[Introduction]
To improve the critical current density (Jc) of REBa2Cu3Oy(RE123) superconductors (SCs) for practical use, it is important to achieve densification and bi- or tri-axial grain alignment. The thin film epitaxial growth technology is used for REBa2Cu3Oy [RE123] coated conductors with high critical current density (Jc). However, the thickness of the RE123 layer has the order of microns and material cost of the coated conductors is very high. Our group focuses on magnetic alignment based on the modulated rotating magnetic field (MRF) as a biaxial/triaxial alignment process. Biaxial alignment of RE123 grains with twinned microstructure has been achieved under the MRF of solenoidal SC magnet (SC-MRF) in an epoxy resin [1, 2]. One of the current issues in SC-MRF is development of continuous production process, and the linear drive type MRF (LDT-MRF) has been developed as a novel MRF which is appropriate for a continuous production. This equipment generates an MRF of ~ 0.8 T by reciprocating motion of a permanent magnet array [3]. Another issue is development of the biaxial aligned RE123 ceramics based on the colloidal process. Generally, viscosity levels of colloidal solutions are 2-4 orders of magnitude lower than that of epoxy resin used in our previous studies[1, 2]. We need to develop RE123 compounds appropriate for the biaxial magnetic alignment using the colloidal solution with the lower viscosities under the permanent magnet level. Y123 is relatively inexpensive in RE123, and appropriate for practical use. However, Y123 shows the smallest magnetic anisotropy in RE123 and its biaxial orientation was not achieved within the previous work[1]. In this study, we clarified the degrees of orientation of YBa2Cu3Oy [Y123] particles under the SC-MRF and LDT-MRF in different types of epoxy resin with various viscosities.
[Experimental detail]
Y123 polycrystals were synthesized by the standard solid-state reaction. Incidentally, the pelletized powders were sintered at 920 ℃ in air for 24 h as the final sintering process. The obtained Y123 pellets were annealed at 300℃ in flowing oxygen gas to achieve y ~ 7, and were pulverized in an agate mortar to obtain powders with ~10 μm in averaged grain size. Y123 powders were mixed with epoxy resins at weight ratio of 1:10 and aligned under SC-MRF of 1 T and LDT-MRF. Here, we used two different types of epoxy resins. Resin A shows a higher initial viscosity (ηinit ~ 40 Pa・s). Resins B and C show lower initial viscosities (ηinit ~ 0.5 Pa・s) and different curing times (17 h for Resin B, 37 h for Resin C). The orientation axes and degrees of orientation of the magnetically aligned powder samples of Y123 were determined from (103) pole figure measurements.
[Results and Discussion]
Figs. 1(a), (b), and (c) show (103) pole figures of the Y123 powder samples aligned under SC-MRF of 1 T in Resin A, Resin B, and Resin C, respectively. Note that the measurement plane for the (103) pole figure is a plane perpendicular to the direction of the static magnetic field component. The biaxial orientation degree (Fs) was evaluated as a ratio of summation of intensities of the four-symmetric peaks related to bi-axial alignment to summation of whole peak intensities. Within our experiment of the uni-axial alignment for RE123, Fs is equivalent to ~ 9 %. Fs for Resin A showed 13.2 %, which is close to the uni-axial alignment. Fs for Resin B and Resin C showed 20.8 % and 31.2 %, respectively indicating that Y123 powders were partially bi-axial aligned in Resin B and Resin C. It was suggested that the orientation degrees were improved by using epoxy resin with the lower viscosities. In this study, we will report the change in the orientation degrees for the magnetically aligned Y123 powder samples as functions of viscosity and magnetic field strength of MRF.
[1] Horii et al., SuST 29 (2016) 125007.
[2] Ali et al., J. Appl. Phys. 134 (2023) 163901.
[3] Horii et al., J. Ceram. Soc. Jpn. 126 (2018) 885.
Fig. 1 (103) pole figures of the magnetically aligned powder samples of Y123 under 1 T of SC-MRF in (a) Resin A, (b) Resin B, and (c) Resin C, respectively.
Keywords: Cuprate superconductor, Magnetic alignment, RE123