.ix .

J Am Cerom Soc 73 1101 3110-12 (1990)

Superconductive Properties of the YBa 2Cu408 Superconductor

Asok K. Sarkar,*,* Gregory Kozlowski,+ and lman Maartense* University of Dayton, Research Institute, Dayton, Ohio 45469-0001;

Wright Research and Development Center, Aero Propulsion and Power Laboratory, Wright-Patterson Air Force Base, Ohio 45433-6533

A bulk density of 85% of the theoretical density was achieved by sintering a powder compact of YBa2Cu408 (124) at 850°C in flowing oxygen at 1 atm (=lo5 Pa). This value is very close to that obtained by the hot isostatic pressure technique (90%). The superconducting properties of the sample were charac- terized by magnetization and ac susceptibility techniques. The magnetization critical current density at 20 K in zero field was determined to be -5 X 104A/cm2, and the supercon- ducting transition temperatures were found to be 77 K for the bulk material and 82 K for the granular phase. The powder X-ray diffraction and ac susceptibility studies revealed the sintered 124 material to be single phase. [Key words: super- conductors, sintering, susceptibility, magnetization.]

I. Introduction

EVERAL new stable ternary oxide superconducting com- S pounds have been discovered in the Y-Ba-Cu-0 system since the discovery of the historical YBazCu307 compound (hereinafter called 123). They have been called the 124 compound for YBa2Cu408 and the 247 compound for Y2Ba4Cu7OI5. These new compounds were at first believed to be formed in bulk form only at oxygen pressure'.' greater than 1 bar (lo5 Pa) as opposed to the atmospheric pressure synthesis of the 123 compound. Presently, it is possible to synthesize both the 124 and 247 compounds at 1 atm3z4 or even lower oxygen partial pre~sure.~ The ease of preparation of the single- phase 124 compound has now made it possible to examine its superconductive properties in greater detail. Various inno- vative ways of synergistically combining the properties of the 123/124 pair have also been reported in the literature'.' and more are expected to appear in the future. In addition to being structurally related to the parent 123 compound, the 124 compound is also of great technological interest. Stability of the oxygen content of the 124 compound at higher tempera- tures and the possibility of increasing its T, from 78 to 90 K by calcium doping' will make the 124-type compounds very attractive to the scientific community. However, one of the disadvantages likely to be encountered with this material is that very high pressure and temperature will be required to prepare a fully dense material of this compound.

In this communication, we show that it is possible sub- stantially to sinter this material under ambient oxygen pres- sure (to -85% of the theoretical density). We also report the results of both the dc and ac magnetic characterization studies performed on bulk sintered specimens prepared from com- mercially available 124 powder.

W. Hammetter-contributing editor

Manuscript No. 197499. Received June 20, 1990; approved August 6,1990. Support for A.K.S provided by the Air Force University Resident Re-

search Program; support for G.K. provided by the National Research Coun- cil; support for I.M. provided by the Wright Research and Development Center, Wright-Patterson Air Force Base, Ohio.

*Member, American Ceramic Society. *University of Dayton. 'Wright Research and Development Center.

11. Experimental Procedure

High-purity 124 powder was obtained from a commercial source.+ The powder was synthesized using a combination of vacuum and low-pressure oxygenation process. The as- received precursor powder was found, by X-ray diffraction (XRD), to be poorly crystallized and it also contained traces of unreacted 123 compound and CuO. The crystallinity and the phase purity of the powder improved substantially after the powder was annealed in flowing oxygen at 800°C for 40 h; no impurity phases were then detectable by XRD.

Approximately 5 g of the powder was pressed into a -20- mm-diameter and -3-mm-thick pellet applying -90 000 psi (620 MPa) pressure in a tungsten carbide mold. The pellet was sintered on an alumina dish inside a tube furnace at 850°C for 24 h and then slowly cooled in flowing oxygen. The process was repeated by turning the pellet over on the dish, so that a more uniform oxygenation of the entire sample could be achieved. The pellet underwent volumetric shrinkage, but no weight loss was detected after this prolonged sintering. The bulk density of the sintered pellet was -85% of the theoreti- cal X-ray density (6.164 g/cm3).

The magnetization measurements were performed on a commercial SQUID magnetometer at a temperature of 20 K and in fields up to 30 kOe ( ~ 2 . 4 x 106A/m). A rectangular specimen taken from inside the pellet was cooled in zero field from above the transition temperature and then the appro- priate data were collected. The details of the complex ac susceptibility, powder XRD, and microstructural characteri- zation via scanning electron microscopy (SEM) techniques are given elsewhere.'

111. Results and Discussion

The powder XRD pattern for the sintered 124 pellet indi- cates that the material is single phase based on the published pattern for the 124 phase.* Moreover, since no weight loss and impurity phases were detected, even though the pellet under- went prolonged heating at 850°C during sintering, the 124 phase is thermodynamically stable up to 850°C in 1 atm of oxygen. Oxygen loss, if any occurred at high temperature, was easily recoverable in the dense sintered sample. From thermogravimetric analysis it has been deduced that 124 starts to decompose into 123 and CuO at 880°C in 1 atm of ~ x y g e n . ~ It is shown here that even without the presence of a liquid phase, it is possible to sinter 124 powder to high den- sity, although the density could certainly be improved further by hot-pressing or hot isostatic pressing. Alexander et a1." have recently shown that phase-pure 124 specimens can be prepared by the hot isostatic pressing technique. However, their samples swelled after the final treatment, resulting in a final density of only 90% of the theoretical density, and the expected higher density was not achieved. The process of densification is very important for a practical superconductor material, in order to optimize its critical current density.

'Superconductive Components, Inc., Columbus, OH.

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October 1990 Communications of the American Ceramic Society 3111

6 N

E

$ 5 t E!

Fig. 1. sintered 124 pellet.

Secondary electron SEM image (fracture surface) of the

The SEM photograph of the fracture surface of the sin- tered specimen shown in Fig. 1 also corroborates the fact that a dense microstructure can indeed be attained by conven- tional sintering of 124 powders. The average grain size in the material is estimated to be -1 pm with some agglomeration into clusters up to -5 pm in size.

Magnetization-hysteresis-loop data collected at 20 K in fields up to 30 kOe are shown in Fig. 2. The magnetization critical current density of the sample calculated from this loop using Bean's model" is -5 x 104A/cm2 in zero magnetic field and decreases with increasing magnetic field up to 30 kOe. The plot of magnetization critical current versus magnetic field is shown in Fig. 3. The critical current density values, although not very impressive (the actual size of the

are quite comparable to the values published by Jin et al.4 for bulk 124 specimens prepared by a different synthesis route.

0

-0.2

5 rn -0.4

7 -o.6

2 specimen instead of the grain size was used in this analysis), -0.8

0 0 5 10 15 20 25

APPLIED FIELD (kOe)

Fig. 3. the sintered 124 sample, at 20 K.

Plot of critical current density vs applied magnetic field for

curve # ~

1 2 3 4 5 6 7

0 04 0 16 0 40 1 0 1 6 2 4 3 6

However, the rapid -failoff of critical current density with magnetic field (Fig. 3) points to the less than optimum weak- 0 20 40 60 80

-1 0 1

link behavior of this ceramic superconductor. TEMPERATURE (K) The weak-link characteristics of the superconductive be- havior in the sintered material are also evident in the ac field dependence of the magnetic susceptibility data, presented in Fin. 4. The ac loss data define a bulk T, of 77 K, by extrauola-

(a)

tion of the field dependence of the temperature i t which the ac loss maxima occur, to zero field. This method of determin- ing T, is found to be equivalent to the electrical transport

-30 -20 -10 0 10 20 30 APPLIED FIELD ( k O e )

Fig. 2. Magnetization curve at 20 K for the sintered 124 sample.

I

100

0

-0.02

-0.04

0.4 t 0.3 -

v) v)

2 0.2 -

0 a

1 I

0 20 40 60 80 100

TEMPERATURE (K)

Fig. 4. Temperature dependence of ac susceptibility with varying ac field ( h ) for the sintered 124 specimen at dc field H = 0: (a) real part; (b) imaginary part (loss). The upper 4% of the data are shown on the right-most axis in (a).

3112 Communications of the American Ceramic Society Vol. 73, No. 10

measurement of the bulk zero-resistivity transition tempera- ture at a low current den~i ty .~

The onset temperature of the field-independent diamag- netic behavior in Fig. 4 defines a granular T, of 82 K, which is close to the value reported for the 124 compound.’ The sus- ceptibility data near T, are shown on an expanded scale on the right of Fig. 4(a). It can be seen in the region between 82 and 92 K, where the presence of any superconducting 123 phase would be revealed, that an upper bound of -0.1% can be placed on the volume fraction of that phase. This result is in accord with the XRD data, thus establishing the excellent phase purity of this material.

Between 77 and 82 K, no intergranular coupling is apparent in the sample, i.e., the ac-field dependence indicative of weak-link coupling at grain boundaries is absent. When this lack of long-range connectivity occurs in sintered 123 mate- rial, it is caused by the presence of insulating impurities in the grain boundaries. These can be 123 decomposition products formed during partial melting at the sintering tem- perature, but a similar origin in the present 124 sample is improbable, because of the relatively low processing tempera- tures used here. A more likely source of grain-boundary im- purities would be the reaction products resulting from exposure of the 124 precursor powder to atmospheric mois- ture and C02 . Such impurities, expected to be mainly CuO, BaC03, and Y2BaCuOS, as in the case of 123,12 would not have reformed into either 123 or 124 material during sintering.

The small grain size of the ceramic could be another cause of this “coupling gap” near T,. Particles smaller than the pen- etration depth would not take part in the bulk supercon- ductivity until the temperature has decreased sufficiently. The apparently small effective volume fraction (-2%) of granular superconductor seen above 77 K in Fig. 4 is consis- tent with this hypothesis.

IV. Conclusion

We have shown that sintering of 124 powder at ambient pressure is capable of producing material with good bulk su- perconductive properties, despite the small grain size and the probable presence of some grain-boundary impurities. Efforts

are now under way to increase the final grain size and to reduce or eliminate the inert material in the intergranular re- gions, in order to determine the extent of improvement in the bulk properties which are possible within the ambient pres- sure sintering process.

Acknowledgments: We thank C. E. Oberly and P. M. Hemenger for their cooperation during this work and especially thank S. Hutson of Super- conductive Components, for providing the 124 powder.

References ‘D. E. Morris, J. H. Nickel, J.Y.T. Wei, N. G. Asmar, J.S. Scott, U. M.

Scheven, C.T. Hultgren, A. G. Markelz, J. E. Post, P. J. Heaney, D. R. Ve- blen, and R. M. Hazen, “Eight New High Temperature Superconductors with the 1:2:4 Structure,” Phys. Rev. B, 39 [lo] 7347-50 (1989).

*D. E. Morris, N. G . Asmar, J.Y.T. Wei, J. H. Nickel, R . L. Sid, J . S. Scott, and J. E. Post, “Synthesis and Properties of the 2:4:7 Superconduc- tors R2BarCu,01j-x(R = Y, Eu, Gd, Dy, Ho, Er),” Phys. Rev. B, 40 [16] 11?06-409 (1989).

D. M. Pooke, R. G. Buckley, M. R. Presland, and J. L. Tallon, “Bulk Su- perconducting YzBarCu701j_d and Y1Ba2Cu408 Prepared in Oxygen at 1 p n , ” Phys. Rev. B, 41 [lo] 6616-20 (1990).

S. Jin, H.M. O’Bryan, P.K. Gallagher, T.H. Tiefel, R. J. Cava, R.A. Fastnacht , and G.W. Kammlot t , “Synthesis and Propert ies of t he Y?a2Cu4O8 Superconductor,” Physica C, 165, 415-18 (1990).

U. Balachandran, M. E. Bizneck, G.W. Tomlins, B.W. Veal, and R. B. Poppel, “Synthesis of 80 K Superconducting YBa2Cu408 via a Novel R y t e , ” Physica C, 165, 335-39 (1990).

S. Jin, T. H. Tiefel, S. Nakahara, J. E. Graebner, H. M. O’Bryan, R.A. Fastnacht, and G.W. Kammlott, “Enhanced Flux-Pinning by Phase Decom- poTition in Y-Ba-Cu-0,” Appl. Phys. Lett., 56 [13] 1287-89 (1990).

D. E. Morris, J. H. Nickel, B. Fayn, A . G . Markelz, R. Gronsky, M. Fendorf, and C. P. Brumester, “Conversion of 124 into 123 + CuO: Microstructure and Phase Diagram”; presented at the Materials Research Society Fall Meeting, Boston, MA, Nov. 1989.

‘T. Miyatake, S. Gotoh, N. Koshizuka, and S. Tanaka, “T, Increased to 909K in YBa2Cu40R by Ca Doping,” Nature (London), 341, 41-42 (1989).

A. K. Sarkar, B. Kumar, I. Maartense, and T. L. Peterson, “The Effects of Long-Term Annealing on Superconductive Properties in the Bi-Sr-Ca-Cu- 0 System,” J. Appl. Phys., 65 [6] 2392-97 (1989).

’OK. B. Alexander, R. K. Williams, and S. J. Pennycook, “Stacking Varia- tion in Bulk Y2Ba4CugOI6 Materials”; pp. 106-107 in Proceedings of the 12th International Congress on Electron Microscopy. San Francisco Press, Sa; Francisco, CA, 1990.

C. P. Bean, “Magnetization of Hard Superconductors,” Phys. Rev. Lett., 8 [6] 250-53 (1962).

‘*A. H. Morrish, X. Z. Zhou, M. Raudsepp, 1. Maartense, J. A. Eaton, and Y. L. Luo, “Deterioration of the High-Temperature Oxide Superconductors in Various Ambient Environments,” Can. J. Phys., 65 808-809 (1987). 0