Effect of Ni substitute in off-stoichiometric Bi(Pb)-Sr-Ca-Cu(Ni)-O superconductor. Excess conductivity, XRD analysis and thermal behaviour

Abstract Two phases (2223 and 2212) are identified in Bi1.8Pb0.3Sr2Ca2(Cu1-xNix)3.3Oy superconductor system, sintered at 847 °C for 322 h, in partial nitrogen atmosphere. The volume fraction of 2223-phase is strongly dependent on Ni doping: 78.37% for x = 0.002, 70.29% for x = 0.005% and 51.13% for x = 0.015. The unit cells of 2223 and 2212 phases were indexed as tetragonal structures, having different lattice constants. Plots of resistance versus temperature (four points method) on cooling to 77 K, evidenced that the critical temperature for the transition to the superconductor phase, Tc, is linearly decreasing from 106.21 to 93.47 K when the Ni content is varying from x = 0.002 to x = 0.03. From log-log plots of the excess-conductivity we calculated the cross-over temperatures between 3D and 2D dimensionality as well as from 2D to SWF (short wavelength fluctuation) behaviour, the coupling factor and the coherence length for all the samples. Thermal analysis of the resulting samples (after the last sintering) was performed by heating each sample from room temperature (RT) to 1000 °C at a rate of 10 K min−1 in dynamic air atmosphere (150 cm3 min−1). A clear dependence on Ni content is seen by TG and DSC, but a relative thermal equilibrium between the two phases, 2223 and 2212, in RT-869 °C range, is observed. Strong endothermic effects (melting accompanied by small decomposition processes) begin at around 869 °C for all Ni doped samples. The results for the specific heat capacities, calculated from DSC plots, are also presented. Contribution of the crystal lattice to the estimated specific heat capacity was in conformity with the Einstein model, the Einstein temperature values being dependent on Ni content.