-field pinning (B/Bc2≥0.5) appeared to be controlled by the intragranular pins. Compared to Nb3Sn, the grain sizes are large and the contribution from grain boundary pinning is small. Inhibiting grain growth through lower-temperature processing and increasing the density of intragranular defects both appear feasible means of further increasing Jc.

Samples of YBa2Cu307 have been prepared with rather sharp inductive transitions having in the best cases breadths of 7 K and midpoint Tc values of 88 K. The resistive Tc midpoints are 92-95 K with transition widths of ± 1-2 K. Flux shielding at 4.2 K is normally 100% and flux expulsion at 4.2 K reaches 95%. However, even small fields of order 1 mT decouple some 15%-20% of the volume, allowing flux to enter the samples. Resistive HC 2 measurements suggest that Hc 2 (0) varies from < 1 to > 300 T, depending on the criterion chosen. ac susceptibility measurements suggest that HC2(0) is -60 T. Magnetization current densities are relatively high (150-200 A/mm2 at 1-10 T at 4.2 K) but measured transport current densities are small (.;; 1 A/mm2). Magnetization current densities at 77 K are about two orders of magnitude lower. The samples were seen to be heavily twinned by light microscopy (scale of 1-5 JLm) and by transmission electron microscopy (scale of -250 nm). It is concluded that these measurements are consistent with a model of superconducting regions of reduced dimensionality coupled by tunneling.

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