Bi(Pb)-2223 phase formation in Ag-sheathed tapes: the role of oxygen partial pressure during sintering

摘要

(Bi, Pb)(2)Sr2Ca2Cu3O10 phase formation has been studied in Ag-sheathed multifilamentary tapes processed in different oxygen partial pressures 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 prepared for the series were characterized by the critical current density j(c), ac susceptibility and x-ray diffraction. Detailed investigations 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 occurs at initial stages of the heat treatment, in which (Bi, Pb)(2)Sr2CaCu2O8, alkaline earth cuprates and Pb compounds act as the precursor phases. The phase formation rate depends sensitively on the established oxygen partial pressure with an optimum partial pressure being approximately 10 vol.% oxygen. The critical current density j(c) varies more sensitively with the applied oxygen partial pressure and temperature than the Bi(Pb)-2223 phase fraction as determined by x-ray diffraction. Optimum j(c) values can only be obtained in a fairly small temperature/oxygen partial pressure window. One explanation for the strong decrease in j(c) is the poor crystalline quality of the grains. By TEM columnar defects were observed with diameters of typically 50 nm extending parallel to the c-axis of the grains, The individual grains contain a high density of these defects yielding still strong x-ray diffraction peaks but obviously poor superconducting properties. The formation rate and defect density depends sensitively on both the temperature and the oxygen partial pressure during sintering. The phase composition at initial stages of the heat treatment, particularly the phase fraction of the lead oxide compounds, can be controlled by the applied oxygen partial pressure. The reason for this is that Pb changes its oxidation state from 4+ to 2+, A stability line was determined for lead oxide compounds with lead being Pb4+, e.g. Ca2PbO4. [References: 18]