CRITICAL CURRENTS AND PINNING IN POWDER METALLURGICALLY PROCESSED CHEVREL-PHASE BULK SUPERCONDUCTING SAMPLES

Critical current density (J(c)) has been measured in hot isostatically and uniaxially pressed bulk samples of PbMo6S8 (PMS) and SnMo6S8 (SMS) Chevrel phase superconductors (CPS). The obtained pinning force densities (F(p)(B, T)) show a scaling in a wide range of temperature (T = 4.2 K-T(c)) and field (B = 0-9 T), which is in good agreement with the predictions of the flux line shear (FLS) model. However, the magnitude of F(p) is an order of magnitude smaller than the expected values. The PMS sample hot pressed at relatively high temperatures (1200-degrees-C) shows long tails in the scaled F(p)(B,T) data. In contrast, the SMS samples sintered at similar temperatures show double peaks in F(p)(B) for temperatures below 10.5 K. For both compounds the pseudo upper critical field (B(c2)*(T)) extracted from the J(c) data, especially near T(c), shows deviation from the expected ideal B(c2)(T) dependence. The results point out a correlation of J(c) and B(c2)* with the morphology of the grain boundaries in terms of granularity and degradation of the superconducting properties.