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

Abstract

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 and the volume fraction of was varied. The samples prepared for the series were characterized by the critical current density , 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 , 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 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 values can only be obtained in a fairly small temperature/oxygen partial pressure window. One explanation for the strong decrease in 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 , e.g. .

Microstructure, texture and critical current of Ag-sheathed 2223 multifilament tapes

“…Figure 1 demonstrates that J c is more sensitive to the heat treatment temperature than the 2223 fraction and the texture. Müller et al [18] have reported similar phenomena but their work did not include texture observations. In the low temperature range (<826 • C) the phase purity and texture are the main controlling factors for J c .…”

Present Status and Future Perspective of Bismuth-Based High-Temperature Superconducting Wires Realizing Application Systems

“…The effect of the intergrowths on I c has been discussed in the literature but is not yet well understood. 21,22) As it is considered that the presence of Bi-2212 grains in the oxide filaments reduces the current carrying capacity of Bi-2223 wires, Bi-2212 grain reduction is important.…”

Magnetic properties of superconducting ceramics Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O 10+d prepared by different methods

“…An excess of PbO corresponding, in weight, to a percentage of the PbO used firstly was added at the beginning for the third method and at the final calcination step for the other methods. The precursor was made from a mixture of SrCO 3 , CaCO 3 , and CuO in the first method, while in the second method only SrCO 3 and CaCO 3 were used. For the first and the second method respectively, the first calcination was at 920 °C during 48 hours and at 1250 °C during 5 to 10 hours, the second calcination at 835 °C during 144 hours and at 845 °C during 5 to 10 hours, the final calcination at 835 °C during 60 hours and at 845 °C during 60 hours.…”

“…Generally the phases were obtained with an excess of Bi and in one case with Ca in excess and Sr deficient. Critical current densities of Bi(Pb)-2223 silver sheathed tapes have been correlated by Müller et al [3] to conditions of preparation in the powder in tube method. Their results show that higher values of the critical current density are obtained with higher partial pressure of oxygen and higher sintering time leading in the same time to higher Bi(Pb)-2223 fraction.…”