High-rate sputter deposition and heat treatment of thick Nb-Al-Ge and Nb-Al superconductors
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deposition v a r i a b l e s included deposition t e m p e r a t u r e s from - 196~ to 750~ and s u b s t r a t e b i a s pot e n t i a l s f r o m 0 t o - 75 V. Deposit heat t r e a t m e n t t e m p e r a t u r e s w e r e from 300~ to 1240~ for t i m e s from 1 h to 20 d a y s . Nb-A1-Ge and Nb-A1 a r e of p a r t i c u l a r i n t e r e s t for f a b r i c a t i o n by high r a t e sputtering because they have v e r y high r e p o r t e d c r i t i c a l t e m p e r a t u r e s (20.7 K (Ref. 3) and 18.8 K (Ref. 4)) and v e r y high c r i t i c a l fields (410 kOe (Refs. 5, 6) and 295 kOe (Ref. 5)), but c r i t i c a l c u r r e n t s (except for thin s p u t t e r e d films) have been disappointingly low. M o r e o v e r , t h e r e have been s h o r t comings in the methods used so far to f a b r i c a t e NbA1-Ge and Nb-AI superconducting compounds into u s e ful p r o d u c t s . F u r t h e r i n t e r e s t in s p u t t e r - d e p o s i t e d s u p e r c o n d u c t o r s containing g e r m a n i u m was g e n e r a t e d by the r e c e n t d i s c o v e r y of the m a t e r i a l with the highest known c r i t i c a l t e m p e r a t u r e ; 23.2 K (Ref. 7) for s p u t t e r - d e p o s i t e d Nb3Ge. High field s u p e r c o n d u c t o r s as a whole a r e inherently difficult to f a b r i c a t e because they a r e e x t r e m e l y b r i t t l e i n t e r m e t a l l i c compaunds. F u r t h e r m o r e , the diffusion methods used to make c o m m e r c i a l high-field s u p e r c o n d u c t o r s , e.g., Nb3Sn, do not s e e m to be s a t i s f a c t o r y for making Nb-A1-Ge or Nb-A1. Methods used so f a r to make Nb-A1-Ge and Nb-A1 s u p e r c o n d u c t o r s include a r c melting, 4'7'8 coating and heat t r e a t m e n t to allow diffusion and chemical r e a c t i o n , %1~ heat t r e a t m e n t of p r e s s e d powders, S swaging and r o l l i n g powders in tubes and heat t r e a t m e n t , 11,1~ c o e v a p o r a VOLUME 7A, SEPTEMBER 1976-1375
tion of the e l e m e n t s , ~3 r f o r de diode c o s p u t t e r e d t a r g e t s of the e l e m e n t s 3,14,15 o r a l l o y s ~5 to m a k e thin f i l m s , and r f s p u t t e r i n g f r o m c a s t o r s i n t e r e d e l e m e n t a l powder t a r g e t s to m a k e thin s u p e r c o n d u c t i n g films.16-21 The n a t u r e of s p u t t e r d e p o s i t i o n p e r m i t s m e t a s t a b l e s o l i d s o l u t i o n s of n e a r l y any c o m p o s i t i o n to be m a d e , w h i c h a p p e a r e d t o a l l o w p r e p a r a t i o n of N b - A I - G e and Nb-A1 s u p e r c o n d u c t o r s with i m p r o v e d s u p e r c o n d u c t i n g p r o p e r t i e s . F o r e x a m p l e , N b - A I - G e and Nb-A1 s u p e r c o n d u c t o r s having t h e h i g h e s t c r i t i c a l t e m p e r a t u r e s at p r e s e n t c o n s i s t of h e a t - t r e a t e d , m e t a s t a b l e s o l i d s o l u t i o n s f o r m e d by quenching f r o m high t e m p e r a t u r e s . 22'23 Furthermore, solid solution compositions predicted
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