Sintering period dependence of superconducting properties of (Bi 0.7Pb0.3)1-ySr1Ca1 Cu1.8Ox system

The (Bi_0.7Pb_0.3)_1-ySr₁Ca₁ Cu_1.8 superconductor sintered above 855°C is shown to have a microstructure consisting of superconducting platelike grains embedded in a matrix formed by semiconducting rodlike grains due to the melting of Bi₂O₃, whereas it has a microstructure consisting of a small number of semiconducting rodlike grains embedded in the superconducting matrix when it is sintered at 845°C. The amount of high-T_c phase increases and that of low T_c phase decreases as the sintering period increases, making it possible to fabricate superconductors with T _c above 100 K by sintering at 845°C in air for 240 h. The sintering time can be shortened by sintering first below 850°C to obtain a microstructure consisting of the platelike grain matrix and resintering at higher temperature to enhance the growth of the high-T_c phase. Superconductors with a T _c of 106 K have been fabricated by sintering at 845°C for 48 h followed by sintering at 855°C for 24 h