K. Togano

National Institute for Materials Science, Tsukuba, Ibaraki, Japan

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Publications (411)583.67 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: MgB2 is expected to operate in heliumfree condition to replace the practical metal superconducting wires in liquid helium condition. But the critical current properties for MgB2 wires are lower for largescale applications at present. Internal Mg diffusion(IMD) method is promising to fabricate high performance MgB2 wires. Recently, we fabricated high Jc thin MgB2 wires by applying IMD method and using 4Ccoated high quality B powder. During the heat treatment most of the B layer was reacted with Mg to form MgB2. Highest Jc of 1100 A/mm2 and 760 A/mm2 were obtained at (4.2 K, 10 T) and (20 K, 5 T), respectively for the wire fabricated with the Ccoated B powder. Engineering Jc( Je) was higher than 100 A/mm2 at 4.2 K and 10 T. These values of Jc and Je are higher than those of wellknown SiCadded wire. SiC addition brings about Mg2Si precipitates which act as barriers of superconducting currents and decrease Jc, while the wire fabricated with Ccoated B powder contains no such precipitates. Among many iron arsenide superconductors, Kdoped BaFe2As2(Ba122) and SrFe2As2(Sr122) are the most interesting superconductors for high field magnet applications due to their high Bc2 of over 50 T and relatively small anisotropy.We fabricated Ba(Sr)122 tapes by applying ex situ powderintube(PIT) technique. We found that the uniaxial pressing brings about a dramatic improvement of transport Jc for ex situ PIT processed Ba(Sr)122 tape when it is properly combined with flat rolling and heat treatment. The pressure was changed between 0.4 GPa to 4 GPa. The increase of uniaxial pressing from 0.4 GPa to 4 GPa significantly enhanced Jc values from 210 A/mm2 to 860 A/mm2 at 4.2 K and 10 T. Microstructure analysis with SEM indicates that the packing density of Ba122 core pressed under 4 GPa is higher than that of the tape pressed under 0.4 GPa. This suggests that the packing density is one of the most important parameters that influence Jc of ex situ PIT processed Ba122 superconducting tapes.
    Journal of the Japan Institute of Metals 08/2014; 78(8):287. · 0.37 Impact Factor
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    ABSTRACT: We have previously reported that the addition of Mg powder to the B powder layer (B layer) of internalMgdiffusion (IMD)processed MgB2 wires can decrease the amount of unreacted B particles, and hence increase the critical current density (Jc). As the amount of Mg powder is increased, the diameter of the central Mg rod must be reduced in order to maintain an overall Mg:B molar ratio of 1:2, corresponding to stoichiometric MgB2. If this ratio is achieved by the Mg powder alone, then the required diameter of the Mg rod is zero, which means that the IMD process becomes the powderintube (PIT) process. A hybrid process intermediate between the IMD and PIT processes is proposed as a new approach for fabricating MgB2 wires. In the present study, the critical current and microstructure of MgB2 wires fabricated using this method are investigated. It is found that the method yields a higher engineering critical current density (Je; D Jc �MgB2 area fraction, where the MgB2 area fraction corresponds to the ratio of the MgB2 crosssectional area to the total crosssectional area of the wire) than that for either the IMD or the PIT method. Compared with the IMD method, the MgB2 layer thickness (the thickness of the MgB2 layer in the transverse cross section) is increased and the diameter of the central hole is decreased, thus increasing the MgB2 area fraction The proposed method also achieves a much higher MgB2 layer density, and thus a much higher Jc, than is possible using the PIT method. The combination of these factors leads to the enhanced Je value of MgB2 wires.
    Superconductor Science and Technology 03/2014; 27:055017. · 2.76 Impact Factor
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    ABSTRACT: We report the temperature dependence of the transport critical current density for Ag-sheathed (Sr,K)Fe2As2 superconducting wires fabricated by an ex situ powder-in-tube process. The microstructure of the core region of pressed (Sr,K)Fe2As2 superconducting wires consists of almost (Sr,K)Fe2As2 single phase and very high densification. On the other hand, the core region of flat-rolled tape contains impurity phases of unreacted Fe-As and metastable phases. The transport critical current density, Jc, at 4.2 K is ̃2.6 × 104 A cm-2 in 10 T, while Jc at 20 K is ̃1.1 × 104 A cm-2 in self-field and ̃3.3 × 103 A cm-2 at 10 T. We discuss the temperature dependence and angular dependence of the critical current density.
    Superconductor Science and Technology 01/2014; 27(2). · 2.76 Impact Factor
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    ABSTRACT: The recently discovered iron-based superconductors are potential candidates for high-field magnet applications. However, the critical current densities (Jc) of iron-based superconducting wires remain far below the level needed for practical applications. Here, we show that the transport Jc of Ba1-xKxFe2As2/Ag tapes is significantly enhanced by the combination process of cold flat rolling and uniaxial pressing. At 4.2 K, Jc exceeds the practical level of 10(5) A/cm(2) in magnetic fields up to 6 T. The Jc-H curve shows extremely small magnetic field dependence and maintains a high value of 8.6 × 10(4) A/cm(2) in 10 T. These are the highest values reported so far for iron-based superconducting wires. Hardness measurements and microstructure investigations reveal that the superior Jc in our samples is due to the high core density, more textured grains, and a change in the microcrack structure. These results indicate that iron-based superconductors are very promising for high magnetic field applications.
    Scientific Reports 01/2014; 4:4065. · 5.08 Impact Factor
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    ABSTRACT: MgB2 has a superconducting transition temperature (Tc) of 39 K, which is much higher than that for practical metallic superconductors. Thus, it is hoped that MgB2 can not only replace metallic superconductors, but can be used under liquid-helium-free conditions, for example, at temperatures of 10-20 K that can easily be achieved using cryocooling systems. However, to date, the reported critical current density (Jc) for MgB2 wires is not high enough for large-scale applications in liquid-helium-free conditions. In the present study, successful fabrication of high-performance MgB2 superconducting wires was carried out using an internal Mg diffusion (IMD) process, involving a p-dimethylbenzene (C8H10) pre-treatment of carbon-coated B powder with nanometer-sized particles. The resulting wires exhibited the highest ever Jc of 1.2 × 105 A cm-2 at 4.2 K and 10 T, and an engineering critical current density (Je) of about 1 × 104 A cm-2. Not only in 4.2 K, but also in 10 K, the Jc values for the wires fabricated in the present study are in fact higher than that for Nb-Ti wires at 4.2 K for the magnetic fields at which the measurements were carried out. At 20 K and 5 T, the Jc and Je were about 7.6 × 105 A cm-2 and 5.3 × 103 A cm-2, respectively, which are the highest values reported for MgB2 wires to date. The results of a detailed microstructural analysis suggested that the main reason for the superior electrical performance was the high density of the MgB2 layer rather than just the small grain size, and that the critical current could be further increased by suitable control of the microstructure. These high-performance IMD-processed MgB2 wires are thus promising superconductors for applications such as magnetic resonance imaging and maglev trains that can operate under liquid-helium-free conditions.
    Superconductor Science and Technology 12/2013; 26(12):5003-. · 2.76 Impact Factor
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    ABSTRACT: We report that the transport critical current density Jc of ex situ powder-in-tube (PIT) processed (Ba, K)Fe2As2 (Ba-122) tapes can be significantly enhanced by applying uniaxial cold pressing at the final stage of deformation. The tapes were prepared by packing high quality precursor powder into a Ag tube, cycles of rolling and intermediate annealing, and pressing followed by the final heat treatment for sintering. The Jc values in applied magnetic fields were increased by almost one order of magnitude compared to the tapes processed without pressing, exceeding 104 A cm-2 at 4.2 K. We achieved the highest Jc (at 4.2 K and 10 T) of 2.1×104 A cm-2 among PIT-processed Fe-based wires and tapes reported so far. The Jc-H curves measured at higher temperatures maintain small field dependence up to around 20 K, suggesting that these tapes are promising for applications at higher temperatures as well as at liquid helium temperature. The microstructure investigations reveal that there are two possible causes of the large Jc enhancement by pressing: one is densification and the other is the change of crack structure. Optimization of processing parameters such as the reduction ratio of rolling and pressing is expected to yield further Jc enhancement.
    Superconductor Science and Technology 11/2013; 26(11):5007-. · 2.76 Impact Factor
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    ABSTRACT: Internal Mg diffusion (IMD)-processed MgB2 wires are known to have a high-density MgB2 core, which obviously leads to high critical current density (Jc). In this work, we first fabricated 37-filament MgB2 wires by IMD processing, and we found that Jc for SiC-added 37-filament IMD-processed MgB2 wires achieved a value of 7.6 × 104 A cm−2 at 4.2 K and 10 T, which is higher than those of multi-filament wires with a smaller number of filaments (1 filament, 7 filaments) fabricated under the same conditions. We also investigated the critical current properties of both undoped and SiC-added 37-filament IMD-processed MgB2 wires under different magnetic fields and temperatures. The undoped 37-filament IMD-processed MgB2 wires achieved Jc value of 1.6 × 105 A cm−2 at 20 K and 3 T. The engineering critical current density (Je) achieved 2.8 × 103 A cm−2 at 20 K and 3 T. The microstructures observed by scanning electron microscopy support the view that the decrease in unreacted B particles by the short diffusion distance is mainly responsible for the high Jc obtained.
    Superconductor Science and Technology 09/2013; 26(10):105027. · 2.76 Impact Factor
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    ABSTRACT: We found that the transport Jc of the ex-situ PIT processed (Ba,K)Fe2As2 (Ba-122) wire with single Ag sheath can be significantly enhanced by repeating the combined process of rolling and heat treatment. The transport Jc (4.2 K and 10 T) of 4.4 x 103 A/cm2 (Ic =15.7 A) was obtained for a thin tape of 0.3 mm in thickness, which is the highest value reported so far for the PIT processed 122 wires and tapes with a Ag sheath and processed by a conventional route. The measurement by a hybrid magnet showed that Jc-H curve maintains very small field dependence up to the strong magnetic field of 28 T as expected from the previously reported high Hc2 value. The core of the thin tape shows dense grain structure with less cracks and voids and indicates the development of c-axis alignment, although it is still incomplete. The researches to elucidate the origin of Jc enhancement and to have further improvement of transport Jc are now ongoing. The process is simple using a Ag single sheath and, therefore, more realistic technique for long length wire production.
    Superconductor Science and Technology 02/2013; 26(6). · 2.76 Impact Factor
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    ABSTRACT: The internal Mg diffusion (IMD) process produces a MgB2 layer with higher density than that achieved with the traditional Powder-in-tube (PIT) method; this makes the IMD process an attractive one for the fabrication of superconducting MgB2 wires with higher critical current densities (Jc). We have recently shown that co-addition of SiC and some liquid aromatic hydrocarbons (toluene and dimethylbenzene) can enhance the Jc of IMD-processed mono-core MgB2 wires. In the present contribution, we discuss the Jc of the IMD-processed mono-core MgB2 wires fabricated under different conditions, viz. with pure B powder, B powder with additive-like SiC or toluene, and B powder with SiC + toluene co-addition. The results from these samples indicated that the SiC + toluene co-addition gave the best Jc value, as in the previous results. The composition and microstructure suggested that carbon substitution for boron and reductions of grain size by SiC and/or toluene additives are responsible for the Jc enhancements of IMD-processed MgB2 wires.
    Physica C Superconductivity 01/2013; 484:167–170. · 0.72 Impact Factor
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    ABSTRACT: The internal Mg diffusion (IMD) process produces a high-density MgB2 layer with high critical current properties, which makes it an attractive and promising method for fabricating MgB2 wires. We have obtained high critical current properties in our previous research. However, IMD-processed MgB2 wires can have unreacted B particles remain in the reacted layer due to the long Mg diffusion distance in the B layer during heat treatment. A reduction in the amount of unreacted B particles is expected to enhance the critical current properties. In this study, we attempted to disperse Mg powder in the B layer as an additive in order to decrease the Mg diffusion distance. We found that a 6 mol% Mg powder addition to a B layer drastically decreased the amount of unreacted B particles and enhanced the critical current density to twice the value for IMD-processed MgB2 wire with no Mg powder added. An analysis is presented that relates the microstructure to the critical current density.
    Superconductor Science and Technology 10/2012; 25(12):125014. · 2.76 Impact Factor
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    ABSTRACT: We report the temperature dependence of the transport critical current density for Ag-sheathed Ag-added (Ba, K)Fe2As2 superconducting wires fabricated by an ex situ powder-in-tube process. The microstructure of the core region of (Ba, K)Fe2As2 superconducting wires consists of the 122 phase, unreacted FeAs, and metastable phases. The transport critical current density, Jc, at 4.2 K is ~1.0 × 104 A cm−2 (Ic = 60.7 A) in self-field and ~1.1 × 103 A cm−2 (Ic = 6.6 A) in 10 T, while Jc at 20 K is ~4.3 × 103 A cm−2 (Ic = 25.8 A) in self-field and ~3.2 × 102 A cm−2 (Ic = 1.9 A) at 10 T. We discuss the temperature dependence of the critical current density. Compared to MgB2, Nb–Ti and Nb3Sn, we found that (Ba, K)Fe2As2 wires had much higher Ic retention in high magnetic fields up to 23 T.
    Superconductor Science and Technology 10/2012; 25(12):125010. · 2.76 Impact Factor
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    ABSTRACT: Soon after the discovery of Fe based superconductors, attempts of wire fabrication started using a powder-in-tube (PIT) technique, because their high transition temperature and extremely high upper critical field bring the hope of high field applications. Although the transport critical current density, JcJc, reported in the early stage was disappointingly low, it has been rapidly improved during the past one year to the orders of 104104 and 105 A/cm2 by optimizing various processing parameters of the PIT technique. This paper reports one of the works which brought such rapid progress. (Ba,K)Fe2As2 superconducting wires were fabricated by an ex situ powder-in-tube (PIT) process using a melt processed precursor material. The paper also discusses the influence of Ag addition, critical current irreversibility, critical current anisotropy of the tape sample, and finally future prospect in the development of Fe based superconducting wires.
    Solid State Communications 04/2012; 152(8):740–746. · 1.53 Impact Factor
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    ABSTRACT: Mechanical properties of MgB2 monocore superconducting wires fabricated by an internal Mg diffusion (IMD) process and a powder-in-tube (PIT) process were investigated. The uniaxial tensile strain dependence of critical current (Ic) and the transverse compressive stress dependence of Ic were investigated at 4.2 K, 10 T. The IMD-processed MgB2 wire showed a large irreversibility tensile strain limit (ϵirr = 0.67%) and transverse compressive stress tolerance of 206 MPa, whereas the PIT-processed wire had 0.51% and 160 MPa. Furthermore, the tensile strain sensitivity of Ic for the IMD-MgB2 wire was smaller than that for the PIT-MgB2 wire. Consequently, mechanical properties of IMD-processed MgB2 superconducting wires are better than those of PIT-processed wires.
    Superconductor Science and Technology 01/2012; 25(5). · 2.76 Impact Factor
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    ABSTRACT: Microstructure in a high-density MgB2 wire fabricated by an internal Mg diffusion (IMD) process has been investigated by electron microscopy imaging and analysis at different scales. In the IMD process, a pure Mg rod was used as Mg source, and nanosized SiC powders were mixed with amorphous B powders. In the case of a heat treatment at 640 °C for 1 h carried out after rolling and drawing processes, the wire has two microstructural features that degrade critical current density: uncrystallized zones composed mainly of unreacted B and SiC powders, and cracks partly filled with course Mg2Si crystals. Those cracks were formed in the uncrystallized zones as well as in crystallized MgB2 zones. It indicate that the cracks formed by the mechanical milling and drawing remain after the heat treatment.
    Physica C Superconductivity 11/2011; 471:1137-1141. · 0.72 Impact Factor
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    ABSTRACT: Several reaction-induced diffusion processes to fabricate high-density MgB<sub>2</sub> materials are developed, and the critical current density ( J <sub>c</sub>) has been notably enhanced. In this study, microstructure in high-density MgB<sub>2</sub> wires fabricated by an internal Mg diffusion (IMD) process has been investigated. The inner reacted region of the wire heat-treated at 640°C for 1 h shows dense polycrystalline MgB<sub>2</sub> of 20-200 nm in grain sizes. Fine MgO and Mg<sub>2</sub>Si particles of 10-30 nm in sizes are dispersed in this region. On the other hand, the outer region near the Ta sheath is composed of unreacted B and SiC powders, fine MgO particles and small voids. Sizes of voids in the IMD MgB<sub>2</sub> wire are small compared with the PIT MgB<sub>2</sub> wire. Oxidation of Mg in the IMD process forms fine dispersion of MgO which may be effective for flux pinning.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
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    ABSTRACT: We have fabricated ex situ processed MgB<sub>2</sub> round wires and tapes sheathed with various metals using chemically treated powders. The critical current density ( J <sub>c</sub>) properties of the as-fabricated wires and tapes are not so different, that is, independent of the sheath materials. On the other hand, the transport J <sub>c</sub> properties of the heat-treated wires and tapes strongly depend on the sheath materials. Heat treatment of those wires and tapes brings about the sintering of MgB<sub>2</sub> and slight enhancement in magnetization hysteresis when using Ta sheath, compared with Fe sheath. In contrast, the transport J <sub>c</sub> properties of the wires sheathed with Ta as barrier are drastically deteriorated by heat treatment, whereas Fe-sheathed tapes show drastic J <sub>c</sub> enhancement. Microstructural observation reveals that many cracks are generated in the MgB<sub>2</sub> core by heat treatment only for the heat treated wires sheathed with Ta as barrier. Such cracks act as obstacles for supercurrent flow.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
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    ABSTRACT: We fabricated mono-, seven-, and 19-filament MgB<sub>2</sub> composite wires by applying an internal Mg diffusion (IMD) process to Mg cores surrounded with B or B+SiC powder. A Ta or Nb tube and a Cu-Ni tube were used as the inner and outer sheaths, respectively. The wires were heated at 600-700 °C for 0.25-10 h. During the heat treatment, Mg diffused into the B layer and reacted with B to form MgB<sub>2</sub> and some impurity phases such as MgB<sub>4</sub>. The thickness of the reacted MgB<sub>2</sub> layer increased rapidly with increasing heat-treatment temperature and/or heat treatment time. The 10 mol% SiC monofilamentary wires had a critical current density Jc (calculated for the reacted layer) of over 10<sup>5</sup>A/cm<sup>2</sup> at 4.2 K and 10 T in the early stage of heat treatment at 600-640 °C, despite the critical current Ic being as low as ~ 10 A. At this stage, only the B area near the Mg core reacted with Mg to form a thin MgB<sub>2</sub> layer; a large amount of B remained unreacted. The Vickers hardness of the reacted MgB<sub>2</sub> layer in the IMD-processed wires is about 1300, which is much higher than that in powder-in-tube (PIT) processed wires. This suggests that the MgB<sub>2</sub> layer has a much higher density than the PIT-processed wire. Excellent Jc values with high Ic values can be obtained for multifilamentary wires when they were heated at ~ 640°C for 1 h. In this case, the B layer reacts almost completely with Mg to form MgB<sub>2</sub>. The seven- and 19-filament wires had Jc values of 0.7-1 × 10<sup>5</sup> A/cm<sup>2</sup> at 4.2 K and 10 T and 1.3 × 10<sup>5</sup> A/cm<sup>2</sup> at 20 K and 3 T. These high Jc values are attributable to the high-density MgB<sub>2</sub> layer produced by the diffusion method.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
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    ABSTRACT: We report large transport critical current densities observed in Ag-added (Ba,K)Fe2As2 superconducting wires prepared by an ex-situ powder-in-tube (PIT) process. The wire has a simple composite structure sheathed only by Ag. A precursor bulk material prepared by a melting process was ground into powder and put into a Ag tube. The composite was then cold worked into a wire and heat treated at 850$^\circ$C for sintering. Transport critical current densities, Jc, at 4.2 K are 1.0\times104 A/cm2 (Ic = 60.7 A) in self field and 1.1\times103 A/cm2 (Ic = 6.6 A) in 10 T. These are the highest values reported so far for the Fe-based superconducting wires.
    Applied Physics Express - APPL PHYS EXPRESS. 03/2011;
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    ABSTRACT: Superconducting materials have contributed significantly to the development of modern materials science and engineering. Specific technological solutions for their synthesis and processing helped in understanding the principles and approaches to the design, fabrication and application of many other materials. In this review, we explore the bidirectional relationship between the general and particular synthesis concepts. The analysis is mostly based on our studies where some unconventional technologies were applied to different superconductors and some other materials. These technologies include spray-frozen freeze-drying, fast pyrolysis, field-assisted sintering (or spark plasma sintering), nanoblasting, processing in high magnetic fields, methods of control of supersaturation and migration during film growth, and mechanical treatments of composite wires. The analysis provides future research directions and some key elements to define the concept of 'beautiful' technology in materials science. It also reconfirms the key position and importance of superconductors in the development of new materials and unconventional synthesis approaches.
    Science and Technology of Advanced Materials 02/2011; 12(1):013001. · 3.75 Impact Factor
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    ABSTRACT: Superconductivity, at 2-3 K, was observed in a novel phase of the ternary Li-Rh-B system. The structural phase exhibits a large noncentrosymmetric cubic unit cell with the a-parameter being within 1.208 \leq a \leq 1.215 nm. This phase is stable over a wider compositional range of LixRhBy (0.6 < x < 2, 1 < y < 2). The superconductivity, as well as the unit cell volume, is sensitive to the Li/B content but it is manifested with Tc \geq 1.8 K over a wider compositional range: the highest Tc \approx 3 K occurs for x : y \approx 0.9:1.5 with a \approx 1.209 nm. The superconducting shielding fraction of most samples is almost 80% of that of Sn. The lower critical field, Hc1(0), is ~65 Oe while the upper one, Hc2(0) is determined from extrapolation to be higher than 14 kOe. We discuss the influence of pressure on Tc and also the influence of the lack of inversion symmetry on the superconducting properties.
    Journal of the Physical Society of Japan 02/2011; 80. · 2.09 Impact Factor

Publication Stats

3k Citations
583.67 Total Impact Points

Institutions

  • 2001–2014
    • National Institute for Materials Science
      • Photocatalytic Materials Center
      Tsukuba, Ibaraki, Japan
    • National Institute of Advanced Industrial Science and Technology
      • Nanoelectronics Research Institute
      Tsukuba, Ibaraki, Japan
  • 2001–2008
    • The University of Tokyo
      • • Department of Advanced Materials Science
      • • Institute for Solid State Physics
      Edo, Tōkyō, Japan
  • 2007
    • Federal University of Rio de Janeiro
      • Instituto de Física (IF)
      Rio de Janeiro, Rio de Janeiro, Brazil
  • 2003–2006
    • Tohoku University
      • Institute for Materials Research
      Sendai, Kagoshima-ken, Japan
    • Seoul National University
      • Department of Materials Science and Engineering
      Seoul, Seoul, South Korea
    • University of Science and Technology, Beijing
      Peping, Beijing, China
  • 1988–2006
    • Tsukuba Research Institute
      Edo, Tōkyō, Japan
  • 2004–2005
    • Toshiba Corporation
      Edo, Tōkyō, Japan
  • 1998–2002
    • University of Tsukuba
      Tsukuba, Ibaraki, Japan
  • 1999–2001
    • Hitachi, Ltd.
      Edo, Tōkyō, Japan
  • 1997–1999
    • Okano Cable Co., Ltd.
      Зама, Kanagawa, Japan
  • 1994
    • Korea Institute of Machinery and Materials
      Sŏul, Seoul, South Korea
  • 1989
    • Tokai University
      Hiratuka, Kanagawa, Japan
    • National Institute of Standards and Technology
      Maryland, United States