Physica C 470 (2010) Contents lists available at ScienceDirect. Physica C. journal homepage:

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1 Physica C 47 (21) Contents lists available at ScienceDirect Physica C journal homepage: Soldering of Bi-2223/ high temperature superconducting tapes with Pb Bi alloy paste Guo Wei a,b, *, Zou Guisheng a, Wu Aiping a, Bai Hailin a, Ren Jialie a a Department of Mechanical Engineering, Key Laboratory for Advanced Manufacturing by Materials Processing Technology, Tsinghua University, Ministry of Education of PR China, Beijing 184, China b The School of Mechanical Engineering and Automation, Beijing University of Aeronautics and Astronautics, Beijing 1191, China article info abstract Article history: Received 13 June 29 Accepted 4 November 29 Available online 1 November 29 Keywords: Bi-2223/ Solder Microstructure CCR Resistance Tensile strength Soldered joints of Bi-2223/-sheathed high temperature superconducting multifilamentary tapes were fabricated using 63 wt.% 34 wt.%pb 1 wt.%bi 2 wt.% paste. The soldered joints were observed by scanning electron microscope (SEM) and X-ray diffraction (XRD). Moreover, the electrical properties of joints were evaluated by current voltage curves, and the tensile strengths of the joints were also tested. The results show that the soldered joint consists of sheath compound layer Pb 2 and solder layer compound layer sheath. The joints are obeyed with Ohms Law and the magnitude of the joint resistance, which deceases with the increase of the overlap length, can reach the order of 1 8 X. The tensile strength of the joints with a brittle fracture mode is a little lower than that of the original tapes. Ó 29 Elsevier B.V. All rights reserved. 1. Introduction Owing to recent remarkable progress of fabricating high temperature superconductor (HTS) Bi-2223/-sheathed tape technologies [1], various R&D of projects of HTS power applications such as power cables, transformers, motors are ongoing in many countries [2 6]. Bonding technology is often needed in these applications to make long HTS superconducting tape or superconducting loop. As an economical, reliable and facilitate method, soldering is still a widely used one to connect the superconducting tapes [7 9]. In the process, the soldered joint is fabricated at low temperature, and thus the superconducting property of Bi-2223/sheathed tape does not degrade greatly. The Pb based solder is one of good representative solders for superconducting tape. However, the comprehensive researches on both mechanical and electrical properties, especially on the metallurgic analysis of the soldered joints have less been reported. In the present work, a series of soldering experiments with 63 wt.% 34 wt.%pb 1 wt.%bi 2 wt.% paste solder were performed in a special stove which could offer a definite load on the joint. The microstructures of the soldered joints between the * Corresponding author. Address: The School of Mechanical Engineering and Automation, Beijing University of Aeronautics and Astronautics, Beijing 1191, China. Tel.: address: gwei@tsinghua.edu.cn (G. Wei). superconducting tapes were investigated. Moreover, the mechanical property and the electrical property of the joints were evaluated by tensile strengths and the resistances or the voltage current (V I) curves of the joints, respectively. 2. Experimental The Bi-2223/-sheathed superconducting tapes used in the experiments are provided by Beijing Innova Superconductor Technology Co. Ltd. The critical current (at 77 K in zero magnetic field) of tape is about 9 A. The tape contains 61 superconducting filaments and is 4.3 mm wide and.22 mm thick. Before soldering, the oxide film outside of the HTS tape was peeled off by abrasive papers and a rectangular bonding window was prepared by etching method, and the filler solder was put in the rectangle window. The dimension of the bonding window and the sketch of assembly joint are shown in Fig. 1. Soldering process was performed in a special soldering stove in air, and the soldering temperature was 2 C which is a little higher than the melting point of the solder (183 C). Meanwhile, an approximate 1 MPa uniaxial press was applied on the superconducting tape. After soldering, the voltage current (V I) curves of the joints were measured by standard four probe method with a 1 lv/cm criterion in liquid nitrogen under zero magnetic field. The system was connected to a computer for data acquisition. Here we defined critical current ratio (CCR) as the ratio of the critical current of the joint to that of original tape. The polished surfaces of the bonded /$ - see front matter Ó 29 Elsevier B.V. All rights reserved. doi:1.116/j.physc

2 116 G. Wei et al. / Physica C 47 (21) Fig. 1. The dimensions of the rectangular bonding window and the sketch of assembly joint. couples were examined by means of scanning electron microscope (SEM) of Leo153 type with electron backscatter diffraction mode. Moreover, identification of various phases present in the interlayer of the joint was examined by XRD of D/max-25X type. Note that the upper superconducting tape of the joint should been removed through mechanical or etched methods before performing the XRD tests. In order to identify the phase constituents of the soldered joint layer upon layer, the sand papers were used to peel off the joint zone layer by layer before each XRD tests. Last, the tensile Joint 1 Joint 2 Joint 3 Fig. 4. Microstructures of the transverse and longitudinal sections of soldered joints (a) transverse section (b) longitudinal section. Voltage/uV Current/A Fig. 2. The V I curves of the soldered joints CCR Resistance 25 2 Fig. 5. The inner microstructure of soldered joint. CCR Resistance/1-8 Ω strength of the joint was tested on the tensile machine typed SHI- MADIU AG-X. One end of the tape was held by the jig and the other end was pulled with 1 mm/min. During the test process, the displacement and the load were recorded by a computer. 3. Results and discussion Overlap length/mm Fig. 3. The effects of the overlap length on the CCR and the resistance of soldered joints Electrical properties of the soldered joints The V I curves of the soldered joints with 2 mm overlap length are shown in Fig. 2, and three curves are shown in the figure. The relations of the voltage and the current of these joints, i.e., joints 1

3 G. Wei et al. / Physica C 47 (21) Table 1 The phase compositions in various layers of the joint by EDS analysis. (Atomic %) Pb Reacted layer A Reacted layer B Pb filler layer are approximately linear, which indicates the soldered joints accord with Ohms Law. Thus, the electrical properties of the joints are just like traditional conductors. The average electrical resistance of the joints is about X derived from Ohms Law. Fig. 3 shows the effects of the overlap length on the CCR and the resistance of the soldered joints. It can be seen that the CCRs of the joints are direct proportional to the overlap length. In contrary, the resistances of the joints decrease with the increase of the overlap length. Note that the magnitude of the resistance is as small as the order of 1 8 X. Fig. 4a and b are microstructures of the transverse and longitudinal sections of the soldered joints, respectively. From Fig. 4b, the upper and lower superconducting tapes have been connected through Pb Bi solder, and about 4 lm thickness diffusion reaction layer come into being between the tapes. The inner microstructure of solder joint is characteristic of eutectic phases shown in Fig. 5. Several obvious symmetrical layer structures are formed in the soldered joint e.g. reacted layer A, reacted layer B and Pb filler layer. To investigate the phase composition in the soldered joint, point scanning experiments by EDS analysis in various layers are carried out, and the results are given in Table 1. From the data, the atomic number ratio of element and the element in the reacted layers A and B adjacent to the superconducting tape is 3:1, and the Pb filler layer consists of at.% at.%pb 24.3 at.% eutectic compounds. Meanwhile, line scanning by EDS analysis has been performed across the bonding interface, and three main elements (silver, lead, tin) diffusion curves are shown in Fig. 6a c. The silver line scanning result reveals that silver element diffuses richly from both sides to the middle soldered zone ( Pb filler layer), and forms a homogeneous silver element distribution in the Pb filler zone. And the lead element mainly distributes in the Pb filler layer. Meanwhile, the tin element diffuses from the Pb filler layer to the reacted A and B layers, which coincides with the point scanning results mentioned before Microstructures of the joint 3.3. XRD results of the joint To identify the phase constituents of various reacted layers on the soldered joint definitely, a series of XRD tests have been performed on each reacted layers through peeling off the outside layers by sand papers. Note that this method cannot definitely distinguish the different reacted layers, but main phase composi- Fig. 6. The results of the element line scanning by EDS analysis (a) silver (b) lead (c) tin.

4 118 G. Wei et al. / Physica C 47 (21) tions at different layers of the whole joint will be identified. The XRD results of the joint are given in Fig. 7. Fig. 7a is the phase constituents near reacted A and B zones. From the XRD results, the main phase constituent is, which is corresponding to the EDS results on the reacted layers A and B shown in Table 1. And a small amount of Pb 2 phase is also detected, which is main phase composition in the Pb solder zone. Moreover, some Bi superconducting phase is detected, which is main phase composition of the superconducting tape. With the increase of detected depth, the inner compositions of the soldered joint are detected shown in Fig. 7b e. From these XRD results, some phenomena should be noticed. First, Pb 2 phase increases and phase decreases with the detected depth increase initially (shown in Fig. 7b and c), and it can been confirmed that Pb 2 and phases are main phase compositions in the Pb eutectic compounds, which are corresponding to the EDS results on the Pb solder layer shown in Table 1. Second, phase exists in almost all XRD results (Fig. 7b e), which shows element in the sheath diffuses cross the whole joint and is not been oxidized during the soldered processing. In summary, the whole joint is composed of sheath compound (reacted layers A and B) Pb 2 and ( Pb solder layer) compound (reacted layers A and B) sheath. The electric property of the joint is mainly determined by (a) 2 15 Bi-2223 Pb 2 (b) 8 6 Pb (c) (d) Pb Pb 2 Bi (e) Pb 2 Bi Fig. 7. The XRD results of the joint.

5 G. Wei et al. / Physica C 47 (21) Tensile strength/kn.1.8 Original tape.6 Joint4.4 Joint1.2. Joint2 Joint Displacement/mm Fig. 8. The tensile curves of the soldered joints and the original tape. are between 75 and 85 MPa, a little lower than that of the original tape (about 1 MPa). The fracture type of solder joints belongs to fragile fracture mode from the profiles of the curves. The photograph of the fracture position of the joints is shown in Fig. 9. The fracture positions of the joints are at the edges of the overlap part where the stress concentration exists because of the special structure type of the joints. 4. Conclusions The soldered joints of Bi-2223/ multifilamentary superconducting tapes were investigated. The magnitude of the joints resistance is as small as the order of 1 8 X, which deceases with the increase of the overlap length. In contrast, the critical current ratio of the joints is proportional to the overlap length of the joint. The result shows that the layer structures are formed in the soldered joint, and the whole soldered joint is composed of sheath compound (reacted layers A and B) Pb 2 and ( Pb solder layer) compound (reacted layers A and B) sheath. And the electric property of the joint is mainly determined by compound zone (reacted layers A and B), Pb 2 and zone ( Pb solder layer). Finally, the tensile strengths of the soldered joints are about MPa which is a little lower than that of the original tape, and the fracture mode was a brittle one. Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant Nos and 56355) and China Postdoctoral Science Foundation (Grant No ). References Fig. 9. The photograph of the tensile testing fracture positions of the joints. compound zone (reacted layers A and B), Pb 2 and zone ( Pb solder layer), because element owns much lower resistivity comparing with the other reacted phases Mechanical properties of joints Fig. 8 shows the tensile curves of the soldered joints. Four tensile curves of the joints and the tensile curve of the original tape are given in the figure. The tensile strengths of the soldered joints [1] S.S. Oh, H.S. Ha, D.W. Ha, H.M. Jang, C. Park, et al., Cryogenics 42 (22) 377. [2] A.P. Malozemoff, D.T. Verebelyi, S. Fleshler, D. Aized, D. Yu, Physica C 386 (23) 424. [3] M. Watanabe, T. Masuda, H. Yumura, H. Takigawa, Y. Ashibe, et al., Physica C (27) 132. [4] H. Takigawa, H. Yumura, T. Masuda, M. Watanabe, Y. Ashibe, et al., Physica C (27) [5] S. Kato, T. Hashimoto, H. Hasegawa, S. Hirano, S. Nagasawa, et al., Physica C (22) [6] T. Masuda, T. Kato, H. Yumura, M. Watanabe, Y. Ashibe, et al., Physica C (22) [7] C. Gu, C. Zhang, T.M. Qu, Z. Han, Physica C (25) [8] S.Y. Oh, H.R. Kim, Y.H. Jeong, O.B. Hyun, C.J. Kim, Physica C (27) 464. [9] J.H. Jang, C.J. Kim, O.B. Hyun, H.W. Park, Physica C (27) 455.

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