Metallurgical considerations for optimizing the superconducting properties of Nb 3 Al
- PDF / 862,077 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 31 Downloads / 158 Views
EAGAR, AND R. M. ROSE
The e f f e c t s of h e a t t r e a t m e n t and c o m p o s i t i o n on t h e 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 t e m p e r a t u r e of NbsA1 w e r e s t u d i e d in conjunction with an X - r a y d i f f r a c t i o n i n v e s t i g a t i o n of o r d e r ing and o t h e r s t r u c t u r a l e f f e c t s . It w a s d e t e r m i n e d that an o r d e r i n g a n n e a l of 750~ f o r a t l e a s t 48 h r a c h i e v e d a m a x i m u m m i d t r a n s i t i o n T c of 18.5 K in a r c - m e t t e d s a m p l e s of NbsA1. The c h a n g e s in l a t t i c e p a r a m e t e r and o r d e r p a r a m e t e r d u r i n g h e a t t r e a t m e n t i n d i c a t e that the ~ - t u n g s t e n c r y s t a l s t r u c t u r e b e c o m e s m o r e o r d e r e d a s the m a x i m u m T c i s a p p r o a c h e d . F u r t h e r , the T c of 18.5 K could not b e r e a c h e d in NbaA1 m a t e r i a l having n o n s t o i c h i o m e t r i c c o m p o s i t i o n . In such m a t e r i a l , t h e r e w a s a d i r e c t c o r r e l a t i o n b e t w e e n i n c r e a s i n g p e r c e n t a g e of s e c o n d - p h a s e Nb2A1 p r e s e n t and d e c r e a s i n g T c a f t e r t h e m a x i m u m T c had b e e n o b t a i n e d . T h e i n f o r m a t i o n o b t a i n e d d u r i n g t h i s i n v e s t i g a t i o n l e d to t h e d e v e l o p m e n t of a s i m p l e e m p i r i c a l m o d e l to e x p l a i n the change in the T c of NbsA1. A l s o b a s e d on t h i s w o r k , a m e t h o d w a s evolved f o r p r o d u c i n g c o n d u c t o r m a t e r i a l b a s e d on t h e high T c Nb3A1 a l l o y (Appendix I).
HIGH c r i t i c a l
c u r r e n t d e n s i t y , high c r i t i c a l f i e l d , and high c r i t i c a l t r a n s i t i o n t e m p e r a t u r e s u p e r c o n d u c t o r s a r e of i n t e r e s t b e c a u s e of the obvious p r a c t i c a l e c o n o m i c b e n e f i t s t h e s e p r o p e r t i e s can m a k e to p o t e n t i a l a p p l i c a t i o n s . F o r i n s t a n c e , o p e r a t i o n at 12.0 K r a t h e r than 4.2 K b r i n g s a t h r e e f o l d i n c r e a s e in the C a r n o t e f f i c i e n c y of the r e f r i g e r a t i o n s y s t e m being u s e d . In m o s t c a s e s , the n e c e s s a r y high T c a l l o y s h a v e a b r i t t l e ~-tungsten or rock salt crystal structure, making them difficult to f a b r i c a t e into any u s e f u l c o n f i g u r a t i o n . T h e m a j o r i t y of d e v e l o p m e n t w o r k t o - d a t e h a s b e e n c o m p l e t e d u s i n g Nb3Sn. K u n z l e r et a l . ~ and Olsen e t al.2 u s e d a p o w d e r c o r e w i r e p r o c e s s in which n i o b i u m and tin p o w d e r s w e r e m i x e d and p a c k e d into a n i o b i u m tube, d r a w n into a fine w i r e and s i n t e r e d into a Nb3Sn p h a s e by h e a t i n g the w i r e at e l e v a t e d t e m p e r a t u r e s . L a t e r Hanak e t al.3 u s e d a h a l i d e v a p o r d e p o s i t i o n p r o c e s
Data Loading...
