Abstract Body

High temperature superconductor (HTS) tapes [1-3] have been applied to develop magnets and coils. To generate strong magnetic fields using such HTS magnets or coils, HTS tapes of several or several-tens of kilometers in length are required, whereas the piece length of commercial Bi₂Sr₂Ca₂Cu₃O_8+x (Bi2223) multifilamentary tapes is less than 1 km. Therefore, many researchers have been focused on the development of HTS joint techniques to connect the tapes for magnets [4-9], which are a fundamental technology for practical applications such as nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). Recently, Kanazawa (Jin) et al, reported a method for a Bi2223/Bi2223 joint with critical current (I_c) of 12.2 A and 177 A at 77 K and 4.2 K, respectively [6-8]. In the work, we characterized the nanostructures of the Bi2223/Bi2223 joining region using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Particular attention was paid to the composition and crystal orientation of the joining region.

Bi2223 tapes with a width of 4 mm produced by Sumitomo Electric company were used for preparing a joint sample. The Bi2223 tapes were peeled to prepare a joint surface on which Bi2223 filament and Ag were exposed. The surfaces of the peeled Bi2223 tapes were brought into contact with each other, and the region was rolled with Pt tapes. The contact region was heated up to 890°C in a furnace and cooled down to room temperature [6-8]. We prepared 12 sections of the joint sample were prepared in total using an Ar ion beam in a JEOL IB-09010CP system, and each cross-section were examined in a Hitachi SU8000 SEM system with an energy-dispersive X-ray spectroscopy (EDS). TEM specimen was prepared from a cross-section corresponding to distance of 13 mm from the sample edge of the joint sample using a focused ion beam (FIB)-microsampling technique in a Hitachi NB5000 SEM-FIB system. The specimen was examined in a Topcon EM-002BF operated at an accelerating voltage of 200 kV.

Figure 1(a) shows a SEM image of the center region of cross-section corresponding to distance of 13 mm from the edge of the joint sample. The Ag sheaths appear brightest, and a region indicated by an arrow is slightly brighter than those of the Bi2223 filaments in fig. 1(a). The results of EDS analysis indicated that such the region denoted by the arrow was Bi, Sr-rich and Ca, Cu-poor compared to corresponding regions of Bi2223 filaments. A cross-sectional TEM specimen was prepared from the square region in fig. 1 using FIB-microsampling technique.

Figures 1(b) and (c) show cross-sectional TEM images corresponding to the square region in fig.1(a), and insets (i)-(iv) show selected area diffraction patterns (SADPs) with plane indexes. These TEM images correspond to the same region, but the TEM specimen was tilted a few degrees relative to the images shown in fig. 1(b) to (c). Insets (i) and (ii) were taken from the upper and the lower grains in fig. 1 (b), respectively, and insets (iii) and (iv) were also taken from those grains in fig. 1(c), respectively. The upper grain is identified as Bi₂Sr₂CaCu₂O_8+x (Bi2212) and the lower grains as Bi2223 from the SADPs (i) and (iv), respectively. These Bi2212 and Bi2223 phases were detected using X-ray diffraction taken from the joining region in the previous paper [6-8]. These grains are connected without secondary phase or gaps. Furthermore, the c-axes of the Bi2212 and the Bi2223 grains shown in figs. 1(b) and (c) are well aligned. These results indicate that Bi2212 grains should be transformed from Bi2223 grains at the joining region without a change of the crystal orientation during the heating process, as predicted in the previous report [6-8]. However, the joining region with the B2212 grains is likely to be approximately 3 mm along the width direction of Bi2223 tape and 1 mm along the length direction of the Bi2223 tapes. To increase the critical joint current, such the formation region of Bi2212 grains between the Bi2223 filaments should be enlarged in those joint Bi2223 wires.

References

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[2] Hikata T, Sato K and Hitotsuyanagi H 1989 Jpn. J. Appl. Phys., 28 L82-L84
[3] Larbalestier D C, Gurevich A, Feldmann D M and Polyanskii A 2001 Nature, 414 368-377
[4] Park Y, Lee M, Ann H, Choi Y H and Lee H 2014 NPG Asia Mater., 6 e98
[5] Ohki K, Nagaishi T, Kato T, Yokoe D, Hirayama T, Ikuhara Y, Ueno T, Yamagishi K, Takao T, Piao R, Maeda H and Yanagisawa Y 2017 Supercond. Sci. Technol., 30 115017
[6] Jin X, Suetomi Y, Piao R, Matsutake Y, Yagai T, Mochida H, Yanagisawa Y and Maeda H 2019 Supercond. Sci. Technol., 32 035011
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[8] Kanazawa S 2021 IEEE Trans. Appl. Supercond., 31 7000104
[9] Takeda Y, Motoki T, Kitaguchi H, Nakashima T, Kobayashi S, Kato T and Shimoyama J 2019 Appl. Phys. Express, 12 023003

Acknowledgment

This work was supported by the Japan Science and Technology Agency (JST)-MIRAI Program Grant Number JPMJMI17A2, Japan.

Fig. 1 (a) shows SEM image of center region of cross-section corresponding to distance of 13 mm from the edge of the joint sample. Square region was prepared to TEM specimen. Fig. 1(b) and (c) shows cross-sectional TEM images of interface between Bi2212 and Bi2223 grains under the [1-10] zone axis of Bi2212 condition, and the [010] zone axis of Bi2223 condition, respectively. (i) – (iv) selected area diffraction pattens with the plane indexes, (i) and (ii) are taken from the Bi2212 and the Bi2223 grains in (b) (iii) and (iv) from those grains in (c).

Keywords: superconducting joint, Bi2223, nanostructure, SEM, TEM