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INFLUENCE OF Nb4O SUBOXIDES IN THE SUBSTRATE UPON PROPERTIES OF Nb3Sn TAPE
SUPERCONDUCTOR
J. Neuschl, D. Synak, M. Jergel, I. Hlásnik, J. Ivan
To cite this version:
J. Neuschl, D. Synak, M. Jergel, I. Hlásnik, J. Ivan. INFLUENCE OF Nb4O SUBOXIDES IN THE
SUBSTRATE UPON PROPERTIES OF Nb3Sn TAPE SUPERCONDUCTOR. Journal de Physique
Colloques, 1984, 45 (C1), pp.C1-413-C1-416. �10.1051/jphyscol:1984184�. �jpa-00223740�
Colloque C l , supplément au n" 1, Tome 45, janvier 1984 page Cl-'f 13
INFLUENCE OF N b
40 SUBOXIDES IN THE SUBSTRATE UPON PROPERTIES OF N b j S n TAPE SUPERCONDUCTOR
J . Neuschl, D. Synak, M. J e r g e l * , I . Hlasnik* and J . Ivan**
Kablo k.p., 816 21 Bratislava, Czechoslovakia
*Electrotechnical Institute CEFR, Slovak Academy of Sciences, 842 39 Bratislava, Czechoslovakia
**Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, 336 06, Bratislava, Czechoslovakia
Résumé - Nous décrivons les propriétés d'un ruban supraconduc- teur de Nb
3Sn préparé par la diffusion de Sn dans un ruban de NbZr. Par un traitement thermique approprié en plus des parti- cules de Zr02 le sous-oxyde Nb
40 a été formé. En conséquence, l'épaisseur de la couche de Nb
3Sn et le courant critique ont augmenté, et les propriétés mécaniques ont été améliorées par rapport aux propriétés d'un ruban de Nb
3Sn sans Nb4Û.
A b s t r a o t - The p r o p e r t i e s o f t a p e d i f f u s i o n p r o c e s s e d Nb~Sn s u p e r o o n d u o t o r p r e p a r e d o n NbZr s u b s t r a t e a r e d e s o r i b e d . By a p p r o p r i a t e b e a t t r e a t m e n t b e s i d e s t h e ZrO. p a r t i c l e s t h e Nbj^O s u b o x i d e s a r e f o r m e d t o o . C o n s e q u e n t l y , t h e v a l u e s o f t h e Nb_Sn l a y e r t h i o k n e s s a n d t h a t o f t h e o r i t i o a l o u r r e n t h a v e i n c r e a s e d a n d sone m e c h a n i c a l p r o p e r t i e s a r e b e i n g i m p r o v e d , when compared w i t h p r o p e r t i e s o b t a i n e d on s u b s t r a t e s w i t h o u t Nbj.0 p a r t i c l e s .
I . INTRODUCTION
I t i s a l r e a d y w e l l - k n o w n t h a t h i g h J v a l u e s i n t h e d i f f u s i o n p r o c e s - s e d Nb«Sn t a p e may be o b t a i n e d b y u s i n g t h e o x i d i z e d Nb-Zr s u b s t r a t e a s w e l l a s b y a d d i n g m a i n l y Cu i n t o t h e Sn b a t h . The Z r 0
2p a r t i o l e s i n t h e s u b s t r a t e s e r v e t h e n a s new p i n n i n g o e n t r e s . As a r e s u l t , b o t h t h e e l e c t r i o a l a n d m e o h a n i c a l p r o p e r t i e s o f Nb„Sn s u p e r c o n d u c t o r may be i m p r o v e d s u b s t a n t i a l l y £1J .
I n o u r p r e v i o u s e x p e r i m e n t s u s i n g t h e p u r e Nb s u b s t r a t e \ZJ we h a v e o b s e r v e d b y SEM, t h a t t h e N b - s u b o x i d e s ( c o n t e n t o f 0
2< 0 . 2 5 w t . # ) and t h e NbO p a r t i c l e s ( c o n t e n t o f 0 _ > 0 . 2 5 w t . # ) may b e f o r m e d . As a r e s - u l t . t h e t h i c k n e s s and t h e I v a l u e s o f Nb-Sn l a y e r h a v e i n c r e a s e d i n
o j
optimal case up to 10O£. The presence of 0„ in the Nb-lattice oauses increased strength and miorohardness of substrate. In most oases the ductility is deoreased but by suitable heat treatment it may be inc- reased again from about k% - 6$ to more than 10$ and, the strength from 600 MPa - 65O MPa to about 750 MPa - 900 MPa. Plausible explana- tion is the presence of Nb-0 phases. The diffusion activation energy
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984184
CI-414 JOURNAL DE PHYSIQUE
of S n on the grain boundaries of these particles is much lower than that of the volume diffusion r37. .- 4
In the following part we tried to repeat our previous results obtain- ed on the -substrate [ 2 ] also on Nb-Zr02 substrate.
11. EXPERIMENTAL
As a tape substrate the -1.5 wt.$ Zr alloy (the O2 content being (0.01 to 0.02 ) lit,$ 02)has been used with the dimension 10 tl~ll x 0.02 mm. Samples of the substrate were externally and internally oxidised.
After that a secondary heat treatment has been applied in vacuum of 5x1 o - ~ kPa at temperatures 2 0 0 ~ ~ to ~ M O C during 5 hours. The Nb3Sn layers were prepared by hot-dipping the substrates in molten Sn-25 wt.5 Cu bath followed by heat treatment i n vacuum of kF'a at
~ O O O C
during 20 min.
Critical currents were measured in transverse magnetic field of 5 T, perpendicular to the broad side of the sample at 4.2 K. The substrate specific resistance was measured at room temperature and, for the mi- crohardness Jim measurement the test load of 10 g; was used. The value of strength woe and the W S n layer thiclmess were also measured.
3 III. RESULTS
As the investigation by SEM has shown, the substrate structure may be influenced substantially by the secondary heat treatment.
In the samples without secondary treatment globular Zr02 particles may be found i n the (A-Nb (solid solution) matrix, with diameter 3 0
n m to 200 nm. Also traces of Nb-0 particles may be seen with dimens-
ions 0.2 jam to 1 . 0 pa, M g . la. The heat treatment up to 2 5 0 ~ ~ does not cause any substantial ohange in the structure exoept that the
boundaries of Nb-0 partioles are more visible. At temperatures 2 5 0 ~ ~ to 4 0 0 ~ ~ a quantity of new partioles with dimensions 0 . 1 pm to 0.5
p may be seen, Fig, lb. We have found by selective electron diffra- ction that the particles are Nb-suboxid Nb40, which is in a-eement with Niebuhr [ 4 ] . The Nb40 cubic lattioe parameter ia a . n 0.69 nm.
The number of smallest 2rO2 particles bas deoreased and, on the other hand, the number of larger Zr02 particles, 100 nm to 200 nm i n diameter has inareased. For temperatures more than 4 0 0 ~ ~ the NbkO phase is unstable and it is decomposited. As a result, the 02-conce- ntration gradient is lowered i n the substrate. The number of larger
Zr02 particles is rising with their nonuniform distribution within
the substrate, because of their clustering on the energetioally fa-
vourable places - the Nb40 particles boundaries, Ng. 10.
Ng. 1 - Scanning electron micrograph (30.000%)
The desoribed change in the substrate structure has also influence upon fornrrtion and growth of the N b Sn superconducting layer, Ng. 2.
3
Ng. 2 - Electrioal and meohanioal properties of secondary heat treated Nb-Zr02 substrate and 1Qb Sn supercon-
duc tor 3
c*
During the heat treatment at T <250°c
-
- .;I 1.. -/ I the 02-content is lowered i n the 6-Nb matrix and it is trapped on the boun-
- daries of Nb40 particles. The value of
E '4
specific resis tanoe deoreases. For
$2 22
- cr n --. i T = 2 5 0 ~ ~ to 4 0 0 ~ ~ new NbkO partiole.
m i are formed. Beoause of precipitation
hardening the strength GOe2 and micro- hardness &I increase with further low- e r i q of . Boundaries of Nb40 parti-
\ I 01es serve as diffusion pathes for Sn and consequently the Nb Sn layer thic-
3
kness and oritical current values inc- rease. For T > ~ O O ~ C , O2 diffuses into
2m
inside parts of the substrate, Nb40
o ~ m x a u a s a m o m , partioles are decomposed and new 2rO2
T''cl particles are formed. The values of
and Hm decrease and that of increase at the same time. The
activation energy of Sn-diffusion is inoreased and, therefore, the
values of layer thiokness and & deorease.
C1-416 JOURNAL DE PHYSIQUE
EYgure 3 shows fractures of Nb S n samples, 3a - without secondary 3
heat treatment, 3b - sample heat treated at 4 0 0 ~ ~ . The Nb Sn layer 3
thickness is 3.5 )un to 4.0 fun in the first oase but about 6.0 p m i n oase b. The dimension of the N b Sn grains is ohanged slightly from
3
0..2 p to 0.4 )un towards direction substrate - Nb Sn layer interface 3
to Nb Sn layer surface.
3
a - without secon- dary heat treat- ment
t
