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[Yasuaki Takeda](https://orcid.org/0000-0001-7217-9853), [Tomoko Eguchi](https://orcid.org/0009-0005-9630-3712), Ariane Keiko Albessard, [Gen Nishijima](https://orcid.org/0000-0001-7493-0559)

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[Evaluation of Microstructure, Resistance, and Critical Current of REBCO Superconducting Joints Fabricated by Slurry Process](https://mdr.nims.go.jp/datasets/a2fe7f5a-9279-4842-b7c7-a7cc81e5b1fe)

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1 EUCAS2025-2-MP-JC1.201  Evaluation of Microstructure, Resistance, and Critical Current of REBCO Superconducting Joints Fabricated by Slurry Process  Yasuaki Takeda, Tomoko Eguchi, Ariane Keiko Albessard, and Gen Nishijima    Abstract—The properties of superconducting joints for REBa2Cu3Oy (REBCO, RE = rare earth) superconducting tapes fabricated using a slurry process were investigated. Microstructural observation revealed that a joining layer fabricated using a slurry was well-connected to the REBCO layer of the joined tape. The resistance and critical current of closed-loop samples containing slurry-processed superconducting joint were evaluated using current decay measurements. The joint resistance was found to be lower than 10−12 and 10−11 Ω at 4 and 77 K, respectively, in the self-field. It is inferred that a persistent current can flow through a slurry-processed superconducting joint. Conversely, in-field critical current was low and exhibited hysteresis due to weak links in the joining layer.  Index Terms—2G HTS conductors, coated conductors, resistance measurement, critical current  I. INTRODUCTION he development of superconducting joints between REBCO coated conductor tapes for operation of REBCO superconducting magnets in the persistent mode has seen significant progress [1][2][3]. To fabricate superconducting joints, the exposed REBCO thin film layers of tapes are joined via a newly formed REBCO layer through REBCO crystal growth at the joining interface [4][5][6][7][8]. Epitaxial growth of the REBCO thin film layer is the most common process used for joining. This can help obtain superconducting joint samples with resistances below 10−12 Ω at currents of 101–102 A. The joining methods developed so far require further improvement [3]. In particular, the epitaxial growth method poses technical challenges to the joining process. This is because to join the REBCO layers at the atomic level, the ab-plane and the c-axis of the two layers must be aligned within a  Submitted for review October 13, 2025. This work was partly supported by JST-Mirai Program Grant Number JPMJMI17A2 and JSPS KAKENHI Grant Number JP22K14482, Japan. (Corresponding author: Yasuaki Takeda.)  Yasuaki Takeda and Gen Nishijima are with National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan (e-mail: TAKEDA.Yasuaki@nims.go.jp).  Tomoko Eguchi and Ariane Keiko Albessard are with Toshiba Corporation, Kawasaki, Kanagawa 212-0001, Japan (e-mail: tomoko.eguchi.t59@mail.toshiba). Color versions of one or more of the figures in this article are available online at http://ieeexplore.ieee.org few degrees of misorientation. Considering that joining will routinely be performed at manufacturing facilities of superconducting magnets, an easy-to-use joining method must be established. Recently, we have been developing a method for joining REBCO tapes using a slurry [9]. A polycrystalline joining layer is fabricated using a slurry between the exposed REBCO layers. This method is technically simple because it does not require epitaxial growth. In addition, slurry-processed joints with compact sizes and a variety of shapes that are suitable for implementation in persistent-mode magnets can be fabricated owing to few restrictions on the shapes of the joining layer and joint configurations. In this study, we evaluated the microstructure, joint resistance (Rj), and joint critical current (Icj) of slurry-processed REBCO superconducting joints. Loop samples closed with the slurry-processed joint were fabricated using a REBCO tape. Rj and Icj of the joints were evaluated by performing current decay measurements. Following the measurements, the microstructure of one of the joints was observed. II. EXPERIMENTAL A. Preparation of closed-loop samples Fig. 1(a) shows a schematic of the fabricated joint structure. To demonstrate a slurry-processed joint, we prepared a bridge with a joining layer. The use of a bridge was effective in decomposing organic components in a slurry [6]. That is, we coated a slurry onto a bridge and then calcined the coated bridge at high temperatures for the decomposition. The closed-loop sample consisted of a 1 m long and 4 mm wide REBCO tape and a 12 mm wide REBCO tape for the bridge. EuBCO tapes with BaHfO3 artificial pinning centers (Fujikura FESC-S04 and FESC-S12) [10] were used as the REBCO tapes. Both tapes did not have a copper-stabilizing layer. According to the inspection report, the critical current of the 4 mm wide tape at 77 K in the self-field was 204 A with a 10−6 V cm−1 criterion. To expose the REBCO layer, the silver layer was removed from both ends of a 4 mm wide tape and the entire bridge by chemical etching. The exposed REBCO layers were to be placed facing each other. To form a joining layer between the opposing REBCO layers, a slurry was deposited by dip-coating onto the REBCO layer of the bridge. The slurry was prepared by mixing EuBCO powder (TEP Co, Ltd.) and a metal-organic deposition (MOD) solution (Kojundo Chemical T 2 EUCAS2025-2-MP-JC1.201  Laboratory Co., Ltd.). To decompose organic components and form a microcrystalline layer on REBCO grains from the MOD solution [3][6], the coated bridge was calcined at 840°C in oxygen flow. We made the joint configuration using the calcined bridge. The joining part was fixed by bolt-tightening, as shown in Fig. 1(b). This setup enables the application of uniaxial pressure to the joint during the heat treatment to form a superconductive joining layer. After a heat treatment at 840°C in 0.05%O2/Ar flow and annealing at 450°C in oxygen flow, the bolts were loosened to release the applied pressure for the characterizations.  We fabricated four single-turn closed-loop samples, labeled as #1, #2, #3, and #4. Fig. 1(c) shows a photograph of #1. For the current decay measurements, we fixed the torsion section using a support structure [11]. The loop mounted on the sample holder had a diameter of 100 mm. The self-inductance (L) of the sample was 4.7 × 10−7 H, which was evaluated using a loop sample with the same shape in our previous study [12].  Table Ⅰ lists the fabrication conditions for the slurry-processed joint of the samples. The bridge length (d) is defined as shown in Fig. 1(a). In a preliminary experiment, using a pressure measurement film (Fujifilm Prescale), we estimated the uniaxial pressure applied to the joint by the bolt-tightening torque of 2.0 N m to be about 3–5 × 107 Pa (30–50 MPa) at room temperature. Considering that the coefficient of thermal expansion of stainless-steel bolts [13] is higher than that of the Hastelloy C-276 substrate [14], the applied pressure at 840°C would be lower than that at room temperature.  B. Evaluation of closed-loop samples  Current decay measurements were performed for the samples at 4 and 77 K using a joint resistance evaluation system [12]. A magnetic field (B) of 0.02–1 T was applied to the joint for in-field measurements. The direction of the field is illustrated in Fig. 1(c). This direction was fixed in the evaluation system used. The decay of the loop current (Iloop), that is, the time (t) dependence of Iloop was measured at a sampling rate of 1 Hz. We introduced Iloop into the sample through magnetic induction using a copper coil (injection coil) located at the center of the loop. The initially introduced Iloop was controlled by an injection coil current (ICC). The initial Iloop value was about 20 times ICC.  To evaluate Rj, the experimentally obtained Iloop–t data points were fitted to the equation of 𝐼loop(𝑡) =𝐼loop(0) exp(−(𝑅j 𝐿⁄ )𝑡)  [15][16]. To determine Icj, the voltage of the joint (V) was calculated from the Iloop–t data points using 𝑉 = −𝐿(Δ𝐼loop Δ𝑡⁄ ). From the obtained V–Iloop, Icj was determined under a voltage criterion (Vc) of 10−8 V [17]. After the current decay measurements, the microstructure of the joint in sample #1 was evaluated. The polished surface of the cross-section of the joint was observed using an electron probe microanalyzer (EPMA, JEOL JXA-8530F Plus). We obtained a backscattered electron image and corresponding elemental maps, as shown in Fig. 2.  TABLE Ⅰ FABRICATION CONDITIONS OF THE JOINT FOR THE CLOSED-LOOP SAMPLES  Sample Bridge length, d (mm) Bolt-tightening torque (N m) #1, #2 10 2.01) #3, #4 15 6.0 1)The pressure applied to the joint at room temperature was estimated to be about 3–5 × 107 Pa (30–50 MPa) in a preliminary experiment.  III. RESULTS AND DISCUSSION A. Microstructure of the slurry-processed joint in #1 Fig. 2(a) shows a backscattered electron image of the polished surface of the joint cross-section in sample #1. The upper and lower parts correspond to the bridge and the joined    Fig. 2. (a) Backscattered electron image of polished surface of cross-section of joint in #1. (b) EPMA elemental maps for Eu, Ba, Cu, and O. The evaluated region is shown in (a). 10 mBridgeJoinedtapeJoininglayerREBCO layer (b)REBCO layer Void(a)Eu BaCu O(b)    Fig. 1. (a) Schematic of the structure of the slurry-processed joint. (b) Schematic of the side view during the heat treatment with applying a uniaxial pressure to the joint. The pressure was controlled by bolt-tightening torque. (c) Photograph of a single-turn REBCO closed-loop sample with the joint. Magnetic field (B) is applied to the joint as shown in the figure.  Bridge (12 mmw REBCO tape)4 mmwREBCOtapeExposedREBCO layerJoining layer:REBCO polycrystalline filmAgAgSubstrateSubstrated(a)UniaxialpressureJointBoltLoop(b)JointSupport structure for torsion section(c)Field (B)3 EUCAS2025-2-MP-JC1.201  tape, respectively. The central region corresponds to the joining layer. It is observed that the REBCO layers of the bridge and tape are dense and do not exhibit any cracks or decomposition. This implies that degradation of the REBCO layer due to the thermomechanical load, as previously reported in [18], is negligible. In the joining layer, many voids corresponding to the black region are observed, implying that the joining layer has a porous microstructure. However, REBCO grains appeared to be well-connected to each other and to both REBCO layers. This is probably attributed to the slight grain growth by the microcrystalline layer on the grains during the heat treatment. Fig. 2(b) shows the EPMA elemental maps of Eu, Ba, Cu, and O at the joining interface between the bridge and the joining layer shown in Fig. 2(a). The chemical composition of the joining layer is comparable to that of the REBCO layer of the bridge. We observed no cracks or secondary phases at the joining interface. At this magnification, the grains of the joining layer appeared to be well-connected to the REBCO layer. As the grains are most likely misoriented with respect to the REBCO layer, further microstructural evaluation is required to clarify the connection state. B. Evaluation of self-field characteristics of the joints Figs. 3(a) and 3(b) show the time dependence of Iloop for samples #1–4 in the self-field at 4 and 77 K, respectively. Table Ⅱ summarizes the self-field Icj and Rj at 4 and 77 K. In most results shown in Fig. 3, Iloop > Icj was introduced at t = 0, resulting in a fast current decay observed at t < 102 s. From this fast decay, Icj was determined. After the fast decay, a slow decay of Iloop was observed. In these measurements, Rj was evaluated using the data points at 3.5–4.0 × 103 s. The low Rj values of 1.7–2.9 × 10−13 Ω at 4 K and 2.3–3.4 × 10−12 Ω at 77 K correspond to persistent currents flowing in the samples. This implies that a persistent current can flow through the slurry-processed joint in the superconducting state. The higher Rj values at 77 K can be attributed to thermally activated flux motion at the joint, as implied in our previous study [19].  As demonstrated for #3 at 89 A in 4 K and for #4 at 15 A in 77 K, when Iloop < Icj was introduced, flatter Iloop–t curves were observed. In these measurements, Rj was evaluated using the data points at 2.0–4.0 × 103 s based on the lower signal-to-noise ratio. Similar to previous studies, lower Rj values were observed owing to the lower load factor [11][17][19][20]. Samples #3 and #4 showed higher Icj values than #1 and #2, indicating that modification of the fabrication conditions was effective for increasing Icj. The longer bridge length for #3 and #4 increased the effective joining area. The higher bolt-tightening torque for these samples likely produced higher pressure to the joint during the heat treatment, resulting in the formation of a denser joining layer.   TABLE Ⅱ CRITICAL CURRENT AND RESISTANCE OF THE JOINT FOR THE SAMPLES IN THE SELF-FIELD AT 4 AND 77 K  Sample 4 K, self-field 77 K, self-field Icj (A) Rj (Ω) Icj (A) Rj (Ω) #1 63 1.7 × 10−13 (62 A)2) 14 2.3 × 10−12 (12 A)2) #2 65 1.8 × 10−13 (64 A)2) 12 3.4 × 10−12 (10 A)2) #3 101 2.9 × 10−13 (99 A)2) 1.2 × 10−14 (89 A)3) 24 3.0 × 10−12 (22 A)2) #4 97 2.7 × 10−13 (95 A)2) 22 3.1 × 10−12 (19 A)2) 7.9 × 10−15 (15 A)3) 2) Rj was evaluated using the data points at 3.5–4.0 × 103 s. 3) Rj was evaluated using the data points at 2.0–4.0 × 103 s.  C. Evaluation of in-field characteristics of the joints at 4 K We evaluated the in-field Icj values at 4 K in the magnetic-field range of 0–1 T. Fig. 4 shows the magnetic field dependence of Icj at 4 K for #1, #3, and #4. Rapid decreases in Icj were observed in these samples with increasing fields up to 0.1 T. This implies that the presence of weak links in the polycrystalline joining layer, similar to a polycrystalline bulk [21][22]. By contrast, as the field increased from 0.1 to 1 T, the decreases in Icj in #3 and #4 were relatively small. To investigate the presence of weak links in the joining layer, we attempted to observe the hysteresis of the in-field Icj. Fig. 5 shows the magnetic field dependence of Icj for #3 at 4 K. We evaluated Icj as the field was increased from 0 to 0.3 T, as shown by the open symbols. We also evaluated Icj as the field was decreased from 0.1, 0.2, and 0.3 T to 0, as shown by the closed symbols. For all measurements, the in-field Icj exhibited hysteresis, demonstrating that the width of the hysteresis increased with the maximum applied field. This suggests the     Fig. 3. Time dependence of Iloop for the samples #1–4 in the self-field at (a) 4 K and (b) 77 K. Initially introduced Iloop was controlled by injection coil current (ICC). A slow decay of Iloop was observed in all samples at 4 and 77 K. This implies that a persistent current can flow in the slurry-processed joint. Samples #3 and #4 showed higher Iloop, corresponding to higher Icj. 4 EUCAS2025-2-MP-JC1.201  presence of weak links due to the polycrystalline nature of the joining layer [22][23].  Fig. 6 shows the time dependence of Iloop for samples #3 and #4 at 4 K and 1 T. By applying a magnetic field of 1 T, Iloop decreased significantly compared to that in the self-field, as shown in Fig. 3(a). This corresponds to a decrease in Icj upon field application, as shown in Fig. 4. However, flat curves were observed at t > 104 s. As shown in Fig. 6, the Rj values at 4 K and 1 T for #3 and #4 obtained using the data points at 1.0–1.4 × 104 s were 6.3 × 10−13 Ω at 7.7 A and 8.6 × 10−13 Ω at 6.7 A, respectively. These Rj values are comparable to those in the self-field, as listed in Table Ⅱ. These results imply that even in magnetic fields, a persistent current can flow through the slurry-processed joint in the superconducting state.  D. Discussion and future work As shown in Fig. 2, the REBCO layer of the tape is well-connected to the grains in the joining layer at the interface. This microstructure allows a persistent current to flow through the slurry-processed joint in the superconducting state. However, Icj must be improved, particularly in magnetic fields. The Icj value was notably lower than the critical current of the original tape and that of the superconducting joints fabricated using epitaxial growth [4][6][7][8]. We attempted to estimate rough critical current ratio (CCR: Icj divided by tape Ic) [2] at 77 K in the self-field. We used the Icj (Vc = 10−8 V) and tape Ic at 10−8 V cm−1 (162 A), which is extrapolated from that at 10−6 V cm−1 using a power law and a typical n value of 20 [1]. The rough CCR value was estimated to be only 7.4–15%. As implied in this study, an increase in the effective joining area can be a promising approach for increasing Icj. Additionally, the formation of a denser joining layer can also be effective in obtaining higher Icj. This densification effect is consistent with that observed in our previous study of (Bi,Pb)2Sr2Cu2Cu3Oy (Bi-2223) superconducting joints [24]. In the following study, we plan to investigate the effect of the joining area and densification on Icj in the slurry-processed joint. By contrast, another strategy may be required to increase the in-field Icj. Considering the random orientation of grains of a joining layer, grain alignment may be one of the approaches to increase the in-field Icj [22].  We found that the samples fabricated under the same conditions exhibited similar Icj and Rj characteristics. This implies the relatively high reproducibility of the fabrication of the slurry-processed joints. Such high reproducibility is in part due to the lack of requirement for precise alignment of the c-axis of the REBCO layers and the direction of the applied uniaxial pressure. Therefore, the slurry process may be suitable for an on-site joining method. Various joint configurations can be achieved using the slurry process. In principle, a joining layer can be directly formed between REBCO layers without a bridge. This will allow fabrication of joints with compact sizes or shapes suitable for implementation in persistent-mode magnets. It is necessary to investigate the field angular dependence of Icj and Rj, that is, the dependence of Icj and Rj on the magnetic field direction. In previous studies, the angular dependence of Icj and Rj for Bi-2223 and REBCO superconducting joints was evaluated [17][25][26]. Both Icj and Rj exhibited strong angular dependence, reflecting an anisotropic microstructure. By contrast, the slurry-processed joint will exhibit flat angular dependence of Icj and Rj owing to the random orientation of the grains of the joining layer. Such flat angular dependence will be a considerable advantage of slurry-processed superconducting joints. Considering electromagnetic forces applying to the joints in persistent-mode magnets, the mechanical properties of the slurry-processed joints should be evaluated. If the mechanical strength is insufficient due to the brittleness of the porous joining layer, reinforcement may need to be considered [3].    Fig. 4. Magnetic field dependence of Icj for the samples #1, #3, and #4 at 4 K with increasing the field. The Icj values at B = 0 correspond to self-field Icj shown in Table Ⅱ.    Fig. 5. Magnetic field dependence of Icj for #3 at 4 K with increasing and decreasing the field. In-field Icj exhibited hysteresis. This suggests the presence of weak links of the joining layer.    Fig. 6. Time dependence of Iloop for #3 and #4 at 4 K and 1 T. Flat curves were observed at t > 104 s, corresponding to the persistent current in the samples. 5 EUCAS2025-2-MP-JC1.201  IV. CONCLUSION We developed and evaluated slurry-processed REBCO superconducting joints. Microstructural observations showed that the polycrystalline joining layer fabricated using the slurry was well-connected to the REBCO layer of the tape. The joint resistance was evaluated to be low based on current decay measurements. The critical current rapidly decreased with increasing magnetic field. 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