Voltage-current characteristics of the thallium-based ceramic superconductors

Voltage-current (V-I) characteristics of the ceramic Tl2Ba2CaCu2O8 (Tl-2212 ) and Tl2Ba2Ca2Cu3O10 (Tl-2223) superconductors were studied in magnetic fi elds up to 80 mT at different temperatures just below the critical temperat ure. The V-I data were fitted to a power law expression V = I-beta(T,I-B) i n which the exponential parameter beta is found to decrease rapidly with in crease of temperature and magnetic field. In zero magnetic field the slopes of beta versus T graphs change drastically in a narrow temperature range i n which beta decreases from about 3 to about 2. At temperatures below about 0.8-0.9 T-c the critical current I-c rapidly decreases with magnetic field s up to about 20 mT. This is followed by a more gradual change up to about 60 mT before I-c decreases rapidly to zero in magnetic fields higher than 6 0 mT. These features of the Tl-based ceramic superconductors are quite simi lar to those of superconducting tapes. We propose that the lattice anisotro py, preferred orientation and behaviours of the weak and strong links betwe en the grains play an important role in this similarity.