Effect of oxygen on microstructures of high-rate sputter-deposited Nb 3 Sn superconductors
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film A15 type s u p e r c o n d u c t o r s with good s u p e r conducting p r o p e r t i e s have r e c e n t l y been made by highr a t e s p u t t e r deposition techniques. 1,2 A m i c r o s t r u c t u r a l study has a l s o been done on s p u t t e r deposited Nb3A1, d e m o n s t r a t i n g the fine, defect free g r a i n s t r u c t u r e s that can be obtained. 3 It was felt that an even finer g r a i n size could be obtained by stabilizing the g r a i n boundaries with p r e c i p i t a t e s . By adding oxygen (by way of oxides) to the sputtering t a r g e t , oxidized p a r t i c l e s could be formed in the sputter deposit, t h e r e by stabilizing the g r a i n s i z e . This p a p e r r e p o r t s on the m i c r o s t r u c t u r a l a n a l y s i s of sputter deposited NbaSn and Nb3Sn containing oxygen. The effect of oxygen on the m i c r o s t r u c t u r e f o r m e d under v a r i o u s sputt e r i n g conditions is d e m o n s t r a t e d . The superconducting t r a n s i t i o n t e m p e r a t u r e T c and its r e l a t i o n to the m i c r o structure are also reported. EXPERIMENTAL PROCEDURES The s p u t t e r deposits were p r e p a r e d by a dc t r i o d e technique d e s c r i b e d p r e v i o u s l y . 1 The s u b s t r a t e was e i t h e r s t a i n l e s s s t e e l or Nb-Ti alloy. The t h e r m a l explansion coefficient of the N b - T i n e a r l y matched that of Nb3Sn so the deposits could be annealed without c r a c k i n g . The s u b s t r a t e s could be maintained at room t e m p e r a t u r e or heated to 800~ The t a r g e t s were made by c o m p r e s s i n g a powder m i x t u r e . The t a r g e t s w e r e e i t h e r a Nb-Sn mixture for Nb3Sn d e p o s i t s or a Nb-Sn-SnO2 m i x t u r e that had a f o r m u l a of Nb3 (Sno.89Oo.H) and which contained n e a r l y 3 at. pct oxygen. The oxygen contents in the deposits, b a s e d on neutron activation a n a l y s i s v a r i e d slightly from s p e c i men to s p e c i m e n . These d e p o s i t s a r e designated as Nbs(SnO). The superconducting c r i t i c a l t e m p e r a t u r e , Tc, was m e a s u r e d by a standard four-point r e s i s t i v i t y technique. The Nb3Sn m a t e r i a l was p r e p a r e d for TEM e x a m i n a tion by f i r s t e l e c t r o l y t i c jetting with a solution of 2 pct HF, 5 pct H3SO4, m e t h y l - b u t y l alcohol followed by a final e l e c t r o l y t i c polish using the s a m e solution. The J. L. BRIMHALLand R. WANGare Staff Scientist and Senior Research Scientist, respectively,PacificNorthwest Laboratory, Battelle Memorial Institute, Richland, WA 99352. Manuscript submitted August ! 4, 1978. METALLURGICALTRANSACTIONSA
bath t e m p e r a t u r e was always 700~ the m a t e r i a l t r a n s f o r m s to a fine g r a i n e d A15 phase (Fig. 3). The g r a i n s i z e is u n i f o r m l y d i s t r i b u t e d in the r a n g e 0.1 to 0.15 g m and t h e r e is no s i g n i f i c a n t size d i f f e r e n c e b e t w e e n Nb3Sn and Nbs(SnO). The A15 g r a i n siz
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