SUPERCONDUCTIVITY AND SUBSTITUTION OF BI-1212

For superconductors of type Bi-1212 the influence of substitution of ( i) Pb for Bi, (ii) rare earth elements (Ln) for Y and (iii) Ca for Y w as studied. It is shown that a partial Pb-->Bi substitution stabilizes the 1212 type and an incommensurate superstructure develops (lambda a lmost-equal-to 4.8). Similar to the Y-123 case a Ln-->Y substitution i s working without destroying the superconductivity. The highest transi tion temperature of 90 K is obtained with a combined Ca-->Y and Pb-->B i substitution in materials of composition (Bi, Pb, Cu)Sr2(Y, Ca)Cu2O7 -z. Bi-based 1212 powdered materials are characterized by non-supercon ducting grain boundaries, resulting in a pronounced weak link behavior . This situation can be improved by introducing an intermediate step o f partial or total melting in the conditions of preparation.