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A. M. STEWART,

AND

MICHAEL

R. DANIEL

Nb3Ga l a y e r s can be s y n t h e s i z e d by diffusion f r o m A g - G a a l l o y s at 1100~ T h i s is cons i s t e n t with the h y p o t h e s i s that the o r i e n t a t i o n of t w o - p h a s e t i e l i n e s in a t e r n a r y s y s t e m , s u c h a s A g - N b - G a , p l a y an i m p o r t a n t r o l e in d e t e r m i n i n g w h e t h e r s u p e r c o n d u c t i n g l a y e r f o r m a t i o n will o c c u r . Although the s u p e r c o n d u c t i n g t r a n s i t i o n o n s e t was 12.3 K, the Nb3Ga l a y e r growth r a t e s in this s y s t e m a r e too slow f o r p r a c t i c a l a p p l i c a t i o n . C h e m i c a l m o d i f i c a t i o n of the p h a s e d i a g r a m h o w e v e r d o e s a p p e a r to be a f e a s i b l e a p p r o a c h for p r o m o t i n g the diffusion s y n t h e s i s of A15 p h a s e l a y e r s f o r m u l t i f i l a m e n t a r y c o n d u c t o r s .

S O L I D s t a t e s y n t h e s i s is a key s t e p in the f a b r i c a t i o n of high T c m u l t i f i l a m e n t a r y s u p e r c o n d u c t o r s . Our r e c e n t s t u d i e s of the C u - N b - G e and C u - N b - S n s y s t e m s I s u g g e s t that p h a s e d i a g r a m g e o m e t r y p l a y s an i m p o r t a n t r o l e in d e t e r m i n i n g w h e t h e r s u p e r c o n d u c t ing/3 p h a s e (A15 s t r u c t u r e ) l a y e r s can be m a d e by the b r o n z e diffusion t e c h n i q u e } B r i e f l y , we found that the fl p h a s e f o r m e d in b r o n z e - N b diffusion c o u p l e s so long a s the t i e l i n e s e q u e n c e b r o n z e - f l - N b was s t a b l e in the r e s p e c t i v e t e r n a r y s y s t e m . If, h o w e v e r , no t w o - p h a s e t i e l i n e s connected the b r o n z e and/3 p h a s e s , the diffus i o n p a t h followed an a l t e r n a t e r o u t e t h r o u g h o t h e r p h a s e f i e l d s in the s y s t e m , and the /3 p h a s e did not a p p e a r in the diffusion couple. We s u g g e s t e d that c h e m i c a l m o d i f i c a t i o n o r r e p l a c e m e n t of the b r o n z e with a n o t h e r e l e m e n t could f a v o r a b l y modify the p h a s e r e l a tions to p r o m o t e the e x i s t e n c e of the r e q u i r e d t i e l i n e s . T h e a p p r o a c h , in fact, d o e s work: /3-Nb3Ga l a y e r s f o r m by d i f f u s i o n when A g is s u b s t i t u t e d for Cu in the C u - N b - G a s y s t e m . The p h a s e r e l a t i o n s and diffusion l a y e r f o r m a t i o n in the A g - N b - G a s y s t e m at l l 0 0 ~ a r e d e s c r i b e d below. E X P E R I M E N T A L PROCEDURE

cared by d r a w i n g Nb c y l i n d e r s in which the A g - G a a l l o y s had b e e n i m b e d d e d . P h a s e i d e n t i f i c a t i o n s w e r e m a d e by s t a n d a r d m e t a l l o g r a p h i c , e l e c t r o n m i c r o p r o b e , and D e b y e - S c h e r r e r X - r a y d i f f r a c t i o n (Ni f i l t e r e d , Cuka r a d i a t i o n ) a n a l y s e s