Effects of Ag-doping on critical current densities in high Tc superconducting materials of YBa2Cu3O7−x

Mat. Res. B u l l . , Vol. 24, p p . 1369-1373, 1989. P r i n t e d in the USA. 0925-5408/89 $3.00 + .0O C o p y r i g h t (c) 1989 Pergamon P r e s s ple.

E F F E C T S OF A g - D O P I N G ON C R I T I C A L C U R R E N T D E N S I T I E S IN HIGH T S U P E R C O N D U C T I N G M A T E R I A L S OF Y B a 2 C u 3 0 7 c x

B e i l i n g Shao, A n s h e n g Liu, Y i y u Z h o u , J i a n g u o Z h a n g a n d J u n s h e n g Wang, General Research I n s t i t u t e for N o n - f e r r o u s Metals, B e i j i n g , P R C

( R e c e i v e d June 29, 1989; Communicated b y H. Tu)

ABSTRACT TO e n h a n c e t h e c r i t i c a l current d e n s i t y of Y B C O superconductors the attempt to a d d Ag, A g z O , AgNOs i n t o the materials has b e e n made. N o obvious difference on m i c r o s t r u c t u r e s , s u c h as grain size, o r i e n t a tions, twins, b o u n d a r i e s , m i c r o c r a c k s , w e r e o b s e r v e d between pure and doped YBCO specimens. ALCHEMI w a s u s e d to l o c a t e the impurity atoms i n s i d e Y B C O u n i t cells. M o s t o f A g in Y B C O Ag-system doped specimens to h e f o u n d f i l l i n g in t h e s i n t e r p o r e s c o n t r i b u t e s to strengthening the m a t e r i a l s a n d p u r i f y i n g t h e Y B C O m a t r i c e s . S o m e A g + c a t i o n s e n t e r ing Y B C O l a t t i c e s a n d o c c u p y i n g the Cu z+ o r C u s• s i t e s w e r e d e t e c t e d b y A L C H E M I . T h e A g + s u b s t i t u t i o n f o r C u z+ o r C u 3÷ w i l l d e t e r i o r a t e t h e superconductivity d u e to t h e d e c r e a s e of o x y g e n absorption abilities of Y B C O c r y s t a l s . T h e t o t a l e f f e c t o f A g - d o p i n g d e p e n d s o n t h e c o m p e t i t i o n o f t h e A g d o p a n t s w i t h t w o k i n d s of d i s t r i b u t i o n s . M A T E R I A L S INDEX:

superconductors,

silver, o x i d e s

Introduction An adequate v a l u e of t h e c r i t i c a l c u r r e n t d e n s i t y Jc is i n d i s p e n s a b l e for t h e a p p l i c a t i o n o f s u p e r c o n d u c t o r s a n d t h e p r o d u c t i o n of m a t e r i a l s w i t h h i g h Jc is s t i l l p r o b l e m a t i c a l u p to now. T h e Jc v a l u e s in s i n t e r e d o x i d e s o f Y B C O which h a v e b e e n r e p o r t e d s o far a r e a t m o s t 1.1 x 104 A / c m z at B = 0 a n d T = 77k(1). Investigations on improving Jc of HTc superconductors have been done by many s c i e n t i s t s in t h e w o r l d . O n e o f t h e a t t e m p t s is d o p i n g Y B C O b u l k m a t e r i a l s w i t h m e t a l s o r m e t a l o x i d e s . M i z u t a n i e t al h a v e c o m p o u n d e d a s u p e r c o n d u c t o r o f Y B C O -30% Ag doped and declared that Ag-doping can increase the critical current d e n s i t y Jc a n d t h e s t r e n g t h of t h e s i n t e r e d Y B C O s u p e r c o n d u c t o r (2). Y ~ a m o t o has studied on YBCO-AgzO doped superconductors and pointed out that AgsOdoping has enhanced the critical current density because the Ag decomposed f r o m A g z O c o m p r e s s e s t h e Y B C O p a r t i c l e s t i g h t l y together, d e n s i f y i n g t h e Y B C O s i n t e r e d bulk, a n d a b s o r b s t h e i m p u r i t i e s in Y B C O p o w d e r (3). A d e t a i l e d s t u d y o n Y B C O - A g , A g 2 0 , AgNOs d o p e d b u l k m a t e r i a l s h a s b e e n c a r r i e d o u t in t h i s work. The microstructures of the specimens with different doped materials and vario u s d o p i n g a m o u n t s h a v e b e e n o b s e r v e d a n d a n a l y s e d . In a d d i t i o n t o M i z u t a n i ' s and Yameuaoto's s t a t e m e n t s above, w e have found some Ag ÷ cations are located

1369

1370

B. S H A O ,

e t B/.

Vol. 24, No. Ii

in the Y B C O cells and substituted for Cu z÷ or Cu s÷ cations. The Ag ÷ c a t i o n s in Y B C O c e l l s are located by an atom l o c a t i o n c h a n n e l l i n g e n h a n c e d m i c r o a n a l y sis (ALCHEMI). These Ag* c a t i o n s in Y B C O cells are d e t r i m e n t a l to Y B C O superc o n d u c t i v i t y d u e to the lower o x y g e n absorption. A g - s y s t e m - d o p i n g , therefore, has its d i s a d v a n t a g e o u s side either. Materials

and M e t h o d s

The p o w d e r with a nominal c o m p o s i t i o n of YBasCu30T was p r o d u c e d by g r i n d i n g and heat t r e a t i n g p o w d e r s of Y2Os. BaC03 and CuO in the appropriate amounts. Several a m o u n t s of Ag(2 to 4 wt%), A g 2 0 ( 2 to 4 wt%), AgN03 (i to 20 wt%) w e r e added into the Y B C O powder separately and the mixtrures were pressed and, then, s i n t e r e d at a t e m p e r a t u r e of 8 3 0 - 9 3 0 °c to form the bulk s p e c i m e n s . T h e bulk samples w e r e g r o u n d and polished m e c h a n i c a l l y and then ion m i l l e d to p r e p a r e TEM specimens. An a n a l y t i c a l e l e c t r o n m i c r o s c o p e (JEM-2000FX) e q u i p p e d w i t h an e n e r g y d i s p e r s i v e X - r a y s p e c t r o s c o p e (LINK system) was used to observe the m i c r o s t r u c t u r e s of all the Y B C O - A g d o p e d s y s t e m s p e c i m e n s and to c a r r y out the atom l o c a t i o n c h a n n e l l i n g e n h a n c e d m i c r o a n a l y s i s (ALCHEMI). In A L C H E M I i m p u r i t y atoms w e r e located by m e a s u r i n g the c h a r a c t e r i s t i c X - r a y intensities with different diffraction conditions. The p r i n c i p l e of i m p u r i t y atom site l o c a t i o n by the A L C H E M I has b e e n r e p o r t e d in r e f e r e n c e s (4). Experimental

Results

(1)No o b v i o u s d i f f e r e n c e on m i c r o s t r u c t u r e s , such as Y B C O c r y s t a l morphology, g r a i n size, orientations, twin structures, g r a i n boundaries, m i c r o c r a c k s and so on, was observed between pure Y B C O b u l k s p e c i m e n s and several Ag doped ones. In b o t h p u r e and doped s p e c i m e n s a few Y2BaCuOs grains were found to be e m b e d d e d in Y B C O g r a i n s , s e e Fig.l. (2)In the YBCO-Ag, Ag20, AgNOs doped specimens Ag was m o s t l y l o c a t e d inside pores of the s i n t e r e d specimens. EDS a n a l y s i s revealed in all the Ag-system doped s p e c i m e n s there w e r e some slnter pores filled w i t h Ag. A few of t i n y Y B C O c r y s t a l s was embeded, Fig.2, and the i m p u r i t i e s (Si, Fe--.) w e r e a b s o r b e d by Ag in pores. NO o b v i o u s Ag s e g r e g a t i o n was found on the Y B C O g r a i n b o u n d a ries far from s i n t e r pores. (3)By EDS analysis, an a m o u n t of Ag inside Y B C O g r a i n s w e r e d e t e c t e d w i t h the maximum of about 1.6 at% Ag, Fig.3(a,b). W i t h A L C H E M I the l o c a t i o n s of Ag ÷ ca-

FIG.I M o r p h o l o g y of YzBaCuOs g r a i n s e m b e d d e d in a YBazCu3OT-x g r a i n

FIG.2 Small Y B a z C u a O T - x g r a i n s e m b e d d e d in Ag in the i n t e r i o r of a s i n t e r p o r e

Vol. 24, No. i i

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a n a l y s i s r e s u l t of

t i o n s in Y B C O u n i t c e l l s w e r e studied.The incident electron beam was almost parallel to t h e (001) p l a n e o f Y B C O o r t h o r h o m b i c structure. The spectra were acquired under several chosen incident orientations, one at a nonchannell i n g c o n d i t i o n a n d o t h e r s at the a n g l e s j u s t g r e a t e r o r s m a l l e r t h a n the B r a g g angle. T h e c h a r a c t e r i s t i c X - r a y i n t e n s i t i e s of Y w e r e s e t e q u a l i n all t h e acq u i s i t i o n s . T a b l e 1 s h o w e d a s e r i e s of intensity data of a YBCO-4 wt% AgzO doped specimen using ALCHEMI with total Y-LG c o u n t o f 20000. TABLE 1 Integral Counts of Cu-Ku , Cu-K~ , Ag-Lu YBC0-4% Agz0 doped specimen Cu-Ka 1 2 3

xl0 z

385.2 368.4 363.8

Cu-K~ 52.5 51.0 49.5

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It i n d i c a t e d t h a t t h e c h a r a c t e r i s t i c i n t e n s i t i e s of A g a n d Cu i n c r e a s e d /dicreased simutaneously when the diffraction condition changed. More measurem e n t s w i t h s o m e o t h e r Y B C O - A g d o p i n g s p e c i m e n s w e r e c a r r i e d o u t a n d a s a m e increase~decrease t r e n d b e t w e e n A g a n d C u c h a r a c t e r i s t i c i n t e n s i t i e s w i t h t h e d i f f r a c t i o n c o n d i t i o n c h a n g e s w a s s h o w n for e v e r y specimen. It c o u l d be c o n c luded, t h e r e f o r e , t h a t A g ÷ c a t i o n s o c c u p i e d C u z÷ or C u 3÷ c a t i o n sites after t h e y e n t e r e d i n t o t h e Y B C O cells. (4)No d i s t i n c t d i f f e r e n c e of m i c r o s t r u c t u r e s in t h e specimens with different d o p e d m a t e r i a l s ( A g, AgzO, A g N O 3 ) w a s o b s e r v e d . It m i g h t be all A g z O a n d AgNOs d e c o m p o s e d a n d d i s t r i b u t e d in Y B C O s p e c i m e n s in t h e s a m e manner with Ag doping. F r o m t h e i n v e s t i g a t i o n s of the s p e c i m e n s d o p e d w i t h AgNOs f r o m 1 to 20 wt%, it w a s f o u n d t h a t no m o r e A g g o t i n t o Y B C O unit cells w i t h the i n c r e a s e of AgN03 a m o u n t a n d all t h e e x t r a A g e n t e r e d s i n t e r pores. In t h e 20 w t % AgNOs doped specimens, almost every pore was filled with Ag. A m o r p h o u s l a y e r s w i t h

1372

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Vol. 24, No. 11

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FIG.4 A m o r p h o u s layer of Ag,Y, Ba, Cu on a i n t e r f a c e of a sinter pore w i t h the surr o u n d i n g Y B C 0 g r a i n s (a) m o r p h o l o g y (b) EDS result from the a m o r p h o u s layer.

Ag,Y,Ba, Cu formed on the i n t e r f a c e s b e t w e e n the Y B C O g r a i n s and the Ag g r a i n s in sinter pores, w h i c h was h a r m f u l to the s u p e r c o n d u c t i v i t i e s , F i g . 4 . Discussion W i t h the e x p e r i m e n t a l results, it can be c o n c l u d e d that A g - s y s t e m d o p i n g w i l l purify the Y B C O m a t r i c e s and d e n s l f y and s t r e n g t h e n the bulk samples and, therefore, i m p r o v e the Jc, b e c a u s e the d o p e d Ag has f i l l e d the sinter pores and c o m p r e s s e d the Y B C O matrices. But it is also e v i d e n t i a l by A L C H E M I that an a m o u n t of Ag + c a t i o n s has e n t e r e d Y B C 0 c e l l s and o c c u p i e d the Cu c a t i o n sites. Obviously, the Ag + c a t i o n s s u b s t i t u t i n g for Cu c a t i o n s w i t h a v a l e n c e of two or three w i l l lower the o x y g e n a b s o r p t i o n a b i l i t i e s of YBC0 unit cells and, t h u s , d e t e r l o r a t e the s u p e r c o n d u c t i v i t i e s of the materials. F r o m all the c o n s i d e r a t i o n s m e n t i o n e d above, w e ' d like to s u g g e s t that the doping w i t h an app r o p r i a t e a m o u n t of Ag s y s t e m m a t e r i a l s is advantageous to e n h a n c e the Jc values at least for those Y B C 0 m a t e r i a l s w i t h low b u l k d e n s i t i e s and small Jc values. It has b e e n d e m o n s t r a t e d by the m e a s u r e d Jc v a l u e s of a s e r i e s YBCOAg system s p e c i m e n s w h i c h show that a m a x i m u m i n c r e a s e of Jc, a b o u t twice of the Jc v a l u e of u n d o p e d specimens, has b e e n o b t a i n e d in the 4 wt% Ag doped specimens. But the experiment phenomena that the i m p r o v e m e n t s of Jc v a l u e s from Ag d o p i n g for m u c h d e n s e r Y B C 0 m a t e r i a l s w i t h a fair Jc v a l u e ( for example, several t h o u s a n d s A / c m 2 ) are not d i s t i n c t and that the Jc of 20 wt% AgN0a d o p e d s p e c i m e n s almost drops to zero i m p l y that in some cases the Ag + c a t i o n s s u b s t i t u t i n g for Cu c a t i o n s a f f e c t the Jc d o m i n a n t l y . M o r e w o r k on A g - d o p l n g is still needed. Conclusion. (1) A g - d o p i n g d o e s not c a u s e any d l s i c t m i c r o s t r u c t u r e c h a n g e s of Y B C 0 superconducting materials. Ag d o p a n t s m a i n l y locate at the pores of the sintered b u l k s and d e n s i f y and s t r e n g t h e n the bulks and, therefore, i m p r o v e the c r i t i cal c u r r e n t d e n s i t i e s of the materials. (2) Some Ag d o p a n t s in the form of Ag ÷ c a t i o n s e n t e r into Y B C 0 u n i t c e l l s and o c c u p y the Cu c a t i o n sites and c a u s e the o x y g e n d e f i c i e n c y of Y B C 0 cells, det e r i o r a t i n g the s u p e r c o n d u c t i v i t i e s of the materials.

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YBCO S U P E R C O N D U C T O R S

1373

(3) The total e f f e c t s of A g - d o p i n g on the J¢ d e p e n d on the c o m p e t i t i o n of the Ag d o p a n t s l o c a t i n g at the s l n t e r p o r e s and the Y B C O cells. The further work on r e v e a l i n g this c o m p e t i t i o n is still needed. Acknowledgement This w o r k was s u p p o r t e d by The N a t i o n a l Superconductivity.

Center

for R e s e a r c h and D e v e l o p m e n t

on

References i. Y.Ikeno, Proc. J a p a n - U S W o r k s h o p on H i g h Field Supercon. Mater. and S t a n d a r d P r o c e d u r e s for H - F Supercon. Mater. Testing, F u k o k a (1987) 2. J . C . H . S p e n c e and J.Tafto, J. Microsc., Vol.130, 147 (1983) 3. U . M i z u t a n i , " I n c r e a s e of Jc by C o m p o s i t i n g Y B C O and Metals" N i p o n k. Superc., p.7 (1988.9) 4. T . Y a m a m o t o , " I n c r e a s e of Jc in Y S u p e r c o n d u c t i n g Wire Adding AgzO" Nipon K e i z a i S u p e r c o n d u c t o r s , p.6 (1988.10)