Y. Iijima

Fujikura Ltd., Edo, Tōkyō, Japan

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Publications (99)106.7 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: REBa2Cu3Oy, (RE : rare earth elements, RE123) coated conductors are expected to show high performance in superconducting applications, due to their high mechanical strength and high current density in magnetic fields. Fujikura has developed ion-beam-assisted-deposition (IBAD) and pulsed-laser-deposition (PLD) technique, and today we routinely manufacture coated conductors with length over 500 m and Ic over 500 A/cm-width at 77 K, self field. We have also been able to fabricate long conductors with higher Ic by thickening RE123 layer. Although Ic performances at 77 K, self field are important, but performances in magnetic fields at lower temperatures below 77 K are more important for coil applications. In this work, we evaluated coated conductors with thick RE123 layer in magnetic fields at low temperatures. All samples are fabricated with long length. We measured samples by four-probe transport method at wide temperature range from 77 K to 10 K by using cryo-cooled cryostat. A conductor with 5.5 μm thick RE123 exhibits high Ic values of 937 A/cm-width at 77 K, self field, 637 A/cm-width at 50 K, 5T and 976 A/cm-width at 40 K, 5T.
    Journal of Physics Conference Series 05/2014; 507(2):022007.
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    ABSTRACT: To elucidate the self-epitaxy mechanism of pulsed-laser deposition-CeO2, a hypothetical relationship with the substrate was derived based on the ion-beam-assisted deposition layer-processing method: the smaller the misorientation angle, the larger the crystallite size. In-plane misorientation angle dependences of crystallite sizes of ion-beam-assisted deposition-MgO and LaMnO3 as substrates for CeO2 deposition, obtained using X-ray diffraction and transmission electron microscopy, indicated that the hypothesis is plausible. This relationship is regarded as a prerequisite for self-epitaxy because large crystallites with small strains would be energetically favorable when CeO2 particles crystallize on them. Eventually, they will grow to dominant grains, which is a possible self-epitaxy mechanism.
    IEEE Transactions on Applied Superconductivity 06/2013; 23(3):6601005-6601005. · 1.32 Impact Factor
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    ABSTRACT: REBCO coated conductors having a performance greater than 300 A/cm at 77 K have been developed for superconducting applications. The conduction-cooled coils fabricated using REBCO coated conductors are expected to have the high performance because the specific heats of the constituent materials increases greatly with the operating temperature. From the point of view of the coil design using REBCO coated conductors, we must consider the critical current versus the magnetic field characteristics and their temperature dependence.In this study, we fabricated the two-layer stacked racetrack coil whose outer diameter was 92 × 172 mm with 56 turns. The coil used the REBCO coated conductor, whose critical current was around 200 A/5 mm at 77 K self field, manufactured by Fujikura Ltd. We compared with the calculated and measured the voltage distribution of one racetrack coil, which was attached with voltage taps each turn, at 77 K in self field. We investigated the Ic of the two-layer coil at 65 K and 50 K under conduction-cooled conditions. The Ic values of the two-layer coil in self field using a criterion of 10−7 V/cm were achieved 220 A at 65 K and 386 A at 50 K, which the central magnetic fields were 0.13 T at 65 K and 0.23 T at 50 K, respectively.
    Physica C Superconductivity 01/2013; 484:292–294. · 1.11 Impact Factor
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    ABSTRACT: We have investigated electric field criterion of in-plane critical current density in a coated conductor characterized by scanning Hall-probe microscopy (SHPM). From remanent field distribution and its relaxation measurements, we could obtain critical current distribution and induced electric field simultaneously by considering the Biot-Savart law and the Faraday’s law, respectively. These results lead us to evaluate a distribution of local critical current density and the corresponding criterion of electric field. As a result, it was found that the electric field criterion for the SHPM analysis was several orders lower than that used in the conventional 4-probe resistive method. However, the data point obtained by the SHPM shows good agreement with E–J curve analytically extended from the measurements by the 4-probe method. This means that we could characterize in-plane distribution of critical current density in a coated conductor at an electric field criterion quantitatively by this method in a nondestructive manner. These findings will be very important information since the uniformity of local critical current density in a coated conductor at extremely low electric fields is a key issue (1) especially for DC applications, (2) for quality control of coated conductors, and (3) for the standardization of the characterization of critical current among different methods.
    Physica C Superconductivity 01/2013; 484:139–141. · 1.11 Impact Factor
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    ABSTRACT: Observations of fractured cross sections, which are frequently used to determine the grain size distribution in Nb<sub>3</sub>Sn, cannot be performed for Nb<sub>3</sub>Al since transgranular fracture is the dominant fracture mode. Instead, we have evaluated the grain size of Nb<sub>3</sub>Al using electron backscatter diffraction (EBSD). To check the validity of EBSD analysis for A15 grain structures, bronze-route Nb<sub>3</sub>Sn filaments were also examined by EBSD. The deformation-and-transformation of bcc supersaturated-solid solution Nb(Al)<sub>ss</sub> was found not only to homogenize and refine the grain size of the resultant Nb<sub>3</sub>Al but also to increase a ratio of high angle grain boundaries to low angle grain boundaries. The texture studies were made by pole figure works: the Nb(Al)<sub>ss</sub> retains a 〈110〉 fibrous texture as well as the filament barrier (Nb, Ta) does. The transformed Nb<sub>3</sub>Al also has the weak 〈110〉 fibrous texture, whereas Nb<sub>3</sub>Sn has almost never texture. A clear relationship between Jc and the inverse of grain size was not observed.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.32 Impact Factor
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    ABSTRACT: We have investigated critical current I<sub>c</sub> properties of 600 A/cm-class GdBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> (GdBCO) coated conductor (CC) deposited on IBAD template. 2.5 μm thick GdBCO layer has been obtained by pulse laser deposition without degradation of critical current density by an improved heating condition in reel-to-reel deposition process. This enables stable production of long length CC with I<sub>c</sub> (77 K, self-field) of 600 A/cm-w. In-filed I<sub>c</sub> has also shown high value e.g., I<sub>c</sub>@77 K, 3 T=33 A/cm-w, I<sub>c</sub>@20 K, 27 T=107 A/cm-w. These experimental results obtained in a wide range of bias conditions of temperature T, magnetic field B and field angle θ are useful as a fundamental data for design of superconducting devices such as coil winding and magnet. Furthermore, we have also shown that our proposed analytical expression of electric field vs. current density (E-J) characteristics agrees well with the experimental results. This analytical expression allows us to predictI<sub>c</sub>-B-T property as well as E-J characteristics at arbitrary conditions of B and T.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.32 Impact Factor
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    ABSTRACT: The quench characteristics, particularly the normal-zone propagation and increase in hot-spot temperature, of REBa<sub>2</sub>Cu<sub>3</sub>O<sub>x</sub> (REBCO) coated conductors are important issues for safe operation in the superconducting applications. Although experimental data on the quench characteristics of REBCO coated conductors have been reported only limited data is available specifically on REBCO coated conductors laminated with Cu tape of thickness no less than 0.1 mm, which are expected to be used for high-current operation in future superconducting applications. Here we reported experimental measurements of the normal-zone propagation velocity (NZPV) in REBCO coated conductors laminated with Cu tape of 0.1 mm thickness at 77, 65 and 50 K in a background magnetic field. We also compared these results with the NZPV at 77 K of REBCO coated conductors laminated with Cu stabilizer tape of thickness 0.05 and 0.3 mm.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.32 Impact Factor
  • Y. Iijima, M. Kimura, T. Saitoh
    MRS Online Proceeding Library 01/2011; 587.
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    ABSTRACT: To enhance the non-Cu critical current density Jc at 15 T and 4.2 K (1000 A mm − 2 at present) we have endeavoured to refine the grain size of rapid heating, quenching and transformation (RHQT)-processed Nb3Al. In the present study, the grain boundary structures of RHQT-processed Nb3Al were examined by electron backscatter diffraction (EBSD) because transgranular fracture prevents the observation of fractured cross sections of Nb3Al to statistically determine the grain size. The grain size distributions of body-centred-cubic supersaturated-solid-solution Nb(Al)ss and A15 Nb3Al filaments were measured for grains misoriented by more than 2°, 5° and 15°. A mixed grain structure, which consists of a few large grains (>25 µm) and many small grains (<1 µm), was observed for an Nb3Al filament that had been transformed from non-deformed Nb(Al)ss. Plastic deformation that had been made between the rapid heating and quenching steps and the transformation step apparently homogenized the grain size distribution and then reduced the average grain size. The misorientation angle distributions of Nb(Al)ss and Nb3Al were also measured and compared with each other. A clear relationship between the Jc and the inverse grain size was not confirmed for the RHQT Nb3Al conductors examined in the present study, which indicates the importance of making a filament compositionally homogeneous to obtain a high Jc.
    Superconductor Science and Technology 10/2010; 23(12):125001. · 2.80 Impact Factor
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    ABSTRACT: Rapid heating/quenching and transformation (RHQT)-processed Nb3Al wires possess better strain tolerance than Nb3Sn wires and exhibit similar high-field properties. Therefore, Nb3Al wires might be promising candidates for use in future high-field accelerator magnets. For this reason, we have been developing RHQT-processed Nb3Al wires for a number of years. During this development, magnetization measurements on several samples have been carried out as a function of either temperature or the magnetic field. This paper presents some of the results of the magnetization measurements performed.
    Journal of Physics Conference Series 07/2010; 234(2):022040.
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    ABSTRACT: Increase of production rate and improvement of quality for RE123 coated conductors have been tried. In-plane texturing of MgO was attempted by the IBAD system with the world largest ion source. As a result of optimizing condition in large deposition area, the dramatically high throughput of 1000 m / h was realized to obtain the IBAD-MgO with Δ < 10°. Furthermore, simple buffer structure was demonstrated. Well textured CeO2 layer with Δ of around 4° was obtained by directly deposited on IBAD-MgO layer in spite of large lattice mismatch of 28% between CeO2 and MgO. Several over 100 m buffer substrates with the architecture of / PLD-CeO2 (60 m / h) / IBAD-MgO (333~1000 m / h) / Y2O3 (500 m / h) / Al2O3 (150 m / h) / Hastelloy / were already prepared. On these production substrates, GdBCO layer was deposited by the large PLD system at high throughput. The 260 m long GdBCO tape with Ic > 600 A except some locations was obtained at the throughput of 15 m / h. In addition to the speed-up, the very high Ic of 1040 A was also achieved by the hot-wall heating PLD system.
    Journal of Physics Conference Series 07/2010; 234(2):022016.
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    ABSTRACT: We have recently developed a thin-type Cu clad stabilized Nb<sub>3</sub>Al flat-wire which can be more densely wound into a coil. Using this flat-wire we have upgraded an existing 14 T Nb-Ti/Nb<sub>3</sub>Sn magnet with a ?? 30 mm clear bore, by replacing the inner coil, to a compact 15 T Nb-Ti/Nb<sub>3</sub>Al magnet. The Nb<sub>3</sub>Al inner coil was connected in series with the Nb-Ti outer coil and excited by a common power supply to generate 15 T in a ?? 40 mm clear bore, after a training of Nb-Ti coil. The resultant higher field and wider space will be used for critical current measurements of various superconducting wires being developed in NIMS.
    IEEE Transactions on Applied Superconductivity 07/2010; · 1.32 Impact Factor
  • 01/2010;
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    ABSTRACT: In Japan we started a national project to develop a 66/6.9 kV–20 MVA transformer with REBCO superconducting tapes in 2006. This paper gives an overview of progress of the development of superconducting transformers in Japan and also describes the fundamental technologies studied before now to realize a 66/6.9 kV–20 MVA transformer as follows. To reduce the ac loss in REBCO superconducting thin tapes, authors proposed a new method different from the conventional technique of reducing the ac loss in superconducting multifilamentary wires. It consists of scribing process into a multifilamentary structure by laser or chemical etching, and a special winding process. Making a multilayered solenoidal coil with laser-scribed REBCO tapes, we verified the ac loss reduction in proportion to a filament width even in coil configuration. In addition, to realize a current capacity more than the rated secondary current of 2.4 kA, we first investigated the workability of REBCO tapes in the actual winding process with forming a transposed parallel conductor, where REBCO tapes were bent edgewise at transposing points. Making a test coil of a 24-strand parallel conductor, we verified no degradation of the critical current and nearly uniform current distribution among the tapes. The result suggests the applicability of the method of enhancing the current capacity by forming a parallel conductor with REBCO tapes. Further, to realize the dielectric strength regulated for the Japanese standards, i.e. lightning impulse withstand level of 350 kV and excess ac voltage of 140 kV, we made test coils and carried out dielectric breakdown tests. As a result, we got hold of the required insulation distance at the important points from the viewpoint of insulation design.
    Physica C Superconductivity 10/2009; 469:1726-1732. · 1.11 Impact Factor
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    ABSTRACT: We have visualized non-uniform current flow in RE123 coated conductors by using a scanning Hall-probe magnetic microscopy (SHPM). Newly developed SHPM system allows us to measure two-dimensional magnetic field distribution with high spatial resolution in micro-meter scale. Corresponding current density distribution can be obtained from the magnetic field image by solving inverted Biot–Savart’s law. One of the most important advantages of the present system is to visualize the current density distribution in practical high transport current and also in wide scanning area. For example, the system has current leads with large capacity up to 500 A, and the operating distance can be 15 cm by 15 cm with a micro-meter step distance. Using the SHPM system, we have successfully visualized current density distributions in the coated conductor, and clarified different kinds of non-uniform current flow. Those insights are very useful to identify local defects as well as non-uniform tape quality. These results indicate that the SHPM system is a powerful diagnostic tool not only to observe spatial inhomogeneities of transport property but also to understand their reason in practical coated conductors.
    Physica C Superconductivity 10/2009; 469:1450-1453. · 1.11 Impact Factor
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    ABSTRACT: We have carried out observations of current flow in processed RE123 coated conductors using a scanning Hall-probe microscope. Using this system, the position of defects and the resulting disturbed current flow could be observed. The two-dimensional current distribution in a 10-filament formed coated conductor (400 mum width and 100 mum space) was also investigated. Current sharing and the current distribution in each filament were quantitatively estimated. Furthermore, current penetration in quasi-static AC current mode was also measured.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.32 Impact Factor
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    ABSTRACT: A Nb<sub>3</sub>Al strand, produced by the ldquorapid-heating, quenching and transformation annealing (RHQT)rdquo process, has been developed in collaboration between JAEA and NIMS aiming at application to the conductor in a fusion DEMO reactor. A detailed description of critical current characteristics of the Nb<sub>3</sub>Al RHQT strand may assist in predicting accurate conductor performance. Calculations of the critical current density are adjusted to 5 scaling laws. Advantages of the newly developed strand compared to the Nb<sub>3</sub>Sn ITER strand and the Nb<sub>3</sub>Al Insert strand include improved strain tolerance and a higher current density at around 16 T. Performance parameters of the Nb<sub>3</sub>Al RHQT strand include 450 A/mm<sup>2</sup> at 15.9 T, 5.7 K and -0.4%. These performance parameters fulfill the requirements of the TF conductor of the fusion DEMO reactor. Therefore, the Nb<sub>3</sub>Al RHQT strand represents an improved performance with prospective application in the fusion DEMO reactor. In addition, the Nb<sub>3</sub>Al RHQT strand has the potential to enhance further the critical current density.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.32 Impact Factor
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    ABSTRACT: We have been developing rapid heating, quenching and transformation (RHQT) processed Nb<sub>3</sub>Al wires for next-generation accelerator magnets that require magnetic fields between 12 and 17 T. However, it has recently been found that the conventional Nb-matrix wire shows a rather strong magnetic instability at low fields. Taking this into consideration, we have begun developing Ta-matrix Nb<sub>3</sub>Al wires. We have fabricated a precursor wire with a Ta-matrix volume fraction of 0.8, and the mechanical properties of the RHQ-processed wire were examined. The effects of the RHQ condition and the area reduction (AR) after RHQ on the critical current density ( J <sub>c</sub>) , and magnetization of the wire have also been examined. This paper describes the properties for the Ta-matrix wire in comparison with those of the Nb-matrix wire.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.32 Impact Factor
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    ABSTRACT: As one of the programs in the Ministry of Economy, Trade and Industry (METI) R&D project on coated conductors, we developed a three-phase 6.6 kV superconducting fault current limiter (SFCL) and conducted some evaluation tests. The developed SFCL mainly comprised a set of three-phase current-limiting coils installed in a sub-cooled nitrogen cryostat with a Gifford-McMahon (GM) cryocooler, circuit breakers, and a sequence control circuit. Two tapes were wound in parallel in each limiting coil to obtain a rated current of 72 A rms. AC characteristics of each coil were measured, and relevant performance metrics were obtained. The whole system was installed in a cubicle. Short circuit experiments were then conducted with a short circuit generator. In a three-line ground fault test, the SFCL successfully restricted a short circuit current of over 1.56 kA to about 840 A with an applied voltage of 6.6 kV. The system integration ability and the obtained data show the promise of this approach for practical implementation. The SFCL was ready for user field tests.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.32 Impact Factor
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    ABSTRACT: A new configuration of Nb<sub>3</sub>Al conductor with a fine multifilamentary structure is presented. The wire is fabricated by a simple re-stacking (RS) method, in which rapid-quenched ductile BCC Nb-Al phase wires are re-stacked into a stabilizing Cu tube and co-drawn into a final form. Such a simple stabilization method will be effective to cut the Nb<sub>3</sub>Al fabrication cost. The developed RS Nb<sub>3</sub>Al conductor has a non-Cu J <sub>c</sub> of almost about 1000 A/mm<sup>2</sup> at 15 T, 4.2 K. The effective filament diameter is 14 mum and the hysteresis loss (+/- 3 T) is 370 mJ/cm<sup>3</sup>, which is small enough with respect to the ITER target specification.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.32 Impact Factor

Publication Stats

931 Citations
106.70 Total Impact Points

Institutions

  • 1993–2011
    • Fujikura Ltd.
      Edo, Tōkyō, Japan
  • 2004–2010
    • National Institute for Materials Science
      Tsukuba, Ibaraki, Japan
  • 2009
    • Japan Atomic Energy Agency
      • Quantum Beam Science Directorate
      Muramatsu, Niigata, Japan
  • 2003–2009
    • Kyushu University
      • Graduate School of Information Science and Electrical Engineering
      Hukuoka, Fukuoka, Japan
    • Kyushu Institute of Technology
      • Faculty of Computer Science & Systems Engineering
      Edo, Tōkyō, Japan
  • 2006–2007
    • Yokohama National University
      • Department of Physics, Electrical and Computer Engineering
      Yokohama, Kanagawa, Japan
  • 2005
    • Waseda University
      • Department of Electrical Engineering and Bioscience
      Tokyo, Tokyo-to, Japan
  • 2001
    • Fisheries Research Agency
      Yokohama, Kanagawa, Japan
    • Central Research Institute of Electric Power Industry
      Edo, Tōkyō, Japan
  • 1989–2001
    • Tsukuba Research Institute
      Edo, Tōkyō, Japan