TRANSPORT CRITICAL CURRENTS IN GRANULAR HIGH-TEMPERATURE SUPERCONDUCTORS

The transport critical current I(c) of more than 100 samples of high temperature superconductor bulk material was measured as a function of temperature using a contactless magnetic induction method. All samples (mainly YBa2Cu3O7-delta, but also Tl-Ba-Ca-Cu-O and (Bi, Pb) - Sr-Ca-Cu-O) show a temperature dependence of I(c) which can be described by I(c)(T) = I(c),0(1 - T/T(c))m with m almost-equal-to 0.9 for 0 < T < 0.9T(c). For temperatures closer to T(c) we find evidence for m almost-equal-to 1.5. Systematic investigations of a number of YBa2Cu3O7-delta samples with different cross-sectional areas (A = 0.15 . . . 2.71 X 1.8 mm2) prepared under identical conditions show that I(c) is proportional to A only for small cross-sectional areas. Furthermore, we find a maximum of I(c) as a function of starting material powder size and a weak dependence on the uniaxial pressure applied for sample preparation.