Superconducting transitions caused by disordering and reordering in Nb3(Al, Ge) crystals

Scripta M E T A L L U R G I C A

Vol. 22, pp. 1-4, 1988 Printed in the U.S.A.

Pergamon Journals, Ltd. All rights reserved

S U P E R C O N D U C T I N G TRANSITIONS CAUSED BY DISORDERING AND R E O R D E R I N G IN Nb3(A1,Ge) CRYSTALS

H. Yoshida,

H. Kodaka

and

Y. Hayashi

R e s e a r c h Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka 590-04, Japan (Received June 16, 1987) (Revised October 19, 1987)

Several intermetallic compounds with the A15 structure, such as Nb3Sn , N b 3 A I and N b 3 G e , are known to s h o w a high superconducting transition temperature, Tc, and a high upper critical m a g n e t i c field, Hc2. The T c of the A15 s u p e r c o n d u c t o r s is known to depend on the degree o H order in the crystals (I), because s u p e r c o n d u c t i v i t y arises from the Nb chains formed by the long range order of Nb-Nb pairs in the A15 structure (Fig.l). Among many A15 s u p e r c o n d u c t o r s Nb]Sn has already been developed for commercial wires and tapes for s u p e r c o n d u c t i n g magnets, and Nb3AI is under development as a material for higher m a g n e t i c fields. The optimum condition of heat treatment was usually selected for practical purposes, and is sometimes different from that giving the ideal state of the crystals. The T_ is also known to decrease with neutron irradiation because of the i r r a d l a t l o n - l n d u c e d d l s o r d e r l n g in the A15 s u p e r c o n d u c t o r s (2-5). The aim of this report is (i) to show the recent results on the T c of Nb3(AI,Ge) crystals appearing during the isochronal a n n e a l i n g after the d i s o r d e r i n g because of neutron irradiation as well as pre-annealing, and (ii) to discuss tentatively the possibility of a higher T c component in the system. The samples of Nb3AI and Nb3(AI,Ge) were prepared by arc melting. The T c were p r e v i o u s l y m e a s u r e d by the inductance method (6) for the crushed pieces from the bulk samples before and after the usual heat treatments (5h at 1500°C; 2h at 900°C). Their chemical compositions were m e a s u r e d by radioactivation analysis, and their crystal structures were analyzed by the powder method of X-ray diffraction. The neutron i r r a d i a t i o n was carried out using the in-core i r r a d i a t i o n facility at the Kyoto U n i v e r s i t y Reactor, KUR. The crushed powder specimens (50 mg) of the Nb3(AI,Ge) samples were sealed in small quartz tubes (@=2.5 mm, 1=25 mm) to fit the inductance coil (5000 turns). The s u s c e p t i b i l i t y m e a s u r e m e n t s were p e r f o r m e d at an AC frequency of 4 kHz using a LCR meter and the percent change in inductance, normalized by the value at 4.2 K, during slow heating (<0.5°/min) from 4.2 K was recorded as ~ L/L4 2 (6). The neutron i r r a d i a t i o n and the isochronal annealing, as well as the i~6thermal a n n e a l i n g at 700°C for disordering, were carried out for the specimens sealed in the quartz tubes. It was p r e v i o u s l y e s t i m a t e d that the s u p e r c o n d u c t i n g t r a n s i t i o n curves obtained from the inductance change indicate the c h a r a c t e r i s t i c s of the m u l t i p l e - c o m p o n e n t superconductors, and the amount of inductance change was p r o p o r t i o n a l to the volume of s u p e r c o n d u c t o r for the same sample (6).

1 0036-9748/88 $3.00 + .00 Copyright (c) 1988 Pergamon Journals Ltd.

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Unit cell of A15 s t r u c t u r e

eNb

OAf

9 Fig. 2 s h o w s the s u p e r c o n d u c t i n g transition curves measured for the Nb3(AI O 7 7 G e 0 2.3) s p e c i m e n during the isochronal 4~nneal~ngs (for lh at 50 ° steps f ~ o m 2v5"0~C) after the i r r a d i a t i o n of 3.6x10 '~ n/cm ~ (E>0.1 MeV) at the reactor ambient t e m p e r a t u r e (~85°C) The original s u p e r c o n d u c t i n g t r a n s i t i o n curve of the a n n e a l e d specimen ( 5h at 1600°C and 2h at 900°C) before the i r r a d i a t i o n showed a sharp t r a n s i t i o n and a Tc(onset) of 19.1 K, while the curve r e m a r k a b l y shifted to lower t e m p e r a t u r e until a Tc(onset) of 6.9 K was achieved after the i r r a d i a t i o n . During the p o s t - i r r a d i a t i o n isochronal annealing, the t r a n s i t i o n curves a p p e a r e d to recover in two stages (5) as seen in the figure; i.e. the a n n e a l i n g s b e l o w 500°C induced a small r e c o v e r y of the transition, p r o d u c i n g a s i m i l a r l y shaped curve, and the large r e c o v e r y of Tc(onset) and gradual t r a n s i t i o n curves a p p e a r e d above that temperature. After isochronal a n n e a l i n g up to 750°C, the Tc(onset) of the Nb3(AI 0 77Ge0 23 ) s p e c i m e n rose to a higher t e m p e r a t u r e than the o r i g i n a l Tc(onset) ~ e f o r e ' t h e irradiation. In the case of Nb3AI the curves r e c o v e r e d after isochronal annealing up to 7 5 0 ° C , approximately to the s a m e c u r v e as b e f o r e the irradiation, w h i c h is similar to the case for the in-situ N b 3 S n s u p e r c o n d u c t o r r e p o r t e d p r e v i o u s l y (4). These p h e n o m e n a can be u n d e r s t o o d as follows: The T o ( o n s e t ) of A15 s u p e r c o n d u c t o r s d e c r e a s e s w i t h an i n c r e a s e in the s i t e e x c h a n g e d i s o r d e r i n g induced by atomic c o l l i s i o n sequences during n e u t r o n irr a d i a t i o n (2-5). R e c o v e r y of the s u p e r c o n d u c t i n g phases occurs h e t e r o g e n e o u s l y by r e o r d e r i n g during the p o s t - i r r a d i a t i o n anneals up to 800°C (5); the transition curves become broad, showing m u l t i p l e components; and the Tc(onset) rises to higher t e m p e r a t u r e s c o r r e s p o n d i n g to the c o m p o n e n t s having higher Tc, if they were formed, as seen in Fig. 2. The present results suggest the e x i s t e n c e of some c o m p o s i t i o n s with higher T c in the system. Fig. 3 s h o w s a n o t h e r e x a m p l e of d i s o r d e r i n g / r e o r d e r i n g for the s a m e sample. W h e n the a r c - m e l t e d s p e c i m e n h a v i n g a T c ( o n s e t ) of 18.9 K was annealed at 700°C, the s u p e r c o n d u c t i n g t r a n s i t i o n curves showed Tc(onset) = 19.3 and 19.7 K and 1.5 and 1.4 times in the amounts of the i n d u c t a n c e change for the a n n e a l i n g times of 2 and 10 hours, respectively. However, a n n e a l i n g for 40 hours r e s u l t e d in a c o m p l e t e d i s a p p e a r a n c e of the transition. The same p h e n o m e n o n was o b s e r v e d in Nb3AI and in the ternary specimens with d i f f e r e n t compositions. It is e s t i m a t e d from the X-ray d i f f r a c t i o n data that the r e l a t i v e intensities of the (100) and (200) peaks of the A15 c o m p o u n d indicate only a small portion of s i t e - e x c h a n g e d i s o r d e r o c c u r r i n g after a n n e a l i n g for 52 hours, a l t h o u g h a 40% increase i n intensity of the (330) peak appears b e c a u s e of the p r e s e n c e of a small amount of t e t r a g o n a l Nb2AI phase. This

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i

/ L4. 2

aL

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(%)

1 C Nb3(AI 0 77Ge 0 23 ) •

0

0 : as-~rrad. l : 250"C l 2 : 300% l 3 : 350°C l 4 : 400_°c I 5 : 450Uc I

-

post-irradiation isochronal

0 6

6 7 8 9 I0 II

h h h h h

: : : : : :

500°C 550°C 600°C 650°C 7000C 75006

I l 1 l I I

h h h h h h

annealing

0 4 0 2 0

N

4

6

f

I

I

I

f

I

f

I

I

8

10

12

14

16

18

20

22

24

TEMPERATURE Fig. 2 Superconducting transition curves anneal~gs ( 1 ~ h a t 50 ° s t e p s f r o m 3.6x10 "~ n/cm z (E>0.1 MeV).

L

/ L4. 2

26

(K)

of Nb3(AI 0 77Gen ~) 250°C~ afte9 neu{ron

during isochronal irradiation of

(%)

1 0 8

Nb3(AIo

•

• 77Geo 2 3 )

i sochronal

0 6

l Z 3

45

annealing

6

after

7

pre_anneal in 9

8 9

250°C 300°C

10 : 11 :

700°C 750°C

1 h 1 h

350°C 400°C 450% 5oo°c 5so°c 600°c 650%

0 4

4

7

9

0 2

oc

s

lo

12

14

TEHPERATURE Fig.

IG

ls

2'o

2'2

2'4

(K)

3

Superconducting transition curves of Nb~(AI N ~7Ge~ 23 ) d u r i n g annealings after pre-annealing ( 40 h ~t 700°C )["

isochronal

2

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suggests that the annealed state showing a loss of superconductivity does not correspond mainly to new phases and site-exchange disorder, but to another kind of disorder formed homogeneously during the isothermal annealing. When the isochronal annealing was subsequently carried out starting from this disordered state, the superconducting transition appeared again but in a different manner from the case of post-irradiation annealings. The transition curves a p p e a r e d to be sharp and at s l i g h t l y h iIg h e r t e m p e r a t u r e s than the original T~. The curves showed an increase in the amount of inductance change up to 400°~, and a decrease above 700°C. These phenomena can be understood as follows: the annealing-induced disordering might have introduced a random arrangement of the Nb-Nb pairs, i.e. reduction in a kind of long range order. During slow annealings the intrinsic superconducting transition of a certain c o m p o s i t i o n might a p p e a r from the h o m o g e n e o u s l y d i s o r d e r e d state, which m i g h t show s l i g h t l y h i g h e r T c than that of the a s - m e l t e d state c o o l e d rapidly. The amounts of inductance change in the transition curves roughly indicate the volume fractions of the ordered region in the crystal. Two kinds of disordering/reordering in the Nb~(AI~ 77Ge 0 23 ) superconductor can be seen in Figs. 2 and 3. The annealing-in~uce~'~Isor~ering/reordering corresponds to the homogeneous formation of Nb chains, which produces sharp transitions and a volume fraction dependent change in s u s c e p t i b i l i t y w i t h o u t a d e c r e a s e in T_~ When the c h e m i c a l c o m p o s i t i o n d e v i a t e s from the stoichiometric composition" to the Nb-rich side, vacancies form in the A1 sites to compensate the composition and one of the neighboring Nb atoms falls into the vacant site, resulting in site-exchange disorder with a large decrease in T_. The irradiation-induced disordering mainly introduces the site-exchange dzsorder because of displacement and replacement collisions, in addition to the displacement cascade damage (5). During the post-irradiation annealings the reordering occurs heterogeneously and forms many local regions with the possible compositions giving a gradual transition corresponding to the mixed compositions. The present results lead us to the possibility of finding a composition with higher T c in the Nb-AI-Ge system. Acknowledgmen£ The authors would like to express their thanks to Mr. T. Aida for his help in the measurements. This work was in part supported by the Grant in Aid from the Japanese Ministry of Education and the Iwatani Foundation. References I. 2 3. 4. 5. 6.

D.E. Cox, S. Moehlecke, A.R. Sweedler, L.R. Newlirk and F.A. Valencia, Superconductivity in d- and f-band Metals, p.461, Plenum Press, New York & London, (1976). A.R. Sweedler and D.E. Cox, Phys. Review B, 12, 147 (1975). B.S. Brown, Advance in Cryogenic Engin. Materi. Voi.23, p.74 (1989). H. Kodaka, K. Miyata, H. Yoshida, H. Yamaoka, M. Fukumoto and T. Okada, J. Nucl. Materi. 133&134, 819 (1985). H. Yoshida, H. Kodaka, M~ Takeda and T. Okada, Materials Science Forum Voi.15-18, p.1141, Trans Tech Publ., Switzerland (1987). H. Kodaka, K. Miyata, H. Yoshida and H. Yamaoka, Proc. Int. Cryo. Engin. Conf.(Kobe), p.234, Elsever Sci. Publ., London, (1982).