BI(PB)-2223 PHASE-FORMATION IN AG-SHEATHED TAPES - THE ROLE OF OXYGENPARTIAL-PRESSURE DURING SINTERING

(Bi, Pb)(2)Sr2Ca2Cu3O10 phase formation has been studied in Ag-sheathe d multifilamentary tapes processed in different oxygen partial pressur es during sintering. In the furnace the total pressure was always kept at 10(5) Pa and the volume fraction of O-2 was varied. The samples pr epared for the series were characterized by the critical current densi ty j(c), ac susceptibility and x-ray diffraction. Detailed investigati ons of the microstructure were carried out by SEM and TEM for samples sintered at 8 and 10% oxygen. The formation of the Bi(Pb)-2223 phase o ccurs at initial stages of the heat treatment, in which (Bi, Pb)(2)Sr2 CaCu2O8, alkaline earth cuprates and Pb compounds act as the precursor phases. The phase formation rate depends sensitively on the establish ed oxygen partial pressure with an optimum partial pressure being appr oximately 10 vol.% oxygen. The critical current density j(c) varies mo re sensitively with the applied oxygen partial pressure and temperatur e than the Bi(Pb)-2223 phase fraction as determined by x-ray diffracti on. Optimum j(c) values can only be obtained in a fairly small tempera ture/oxygen partial pressure window. One explanation for the strong de crease in j(c) is the poor crystalline quality of the grains. By TEM c olumnar defects were observed with diameters of typically 50 nm extend ing parallel to the c-axis of the grains, The individual grains contai n a high density of these defects yielding still strong x-ray diffract ion peaks but obviously poor superconducting properties. The formation rate and defect density depends sensitively on both the temperature a nd the oxygen partial pressure during sintering. The phase composition at initial stages of the heat treatment, particularly the phase fract ion of the lead oxide compounds, can be controlled by the applied oxyg en partial pressure. The reason for this is that Pb changes its oxidat ion state from 4+ to 2+, A stability line was determined for lead oxid e compounds with lead being Pb4+, e.g. Ca2PbO4.