Critical current density of a YBa2Cu3Ox by melt (QMG) process

PMEA

Physica C 185-189 (1991) 2467-2468 North-Holland

CRITICAL CURRENT DENSITY OF A YBa2Cu30 x BY MELT (QMG) PROCESS

Keiichi Kimura, Mitsum Morita, Masamoto Tanaka,Seiki Takebayasi, Katsuyoshi Miyamoto and Kiyoshi Sawano R&D Laboratories- I, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki, Kanagawa, 211, Japan

We investigated the macroscopic and microscopic critical current densities of a melt processed materials by a magnetic measurement. By the Quench and Melt Growth (QMG) process, large YBa2Cu30 x crystals without a large angle grain boundary can be obtained. Several types of powders less than 1001.tm were prepared by pulverizing and classilying QMG crystals. These powders were aligned in a magnetic field and the magnetization was measured. The magnetization hysteresis of the prepared powders was proportional to their average diameter. The critical current density of the powders estimated from the Bean model is higher than that of the bulk crystal. This indicates that the current in the grain is not uniform. A current is probably prevented by cracks mostly parallel to the ab-plane or subgrain boundaries. 1. INTRODUCTION Recently, a bulk superconductor larger than 20 cm 3

weighed and inserted into acrylic tubes, then aligned in a

up of 8 steps from 10-100 gm. These powders were

without a grain boundary has been obtained by the QMG

magnetic field up to !2T in the molding resin. The

process with seeding technique [1][2]. The large bulk

magnetizations of the samples prepared in these ways

without a grain boundary has high critical current density (Jc) and interesling magnetic properties due to flux

were measured by the vibrating sample magnetometer at liquid nitrogen temperature.

pinning. So several applications such as magnets and 3 R E • TT~'v~ The magnetizations of several plate-shaped specimens with appro×imate size of ]×3×4 mm 3 cut from the QMG-

bearings are proposed [3][4]. The QMG process is one of the melt processes. The Jc of the QMG-processed material with the size of 10-1cm 3 is measured to be 1-3x104 A/cm2 at 77K and 1T [1].

processed material were measured before pulverizing.

Since this value is sail lower than that of thin films [5], it

And the Jc was estimated by the Bean model [6]. The Yc

is suspected that weak-finks still exist even in a single grain. The obtained Jc probably represents the average

was 2xl04

current density throughout the grain. So we have tried to estimate the Yc in a local area.

parallel to the c-axis at 77K and IT. Next, the bulk with a single grain was pulverized,

A/era 2 in the magnetic field applied

perpendicu]ar to the c-axis and 1.5x104 Alcm 2 in the field

classified and aligned as mentioned. The particles were 2 EXPERIMENT The single-grained bulk was made by the QMG

aligned with the c-axis parallel to the applied field [7]. Figure I shows the magnerzafion hysteresis against the average diame[ers.

with finely dispersed Y2BaCuO5 phase. This is because

The magnetization hysteresis is

proportional to i~s diameter. 7~is can be e×p~ained by

the YBa2Cu3Ox phase is formed by pefitectc reaction.

Bean's critical state model. The range in w~ch the linear

And about 40mo1% of excess Y2BaCuO 5 is intentionally add to the stoichiomelfic composition of YBa2Cu30 x.

relationship is maintained is wider than that of the powders made by pulverizing sintered materials [8t. Tt~s

(See ref. [1] or [2] about the details of QMG process.)

is because particles made from the QMG-processed

These sample were pulverized and classified by sieves

materials consist of a single grain, while particles made

utilizing ultrasonic vibraten. The c]a3sifieaton was made

from the sintered mate6Ms comain several grains in them.

0921-4534/911503.50 © 1991 - Elsevier Science Publishers B.V. A~I fights rescrvcd.

2468

K lOmura et at / Critical current density of a YBa2Cu30x by melt (QMG) process

1 0 1

•

'

"

'

"

'

"

TABLE Sample size and Jc with in the magnetic field parallel to the c-axis at 77K and 1T

'

E

(k3

5

SIZE (ram)

®

Powder

Q

"tO

m =E

0 =

d

.

.

.

20

.

,

40

,

o0

i

80

~(Mcm2) 3x1~

< ~ 0.1

Small plate

lx3x4

1.5xl~

Larl~e bulk

• 45x ~


,

100

Diameter (grn) FIGURE 1 Magnetic hysteresis in the magnetic field parallel t o the caxis vs average diameter of powders at 77K and 1T The magnetization hysteresis in the magnetic field applied parallel to the c-axis is about twice as large as that

The Jc of the powders is high compared to the bulk

because of the exclusion of weak-links by pulverizing. But this value may be still underestimated, because the powder was not aligned sufficiently. Actually, a higher Jc was obtained by measuring samples which were cut to 100gin.

in the field perpendicular to the c-axis. It is considered that the magnetization hysteresis in the field perpendicular

5 CONCLUSIONS

to the c-axis is due to the current parallel to the e-axis

Magnetic measurements were made on the powders

which is lower. In the ease of the current parallel to the ab-plane, the Jc was estimated to be 3x104 A/cm2 at 77K

which were pulverized the QMG-processed materials. The

~,~d IT using the Bean model from a straight line of Fig.1 and assuming that the particles ~re spherical.

diameter. This dependence can be explained based on the

magnetizaion hysteresis was proportional to the particle Bean model. It was found that the Jc throughout a grain is lower than that in a local area by this model. This is

4 DISCUSSION Table 1 shows the shape of the samples and their Jc.

probably caused by the cracks formed during the heat treatment and/or the subgrain boundaries.

These values are of the samples without a grain boundary and at 77K under a magnetic field of 1T parallel to the eaxis. The Jc of a large bulk was estimated by measuring trapped magnetic flux on the surface using Hall elements [9]. So all values represent the current density throughout a single grain. If the current in a sample is uniform, the Jc is determined by the flux pinning and should be constant regardless sample size. But the Jc decreases with

REFERENCES 1. M.Morita et al ; Phisica C, 172(1990) 383. 2. M.Morita et al ; Proceedings of ISS'90,(1990) 733. 3. F.C.Moon et al ; Jpn.J.Appl.Phys., 29 (1990) 1257. 4. R.Takahara et al ; Proceedings of ISEM-Sendai.

increasing the sample size. This indicates that the Jc in a local area is higher than that of a large sample. This suggests that the current in a large bulk i8 n o t uniform.

6. C.P.Bean ; Phys. Rev. Lett. 8 (B962) 250.

One of the factors which prevents the current is considered to be cracks formed during the heat treatment.

7. J.D.Livingston et al ; J. Appl. Phys., 64 (1988) 5806.

On the other hand, it is found that a single grain made by

8. E. Simizu and D.ito ; ~-oceeding of ISS'88,(1988) 45I,

the QMG process has subgrains (-ram) and crystallites (lO0~m) caused by the deviation of crystallographic

9. K.Sawano et aJ ; To be published to 3pn,J.Appl.Phys.

direction [10]. These probably reduce the Jc of a bulk.

10.M.Kimura et at ; Phy.~ica C , ~74 (]99t) 263.