Yasuhiro Iijima

Fujikura Ltd., Edo, Tōkyō, Japan

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Publications (79)69.36 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: 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. · 0.72 Impact Factor
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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 01/2013; 23(3):6601005-6601005. · 1.20 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. · 0.72 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; · 1.20 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; · 1.20 Impact Factor
  • IEEE Transactions on Applied Superconductivity 01/2011; 21:3393-3397. · 1.20 Impact Factor
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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 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; · 1.20 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; · 1.20 Impact Factor
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    ABSTRACT: We designed and fabricated a 7.5 kW-360 rpm synchronous motor with a superconducting rotor. The rotor was 6-pole type and has no iron core. The armature winding was wound with copper wires. 6 pieces of racetrack-shaped field coils were wound with 5 mm wide RE<sub>1</sub>Ba<sub>2</sub>Cu<sub>3</sub>O<sub>7-delta</sub> (REBCO, RE:Rare Earth) superconducting tapes produced by an ion beam assisted deposition method and a pulsed laser deposition method. The total length of REBCO tape was 400 m per a field coil. We confirmed the good transport properties of the respective field coils up to 80 A at 40 to 50 K before and also after the assembly to a motor. The maximum magnetic field in the field coils was 0.67 T. The superconducting rotor was cooled down to around 40 K by forced-flowed helium gas and the completed motor was tested in a similar way to conventional motors. As a result of a load test, the designed operation of 7.5 kW at 360 rpm was verified for the rated field current of 60 A. Even when the field current was increased up to 70 A at 40 K, the temperature at each part of the superconducting rotor was held constant and the quite stable operation up to 11 kW at 360 rpm was also achieved.
    IEEE Transactions on Applied Superconductivity 07/2009; · 1.20 Impact Factor
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    ABSTRACT: Intensive research work has been carried out in order to develop industrially available HTS REBCO-coated conductors under the NEDO project in Japan. Recently, several groups in the project succeeded in the development of high performance coated conductors. Their characteristic features have been evaluated in terms of mechanical properties and their influence on critical currents. The mechanical properties at RT and 77 K were analyzed on the basis of the rule of mixtures. The force-free strain (Aff) was analytically deduced, which indicates the strain at which the residual stress exerted on the superconducting layer becomes zero. Tensile strain dependence on critical currents could be divided into elastic and brittle regions. The reversible strain limit (Arev) was defined as a strain at which the critical current recovers elastically to the level of 99% Ico. Within the elastic region, the critical current showed a convex strain dependence, which is explained as Ekin's intrinsic strain effect. The degradation beyond the reversible strain limit was attributed to a fracture of the superconducting layer. As a whole, the present study made clear quantitatively the tensile strain behavior of critical currents and proposed a reasonable definition for the reversible strain limit.
    Superconductor Science and Technology 01/2009; 22(2):025015. · 2.76 Impact Factor
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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 01/2009; 469:1726-1732. · 0.72 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 01/2009; 469:1450-1453. · 0.72 Impact Factor
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    ABSTRACT: One of the programs in the Ministry of Economy, Trade and Industry regarding R&D for developing YBCO conductors is to evaluate the suitability of the conductors in several applications. This paper focuses on one of the expected power applications, namely, a fault current limiter (FCL). YBCO tape conductors with ion beam assisted deposition (IBAD) substrates are used in this work. In order to increase the resistivity of the conductor, which is preferable for FCL applications, the thickness of a protective layer made of silver was decreased as much as possible. After obtaining the required current limiting performance in short sample experiments, a model coil was developed aiming at 6.6 kV-class FCLs. Short circuit experiments were conducted with a short-circuit generator. The coil successfully suppressed a short-circuit current of over 1.4 kA to about 500 A under an applied voltage of 3.8 kV, which is the nominal phase-to-ground voltage. The coil also suppressed a short-circuit current of 17 kA down to 700 A. The experimental results are as expected and show promise toward FCL applications.
    Journal of Physics Conference Series 03/2008; 97(1):012308.
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    ABSTRACT: We fabricated a long-length GdBCO(GdBa2Cu3O7-δ) coated conductor by the ion-beam assisted deposition (IBAD) and the pulsed laser deposition (PLD) methods. The GdBCO tape was over 400 m in length with 1.5-2.0 MA/cm2 of the critical density. The critical currents of the tape were over "304.8 A/cm" along 368 m in length. So we have achieved "Ic × L = 112,116 Am". For the IBAD process, we have already developed 500 m-length IBAD-Gd2Zr2O7 (IBAD-GZO) tapes with highly aligned in-plane texture, and continuously fabricate 250 m-length tapes at a rate of 0.5~1 km/month. In addition, we have also developed a new type template for the IBAD-GZO, to enable the production rate fast. Structure of the template is IBAD-GZO/IBAD-MgO/Y2O3. It is note that MgO (111) is parallel to the substrate normal and three fold symmetry. For this structure, the film thickness of the IBAD-GZO could lessen from 1.2 μm to 0.3 μm. By the new IBAD buffer layer, a production rate is expected to be over 30 m/h.
    Journal of Physics Conference Series 03/2008; 97(1):012273.
  • Source
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    ABSTRACT: One of the programs in the Ministry of Economy and Trade and Industry (METI) project regarding R & D on YBCO conductor is to evaluate the applicability of the developed conductor toward several applications. This paper focuses on a fault current limiter (FCL) as one of the expected power applications. YBCO tape conductors with ion beam assisted deposition (IBAD) substrate are used in this work. In order to obtain high resistance of the conductor, which is preferable to an FCL, the thickness of the protecting layer made of silver was decreased as possible. Then high-resistive metal stabilizing layer is attached on the silver layer to improve stability. Obtaining the relevant current limiting performance on short sample experiments, model coils were developed to aim the 6.6 kV-class FCL. Short circuit experiments were implemented with a short circuit generator. The coil successfully restricted the short circuit current over 17 kA to about 700 A by the applied voltage of 3.8 kV, which is nominal phase-to-ground voltage. The experimental results show good agreement with computer analyses and show promising toward the application.
    Physica C Superconductivity 01/2008; 468:2046-2049. · 0.72 Impact Factor
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    ABSTRACT: Four 5 mm wide YBCO conductors were assembled face to face to obtain a stacked YBCO conductor (YBCO stack). The space between the conductors composing the YBCO stack was set at 1.0 mm to investigate the influence of the space on AC loss characteristics. The magnetization losses without transport current, the transport losses without external magnetic field, and total AC losses of the YBCO stack carrying various transport currents in transverse magnetic fields with various orientations were measured. The measured AC losses in the YBCO stack were compared with those in a 0.2 mm space YBCO stack composed of the same conductors used in the 1.0 mm space YBCO stack as well as a single YBCO conductor. Magnetization loss without transport current in a YBCO stack decreases with decreasing space between the conductors in low and intermediate magnetic field. Transport loss without external magnetic field in a YBCO stack increases with decreasing space between conductors. There are three characterized regions, when comparing the total AC losses in the two YBCO stacks with different space between the conductors composing the stacks: (a) in the low magnetic field region, the total AC losses in the YBCO stack with smaller space are larger than those in the YBCO stack with larger space, (b) in the intermediate magnetic field region, the total AC losses in the YBCO stack with smaller space are larger than those in the YBCO stack with larger space, (c) in the high magnetic field region, the total AC losses in both YBCO stacks agree well with those in the single YBCO conductor as well as Brandt and Indembom's analytical values.
    Superconductor Science and Technology 12/2007; 21(1):015020. · 2.76 Impact Factor
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    ABSTRACT: Dependence of AC transport current losses on bending strains of two kinds of YBCO coated conductors (CCs) manufactured by IBAD/MOCVD and IBAD/PLD processes were investigated. The bending strains were applied to CCs samples by mounting the samples on U-shape sample holders consisted of straight and half-cylindrical sections. The bending strains to the superconductor layer were in tensile and compressive directions. The experimental results showed that the bending dependence of the AC transport current loss could be characterized by the amount of the deteriorations of the conductor critical currents dependent on structure changes of the superconductor layers due to the bending strains, regardless of the CCs manufacturing processes and the directions of the strains.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor
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    ABSTRACT: We developed a 15 kW synchronous motor with a fixed superconducting field winding. It was 8-pole type. Each field coil was a racetrack-shaped single pancake wound with a YBCO superconducting tape. It was fabricated by IBAD-PLD method and 10 mm in width. The rotating armature was composed of a copper winding and an iron core. The field coils were conduction-cooled down to 20 to 30 K through a copper pipe attached to the copper coil flange, in which helium gas was forced-flowed. The completed motor was first tested in the Suzuka factory of Japan motor and Generator Co. Ltd. before shipping as usual. Then it was moved to the test site of the Nagasaki shipyard of Mitsubishi Heavy Industry Ltd. and installed into a ship propulsion test system, which had a propeller with a diameter of 0.5 m. We verified the quite stable underwater operation and the output power of 15 kW-360 rpm as designed.
    IEEE Transactions on Applied Superconductivity 07/2007; · 1.20 Impact Factor

Publication Stats

447 Citations
69.36 Total Impact Points

Institutions

  • 1999–2011
    • Fujikura Ltd.
      Edo, Tōkyō, Japan
  • 2003–2009
    • Kyushu University
      • Graduate School of Information Science and Electrical Engineering
      Hukuoka, Fukuoka, Japan
    • Japan Fine Ceramics Center
      Nagoya, Aichi, 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
  • 2002–2003
    • Kyushu Institute of Technology
      • Faculty of Computer Science & Systems Engineering
      Edo, Tōkyō, Japan
  • 2001
    • International Superconductivity Technology Center
      Sakado, Saitama, Japan