X. P. Chen

Effect of bending strain on the critical current of Bi-2223/Ag tapes with different structures

We have evaluated the effect of bending strain on the critical current of Bi-2223/Ag tapes with different structures. The measured results were analysed using a symmetric pure bending model. The irreversible degradation indicates that the Ic reduction is caused mainly by crack formation and propagation in the brittle Bi-2223 ceramic core. Experimented results have shown that the number of filaments, as well as the structure of the tapes, could have strong effects on the measured apparent critical values of the tapes. Multifilamentary tapes showed better performance compared with single core tapes. The bending strain tolerance of the tape could be improved by adding a metal strip on the upper side of the tape.

Phase evolution of lead oxide compounds corresponding with oxygen release and absorption processes of BSCCO powders during heat treatment

The phase evolution of lead oxide compounds in (Bi,Pb)2Sr2Ca2Cu3Ox (BSCCO) precursor powder during heat treatment was studied. The BSCCO powder oxygen release and absorption processes occurring during heat treatment were investigated by using an oxygen analyser. The experimental results show that the oxygen release and absorption processes are related to evolution of the lead oxide compounds in the BSCCO powder. Oxygen absorption may result from the segregation of Pb from the (Bi,Pb)-2212 phase as well as the formation of the Pb3Sr2.5Bi0.5Ca2CuOy (3321) and Ca2PbO4 phases. Oxygen release could be related to decomposition of the 3321 and Ca2PbO4 phases as well as incorporation of Pb into the (Bi,Pb)-2212 phase. The formation and decomposition of the 3321 and Ca2PbO4 phases, which depend on temperature and oxygen partial pressure, have also been identified.

Microstructural evolution of Bi-2223/Ag tapes during the cooling process after the first heat treatment

The microstructural evolution of Bi-2223/Ag tapes during cooling after the first heat treatment is investigated by SEM, XRD and EDS. Cooling was carried out after samples were heat treated at 840 °C for 10 h. Samples were quenched in air from different temperatures during the cooling process. Investigation showed that the volume fraction of Bi-2223 increased during the initial stages of cooling, reaching a maximum of 93% at 780 °C, and then began to decrease as the temperature fell below 780 °C. CuO particles precipitated out from the liquid phase and grew into big particles at temperatures below 750 °C. The Ca2PbO4 phase formed rapidly at the beginning of the cooling process, while the 3321 phase began to form only below 750 °C. The formation and amount of the lead-rich phases are associated with the amount of the liquid present at high temperatures.

Heating rate effect on the evolution of texture in (Bi, Pb)2Sr2Ca2Cu3O10 Ag-sheathed tapes

Bi2223/Ag tapes have been produced by the powder-in-tube technique. The evolution of texture during the first heat treatment has been studied by means of in situ synchrotron radiation diffraction. Using various heating rates (1, 2 and 4 °C min−1), it was found that the kinetics of preferential orientation development during the early sintering period was slower when low heating rates were used. Microstructural observations indicate that this effect is related to the enhanced growth of Bi2212 grains during the final stage of the heating ramp. Increasing the heating rate results in a faster disappearance of the Bi2212 phase, which in turn is supposed to induce a faster rate of liquid phase formation at the beginning of the reaction process leading to the development of the Bi2223 phase. This feature is also believed to play a role in the texturing kinetics.

Observation of the formation and growth of secondary phases in Bi-2223/Ag tapes

Abstract Formation and growth of secondary phases in Bi-2223/Ag tapes have been investigated. Samples were heat treated at several temperatures for different periods under reduced oxygen partial pressure. The composition and microstructural evolution of secondary phases during the sintering process were characterized by SEM, XRD and EDS. Using high resolution SEM, the (Ca,Sr)2CuO3 (2:1), CuO and (Ca,Sr)14Cu24O41 (14:24) phases could be clearly identified, and details of the Bi-2212, Bi-2223 structure, as well as the quenched liquid region could be also observed. Experiments showed the liquid region of quenched samples contained small Cu-rich precipitates, and the majority of the liquid was rich in Pb, Bi and Ca. The increase of the liquid region area fraction and the disappearance of the 2212 phase both take place quickly, in contrast to the slower development of the total secondary phase area fraction. The amount of the large secondary phase particles was lowest for samples sintered at 830 °C in 8.5% O2. At higher temperatures, the amount of the 2:1 phase increased and the particle size of the 14:24 became larger, while at lower temperatures, the CuO particles appeared at very beginning and tended to grow rather fast.