Site:           Tokai University, Shonan Campus
                1117 Kitakaname, Hiratsuka-shi
                Kanagawa 259-12, Japan
                http://www.u-tokai.ac.jp/English/index.html
Date Visited:   4 June 1996
WTEC Attendees: R. Sokolowski (report author), 
                R.D. Blaugher, 
                J. Daley, 
                P.M. Grant, 
                H. Morishita, 
                R. Schwall
Hosts:          Professor Kyoji Tachikawa, Faculty of Engineering, 
                   Department of Material Science and Technology
                Professor Naoki Maki, Department of Electrical Engineering

BACKGROUND

The Tokai University Educational System (TUES) founded in 1942 by Dr. Shigeyoshi Matsumae has expanded into a comprehensive educational institution encompassing three universities -- Tokai University, Kyushu Tokai University, and Hokkaido Tokai University -- and a number of affiliated schools and research institutes in Japan and abroad, including junior colleges, secondary educational schools, an elementary school, and kindergartens. TUES has also established overseas hubs of activities: Tokai University European Center in Denmark, Tokai University Boarding School in Denmark, Tokai University Budo Center in Austria, and Tokai University in Honolulu, Hawaii. TUES has concluded academic exchange agreements with 16 foreign universities in 9 countries for the exchange of students and teaching and research personnel. Foreign students studying in TUES institutions number more than 400.

SUPERCONDUCTIVITY RESEARCH

Funding for research and development on superconductivity at Tokai University amounts to about ¥10 million per year (~$100,000 at $1 = ¥100), half of which comes from the university, which is privately funded, and the other half from the Ministry of Education and the Science and Technology Agency. Nearly 90% of funding is applied to conductor and materials development. External organizations collaborate with Tokai by providing use of their facilities and by fabricating conductors. Two professors and 21 students (1 doctoral candidate, 10 master's candidates, and 10 undergraduates) were working on superconductivity at the time of this WTEC visit.

The superconductivity research at Tokai University can be classified into three main categories:

1. Metallic superconductors, aimed at high field A15 superconductors and superconductors for ac use.
2. High T_c superconductors, involving the RE-123 oxides, Bi-2212 oxides, and Tl-1223 oxides. Professor Tachikawa's most significant contribution to achieving practical HTS conductors has been in the development of the diffusion process. The performance level(s) achieved in Bi-2212 is 30,000 A/cm² (4.2K, 10T) for a 0.15 cm² cross-section, and in Tl-1223 are 15,000 A/cm² (77 K, 0 T) and 1,500 A/cm² (77 K, 1.5 T) for a 0.075 cm² cross-section.
3. Versailles Project on Advanced Materials and Standards (VAMAS) international cooperation, consisting of six tasks:
   • critical current measurement in Nb₃Sn multifilamentary wires
   • ac loss measurement in Nb-Ti wires
   • upper critical field measurement in Nb-Ti wires
   • tensile measurement at 4.2 K in SUS 316 LN and YUS 170 steels
   • fracture toughness measurement at 4.2 K in the same steels
   • critical current measurement in high T_c oxide tapes.

Metallic Superconductors

The primary goal of the metallic superconductor program is to develop better A15 compounds for use in NMR applications. The most significant advance in processing techniques is the use of melt diffusion processing for preparing the intermediate compound Nb₆Sn₅, to which Ti and Ge can be added for enhanced performance and Cu can be added to reduce the optimum heat treatment temperature. Fig. Tokai.1 is a flowchart of the sample processing sequence. Nb₃Sn processed in this way exhibits improved properties over Nb₃Sn processed by the bronze route. Normal state resistivity is increased threefold (Fig. Tokai.2), and the potential of 21 tesla operation is clearly evident (Fig. Tokai.3).

Fig. Tokai.1. Flowchart of a sample processing sequence.

Fig. Tokai.2. Normal state resistivity increased threefold (H. Sekine, K. Itoh and K. Tachikawa, J. Appl. Phys., 63 [1988]:2167).

Fig. Tokai.3. Potential of 21 tesla operation.

Superconductivity for Electric Power Applications

The Nb-Ti cable developed for ac use consists of 6 elemental Nb-Ti wires around a central SUS 316 LN steel wire. Figure Tokai.4 shows the characteristics of the elemental wire and cable dimensions. The small filament diameter and the alloyed Cu matrix (Cu-2.5 without Si) contribute to favorable performance in ac application. The Si in the matrix also diffuses to the filament/matrix interface, creating a thin Si layer that provides a barrier against adverse interdiffusion, which would result in impaired performance of the superconducting filaments. Tests of this cable's ac properties were made on a 100 kVA test coil (winding ID/OD of 45 mm/111.5 mm, and height of 70.3 mm) that was wound and assembled at Furukawa and tested at CRIEPI.

Elemental Wire
Wire Diam.:         0.203 mm           Cable:              Six elem. wires
Matrix:             Cu-2.5wt% Si       Central Wire:       SUS 316 LN
Filament:           Nb-50wt% Ti        Cable OD:           0.75 mm
Filament diam.:     0.14 mm            Cable Length:       467 m
No. of filaments:   164,730
Cu ratio:           1.08
Twist pitch:        1.9 mm

Central Cu/bronze-processed Nb₃Sn wire also has submicron filament diameter, over 17,000 filaments, and an overall diameter less than 0.3 mm (Fig. Tokai.5). The addition of Ta to Nb filaments prevents their ribbon-like deformation, reduces the proximity effect, and enhances the critical current density (Fig. Tokai.6). Additional alloying of small amounts of Ge increases J_c even further and contributes to significant reductions in hysteresis loss. Wire incorporating these improvements that was made at Hitachi cable for the Super-GM project exhibits substantially smaller hysteresis losses when compared to non-alloyed Nb/Cu-5Sn (Fig. Tokai.7).

Outer diam.     0.284 mm
Fil. Diam.      0.70 µm
No. of fil.     109 x 162 = 17,658

Fig. Tokai.6. Reductions in hysteresis loss.

Core/Bronze    Nb/Cu-5Sn    Nb-0.5Ta/Cu2.7Sn-2Ge
Local Bronze
/Core Ratio    2.05         4.7
Filam. Diam.   0.70 µm      0.20 µm
No. of Filam.  17,658       75,990
Wire Diam.     0.284 mm     0.284 mm
Twist Pitch    2.3 mm       2.3 mm

High Tc Oxide Superconductors

With demonstrated success in melt diffusion processing of Nb-Sn compounds, Professor Tachikawa's group has applied diffusion processing to oxide superconductors. By applying a lower melting point layer onto a substrate having a higher melting point, an HTS layer can be formed with the attributes of shorter reaction time, greater thickness, and improved homogeneity. This technique has been applied successfully to RE-123 compounds, Bi-2212, and Tl-1223.

Figure Tokai.8 shows the required layer stoichiometries needed to form these compounds. Fig. Tokai.9 gives actual data representing the compositional variation across the various interfaces for the case of Nd-123, where the product layer is around 150 microns. Fig. Tokai.10 is a schematic of a phase diagram and the development of the reaction layer by diffusion. Processing Gd-123, which has a smaller ionic radius than Nd, results in a 300 micron 123-layer under identical processing conditions.

Fig. Tokai.8. Required layer stoichiometries needed to form RE-123, Bi-2212 and Tl-1223 compounds.

Fig. Tokai.9. Actual data representing the compositional variation across the various interfaces for the case of Nd-123.

Fig. Tokai.10. Schematic of a phase diagram and the development of the reaction layer by diffusion.

Tl-based high T_c oxides are also easily synthesized by diffusion reaction (Fig. Tokai.11) in a short reaction time. TlF substituted for Tl₂O₃ produces a dense structure and acts as an effective flux to transform the 2223 phase into the 1223 phase. Without this fluorine addition, 1223 is hardly formed, even after very long reaction times. Fluorine is lost in the reaction. In F-substituted specimens, the c-axis lattice parameter of the 1223 phase formed reaches a slightly smaller value than that reported (Fig. Tokai.12), and transport I_c at 77 K is also vastly improved (Fig. Tokai.13), even more so when the sample is cooled slowly after reaction (Fig. Tokai.14). Annealing in flowing O₂ at 600°C after reaction decreases normal state resistivity and improves transport I_c at 77K. J_c of about 15,000 A/cm² has been obtained in thick layers through F addition and post-annealing in O₂.

Fig. Tokai.11. Tl-based T_c oxides synthesized by diffusion reaction.

Fig. Tokai.12. F-substituted specimens with c-axis lattice parameter of the 1223 phase.

Fig. Tokai.13. Transport I_c at 77 K.

Fig. Tokai.14. Improved Transport I_c at 77 K when cooled slowly after reaction.

Power Generation, Storage, Transmission & Distribution

Other than the conductor-related research mentioned above which includes ac loss reduction in metallic superconductors and enhanced operating temperature and J_c enhancement in Tl-compounds there are no specific R&D applications programs in these areas of electric power technology. At this time there are no plans to conduct any research on improving the ac loss characteristics of high temperature superconductors.

Other Applications

Two application-specific research activities at Tokai University have involved using the diffusion techniques outlined above to produce HTS current leads and magnetic shielding tubes. The current leads are made of Bi-2212 and can be operated from 4.2 K to 30 K. The I_c for 3 mm diameter rods at 30 K and 0.5 T are 200 A, and the associated heat leak through 50 mm long rods of this diameter were measured to be 19 mW, which is acceptable for the available refrigeration power at 4.2 K. See Fig. Tokai.15 for the current-field characteristics of Bi-2212 produced by the diffusion process. The magnetic shielding tubes were made from Y-123. Data on the flux exclusion performance are available in Adv. Cryogenic Engineering 140(1994): 253-260.

No detailed information was available for target costs or markets; however, the expectations for HTS products in the marketplace are as follows:
5 years -- tapes and small magnets, bulk magnets, film-based devices
10 years -- fault-current limiters
20 years -- motors, generators, transmission lines, MRI, levitated trains, etc.

The principal opportunities for new magnets are for high field magnets and cryocooler refrigerated magnets with HTS displacing LTS/HTS hybrids ten years later.

Fig. Tokai.15. Current-field characteristics of Bi-2212 produced by the diffusion process.