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Japanese Journal of Applied Physics 

Vol. 27, No. 2. February, 1988, pp. L209-L210 



A New High-r c Oxide Superconductor without a Rare Earth Element 

Hiroshi Maeda, Yoshiaki Tanaka, Masao Fukutomi and Toshihisa Asano 
National Research Institute for Metals, Tsukuba Laboratories, Ibaraki 305 
(Received January 22, 1988; accepted for publication January 23, 1988) 

We have discovered a new high-7; oxide superconductor of the Bi-Sr-Ca-Cu-O system without any rare earth de- 
ment. The oxide BiSrCaCuAr has T e of about 105 K, higher than that of YBa 2 Cu 3 0 7 by more than 10 K. In this oxide, 
the coexistence of Sr and Ca is necessary to obtain high T Q . 

KEYWORDS: oxide superconductor, Bi-Sr-Ca-Cu-0 system, rare earth, high T c , new stable superconductor 



Soon after the discovery of high-7; superconductors of 
the layered perovskites (LaBa^CuO/* and (LaSr^CuCV* 
with T c of about 40 K, YBa 2 Cu 3 C>7 3) with T c of 94 K was 
synthesized. The discovery of these materials stimulated 
many researchers to investigate new oxide superconduc- 
tors of still higher T c and extensive studies have been car- 
ried out to search for these oxides. Up to now, however, 
no new stable supercondutors with T c higher than that of 
YBa 2 Cu 3 0 7 have been reported. The values of T c have 
not improved by the substitution of other rare earth 
elements for yttrium. 

In order to find high-7; superconductors, we believe 
that it is important to investigate other classes of oxides 
which do not include rare earth elements. This led us to 
study the superconducting oxide system including the 
Vb-element group such as Bi and Sb of trivalent 
elements, and we discovered a new high-7; superconduct- 
ing material BiSrCaCu 2 O r . This oxide has T c of about 
105 K, being higher than that of YBa 2 Cu 3 0 7 by more 
than 10 K. 

The value of T c in the Bi-Sr-Cu-O oxide system which 
does not include Ca is very low being about 8 K. 4,5) In 
order to obtain high T C9 the coexistance of Sr and Ca in 
the Bi oxide system is found to be absolutely necessary. 

The Bi-Sr-Ca-Cu-O oxide samples were prepared 
from powder reagents of Bi 2 0 3 , SrC0 3 , CaC0 3 and CuO. 
The appropriate amounts of powders were mixed, calcin- 
ed at 800-870°C for 5 h, thoroughly reground and then 
cold-pressed into disk-shape pellets (20 mm in diameter 
and 2 mm in thickness) at a pressure of 2 ton/ cm 2 . Most 
of the pellets were sintered at about 870°C in air or in an 
oxygen atmosphere and then furnace-cooled to room tem- 
perature. 

The electrical resistivity was measured by the standard 
four-probe method for a bar-shaped specimen of about 
1x2x20 mm 3 cut out from the pellets. Magnetization 
measurements were carried out with a vibrating sample 
magnetometer. The temperature was measured by 
Au7%Fe-Chromel thermocouples. Figure 1 shows the re- 
sistivity vs temperatue curves of BiSrCaCu 2 O x oxides 
thus prepared. Specimen (a) was sintered at a relatively 
low temperature of 800 °C for 8 h while specimen (b) was 
sintered at a higher temperature of 882°C for 20min 
followed by annealing at 872°C for 9 h. In the case of the 
lower sintering temperatue, the onset temperature (7*?°) 
of the superconducting transition is about 83 K and the 
zero resistance state (r? ff ) is reached at 75 K (low-7; 



phase). On the other hand, in the case of a higher sinter- 
ing temperature, a high-7; phase appears, the onset tem- 
perature of which is about 120 K and T c extraporated to 
zero resistance is as high as 105 K. The value of Tf is 
higher than that of YBa 2 Cu 3 0 7 by more than 10 K. Since 
a little amount of the low-7; phase still remained in the 
sample, a complete zero resistance state is achieved at 
75 K which corresponds to that of the low-7; phase. We 
have not succeeded in synthesizing the oxides with a 
single phase of the high-7; material at this moment. 
From our preliminary experiments, we know that sinter- 
ing at high temperatures for a short duration of time is 
effective enough to increase the relative amount of the 
high-7; phase. This may indicate that the high-7; phase is 
stable at elevated temperatures. 

Figure 2 shows the magnetization vs temperature curve 
for the specimen (b) in Fig. 1 which was sintered at the 
higher temperatures. A Meissner effect showing a perfect 
diamagnetic state is observed exactly in the same tempera- 
ture range as in curve (a) shown in Fig. 1. We conclude, 
therefore, that the present high-T c phase is indeed super- 
conducting. 

The high-r c phase appears near the composition ratios 
of Bi:Sr:Ca= 1:1:1. As the composition deviates from 




Temperature ( K ) 
Fig 1 Temperature dependence of resistivities in BiiS^CaiCujO, 
ides (a) sintered in air at 800°C for 8 h, then cooled in a furnace i 
(b) sintered at 882°C for 20 min followed by annealing at 872°C 
9h. 



L209 



Japanese Journal of Applied Physics 

Vol. 27, No. 2,.February, 1988. pp. L209-L210 



A New High-Jc Oxide Superconductor without a Rare Earth Element 

Hiroshi Maeda, Yoshiaki Tanaka, Masao Fukutomi and Toshihisa Asano 
National Research Institute for Metals, Tsukuba Laboratories, Ibaraki 305 
(Received January 22, 1988; accepted for publication January 23, 1988) 

We have discovered a new high-r c oxide superconductor of the Bi-Sr-Ca-Cu-O system without any rare earth ele- 
ment. The oxide BiSrCaCuA, has T c of about 105 K, higher than that of YBa^O, by more than 10 K. In this oxide, 
the coexistence of Sr and Ca is necessary to obtain high T c . 

KEYWORDS: oxide superconductor, Bi-Sr-Ca-Cu-0 system, rare earth, high T ef new stable superconductor 



Soon after the discovery of high-7 c superconductors of 
the layered perovskites (LaBa^CuCV and (LaSr) 2 Cu0 4 2) 
with 7 C of about 40 K, YBa 2 Cu 3 07 3) with 7 C of 94 K was 
synthesized. The discovery of these materials stimulated 
many researchers to investigate new oxide superconduc- 
tors of still higher 7 C and extensive studies have been car- 
ried out to search for these oxides. Up to now, however, 
no new stable supercondutors with 7 C higher than that of 
YBa 2 Cu 3 0 7 have been reported. The values of 7 C have 
not improved by the substitution of other rare earth 
elements for yttrium. 

In order to find high-7 c superconductors, we believe 
that it is important to investigate other classes of oxides 
which do not include rare earth elements. This led us to 
study the superconducting oxide system including the 
Vb-element group such as Bi and Sb of trivalent 
elements, and we discovered a new high-7 c superconduct- 
ing material BiSrCaCu 2 0,. This oxide has 7 C of about 
105 K, being higher than that of YBa 2 Cu 3 0 7 by more 
than 10 K. 

The value of T c in the Bi-Sr-Cu-O oxide system which 
does not include Ca is very low being about 8 K. 4,5) In 
order to obtain high 7 C , the coexistance of Sr and Ca in 
the Bi oxide system is found to be absolutely necessary. 

The Bi-Sr-Ca-Cu-O oxide samples were prepared 
from powder reagents of Bi 2 0 3 , SrC0 3 , CaC0 3 and CuO. 
The appropriate amounts of powders were mixed, calcin- 
ed at 800-870°C for 5 h, thoroughly reground and then 
cold-pressed into disk-shape pellets (20 mm in diameter 
and 2 mm in thickness) at a pressure of 2 ton/ cm 2 . Most 
of the pellets were sintered at about 870°C in air or in an 
oxygen atmosphere and then furnace-cooled to room tem- 
perature. 

The electrical resistivity was measured by the standard 
four-probe method for a bar-shaped specimen of about 
1x2x20 mm 3 cut out from the pellets. Magnetization 
measurements were carried out with a vibrating sample 
magnetometer. The temperature was measured by 
Au7%Fe-Chromel thermocouples. Figure 1 shows the re- 
sistivity vs temperatue curves of BiSrCaCu 2 O x oxides 
thus prepared. Specimen (a) was sintered at a relatively 
low temperature of 800°C for 8 h while specimen (b) was 
sintered at a higher temperature of 882°C for 20min 
followed by annealing at 872°C for 9 h. In the case of the 
lower sintering temperatue, the onset temperature (7? n ) 
of the superconducting transition is about 83 K and the 
zero resistance state (7? ff ) is reached at 75 K (Iow-7 c 



phase). On the other hand, in the case of a higher sinter- 
ing temperature, a high-7 c phase appears, the onset tem- 
perature of which is about 120 K and 7 C extraporated to 
zero resistance is as high as 105 K. The value of Tf is 
higher than that of YBa 2 Cu 3 0 7 by more than 10 K. Since 
a little amount of the low-7 c phase still remained in the 
sample, a complete zero resistance state is achieved at 
75 K which corresponds to that of the low-7 c phase. We 
have not succeeded in synthesizing the oxides with a 
single phase of the high-7 c material at this moment. 
From our preliminary experiments, we know that sinter- 
ing at high temperatures for a short duration of time is 
effective enough to increase the relative amount of the 
high-7 c phase. This may indicate that the high-r c phase is 
stable at elevated temperatures. 

Figure 2 shows the magnetization vs temperature curve 
for the specimen (b) in Fig. 1 which was sintered at the 
higher temperatures. A Meissner effect showing a perfect 
diamagnetic state is observed exactly in the same tempera- 
ture range as in curve (a) shown in Fig. 1. We conclude, 
therefore, that the present high- 7c phase is indeed super- 
conducting. 

The high- 7 C phase appears near the composition ratios 
of Bi:Sr:Ca= 1:1:1. As the composition deviates from 



-i — r- — i i 




B^SrtCaiCuzOx 



j^105K 

■ u. 



40 60 120 160 

Temperature ( K ) 



200 



240 



Fig. 1 . Temperature dependence of resistivities in BhS^C^Cu A, ox- 
ides (a) sintered in air at 800°C for 8 h, then cooled in a furnace and 
(b) sintered at 882°C for 20 min followed by annealing at 872°C for 
9h. 



L209 



L2I0 



Hiroshi Maeda, Yoshiaki Tanaka, Masao Fukutomi and Toshihisa Asano 



n (emu/9) 
0.080 

0.040 

o.ooo 

-0.040 
-0.080 
-0.120 
-0. 160 
-0.200 



50 



BiiSnCai ClgOx 

FIELD CkOe); JJ» -\ 

EMU FULL SCALE; FS= .01 
SAHPL. TIME (S>; TIS= 1 



170 



210 250 
T ( K ) 



Fig. 2. Magnetization of Bi^r.C^Cu^, for the sample (b) in Fig. 1 in a field of 100 Oe. 



10? 



a. 
o 

'</> 
c 

Of 



K 1 Sr 1 Ca 1 Oi 2 0x 




» ' ' ■ ' 



i * t 1 l l I I J I II I 



56 



50 



45 



25 



20 



15 



40 35 30 

2 6 (degree ) 

Fig. 3. X-ray (Cu ka) diffraction pattern of the Bi^Ca.CuA oxide superconductor for the sample (b) in Fig. 1. 



this ratio, a low-r c phase tends to appear irrespective of 
the sintering conditions. In BiSrCaCu^O* oxides, the 
oxide of y=l is not superconducting. According to the 
results of the X-ray diffraction analyses, the starting 
material corresponding to the composition of 
BijSriCaiCuaOjc seems to form a single phase. While in 
the nominal composition of oxides with j>>2, unreacted 
CuO remained in the sample. A typical X-ray diffraction 
pattern for the oxide of y=2 (sample (b) in Fig. 1) is 
shown in Fig. 3. Although the structure of this oxide is 
not identified yet, it appears to be different from those of 
(LaSr)2Cu04 and YBa 2 Cu 3 07. 

This material having high T c above 105 K may have 
potential application in various industrial fields in the 
near future. It should be noted that these oxides are ex- 
tremely stable in water and moisture and that no change 
in the superconducting properties has been observed even 
after the thermal cyclings between 4 K and room tempera- 
ture or above. 

Furthermore, the oxide has two phases with different 



T c and their structures seem to be different from those of 
high-r c oxide superconductors discovered up to now. We 
believe that this new oxide will contribute greatly to 
elucidating the high-Ii superconducting mechanism. 

Acknowledgements 

We would like to thank Dr. M. Uehara for the measure- 
ments of magnetization and Dr. K. Ogawa for his useful 
discussions. 

References 

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4) J. Akimitsu, A. Yamazaki, H. Sawa and H. Fujiki: Jpn. J. Appl. 
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