COMPOSITION, STRUCTURE AND CURRENT CARRYING CAPACITY OF Nb3Sn PHASE IN MULTIFILAMENT SUPERCONDUCTORS

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COMPOSITION, STRUCTURE AND CURRENT CARRYING CAPACITY OF Nb3Sn PHASE IN

MULTIFILAMENT SUPERCONDUCTORS

A. Nikulin, A. Shikov, A. Skvortsov, V. Sergeev, I. Davydov, V. Kovaleva, E.

Klimenko, V. Kruglov, O. Malafeeva, V. Medkov

To cite this version:

A. Nikulin, A. Shikov, A. Skvortsov, V. Sergeev, I. Davydov, et al.. COMPOSITION, STRUCTURE AND CURRENT CARRYING CAPACITY OF Nb3Sn PHASE IN MULTIFILA- MENT SUPERCONDUCTORS. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-407-C1-411.

�10.1051/jphyscol:1984183�. �jpa-00223739�

C O M P O S I T I O N , S T R U C T U R E A N D C U R R E N T C A R R Y I N G C A P A C I T Y OF N b

S n P H A S E IN M U L T I F I L A M E N T S U P E R C O N D U C T O R S

A.D. Nikulin, A.K. Shikov, A.I. Skvortsov, V.S. Sergeev, I.I. Davydov, V.A. Kovaleva, E. Yu. Klimenko*. V.S. Kruglov*, O.V. Malafeeva and V.V. Medkov

Union Research Institute of Non-Organic Materials, Moscow, U.S.S.R.

*I.V. Kurchatov Institute, 123182, Moscow, U.S.S.R.

Résumé - Nous avons étudié les procédés de formation et les propriétés caracté- ristiques de la composition et de la structure du Nb^Sn formé dans des supracon- ducteurs multifilamentaires lors d'un recuit entre 7Ô0-820°C pendant 200 h.

La capacité de transport de courant du matériau cité ci-dessus a été étudiée dans un champ < 18 T.

Il a été établi que la meilleure densité de courant critique du Nb3Sn inter- métallique dans des champs magnétiques faibles et élevés est obtenue avec des temps de recuit différents.

Abstract - The study was made into processes of formation and characteristic pro- perties of the composition and structure of NbqSn formed in multifilament super- conductors during annealing within 700-820°C for up to 200 h.

The current carrying capacity of the above materials was studied in the field of < 18 T.

It is established that the best critical current density of intermetallic Nb3Sn in low and high magnetic fields is achieved through dissimilar annealing schedules.

Introduction

The method of solid phase diffusion proved most simple and efficient for the production of NbgSn based multifilament superconductors to be used commercially.

Superconductors of this kind are most advisable for the creation of magnetic fields of the induction of 14-15 T, however, their usage in the fields of 5-8 T is also justified. The latter is related to the fact that as compared to niobium-titanium alloys Nb

Sn base superconductors in fields of 7-8 T have higher critical density of current and temperatures of transition to the normal state.

To reach the best critical currents in fields of 5-8 T the tin contents of a conductor matrix must be increased (1,2), it is advisable to increase the surface of Nb/Cu-Sn interaction through reduced sizes of niobium filaments and concurrents increase of their number (3) and, which is most important, the temperature of super- conductor heat treatment must be lowered to obtain fine-grained Nb,Sn layers that have the highest force of fluxoid pinning (4).

The studies into materials of the indicated structure in higher fields showed that growth of the magnetic field leads to a reduction of the current carrying capa- city that is so sharp, that the possibility of their usage in practice is open to ques- tion.

This paper shows that along with other important factors the heat treatment sche- dules affect the current carrying capacity of a conductor in low and high magnetic fields in a different manner.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984183

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Experimental Technique

For t h e i n v e s t i g a t i o n t h e use was made o f superconductor specimens t h a t c o n s i s t e d o f bronze m a t r i c e s o f t h e t i n c o n c e n t r a t i o n u p t o 10 % mass Sn w i t h i n c o r p o r a t e d ,7000 n i o b i u m f i l a m e n t s 2-5 mcm t h i c k t h e volume f r a c t i o n o f which was 20 %. The superconduc- t o r s were vacuum h e a t t r e a t e d a t temperatures o f 600 ... 820eC f o r < 200 h. The t h i c k n e s s o f Nb3Sn l a y e r s was determined i n a m e t a l l o g r a p h i c microscope u s i n g m e t a l l o g r a p h i c spe- cimens o f t h e superconductor c r o s s s e c t i o n s . P r e c i s i o n x - r a y s t u d i e s were c a r r i e d o u t i n an d i f f r a c t o m e t e r DRON-2M u s i n g monochromatized Cuks - r a d i a t i o n . The c r i t i c a l tem- p e r a t u r e was determined w i t h a r e s i s t i v e method from t h e m i d d l e o f t h e t r a n s i t i o n curves.

The c u r r e n t c a r r y i n g c a p a c i t y was measured a t 4.2 K i n t r a n s v e r s e magnetic f i e l d s up t o 1 8 T b y t h e s t a n d a r d technique.

Experimental R e s u l t s and D i s c u s s i o n

D u r i n g heat t r e a t m e n t o f superconductors c o n s i s t i n g o f n i o b i u m f i l a m e n t s i n a bronze m a t r i x i n t e r m e t a l l i c Nb3Sn forms a t t h e i n t e r f a c e Nb/Cu-Sn ; due t o t h e d i f f u s i o n c h a r a c t e r o f t h e i n t e r a c t i o n t h e Nb3Sn t h i c k n e s s grows w i t h an i n c r e a s e i n t h e a n n e a l i n g t e m p e r a t u r e and t i m e .

However, an i n c r e a s e d amount o f a s u p e r c o n d u c t i n g phase i n m u l t i f i l a m e n t su- perconductors does n o t n e c e s s a r i l y r e s u l t i n t h e i r i n c r e a s e d c u r r e n t c a r r y i n g capa- c i t y . Thus, f o r d i f f e r e n t l y designed conductors t h e s a l i e n t f e a t u r e i s t h a t w i t h an i n c r e a s e i n t h e t i m e o f anneal t h e c u r r e n t c a r r y i n g c a p a c i t y i n t h e f i e l d s o f 7-8 T grows a t f i r s t , reaches i t s maximum and t h e n s t a r t s f a l l i n g , a l t h o u g h t h e amount o f t h e s u p e r c o n d u c t i n g phase c o n t i n u e s t o grow ( f i g . 1) ; w i t h a r i s e i n t h e tempera- t u r e o f anneal t h o s e maxima b e i n g s h i f t e d t o t h e r e g i o n o f l e s s t i m e o f h e a t t r e a t m e n t and r e a c h i n g l o w e r v a l u e s o f t h e c r i t i c a l c u r r e n t .

F i g . 1. Dependence o f c r i t i c a l c u r r e n t i n f i e l d o f 8.3 T on t i m e o f a n n e a l i n g a t d i f f e r e n t temperatures f o r c o n d u c t o r s 0.16 mn x 1.93 mm c o n t a i n i n g 6859 f i l a m e n t s i n Cu-10 % m a t r i x .

W i t h an i n c r e a s e o f t h e m a g n e t i c f i e l d t h e d e t r i m e n t a l e f f e c t o f t h e r i s e i n t h e t e m p e r a t u r e o f a n n e a l i n g on t h e c r i t i c a l c u r r e n t i s g r a d u a l l y reduced and a f t e r t h e f i e l d s o f 12-13 T have been reached, t h e b e s t c u r r e n t c a r r y i n g c a p a c i t y i s a t t a i - ned i m m e d i a t e l y f o l l o w i n g t h e h e a t t r e a t m e n t a t a h i g h e r temperature. T h i s i s c l e a r - l y seen from f i g . 2 t h a t shows t h e dependence o f t h e c r i t i c a l c u r r e n t on t h e a p p l i e d m a g n e t i c f i e l d f o r c o n d u c t o r s 1 nm d i a c o n t a i n i n g 7225 f i l a m e n t s i n a Cu-9 % Sn m a t r i x and 24 h annealed a t 800°C and 72 h annealed a t 750°C.

I t i s i n t e r e s t i n g t o d i s c u s s t h e a p p l i e d magnetic f i e l d dependencies o f t h e

p i n n i n g f o r c e = j c . H t h a t t a k e s i n t o account t h e area o f t h e superconducting

phase across t h e c o n d u c t o r . The s i m i l a r dependencies a r e g i v e n i n f i g . 3 f o r conduc-

t o r s 1 mm d i a , c o n t a i n i n g 7225 n i o b i u m f i l a m e n t s i n a Cu-10 % Sn m a t r i x .

F i g . 3 Dependence o f p i n n i n g f o r c e on magnetic f i e l d a p p l i e d f o r conductors annealed under d i f f e r e n t c o n d i t i o n s .

As t h e a n a l y s i s o f t h e curves shows t h e h i g h e s t p i n n i n g f o r c e i n f i e l d s < 12-13 T i s reached f o l l o w i n g anneal a t 720-750°C, however, i n h i g h e r f i e l d s i t i s h i g h e r f o r a conductor as h e a t t r e a t e d a t 800°C f o r 48 hours. The d a t a p r e s e n t e d i n d i c a t e once a g a i n t h a t t h e d e t e r m i n i n g p a r t i n t h e c u r r e n t - c a r r y i n g c a p a c i t y i s p r i m a r i l y p l a y e d n o t by t h e amount b u t t h e q u a l i t y o f a superconducting phase, i . e . , i t s s t r u c t u r e and c o m p o s i t i o n .

E a r l i e r we s t u d i e d ( 3 ) t h e i n f l u e n c e o f t e m p e r a t u r e and d u r a t i o n o f h e a t t r e - atment on t h e s i z e o f i n t e r m e t a l l i c Nb3Sn g r a i n . An i n c r e a s e i n t h e t i m e o f anneal .from 12 t o 200 hours a t 600°C was shown t o l e a d t o an i n c r e a s e i n t h e average g r a i n s i z e f r o m 0.05 t o 0.09 mcm. A r i s e i n t h e t e m p e r a t u r e o f a 10 h anneal f r o m 600 t o 8 0 0 ~ ~ r e s u l t e d i n an i n c r e a s e o f t h e average g r a i n s i z e from 0.05 t o 0.17 mcm.

I n v e s t i g a t i o n s i n t o t h e i n f l u e n c e o f heat t r e a t m e n t schedules on t h e c o m p o s i t i o n o f t h e generated Nb3Sn l a y e r s showed t h a t none o f t h e anneal schedules p e r m i t s s t o i - c h i o m e t r i c i n t e r m e t a l l i d e t o be produced. However, w i t h a r i s e o f t h e anneal tempe- r a t u r e t h e t i n c o n t e n t o f a l a y e r i s i n c r e a s e d , t h e l a t t i c e parameter grows and t h e composiiion o f a s u p e r c o n d u c t i n g phase approaches t h e s t o i c h i o m e t r i c one ( f i g . 4a).

The dependence o f a l a t t i c e parameter on t h e t i m e o f anneal i s more c o m p l i c a t e d

( f i g . 4 b ) . W i t h l o w h o l d i n g t h e l a t t i c e parameter i s e n l a r g e d ( t h e l a y e r i s more

s t o i c h i o m e t r i c ) , t h e n t h e t i n c o n c e n t r a t i o n o f Nb3Sn i s somewhat reduced and w i t h

t h e f u r t h e r i n c r e a s e o f h o l d i n g t h e c o m p o s i t i o n o f l a y e r i s observed t o approach t h e

s t o i c h i o m e t r i c one.

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Fig. 4 Dependence o f l a t t i c e parameter and c r i t i c a l temperature o f Nb3Sn l a y e r s on temperature ( a ) and time ( b ) o f formation f o r conductors 1 mm d i a containing 7225 filaments i n Cu-10 % Sn matrix.

I t should be pointed o u t , t h a t increased temperature and time of annealing lead t o t h e e q u i l a z a t i o n of t h e i n t e r m e t a l l i d e composition across t h e l a y e r and hence, t h e reduction of t h e t i n concentration g r a d i e n t , t h e value o f which i s t h e highest a t low anneal temperatures.

The values of t h e c r i t i c a l temperature ( f i g . 4) c o r r e l a t e well with t h e charac- t e r i s t i c s of t h e Nb3Sn l a y e r composition, although t h e comparison between those dependencies on time and temperature of anneal shows d i f f e r e n c e s in some d e t a i l s . However, i t i s important t h a t a r i s e in t h e anneal temperature permits t h e pro- duction of Nb3Sn l a y e r s of t h e composition t h a t i s c l o s e r t o t h e s t o i c h i o m e t r i c one and, hence, having a higher c r i t i c a l temperature, and, hence, a higher upper c r i t i c a l f i e l d .

Thus, s i n c e a lower temperature of anneal provides f o r a fine-grained Nb3Sn l a y e r , and i t s c r i t i c a l temperature and hence i t s upper c r i t i c a l f i e l d a r e not a t t h e i r maxima following t h e heat treatment o f t h i s kind, t h e s t r u c t u r e o f Nb3Sn should be considered t o be o f p r e v a i l i n g s i g n i f i c a n c e in providing t h e c u r r e n t carrying c a p a c i t y in t h e f i e l d s of 7-8 T . I t i s t h e fine-grained s t r u c t u r e , t h a t permits having a l a r g e r number of pinning c e n t r e s in a Nb3Sn l a y e r , grain bounda- r i e s a r e c e n t e r s of pinning. The higher i s t h e magnetic f i e l d in which t h e c r i t i c a l c u r r e n t of a conductor i s measured, t h e g r a t e r appears t o be t h e s i g n i f i c a n c e of t h e c r i t i c a l temperature and upper c r i t i c a l f i e l d of Nb3Sn f o r t h e c u r r e n t carrying capacity. Since t h e s e c h a r a c t e r i s t i c s of a Nb3Sn l a y e r a r e a t t h e i r maximum a f t e r i t s anneal a t a higher temperature, i n f i e l d s higher than 12-13 T t h e highest cur- r e n t p r o p e r t i e s w i l l be reached p r e c i s e l y subsequent t o such an anneal d e s p i t e t h e l a r g e r grain s i z e of t h e i n t e r m e t a l l i d e . I t i s q u i t e possible t h a t t h e c u r r e n t car- rying capacity of t h e f i n e grained Nb3Sn l a y e r s of t h e composition c l o s e t o t h e s t o i c h i o m e t r i c one w i l l be s t i l l higher i n t h e f i e l d s of 12-13 T.

In (5,6) i t i s shown t h a t t h e c u r r e n t c a r r y i n g c a p a c i t y i n high f i e l d s can be increased through a1 loying elements introduced i n t o matrix and strand material

However, conclusions about heat treatment schedules a r e l i k e l y t o be v a l i d f o r a1 loyed superconductors a s we1 1 .

I t should be a l s o pointed out t h a t t h e value of t h e c r i t i c a l temperature and

t h e upper c r i t i c a l f i e l d of Nb3Sn l a y e r s i s a l s o influenced by compressive s t r e s s e s

t o t h o s e s t r e s s e s and t h e r e f o r e a l s o have h i g h e r c u r r e n t c a r r y i n g c a p a c i t y i n h i g h m a g n e t i c f i e l d s .

Thus, t o a c h i e v e t h e b e s t c u r r e n t p r o p e r t i e s i t i s a d v i s a b l e f o r each range o f f i e l d s t o use s p e c i f i c superconductors t h a t d i f f e r i n design, a l l o y i n g element c o n t e n t and h e a t t r e a t m e n t schedul es .

The o p t i m i z a t i o n o f t h e s e t o f t h e s e v a r i a b l e s w i l l make i t p o s s i b l e t o have Nb3Sn base superconductors w i t h t h e p r o p e r t i e s r e q u i r e d f o r t h e p r o d u c t i o n o f h i g h e r f i e l d s t h a n have been reached u s i n g t h e m a t e r i a l s considered.

The a u t h o r s a r e g r a t e f u l t o N. I. Kozlenkova and N.G. V a s i l i e v f o r t h e measurement o f t h e c r i t i c a l temperature.

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6. H. Sekine, X . Tachi kava, Y . Iwasa. "Improvements o f c u r r e n t - c a r r y i n g o f t h e composite-processed Nb3Sn i n h i g h t magnetic f i e l ' d s . Appl. Phys. L e t t . , v.35, N 6, 1979, p. 472-473.

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