Retropgrade Densification in Bi sub 2 Sr sub 2 CaCaCu sub 2 O sub 8 Superconductors.

Bi sub 2 Sr sub 2 CaCu sub 2 O sub 8 was prepared using the mixed oxide carbonate method and sintered at temperatures ranging from 850 to 911C. The samples were characterized for density, mechanical strength, phase composition, microstructure and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated whereby the material first becomes less dense as the sintering temperature is raised and only in a narrow temperature range from 900-905C does the material densify; then with the formation of a liquid phase. The retrograde densification mechanism is shown to be that of the formation of thin platelike crystallites which grow in a randomly oriented fashion thus pushing the structure apart. This retrograde densification coupled with a narrow sintering range overlapping the melting temperature make this compound a difficult one to process.