DOUBLE SUPERCONDUCTING TRANSITIONS IN YBA2CU3OX VERSUS OXYGEN-CONTENT

A homogeneous batch of YBa2Cu3Ox porous ceramics was studied in detail as a function of the annealing temperature T(a) X-ray patterns and op tical micrographs show definitely one single phase. Well separated dou ble transitions are however observed, most clearly in the specific hea t curves, except for T(a) almost-equal-to 460-degrees-C. Initial value s are T(c1) = 87 K and T(c2) = 92 K for T(a) = 300-degrees-C. Upon inc reasing the annealing temperature to T(a) = 500-degrees-C, T(c1) incre ases to 93.5 K while T(c2) remains almost constant. Beyond T(a) = 500- degrees-C, both T(c1) and T(c2) decrease. The reversible splitting of superconducting transition appears to be due to the complex kinetics o f oxygen uptake. T(c1) is attributed to the external layer of the grai n and T(c2) to the grain cores.