Abstract Body

To achieve high critical current densities in both self-field and in-field conditions for high-T_c cuprate superconductor REBa₂Cu₃O_y (RE123), the densification and orientation of a significant number of grains are required. In the study of the practical utilization of RE123 as a superconducting wire, our group is recently focusing the magnetic alignment methods to lead biaxial orientation of the RE123 grains. The strong points of magnetic alignment are that it does not require highly oriented template material, and it is a room temperature process. These features of magnetic alignment open new possibilities for fabricating RE123 thick (> 10 μm) films.

In the magnetic alignment method, the expectation is that the easy and hard axes align perpendicular to the static magnetic field and the rotating magnetic field, respectively. When the grain shape is approximately spherical, the following formula can be used to estimate the required magnetic alignment time τ [Ref. 1]: τ^-1 = χaB² / 6ημ₀, where χ_a is the dimensionless difference between the magnetic susceptibility along the easy axis and that perpendicular to the easy axis, B is the magnetic field strength, η is the medium viscosity, and μ₀ is the vacuum permeability. Roughly ten years ago, superconducting magnets were essential for achieving the biaxial magnetic alignment of several RE123s [Ref. 2]. Recently, our group developed an original apparatus that can create a linear drive type of modulated rotating magnetic fields (LDT-MRF) using the permanent magnet arrays [Ref. 3]. This apparatus is simple and has relatively low costs. As an important achievement to date, this LDT-MRF equipment had achieved a static magnetic field of 0.9 T and a rotating magnetic field of 0.8 T [Ref. 4]. Traditionally, such research has been conducted using batch processes. However, the application of magnetic alignment for the superconducting wire needs a continuous process. In this study, we integrated a sample transport system into the LDT-MRF apparatus and evaluated its performance in a continuous magnetic alignment process.

DyBa₂Cu₃O_y (Dy123, y ~ 7) powders (ave. particle size ~ 2－4 μm) were selected as the test sample of magnetic alignments by using the LDT-MRF apparatus, showing its relatively large magnetic anisotropy among the RE123 compounds [Ref.2]. The epoxy resin was used as the dispersion medium for Dy123 grains. The initial viscosity of the epoxy resin is η_int ~ 40 Pa⋅s, and less than five times η_int even one hour after mixing the base agent and hardener. The Dy123 powder and epoxy were rapidly mixed and transferred to the sample space with the acrylic mold. During the operation of the LDT-MRF apparatus, the sample tray moved (// linear driving motion) at a speed of approximately 0.7 mm/min. The time when the sample entered/escaped the magnetic field region of the LDT-MRF apparatus was defined as t_sta/t_end, respectively. Here the t_sta/t_end of Sample I was ~ 8 min./~ 25 min. The t_sta/t_end of Sample II was ~ 20 min./~ 37 min. The degree of orientation for the Dy123 powder sample after magnetic alignment was evaluated by using the (1 0 3) pole figures.

Figures 1(b-c) show the pole figures at the bottom surface of the sample I and Sample II after magnetic alignment by using the LDT-MRF apparatus. The four-fold rotationally symmetric spots in each pole figures indicate the biaxial orientation of Dy123 grains with twin microstructures. In both the results for samples I and Sample II, four spots and their centers of gravity were shifted downward in parallel on this paper. This parallel shift suggests that the orientation state of the Dy123 grains has been tilted. The inclination angle of the Dy123 grains in Sample I (~ 10°) was smaller than that of Sample II (~ 20°). Namly, the inclination angle of the grains was suppressed due to a higher viscosity of the sample when it is escaped from the magnetic field. In the future, a continuous process of magnetic alignment may become possible by tuning the magnetic anisotropy of a target material and medium viscosity for the dispersion to the state of the MRFs. In this presentation, we will report details of the methods and results.

References

[1] T. Kimura, Polym. J. 35, 823 (2003).

[2] S. Horii, T. Nishioka, I. Arimoto, S. Fujioka, and T. Doi, Supercond. Sci. Technol. 29, 125007 (2016).

[3] S. Horii, I. Arimoto, and T. Doi, J. Ceram. Soc. Jpn 126, 885 (2018).

[4] W. B. Ali, S. Adachi, F. Kimura, and S. Horii, J. Phys. Conf. Ser., 012016, 2545 (2023).

Acknowledgment

We are grateful to Mr. Kotakebayashi and Mr. Hiratsuka (the members of KUAS Machine Shop) for their support with modifying the magnetic alignment system.

Figure 1. (a) Schematic diagram of the apparatus for magnetic alignment using modulated rotating magnetic fields. (b-c) (103) pole figures at the bottom surface of the sample after magnetic alignment by using the apparatus of Figure 1a.

Keywords: Cuprate superconductor, REBCO, Magnetic alignment, Pole figure