FIELD ION MICROSCOPY AND ATOM PROBE MICROANALYSIS OF Nb3 Sn WIRES

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FIELD ION MICROSCOPY AND ATOM PROBE MICROANALYSIS OF Nb3 Sn WIRES

Jérôme Rose, M. Goringe, G. Smith, A. Moore

To cite this version:

Jérôme Rose, M. Goringe, G. Smith, A. Moore. FIELD ION MICROSCOPY AND ATOM PROBE MICROANALYSIS OF Nb3 Sn WIRES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-281- C2-285. �10.1051/jphyscol:1986242�. �jpa-00225676�

FIELD ION MICROSCOPY AND ATOM PROBE MICROANALYSIS OF Nb,Sn W I R E S

J . D . ROSE, M . J . GORINGE, G.D.W. SMITH and A.J.W. MOORE*

Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, GB-Oxford OX1 3PH, Great Britain

A b s t r a c t

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Nb3Sn composite w i r e s a r e used e x t e n s i v e l y i n s u p e r c o n d u c t i n g a p p l i c a t i o n s . F a b r i c a t i o n i s by a method c a l l e d t h e "Bronze Process". T h i s i n v o l v e s t h e i n s e r t i o n of niobium r o d s i n t o h o l e s d r i l l e d i n a bronze block. T h i s composite i s t h e n swaged and drawn, rebundled and f u r t h e r reduced t o a f i n a l f i l a m e n t d i a m e t e r of 2 t o 5pm, t h e f i n a l w i r e having a n o v e r a l l d i a m e t e r of about lmm and c o n t a i n i n g around 6000 i n d i v i d u a l f i l a m e n t s . T h i s i s t h e n formed i n t o i t s f i n a l s h a p e and p a r t i a l l y r e a c t e d , a t 700°C f o r 2 t o 3 days, t o produce Nb3Sn by s o l i d s t a t e d i f f u s i o n .

Recent work by Suenaga et a l . [l] and Taf t o e t a l . [ 2 ] has i n d i c a t e d t h a t t h e g r a i n boundary r e g i o n s of Nb3Sn wires c o n t a i n amounts of c o p p e r s i g n i f i c a n t l y above t h a t i n t h e g r a i n i n t e r i o r . The l e v e l of t i n i s a l s o s u g g e s t e d t o be h i g h e r i n t h e r e g i o n of t h e boundaries. Both t h e s e r e s u l t s , i f confirmed, would have i m p l i c a t i o n s f o r t h e mechanism of f l u x p i n n i n g , and a l s o f o r t h e mechanism of growth of t h e m a t e r i a l . T h i s paper r e p o r t s a f i r s t i n v e s t i g a t i o n of NbJSn i n t h e FIM and s u b s e q u e n t l y i n t h e time of f l i g h t atom probe w i t h t h e o b j e c t i v e of c l a r i f y i n g t h e c o m p o s i t i o n a t and n e a r g r a i n boundaries.

I n t h e p r o d u c t i o n of Nb3Sn f o r i t s u s u a l a p p l i c a t i o n s , t h e w i r e i s n o t f u l l y r e a c t e d , s o a s t o l e a v e a c e n t r a l c o r e of niobium f o r s t r e n g t h and s t a b i l i s a t i o n . F o r t h i s i n v e s t i g a t i o n , however, f u l l y r e a c t e d w i r e was r e q u i r e d o t h e r w i s e o n l y t h e c e n t r a l c o r e of niobium would have imaged. To a c h i e v e f u l l r e a c t i o n , t i m e s of around 28 days a t 700°C were r e q u i r e d . The p r o d u c t i o n of t i p s by s t a n d a r d FIM p r e p a r a t i o n t e c h n i q u e s [ 3 ] proved t o be i m p o s s i b l e f o r t h e s e composite w i r e s . The t e c h n i q u e developed was t o mount a Icm l e n g t h of w i r e i n a n i c k e l t u b e , and t o d i s s o l v e away some of t h e m a t r i x u s i n g d i l u t e n i t r i c a c i d . Then a 90%/10X HN03/H~

c h e m i c a l p o l i s h was used, i n c o n j u n c t i o n w i t h a m i c r o p o l i s h i n g t e c h n i q u e , s o t h a t only one of t h e f i l a m e n t s was l e f t s t a n d i n g o u t from t h e r e s t . The very f i n e d i a m e t e r of t h e w i r e s meant t h a t t h e end form of t h e p r o t r u d i n g f i l a m e n t was d i f f i c u l t t o r e s o l v e o p t i c a l l y , s o TEM was u s e d t o a s s e s s t h e q u a l i t y of t h e t i p s . P o l i s h i n g of t h e s e u l t r a f i n e w i r e s was very d i f f i c u l t , and only a l i m i t e d p r o p o r t i o n of specimens imaged. However, t h e specimens t h a t imaged s u c c e s s f u l l y began t o a p p e a r a t around 1-3kV. The p o l i s h i n g c o n d i t i o n s were such t h a t r e g i o n s

" p e r m a n e n t address : 1, S t o r y S t r e e t , P a r k v i l l e , V i c t o r i a 3052, A u s t r a l i a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986242

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f u r t h e r back down t h e t i p were t h i n n e d , o f t e n c a u s i n g premature f r a c t u r i n g , a l t h o u g h t h e specimens would u s u a l l y s t i l l image a g a i n a t a h i g h e r v o l t a g e .

The new Oxford atom probe [ 4 , 5 ] (VG S c i e n t i f i c FIM100) was used f o r t h e atom probe work. T h i s was b e c a u s e of i t s s u p e r i o r mass r e s o l u t i o n (FWHM 1/2000), compared t o

t h e o l d e r i n s t r u m e n t . The g r e a t e r r e s o l u t i o n was needed s o t h a t t h e niobium peak a t m/n=31.0 d i d n o t o b s c u r e low c o n c e n t r a t i o n s of copper ( i f p r e s e n t ) a t m/n=31.5.

3- FIELD I O N MICROSCOPY AND COMPUTER SIMULATION OF IMAGES

F i e l d i o n micrographs were r e c o r d e d w i t h neon imaging g a s a t a p r e s s u r e of about 1 0 - ~ t o r r , 50K t i p t e m p e r a t u r e and a t a v o l t a g e of about 13kV. A t y p i c a l image i s shown i n f i g - l a .

The t e x t u r e of Nb3Sn w i r e s produced by t h e "Bronze P r o c e s s " i s known t o be s t r o n g l y

4110') [6], s o t h e c e n t r a l p o l e on t h e images was p r o v i s i o n a l l y i d e n t i f i e d a s (110).

The images show a r e g u l a r i t y of r i n g s and a l s o s i g n s of a l t e r n a t e l y b r i g h t and d a r k e r r i n g s , b o t h of which a r e i n d i c a t i o n s of a n o r d e r e d phase. S t r i k i n g f e a t u r e s of t h e images a r e t h e p a r a l l e l rows of atoms l y i n g t r a n s v e r s e t o t h e zone l i n e s which r a d i a t e away from t h e c e n t r a l p o l e .

The s t r u c t u r e of Nb3Sn i s c l a s s i f i e d a s b e i n g of t h e Cr3Si type. I n t e r p r e t a t i o n of FIM images from such a complex s t r u c t u r e i s n o t s t r a i g h t f o r w a r d , and computer s i m u l a t i o n s were r e q u i r e d t o h e l p i n t h e i n d e x i n g . S i m u l a t i o n s were c a r r i e d o u t on a S i r i u s computer u s i n g a s h e l l model [ 7 ] . P a t t e r n s were g e n e r a t e d w i t h only t i n atoms imaging, f i g . 2 , o r w i t h b o t h s p e c i e s imaging, f i g . 3 . ( I n each c a s e only a q u a d r a n t o f f h e s i m u l a t e d image i s shown, w i t h t h e (100) p o l e l o c a t e d a t t h e bottom l e f t , and (101) and (011) p l a n e s a t t h e t o p l e f t and bottom r i g h t r e s p e c t i v e l y ) . The b e s t match w i t h t h e FIM micrographs was o b t a i n e d f o r both s p e c i e s imaging, but w i t h t h e t i n atoms b r i g h t e r t h a n t h e niobium atoms, fig.4. The s i m u l a t i o n s show t h a t t h e most prominent rows of t i n atoms l i e between t h e (110) and (211) p o l e s s o t h e i n d e x i n g of f i g . l b i s done i n a c c o r d a n c e w i t h t h i s assumption.

4 - MICROANALYSIS RESULTS

The atom probe a n a l y s i s of t h e m a t e r i a l was done a t 50K and 3.10-l1 t o r r w i t h a p u l s e f r a c t i o n of 20%. A n a l y s i s of t h e bulk m a t e r i a l gave a t most 1 copper i o n i n 1000, f i g . 5 , t h e o v e r a l l s t o i c h i o m e t r y c o r r e s p o n d i n g t o 78.4% niobium (+3.1%), and 21.6% t i n (+ 1.6%). Some hydrogen and oxygen i o n s were a l s o recorded, b u t it is b e l i e v e d t h a t t h e s e were a r t i f a c t s of t h e t e c h n i q u e r a t h e r t h a n b e i n g genuine f e a t u r e s of t h e specimen composition. Only one g r a i n boundary r e g i o n was found and a n a l y s e d , f i g . 6 . The composition i t gave was 7 % copper ( f 2%), 65% niobium (f 6%) and 28% t i n (f 4%) ( a l l r e s u l t s a r e t o one s t a n d a r d d e v i a t i o n ) . I n t h e m a t r i x c l o s e t o t h e boundary, t h e copper l e v e l d e c r e a s e d t o z e r o l e a v i n g 81% niobium ( + 6%) and 1 9 % t i n (+ 3%). The anomalously low v a l u e of t i n c o n c e n t r a t i o n t h a t i s o b t a i n e d from t h e m a t r i x i s probably due t o t h e d i f f e r e n t e v a p o r a t i o n p o t e n t i a l s of Nb and Sn. T h i s s u g g e s t s t h a t t h e p u l s e f r a c t i o n of 20% i s s t i l l n o t high enough t o p r e v e n t t h e D.C. s t a n d i n g v o l t a g e from removing some t i n atoms i n t h e i n t e r v a l s between t h e high v o l t a g e p u l s e s . The r e l a t i v e d i f f e r e n c e s i n t i n c o n t e n t between t h e m a t r i x and t h e boundary a r e probably s t i l l s i g n i f i c a n t , however.

5

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The p r e v i o u s work c a r r i e d o u t on g r a i n b o u n d a r i e s i n t h i s m a t e r i a l was by Auger s p e c t r o s c o p y and ALCHEMI (atom l o c a t i o n by c h a n n e l l i n g enhanced m i c r o a n a l y s i s ) , and i n d i c a t e d approximately 25% and 1.2% copper r e s p e c t i v e l y , p r e f e r e n t i a l l y a t t h e g r a i n boundaries. Bulk a n a l y s i s [ 8 ] h a s shown t h a t t h e r e i s u s u a l l y about 0.5%

copper i n t h e Nb3Sn. I f i t i s assumed t h a t a g r a i n boundary h a s a w i d t h of 2nm and t h e NbgSn h a s an average g r a i n s i z e of a b o u t 100nm, t h e n i f a l l t h e copper i s p r e f e r e n t i a l l y i n t h e boundary a b o u t 6% a t o m i c c o n c e n t r a t i o n should be found i n t h i s region. T h i s compares v e r y w e l l w i t h t h e r e s u l t s found from t h e p r e s e n t i n v e s t i g a t i o n . The work by Suenaga and J a n s e n [ l ] a l s o i n d i c a t e s a 5% h i g h e r c o n c e n t r a t i o n of t i n i n t h e g r a i n boundary r e g i o n which a l s o compares w e l l w i t h t h e r e s u l t s found i n t h i s i n v e s t i g a t i o n .

copper can Zener pin the boundaries during reaction to help maintain a small grain size. Secondly by creating a local chemical inhomogeneity, it can increase the effectiveness of the boundary as a flux pinner (as also could a locally higher concentration of tin). Controlled use of such phenomena may prove to be of value in the further development of superconducting materials.

Acknowledgements

The authors thank the SERC for provision of funds, Dr.J.A.Lee of AERE Harwell, U.K.

for suppling the composite wires, and other members of the Oxford FIM group for their assistance and advice during this project.

References

[l] Suenaga M. and Jansen W., Appl.Phys.Lett. (1983), 15.

[2] Tafto J., Suenaga M. and Welch D.O., J.Appl.Phys. 5 (1984), 4330.

[3] Brenner S.S. and Miller M-K., J.of Metals, March 1983, p55.

[4] Cerezo A., Smith G.D.W. and Waugh A.R., J.de Physique,

2

[5] Cerezo A., Godfrey T.J., Moore A.J.W. and Smith G.D.W., J.de Physique, 41

(1984), C9-329.

[6] Togano K. and Tachikawa K., J.Appl.Phys. 2 (1979), 345.

[ 7 ] Moore A. J.W., in "Field Ion Microscopy" eds J. J.Hren and S.Ranganathan, Plenum Press, New York (1968) p69.

[8] Suenaga M., "Metallurgy of Continuous Multif ilamentary A15 Superconductors", in Superconductor Materials Science, eds. S.Foner and B.B.Schwartz, Plenum Press, New York (1980), p201.

Fig.1 (a) Neon field ion micrograph of a NbgSn specimen containing a grain boundary (50K, 13kV).

(b) Crystallographic indexing of (a), showing location of grain boundary.

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Fig. 2 Computer simulation of Nb3Sn Fig.3 Computer simulation of Nb3Sn FIM image with only the tin FIM image with both tin and

atoms appearing. niobium atoms appearing

equally bright.

Fig. 4 Computer simulation of Nb3Sn FIM image in which the tin atoms appear brighter than the niobium atoms.

(1000 ions).

m/n 65

Log No. of ions

0 -

Log No. of ions

0

9 m/n 66

8 1 N L +

S""

Fig. 6a Atom probe mass spectrum of grain boundary region.

30 mln 48

*= Nb2+

"cuz+

"cu"

Fig. 6b Enlargement of fig.6a, showing resolution of copper isotopes.

cu2+

cu2+

(280 ions).