Abstract Body

Since REBa₂Cu₃O_7-δ. (REBCO) coated conductors (CCs) have the advantage of a high critical current density in high magnetic fields, they are expected to be applied to high field magnets, however they often burn out due to thermal runaway if there is a local defect. Although the "no insulation coil" was developed to protect the REBCO coil against thermal runaway, it has the disadvantage of low magnetic field stability due to current transfer between neighboring tapes in the windings [1]. Recently, “smart insulation (SI) coil” has been proposed to achieve both stability in insulated coils and protection in NI coils through using the metal-insulator transition (MIT) materials, which changes the coil from an insulated to no insulation coil only when thermal runaway occurs [2]. As a candidate of MIT materials, we focus on Co-based perovskite oxide (Pr_0.8Sm_0.2)_0.6Ca_0.4CoO₃ (PSCCO), which is fabricated using the sol-gel method [3]. In this study, we fabricated polycrystalline bulks of PSCCO and measured their resistivity ρ and MIT temperature T_MI at low temperatures. Furthermore, we prepared the PSCCO slurry, and applied it to REBCO CCs and measured the contact resistance R_s between them.

Polycrystalline PSCCO were fabricated by the sol-gel method [3]. Firstly, gels were synthesized from an ethanol suspension (sol) of metal acetates by a ultrasound irradiation. The gels were dried and then sintered at 800 °C in air for 12 hours. Next, pellets formed from the ground powder were annealed at 1200 °C in oxygen gas flow. The pellets were cut into rectangular shape by a diamond saw for a resistivity measurement. On the other hand, the PSCCO slurry was fabricated using N-Methyl-2-Pyrrolidone (NMP) as a solvent and polyvinylidene fluoride (PVDF) as a binder. To begin with, the PSCCO sample was ground into the powder using an alumina mortar and pestle for one hour. Secondly, NMP and PVDF were added to a sample vial and heated on a hot plate at 100 °C for 40 minutes to dissolve the PVDF. Then, the PSCCO slurry was prepared by adding the PSCCO powder to the sample vial and mixing it with an automatic mixer for one minute. Finally, the slurry was painted onto the REBCO tapes using a toothpick and dried them on a hot plate at 100 °C for one hour. The temperature dependence of ρ in the polycrystalline samples and R_s in the contact between the CCs were measured from 10 to 270 K using the DC four-probe method. The T MIT was determined as the temperature at which the temperature derivative of ρ and R_s were maximum.

According to the transport measurement, the ρ in PSCCO bulk was decreased sharply at T_MIT= 74 K with increasing temperature. The ρ of PSCCO changed by more than three orders of magnitude from 3360 mΩcm at 270 K to 4.6 mΩcm at 10 K. On the other hand, R_s between REBCO tapes was gradually decreased by over four orders of magnitude due to the MIT of PSCCO from 10 K to 270 K. The R_s was 1.6 × 10⁶ mΩcm² at 10 K and 7.5 × 10¹ mΩcm² at 270 K, respectively. The T_MIT obtained from the R_s measurement was 76 K. It was found that the change in ρ and R_s differed by one order of magnitude. This difference is considered to be due to the interfacial resistance between the REBCO CCs and the dried PSCCO slurry.

We will discuss the R_sin the PSCCO-coated REBCO tapes in comparison with the earlier reports about other MIT materials such V₂O₃ [4].

References

[1] S. Hahn et al., IEEE Trans. Appl. Supercond. 21, 1592 (2011).
[2] H.-W. Kim et al., IEEE Trans. Appl. Supercond. 27, 461704 (2017).
[3] M. Tahashi et al., Jpn. J. Appl. Phys. 61, 018003 (2022).
[4] M. Bonura et al., IEEE Trans. Appl. Supercond. 33 8800106 (2023).

Acknowledgment

This work was supported by Grant-in-Aid for JSPS Fellows Grant Number JP24KJ0385.

Keywords: Quench protection, Smart insulation, Metal-insulator transition (MIT), Contact resistance