ROLE OF LEAD SUBSTITUTION IN THE PRODUCTION OF 110-K SUPERCONDUCTING SINGLE-PHASE BI-SR-CA-CU-O CERAMICS

We have systematically investigated the influence of the three parameters (temperature of sintering, lead content, and duration of the sintering) on the growth of the (2:2:2:3) phase in ceramics of nominal composition Bi 2-xPbxSr2Ca2Cu3O 10-x/2, and have determined the compositions and thermal conditions leading to ceramics with Tc≊110 K and containing more than 95% of this phase. We show that this result can be obtained by usual calcination near 880°C in air, without recurring to the introduction of excess in the constituants, providing that intermediate grinding are effected, in order to reactivate the formation of the (2:2:2:3) phase which otherwise saturates for long durations of sintering. The suitable nominal compositions (0.3≤x≤0.4) correspond to those that yield inside the grains a lead content in the vicinity of the limit of solubility for lead. From the results of the preceding investigations and of experiments effected on lead-substituted single crystals of the (2:2:1:2) phase, we have clarified the role played by lead, at the microscopic level, in the enhancement of the formation of the (2:2:2:3) phase. The inferred role is the establishment of strong bonding between the slabs which are the buildings blocks of the structure of the superconducting phases, these slabs being almost independent in the lead-free compounds. The origin of the stronger bonding is discussed.