Densification and microstructural btudies on Bi1.6Pb0.4Sr2Ca2Cu3Oy system

Mat. Res. B u l l . , Vol. 25, p p . 1057-1063, 1990. P r i n t e d in t h e USA. 0025-5408/90 $3.00 + .00 C o p y r i g h t (c) 1990 Pergamon P r e s s p l c .

DENBZFZCATION

AND MZCROBTRUCTUR6~I. 8TUDZE8 ON

Bl~.~Pbo.4SrL.Ca=Cu=Oy'

SYSTEM

K i r a n J a i n , D . K . s u r i , K.C.Nagpal, S.U.M.Rao, Materials division N a t i o n a l Physical Laboratory New D e l h i , I n d i a .

B.K.Das

( R e c e i v e d A p r i l 4, 1990; Communicated b y C . N . R . Rao)

ABSTRACT I n processing o x i d e superconductors it is i m p o r t a n t to o b t a i n the highest possible density without having adverse e ~ e c t on the e l e c t r i c and magnetic p r o p e r t i e s o~ the m a t e r i a l . The present study r e p o r t s some o£ the systematic r e s u l t s on d e n s i ~ i c a t i o n , g r a i n growth and o t h e r m i c r o s t r u c t u r a l changes d u r i n g £ormation o£ high T= phase in Bi~.~Pbo.4Sr=CamCuaOy syslem. The samples were prepared by c o n v e n t i o n a l ceramic t e c h n i q u e , s i n t e r e d a t 845"C £or 20-240 hr. The d e n s i t y in the sample was observed t o decrease w i t h the increase in s i n t e r i n g t i m e , though the high Tc (110 K) phase content s h o w s increase through X-ray a n a l y s i s . The r e s u l t s on c r i t i c a l current d e n s i t y are a l s o discussed. MATERIAL

I N D E X : bismuth, lead, oxides, superconductors

INTRODUCTION Since the d i s c o v e r y o~ a s l i g h t s i g n a l o~ a high T= superconducting phase (T=-IIOK) in the Bi-Sr-Ca-Cu-O system many s t u d i e s have been c a r r i e d out to o b t a i n the s i n g l e phase m a t e r i a l . Takano e t a l . ( 1 ) and subsequently Endo et al.(2) have r e p o r t e d t h a t the i n t r o d u c t i o n o~ Pb and s i n t e r i n g in low oxygen p a r t i a l

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pressure produces good s i n g l e phase samples in a r e p r o d u c i b l e way. Ono (3) has r e p o r t e d t h a t the 110K phase c r y s t a l l i z e s in air in the temperature range where p a r t i a l m e l t i n g takes place due to which t h e r e i s a narrow temperature range where a s i n g l e phase can be o b t a i n e d . S i n t e r i n g time as high as 300 h r . i s r e p o r t e d to give a s i n g l e phase m a t e r i a l ( 4 ) . H i g h l y o r i e n t e d m i c r o s t r u c t u r e and a s i g n i f i c a n t improvement in c r i t i c a l c u r r e n t d e n s i t y were r e p o r t e d by i n t e r m e d i a t e pressing ( 5 , 6 ) . A systematic study was undertaken on d e n s i f i c a t i o n and m i c r o s t r u c t u r e in order to gain an understanding about the g r a i n shape, s i z e , o r i e n t a t i o n , sample p o r o s i t y e t c . which are b e l i e v e d t o e f f e c t the t r a n s p o r t p r o p e r t i e s o f the f i n a l product. A small v a r i a t i o n in the processing parameter i s observed to affect the d e n s i £ i c a t i o n and m i c r o s t r u c t u r e of the sintered p r o d u c t . In t h i s paper, we r e p o r t the d e n s i f i c a t i o n and m i c r o s t r u c t u r e s t u d i e s in Bi-system and c o r r e l a t e t h e m w i t h observed c r i t i c a l current density. EXPERIMENTAL TECHNIQUE SAMPLE PREPARATION P o l y c r y s t a l i n e samples of the compound B i l . b P b o . 4 Sr=Ca=Cu=Oy were prepared v i a a s o l i d s t a t e route. Appropriate amounts o f Bi=O=,CaCO=,SrCO=,CuO,PbO ( a l l 99.9% pure) were weighed to make 100 gm. batch, t h o r o u g h l y mixed to homogenize them in a p o l y p r o p y l e n e j a r and b a l l milled for 24 hours, using y t t r i a s t a b i l i z e d z i r c o n i a b a l l s as the g r i n d i n g media. The mixed powder was placed on an alumina t r a y and f i r e d a t 830-840°C f o r 20 h r s . i n a i r and s l o w l y cooled to room temperature. The black powder so o b t a i n e d was again b a l l m i l l e d f o r two days to reduce the p a r t i c l e s i z e . P e l l e t s (16mm d i a , 2mm t h i c k ) were cold pressed under 4 ton/cm= pressure w i t h o u t b i n d e r . These p e l l e t s were s i n t e r e d f o r d i f f e r e n t time periods (20-240 h r s . ) a t 845°C. CHARACTERIZATION METHODS The d e n s i t y of each sample was determined by the l i q u i d displacement method. Xylene was used as the immersion l i q u i d . The r e s i s t i v i t y of the sample was measured on a r e c t a n g u l a r bar (lmmxlmmxlSmm) by the f o u r probe t e c h n i q u e . E l e c t r i c a l contacts to the sample were made by f i n e copper w i r e a t t a c h e d to the sample w i t h conducting s i l v e r p a i n t . Measurements were done a p p l y i n g I mA ac c u r r e n t (73 Hz) across the sample, v o l t a g e was measured w i t h a Lock i n A m p l i f i e r (PAR Model 5210) and recorded on a X-Y r e c o r d e r . Temperature of the sample was measured using a Cu-Constanton thermocouple. The c r i t i c a l c u r r e n t d e n s i t y was measured, by d i p p i n g the sample i n l i q u i d n i t r o g e n , using a pulse technique a t zero magnetic f i e l d . A d c pulse c u r r e n t ( I sec) was passed through the sample and v o l t a g e was measured through a n u l l d e t e c t o r . Sample was t h i n n e d down to a t h i c k n e s s less than 0 . 5 mm to reduce the h e a t i n g a t the c u r r e n t c o n t a c t s . I= was d e f i n e d as the current

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OXIDE S U P E R C O N D U C T O R S

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which induced I ~v across a 10 mm l e n g t h of the sample and critical c u r r e n t d e n s i t y J= was c a l c u l a t e d by d i v i d i n g I~ by the c r o s s - s e c t i o n a l a r e a . DiFFerent phases present in the samples a f t e r s i n t e r i n g were i d e n t i f i e d by the X-ray d i F F r a c t i o n (XRD) t e c h n i q u e using Cu-K= r a d i a t i o n . M i c r o s t r u c t u r a l s t u d i e s were c a r r i e d out with SEM(model J E O L - 3 5 CF)and the compositional a n a l y s i s by EDS (model KEVEX 7000-77) a t t a c h m e n t t o the SEM. RESULTS AND DISCUSSION The average p a r t i c l e s i z e of c a l c i n e d powder was 1.6 ~m a f t e r g r i n d i n g For 48 h r s . Maximum p a r t i c l e s i z e present in the powder was 4 ~m in s i z e . F i g 4 s h o w s the bulk d e n s i t y oF the samples s i n t e r e d From 20 h r s . t o 250 h r s . During the i n i t i a l stage

3"E

o

3',~-

4(

3.C

2.6 20

I

I

643

I

I

I

I

i

!

~0 Sinlering Time (hrs.) tO0

140

I

A

I~'~I

220

FIG.I V a r i a t i o n oF b u l k d e n s i t y w i t h s i n t e r i n g time at a t e m p e r a t u r e (845 ° C)

B

c

260

FIG. 2 Photographs oF samples s i n t e r e d For (A) 20 hrs (B) 150 hrs and (C) 250 h r s .

oF s i n t e r i n g up t o 440 h r s . the d e n s i t y oF the samples drops r a p i d l y w i t h the s i n t e r i n g t i m e and l a t e r the rate oF drop in d e n s i t y was l e s s . I n s t e a d o£ the s h r i n k a g e u s u a l l y observed d u r i n g sintering oF ceramic m a t e r i a l s , c o n s i d e r a b l e expansion in the samples s i z e was observed as shown in Fig. 2 which shows a photograph o£ the samples F i r e d For 20 , 150 and 250 hrs. This k i n d oF s i z e expansion i s observed by Hatano e t a l (7) and o t h e r s (8) a l s o , and may be due to l i q u i d phase Formation and Pb l o s s . F i g . 3 shows the n o r m a l i s e d r e s i s t i v i t y vs t e m p e r a t u r e curve f o r a sample s i n t e r e d a t 845°C For 150 h r s . The two stepped t r a n s i t i o n r e g i o n , n o r m a l l y observed in undoped Bi system was not p r e s e n t i n these samples. I n s t e a d a sharp drop in resistivity above lOOK was o b t a i n e d . F i g . 4 s h o w s c r i t i c a l current density (J=) o b t a i n e d For samples s i n t e r e d For 20-250 h r s . The J= values increased w i t h s i n t e r i n g time f o r i n i t i a l 20-450 h r s . oF F i r i n g , but a f t e r 150 hrs i t s t a r t e d t o decrease w i t h time. Maximum J= o b t a i n e d was 280 A/cm=. This Fall in critical current density a f t e r 450 h r s . oF s i n t e r i n g may be due to the poor d e n s i t y oF these m a t e r i a l s , which may cause a r e d u c t i o n in grain to grain

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et al.

Vol.

25,

No. 8

c o n t a c t in these samples. Jc

Vs

Time

28C 24C .009

.007

~ 120' .004

8O .o01 4 O

-.oot

I I I ....

I

i 0

I

t00

150

200

2~K)

300

,

20

|

|

iO0

Tempe~lure (K)

I

i

I

140

i

|

!

220

180

260

Time in hrs.

FIG. 4 V a r i a t i o n of c r i t i c a l c u r r e n t d e n s i t y w i t h s i n t e r i n g time a t 845° C

FIG. 3 Normalised r e s i s t i v i t y vs. temperature curve f o r a sample s i n t e r e d a t 845° C/64 hrs

F i g . 5 shows the r e s u l t s of the X-ray d i f f r a c t i o n p a t t e r n of these samples from 2e=3°-40=. The p e a k s i n d i c a t e d by s o l i d c i r c l e s agreed ( 1 , 2 ) w i t h those of the high Tc phase(T==110 K) Bi=Sr=Ca=Cu=Oy w h i l e the peaks i n d i c a t e d by o p e n c i r c l e s agreed (9) w i t h those of the low Tc phase (T==8OK) BieSr=CaCueOy. From

i

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9O 8(:

i

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i

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FIG. 5 XRD p a t t e r n f o r samples s i n t e r e d a t 845° C f o r (a) 20 hrs (b) 40 hrs (c) 72 hrs (d) 140 hrs (e) 250 hrs. the X-ray a n a l y s i s ( f i g 5a-e) i t

i

60

i

i

I

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ioo 14o 18o Time in hrs.

,

i

220

i

260

FIG. 6 Volume f r a c t i o n of high Tc phase w i t h s i n t e r i n g time.

was r e v e a l e d t h a t in

the

initial

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OXIDE S U P E R C O N D U C T O R S

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20-60 hrs of f i r i n g 80 K phase was predominant but as the firing time was increased the p r o p o r t i o n of 110 K p h a s e c o n t i n u o u s l y i n c r e a s e d . In the sample s i n t e r e d f o r 150 hrs no o t h e r impurity phase except 80 K phase was p r e s e n t . F r a c t i o n of high T© phase was c a l c u l a t e d using the X-ray d i f f r a c t o g r a m s from the relative i n t e n s i t i e s ( I ) of (002) l i n e s f o r high T= (14.~) and low T= ( I ~ . 7 ) phases r e s p e c t i v e l y . High T= phase f r a c t i o n was c a l c u l a t e d from e q u a t i o n (1) as given b y ( 1 0 , 1 1 ) . High T= phase f r a c t i o n = 1 4 . 8 / ( 1 4 . 8 + I ~ . 7 )

(a)

(b)

(d)

~c)

(e)

SEM photographs f o r for (d)

20 72

hrs, hrs

fracture (e) 140

(1)

FIG. 7

(a)

(@)

p o l i s h e d surface of a sample s i n t e r e d

surface for (b) hrs (f) 250 hrs.

20

hrs

(c)

40

hrs

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8

F i g . 6 shows the f r a c t i o n o f high T= phase as o b t a i n e d using e q . ( 1 ) , which i n d i c a t e s the development of high T= phase w i t h s i n t e r i n g t i m e . SEM photographs o f the f r a c t u r e d surface of the samples are shown in fig 7(b-e). The m i c r o s t r u c t u r e o f p o l i s h e d sample s i n t e r e d f o r 20 hrs ( f i g 7a) shows a few dark coloured g r a i n s w h i t h i n a m a t r i x of gray c o l o u r e d g r a i n s . EDS a n a l y s i s on t h i s dark g r a i n shows t h a t t h i s phase c o n s i s t s o n l y of Sr,Ca,Cu. This phase may be Sr=-.CuxOv a l s o observed by Takano et a l . ( 1 ) . With the increase in s i n t e r i n g time this p h a s e goes on decreasing i n c o n c e n t r a t i o n . As shown in fig 7(b-e) the m i c r o s t r u c t u r e of these samples c o n s i s t of two types o£ g r a i n s , one type of g r a i n s are in the shape of thin plates with aspect r a t i o about 10 and o t h e r s are in the shape o f p l a t e s w i t h aspect r a t i o almost one. In a l l these samples the g r a i n s are not o r i e n t e d i n any p a r t i c u l a r d i r e c t i o n . They had grown randomly l e a v i n g l a r g e pores a t g r a i n boundaries. M o s t of the pores are lying at the g r a i n boundaries, no i n t r a - g r a n u l a r pores are v i s i b l e . ~-I

-Ol &

•.,~oK,..o.,.

24

o Low TC(8OK) Ohose

2c .c

8 •

i 4 ,

o.ee

2e/deg

FIG. 8 XRD p a t t e r n for a sample for 150 hrs and subjected regrinding & resintering

0.92

096

Curmnl DInldly A / c m I (XlOI1

sintered to for 250

hrs.

FIG. 9 c u r r e n t d e n s i t y vs v o l t a g e f o r the same sample (as used in f i g . 8 )

To summarize the r e s u l t s , T=(O) more than 100 K have been o b t a i n e d r e p r o d u c i b l y i n m o s t of the samples. The high T© phase content increased w i t h i n c r e a s e in s i n t e r i n g time but the r a t e of f o r m a t i o n and g r a i n growth i s very slow. S u c h slow k i n e t i c s o f g r a i n growth may be due to the two processes occuring s i m u l t a neously (a) g r a i n growth (b) conversion of low T= to high T= phase. I t was r e p o r t e d by Hatano et a l . ( 7 ) a l s o t h a t the density o f the samples decreases as the high T= phase i s formed, then the samples become porous. Due t o t h i s increase in p o r o s i t y , Jc w i l l a l s o get e f f e c t e d . C r i t i c a l c u r r e n t can be improved by decreasing the volume f r a c t i o n o f the i n s u l a t i n g phase and by o p t i m i s i n g the s y n t h e s i s c o n d i t i o n s s u c h as s t a r t i n g composition and s i n t e r i n g period. One sample f i r e d f o r 150 hrs. was crushed, p e l l e t i z e d and s i n t e r e d again a t &45°C f o r 250 h r s . It shows a drastic improvement in d e n s i t y , and c r i t i c a l c u r r e n t d e n s i t y . D e n s i t y of

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OXIDE S U P E R C O N D U C T O R S

t h i s sample increased to 4.8 gm/cm= increased to 980 A/cmz. The c u r r e n t t h i s sample was shown in Fig 8. The t h i s sample (Fig 9) shows t h a t more was present i n t h i s sample.

1063

while c r i t i c a l current density d e n s i t y Vs voltage curve f o r X-ray d i £ f r a c t i o n p a t t e r n o£ than 80% high T= (i10K) phase

CONCLUSION The process o£ £ormation of high T= phase i s very slow. The low T= phase £ormed during c a l c i n a t i o n i s trans£ormed i n t o the high Tc phase, and t h i s phase t r a n s f o r m a t i o n requires prolonged heat treatment. This process is u s u a l l y accompanied by sample expansion. From the p r e l i m i n a r y r e s u l t s i t i s indicated that i£ before the £ i n a l s i n t e r i n g high T© phase can be present in the sample the d e n s i t y , c r i t i c a l c u r r e n t d e n s i t y as w e l l as high T= phase content can be increased. I t would be very u s e f u l to study the g r a i n growth o£ 2223 phase in which 2 2 1 2 p h a s e h a v e a l r e a d y converted to 2223 or not £ormed at all,since this may be i n h i b i t i n g the g r a i n growth and d e n s i £ i c a t i o n o£ high T~ phase. Acknowledgement We are thank£ul to Mr.R.C.Goel and Dr.A.K.Gupta f o r conducting p a r t i c l e size and r e s i s t i v i t y measurements. We are also thank£ul to D r . S . K . J o s h i , D i r e c t o r , N a t i o n a lPhysical Laboratory New D e l h i £or his personal i n t e r e s t and encouragement during the progress of this work. Re£erences I.

M.Takano, J.Takada, K.Oda, H . K i t a g u c h i , Y.Miura, Y.Ikeda, Y.Tomii and H.Mazaki, J p n . J . A p p l . P h y . 27,LI041(1988) 2. U.Endo, S.Koyama and T.Kawai, J p n . J . A p p l . P h y . 27,L1476(1988) 3. A.Ono, J p n . J . A p p l . P h y s . 27,L2276(1988) 4. K.Togano, H.Kumakura and D . R . D i e t d e r i c h , C r y o g e n i c s 29,286(1989) 5. A . I t o , M.Matsuda,Y.lwai, M . I s h i , M.Takata, T.Yamashita and H. Koinuma, J p n . J , A p p l . P h y . 2__88,L380(1989) 6. Y.Tanaka, T.Asano, K . J i k i h a r a , M.Fukutomi, J.Machida and H.Maeda, J p n . J . A p p l . P h y s . 27,L1655(1988) 7. T.Hatano, K.Aota, S.Ikeda, K.Nakamura and K.Ogawa, Jpn.J.App. Phy. 27,L2055(1988) 8. A.Maeda, K.Noda, K.Uchinokura and S.Tanaka, Jap. J.App.Phy. 2_88,L576(1989) 9. M.Onada, A.Yamamoto, E.T.Muromachi and S.Takekawa, Jpn. J.Appl. Phys.2_Z,L833(1988) 10. H.Shimojima, K.Tsukamoto and C.Yamagishi, Jpn.J.Appl.Phys. 2~ L226(1989) 11. Y.Kozono, T.Ohno, M.Kasai, M.Hanazono and Y.Sugita, Jpn.J.Appl Phys. 2_88,L646(1989)