Voltage-current characteristics of the thallium-based ceramic superconductors

Voltage-current ( V - I ) characteristics of the ceramic Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212) and Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223) superconductors were studied in magnetic fields up to 80 mT at different temperatures just below the critical temperature. The V - I data were fitted to a power law expression V = I ( T , B ) in which the exponential parameter is found to decrease rapidly with increase of temperature and magnetic field. In zero magnetic field the slopes of versus T graphs change drastically in a narrow temperature range in which 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 fields 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 80 mT. These features of the Tl-based ceramic superconductors are quite similar to those of superconducting tapes. We propose that the lattice anisotropy, preferred orientation and behaviours of the weak and strong links between the grains play an important role in this similarity.