Superconducting Glass‐Ceramics in the Bi‐Sr‐Ca‐Cu‐O System

Bi 1.5 SrCaCu 2 O z was prepared in the glassy state by rapid quenching of the melt. The recrystallization of the glass during various heat treatments was studied by differential thermal analysis, X‐ray diffraction, scanning electron microscopy, and resistivity measurements. Activation energies and frequency factors for the crystallization events below 600°C were determined by nonisothermal differential scanning calorimetry. Heating at 450°C formed mainly the Bi 2+ x Sr 2− x ‐CuO z solid solution (“R”). Between 765° and 845°C, R reacted slowly with the glass to form the 80 K superconductor Bi 2 (Sr,Ca) 3 Cu 2 O z and CuO. Heating for 7 d at 845°C, followed by slow cooling, eliminated a low‐temperature resistive “tail” and raised the temperature of zero resistance to 77 K. Heating at 867°C caused partial melting, with segregation of a Bi‐ and Cu‐rich liquid, and loss of the superconducting phases.