Influences of Ca nonstoichiometry on the superconducting properties of Bi-2212 superconducting bulks

Polycrystalline bulks of Bi2Sr2CaxCu2.0O8+δ (Bi-2212) with x = 0.90, 0.95, 1.00 and 1.05 were fabricated by spark plasma sintering technique. The influences of Ca nonstoichiometry on the microstructures, carrier concentration, as well as the related superconducting properties were systematically investigated. XRD analyses revealed the corresponding change of lattice parameters of Bi-2212 phase with the Ca nonstoichiometry. The average particle radius in the sintered bulks underwent an obvious change with the increase of Ca content, which could be attributed to the variation of thermodynamic properties. AC susceptibility measurements exhibited the optimization of intergrain connections in Ca = 0.95 bulk, which lead to the optimization of both self- and in-field critical current density, Jc of this system. The optimization of microstructures also caused the enhancement of surface pinning. Based on the enhancements of both intergrain connections and flux pinning properties, an obvious improvement of critical current density was obtained with the optimal doping content of Ca = 0.95.