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

Bulk Bi1.80Pb0.34Sr1.87Ca2.02CU3.0Oy (2223) superconductors with a rel ative density of about 90% were prepared by hot pressing. The hot-pres sed 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, res pectively. 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 de pendence of J(c). It was concluded that one of the reasons for the inc rease in J(c) in this sample was the disappearance of a Bi, Pb-rich ph ase, probably the 2212 phase as observed in scanning transmission elec tron 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 trea tment at 885 degrees C. Subsequently the latter phase, apparently a li quid phase, was extruded from the sample by uniaxial pressure. The 222 3 phase scarcely decomposed by a heat treatment at 885 degrees C in ai r for 15 min when uniaxial pressure was applied. On the contrary, in t he case of no uniaxial pressure, the same heat treatment caused the de composition of the 2223 phase to the 2212 phase and other phases. It w as found that residual defects existed near sub-grain boundaries in th e 2223 grains in as hot-pressed samples observed by transmission elect ron microscopy. The sub-grain boundaries were produced by the generati on and propagation of dislocations along the a-b-plane due to bending stresses during hot pressing. The disorder near sub-grain boundaries w as relieved by a short period heat treatment at 885 degrees C. This wa s an additional reason for the increase in J(c) in the heat-treated sa mple.