Vacuum annealing effects on Bi1.8Pb0.2Sr2Ca2Cu3O9.9+y high temperature superconductor

PIIVBG

Physica C 185-189 (1991) 835-836 North-Holland

VACUUM ANNEALING SUPERCONDUCTOR Yoshlhtde

EFFECTS

KIMISHINA a n d J u n

ON

Blz.aPbe.2Sr2Ca2Cu3Og.9÷y

HIGH

TEMPERATURE

NAKANO

Department of Physics, Faculty of Engineering, Yokohama N a t i o n a l T o k l w a d a l 1 5 6 , H o d o g a y a - k u , Y o k o h a m a 2 4 0 , JAPAN The l a t t i c e fraction rate

University,

onset temperature Te 0n a n d v o l u m e fs were investigated for the Biz.ePbe.2Sr2Ca2Cu309.9÷~-8 system. Here y~J).3 and 6 Is the amount of oxygen deficiency per chemical formula unit by vacuum annealing. The s t e e p c h a n g e s of a, c , Te 0n a n d f s w e r e o b s e r v e d a t 0 < 8 < 0 . 3 , a n d t h e f s b e c a m e z e r o at 8=ac~1.6. The above experimental results were discussed on the basis of hole donor reduction by the oxygen deficiency.

1.

parameters a and c, superconducting of high T¢ superconductive phase

INTRODUCTION

In the previous paper, the deoxldation effects by vacuum annealing were reported for Bi2Sr2CaCu2Oe+y (Bi2212)system (1). In that system, the steep decrease of superconducting onset temperature T¢ 0~ a n d v o l u m e f r a c t i o n rate fs were observed in 0<8<0.2, where 8 Is the oxygen deficiency per chemical formula unit, and they were explained by the h o l e donor r e d u c t i o n . While the decomposition t o the o t h e r n o n - s u p e r c o n d u c t i v e o x i d e s oc~ured in 0.2<651. In the p r e s e n t s t u d y , the vacuum a n n e a l i n g e f f e c t s were i n v e s t i g a t e d f o r Bil.ePbe.2Sr2CatCu309.9÷x (Bi2223) system. The Bi2223-phase i s composed o f t h r e e CuO2-1ayers sandwltched between t w o - C a - l a y e r s , or (BI-Pb)O-SrOand C a - l a y e r ( 2 ) . To e l u c i d a t e the r o l e of e x c e s s oxygen f o r the superc o n d u c t i v i t y i n t h i s system, t h e X-ray d i f f r a c t i o n s and m a g n e t i z a t i o n M were measured f o r oxygen d e f i c i e n t Bi2223samples. 2. SAMPLE PREPARATION The o r i g i n a l ( 8 = 0 ) Bi2223-sampx3 was o b t a i n e d by t h e following procedure, The powdered B1203, Pb02, SrC03, CaC03 and CuO were mixed with t h e r a t i o of 0 . 9 2 : 0 . 3 4 : 1 . 9 1 : 2 . 0 3 : 3 . 0 6 . The mixture was p r e s s e d and p r e f i r e d in t h e furnace a t 1116 K f o r 50 hours in a i r . Next, t h e p r e s l n t e r e d sample was ground, r e p r e s s - ed and f i r e d a t 1116 K f o r 100 hours. Each v a c u u m a n n e a l i n g sample was o b t a i n e d from t h e above o r i g i n a l one by firing at 823~1023 K for 0~.30 minutes under 0.1 Torr. From t h e elementary analysis by the X-ray micro-analyzer,

i t w a s known t h a t t h e original sample had about 0.3 excess oxygen per formula unit, and the chemical formula of vacuum a n n e a l i n g sample was o b t a i n e d as Btt.aPbe.2Sr2Ca2Cu3Ote,2-8 (0<8~!.6). Thus the vacuum a n n e a l i n g r e s u l t e d In the d e o x l d a t l o n f o r the Bi2223-system, as In the Bi2212-system (1). 3. EXPERIMENTAL RESULTS [ i ] L a t t i c e parameters The l a t t i c e p a r a m e t e r s a and c in 05 8<0.8 a r e given in F i g u r e A. ';hey were determined from the CuK~ X-ray powder diffractions.

5.421

o a

37.2

c

37.1

5.4t I

0

I

I

z

O.5

8

F i g u r e I The a-dependence p a r a m e t e r s a and c.

of

lattice

As shown in F i g u r e I , both of a and c i n c r e a s e d in 0<8<0.3. A b o v e 8=0.3, t h e a was n e a r l y c o n s t a n t , while the c g r a d u a l l y d e c r e a s e d with 6. At S=0.2~ 0 . 3 , the X-ray d i f f r a c t i o n peaks by CaO, Cu20 and p r o b a b l y Bl1.sPbe.2-

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Y. Kimbhima, Y. Nakano / Vacuum annealing effects on Bit.~Pbo.~Sr2Ca2CujOg.9+y

836

SreCaOs., began to appear became dominant at 6~1.6.

and

(3)

[2] Transition temperature The superconducting onset temperat u r e T¢ on o b t a i n e d f r o m t h e t e m p e r a t u r e dependence of magnetization M were g i v e n i n F i g u r e 2. The M was measured by the vibrating sample magnetometer.

from the ratio of the initial gradient o f M-H c u r v e f o r t h e oxygen deficient (0<851.6) sample and that for the original(6=0) s a m p l e , w h e r e M i s t h e 10 0e field cooling magnetization. The results of fs(6) are given In Figure 3. The fs also steeply decreased in 0<6<0.3 as the Tc 0n, and gradually decreased in 0.3<651.6. At 6=1.6, the fs became zero.

@O$ •

•

0@

6%

,

I

,

S Flgure 2 The 8-dependence of superconducting onset temperature T~0n. In 0<6<0.3. the Tc on rapidly decreased, and recovered to the orlglnal (6=0) one by the reoxldation at 1116 K In alr. The nearly constant Tc 0. in 6>0.3 seems to correspond to the above mentioned decomposition to the other non-superconductlve oxides. [31 Superconductive volume

fraction

rate

The superconductive volume rate fs(B)=Vs(6)/Vs(O) was

fraction obtained

\

0 Figure 3 conductive

1 S

2

The 6-dependence of supervoiume fraction rate fs.

4 . DISCUSSIONS The rapid changes of lattice parameters a and c, superconducting onset temperature Tc 0n a n d superconductive volume fraction rate fs In 0<6<0.3, show t h e deoxldatlon effect on t h e Bll.sPbe. RSr2Ca2CU3Ole.2-system by the vacuum annealing. On t h e assumption that the hole donor is the excess oxygen of 0.3 per formula unit, t h e 8¢, at which the Tc 0n a n d fs disappear, should be about 0.3. The experimental 6c o f a b o u t 1 . 6 is due t o t h e d e c o m p o s i t i o n to the other oxides, mentioned above. When the decomposition t o Bll.sPbe.2Sr2CaOs.9 + (3/2)Cu20 + CaO completes, the 6 equals to 1.8 and close to the present result of 6c~1.6.

REFI~CES (1) Y.Klmlshlma, H.Kitsutaka and J.Nakano, P h y s i c a B, 1 6 5 & 1 6 6 ( 1 9 9 0 ) 1681. (2) M.Takano, J.Takada, K.0da, H. K~aguchi, Y.Mlura, Y.Ikeda, Y. Tomli and H.Mazakt, Jpn.J.Appl. Phys., 27(1988)L1476. (3) H.Nagano, R.Llang, Y.Matsunaga, M.Suglyama, M.Itoh and T,Nakamura, Jpn.J.Appl.Phys., 28(1989)L364.