Processing of High-Temperature Ceramic ... - ACS Publications

Chapter 22 Processing of High-Temperature Ceramic Superconductors: Structure and Properties L. E. Toth1,4, M. Osofsky1,5, S. A. Wolf1, E. F. Skelton1, S. B. Qadri2, W. W. Fuller1,

D. U. Gubser1, J. Wallace1, C. S. Pande1, A. K. Singh3, S. Lawrence3, W. T. Elam1,

B. Bender1, and J. R. Spann3

U.S. Naval Research Laboratory, Washington, DC 20375-5000 2 Sachs-Freeman Associates, Landover, MD 20785 3 Crystal Growth & Material Testing Association, Lanham, MD 20706 Chemistry of High-Temperature Superconductors Downloaded from pubs.acs.org by UNIV LAVAL on 09/19/15. For personal use only.

1

The processing of high T c ceramic superconductors by traditional ceramic techniques is reviewed. All high Tc ceramic superconductors are layered Cu-O compounds that are closely related to each other. In each, Cu-O coordination polyhedra are typical of Cu+2 compounds. A critical step in processing these compounds is the intercalation of oxygen which changes the coordination polyhedra at a few atomic sites and causes a dramatic effect on the superconducting properties. F o l l o w i n g Bednorz and Mùller's p u b l i c a t i o n ( 1 ) on h i g h transition t e m p e r a t u r e , T , ceramic s u p e r c o n d u c t o r s , t h e r e has been a d e l u g e o f p a p e r s on t h e s u b j e c t . Momentum i n c r e a s e d with t h e announcement by Wu e t a l . ( 2 ) o f a T b r e a k t h r o u g h i n e x c e s s o f 90K. S i n c e then a great deal has been learned about superconducting properties, c r y s t a l s t r u c t u r e s and phase r e l a t i o n s h i p s i n t h e Y-Ba-Cu-0 system. Knowledge of p r o c e s s i n g these m a t e r i a l s has improved but n o t to t h e same extent as some o t h e r areas. Each r e s e a r c h group seems t o report a d i f f e r e n t s e t o f processing procedures. I t i s widely acknowledged t h a t i t i s d i f f i c u l t to t r a n s f e r p r o c e s s i n g p r o c e d u r e s from one group t o a n o t h e r and a c h i e v e comparable superconducting properties. In t h i s paper we attempt to r e v i e w some o f the processing procedures. We pay p a r t i c u l a r attention to the u n d e r l y i n g s t r u c t u r e : c r y s t a l and m i c r o s t r u c t u r e . At this time, r e l a t i o n s h i p s between s t r u c t u r e and s u p e r c o n d u c t i n g p r o p e r t i e s are better defined than a r e r e l a t i o n s h i p s between p r o c e s s i n g and properties. T h e r e f o r e , one c a n use the e v o l u t i o n o f the d e s i r e d s t r u c t u r e as a g u i d e t o p r o c e s s i n g . c

c

On sabbatical f r o m the N a t i o n a l Science Foundation, Washington, D C 2 0 5 5 0 P o s t d o c t o r a l fellow, Office o f N a v a l Technology, A r l i n g t o n , V A 2 2 2 1 7 - 5 0 0 0

4

0097-6156/87/0351-0228$06.00/0 © 1987 A m e r i c a n C h e m i c a l Society

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229

Our group has been m a i n l y c o n c e r n e d w i t h t r a d i t i o n a l ceramic processing techniques i n v o l v i n g s o l i d s t a t e particulate reactions such as s i n t e r i n g and h o t p r e s s i n g . A number of o t h e r t e c h n i q u e s , such as c o - p r e c i p i t a t i o n and o r g a n o m e t a l l i c s , have been s u c c e s s f u l l y u s e d by o t h e r s , but w i l l not be d i s c u s s e d h e r e . R e g a r d l e s s of the method o f p r e p a r a t i o n , i t i s b e l i e v e d t h a t a l l m a t e r i a l s must end up with the same structural modifications to ensure good superconducting properties. General Structure : Some 14 d i f f e r e n t ceramic compounds a r e reported to s u p e r c o n d u c t w i t h T ' s i n excess o f 35K. These a r e l i s t e d i n T a b l e I. There a r e 5 d i s t i n c t classes of materials with 4 different crystal structures.

Chemistry of High-Temperature Superconductors Downloaded from pubs.acs.org by UNIV LAVAL on 09/19/15. For personal use only.

c

Table I.

H i g h Temperature Ceramic

Formula La Cu0 2

Reference

(1)

4

La . A Cu0 . / +6 2

x

x

4

ABa Cu 0 2

La Ba 3

3

3

x

2

s

5+

6

Ba,Sr,Ca

(4-6)

Y,Lu,Nd,Sm,Eu,Gd Er,Ηο,Yb

(2,7,8)

Cu O +$ 6

(9)

u

(10)

Y BaCu 0 _ / +6 2

Superconductors

2

6

x

2

There a r e s e v e r a l c h e m i c a l and c r y s t a l s t r u c t u r e s i m i l a r i t i e s i n the h i g h T ceramic superconductors. A general feature i n t h e i r crystal c h e m i s t r y i s the o c c u r r e n c e of a s m a l l number of Cu-0 coordination polyhedra. As d i s c u s s e d by W e l l s ( 1 1 ) , these are the same p o l y h e d r a t h a t one f i n d s i n Cu-0 compounds i n w h i c h Cu has a +2 v a l e n c e . Copper i s l o c a t e d i n one of three c o n f i g u r a t i o n s : (1) at the center of a square a r r a y o f c o p l a n a r oxygen atoms (square p l a n a r ) ; (2) a t the c e n t e r o f the square base o f a pyramid w i t h oxygen a t the v e r t i c e s , ( p y r a m i d a l (4+1)); and (3) a t the c e n t e r o f a distorted octahedra with oxygen a t the v e r t i c e s , (distorted o c t a h e d r a (4+2)), (see F i g 1 ) . I n square p l a n a r , the Cu-0 spacing i s about 1.95Â. In the p y r a m i d a l and d i s t o r t e d o c t a h e d r a , there are 4 s u r r o u n d i n g oxygen at s h o r t d i s t a n c e s (1.95À) comparable t o those found i n square p l a n a r , and 1 o r 2 d i s t a n c e s s i g n i f i c a n t l y l o n g e r (2.3À). The n o t a t i o n (4+1) and (4+2) i s used to denote the f a c t t h a t 4 o f the Cu-0 d i s t a n c e s a r e s h o r t and 1 o r 2 a r e l o n g e r . c

Table II l i s t s the c o o r d i n a t i o n o f oxygen about the central copper atom i n each of the h i g h T c e r a m i c s u p e r c o n d u c t o r s and i t lists typical Cu-0 distances. In the crystal structures, the coordination polyhedra are arranged so that the square planar c o n f i g u r a t i o n s are p e r p e n d i c u l a r to the c - a x i s o f the u n i t c e l l and the l o n g a x i s o f the pyramids and o c t a h e d r a a r e p a r a l l e l t o i t . W i t h i n p l a n e s o f Cu-0 atoms, which are p e r p e n d i c u l a r to the u n i t c

CHEMISTRY OF H I G H - T E M P E R A T U R E

230

Ο

SUPERCONDUCTORS

Copper


Oxygen

a b

c F i g u r e 1. C o o r d i n a t i o n p o l y h e d r a o f copper and oxygen atoms i n high T superconductors: (a) square p l a n a r , (b) p y r a m i d a l (4+1) and ( c ) d i s t o r t e d o c t a h e d r o n (4+2) . The d i s t a n c e χ i s about 1.95À and t h e d i s t a n c e y about 2.3À. c

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231

c e l l ' s c a x i s , the t y p i c a l d i s t a n c e i s 1.95Â. P a r a l l e l t o the c a x i s , the Cu-0 d i s t a n c e s are e i t h e r s h o r t e r or l o n g e r . Ihus a l l s t r u c t u r e s c a n be viewed as h a v i n g p a r a l l e l p l a n e s o f Cu-0 s h e e t s i n w h i c h the Cu-0 s p a c i n g i s 1.95Â. The r a r e e a r t h and oxygen atoms a r e a l s o a r r a n g e d i n p l a n e s p e r p e n d i c u l a r to the c - a x i s . I n Y B a C u 0 , there i s a square p l a n a r r i b b o n at the b a s a l p l a n e s of the u n i t c e l l which i s p a r a l l e l to the c a x i s and i n which the Cu-0 distance is o n l y 1.85À. The longest distance i n t h i s structure i s also p a r a l l e l t o the c a x i s and i s about 2.3Â, e x i s t i n g i n b o t h the d i s t o r t e d o c t a h e d r a and 4+1 pyramidal s t r u c t u r e . F i g u r e 2 shows c r y s t a l s t r u c t u r e s of the f o u r c l a s s e s o f s u p e r c o n d u c t i n g m a t e r i a l s , showing some elements o f the Cu-0 c o o r d i n a t i o n p o l y h e d r a .


2

Table I I .

Comparison o f C o o r d i n a t i o n P o l y h e d r a f o r Cu-0 S u p e r c o n d u c t i n g Ceramic Oxide Phases

Compounds L a

A

2-x x

La Ba 3

3

C u 0

2

χ

4-x/2+S

Cu 0 6

L a _ Α) x

C u

+

Dis t a n c e s

Coordination

0

octahedra

-x/2

2

3

6

5

+

square

8

Cu -0i(x4) Cu -0-,(x4) Cu - 0 ( x l ) Cu - 0 ( x 4 ) Cu - 0 ( x l )

1.,954 1.,954 1.,723 1.,959 2..333

(13)

Cu- 0-, (x4) Cu- 0 ( x l )

1,.94 2..27

(14)

3

-OT(X2)

1,.85 1,.94 1..94 1..96 2..3

(15,16)

3

-0 -0 -0 -0

2

2

planar

3

4

2

4

3

Cu-, Cu., Cu Cu Cu

pyramidal(4+1)

Befs

A

(12)

2

YBa Cu 0

in

1.,937 2.,27

2

pyramidal(4+1)

7

Cu- 0-, (x4) Cu- 0 ( x 2 ) 2

square p l a n a r d i s t o r t e d octahedra (4+2) pyramidal(4+1)

1 4 + 5

2 6

distorted (4+2)

3

2

1

4

2

3

(x4) (x4) (x2) (x2)

The s u p e r c o n d u c t i n g and e l e c t r o n i c t r a n s p o r t p r o p e r t i e s o f t h e s e m a t e r i a l s a r e v e r y s e n s i t i v e to t h e i r oxygen c o n t e n t , thus i t i s i m p o r t a n t to u n d e r s t a n d where oxygen i s added or s u b t r a c t e d i n the u n i t c e l l . T a b l e I I I l i s t s p o s i t i o n s where oxygen i s added and i t s e f f e c t on T . For Y B a C u O , oxygen i s added o r s u b t r a c t e d from b a s a l p l a n e s i n the 0 1/2 0 and 1/2 0 0 p o s i t i o n s . A l s o , oxygen can be o r d e r e d on the 0 1/2 0 s i t e s l e a v i n g the 1/2 0 0 s i t e vacant. T h i s r e s u l t s i n an o r t h o r h o m i c d i s t o r t i o n w i t h b/a « 1 . 7 % , a unique one dimensional character to the structure and excellent s u p e r c o n d u c t i n g p r o p e r t i e s (17). For L a B a C u 0 added oxygen f i l l s the s i t e 1/2 1/2 1/2 between two (4+1) pyramids, thus f o r m i n g two distorted octahedra along the c axis. One octahedron i s e l o n g a t e d and the o t h e r i s compressed. S e v e r a l groups (9,18) have found T o n s e t s o f 70K+ i n L a - B a Cu 0 i n samples a n n e a l e d i n a few atmospheres o f oxygen. c

2

3

x

3

c

the

3

x

3 + x

3

6

6

1 4

1 5

I f s i m p l e v a l e n c e s a r e c o n s i d e r e d , the a d d i t i o n o f oxygen has e f f e c t o f r a i s i n g a p o r t i o n o f the c a t i o n s to a h i g h e r v a l e n c e

232


CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS

Figure 2. Crystal structures of ceramic high T superconductors. (a) K N i F structure showing layers of d i s t o r t e d o c t a h e d r a (4+2). (b) Y B a C u 0 structure with nearly square p l a n a r and p y r a m i d a l (4+1) Cu-0 c o o r d i n a t i o n p o l y h e d r a , (c) Er-Rakho structure with square planar configurations perpendicular t o , and d i s t o r t e d o c t a h e d r a w i t h t h e l o n g e r a x i s parallel to the c axis of the u n i t cell, (d) t h e L a _ S r + C u 0 _ / + £ w i t h (4+1) Cu-0 p y r a m i d a l coordinations. The small solid circles are copper atoms, the l a r g e open c i r c l e s a r e oxygen atoms, and the shaded c i r c l e s a r e the r a r e e a r t h , Y, Ba, o r S r atoms. c

2

4

2

2

x

1

x

2

6

x

2

3

7

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233

s t a t e , or more l i k e l y , h y b r i d i z i n g Cu-0 bonds p r o d u c i n g metallic h o l e - t y p e c o n d u c t i o n . I t i s c o n v e n i e n t , however, to r e f e r t o these m a t e r i a l s as i f they have some C u , even though i n a f o r m a l sense t h i s i o n may n o t e x i s t . + 3

Table I I I .

Compound


L a

A

2-x x

C u

4-x/2+5

3

Effects

6

1 4 + 5

on

Effect

Location

La Ba Cu 0 3

L o c a t i o n o f 8 Oxygen Atoms and Sup e r c o n d u c t i v i t y

+ 3

+ 2

vacant octahedral corners

increases C u / C u r a i s e s T~

vacant corner of octahedron at unit c e l l center

changes Cu c o o r d i n a t i o n , pyramidal to o c t a h e d r a l increases C u / ratio, s e m i c o n d u c t o r to 90K T + 3

ratio

+ 2

r

YBa Cu 0 2

3

vacant s i t e 0 1/2 0

6 > 5 + 5

p e r f e c t s square p l a n a r s i t e s , increases C u / , orthorhombic distortion, 55 t o 90K raises T

at

+ 3

+ 2

r

Processing: A l l these m a t e r i a l s can be p r o c e s s e d i n the same g e n e r a l manner. One s t a r t s w i t h powders of the r a r e e a r t h o x i d e (M 0 ) or Y 0 , copper o x i d e CuO, and the c a r b o n a t e of barium, s t r o n t i u m or c a l c i u m . The c a r b o n a t e s and r a r e e a r t h o x i d e s can be u s e d ^n t h e i r a s - r e c e i v e d c o n d i t i o n , b u t we have f o u n d t h a t CuO requires additional milling to break up coarse particles. In addition, the powders s h o u l d be p r e d r i e d t o remove any adsorbed m o i s t u r e p r i o r to weighing. When d r y i n g , care s h o u l d be taken to avoid agglomerate formation. The weighed powders are then t h o r o u g h l y mixed i n a m i l l or m o r t a r and p e s t l e . The powders a r e then c a l c i n e d i n open, f l a t crucibles. In the c a l c i n i n g step, carbonates are decomposed to the oxides and C0 , and a multicomponent o x i d e i s formed, i . e . 2

3

2

3

2

1/2

Y 0 2

3

+ 2 BaC0

3

+ 3CuO

> YBa Cu 0 2

3

6

5

+

2C0

2

The above f o r m u l a assumes no a d d i t i o n or d e p l e t i o n of oxygen from the ambient d u r i n g the c a l c i n i n g s t e p , an assumption n o t always valid. The powders are not p e l l e t i z e d p r i o r to c a l c i n i n g largely due to a l a r g e m o l a r volume change (-30%) between r e a c t a n t s and products. One problem i n c a l c i n i n g i s t h a t the c a r b o n a t e s r e m a i n s t a b l e and do not always decompose. Furthermore, a t the r e a c t i o n temperatures p a r t i c l e s i n t e r i n g and g r a i n growth o c c u r . Thus, the c a l c i n e d m a t e r i a l s must be r e m i l l e d . By m o n i t o r i n g the c a l c i n i n g s t e p w i t h x - r a y d i f f r a c t i o n , we have f o u n d t h a t 3-4 hr a t 925C i s s u f f i c i e n t time f o r c a l c i n i n g , p r o v i d e d a l l s t a r t i n g powders a r e 10μ or l e s s . Once c a l c i n i n g i s complete, the major molar volume changes have t a k e n p l a c e and the m i l l e d powders can be p e l l e t i z e d f o r s i n t e r i n g . Here a m u l t i p l e o f t r a d i t i o n a l ceramic p r o c e s s i n g steps can be u s e d

234

CHEMISTRY OF H I G H - T E M P E R A T U R E S U P E R C O N D U C T O R S

to form the m a t e r i a l t o i t s d e s i r e d shape. The powders can be cold-pressed, with or without a binder, isostatically pressed, hot-pressed, mixed w i t h b i n d e r s and e x t r u d e d i n t o sheets, tubes, etc. Some members o f our group, i n c o l l a b o r a t i o n w i t h r e s e a r c h e r s at Brookhaven, have c o n s o l i d a t e d powders by plasma and flame s p r a y i n g (19)


Once powders are c o n s o l i d a t e d and f i r e d , the individual p a r t i c l e s s i n t e r t o g e t h e r . T h i s step i s a i d e d by a f i n e powder s i z e (l-ΙΟμ) and a uniform d i s p e r s i o n of powders (no large v o i d s ) . Typically 6-12 hours a t 900-950C w i l l s i n t e r the p a r t i c l e s t o 80% density. The f i n a l step i n p r o c e s s i n g s e v e r a l of the h i g h T ceramic superconductors is critical f o r good p r o p e r t i e s . The sintered samples are s l o w l y c o o l e d i n f l o w i n g 0 and h e l d at some lower t e m p e r a t u r e t o i n c r e a s e the oxygen c o n t e n t o f the compound. For example, c

2

ABa Cu 0 2

3

6

5

(from c a l c i n i n g ) + 5/2

0

2

—>

ABa Cu 0 2

3

6

5 +

£

The e f f e c t s o f t h i s step on s u p e r c o n d u c t i v i t y can be d r a m a t i c . For Y B a C u 0 , the f u l l y oxygenated samples show a complete Meissner e f f e c t and R = 0 at temperatures i n excess of 92K. Without the a d d i t i o n a l oxygen, the t r a n s i t i o n s are broad and the f l u x e x p u l s i o n only partial (17). Table IV gives the composition ranges of i n t e r c a l a t i o n o f oxygen (20). The h i g h e s t v a l u e s o f δ are o b t a i n e d w i t h long term a n n e a l s at about 500C under one atm of oxygen; lower v a l u e s o f δ are o b t a i n e d w i t h lower p a r t i a l p r e s s u r e s . T a b l e IV a l s o shows the l a r g e e f f e c t i n t e r c a l a t i o n has on the room t e m p e r a t u r e electrical c o n d u c t i v i t y , w i t h samples becoming more m e t a l l i c as oxygen i s added. 2

3

Table

7

IV.

Known Ranges o f Oxygen I n t e r c a l a t i o n i n H i g h T Superconductors

Compound La _ A Cu . 2

L a

x

x

4

A

2-x 1+x

C u

2 °6-x/2 +5

La Ba Cu 0 3

3

6

YBa Cu 0 2

3

6

5

Individual

x / 2 + 5

+

1 4 + 5

8

I n t e r c a l a t i o n Range δ 0-0.3 0

"

0

·

2

0-0.4 0.4-1.0 (3

Log

Ceramic

c

Cond. Range (300K) 1-1.8 1

2

" '

5

0.5-1.9 atm)

0-0.5

Compounds

Y B ^ C i ^ 0^ : Processing this compound follows the general p r o c e d u r e o u t l i n e d above. A t t e m p e r a t u r e s above 700C, Y B a C u 0 has a t e t r a g o n a l s t r u c t u r e (21). I t s oxygen c o n t e n t i s b e l i e v e d to be YBa Cu 0 . Below 700C, the u n i t c e l l i s o r t h o r h o m b i c and the 2

2

3

6

5

3

x

22.

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235

composition adjusts towards YBa Cu Ογ . The critical step in processing i s to add t h i s 0.5 oxygen atom per u n i t c e l l . T h i s i s c r i t i c a l because t h e s e oxygen atoms and t h e i r p r o p e r o r d e r i n g on 0 1/2 0 s i t e s d r a m a t i c a l l y improve s u p e r c o n d u c t i n g p r o p e r t i e s . I n such samples the T o n s e t s are 93K, the t r a n s i t i o n w i d t h s IK, and a f u l l Meissner e f f e c t observed, i . e . the sample l e v i t a t e s i n a s t r o n g magnetic f i e l d . The presence o f the o r d e r e d i n t e r c a l a t e d oxygen atoms causes a d i s t i n c t o r t h o r h o m b i c d i s t o r t i o n i n the u n i t c e l l o f about 1.7% (17,21). I t s p r e s e n c e can be r e a d i l y o b s e r v e d from a s p l i t t i n g or s h o u l d e r i n g the main X-ray d i f f r a c t i o n peak a t 32-33°. N o r m a l l y the e x t r a oxygen can e n t e r d u r i n g slow c o o l i n g (l°/min) from 900C i n f l o w i n g oxygen. In some i n s t a n c e s , f u r n a c e c o o l i n g i s sufficient or a l t e r n a t i v e l y one can anneal the sample a t about 500-600C. We have o b s e r v e d , however, t h a t some o f f - s t o i c h i o m e t r i c samples t r a n s f o r m to the o r t h o r h o m b i c i n a v e r y s l u g g i s h manner. At this time, the reasons for t h i s sluggishness in reaction are unclear. As another word of c a u t i o n , we and others have observed that YBa Cu 0 g r a d u a l l y decomposes when heated i n a i r above 950C (22,23) . By a n n e a l i n g a sample i n a i r a t 975C f o r 12 h o u r s , we have nearly completely decomposed the phase i n t o Y BaCuC^ , CuO and p r o b a b l y BaCu0 . R e h e a t i n g t h i s same sample at 975C i n f l o w i n g 0 g r a d u a l l y r e f o r m s the Y B a C u 0 phase. 2

3

0


c

2

3

6

5

2

2

2

2

3

6

5

The i n d i v i d u a l g r a i n s o f samples w i t h a pronounced o r t h o r h o m b i c distortion show s t r i a t i o n s or bands when v i e w e d i n an electron m i c r o s c o p e (see f i g u r e 3) . T h i s i s b e l i e v e d to be due to domain formation; i n one band the "b" a x i s o f the u n i t c e l l i s o r i e n t e d 90° to the "b" a x i s o f the a d j a c e n t band. E l e c t r o n microscopy also shows that small deviations from stoichiometry result in an amorphous second phase f o r m i n g i n the g r a i n b o u n d a r i e s . Because most o f the phases i n e q u i l i b r i u m w i t h Y B a C u 0 are i n s u l a t o r s , a g r a i n boundary phase i s p r o b a b l y an i n s u l a t o r . 2

3

7

We have measured a l a r g e number o f s u p e r c o n d u c t i n g p r o p e r t i e s on well characterized YBa Cu 0 w i t h a 1.7% orthorhombic d i s t o r t i o n . These a r e l i s t e d i n T a b l e V. 2

3

7

Laj_ Ba Cu O ^: When p r o c e s s e d by the g e n e r a l p r o c e d u r e , t h i s sample i s not s u p e r c o n d u c t i n g . M i t z i e t a l . (9_) showed t h a t samples annealed for 24 hours at 450C in 3.5 atm of oxygen are s u p e r c o n d u c t o r s , i f χ > 0.75, w i t h T onsets o f about 90K. We have verified these experiments (19). We find that the resistance behavior of La Ba Cu 0 £ a l s o becomes much more m e t a l l i c after annealing i n 3 atm of 0^ . L a B a C u 0 1 4 + 5 becomes s u p e r c o n d u c t i n g with T onset greater than 75K. The latter material shows a distinct second phase, however, so we cannot rule out the p o s s i b i l i t y t h a t i t i s c o n t r i b u t i n g to s u p e r c o n d u c t i v i t y . Also with x-ray analysis i t is d i f f i c u l t to unambiguously decide i f the c r y s t a l s t r u c t u r e of t h i s m a t e r i a l i s the Er-Rakho (13) or Y B a C u type. In both structures the cations have nearly identical p o s i t i o n s , o n l y the oxygen p o s i t i o n s are s i g n i f i c a n t l y d i f f e r e n t . Both are v e r y similar layered s t r u c t u r e s . The x-ray d i f f r a c t i o n s i g n a t u r e s , which are not s e n s i t i v e to oxygen, are n e a r l y i d e n t i c a l . We see e v i d e n c e f o r the v e r y weak l i n e a t 20=16 as r e q u i r e d f o r the (100) d i f f r a c t i o n i n the Er-Rakho t e t r a g o n a l s t r u c t u r e b u t a l s o some evidence for peak s h o u l d e r i n g which may indicate it is an o r t h o r h o m b i c s t r u c t u r e and not t e t r a g o n a l . I t i s a l s o p o s s i b l e that x

3 + x

6

U +

c

3

3

6

1 4 +

2

4

6

c

2

3

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CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS

Figure defects i n the because

3. Transmission electron m i c r o g r a p h showing planar i n YBa Cu 0 u n i f o r m l y s p a c e d about 2000Â and d i s p e r s e d specimens. These have been i d e n t i f i e d as twins formed o f the s l i g h t d i f f e r e n c e i n the a and b axes. 2

3

7

22.

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T a b l e V.

P h y s i c a l and

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Superconductors

Properties of Y B a C u 0 2

3

7

Lattice

Parameters (A) 3.822+0..002 a 3.888+0,.002 b 11.672+0,.005 c Volume ( A ) 173.4 % Distortion 1.7 6.94 Oxygens p e r u n i t c e l l T o n s e t (K) 93 R=0 (K) 91 100 % flux expulsion ρ (94K) (μΩ-cm) 200 22-36* (dH /dT) (kG/K) 1470-2370* H (0) (kG) H (4.2K) (kG) 0.8 H (0) (kG) 20-26* 105 + J ( 4 K ) (A/cm ) ^ C a l c u l a t e d from c r i t i c a l f i e l d measurements ^ E s t i m a t e d from m a g n e t i z a t i o n s t u d i e s 3


c

c

T c

c 2

c 1

c

2

C

b o t h t e t r a g o n a l and o r t h o r o m b i c phases e x i s t as s u g g e s t e d by Lee et. al. (24). Because of the importance of this material in understanding s u p e r c o n d u c t i v i t y i n high T ceramics, i t s structure s h o u l d be r e f i n e d by n e u t r o n d i f f r a c t i o n . c

L a _ A C u O 4 _ / +s: 2

x

x

Χ

2

A=Sr,Ba,Ca.

The optimum c o m p o s i t i o n s f o r h i g h T s u p e r c o n d u c t i n g p r o p e r t i e s are f o r x=0.1 t o 0.2. Samples are p r e p a r e d a c c o r d i n g to the g e n e r a l procedure although p r o c e s s i n g temperatures of about 1100C are reported. The b e s t samples a r e prepared i n flowing 0 . Slow c o o l i n g and a n n e a l i n g at a lower temperature (500C) i n c r e a s e T by 1 to 2 degrees and a l s o sharpen T . L i k e w i s e , a n n e a l i n g i n vacuum destroys superconductivity. Reannealing in 0 restores superconductivity. The degree o f oxygen i n t e r c a l a t i o n i s s m a l l , 8=0 to 0.2 ( 2 0 ) . c

2

c

c

2

I f χ i s s m a l l , l e s s t h a n 0.075, a n n e a l i n g i n 0^ lowers T (25) . If x=0, samples are n o t superconducting. A i r - q u e n c h i n g samples a n n e a l e d a t 800-1000C cause a v e r y s m a l l p a r t o f the sample to superconduct (27) . This is probably grain boundary superconductivity because while R tends to zero, s u s c e p t i b i l i t y measurements show o n l y a t r a c e o f s u p e r c o n d u c t i v i t y . A t v e r y low t e m p e r a t u r e s , pure La Cu0 t r a n s f o r m s from an o r t h o r h o m b i c to an unknown structure (28) . It has been speculated that this t r a n s f o r m a t i o n may somehow i n h i b i t s u p e r c o n d u c t i v i t y (29^) . c

2

4

Y _ Ba Cu0 _ ^ +5 : ^ ^as been s u g g e s t e d t h a t t h i s phase i s a high T superconductor (10) . This o b s e r v a t i o n has never been c o n f i r m e d , and i n f a c t , i t i s d o u b t f u l t h a t t h i s p a r t i c u l a r phase even e x i s t s . N e v e r t h e l e s s , the c r y s t a l s t r u c t u r e of t h i s f a m i l y o f 2

x

1 + x

c

6

x

2

238

CHEMISTRY OF H I G H - T E M P E R A T U R E S U P E R C O N D U C T O R S

(

!u

(

)

s

a

compounds, -x^1+x - 2 - 6-x/2+δ ^ l a y e r e d one w i t h c o o r d i n a t i o n p o l y h e d r a , l i k e those found i n the h i g h T s u p e r c o n d u c t o r s and the compounds' e l e c t r i c a l c o n d u c t i v i t i e s show t h e same s e n s i t i v i t y t o oxygen i n t e r c a l a t i o n . Thus the compounds are o b v i o u s c a n d i d a t e s f o r superconductivity. c

Summary : H i g h T ceramic superconductors have layered crystal structures with Cu-0 coordination polyhedra typical o f Cu+2. Intercalation of additional oxygen is critical to the s u p e r c o n d u c t i n g p r o p e r t i e s . A l l these m a t e r i a l s can be p r o c e s s e d i n a s i m i l a r manner by t r a d i t i o n a l c e r a m i c p r o c e s s i n g t e c h n i q u e s . c


Acknowledgments The authors acknowledge t h e h e l p of W. L e c h t e r and R. Rayne i n sample p r e p a r a t i o n and u s e f u l d i s c u s s i o n s w i t h D. Schrodt, M. Kahn, D. Lewis and W. L e c h t e r . B. Wood and R. C a r p e n t e r helped i n manuscript p r e p a r a t i o n .

Literature Cited 1. J.G. Bednorz and K.A. Muller, Z. Phys. Rev. Lett, Β 64, 189 (1986). 2. Μ.Κ. Wu, J.R. Ashburn, C.T. Torng, P.H. Hor, R.L. Meng, L. Gao, Z. Huang, Y.Q. Wang and C.W. Chu, Phys Rev Lett, 58, 908 (1987). 3. P.M. Grant, S.S.P. Parkin, V.Y. Lee, E.M. Engler, M.L. Ramirez, J.E. Vazquez, G. Lim, R.D. Jacowitz and R.L. Greene, Phys Rev Lett, June 1987 (to be published). 4. S. Uchida, H. Takagi, K. Kitazawa and S. Tanaka, Jpn J Appl Phys Lett 26,L1,(1987). 5. C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J Huang, and Y.Q. Wang, Phys Rev Lett, 58, 405 (1987). 6. D.U. Gubser, R.A. Hein, S.H. Lawrence, M.S. Osofsky, D.J. Schrodt, L.E. Toth and S.A. Wolf, Phys Rev Lett, Β 35, 5350 (1987) and references therein. 7. P.H. Hor, R.L. Meng, Y.Q. Wang, L. Gao, Z.J. Huang, J. Bechtold, K. Forster and C.W. Chu, Phys Rev Lett, 58, 1891 (1987) 8. D.W. Murphy, S. Sunshine, R.B. VanDover, R.J. Cava, B.Batlogg, S.M. Zahurak and L.F. Schneemeyer, Phys Rev Lett, 58, 1888 (1987) . 9. D.B. Mitzi, A.F. Marshall, J.Z. Sun, D.J. Webb, M.R. Beasley, T.H. Geballe and A. Kapitulnik (submitted to Phys Rev). 10. Shiou-Jyh Hwu, S.M. Song, J. Thiel, K.R. Poeppelmeier, J.B. Ketterson and A.J. Freeman, Phys Rev Lett, Β 35, 7119 (1987). 11. A. F. Wells, Structural Inorganic Chemistry 5th Ed. Claredon Press: Oxford, 1984. 12 R. J. Cava, A. Santoro and D.W. Johnson Jr., (to be published). 13. L. Er-Rakho, C. Michel, J. Provost and B. Raveau, J. Solid State Chem, 37, 151 (1981). 14. N. Nguyen, L. Er-Rakho, C. Michel, J. Choisnet and B. Raveau, Mate Res Bull, 15, 891 (1980). 15. M.A. Beno, L. Soderholm, D.W. Capone II, D.G. Hink, J.D. Jorgensen, Ivan K. Schuller C.U. Segre, K. Zhang and J.D. Grace, Appl Phys Lett (to be published).

22.

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16.

F. Beech, S. Miraglia, A. Santoro and R.S. Roth, (to be published). L.E. Toth, E.F. Skelton, S.A. Wolf, S.B. Qadri, M.S. Osofsky, B.A. Bender, S.H. Lawrence and D.U. Gubser, (submitted for publication to Phys Rev) and also E.F. Skelton, S.B. Qadri, B.A. Bender, A.S. Edelstein, W.T. Elam, T.L. Francavilla, D.U. Gubser, R.L. Holtz, S.H. Lawrence, M.S. Osofsky, L.E. Toth and S.A. Wolf, MRS Proceedings, California 1987 (to be published). Unpublished result, NRL. J.P. Kirkland, R.A. Neiser, H. Herman and W.T. Elam, (unpublished). C. Michel and B. Raveau, Rev Chim Minerals, 21, 407 (1987). Ivan K. Schuller, D.G. Hinks, M.A. Beno, D.W.Capone II, L. Soderholm, J.-P. Locquet, Y. Bruynseraede, C.U.Segre, and K. Zhang, (submitted for publication, Solid State Commun.) Unpublished result, NRL. T.L. Aselage, B.C. Bunker, D.H. Doughty, M.O. Eatough, W.F. Hammetter, K.D. Keefer, R.E. Loehman, B. Morosin, E.L. Venturini and J.A. Voigt, MRS Proceedings, California 1987 (to be published). Sung-Ik Lee, John P. Golben, Sang Y. Lee, Xuao-Dong Chen, Yi-Song, Tae W. Noh, R.D. McMichael, Yve Cao, Joe Testa, Fulin Zuo, J. R. Gaines, A.J. Epstein, D.L Cox, J.C. Garland, T.R. Lemberger, R. Sooryakumar, Bruce R. Patton and Rodney T. Tettenhorst, MRS Proceedings, California 1987 (to be published). J.M. Tarascon, L.H. Greene, W.R. Mckinnon, G.W. Hull and T.H. Geballe, Science, 235, 1373 (1987). P.M. Grant, S.S.P. Parkin, V.Y. Lee, E.M. Engler, M.L. Ramirez, J.E. Vazquez, G. Lim, R.D. Jacowitz and R.L. Greene, Phys. Rev. Lett. (to be published). E.F. Skelton, W.T. Elam, D.U. Gubser, V. Letourneau, M.S. Osofsky, S.B. Qadri, L.E. Toth and S.A. Wolf,(submitted Phy. Rev. Lett.). R.V. Kasowski, W.Y. Hsu and F. Herman, Solid State Commun., (to be published).

17.


18. 19. 20. 21. 22. 23.

24.

25. 26. 27. 28.

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