Uniformity of Orientation Degrees in Dy123 Superconductor through Linear Drive-Type Modulating Rotating Magnetic Field

The rare earth-based cuprate superconductor REBa₂Cu₃O<i>_y</i> (RE123) has garnered significant attention due to its high critical temperature (<i>T</i>_c) of approximately 90 K, making it suitable for various applications like liquid nitrogen-operated superconducting bulk magnets and cables. This material features an anisotropic crystal structure, comprising alternating 2D superconducting CuO₂ layers and 1D Cu-O chains. This anisotropy results in varying critical current densities (<i>J</i>_c) in different directions, with J_c//<i>c</i> being lower than <i>J</i>_c//<i>ab</i> [1]. To enhance the practical use of RE123, it's crucial to achieve biaxial or triaxial orientation and densification of its grains. One non-epitaxial method for grain orientation enhancement is magnetic alignment. Magnetic alignment doesn't require a vacuum process and can be done at room temperature. It involves the generation of a modulated rotating magnetic field (MRF) with two or more axes. In this context, permanent magnets (PMs) have proven to be a cost-effective solution for creating a low-cost MRF and aligning RE123 grains.<br/>A continuous production technique known as linear drive-type modulated rotating magnetic field (LDT-MRF) employs a PM array to achieve triaxial grain alignment [2]. This equipment can generate an MRF of ~ 0.8 T without physically moving the sample [3]. Using this approach, DyBa₂Cu₃O<i>_y</i> (Dy123) grains were successfully biaxially aligned, as evidenced by a four-fold symmetric (103) pole figure at <i>Ψ</i> ~ 45°, similar to results achieved with a 10 T-MRF from a superconducting solenoidal magnet [4]. In prior work [3], we noticed location-dependent orientation degrees in the biaxially aligned sample. The center part displayed higher in-plane and <i>c</i>-axis orientation degrees of around 8.5° and 6.0°, respectively. The side portions also exhibited higher orientation degrees but with a shift of the four-fold symmetric spots toward the right and leftward of the (103) pole figures. To understand this shift in diffraction spots, 3D simulations using the Finite Element Method (FEM) software were conducted to analyze the behavior of flux lines in the air gap between the magnet arrays. The simulation revealed that the flux lines inclined toward the outside of the magnet array along the <i>Y</i>-axis (the width of the magnet array), causing a <i>θ</i> ~ 6° shift in diffraction spots from the center. This indicated non-uniformity in the MRF at the side parts of the magnet array, resulting in flux line leakage outside the array.<br/>To address this non-uniformity, we designed a new magnet array with an increased width along the <i>Y</i>-axis, expanding it from 20 mm to 48 mm. The 3D simulation of this new design showed that the inclination angle remained under ~ 5° up to<i> Y</i> = 16.5 mm from the center, ensuring high orientation degrees. Biaxial alignment experiments with Dy123 powders in the newly designed magnet array consistently displayed four-fold diffraction spots at <i>Ψ</i> ~ 45°, extending from the center to <i>Y</i> = 10.5 mm, with a slight 3° shift at <i>Y</i> = 14 and 16.5 mm. Importantly, the experimental results quantitatively matched the simulation results, demonstrating a homogeneous MRF within the range of 0 ≤ <i>Y</i> ≤ 16.5 mm in the newly designed magnet array, resulting in an in-plane orientation degree of ~ 8.4°. This research suggests that LDT equipment, in combination with improved magnet array designs, has the potential to advance RE123 superconducting tape production. However, challenges in material production and equipment limitations still need to be addressed. The presentation will feature 3D simulation results for the previous and newly designed magnet arrays, along with an explanation of how increasing the magnet array's width helped overcome location-dependent orientation degree issues.<br/>Reference:<br/>[1] lye et al., Jpn. J. Appl. Phys. 26, (1987). [2] Horii et al., J. Ceram. Soc. Jpn. <b>126</b>, (2018). [3] W. B. Ali et al., J. Appl. Phys. <b>134</b>, (2023) (to be published) [4] Horii et al., Supercond. Sci. Technol. <b>29</b>, (2016).