Study of Factors in Joint Resistance for GdBCO Coated Conductors

Development of a coated conductor tape joint with good quality and low joint resistivity, Rsj, in terms of transport and mechanical properties, was attempted by direct bonding at the interface of the Cu–Cu stabilizers in overlapped GdBCO CC tapes. In this study, we attempted to achieve a low Rsj by introducing hybrid joining, soldering and ultrasonic welding (UW), and its mechanism was analyzed theoretically. Coated conductor tapes were experimentally joined using various methods of soldering, UW, and combinations of the two. As a result, a much lower Rsj of about 57 nΩ · cm2 was obtained for RCE-DR-processed GdBCO CC tape joints using the hybrid joining method. The mechanical properties of the jointed CC tapes were also evaluated at room temperature and 77 K under self-field. Load–displacement curves of joined CC tapes followed the curve of the single CC tape. Critical current and joint resistance, Rj, of hybrid-joined CC tape were retained after double bending at room temperature up to 20 mm bending diameter.

Section: Introductionmentioning

confidence: 99%

Pursuing low joint resistivity in Cu-stabilized REBa₂Cu₃O_δcoated conductor tapes by the ultrasonic weld–solder hybrid method

Shin¹,

Kim²,

Dedicatoria

2015

“…Furthermore, the resistance at the Ag/REBCO interface was identified as the predominant source of the total joint resistivity [19]. Thus, the absence of resistive layers within the joint minimizes the metal resistance and interface contact resistance through the direct connection of Ag-protecting layers of CC tapes via diffusion joints, which can have an R j range of 4.9-10 nΩ cm 2 for YBCO CC joints and 23-60 nΩ cm 2 for GdBCO CC joints [5][6][7][20][21][22][23][24][25]. Watanabe et al have reported that higher R j in GdBCO CC than YBCO CC joint was caused by the reduction of oxygen at the GdBCO layer and the diffusion of Gd to the Ag layer [20].…”

Section: Introductionmentioning

confidence: 99%

“…Thus, the absence of resistive layers within the joint minimizes the metal resistance and interface contact resistance through the direct connection of Ag-protecting layers of CC tapes via diffusion joints, which can have an R j range of 4.9-10 nΩ cm 2 for YBCO CC joints and 23-60 nΩ cm 2 for GdBCO CC joints [5][6][7][20][21][22][23][24][25]. Watanabe et al have reported that higher R j in GdBCO CC than YBCO CC joint was caused by the reduction of oxygen at the GdBCO layer and the diffusion of Gd to the Ag layer [20]. Although diffusion joint can produce lower R j , it employs a complex and long fabrication process at high temperatures, and additional oxygen annealing is applied to restore the superconductivity of the diffusion joints.…”

Section: Introductionmentioning

confidence: 99%

Applicability of ultrasonic welding to Ag-stabilized REBCO CC joints and evaluation of their electromechanical properties

Nisay¹,

Diaz²

2022

Manufacturing a single long-length rare-earth barium copper oxide (REBCO) coated conductor (CC) tape with uniform critical current (Ic) is still a challenge; therefore, joining between multiple-length CC tapes is needed in the production of longer CC tapes for superconducting cables, coils, and magnets. Various joining techniques have been developed to achieve acceptably low joint resistance (Rj) with no Ic degradation and good electromechanical properties. The authors established ultrasonic welding (UW) and hybrid welding (HW) methods for joining Cu-stabilized CC tapes with different configurations. However, these methods have yet to be applied to Ag-stabilized CC tapes to produce compact joints and longer tapes during in-line production. This study used the UW and HW methods to fabricate Ag-stabilized CC joints to have low Rj without Ic degradation through a direct connection at the overlapped Ag layers between both CC tapes. Two CC tape samples with different thicknesses of ~2 µm and ~6 µm Ag stabilizers were supplied to produce UW and HW Ag-stabilized CC joints. These were compared with soldered CC joints fabricated by a mechanically controlled soldering method. In the case of UW, the design of experiment (DOE) using the Taguchi method was used to systematically determine the optimized weld parameter combination that yields a lower Rj without any Ic degradation. This study is a systematic attempt to evaluate the applicability of UW for solid-state joining between thin Ag layers, unlike the conventional joining of thick Cu stabilizers. Moreover, the electromechanical properties of differently processed Ag-stabilized REBCO CC joints were evaluated using both lap-shear and double-bending tests. As a result, the optimum UW parameter combinations were obtained to achieve Ag-stabilized CC joints. However, some variations were probably due to the differences in the production batch and thickness of the Ag layers. The UW Ag-stabilized CC joints showed superior joint characteristics and electromechanical properties compared to soldered CC joints. Good solid-state bonding of the Ag layers was observed through microscopic observation of the cross-section at the joint region of the UW CC joint. The joint characteristics and electromechanical properties of the UW Ag-stabilized CC joints can be further improved using the HW method.

“…Furthermore, the Sn alloy easily corrodes the REBCO and this will be briefly described in this paper. In addition, we have to consider the surface resistance of the epitaxially grown REBCO layer; non-negligible interface resistance between the REBCO layer and Ag protective cap layer has been reported [16][17][18][19]. Reduction of the oxygen content near the surface of the REBCO layer during heat treatment has also been pointed out as the cause of the degradation of the superconducting properties and increase in the Ag/REBCO interfacial resistance [18].…”

Section: Introductionmentioning

confidence: 99%

“…In addition, we have to consider the surface resistance of the epitaxially grown REBCO layer; non-negligible interface resistance between the REBCO layer and Ag protective cap layer has been reported [16][17][18][19]. Reduction of the oxygen content near the surface of the REBCO layer during heat treatment has also been pointed out as the cause of the degradation of the superconducting properties and increase in the Ag/REBCO interfacial resistance [18]. On the other hand, in the case of BSCCO multifilamentary wires, a relatively low joint resistance is achievable using a similar matrix replacement method [20].…”

Section: Introductionmentioning

confidence: 99%

A new concept for developing a compact joint structure for reducing joint resistance between high-temperature superconductors (HTS) and low-temperature superconductors (LTS)

Banno

Asano

Kato

et al. 2020

It is significantly difficult to develop a superconducting joint between REBCO and low-temperature superconductors (LTS) using a solder matrix replacement technique. This is because the REBCO superconducting layer is highly corrosive to Sn. In this work, TEM observations were first conducted on the reaction interface between the REBCO layer and Sn to reveal the reaction. Then, a new idea to create a compact low-resistance joint to reduce the joint resistance between REBCO and LTS was proposed. In this method, the REBCO tape is rolled in a metal boat, with the Cu stabilizer remaining around the tape. Then, an LTS wire, whose superconducting filament ends are coated with a superconducting solder, is set straight into a boat. Then, the boat is filled with a superconducting solder, so that the joint state between the LTS and superconducting solder matrix remains superconductive. However, the electrical joint between the REBCO tape and the solder matrix is resistive, even if the solder matrix is superconductive. Consequently, the overall joint resistance is determined by the boundary resistance between the REBCO tape and the superconducting solder matrix. However, to achieve a joint resistance below 10–10 Ω, a long joint length of more than 5 m, preferably more than 10 m, will be required. Considering the strain state of the REBCO layer when it is rolled in to a boat, the boat-type joint structure proposed in this work enables the joint size to be significantly compact, even if a length of more than 10 m is required. At present, a joint resistance of 0.7 nΩ was obtained in a field range of less than 0.4 T by using a boat with an inner size dimensions as follows: 50 mm length, 16 mm width, 7 mm height, radius of curvature of 8 mm, and tape length of 2 m.