Ag tape

Physica C 235-240 (1994)3405-3406 North-Holland

PHISIgA

Alloying Effect of Ag Sheath on Microstructure and Superconducting Properties of Bi-2223/AgTape J.-H. Ahn, K.-H. Ha, S.-Y. Lee, J.-W. Ko, H.-D. Kim and H. Chung Korea I n s t i t u t e of Machinery & Metals, 66 Sangnam-dong, Changwon 641-010, Korea

The e f f e c t of Cu and Pb a ddi t i on to Ag sheath on m i c r o s t r u c t u r e and superconducting p r o p e r t i e s was i n v e s t i g a t e d in Bi-2223/Ag composite . The use of Ag-Cu(2.5 and 5~ Cu) a l l o y s as sheath m at er i a l s improved oxide alignment e x h i b i t i n g good w orkabi l i t y and mechanical strength, while no marked d e t e r i o r a t i o n of superconductivity was observed.

I. INTRODUCTION

In powder-in-tube(PIT) processed Bi-2223/Ag composite, the grain alignment can be e a s i l y achived by drawing and repeated r o l l i n g processes. However, the alignment of 2223 grains occurs p r e f e r e n t i a l l y near the oxide/silver i n t e r f a c e , while the grains a t the c e n t e r of the tape are not highly aligned [1-2]. Furthermore, the o x i d e / s i l v e r i n t e r f a c e u s u a l l y becomes wavy due to a large d i f f e r e n c e in w o r k a b i l i t y between s o f t silver and relatively hard oxide, r e s u l t i n g in the diminution of Jc values. In the present work, solution hardened s i l v e r a l l o y s were used as the sheath m a t e r i a l s to minimize the waviness a t the i n t e r f a c e and also improve the alignment of oxide grains in Bi-2223/Ag composite tapes.

2. EXPERIMENTAL METHODS Bi-2223 powders with a composition of B i : P b : S r : C a : C u = l . 8 4 : 0 . 3 4 : l . 9 : 2 . 0 : 3 . 1 were prepared from mixtures of BizO3, PbO, SrC03, CaCO3 and CuO by s o l i d s t a t e processing: calcined twice a t 800°C f or 10 hr and s i n t e r e d at 835°C for 50 hr. The powders were put i nt o s l y e r or s i l v e r

a l l o y tubes of 12 mm out er and 9 mm inner diameters. The tubes were (a) pure Ag, (b) Ag-2.5(wt.%)Cu, (c) Ag-5Cu and (d) Ag-2.5Pb al l oys. The composites were drawn to a diameter of 0.8 mm and then c o l d - r o l l e d into tapes of ~ 0 . 1 mm thickness. The tapes were h e a t - t r e a t e d at 835°C for 50 hr in a i r , followed by cold pressing at 50 MPa. The process was repeated once or twice.

3. RESULTS and DISCUSSION

Workabilty of the examined composites was e x c e l l e n t except in the case of Ag-2.5Pb sheathed composite where wire drawing could not be continued due to fracturing. Vickers hardness of the sheath material increased considerably by the addi t i on of Cu, e . g . , a f t e r 9&a area reduction by drawing, Hv becomes 75, 130 and 150 for Ag, Ag-2.5Cu and Ag-5Cu, respectively. In Fig. l, interface appearance was compared for d i f f e r e n t composites, showing t hat Cu-Ag sheathed composites improve c l e a r l y the uniformity of oxide layer thickness. Such smooth oxide/metal i n t e r f a c e in the case of Cu-Ag sheath is a t t r i b u t a b l e to the increase in hardness by the al l oyi ng. Fig.2 shows X-ray d i f f r a c t i o n p a t t e r n s of 2223/Ag and 2223/Ag-Cu tapes obtained

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J-t~. Ahn et al./Physica C 235-240 (1994) 3405 3406

after third pressing-sintering. In a l l cases, the Bi-2223 is the main phase with 2212 as minor phase. No t h e r phases a r e d e t e c t e d by the a d d i t i o n o f 2 . 5 ~ 5 ~ Cu to Ag sheath. Tc v a l u e s were not much a f f e c t e d r e g i s t e r i n g 97-98K, but normal state resistivity i n c r e a s e d f o r Ag-Cu s h e a t h e d tapes. Jc v a l u e s a r e i n d i c a t e d in Fig. 3. For a l l examined samples, i t i s e v i d e n t t h a t the Jc v a l u e s i n c r e a s e d with increasing the number of pressing-sintering process. In a l l cases, Jc v a l u e s a r e roughly the same, r a n g i n g from 5000 to 6500 A/cmz a f t e r third treatment. In s p i t e o f the h i g h l y uniform m i c r o s t r u c t u r e s o f Ag-Cu s h e a t h e d tapes, the r e s u l t i n g Jc v a l u e s were s t i l l i n s u f f i c i e n t . Higher Jc v a l u e s could be possible in Ag-Cu s h e a t h e d t a p e s by employing optimal s i n t e r i n g conditions which might d i f f e r from non-doped Ag sheathed tapes.

o2223 4,2212

"-r.

2 Theta ( degree )

Fig. 2: X-ray Bi- 2223/Ag+M

d i f f r a c t i o n patterns o f tapes, ob ta ined after

s i n t e r i n g at 835°C for 50 hr.

7000

Ag Sheath

o~" " Ag-2.5Cu "

6000

/oS

5OO0

<

4000

'-" 3000

'

~Ag-5Cu "

2000

1000 0

i

i

i

1

2

3

4

N u m b e r of Heat T r e a t m e n t

Fig. 3: C r i t i c a l current d e n s i t y values o f Bi- 2223/Ag+M tapes, after repea ted p r e s s i n g - s i n t e r i n g at 835°C for 50 hr.

Fig. l : Optical microstructures o f Bi-2223 tapes using sheath o f (a ) Ag, (b ) Ag+2. 5Cu and (c ) Ag+5Cu alloys.

REFERENCES 1- Y. Yamada, Supercond. 2- M. Wilhelm, Physica C

B. Oberst and R. Flfikiger, Sci. T e c h n o l . , 4 ( 1 9 9 1 ) 165 H.W. Heuwfiller and G. Ries, (1991) 185-189