HOT-FORGING WITH HEAT-TREATMENT OF BI-PB-SR-CA-CU-O SUPERCONDUCTORS

Bulk Bi1.80Pb0.34Sr1.87Ca2.02CU3.0Oy (2223) superconductors with a relative density of about 90% were prepared by hot pressing. The hot-pressed sample was then hot-forged at 840 degrees C and a pressure of 9.8 MPa in air for 50 h, and was also heat-treated at 885 degrees C for 15 min during hot forging. The values of critical current density, J(c), were 70, 150 and 600 A/cm(2) at 77 K and 1 kOe for these samples, respectively. For the heat-treated sample, the value of J(c) at 77 K and 0 Oe was estimated to be 7000-10000 A/cm(2) from the magnetic field dependence of J(c). It was concluded that one of the reasons for the increase in J(c) in this sample was the disappearance of a Bi, Pb-rich phase, probably the 2212 phase as observed in scanning transmission electron microscopy. The Bi, Pb-rich phase decomposed to Sr-Ca-Cu-O phases and a phase with the elements Bi and Pb by the short period heat treatment at 885 degrees C. Subsequently the latter phase, apparently a liquid phase, was extruded from the sample by uniaxial pressure. The 2223 phase scarcely decomposed by a heat treatment at 885 degrees C in air for 15 min when uniaxial pressure was applied. On the contrary, in the case of no uniaxial pressure, the same heat treatment caused the decomposition of the 2223 phase to the 2212 phase and other phases. It was found that residual defects existed near sub-grain boundaries in the 2223 grains in as hot-pressed samples observed by transmission electron microscopy. The sub-grain boundaries were produced by the generation and propagation of dislocations along the a-b-plane due to bending stresses during hot pressing. The disorder near sub-grain boundaries was relieved by a short period heat treatment at 885 degrees C. This was an additional reason for the increase in J(c) in the heat-treated sample.