Stress effects for Bi-based superconducting tapes

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Physica C 341-348 (2000) 2587-2588 www.clscvier.nl/locatc/physc

Stress effects for Bi-based superconducting tapes Zhengdao Wang~b Zlmojia Chenb Huaming Wenc Wenquan Wangd 1Cryogenic Laboratory, Chinese Academy of Sciences Beijing 100080 ~Institute of Physics, Chinese Academy of Sciences Beijing 100080 CInstitute of Electrical Engineering Chinese Academy of Sciences Beijing 100080 d Beijing General Research Institute for Non-Ferrous Metals, Beijing 100088

Abstract In this study, the tensile experiments were performed on four types of Bi(2223)/Ag alloy superconducting tapes at 77K. The results showed that, in comparison to pure Ag-sheath, the critical tensile stress of the tapes with Ag alloy sheaths did not improve greatly as we expected. Based on XRD pattern, a poor phase conversion of the superconducting core in the Ag alloy sheaths was found, which might be the main reason affecting the critical stresS.

1. Introduction Though Bi(2223)/Ag superconducting tapes have been used commercially, Ag is a kind of soft and ductile metals. Its strength of anneal-state is very low, which doesn't make superconducting tapes robust enough to handle stresses during handling and operation. Some researchers tried to develop multifilamentaryIL2I conductors and conductors with various alloy sheaths[3'41. However, the difference in experimental data ~om different groups was often ratherlarge, which might be caused by the selecting of different metal sheaths and procedure. In this paper, the tensile experiments were done at 77K on HTSC tapes with four different kinds of Ag alloy sheaths. By XRD patterns we gave an explanation on the experimental results.

2.ResuRs and discussion According to the different sheath materials, the specimen can be divided into four kinds(outer sheath/inner sheath): Ag/Ag, AgCu/Ag, AgCu/AgCu and Ag/AgCu. Table.1 listed the critical ctrrent Ic and n-value at 77K. The relation cm~es of normalized eritical current Ic/Ic(0) versus tensile stress on four kinds of specimens at 77K are shown in Fig.1. From Fig.l, we can find: a)To the tape with Ag/Ag sheaths, when the tesile stress is m~'e than 50MPa, Ic will begin to degrade. We know that the yield stress of Ag is about 60MPa

at 77K. Thinking of some influences of experiments, we can conclude ~ t , to the tapes with Ag-shea~, the critical stress(the stress at which the Ic begins to degrade) is near to yielding s~ength of the sheath. h)To the tapes with Ag alloy, Ic has begun to degrade even if the stress is much lower than yielding strength of the sheaths. Especially to the tapes with the Ag/AgCu, Ic has greatly degraded when the s~ess is only to 20MPa. In our experiments, by use of the Ag alloy sheaths, the critical stress does not have an apparent improvement. We thought it might be caused by the undesirable surface properties between superconducting core and sheaths, and those between outer and inner surface. Some researchers have demonstrated the supercurrent was transported along the area adjacent to the sheath[~]. Furthermore, when the metals of the outer and inner sheaths were different, there might have prestress during the procedure of machine and heat trea|~nent, which also has some influences on the critical stress of tapes.

Tablel the Ic and n value at 77K specimen I¢(A) Ag(out)/Ag(in) 14.24 AgCu(out)/Ag(in) 13.29 11.58 AsCu(out)/A6Cu0n ) Ag(out)/A~M(in) 8.70

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Stress (MPa) Fig. 1 Ic/Ic(0) versus tensile stress at 77K (1,2,3,4 seperatively mean tapes of Ag/Ag, AgCu/Ag, AgCu/AgCu and Ag/AgCu sheath) In order to verify the above explanation, the XRD experiments of the four kinds of specimen were done. Fig.2 was the results of XRD. By analysis, we found the specimen of Ag/Ag had the least impurity phase. The others would degrade according to the sequence: AgCu/Ag, AgCu/AgCu and Ag/AgCu. The results could partly explain the phenomena of Fig. 1.

3.Conclusion Though there are many advantages about the Bibased 2223 of Ag-sheath, it could not provide the necessary strmgth for practical application. In theory, the critical stress can be improved by replacing the sheaths by Ag alloy, and there were also some papers[6] about the assumption. But in our experiments, the critical slress did not improve apparently. By XRD pattern, we found the properties of the alloy sheaths surface between the core and sheath are poorer than that of Ag sheath. So if we want to improve the mechanical properties, not only a high yielding stress sheath is needed, the compatible and possible stress distribution also need to be thought during the procedure of machine and heat U'eaUnent.

Fig.2 XRD patterns o f different tapes (From above to below, according to the sequence of Ag/Ag, AgCu/Ag, AgCu/AgCu and Ag/Ag/Cu) Reference [1] U.Balachandran, A.N. Iyer, P.Haldar, and L.R.Motowildo, J.Met. 9 54-57 1993 [2] M.Suenaga, Y.Fukomuto, and U.Wildgrllber, Appl. Phys. Lett.67 20 1995 [3] K.C.Goretta,M.Jiang, and D.S.Kupperman, Jr. IEEE Trans.Appl.Supercom.,7,No.2 1997 [4] J.Guo, J.A.Lewis, K . C . Goretta, and J.Schwartz, J.Appl.Phys.7$ 7 1995 [5] Y.Feng, Y.E.High, D.C.Larbalstier, Y.S.Sung, and E.E.Hellstrom, Appl.phys.Lett. 62 13 1993 [6] B.Z.Tang, M.Huang, and Y.B.Tang , MT-15 Proceedings, Science Press,1040, 1998