Densification of Bi-Pb-Sr-Ca-Cu-O Bulk Superconductor by Hot Press Sintering and Critical Current Density

Bi1.84Pb0.34Sr2.03Ca1.9Cu3.06Oy bulk superconductors were prepared by hot press sintering at 780°-820°C at 4.9MPa-24.5MPa in air for 2h. The relationship between the porosity (p) of the sample and hot pressing conditions was empirically formulated using the following equations, ln(p)=-3Pt/(4η)+ln(p0), η=η0exp(E/RT), where P is the hot pressing pressure, t is the hot pressing time, η is the viscosity, p0 is the porosity of the compact body, E is the activation energy for viscous flow and T is the temperature. The values of η0 and E were evaluated to be 80Pa⋅s and 184kJ/mol, respectively. With increasing bulk density, the resistivity at room temperature decreased and the critical temperature of each grain, which was evaluated from the temperature dependence of magnetization, decreased. The temperature dependence of resistivity greatly differed among samples with various bulk densities. The critical current density increased with increasing bulk density. The relationship between these superconducting properties and the bulk density was explained in terms of the two factors, that is, the increase in grain coupling strengths and the decrease in oxygen content, which were caused by the densification of the sample.