# Fileset

[Final_version_Fabrication_of_Bi-2223_Superconducting_Joint_by_Hot-Pressing_Process.pdf](https://mdr.nims.go.jp/filesets/11ccb12e-2918-42dc-bf61-b2a9c6168b05/download)

## Creator

[Yasuaki Takeda](https://orcid.org/0000-0001-7217-9853), [Gen Nishijima](https://orcid.org/0000-0001-7493-0559), [Kensuke Kobayashi](https://orcid.org/0000-0003-1748-5799), [Hitoshi Kitaguchi](https://orcid.org/0000-0002-5998-2649)

## Rights

© 2023 IEEE.  Personal use of this material is permitted.  Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Fabrication of Bi-2223 Superconducting Joint by Hot-Pressing Process](https://mdr.nims.go.jp/datasets/8834125f-9a7e-4576-8735-9537dda05bf5)

## Fulltext

3MPo1C-01   Template version 8.0d, 22 August 2017. IEEE will put copyright information in this area See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 1 Fabrication of Bi-2223 Superconducting Joint by Hot-Pressing Process  Yasuaki Takeda, Gen Nishijima, Kensuke Kobayashi, and Hitoshi Kitaguchi    Abstract—Superconducting joints between Bi-2223 tapes were fabricated by a hot-pressing process with a 24 h heat treatment. We developed specialized equipment to apply pressure to the joint during the heat treatment. Hot pressing is effective for producing practical critical current even by the short heat treatment. The su-perconducting joint in a closed-loop showed a resistance of 7.3 × 10−15 Ω at 4.2 K in self-field. The maximum persistent cur-rent that could flow in the loop was estimated to be about 170 A at 4.2 K. The purification and further densification of the intermedi-ate layer in the superconducting joint are issues that need to be ad-dressed. Once these issues are addressed, the hot-pressing process should be a promising method for fabricating high-performance Bi-2223 superconducting joints in high yields within a short pro-cessing time.   Index Terms— Bi-2223 tape, HTS magnets, resistance meas-urement, critical current I. INTRODUCTION ERSISTENT current mode superconducting magnets are used for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). Superconducting joints are indis-pensable for persistent-mode magnets [1], [2]. To realize per-sistent-mode magnets using high-temperature superconducting (HTS) tapes/wires, the formation of superconducting joints be-tween the HTS tapes/wires is necessary, and significant pro-gress has been made in this area in the last decade [2]–[11].  Ag sheathed multifilamentary (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) HTS tapes are commercially available. The most fa-mous is DI-BSCCO® produced by Sumitomo Electric Indus-tries, Ltd. [12], [13]. This tape shows a high critical current (Ic) and has been used for various magnets generating high fields or operating at high temperatures [13]–[16]. However, practical persistent-mode Bi-2223 magnets have not yet been realized, owing to the lack of superconducting joint formation technology.  Manuscript received xxxxxxxx; accepted xxxxxxxx. Date of publication xxxxxxxx; date of current version xxxxxxxx. This work was supported by JST-Mirai Program Grant Number JPMJMI17A2 and JSPS KAKENHI Grant Num-ber JP22K14482, Japan (Corresponding author: Yasuaki Takeda.) Yasuaki Takeda, Gen Nishijima and Hitoshi Kitaguchi are with National In-stitute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0003, Japan (e-mail: TAKEDA.Yasuaki@nims.go.jp).  Kensuke Kobayashi was with NIMS and is with RIKEN Center for Biosys-tems Dynamics Research, Yokohama, Kanagawa 230-0045, Japan. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier will be inserted here upon acceptance. We have previously developed superconducting joints with a high Ic between the Bi-2223 tapes [3], [4]. In the formation of the superconducting joints, most of the Bi-2223 filaments were exposed by low-angle (0.3°) polishing using a tool with a 0.3° tilted surface that we made. To connect the exposed fila-ments, we synthesized an intermediate layer, which was equivalent to an about 0.1 mm thick polycrystalline Bi-2223 film. This thick film was synthesized through the slurry pro-cess, uniaxial pressing at room temperature (RT), and heat treatments. So far, we achieved an Ic of about 300 A at 4.2 K and 1 T using a short sample of the Bi-2223 superconducting joint [4]. We also fabricated a Bi-2223 closed-loop sample with the superconducting joint under the same condition as that of the short sample. Using this closed-loop sample, a low joint resistance (Rj) of less than 10−14 Ω at 4.2 K and 1 T was demonstrated [4]. In a recent study [17], we reported that the densification of the intermediate layer is effective in increasing the Ic of Bi-2223 superconducting joints. This increase in Ic was attributed to the increase in the intergrain critical current density (Jc) of the intermediate layer. The densification was achieved by in-troducing the general two-step heat treatment process. This process comprises the first heat treatment, an intermediate uniaxial pressing at RT, and the second heat treatment. Be-cause of this process, the filling factor of the intermediate lay-er could be increased to about 80%. However, high-pressure intermediate pressing at pressures more than 2 × 108 Pa (about 200 MPa) can mechanically damage the Bi-2223 filaments in the tapes, which decreased the Ic of the superconducting joints. This suggests that methods other than the intermediate press-ing are required to further densify the intermediate layer and improve Ic. A promising densification method is hot pressing. It is well known that polycrystalline Bi-2223 materials swell with the grain growth during a heat treatment, resulting in a decrease in the density [18], [19]. Hot pressing is effective in suppressing this swelling [20], [21]. It has been reported that the Ic of Bi-2223 tapes increased by introducing a hot-pressing process at pressures of 1–6 × 106 Pa.  Attempts to fabricate Bi-2223 superconducting joints by hot pressing have already been reported [22], [23]. The supercon-ducting joints appeared to be formed by hot pressing even with a short heat treatment of about 10 h, although the high-purity Bi-2223 intermediate layer was not synthesized. An Rj value of less than 10–13 Ω at 77 K was observed in a Bi-2223 closed-loop sample made from a DI-BSCCO® tape. However, P   2 practical persistent current values of more than 102 A have not yet been realized.  In this study, we fabricated high-performance Bi-2223 su-perconducting joints using a hot-pressing process. A short heat treatment was performed to reduce the processing time, mak-ing the proposed process a practical one for forming joints. II. EXPERIMENTAL A. Preparation of hot-pressing equipment (HOPE) Specialized equipment for hot pressing, which we have la-belled HOPE (HOt-Pressing Equipment). The equipment had to be smaller than the inner diameter of a tube furnace (40 mm). Fig. 1(a) shows a photograph of a straight lap joint sample with HOPE before it was subjected to a heat treatment. HOPE consists of two stainless-steel (SS) plates (3.0 × 20 × 25 mm) with four holes, four M4 brass bolts, and four alumina nuts. Uniaxial pressure perpendicular to the tape surface was applied on the joint by the axial force of the bolts. The pressure was controlled by the bolt-tightening torque (N) of 0.20 or 0.50 N·m. The surfaces of the SS plates were oxidized in advance by annealing in air at 800 °C to control the partial oxygen pres-sure (PO2) during the heat treatment. The SS plates and alumi-na nuts were durable and could be repeatedly used. However, the brass bolts had to be disposed owing to the oxidization of their surfaces during the heat treatment. We used the brass bolts because the linear thermal expansion coefficient of brass is similar to that of silver, which is the matrix of the Bi-2223 tapes [24]. We believe that by using the brass bolts, the pres-sure applied at RT could be maintained during the heat treat-ment at a holding temperature of 820 °C.  B. Fabrication of samples by hot-pressing process Four samples (three straight lap joint samples and one loop sample) were fabricated using the Bi-2223 tapes without me-chanical reinforcements (DI-BSCCO® type H, 4.2 mm wide and 0.22 mm thick). To apply a uniform uniaxial pressure, we flattened the tapes by polishing both of their sides [17].  For each straight lap joint sample about 10 cm long (SJ-0, SJ-02, and SJ-05), two 6 cm long tapes were used, as in [3] and [17]. The loop sample (CL-02) was a three-turn closed-loop with a praying-hands type superconducting joint using one 1.6 m long tape, similar to the sample in [4]. The diameter of the loop was 100 mm. The self-inductance (L) of CL-02 was estimated to be 1.4 μH. The joining processes of exposing the filaments, the slurry process, and uniaxial pressing at RT were the same as that used in our previous study. The overlap length was 16–17 mm shown in Fig. 1(b). The area of the Bi-2223 intermediate layer was about 70 mm2. Before the heat treatment, uniaxial press-ing on the joint was performed at 2 × 108 Pa and RT to densify the intermediate layer.  Schematic of the fabrication procedure for the closed-loop sample CL-02 is shown in Fig. 1(c). During the heat treatment for the joint, CL-02 was a one-turn loop with the about 0.6 m long temperature transition zone. The joint was inserted in the tube furnace and heat-treated, while the loop part was outside the furnace and held at room temperature [4]. After the heat treatment and removal of HOPE, as shown in the lower part of Fig 1(c), the one-turn loop was wound into three-turn loop with a diameter of 100 mm. This procedure did not damage the tape and joint. This sample geometry is suitable for the evaluation system described in the next section.  In general, because the Bi-2223 formation reaction is slow, a heat treatment of more than 24 h is necessary to ensure high purity when the starting material is precursor powder [25]. To synthesize a high-purity Bi-2223 intermediate layer by a short heat treatment, we premixed Bi-2223 powder with a slurry.  Figs. 2 and 3 show the flow chart for the powder and slurry preparation processes and the θ/2θ x-ray diffraction patterns for the three types of powder measured using a Rigaku Mini-Flex Ⅱ, respectively. The precursor powder produced by TEP Co., Ltd. was used as the starting material. The nominal chem-ical composition of the precursor powder was Bi:Pb:Sr:Ca:Cu = 1.6:0.45:1.9:2.0:3.0. The main phase of the precursor pow-       Fig. 1. (a) Photograph of straight joint sample before heat treatment with HOPE (HOt-Pressing Equipment). (b) Schematic of side view of straight joint sample with HOPE. (c) Schematic of fabrication procedure for closed-loop sample CL-02.   3 der was (Bi,Pb)2Sr2CaCu2Oy (Bi-2212). To synthesize the Bi-2223 powder, the precursor powder was pelletized, heat-treated at a PO2 of 3 kPa and 820 °C, and pulverized. After re-peating twice these steps, the Bi-2223 powder was obtained. By using the two types of powder (i.e., the precursor powder and Bi-2223 powder), we obtained a mixed powder. A slurry was prepared by adding EtOH/1-BuOH as solvent to the mixed powder. The heat treatment profile used during the hot-pressing pro-cess is shown in Fig. 4. The heat treatment at a PO2 of 3 kPa in a tube furnace was completed in about 24 h. To avoid over-shooting the holding temperature, a slow heating rate was used. In addition, a slow cooling rate, which is known to be effec-tive in increasing the Ic of Bi-2223 tapes [26], was also used. The pressure was applied on the joint using HOPE during the heat treatment. The applied pressures (P) for the straight joint samples SJ-02 and SJ-05 at RT were about 5 and 13 MPa (N = 0.20 and 0.50 N·m), respectively. The pressure values were checked using pressure measurement films (Fujifilm Pre-scale). For comparison, SJ-0 was prepared without applying any pressure (P = 0) during the heat treatment. The heat treat-ment profile of SJ-0 was the same as that of the hot-pressed samples. The specifications of the straight joint samples are listed in Table Ⅰ. P was about 5 MPa (N = 0.20 N·m) for the joint in the closed-loop sample CL-02.  C. Characterization Transport measurements in self-field were performed for the three straight joint samples in a liquid nitrogen bath (77 K) using the conventional DC four-probe method. For the hot-pressed samples SJ-02 and SJ-05, the transport measurements were performed twice. First, the voltage–current (V–I) curve was obtained for the samples while HOPE was attached. Sub-sequently, HOPE was removed by loosening the bolts, result-ing in the release of the pressure applied by HOPE. The V–I curve was measured again using the samples without HOPE. Ic was determined using a 0.2 μV voltage criterion. The expo-nent, n, of the empirical power law model was calculated for the voltage range of 0.2 μV ≤ V ≤ 0.7 μV.  After the transport measurements, we observed the micro-structures of the straight joint samples. Note that HOPE had already been removed from the hot-pressed samples at this stage. The polished surfaces of the transverse cross-sections of the three straight joint samples were observed. Secondary and backscattered electron images were obtained using a field emission scanning electron microscope (FE-SEM, Hitachi SU-70). The local chemical composition was analyzed using an   Fig. 2. Powder and slurry preparation flow chart.     Fig. 3.  θ/2θ x-ray diffraction patterns of three types of powder used in this study.    Fig. 4. Heat treatment profile used for all samples in this study. Heat treatment was completed in about 24 h.   TABLE Ⅰ  SPECIFICATIONS OF STRAIGHT JOINT SAMPLES. TRANSPORT MEASURE-MENTS IN SELF-FIELD WERE PERFORMED IN LIQUID NITROGEN BATH  Sample Bolt- tightening torque, N / N·m Applied pressure to joint at RT, P / MPa Transport measurements (77 K) Before or after removal of HOPE Ic / A n / – SJ-0 01) 01) – 0.34 1.89 SJ-02 0.20 ~5 Before 57.8 12.4 After 40.5 12.4 SJ-05 0.50 ~13 Before 36.3 4.64 After 34.6 4.60 1)SJ-0 was prepared without applying the pressure during the heat treat-ment.    4 energy dispersive x-ray spectroscopy (EDS) system attached to the FE-SEM. Current decay method is known to evaluate an Rj value of 10−13 Ω or lower, which cannot be evaluated by transport measurements [1]. Current decay measurements at 4.2 K in self-field were performed for the closed-loop sample CL-02 using the joint resistance evaluation system that we have de-veloped [27]. CL-02 was mounted on the sample holder after the removal of HOPE, because the space for the joint within the holder was less than 25 mm in diameter. After cooling to 4.2 K, loop current (Iloop) was injected in CL-02 by magnetic induction using a copper coil located at the loop center. Time dependence of Iloop was measured using a current transform-er [28].  III. RESULTS AND DISCUSSION A. Transport measurements of straight joint samples at 77 K in self-field Fig. 5 shows the V–I curves at 77 K in self-field for the three straight joint samples. SJ-0 showed an almost linear in-crease in voltage, which meant that SJ-0 was not superconduc-tive. This is attributed to the poor grain connectivity and low intergrain Jc of the intermediate layer. It is probably due to the short heat treatment (24 h) and the use of the Bi-2223-rich powder for synthesizing the intermediate layer [29]. The V–I curves for SJ-02 and SJ-05 showed a typical super-conducting to normal transition. This indicates that Bi-2223 superconducting joints were successfully formed in these hot-pressed samples. It also suggests that hot pressing is effective for improving the grain connectivity and synthesizing a high-Jc intermediate layer. Before the removal of HOPE, SJ-02 showed an Ic of 57.8 A. Although this Ic value is not much higher than those of the previously reported joint samples, all the previous samples were fabricated with longer processing times than SJ-02 [3], [4], [17]. Thus, hot pressing is effective for fabricating Bi-2223 superconducting joints with practical Ic values even by the short heat treatment (24 h). However, considering that the Ic of a virgin tape is more than 150 A at 77 K in self-field, an Ic value of the hot-pressed superconducting joints has room for improvement. After the removal of HOPE, that is, after the release of the pressure, the Ic of SJ-02 decreased by 30%, although the n value did not change. We observed an increase in the thick-ness of the joint of about 20 m after the removal of HOPE. Assuming that this increase was mainly attributable to the ex-pansion of the intermediate layer, the increase corresponded to the decrease in the density of the intermediate layer with mi-crostructural changes, which, in turn, resulted in a decrease in the intergrain Jc. This is a possible reason for the decrease in the Ic of SJ-02 after the removal of HOPE. This decrease in the Ic is not reversible. In preliminary experiments, we found that the Ic was not recovered and further decreased by reapplying HOPE to a joint sample. We expected that a higher P would result in the densifica-tion of the intermediate layer and an increase in Ic. However, SJ-05 exhibited the lower Ic and n values than those of SJ-02. The P value for SJ-05 was probably too high and mechanical-ly damaged the Bi-2223 filaments in the tapes during the heat treatment. This result is similar to the low Ic and n values ob-tained in the case of the samples pressed at high pressures and RT in our previous study [17].  For SJ-05, the increase in the joint thickness after the re-moval of HOPE was about 70 m. This suggests that in the in-termediate layer, both the decrease in the density after the re-moval of HOPE and the densification effect by hot pressing should have been larger in the case of SJ-05 compared with those for SJ-02. However, the Ic and n values of SJ-05 were lower than those of SJ-02 before the removal of HOPE and decreased to a lesser extent after the removal of HOPE. This implies that the Ic and n values of SJ-05 are determined by the superconducting properties of the damaged Bi-2223 filaments in the tapes. B. Microstructural observations of straight joint samples Fig. 6 shows typical secondary electron images of the pol-ished surfaces of the transverse cross-section of the joining part of the samples. The upper and lower parts correspond to the intermediate layer and the Bi-2223 tape, respectively.  Many voids were observed in the intermediate layer of each sample. In the case of the hot-pressed samples, although the microstructural observations were performed after the transport measurements, that is, after the removal of HOPE, the density of the intermediate layer appeared to be higher than that of SJ-0. The filling factor values of the intermediate layer evaluated by image analysis [17] were about 70% for SJ-0 and about 80% for both of SJ-02 and SJ-05.  In the case of SJ-02 and SJ-05, the grains were in good con-tact at the interface between the intermediate layer and fila-ments of the tape. This microstructure contributed to the for-  Fig. 5. V-I curves for three straight joint samples at 77 K in self-field. SJ-0 was not superconductive. In hot-pressed samples SJ-02 and SJ-05, superconducting joints were formed.    5 mation of the superconducting joints in the hot-pressed sam-ples. In contrast, many voids were observed at the joining in-terface in SJ-0. This poor grain connectivity at the interface is one of the reasons why SJ-0 was not superconductive. These results imply that hot pressing is effective not only for densi-fying the intermediate layer but also for forming a joining in-terface wherein the grains are in good contact. Large cracks were observed in the Bi-2223 filaments of SJ-05. In contrast, such cracks were not observed in SJ-0 and SJ-02. These large cracks represent mechanical damage to the fil-aments. This suggests that the superconducting properties of the damaged filaments had a determining effect on the Ic and n values of SJ-05. The microstructural observations also revealed that the puri-ty of the intermediate layer was insufficiently high. Fig. 7 shows a backscattered electron image of the intermediate layer of SJ-02. Voids (black region), plate-like Bi-2223 grains (light gray region), and 2–5 μm sized impurities (dark gray region) were observed. Based on the analyses of the local chemical composition, the secondary phase was determined to be Ca2CuO3. The impurities remained probably owing to the short heat treatment (24 h), because Ca2CuO3 is the precursor to the formation of the Bi-2223 phase [30], [31]. Such impuri-ties reduce the intergrain Jc of the intermediate layer, which, in turn, decreases the Ic of the superconducting joints.  C. Evaluation of Rj and persistent current using closed-loop sample with a superconducting joint Fig. 8 shows the time dependence of Iloop at 4.2 K in self-field for the closed-loop sample CL-02 obtained by the current decay measurements. Iloop was injected at t = 0. An initial de-crease in Iloop of about 0.02 A was observed for 1.5 × 103 s. Thereafter, the time variation in Iloop gradually became smaller. This result is similar to those of the previous current decay measurements [4], [27], [28] and indicates that persistent cur-rent of about 130 A was flowing in CL-02.  Time dependence of Iloop can be fitted using an exponential decay function [1]. Assuming that the resistance of the sample corresponds to that of the superconducting joint, the decay time constant is L/Rj. Based on the fitting of the experimental data points between 4 × 103 and 5 × 103 s, the Rj at 4.2 K in self-field was deduced to be 7.3 × 10−15 Ω, as shown in Fig. 8 (solid line). This value is comparable to that observed for a previous Bi-2223 closed-loop sample with a superconducting joint [4]. It is also comparable to or lower than that of REBCO [28], [32] and LTS [1], [2], [27] superconducting joints. From these results, it can be concluded that the superconducting joint was successfully formed in CL-02 by the hot-pressing process.  After the current decay measurements, the temperature of CL-02 was started to increase while Iloop of about 130 A was still flowing. The temperature was increased from 4.2 to 120 K at a ramp rate of 0.5 K min−1 using a heater installed in the sample holder [27]. During this temperature increase, we measured the temperature of superconducting joint (Tj) of CL-02. Fig. 9 shows Tj dependence of Iloop in self-field. Iloop was constant till Tj = 22 K. When Tj was increased beyond 22 K, Iloop decreased with increasing Tj. Iloop disappeared completely at Tj = 103 K, as shown in inset of Fig. 9. This means that the critical temperature (Tc) of the joint in self-field was 103 K. This value is almost the same as that of a Bi-2223 supercon-ducting joint sample evaluated by transport measurements in our previous study [3].  At Tj values of above 30 K, a linear relationship was ob-served between Tj and Iloop. In this region, Iloop corresponded to the maximum value of the persistent current that can flow in CL-02. At a given temperature, the maximum value of the persistent current would correspond to Ic, which was deter-mined by a very low-voltage criterion such as less than 10−8 V, of the superconducting joint. At Tj of around 70 K, the slope of the Iloop–Tj curve appears to change. Although we observed similar slope changes in preliminary experiments using other Bi-2223 closed-loop samples, the reason of this change is not clear. The gray dashed line in Fig. 9 was derived from the data points between Tj values of 30 and 60 K using the least-squares method. In transport measurements using Bi-2223 su-  Fig. 6. Typical secondary electron images of polished surfaces of trans-verse cross-sections of joining part of straight joint samples. Microstruc-tural observations of hot-pressed samples were performed after removal of HOPE. In SJ-0, many voids at joining interface were observed. Some large cracks were observed only in Bi-2223 filaments of SJ-05.      Fig. 7. Backscattered electron image of intermediate layer of SJ-02. Im-purities 2–5 μm in size (Ca2CuO3) remained probably owing to short heat treatment (24 h).   6 perconducting joint samples, a linear relationship between Ic and Tj was observed at 4.2–90 K [3]. Assuming that the simi-lar linear relationship holds in the Iloop–Tj curve even at low temperatures, the maximum value of the persistent current that can flow in CL-02 is estimated to be about 170 A at 4.2 K. Note that this value was much lower than Ic of a virgin tape (more than 800 A in magnetic fields below 1 T [12]).  D. Yields, processing time, and issues to be addressed  In this study, we demonstrated the Bi-2223 superconducting joints using the hot-pressing process. Although not mentioned here, we have fabricated more than 20 hot-pressed joint sam-ples with the P of about 1–13 MPa in preliminary experiments and confirmed that superconducting joints were formed in each case. In contrast, as demonstrated in SJ-0, non-superconductive joint samples have sometimes been fabricated without applying a hot-pressing pressure. We believe that the superconducting joints can be fabricated in high yields by the hot-pressing process.  The heat treatment to fabricate the hot-pressed supercon-ducting joints was completed within about 24 h. This short heat treatment reduced the processing time for joining by more than a day compared with that for previous studies [3], [4], [17]. Note that the use of a more precisely controlled furnace can further reduce the heat treatment time by allowing for higher heating and cooling rates. Even before the removal of HOPE, the hot-pressed super-conducting joints showed lower Ic than a virgin tape. A few is-sues related to the hot-pressing process need to be addressed to increase Ic. First, the purity of the intermediate layer should be improved. Reducing the impurities in the intermediate layer will be effective for increasing the intergrain Jc, as demon-strated in the case of Bi-2223 tapes [13]. A long heat treatment will reduce Ca2CuO3 with the formation of Bi-2223 [30]. Alt-hough the processing time is extended, a long heat treatment probably leads to an increase in the Ic of the superconducting joints.  Second, the Ic of the hot-pressed superconducting joints can be further increased by densifying the intermediate layer. We will investigate the upper limit for densification by hot press-ing without damaging the Bi-2223 filaments in the tapes. We will also attempt to introduce the two-step heat treatment pro-cess, which is effective for the densification [17]. In preliminary experiments using the closed-loop sample CL-02, we observed the persistent current of about 60 A at 4.2 K with applying 1 T to the joint. Considering a large de-crease of the Ic in a magnetic field similar to the previous study [4], the in-field Ic of the hot-pressed superconducting joints is also required to be increased. Details of the in-field performance will be reported elsewhere. IV. SUMMARY Bi-2223 superconducting joints were successfully formed in samples fabricated by the hot-pressing process with a 24 h heat treatment. An Ic of 57.8 A at 77 K in self-field under a 0.2 μV voltage criterion was observed in the hot-pressed sam-ple. The superconducting joint in the closed-loop sample showed an Rj of 7.3 × 10−15 Ω at 4.2 K in self-field. It was in-dicated that a persistent current was flowing in the loop. The maximum value of the persistent current that could flow in the closed-loop sample was estimated to be about 170 A at 4.2 K.  Issues to be addressed are purification and further densifica-tion of the intermediate layer, which is probably effective for improving the Ic, including in magnetic fields, of the hot-pressed superconducting joints. Once these issues are ad-dressed, the proposed hot-pressing process should be a prom-ising method to fabricate high-performance Bi-2223 super-conducting joints in high yields and within a short processing time.  REFERENCES [1] G. D. Brittles et al., “Persistent current joints between technological superconductors,” Supercond. Sci. Technol., vol. 28, no. 9, Aug. 2015, Art. no. 093001. [2] Y. Takeda et al., “Review of the temporal stability of the magnetic field for ultra-high field superconducting magnets with a particular focus on   Fig. 8. Time dependence of Iloop for CL-02 at 4.2 K in self-field. Solid line represents exponential decay curve with Rj = 7.3 × 10−15 Ω, which was deduced from fitting of experimental data points between 4 × 103 and 5 × 103 s. It was indicated that persistent current was flowing in CL-02.     Fig. 9. Tj dependence of Iloop for CL-02 in self-field to evaluate maxi-mum value of persistent current that can flow in CL-02 at the given tem-perature. Inset shows Tc of joint was about 103 K. Gray dashed line was derived from data points between Tj values of 30 and 60 K using least-squares method. Maximum value of persistent current was estimated to be about 170 A at 4.2 K.   7 superconducting joints between HTS conductors,” Supercond. Sci. Technol., vol. 35, no. 4, Apr. 2022, Art. no. 043002. [3] Y. Takeda et al., “High Ic superconducting joint between Bi2223 tapes,” Appl. Phys. Express, vol. 12, no. 2, Feb. 2019, Art. no. 023003. [4] Y. Takeda et al., “Critical current improvement and resistance evaluation of superconducting joint between Bi2223 tapes,” Supercond. Sci. Technol., vol. 35, no. 2, Feb. 2022, Art. no. 02LT02. [5] Y. Park et al., “A superconducting joint for GdBa2Cu3O7−δ-coated conductors,” NPG Asia Mater, vol. 6, May 2014, Art. no. e98. [6] K. Ohki et al., “Fabrication, microstructure and persistent current measurement of an intermediate grown superconducting (iGS) joint between REBCO-coated conductors,” Supercond. Sci. Technol., vol. 30, no. 11, Oct. 2017, Art. no. 115017. [7] P. Chen et al., “Development of a persistent superconducting joint between Bi-2212/Ag-alloy multifilamentary round wires,” Supercond. Sci. Technol., vol. 30, no. 2, 2017, Art. no. 025020. [8] S. Mukoyama et al., “Superconducting joint of REBCO wires for MRI magnet,” J. Phys.: Conf. Ser. vol. 1054, 2018, Art. no. 012038. [9] X. Jin et al., “Superconducting joint between multi-filamentary Bi2Sr2Ca2Cu3O10+δ tapes based on incongruent melting for NMR and MRI applications,” Supercond. Sci. Technol., vol. 32, no. 3, Feb. 2019, Art. no. 035011. [10] T. Mousavi et al., “Superconducting Joint Structures For Bi-2212 Wires Using a Powder-in-Tube Technique,” IEEE Trans. Appl. Supercond., vol. 31, no. 5, Aug. 2021, Art. no. 6400504. [11] D. Huang et al., “An efficient approach for superconducting joint of YBCO coated conductors,” Supercond. Sci. Technol., vol. 35, no. 7, May 2022, Art. no. 075004. [12] N. Ayai et al., “DI-BSCCO wire with Ic over 200 A at 77 K,” J. Phys.: Conf. Ser., vol. 97, Mar. 2008, Art. no. 012112. [13] K. Sato et al., “Present Status and Future Perspective of Bismuth-Based High-Temperature Superconducting Wires Realizing Application Sys-tems,” Jpn. J. appl. Phys., vol. 51, Dec. 2012, Art. no. 010006. [14] G. Nishijima et al., “Successful Upgrading of 920-MHz NMR Super-conducting Magnet to 1020 MHz Using Bi-2223 Innermost Coil,” IEEE Trans. Appl. Supercond., vol. 26, no. 3, Apr. 2016, Art. no. 4303007. [15] S. Awaji et al., “First performance test of a 25 T cryogen-free superconducting magnet,” Supercond. Sci. Technol., vol. 30, no. 6, May. 2017, Art. no. 065001. [16] G. Nishijima et al., “Transport critical current measurement apparatus using liquid nitrogen cooled high-Tc superconducting magnet with vari-able temperature insert,” Rev. Sci. Instrum., vol. 84, no. 1, Jan. 2013, Art. no. 015113. [17] Y. Takeda et al., “The effect of intermediate layer densification on the critical current of a Bi-2223 superconducting joint,” Supercond. Sci. Technol., vol. 36, no. 3, Jan. 2023, Art. no. 035004. [18] J. Jiang et al., “Through-process study of factors controlling the critical current density of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes,” Supercond. Sci. Technol., vol. 14, pp. 548–556, Jul. 2001. [19] T. Kato et al., “Development of high performance Ag sheathed Bi2223 wire,” Physica C: Supercond., vol. 412–414, no. 2, pp. 1066–1072, Oct. 2004. [20] H. Fujii et al., “Effect of hot uniaxial pressing on the microstructure and critical current density of (Bi, Pb)-2223 tapes,” Supercond. Sci. Technol., vol. 17, no. 2, pp. 263–268, Feb. 2004. [21] E. Hellstrom et al., “Review of overpressure processing Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox wire,” Supercond. Sci. Technol., vol. 18, no. 12, pp. S325–S331, Nov. 2005. [22] W. Guo et al., “Fabrication of joint Bi-2223/Ag superconducting tapes with BSCCO superconducting powders by diffusion bonding,” Physica C: Supercond., vol. 470, no. 9–10, 1, pp. 440– 443, May 2010. [23] Y. Ebara et al., “Electrical Resistance Measurement Method to Investigate Superconducting Joints in Bi-2223 Tape,” IEEE Trans. Appl. Supercond., vol. 27, no. 4, Jun. 2017, Art. no. 9000904. [24] C. Hagart-Alexander, “Chapter 21 - Temperature measurement,” in Instrumentation Reference Book, 4th ed., W. Boyes, Ed. Boston, MA, USA: Butterworth-Heinemann, 2010, pp. 269–326. [25] R. Tajima et al., “Synthesis of Bi2223 by Low PO2 Sintering,” IEEE Trans. Appl. Supercond., vol. 23, no. 3, Jun. 2013, Art. no. 6400604. [26] Y. Yuan et al., “The Origin of the Pb-rich Bi3Sr2Ca2Cu1Ox (3221) Phase During Post Annealing of (Bi, Pb)2Sr2Ca2Cu3Ox/Ag Composite Conductors,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pp. 2530–2533, Jun. 2005. [27] K. Kobayashi et al., “Development of a superconducting joint resistance evaluation system,” IEEE Trans. Appl. Supercond., vol. 30, no. 4, Jun. 2020, Art. no. 9000204. [28] K. Kobayashi et al., “In-Field Evaluation of REBCO Superconducting Joint,” IEEE Trans. Appl. Supercond., vol. 32, no. 6, Sep. 2022, Art. no. 6601404. [29] Y. Takeda et al., “Development of high Jc Bi2223/Ag thick film materials prepared by heat treatment under low PO2,” Supercond. Sci. Technol., vol. 31, no. 7, May 2018, Art. no. 074002. [30] A. Jeremie and R. Flükiger, “Similarities in Bi,Pb(2223) formation starting from different precursors,” Physica C: Supercond., vol. 267, no. 1–2, pp. 10–18, Aug. 1996. [31] R. Flükiger et al., “Phase formation and critical current density in Bi,Pb(2223) tapes,” Supercond. Sci. Technol., vol. 10, no. 7A, pp. A68–A92, Jul. 1997. [32] Y. Yanagisawa et al., “Development of a persistent-mode NMR magnet with superconducting joints between high-temperature superconductors,” Supercond. Sci. Technol., vol. 34, no. 11, Sep. 2021, Art. no. 115006.