First performance test of FeSe_0.5Te_0.5-coated conductor coil under high magnetic fields

Since 2008, when Dr Hosono's group at the Tokyo Institute of Technology discovered superconductivity at 26 K in the iron-based layered material LaFeAsO_1−x F_x [1], iron-based superconductors (IBSs), which are expected to be the next-generation high temperature superconductors, have attracted intense interest from scientists around the world [2–9]. The IBS has a high upper critical field (>100 T), high critical temperature (∼55 K), low anisotropy (1.5–2) and strong current carrying capacity. In the current research, there are four main structures of IBSs: 1111 phase (for example, LnOFeAs), 122 phase (for example, Ba(Sr,K)Fe₂As₂, BaxK_1−x Fe₂As₂), 111 phase (for example, LiFeAs) and 11 phase (for example, FeSe, FeSe_x Te_1−x ) [10]. The 122 phase and the 11 phase are mainly studied among the four structures. The iron-based superconducting coils of 122 phase have been successfully fabricated and test under high fields. As it has a simple structure, low anisotropy, nontoxicity, and promising prospects in high-field applications [11], the 11 phase represented by FeSe_x Te_1−x arouses great interest in researchers.

Most of the substrates to grow FeSe_0.5Te_0.5 films are CaF₂, MgO and SrTiO₃, and high-quality FeSe_0.5Te_0.5 films are typically deposited by pulsed laser deposition (PLD) [12]. Southeast University has investigated the factors affecting the transport properties of superconducting FeSe_0.5Te_0.5 thin films deposited by PLD, such as substrate type, superconducting film thickness, and the proportions of each element [2–6]. In 2013, Braccini studied the superconducting properties of a Fe(Se,Te)-coated conductor, and showed that the superconducting property of a thin film grown on CaF₂ substrate with a T_c of 20 K exhibited a critical current density, J_c, as high as 1 × 10⁶ A cm⁻² at 4.2 K in self-field [13], and a J_c for FeSe_0.5Te_0.5 as high as 10⁵ A cm⁻² at 9 T was measured in Kawale's study [14]. In addition, Bhatt has successfully synthesized large-size FeSe single crystals doped with Fe and Co and studied their transition temperatures and upper critical fields [15]. The FeSe_x Te_1−x -coated conductor shows good high-field performance in the above studies. However, Yuan pointed out that the thermal expansion coefficient of the substrate is much larger than that of the FeSe_0.7Te_0.3 film [5]. Also, when cooling from room temperature to a low temperature of 4.2 K, the larger contraction of the substrate leads to compressive stress on FeSe_0.5Te_0.5 films [16], which will damage the properties of the superconducting layer. When coils are fabricated using the FeSe_0.5Te_0.5-coated conductor, compressive stresses inevitably occur on the film of the FeSe_0.5Te_0.5-coated conductor. Therefore, there is still great difficulty and challenge to develop FeSe_0.5Te_0.5-coated conductor coil. At present, almost all research is mainly focused on improving the performance of FeSe_0.5Te_0.5-coated conductor tape preparation. ReBCO (ReBa₂Cu₃Ox), which is also a coated conductorstructure, has been successfully used in the fabrication of magnets and cables [17–19], but fabrication of a coil of FeSe_0.5Te_0.5-coated conductor and a study of its high-field performance have not been attempted.

In this paper, a hybrid coil with a FeSe_0.5Te_0.5-coated conductor was first successfully fabricated and then tested at liquid helium temperature in background fields from 0 T to 10 T.

2.1. FeSe_0.5Te_0.5 tape encapsulation

2.2. Encapsulated FeSe_0.5Te_0.5 tape characteristics

A short sample cut from the end of the strengthened, encapsulated tape was tested at 4.2 K using liquid helium. The critical current I_c of the FeSe_0.5Te_0.5 encapsulated tape was measured as the background magnetic field parallel to the tape was increased from 0 T to 10 T. The values of I_c were measured by the standard four-probe method with a criterion of 1 μV cm⁻¹. Figure 1 shows the magnetic field dependence of I_c at 4.2 K for the FeSe_0.5Te_0.5 tape. Under a 10 T background field, the I_c of the tape is 17 A at 4.2 K, and the corresponding transport current density J_c about 1.42 × 10⁵ A cm⁻². The critical current of the FeSe_0.5Te_0.5 tape decreases quickly when adding a small background field less than 0.3 T, and then the I_c of the tape decreases slowly as the background field increases, as shown in figure 1. To improve the greatly reduced I_c when adding a small background field, the tape production and fabrication process should be further optimized. However, the transport current I_c is less dependent on the high background field, and I_c decreases by only 12 A when the background field is increased from 4 T to 10 T.

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3.1. Insert coil design

The hybrid coil consists of eight YBCO DPCs and one FeSe_0.5Te_0.5-coated conductor SPC. The parameters of the hybrid coil are listed in table 2. Considering the aperture of the background magnet, the inner diameter of the magnet is set as 45 mm, and the outer diameter is 52.23 mm. The dimensions of the SPC are 51.04 mm in inner diameter and 52.23 mm in outer diameter. Figure 2 shows the magnetic field distribution of the hybrid coil using COMSOL finite element method software. The center field of the hybrid coil is 0.26 T with a current of 100 A. Figure 3 shows the maximum stress on the FeSe_0.5Te_0.5 coil and the YBCO coils with the critical current of FeSe_0.5Te_0.5 tapes with 0–10 T background fields. The coil stress under each background field is less than 1 MPa which is within the allowable pressure range of the wire. Therefore, it is not necessary to apply additional preload on the surface of the hybrid coil.

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Table 2. Parameters of FeSe_0.5Te_0.5 hybrid coil.

Parameters	YBCO coil	FeSe0.5Te0.5 coil
Coil type	DPC	SPC
Tape width (mm)	4.8	10.5
Tape thickness (mm)	0.185	0.15
Tape length (mm)	4026	560
Inner diameter (mm)	45	51.04
Outer diameter (mm)	50.18	52.23
Turn	14	3.5
Coil number	8	1
Coil height (mm)	45.5 × 2	10.5

3.2. Insert coil fabrication

The FeSe_0.5Te_0.5 encapsulated tape was used to fabricate the insert hybrid FeSe_0.5Te_0.5 coil. The FeSe_0.5Te_0.5 encapsulated tape was wound on a stainless-steel frame. Eight YBCO DPCs were divided into two groups connected to both sides of the FeSe_0.5Te_0.5 SPC; a small piece of YBCO tape was used to connect the YBCO DPC and the FeSe_0.5Te_0.5 SPC. The voltage signal lines were placed at the 1st turn and the 3.5th turn. The voltage signal line distance of the FeSe_0.5Te_0.5 coil was set to 560 mm. Figure 4 shows the fabrication process of the hybrid FeSe_0.5Te_0.5 coil. Figure 4(a) shows the FeSe_0.5Te_0.5 encapsulated tape and Figure 4(b) the outer view of the FeSe_0.5Te_0.5 SPC. Figure 4(c) shows the outer view of the hybrid FeSe_0.5Te_0.5 insert coil.

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The hybrid coil was tested at a liquid helium temperature of 4.2 K. The hybrid coil was inserted into a uniform field magnet with a 70 mm aperture and the field was increased from 0 to 10 T. The uniform field was perpendicular to the hybrid FeSe_0.5Te_0.5 coil. A Hall-effect Gauss meter was placed in the center of the FeSe_0.5Te_0.5 SPC to measure the magnetic field. Figure 4(d) is the part of the cold test device. The transport critical current I_c of the FeSe_0.5Te_0.5 coil at 4.2 K was also measured by the standard four-probe method with a criterion of 1 μV cm⁻¹. As the voltage signal line distance of the FeSe_0.5Te_0.5 coil was 560 mm in this study, the quench criterion of the coil was 56 μV.

The voltages of the FeSe_0.5Te_0.5 coil during the cold test with 0–10 T background fields are shown in figure 5. At self-field, the value of I_c reached 108.1 A and the n value was 24. Under the 10 T field, the I_c reached 17.4 A and the n value was 17. Figure 6 shows the cold test result of the hybrid coil. The critical current values are slightly higher than the short FeSe_0.5Te_0.5 encapsulated tape. The most likely reason is that it is not uniform at the end of the FeSe_0.5Te_0.5 tape. The critical current at the tape end will be slightly lower than that of in the entire tape. A Hall-effect Gauss meter in the center of the FeSe_0.5Te_0.5 SPC measured the magnetic field intensity. After the cold test, the hybrid FeSe_0.5Te_0.5 coil was cooled to room temperature and placed the coil for 48 h. Then the cold test of the hybrid coil was repeated. The second coil test result is also shown in figure 6. The transport critical current I_c of the FeSe_0.5Te_0.5 SPC was 106.8 A at self-field and 17.4 A at 10 T field, which was almost the same result as the first test. The test result proves that there is no decrease of transport I_c when the inner diameter of the FeSe_0.5Te_0.5 coil is 51 mm. Besides, the performance of the FeSe_0.5Te_0.5 hybrid coil is stable and repeatable in the two cold tests. This study also proves that the use of a copper protective layer can effectively withstand the high stress on the FeSe_0.5Te_0.5 coil during the cold test, avoiding damage to the superconducting layer due to the larger contraction of the substrate.

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In this study, we successfully fabricated and tested a FeSe_0.5Te_0.5 hybrid coil including one FeSe_0.5Te_0.5 SPC coil with a 51 mm inner diameter and eight DPC YBCO coils with a 45 mm inner diameter. The bare FeSe_0.5Te_0.5 tape was soldered into encapsulated tape, 10.5 mm in width and 0.15 mm in thickness, to withstand the high stress during the cold test of the hybrid coil in this study. The critical current of the FeSe_0.5Te_0.5 coil was measured at self-field and as the background field increased from 0 to 10 T. The transport I_c value is 17.4 A at 4.2 K with the 10 T field, and the corresponding transport J_c was about 1.42 × 10⁵ A cm⁻², which was almost the same performance as the short FeSe_0.5Te_0.5 tape. The results of this study showed that the FeSe_0.5Te_0.5-coated conductor coil can be an option for developing strong magnets. It has good performance in high-field magnet applications.

This work was supported by the National Key R&D Program of China (Grant No. 2018YFA0704300), and the National Natural Science Foundation of China (Grant No. 12275063).

All data that support the findings of this study are included within the article (and any supplementary files).