Physica C 190 ( 1991 ) 84-86 North-Holland PHYSICA

Effect of Ag20 addition on the formation of YIBa2Cu408 I. T h e synthes is c o m b i n e d wi th O2 H I P t e c h n i q u e

Yuichi Watanabe a, Kazuo Yanagida a, Masasuke Takata ~ and Kozo Ishizaki b a Department of Electrical Engineering, and b Department of Mechanical Engineering, Nagaoka University of Technology,

Nagaoka, Niigata 940-21, Japan

The effect of Ag20 addition on the formation of Y~Ba2Cu4Oa (124) under high pressure was examined in terms of reaction rate enhancement. By adding a few wt.% Ag20, the calcined material composed of Y~Ba2Cu3Ov_a and CuO was found to be easily converted to 124 after HIPping at 920 ° C for only 3 h i n a mixed A r + 02 gas of 900 arm (Po2 = 180 atm ). T, ( onset ) and T¢ ( zero ) for the 124 sample were independent of Ag20 content, being 80 and 75 K, respectively.

1. Introduction

Among the superconducting oxides in the Y - B a - C u - O system, YlBa2Cu408 ( t 24 phase) has been of great interest since the material shows higher thermal stability [ 1 ] than that of the well-known YiBa2Cu3OT_,~ (123 phase). How- ever, the conventional solid-state reaction does not seem to be suitable for the synthesis of the 124 phase, since the 124 phase is recognized as a low-temperature stable phase at an ordinary pressure [ 1-4 ]. Then, successful synthesis has been carried out by the solid-state reaction at high oxygen pres- sure in which the reaction rate for the formation of the 124 phase is expected to be enhanced.

In the present work, we examine the effect of Ag20 ad- dition upon the formation of the 124 phase at high oxygen pressure. It will be shown that Ag20 addition is effective for the formation of the 124 phase, leading to reaction-rate enhancement.

2. Experimental procedures

Samples were prepared by the following sequence. At first, raw materials of Y203, BaCO3 and CuO were carefully weighed and mixed together to have the stoichiometric com- position Y: Ba: Cu = 1 : 2: 4. Then, the mixed powders were calcined at 900°C in an electric furnace for 12 h in air. The resulting product was pulverized and further mixed with various amounts (0-20 wt.%) of Ag20 powder. The mixture was pressed into a disk 10 mm in diameter at 1000 kgf/cm 2. Subsequently, the disk was subjected to thermal treatment at high oxygen partial pressure with an O2-HIP apparatus

(Kobe Steel, O2-Professor HIP). All HIP treatments were conducted for 3 h at 900 arm of mixed Ar and 02 gases (Po2 = 180 a tm) at temperatures around 900°C.

The crystal structure of the samples was analyzed by X-ray diffraction measurements (Rigaku, RINT 1500) operating with Cu Kct radiation. The AC magnetic susceptibility at 1 kHz was measured using an impedance analyzer (YHP- 4192A) for the powdered sample.

3. Results and discussion

Figure 1 shows the X-ray diffraction patterns of the cal- cined powder (a) , the HIPped sample without Ag20 addi- tion (b) , and the HIPped samples with the addition of Ag20 of l, 5 and 20 wt.% (c, d and e), respectively. Here, the HIP- temperature was fixed at 920°C at which the 124 phase was found to be stable in the P - T phase diagram [ 1-3]. As seen in the figure, both the 123 phase (marked as × ) and un- reacted CuO (marked as i ) were mainly observed in cal- cined powder (a) . Since the calcination was carried out at 900°C in an ordinary pressure, the formation of 123 with unreacted CuO was quite reasonable from the phase dia- gram. Even though a HIP treatment was carded out, no sig- nificant change took place in the sample without Ag20 ad- dition, as shown in (b) . Considering that the present HIP treatment was carried out only for 3 h, the result could be explained as that the formation of 124 has not sufficiently proceeded because of the low reactivity between 123 and CuO even at high oxygen partial pressure [ 5 ].

On the other hand, the effect of Ag20 addition on the for- mation of the 124 phase was remarkable. As seen in figs. 1 (c-

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Y. Watanabe et aL / Effect of Ag20 on the formation of YtBazCu4081 85

5 d

v

i11

Q~

c

c o

"5 0

a

(e) o0

o%° o o oO ~o°% o Ooo°

to o o ° 1 1 ,MI ° ° o o

• o o o A ° o o

(b)

(a)

I

20 30 28

J ~

I I x x x • - x " x

I 1

40 50 60 (deg., Cu-Ka )

Fig. I. X-ray diffraction patterns (Cu Kct) of calcined powder (a) and HIPped samples added with various amount of Ag20 [(b) 0, (c) 1, (d) 5, and (3) 20 wt.%]. Peaks are assigned to x : YBa2Cu3OT_a; O: YBa2Cu4Os; O: Y2BaCuOs; I1: CuO; ,t: Ag; and A: BaCO3, respectively.

e), the X-ray profiles for the HIPped samples with the ad- dition of Ag20 are obviously different from those of the Ag20- free samples (a) and (b). As seen in fig. 1, the peaks as- sociated with the 123 phase are completely annihilated after HIP treatment and the main peaks observed for HIPped samples were assigned to those of the 124 phase (marked as O ) and metallic silver (marked as • ). The lattice param- eters found for the observed 124 phase were independent of Ag content and were almost identical with previously re- ported values for 124 phases, a = 3.8415 tk, b = 3.8708/~, and c=27.240 A [6].

Figure 2 shows the result of AC magnetic susceptibility measurements on the samples with the addition of Ag20 by 1 and 20 wt.%. As seen in fig. 2, the To(onset) values are independent of Ag content and are identical with that of pure 124, i.e., 80 K [1].

Thus, judging from the combined results for X-ray dif- fraction and AC susceptibility measurements, it can be con- cluded that the added Ag20 is not incorporated into the 124 lattice, which is also the case for the 123 system sintered with the addition of Ag20. It should be noted that added Ag20

U

: 3 u')

I

0 10

I I I I I I I I I I I

A ~

I I I I I I I I I I

20 30 40 50 60 70 80 90 100110120

TEHPERATURE [K]

Fig. 2. Temperature dependence of the AC susceptibility for HIPped samples with the addition of Ag20 by 1 wt.% ( A ) and 20 wt.% (0 ) .

was decomposed into metallic silver even at high oxygen partial pressure. The precipitated metallic Ag might exist in the intergranular space. Thus, it can be supposed that the formation of metallic silver may be associated with the en- hancement of the reaction rate between 123 and CuO for the formation of 124. However, the same effect upon the for- mation of 124 was observed when metallic silver was used as an additive. Therefore, the reaction rate enhancement should not be associated directly with the thermal decom- position of AgzO. As described in the following paper of this series [ 7 ], the precipitated silver may enhance the diffusion of oxygen on the stage of progress in the reaction.

4. Conclusion

The effect of Ag20 addition upon the formation of the 124 phase at high oxygen partial pressure has been investigated. It was confirmed that the addition of Ag20 favoured the for- mation of the 124 phase, leading to reaction-rate enhance- ment. To(onset) and lattice constants for the present 124 phase were independent of the amount of added Ag20 and were identical with those of Ag20-free 124. These facts im- ply that the added Ag20 was not incorporated into 124 lat- tice. Thus, Ag20 should be recognized as a reaction-rate en- hancer in the synthesis of 124.

References

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