ANOMALOUS INCREASES IN THE CRITICAL CURRENTS OF MULTIFILAMENTARY Nb3Sn WIRES PREPARED BY THE BRONZE ROUTE

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ANOMALOUS INCREASES IN THE CRITICAL CURRENTS OF MULTIFILAMENTARY Nb3Sn WIRES PREPARED BY THE BRONZE ROUTE

Y. Hascicek, S. Nourbakhsh, M. Goringe, P. Hudson, H. Jones

To cite this version:

Y. Hascicek, S. Nourbakhsh, M. Goringe, P. Hudson, H. Jones. ANOMALOUS INCREASES IN THE CRITICAL CURRENTS OF MULTIFILAMENTARY Nb3Sn WIRES PREPARED BY THE BRONZE ROUTE. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-395-C1-398.

�10.1051/jphyscol:1984180�. �jpa-00223736�

JOURNAL DE PHYSIQUE

Colloque C l , supplément au n° 1, Tome 45, janvier 198* page Cl-395

ANOMALOUS INCREASES IN THE C R I T I C A L CURRENTS OF MULTIFILAMENTARY Nb3Sn WIRES PREPARED BY THE BRONZE ROUTE

Y . S . H a s c i c e k , S. Nourbakhsh , M.J. G o r i n g e , P.A. Hudson and H. J o n e s

Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, Oxford 0X1 3PU, U.K.

**'Clarendon Laboratory, University of Oxford, Parks Road, Oxford 0X1 3PU, U.K.

Résumé - Plusieurs procédés de traitement thermique ont été proposés pour amé- liorer les courants critiques des fils multifilamentaires de Nb3 Sn préparés par la voie bronze. Dans ce papier nous décrivons les résultats obtenus avec des fils traicés à chaud sous argon à haute pression. Cette opération de traitement thermique améliore à la fois les propriétés mécaniques et élec- triques, en supprimant la formation de porosités de Kirkendall ce qui aug- mente la croissance des couches de Nb3 Sn. Ceci a pour effet une augmentation importante des courants critiques dans les fils.

Abstract - Several heat treatment processes have been suggested to improve the critical currents of multifilamentary Nb3Sn wires prepared by the bronze route1. In this paper we describe the results obtained on wires heat treated under high pressure argon gas. This heat treatment process improves both the mechanical2 and the electrical properties by suppressing the formation of Kirkendall3 porosity which, in turn, enhances the Nb3Sn layer growth. The latter effect results in anomalous increases in the overall critical current density of the wires.

INTRODUCTION

Over the last few decades the demand for high current and high field superconductors has grown considerably as the need for large magnetic fields (both in strength and volume) in high energy physics, fusion energy, magnetic ore separation, magnetic levitation etc. has continued to increase. At the beginning of the last decade A15 superconductors (Nb3Sn and V3Ga), fabricated into filamentary wire form following the discovery of the 'bronze process '5 achieved pre-eminence as their super- conducting properties proved to be superior at high fields to those of the ductile Nb-Ti or Nb-Zr alloy superconductors.

Improving the critical parameters (especially J ) of these wires, which are all structure sensitive6 8, has been the subject of research in many laboratories.

These investigations mainly involved varying the reaction time and temperatures1

and/or adding different third elements into the filaments or matrix in order to improve J 9'10.

In Oxford, we have investigated microstructure2'11'^lz and physical properties12

of the bronze route multifilamentary Nb3Sn wires heat treated under varying conditions in order to find appropriate processing to achieve even higher critical parameters. In addition to adopting the reaction time and temperature which results in the highest critical currents, the wires were heat treated under high pressure argon gas. This latter process enhanced the Nb3Sn layer growth and in turn resulted in anomalous increases in the overall critical current density of the wires.

The improvement is 30-40% at all fields over the highest achieved under normal (atmospheric pressure argon gas) reaction conditions.

now at the Polytechnic Institute of New York

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984180

Cl-396 JOURNAL DE PHYSIQUE

EXPERIMENTAL

The 'bronze r o u t e ' m u l t i f i l a m e n t a r y Nb3Sn w i r e s , which were s u p p l i e d by AERE _Harwell, were h e a t t r e a t e d , a f t e r being wound onto t h e J c sample h o l d e r , a t about 700DC under

3000 p . ~ . i ; ( l - ~ . s . i . ". 6.89 kPa) argon p r e s s u r e f o r v a r y i n g times. The wire and sample h o l d e r were cleaned by a p p r o p r i a t e procedures, published e l s e ~ h e r e ' ~ ' ~ ~ ,

and were s o l d e r e d f i r m l y t o g e t h e r t o withstand Lorentz f o r c e s and f a c i l i t a t e c u r r e n t t r a n s f e r . The sample h o l d e r was then clamped between t h e e l e c t r o d e s of t h e J c t e s t probe15 which was then i n s e r t e d i n t o t h e bore of t h e Clarendon Laboratory 16T super- conductive magnet16' ^17. C r i t i c a l c u r r e n t d e n s i t i e s were determined a t 0.5T

increments up t o 15T a t 4.2K. We s e l e c t e d a small c o i l c o n f i g u r a t i o n f o r t h e J C sample and t h e superconducting/normal t r a n s i t i o n was determined using a 10 1 4 h s e n s i t i v i t y c r i t e r i o n . Using t h i s measuring technique - which i s a m o d i f i c a t i o n of the t r a d i t i o n a l p r a c t i c e s developed i n Oxford f o r Jc(H) d e t e r m i n a t i o n s l 5-17

-

t e s t s d e s c r i b e d h e r e , t o g e t h e r with s e v e r a l o t h e r s , have y i e l d e d c o n s i s t e n t J c r e s u l t s 1 2 .

RESULTS AND DISCUSSIONS

Fig. 1 shows t h e v a r i a t i o n i n t h e o v e r a l l c r i t i c a l c u r r e n t d e n s i t y , J c , with r e a c t i o n time a t a r e a c t i o n temperature of 700°C. At moderate f i e l d s a maximum i s apparent whereas a t high f i e l d s Jc i n c r e a s e s continuously with r e a c t i o n time (with t h e exception of 120 h r s . r e a c t i o n which showed some sample h e a t i n g d u r i n g t e s t ) . The maximum which occurs a t moderate f i e l d s i s due t o g r a i n growth i n t h e Nb3Sn

l a y e r 1 ' 5 ' 1 8

.

I

1.27 100

.

.

_.

3 days

.

i

8 8 10

10 12 H (Tesla) 12 H Cr&)

Fig.1. Ic (Jc) v e r s u s H f o r t h e w i r e s Fig.2. I,, ( J C ) v e r s u s H f o r t h e w i r e s r e a c t e d a t 700°C f o r v a r y i n g times. r e a c t e d a t %700°C under 3000 p . s . i . The w i r e used i n t h i s i n v e s t i g a t i o n argon p r e s s u r e f o r v a r y i n g times.

i s a 5143 f i l a m e n t wire of lmm d i a - meter and i n c l u d e s s i x Cu s t a b i l i - z e r s h a l f way o u t from the c e n t r e 2 . J c i s t h e c u r r e n t d e n s i t y with r e s p e c t t o the t o t a l conductor c r o s s s e c t i o n a l a r e a . Note t h e degraded J c v a l u e s f o r t h e w i r e s r e a c t e d f o r 48 hours which was caused by an un- f o r t u n a t e mishandling of t h e J c sample j u s t before s o l d e r i n g .

Fig. 2 shows t h e v a r i a t i o n of o v e r a l l Jc w i t h H f o r wires r e a c t e d under high p r e s s u r e . The f i e l d dependence of J c i s s i m i l a r t o t h a t of Fig. 1. I n Fig. 3 t h e b e s t J c v a l u e s , o b t a i n e d under d i f f e r e n t h e a t t r e a t m e n t c o n d i t i o n s , a r e compared. The f i g u r e shows t h a t high p r e s s u r e r e a c t i o n i n c r e a s e s o v e r a l l J c very s i g n i f i c a n t l y .

M i c r o s t r u c t u r a l i n v e s t i g a t i o n s , b o t h l a y e r t h i c k n e s s measurements and d e t e r m i n a t i o n of r e s i d u a l Sn c o n t e n t i n t h e m a t r i x , showed t h a t h i g h p r e s s u r e r e a c t i o n i n c r e a s e s t h e NbgSn l a y e r g r o w t h 2 9 1 2 , and r e s u l t s i n a more u n i f o r m l a y e r , p r o b a b l y due t o s u p p r e s s i o n of v o i d f o r m a t i ~ n ~ ' ~ ~ It a l s o r e s u l t s i n t h e l o w e s t r e s i d u a l Sn c o n t e n t i n t h e m a t r i x ( s 1 . 5 a t % S n ) . The l a t t e r e f f e c t may a l s o improve t h e s t a b - i l i t y of such w i r e s ( e s p e c i a l l y w i r e s which do n o t i n c l u d e Cu s t a b i l i z e r s ) under working c o n d i t i o n s t h r o u g h t h e reduced normal s t a t e r e s i s t i v i t y o f t h e c o n d u c t o r [ J c = ( 2 0 / p ~ ) 4 where J c i s s t a b l e ~ r i t i c a l c u r r e n t , p i s normal s t a t e r e s i s t i v i t y R i s t h e d i a m e t e r of t h e w i r e and Q i s t h e h e a t f l u x from t h e s u r f a c e o f t h e c o n d u c t o r ] ^19.

0 .

0

-

100

0 145hrs65O0C

o 96 hrs 700°C 0 .

6 hrs 850°C _D I

1 .

I

F i g . 3. I,, ( J c ) v e r s u s H f o r t h e w i r e s , F i g . 4 . J h I H f v e r s u s H f o r t h e w i r e s i n h e a t t r e a t e d f o r t h e t i m e s i n d i c a t e d , Fig.1. J h i s t h e c r i t i c a l c u r r e n t which had t h e h i g h e s t Jc v a l u e a t d e n s i t y w i t h r e s p e c t t o t h e Nb3Sn

e a c h t e m p e r a t u r e . a r e a , which was d e t e r m i n e d by o t i c a l m e t a l l o g r a p h y t e c h n i q u e s 1s:

Only two l i n e s f o r 6 h r s . r e a c t i o n and 96 h r s . r e a c t i o n a r e s h o w h e r e f o r c l a r i t y , b u t t h o s e f o r 12 h r s . , 24 h r s . , 4 8 h r s . , 74 h r s . and 120 h r s . r e a c t i o n a l l f a l l between t h e s e two.

I f

F i g . 4 shows J;'H' v e r s u s H ( f o l l o w i n g KramerZO, 3, H ( 1 - aJp) = o . ~ K - ~ ( H c ~

-

f o r t h e w i r e s i n F i g . 1. The e x t r a p o l a t e d v a l u e s of Hc2 shows a s m a l l e r s p r e a d w i t h t i m e of r e a c t i o n a t t h i s r e a c t i o n t e m p e r a t u r e (700°C) t h a n a t h i g h r e a c t i o n

t e m p e r a t u r e s w h i l e K i n c r e a s e s g r a d u a l l y . The s l i g h t i n c r e a s e i n Hc2 i s p r o b a b l y due t o d e c r e a s i n g r e s i d u a l Sn i n t h e m a t r i x . High p r e s s u r e r e a c t i o n does n o t seem t o a f f e c t ~ ~ 2 ' ~ .

CONCLUSION

Improvements i n o v e r a l l J c o f 30-40% have been found f o l l o w i n g h e a t t r e a t m e n t s under h i g h p r e s s u r e a r g o n g a s a s compared w i t h s t a n d a r d h e a t t r e a t m e n t s . T h i s p r o c e s s s h o u l d r e s u l t i n even h i g h e r J c s i f combined w i t h a two s t a g e r e a c t i o n 1 .

ACKNOWLEDGEMENTS

The a u t h o r s wish t o t h a n k D r J A Lee and h i s c o l e a g u e s a t AERE Harwell f o r k i n d l y s u p p l y i n g t h e w i r e s used i n t h i s i n v e s t i g a t i o n and f o r h e l p f u l d i s c u s s i o n s . F i n a n c i a l s u p p o r t from t h e T u r k i s h M i n i s t r y of N a t i o n a l E d u c a t i o n (Y.S.H.) and SERC i s g r a t e f u l l y acknowledged,

Cl-398 JOURNAL D E PHYSIQUE

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i It is with deep regret that we report the death of Peter Hudson on 18 August 1983. His co-authors dedicate this contribution to his memory

.