A. Matsumoto

A. Nishimura

H. Kumakura

S. Yoshizawa

Y. Hishinuma

National Institute for Fusion Science (NIFS-Repository)

§ 18. Enhancement of Critical Current Density on Bi-2223 HTS Bulk for Current Lead Application Hishinuma, Y., Nishimura, A., Yoshizawa, S. (AMRC, Meisei Univ.), Matsumoto, A., Kumakura, H. (NIMS) Bi-2223 high critical temperature (TJ superconductor (HTS) material is suitable to apply to the current lead for large-scaled application system such as a nuclear fusion reactor, because it has high critical current density (JJ value of 104 Alcm2 order at 77.3 K and lower thermal conductivity compared with free oxygen copper. In fact, Bi-2223 oxide bulk material is used current lead for the liquid He-free cryocooler-cooled magnet system I). Bi-2223 bulk, however, has some problems to limit the application field. Typical problem is that Je value of the bulk materials is much smaller than that of the wire and tape materials. We thought that the enhancement of the Je value is urgent for the nuclear fusion reactor application. Recently, several studies have been made on the enhancement of Je property for Bi-2223 oxide bulk. Enhancement of Je values by inserted with number of silver (Ag) sheet was reported2),3). Then, we prepared the Bi-2223 sintered bulk composed with metal Ag wire, tried to improve Je property on the Bi-2223 bulk. The Bi-2223f Ag wires composite bulk was prepared hand-made by stacking alternately the calcined powder and the Ag wires of 0.2 mm in diameter. The number of composed Ag wires of 0.2 mm in diameter was fourteen, twenty-eight and forty-two. The stacked samples were molded with metal dies, and the sample size is about 5 mm in width, 22 mm in length and 2 mm in thickness. Fig. 1 shows typical cross-sectional structure of the prepared sample composing with Ag wires of 0.2 mm diameter. The prepared samples were sintered at 840°C for 50 hours in air. Microstructure of the interface region between the oxide and the Ag was observed using a Scanning Electron Microscope ............. ....... ..... .. .. •• ... _e\ O.2mm<J> 42 pieces Imm Fig. 1 Optical microscopic photograph of typical cross-sectional structure of Bi-2223/ Ag wires composite bulk 150~ __ ~ ____ ~ __ ~ ____ ~~ 1500 • 1200 ~ 900 ~ S ~ 600 .---- Transport Ic Q---- Jc 300 10 20 30 40 Number ofAg wires Fig. 2 The number of Ag wires vs. Ie; and Jc values at 4.2 K and self-field. (SEM). In evaluation of superconducting property, the transport critical current (IJ was measured at several conditions as follows; at 77.3 K and self-field, and at 4.2 K and magnetic field up to 5 tesla. Transport Ie was decided by using 1 /.1 V fcm criterion. Critical current density (Je) was obtained from transport Ie and cross-sectional area of Bi-2223 oxide expect for Ag wires. From the results of SEM observation near interface between Bi-2223 oxide and Ag wire, we found that the highly c-axis oriented and densely structured Bi-2223 plate-like grains are formed along the circumferential surface of Ag wires, and the other regions in the bulk are random and porous structure in conventional Bi-2223 oxide bulk materials. The good alignment and very dense structure occurred within 20 j.1 m from the interface between Bi-2223 oxide and Ag wire. This suggests that highly oriented and densely structured Bi-2223 plate-like grains could be formed along surface of metal Ag wire in Bi-2223 oxide bulk. In order to examine superconducting properties of Bi-2223 bulks when composed with Ag wire, the relationships between the number of Ag wires and transport Ie and Je values at 4.2 K and self-field are shown in Fig. 2. The Jc value of Bi-2223 bulk without Ag wire was obtained less than 400 Alcm2 (Ie=50A), and the Je values of sample composed with forty-two Ag wires of 0.2 mm diameter were estimated to be about 800 Alcm2 (Ie=100A). Both transport Ie and Je values are improved with increasing number of Ag wires. In the case of measurement at 77.3K, transport Ie and Je values are improved by composing with Ag wires. All Je values of the sample composed with Ag wires are higher than sample without Ag wire. The composite is the one of the effective method to improve Je property. Reference [1] Ohkura, K. eta\.: Adv. in Superconductivity VI (1994) 735. [2] Hishinuma, Y. et al. : Adv. in Supercond. XII (2000) 548. [3] Hishinuma, Y. et al. : Physica C 351 (200 I) 409. 105 

Enhancement of Critical Current Density on Bi-2223 HTS Bulk for Current Lead Application

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