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- ItemMagnetic flux-trapping of anisotropic-grown Y-Ba-Cu-O bulk superconductors during and after pulsed-field magnetizing processes(Milton Park : Taylor & Francis, 2014) Oka, T.; Yamada, Y.; Horiuchi, T.; Ogawa, J.; Fukui, S.; Sato, T.; Yokoyama, K.; Langer, M.The magnetic flux penetration into the melt-textured Y-Ba-Cu-O high temperature superconducting bulk magnets were precisely evaluated during and after the pulsed field magnetization processes operated at 30 K. The bulk magnets were carefully fabricated by the cold seeding method with use of a single and a pair of seed crystals composed of the Nd-Ba-Cu-O thin films. These seed crystals were put on the top surfaces of the precursors to let the large grains grow during the heat treatments. We observed the flux penetrations which occurred in the lower applied-field regions at around 3.1 T for the samples bearing the twin seeds than those of the single-seeded crystals at around 3.8 T. This means that the magnetic fluxes are capable of invading into the twin-seeded samples more easily than the single-seeds. It suggests that the anisotropic grain growths of parallel and normal to the rows of seed crystals affects the variations of Jc values with different distributions of the pinning centers, results in the preferential paths for the invading magnetic fluxes.
- ItemOccurrence of Flux Jumps in MgB2 Bulk Magnets during Pulse-Field Magnetization(Bristol : IOP Publ., 2020) Sakai, N.; Oka, T.; Yamanaka, K.; Dadiel, L.; Oki, H.; Ogawa, J.; Fukui, S.; Scheiter, J.; Häßler, W.; Yokoyama, K; Noudem, J.; Miryala, M.; Murakami, M.The magnetic flux capturing of MgB2 bulk magnets made by spark plasma sintering process has been precisely investigated to clarify the mechanism of flux motions during the pulse-field magnetization processes. The field trapping ratio B T/B P was evaluated as a key parameter of field trapping ability which strongly relates to the heat generation due to the rapid flux motion in the samples. The time dependence of magnetic flux density revealed the actual flux motion which penetrated the samples. The trapped fields B T and field trapping ratios B T/B P of various samples were classified into three regions of 'no flux flow', 'fast flux flow' and 'flux jump' according to the generation of heat and its propagation. A flux jump was observed late at 280 ms from the beginning of PFM process, while the field penetration B P showed its peak at 10 ms. Considering the heat propagation speed, the long-delayed flux jump should be attributed to the macroscopic barriers against the heat propagation to the surface centre of bulk magnet. © Published under licence by IOP Publishing Ltd.
- ItemPulsed-field Invasion to HTS Bulk Magnets Grown from Two Seeds with Varied Seed-crystal Positions and Numbers(Amsterdam [u.a.] : Elsevier, 2014) Oka, T.; Yamada, Y.; Horiuchi, T.; Ogawa, J.; Fukui, S.; Sato, T.; Yokoyama, K.; Langer, M.The flux-invasion behavior into the melt-processed Y-Ba-Cu-O bulk magnets were precisely measured and analyzed during and after their pulsed-field magnetization processes operated at 30.6 K. The materials were fabricated as the bulk monoliths grown by adopting two seed-crystals, or shifting the seed-crystal positions from the centre of the sample surface, which exhibited the magnetically single-domain distributions. Although the performances of the trapped flux density after activations showed no obvious differences, the flux started invading into the sample bearing two seeds obviously at lower fields than those of normally-grown isotropic crystal. Since the flux penetration behavior were thus clearly different between the samples with the structure grown from two seeds and uniformly grown samples with a seed crystal, it is suggested that the structure results in an effective magnetizing method with less heating than those of conventional samples, which results in the higher performance of field trapping in the bulk magnets than usual.