Physiea C 185-189 (1991) 2497-2498 North-Holland PliYSICA

MiCROSTRUCTURES OF YBaCuO FABRICATED BY MELT-POWDER-MELT-GROWTH PROCESS

N~W 7 goji YAMAGUCHI, Masato MURAKAMI, Hiroyuki FUJIMOTO, Naoki K .... I~UKA and Shoji TANAKA

Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13, Shinonome, Koto-ku, Tokyo 135, dapan

Microstructures of YBaCuO material fabricated by Melt-Powder-Melt-Growth process have been investigated by transmission electron microscopy and using Fourier transforms of crystal latt ice imaoes. The interface between YBapCuaO 7 and Y2BaCuO 5 inclusion is confirmed to be sharp with no other phases or crystallographlc ~istor~ion. Such an interface may act as a strong pinning center and bring a large dc value.

I . INTRODUrTION

We have developed a mel t process named

Melt-Powder-Melt-Growth (MPMG) to obtain high-

Jc bulk YBaCuO mater ia l . 1 Using MPMG process,

superconducting YBa2Cu307 (Y123) wi th f i n e l y

dispersed Y2BaCu05 (Y211) inc lus ions can be

obtained, dc of the MPMG processed material

increases up to 30000 A/cm 2 (a t 77 K with a

magnetic f i e l d of 1 T para l le l to the c-a×is

of Y123) wi th increasing the quant i t y of Y211

inclusions up to 30 % in volume. 2

To inves t iga te the dominant f lu× pinning

cen te r in the m a t e r i a l , m i c r o s t r u c t u r a l

studies by transmission e lec t ron microscopy

(TEM) have been performed. And we have

proposed t h a t the i n t e r f a c e between ¥123

mat r i x and Y211 i n c l u s i o n ac ts as a s t rong

pinning center. 3 '4

In t h i s work, m i c r o s t r u c t u r e s in MPMG

processed mater ia ls have been invest igated by

TEM. Furthermore, two dimensional d iscrete

Fourier transform (2-d DFT) of TEM image has

been performed to evaluate the s t ra in in Y123

crys ta l .

2, EXPERIMENTAL

The m a t e r i a l i n v e s t i g a t e d here conta ins

30 % volume of Y211 inc lus iens wi th a Jc of

30000 A/cm 2 at 77 K with a magnetic f i e l d of

1 T para l le l to the c-axis of Y123. Samples

fo r TEM observations were prepared by Ar + ion

mi l l i ; .g a f te r mechanical th inning. A l l the T~M,L~.,

observations were performed using a microscope

operated at 400 kV having a point resolut ion

of 0.17 nm. The de ta i l s of TEM observations

were described in elsewhere. 3,4

Crys ta l l a t t i c e images were taken and

d i g i t i z e d by TV camera with a size of 255 x

256 p i x e l s from p r i n t e d images. Before 2-d

DFT~ the i n tens i t i es of p ixels beyond a c i r c l e

wi th a radius of 70 p ixe ls were set to zero

gradual ly in order to avoid a r t i f a c t s caused

by 2-d DFTo ]n ~PMG processed materiaqs~ there

are many stacking f a u l t s in Y123 matr ix. 4 i f

an image c o n t a i n i n g a s tack ing f a u l t is

transformed, streaks due to the f a u l t can be

g e n e r a t e d in t he t r a n s f o r m e d p a t t e r n .

The re fo re , reg ions w i t h o u t s tack ing f a u l t ;

were selected fo r 2-d DFT.

3. RESULTS AND DISCUSSION

In the MPMG processed materials~ there are

many stacking fau l t s ~ V123 matri× es shown

in Fig, l . Although such stacking fau l t s may

act as pinning centers, t he i r densi ty does not

inc rease by i n t r o d u c i n g Y211 i n c l u s i o n s .

Therefore~ we proposed that the d i f fe rence of

Jc between the mater ia ls with and wi thout Y211

should be caused by the in ter face between Y123

and Y211. 4

0921-4534/91/$93.50 © 1991 - Elsevier Science ~ublishers B.V. All dgbLs reservea.

2491~ I(. Yo~aguch~ et at / YBaCuO fabricated by melt.powder.melt-growth process

FIGURE l TEM image viewed perpendicular to the c-axis of Y123, Lines are stacking faults, Regions "A" and "B" are transformed by 2-d DFT.

To evaluate the crystallographic strain of

Y123 caused by Y211, two crystal la t t ice

images of regions "A" and "B" in Fig. l are

digitized and masked as shown in Figs, 2 (a)

and (b), respectively. These two images are

transformed by 2-d DFT as shown in Figs. 2 (c)

and (d).

The result of 2-d DFT for the region 50 nm

distant from the interface (Fig. 2 (c)) is the

same as the electron diffraction pattern of

[OlO]-zone with no streak in each spot.

Comparing Fig, 2 (d), the result of the

region next to the interface, with Fig, 2 (c),

no difference can be recognized. Distance

between spots 000 and 006 is IO6 pixels and

that between spots OOO and 200 is I04

respectively. These distances are the same for

Figs, 2 (c) and (d ) . Therefore~ c r y s t a l

l a t t i c e constants a and c of region "A" equal

to those of region "B" within an er ror of 1%.

This means the c rys ta l of Y123 is not strained

at the inter face as large as 1%.

The resul ts of TEM observations and the i r

2d-DFT show t h a t the Y123 m a t r i x is not

d is tor ted even at the interface. I t means thai;

the interface is sharp and the superconducting

(a)~ (c) ~(d)

FIGURE 2 Digitized crystal latt ice images of region "A" (a) and "B" (b). (c) and (d) are the results of 2-d DFT of (a) and (b), respectively.

order parameter of Y123 wil l fall to zero at

the interface rapidly with the width of

coherent length. Since the pinning force is to

be proportional to the gradient of the order

parameter, the strong pinning force is

expected for such sharp interface,

4. CONCLUSION

Microstructures oF MPMG processed material

were investigated by TEM and 2-d DFT. The

interface Fetween Y123 matrix and Y211

inclusion was confirmed to be sharp and is

considered to act as a strong pinning center,

The large Jc of this material is thought to be

brought by this sharp interface.

REFERFNC~% I. H, Fujimoto et alo, Advances in

[uperconduct iv i ty [ I (Springer-Verlag, Tokyo, 1990)~ ppo285.

2. M. Murakam et al.~ Supercond,Sci.Technol, 4 (1991) $43o

3, Ko Yamaguchi et a l , , Jpno J. Appl,Phys. 29 (1990) L1428.

4. Ko Vamaguch~ et alo,J.Mater,Reso in press,