ROLE OF CALCIUM PLUMBATE DURING THE FORMATION OF 2223 PHASE IN THE BI(PB)-SR-CA-CU-O SYSTEM

There are conflicting reports regarding the melting point of Ca2PbO4 in the literature. By performing differential thermal analysis under various atmospheres, we found that the melting point of Ca2PbO4 is severely depressed due to the reduced partial pressure of oxygen. At an oxygen partial pressure of one atm., Ca2PbO4 melts at 1055-degrees-C and this temperature is reduced to about 833-degrees-C in zero oxygen partial pressure (e.g. in nitrogen). Ca2PbO4 is formed as an impurity in the initial stage during the preparation of pure 2223 phase from the Bi(Pb)-Sr-Ca-Cu-O system in air. However, the usual sintering temperatures (840-degrees-850-degrees-C) at which the 2223 phase is synthesized in air are not high enough to melt Ca2PbO4. Thus, the melting of Ca2PbO4 is not responsible for the growth of 2223 phase from a liquid medium. Ca2PbO4 and the lead-enriched 2223 phase were found to be compatible with each other at 845-degrees-C in air. Utilization of reduced partial pressure of oxygen will inhibit the growth of Ca2PbO4 and promote the formation of the lead-containing, 2223 phase by modifying the reaction paths that would not otherwise be possible if higher oxygen partial pressure were used.