Transport current and microstructure in Y1Ba2Cu3O7−x superconductors

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Solld State Communications, Vol. 68, No. 9, pp.841-845, 1988. Printed in Great Britain.

T R A N S P O R T C U R R E N T AND M I C R O S T R U C T U R E SUPERCONDUCTORS

0038-1098/88 $3.00 + .00 Pergamon Press plc

IN Y i B a 2 C u 3 0 7 _ x

B.V. Reddi, K. Jain, B.S. Khurana, S.U.M. Rao, Ram Kishore, R.B. Tripathi, R.K. Kotnala, S.M. Khullar, R.C. Goel, S. Singh and B.K. Das N a t i o n a l P h y s i c a l Laboratory, New Delhi-ll0 012, India

Dr. K r i s h n a n ' s Road,

(Received i0 A u g u s t by C.N.R.

Rao)

Bulk p o l y c r y s t a l l i n e Y i B a 2 C u 3 0 7 _ x samples have been synthesized by ceramic, c h e m i c a l and m e l t i n g techniques. Transport c r i t i c a l c u r r e n t d e n s i t y (Jct) of the samples of ceramic and c h e m i c a l m e t h o d s has been m e a s u r e d and found very low c o m p a r e d to the Jc values c a l c u l a t e d from the m a g n e t i zation m e a s u r e m e n t s and t h e o r e t i c a l d e p a i r i n g critical c u r r e n t density. Here Jct values are i n t e r p r e t e d in terms of the m i c r o s t r u c t u r e . The poor Jct is a t t r i b u t e d to weak links such as, besides grain boundaries, twin boundaries, m i c r o i n h o m o g e n e i t y regions w i t h i n and on the g r a i n . a n d twin boundaries, i n t r i n s i c structural d i s o r d e r and inter and t r a n s g r a n u l a r m i c r o c r a c k s w h i c h may be r e s p o n s i b l e for s u p e r c o n d u c t i n g g l a s s y state and lath or plate type a n i s o t r o p i c grains d i s t r i b u t e d randomly. The work described here further reveals that fine needle shaped grain m o r p h o l o g y is r e a l i s e d by m e l t i n g t e c h n i q u e and higher Jct values could be o b t a i n e d if the long axes needle type grains are aligned along the length of s u p e r c o n d u c t i n g rod or wire.

Introduction S u p e r c o n d u c t o r s w i t h their r e m a r k a b le p r o p e r t i e s - absence of e l e c t r i c a l res i s t a n c e and e x p u l s i o n of m a g n e t i c field have been e x p l o i t e d in the last two decades for a v a r i e t y of m e g a and m i c r o scale a p p l i c a t i o n s i n v o l v i n g g e n e r a t i o n of as high as 1011joules to as small as 10 -22 Joules at liquid h e l i u m (4.2K) using Nb-Ti, Nb3Sn and V3Ga. Recent d i s c o v e r y of Y i B a 2 C u 3 0 7 _ x (123) with T c of 93K 1 has brought a s t r o n o m i c a l change b e c a u s e of its p o s s i b l e use at liquid n i t r o g e n (77K) for obvious e c o n o m i c gains besides the s i m p l i c i t y in o p e r a t i o n of s u p e r c o n d u c t i n g devices. P a r t i c u l a r l y for m e g a s c a l e applications like fusion and MHD power systems, e l e c t r i c i t y storage systems, electric power generators, e l e c t r i c power t r a n s f o r m e r s , m a g n e t i c separators, particle a c c e l e r a t o r s , ship p r o p u l s i o n motors and g e n e r a t o r s , m e d i c a l d i a g o n s t i c s from m a g n e t i c r e s o n a n c e imaging and l e v i t a t e d trains, s u p e r c o n d u c t o r should also possess high values of upper c r i t i c a l field (Hc2) and c r i t i c a l current density (Jc) . F o r t u n a t e l y the e s t i m a t e d H~o(O) values are very high, 88 to 330T2-~ =. Even at 77K, its Hc2 value is as high as 15T5. For p r a c t i c a l a p p l i c a t i o n s , it is equally i m p o r t a n t that the m a t e r i a l should have at least t r a n s p o r t Jc of 104A/cm 2 in 841

such a m a g n e t i c field. Though the magn e t i s a t i o n Jc values (Jcm) of bulk as well as single crystals ar~ v e r y large values, 104 - 106 A / c m 2 6- , very low values of t r a n s p o r t current (Jct) of 10-103 A / c m 2 I,~,9 in bulk s i n t e r e d 123 s u p e r c o n d u c t o r are reported. Such a poor JG' in bulk m a t e r i a l may be due to w e a k ll~ks, such as grain boundaries, w h i c h d e c o u p l e high Jc regions, as pointed out for L a - S r - C u - O i0 . In this paper we r e p o r t J c t m e a s u r ements and m i c r o s t r u c t u r a l study of 123 s u p e r c o n d u c t o r s s y n t h e s i s e d by ceramic, chemical and m e l t i n g techniques. Low Jct values are a t t r i b u t e d to u n f a v o u r a b l e m i c r 0 s t r u c t u r e , causing w e a k links, in these bulk materials, w h i c h calls for r e f i n e m e n t of m i c r o s t r u c t u r e for improving t r a n s p o r t c u r r e n t c a p a b i l i t y of the superconductor. Experimental In ceramic processing, Y203, BaCO 3 and CuO (Analar grade) in 1:2:3 m o l a r ratio of Y, Ba and Cu were used to make sample pellets of 10 m m dia and 2 mm thickness. C a l c i n a t i o n and s i n t e r i n g p r o c e d u r e f o l l o w e d in this work were d e s c r i b e d in detail else-

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MICROSTRUCTURE IN SUPERCONDUCTORS

wherell. P r e s s e d pellets of c a l c i n e d and g r o u n d p o w d e r (0.9 JLLm average size) were s i n t e r e d at 900-950oc in oxygen. In the c h e m i c a l method, p o w d e r was p r e p a r e d by c o p r e c i p i t a t i o n of all the n i t r a t e s of Y, Ba and Cu w i t h aqueous s o l u t i o n of sodium hyd r o x i d e at pH of 13 and 60oc. As m e n t i o n e d in our e a r l i e r paperl2, the hydroxide~ixture thus o b t a i n e d was c ~ n v e r t e d into oxide by b u b b l i n g w i t h pure o x y g e n gas. The oxide powder was dried, calcined, ground, p r e s s e d into tablets and s i n t e r e d at 900oc for 13 hours in oxygen. In all the above cases, c o o l i n g rate was 60OC/hr from the s i n t e r i n g t e m p e r a t u r e e x c e p t in one case of c e r a m i c processing, w h e r e the sample was cooled from 920oc after 12 hrs of s i n t e r i n g at the rate of 200OC/hr. In the m e l t i n g process, the c a l c i n e d powder, p r e p a r e d as m e n tioned e a r l i e r in the ceramic p r o c e s s ing, was i s o s t a t i c a l l y p r e s s e d at room t e m p e r a t u r e in the form of 12 m m dia and 75 m m length rod and s i n t e r e d later in o x y g e n atmosphere. Using ind u c t i o n coil of a RF generator, about 5 m m length from one end of the sintered rod was m e l t e d in air and g l o b u l e s of a p p r o x i m a t e l y 5 mm dia were obtained w h i c h were later oxygen annealed. The samples of 1 x 1 x I0 mm were cut from the s i n t e r e d p e l l e t and Jct were m e a s u r e d using four probe r e s i s t i v e m e t h o d and pulse t e c h n i q u e r e s p e c t i v e l y . All samples showed T c of 93 + IK. Optical m i c r o s c o p e w i t h cross p o [ a r i s e r and SEM (JOEL - 35CF model) were e x t e n s i v e l y used to study m i c r o s t r u c t u r e at room temperature. J-t is d e f i n e d as the Critical c u r r e n t ~ e n s i t y when 1 ~ V / c m is developed. Results T r a n s p o r t c r i t i c a l c u r r e n t density, Jct is zero field at 77.4K of the samples s i n t e r e d at 900°C and 920 - 950°C are shown in figs 1 and 2. It may be noticed in Fig.l that as the s i n t e r i n g period at 900°C i n c r e a s e s from 4 to 13 hrs, J~t of ceramic route samples goes up from 55 to 96 A / c m 2. A sample of c h e m i c a l m e t h o d at the same s i n t e r i n g t e m p e r a t u r e for 13 hrs is found to carry 48 A / c m 2. Jct values of ceramic m e t h o d samples s i n t e r e d at 940°C for 4 and 20 hrs (Fig.2) i n d i c a t e that s i n t e r i n g for 4 hrs i n s t e a d of 20 hrs w o u l d be b e n e f i c i a l since Jet d e c r e a s e s from 82 to 60A/cm 2 for longer s i n t e r i n g period. Still h i g h e r Jct of 250 A / c m 2 is r e a l i s e d in the sample s i n t e r e d at 920oc for 12 hrs and c o o l e d at 200oc/hr. It is i n t e r e s t ing to note in fig.2 that the sample w h i c h was h e a t e d to 950oc just for a second (which o t h e r w i s e f o l l o w e d same h e a t i n g and c o o l i n g rate) is found to carry 160 A / c m 2, m o r e than Jct values of the samples s i n t e r e d at 900°C and 940oc in the work r e p o r t e d here.

Vol. 68, No. 9

50

Ceromic Roule

40

30 ,.¢

*

~2c

~

o 0

J: /oo

t0

.....

i 0

,_./ 40

~z,__j

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1202

Currentdensity (A/cm Fig.l

)

1E

C u r r e n t density vs voltage of sample of ceramic route and chemical route, sintered at 900oc in oxygen atmosphere.

CERAMIC ROUTE

100

80 ~60 o

>o 40 2O 0

.... &.---J 0

Fig.2

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........ ~._ _ _ .~j__ .~ ..J;.t_ 120 160 200 240 280 Currenl Densily A/cm z

C u r r e n t d e n s i t y vs voltage of samples of ceramic route, sintered at 920, 940 and 950°C in oxygen atmosphere.

Discussion It is w e l l k n o w n that the t r a n s p o r t current is very sensitive to the m i c r o s t r u c t u r a l features such as grain and twin boundaries, stacking faults, dislocation, voids (pores), p r e c i p i t a t e s and other defects, f o r m a t i o n of w h i c h is i n f l u e n c e d by p r o c e s s i n g m e t h o d and post treatmentl3. In order that the s u p e r c o n d u c t o r should sustain high current it should have e f f e c t i v e flux pinning centres whose size should be w i t h i n 1 to 5 times of th~ c o h e r e n c e lengths, w h i c h is about 19 A 2 in Y B a 2 C u 3 0 7 - x superconductor. T h o u g h d e n s i f i c a t i o n takes place very fast in the first 3-4 hrs of sintering f o l l o w e d by slow rate of d ~ n s i f i c a t i o n as the sample is s i n t e r e d II, 4 hrs s i n t e r i n g time at 900°C has not densified the sample p r o p e r l y and grains v a r y i n g from s u ~ m i c r o n to 15 m i c r o n s

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MICROSTRUCTURE IN SUPERCONDUCTORS

Could be seen in the optical m i c r o g r a p h (fig 3a). Lack of good i n t e r c o n n e c t ion and wide spectrum of grain sizes randomly o r i e n t e d seem to be the cause for lower Jc whereas more or less uniform size grain ( ~ 1 2 ~m) structure (fig.3b) is observed in the sample sintered for a longer time, 13 hrs. Though the grains are oriented randomly, r e d u c t i o n in the porosity (more densification) and grain structure have helped in raising Jct from 55 to 96 A/cm 2.

Fig. 4 shows optical m i c r o g r a p h s of ceramic m e t h o d samples sintered at 920oc for 13 hrs, 940°C for 4 and 20 hrs and m o m e n t a r y touch of 950°C for a second, whose Jet values are

The sample of chemical m e t h o d (900oc, 13 hrs) exhibits random o r i e n t a t i o n of grains with longer d i m e n s i o n (20 Dm) in one d i r e c t i o n and smaller dimension ( > 5 Dm) in other d i r e c t i o n (fig 3c) c o r r e s p o n d i n g to a or b axis and c axis r e s p e c t i v e l y of orthorhombic phase ii and presence of Na giving rise to poor Jct"

Fig.3

Optical m i c r o g r a p h s of sintered samples in oxygen atmosphere at (a) 900oC/4hrs (b) 900oc/13 hrs (both of ceramic route) and (c) 900°C/13 hrs of chemical route.

Fig.4

Optical m i c r o g r a p h samples of ceramic Qxygen atmosphere) 12 hrs (b) 940oc/4 1 sec.

of sintered route (in at (a) 920°C/ hrs (c) 940oc/

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MICROSTRUCTURE IN SUPERCONDUCTORS

Vol. 68, No. 9

250, 103, 60 and 160 A/cm 2 respectively. In the first and last samples, more or less uniform and well defined grains but with random orientation, and reduced porosity may be responsible for higher Jc over the samples sintered at 900°C (fi i) and 940°C for 4 hrs (103 A/cm ~") and 20 hrs (60 A/cm 2) (fig.2). In the samples sintered at 940oc, longer sintering time (20 hrs) causes the more or less equiaxed grains to change into lath or plate type grains randomly. This type of lath or plate type of grains oriented in all directions is not favourable for carrying high Jc due to the fact that more favourable superconting path (a-b basal plane) in one grain is likely to meet c direction in neighbouring grain which is less favourable to carry that much current because of anisotropic nature of 123 superconductor with variation in superconducting properties in different directions. In all the samples (figs 3 and 4), lamellar twin domains are distinctly seen. The real role of twin boundaries is not understood at present. Added to random orientation problem, other inherent material defects in our bulk material like grain and twin boundaries may be acting as weak links and causing a drastic fall in Jct" The weak link problem is further aggravated if the impurities segregate along these defects. In fact we have noticed with the help of auger electron spectroscope, the segregation of Na and C along the grain boundaries. It is possible that these impurities may also be segregating at the twin boundaries. Microscopic variation ( 0 < x < 0 . 5 ) in oxygen l e v e l ~ -16 and oxygen depletion at the twin b o u n d a r i e s l 7 and deviation from tetragQnality of orthorhombic phase from 0 to 4.0% 18 within the grains, either individually or collectively may be causing superconducting glassy statel9 in which state superconducting loops exist within the areas typically smaller than the grain size. Therefore, it is not surprising if the twin boundaries, besides grain boundariesl0, may also be acting as weak links, as suggested by Deutscher and Muller~0. Presence of other phases such as Y2BaCuO5 CuO, Ba~Cu305 and Cu2 O in varying quantitzes reported by us 11,21 may also hinder the superconducting paths if the size of these precipitates is more than the coherence length. Microcracks (inter and transgranular) in lath type grains as observed by us II formed due to anisotropic thermal expansion and elastic anisotropy variation in grain and twin boundary composition and intrinsic structural disorder may also act as weak links.

Fig.5

SEM photographs of samples (a) melted in a 9 and annealed in oxygen at 900oc and (b) conventional sintering at 900oc/13 hrs (ceramic route) in oxygen.

In the light of the above discussion and discontinuous grain growth in lath or plate type shape randomly due to anisotropic interfacial energy a s mentioned earlierll, it is obvious for high Jc, we should have material free from such weak links on microscale in bulk material, which is impossible to achieve. Alternatively, we should have orientation of grains in such a way that a-b basal plane of orthorhombic phase is in the direction of current flow. Therefore, it prompted us to follow a melting technique to ascertain the features of general morphology of grain structure of the material. Our preliminary experiment without exercising control on heating or cooling rate of the molten rod or any arrangement to unidirectionally solidify the material showed, as it is evident from SEM micrograph (fig.5a) compared to the grain morphology of the sintered pellet at 900oc/13 hrs (fig 5b) that needle shaped grains (~ 2 ~m dia), besides lath type grains, formed randomly during solidification. Therefore, if efforts are made to solidify the material unidirectionally from its melt state, it is possible to obtain all needle shaped grains and align long axes of these fibres or needles in place of randomly oriented

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MICROSTRUCTURE IN SUPERCONDUCTORS

Vol. 68, No. 9

e q u i a x e d m a t e r i a l and r a n d o m o r i e n t a t i o n of lath or plate type grains s e v e r e l y affects JctLow JGt is a t t r i b u t e d to grain boundarles, twin b o u n d a r i e s m i c r o l e v e l i n h o m o g e n e i t y regions w i t h i n the grains and m i c r o c r a c k s w h i c h may act as w e a k links in high Jc regions. Unidirectional solidification t e c h n i q u e to o r i e n t grains w i t h easy s u p e r c o n d u c t i n g paths (a-b basal planes) in the d i r e c t i o n of the s o l i d i f i c a t i o n for high Jct seems to be a p o t e n t i a l synthesis technique.

g r ~ i n s along the length of the rod w h i c h will be able to carry m o r e current. In fact w h i l e p r e p a r i n g this paper, we have come to k n o w that " m e l t - t e x t u r e d g r o w t h t e c h n i q u e 22 , similar to u n i d i r e c t i o n a l solidif i c a t i o n technique, s u g g e s t e d here, is f o l l o w e d to p r o d u c e Y I B a 2 C u 3 0 7 _ x and its Jct is as high as 1700 0A/cm 2 at 77K, s u r p a s s i n g all Jct values r e p o r t e d here and q u o t e d in the literature6,9. This V a l u e 2 2 i s also two orders of m a g n i t u d e less the theo r e t i c a l upper limit set by the dep a i r i n g c r i t i c a l c u r r e n t d e n s i t y at 77K ( J c = H c / 3 ~ l = 5 x 1 0 6 A/cmZ). T h e r e f o r e there is a need for further w o r k in this d i r e c t i o n to improve the c u r r e n t c a r r y i n g c a p a c i t y of the superconductor.

Acknowledgement A u t h o r s thank Mr. Mukul Sharma for his a s s i s t a n c e in setting up the Jc m e a s u r e m e n t unit and Prof. S.K. Joshi, Director, NPL for his encoura g e m e n t and p e r m i s s i o n to p u b l i s h this work.

Conclusion In summary we c o n c l u d e that Jct is g e n e r a l l y found to be more in

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