Effect of magnetic field and gamma irradiation on the electrical properties and structure of the Tl-based ceramic superconductors

The voltage-current ( V - I ) characteristics, critical temperatures and the critical currents of Tl 2 Ba 2 CaCu 2 O 8 (Tl-2212) and Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223) ceramic superconductors were studied in magnetic fields up to 80 mT after several irradiations up to 100 MR. The effect of irradiation on the structure of these materials has also been studied. The normal state resistance increases by 20-30% with irradiation, most of the changes taking place in low doses up to 10 MR. The critical temperatures of Tl-2212 and Tl-2223 decrease by about 8 K up to 30 MR and approach the saturation values of 101 K and 113 K, respectively, with further increase of dose. For Tl-2212 in the 96-98 K temperature range the V - I behaviours become nearly ohmic under a dose of about 30 MR or in an external magnetic field of 20-30 mT. In the same temperature range the critical current decreases rapidly in low doses up to 10-25 MR; above that it decreases only slightly. For both materials, the positions and widths of the x-ray diffraction peaks almost remain the same but the peak intensities decrease by 10-40% with irradiation, the rate of decrease being higher in the 0-30 MR range. These observations indicate that drastic changes in the lattice do not take place with irradiation but substantial electronic excitations and point defects may be generated, at higher rate in low doses, to affect the concentration and arrangements of the carriers and the links between the grains.