Abstract Body

Biaxial-oriented and densified microstructures are necessary to achieve self-field and in-field Jc in high-Tc cuprate superconductors. The typical technique for realization of the biaxial orientation and densification is epitaxial growth technology, such as melt-solidification and thin film growth on highly-oriented substrate. On the other hand, our group focuses on the biaxial magnetic alignment by modulated rotating magnetic field (MRF)[1,2] as a triaxial alignment technique of materials and is currently investigating a material production process based on MRF The advantageous points of the magnetic alignment using MRF are “room temperature process”, “no need to use highly oriented template” and “triaxial grain alignment”. These intriguing features of MRF leads to possibilities of production of biaxially oriented REBa₂Cu₃O_y (RE123) thick films (in tens and hundreds micron levels) with higher Ic.

In our group, the intermittent type MRF[3], which is a rotating magnetic field including the resting process at every 180 degree, has been used and magnetic alignment of RE123 powder samples has been examined in epoxy resin at room temperature. The MRF enables the simultaneous alignment of the first easy and hard axes parallel to the direction of static field component and perpendicular to the plane created by the rotating magnetic field, respectively, in principle. However, a “sample-rotation” system in which samples rotate in the horizontal plane should be installed in order to generate MRF by a superconducting solenoidal magnet (SC magnet) with 300 kg in weight. This magnetic alignment technique is a batch process and is disadvantageous from the practical viewpoint.

Recently, our group has developed the linear-drive type MRF (LDT-MRF) equipment with a permanent magnet array[4], which does not require rotational motion of samples and rotation of the solenoidal magnet. It enables the generation of 0.9 T as the static field component and 0.8 T as the rotating field component. The equipment can continuously provide MRF in air gap between top and bottom parts of the magnet array, which achieves triaxial alignment, for long and sheet-shaped sample without the rotation processes of sample and magnet. In practice, Dy123 powders were biaxially oriented in LDT-MRF[4]. In the present study, we estimate uniformities of MRF in magnet arrays of LDT-MRF by the finite element method and design new magnet arrays to improve the uniformity of MRF on the basis of their electromagnetic simulations.

Figure 1(a) shows a schematic (bird view) of the magnet arrays. Three different magnet arrays were developed in the present study. The magnet arrays A, B and C have different widths (W in Fig. 1(a)), and respective values of W are 18, 36 and 44 mm for the magnet arrays A, B and C. In order to generate MRF, LDT-MRF requires reciprocation motion of the magnet array and its stroke is set to be 50 mm in the present study. Figure 1(b) shows the orientation degrees (F) of the magnetically aligned Y123 powders with 10 mm and 4 mm in size. Incidentally, the magnet array A (W=18 mm) is used and F [%] is determined from a ratio of the summation of intensities of the 4-fold symmetric peaks to the summation of intensities in a whole measured region in (103) pole figure. For reference, the F value for the Dy123 powder sample which was oriented in 1T-MRF using a superconducting (SC) magnet. For reference, FWHM (ΔΦ) values in the rotational angle direction on the four peaks are also shown. Clearly, F values were almost constant, whereas for the 10 mm-sample was the worst in the three. This deterioration in ΔΦ is reasonably explained in terms of the decrease in the uniformity of MRF in a direction parallel to W. The simulation by FEM revealed that the vertical rotating field component was inclined outward and the inclination angle was approximately 5 deg at 5 mm apart from the center. In the case of the 10 mm-sample, due to non-uniform MRF, the worse ΔΦ was obtained.

In the present study, the electromagnetic simulation results for the three magnet arrays and the position dependent F and ΔΦ for the magnetically aligned Dy123 powder samples with the magnet arrays A, B and C will be shown.

References

[1] Kimura et al., Langmuir 21 (2005) 4805.  
[2] Fukushima, Horii et al., Appl. Phys. Express 1 (2008) 111701.
[3] Horii et al., Supercond. Sci. Technol. 29 (2016) 125007.
[4] Horii et al., J. Ceram. Soc. Jpn 126 (2018) 885.

Figure 1. (a)Schematic of the magnet arrays. (b) F and ΔΦ values for the three different Dy123 powder samples; 1T-MRF in SC magnet, 10 mm in size in Array A, and 4 mm in size in Array A.

Keywords: Magnet array, Magnetic alignment, Modulated rotation magnetic field, 3D simulation