Research Digest 27We fabricated SC joint samples by synthesizing an intermediate layer between two Bi-2223 tapes. Ic of the samples was increased by additional pressing and heat treatment (HT2) after the initial heat treatment (HT1). From the microstructural observations, we confirmed that pressing densified the intermediate layer. Figure 1 shows the relationship between filling factor of the intermediate layer (F) and ΔIc = Ic (after HT2) − Ic (after HT1) at 77 K in self-field for the samples. J1, which was prepared by the initial HT1, was plotted as ΔIc = 0. J3, J4, and J6 were fabricated by additional pressing and HT2. ΔIc appears to be positively correlated with F. This suggests that densification of the intermediate layer produces a large ΔIc in the SC joint. It can be concluded that the density of the intermediate layer is a key parameter affecting Ic of the SC joint8).Based on the above results, SC joint samples were fabricated by a hot-pressing process, which is a promising method to densify the intermediate layer. We developed the specialized equipment to apply uniaxial pressure to the joint during the heat treatment as shown in Fig. 2. The hot-pressed samples showed higher Ic even by the shorter heat treatment compared to the samples without applying pressure during the heat treatment. It is suggested that the proposed hot-pressing process is a promising method to fabricate high-Ic SC joints in high yields and within a short processing time9).Fig. 1. [adapted from 8)] Relationship between F (filling factor of intermediate layer) and ΔIc for Bi-2223 SC joint samples.Fig. 2. [adapted from 9)] Photograph of SC joint sample before heat treatment with specialized equipment to apply pressure.References1) K. Hashi et al., J. Magn. Reson. 256, 30 (2015).2) G. Nishijima et al., Rev. Sci. Instrum. 84, 015113 (2013).3) G. D. Brittles et al., Supercond. Sci. Technol. 28, 093001 (2015).4) Y. Takeda et al., Supercond. Sci. Technol. 35, 043002 (2022).5) Y. Takeda et al., Appl. Phys. Express 12, 023003 (2019).6) Y. Takeda et al., Supercond. Sci. Technol. 35, 02LT02 (2022).7) Y. Takeda et al., IEEE Trans. Appl. Supercond. 32, 4301005 (2022).8) Y. Takeda et al., Supercond. Sci. Technol. 36, 035004 (2023).9) Y. Takeda et al., IEEE Trans. Appl. Supercond. 33, 6400207 (2023).1. Outline of ResearchA superconducting (SC) magnet is an electromagnet made from coils of SC wires. SC magnets are widely used for various applications such as nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), etc. Until now, conventional low-temperature superconductors (LTSs) have been used for the magnets. To generate higher magnetic fields and operate SC magnets at higher temperatures, research and development of high-temperature superconductors (HTSs) have progressed. Ag-sheathed multifilamentary (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) HTS tape is commercially available. The tape shows high critical current (Ic) and has been used for magnets generating high magnetic fields, such as a 24 T NMR magnet (1.02 GHz for 1H NMR) developed at NIMS1). A Bi-2223 magnet operated at high temperatures was also developed at NIMS2).SC joint is indispensable for the fabrication of SC magnets operating in the persistent current mode3). Such a persistent-mode magnet enables the energy-saving systems without a power supply. The SC joint for the LTS wires is well established. In the last decade, significant progress has been made in the development of the SC joint between the HTS tapes4).We have successfully developed high-Ic SC joints between Bi-2223 tapes by synthesizing a polycrystalline Bi-2223 intermediate layer5),6). The synthesis comprises a slurry process, uniaxial pressing at room temperature, and heat treatments. A small SC joint sample showed a high Ic value of about 300 A at 4 K and 1 T. However, the high Ic could not be replicated in a SC joint implemented in a Bi-2223 test coil for developing a primitive persistent-mode Bi-2223/LTS NMR magnet7). This is because the key parameters affecting Ic have not yet been clarified. To identify these parameters, it is important to understand the relationships between Ic of a SC joint and the properties for an intermediate layer such as microstructure and density.To clarify the key parameters affecting Ic of a Bi-2223 SC joint, I have studied the relationships between Ic and the properties for an intermediate layer. First, the effect of intermediate layer densification on Ic was quantitatively studied. Second, SC joint samples were fabricated by a hot-pressing process to densify the intermediate layer.2. Research ActivitiesEffect of Properties for Intermediate Layer on Critical Current Characteristics in Bi-2223 High-Temperature Superconducting JointYasuaki TAKEDA
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