Physica C 153 155 (1988) 365-366 North-Holland, Amsterdam

A NOVEL PROCESSING TECHNIQUE AND RELATED PROPERTIES OF YiBa2Cu307_x

Binod KUMAR

University of Dayton Research Inst i tu te Dayton, OH 45469

The cr i t ica l current density, J of the YiBa2Cua07_~ material has attracted considerable attention. The Jc of sintered bulk material mugt b8 raised by orders of magnitude before the material can find large scale commercial applications. The paper discusses inherent processing d i f f i cu l t ies and demonstrates a new approach to resolve them. Experimental data (Meissner and magnetic measurements) from a Y~BagCuRO 7 ~ material synthesized using the proposed approach (enhancement of intergranular c6up?ing) wCll be presented. The data show major improvements in the magnetic properties of the material.

I. INTRODUCTION Since the discovery of high-T c supercon-

ductors by Bednorz and Muller ( I ) , and a subsequent report of above liquid nitrogen temperature T in Y-Ba-Cu-O system by Wu, et al . , (2) an intense race is underway to improve upon the existing high-T~ materials and search for yet higher-T~ materials. To date, reproducible maximum temperature for zero res is t i v i t y is about 95°K for the Y.Ba^Cu~O~ material and i ts derivatives. C~ns~qu~n{1~, most research efforts have been centered around this material. For commer- cial applications the 95°K cr i t ica l temperature is much better than existing superconductor (T~ 23.2°K), however the low cr i t ica l current bf these ceramic supercon- ductors needs to be further improved.

The current density in the polycrystal- line bulk specimens of YIBa~Cu~07 v is limited by the grain bouflda~ie~ An8 low density of the material. Individual grains of the polycrystalline materials are coupled by grain boundaries (weak links) which tend to decouple at high current densities and/or magnetic f ie ld. Although several methods have been proposed to minimize the weak-link effect ~3) synthesis of high current density (10~-I0 ~ Amps/cm ~) polycrystalline bulk material is yet to be accomplished.

This communication reports a processing technique that has greatly improved the quality of the material (YiBa~Cu~07 ~) as measured by the levitat ion he~gh% ~u8 to the Meissner effect and magnetic measurements such as susceptibi l i ty and loss. The tech- nique was developed by analyzing the variable that leads to the development of weak links

2. EXPERIMENTAL The specimens were prepared under vary-

ing experimental conditions. The processing

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technique that produced the best results involves: ( i ) preparation of the supercon- ducting powder by the solid state reaction process as reported earl ier (4), ( i i ) soaking the powder in l iquid nitrogen or gaseous nitrogen at room temperature, ( i i i ) pressing the powder in disc or any other forms, and (iv) sintering the pressed specimens in a flowing oxygen atmosphere at or below 900:C for 8-10 hrs. The quality of the specimens was evaluated by the determination of mag- netic susceptibil i ty, loss, and measuring the levitat ion height of a I gm rare earth per- manent magnet at l iquid nitrogen temperature.

3. RESULTS The processing parameters and levitation

height of six specimens are shown in Table I. A comparison of Specimens 1 and 2 reveals that soaking in l iquid nitrogen increases the levitat ion height 100%. Sintering in flowing oxygen atmosphere, as in Specimen 3, s l ight ly improves the levitation height. Specimens 3 and 4 were processed in identical conditions. The reproducibility of the experiment is estimated to be ± 0.5 mm. Specimens exposed to gaseous nitrogen, 5 and 6, yield similar results.

Typical results of the Z' component of all ac complex susceptibil ity × = ×' - ix" are shown in Figure I (A) and (B) for specimens I and 3, respectively. A strong dependence of × on the applied ac f ield is characteristic of weak-link superconductors, and reflects a low cr i t ica l current density of the bulk material. This weak-link be- havior originates from the grain boundary structure of the ceramics. Its suppression is one of the objectives of improving the processing methods. In this context, Figure I shows that specimen 3, which was prepared by soaking the calcined and ground material in liquid nitrogen and sintering in oxygen,

366 B. Kumar / Novel processing technique and related properties of YI Ba eCu 307 x

TABLE I

PROCESSING PARAMETERS OF THE Y,Ba Cu~O~ MATERIAL AND LEVITATION H~IG~T j /-x

Sintering Levitation Specimen # Treatment Atmosphere Heiqht

I None Air 3.0 mm 2 Soaked in

L. N Air 6.0 mm 3 Soaked in

L. N 02 6.5 mm 4 Soaked in

L. N 02 7.0 mm 5 Soaked in

G. N 02 7.0 mm 6 Soaked in

G. N 2 02 6.5 mm

is clearly superior to specimen I in the following respects. I t shows a much sharper drop in Z' below T - 91°K, and retains i ts c

A

~- 0.5

- 1.0

60

-05

- I .O

i i i

- - 50 OCT 87 J "J~ ,v . . . . 20 JAN 88 ~ / ~ 1 / ~ S ~

/ / / 4.o~ - - ~ / / / . : o

I I I

70 80 90 T E M P E R A T U R E ( *k ) p

(A)

i i i i i i

~ / 0 H = O

4.0 ~ .I 8

~ 1.8

I00

i I I I I I I I [ 60 65 70 75 80 85 90 95 I00

TEMPERATURE (k) (B)

Figure I. Magnetic suceptibi l i ty (×') vs temperature (A) specimen #I (B) specimen #3; the curve parameters are the amplitude of the measuring ac f ie ld in Oe. I t refers to biased dc f ie ld.

fu l l diamagnetic shielding properties (4~×' = - I) near 77°K over the fu l l range of applied ac f ields, whereas specimen I has a much greater f ie ld sensi t iv i ty. The ×" (loss) data confirm this improvement. Furthermore, exposure to ambient air has destroyed almost al l of the intergrain superconducting links and only the diamagnetic properties of the individual grains are observed [broken curve in Figure I (A)]. Specimen 3 did not reveal such degradation effects; this suggests that i t has a much more stable grain boundary structure.

The observed increase in levitation height also implies a larger J~. A direct measurement of the cr i t ica l current density of specimen 3 was attempted, however no rel iable upper l im i t on J could be obtained due to contact heating problems. Nevertheless, a val~e for steady state J of well over 1250 A/cm was indicated at 77¢K in zero f ie ld.

The experimental data lead us to suggest that the intergranular coupling among in- dividual grains has been enhanced by the processing technique. I t is believed that the fundamental problem with the sintered material in regard to the current density is related to the creation of Josephson junction between the superconducting grains. At low current densities and/or magnetic f ie ld these grains tend to be coupled. However, as the current density or magnetic f ie ld is in- creased, the grains decouple. This problem can be resolved by eliminating the insulating layer and developing an intimate contact between the grains. This is achieved by saturating the powdered material by an inert gas so that they become less reactive during handling, storage, and heat treatment processes. The adsorbed gas escapes from the material during f inal heat treatment.

ACKNOWLEDGEMENTS The author expresses gratitude to Dr.

Iman Maartense for conducting magnetic measurements and reviewing the manuscript; and to Mr. S.R. Smith for providing current density data. Preparation of the manuscript was supported by Edison Materials Technology Center under the Contract #CT0060035.

REFERENCES (I) J.G. Bednorz and K.A. MSller, Z. Phys.

B64, (1986) 189. (2) M.K. Wu, J.R. Ashburn, C.J. Torng, P.H.

Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.O. Wang, and C.W. Wu, Phys. Rev. Lett. 58 (]987) 908.

(3) J.W. Ekin, Adv. Ceram. Mat. 2(2B), (1987) 586.

(4) B. Kumar, J.D. Wolf, and S.R. Smith, J. Mat. Sci. Lett. in print.