Mat. Res. Bull., Vol. 25, pp. 1065-1071, 1990. Printed in the USA. 0025-5408/90 $3.00 + .00 Copyright (c) 1990 Pergamon Press plc.

SUPERCONDUCTIVE CRITICAL TEMPERATURE DEPRESSION IN THE (Bi,Pb)2Sr2Ca2Cu3010+8 (2223) PHASE BY

POST-SINTER QUENCHING

Asok K. Sarkar and I. Maartense University of Dayton Research Institute

Dayton, Ohio 45469-0001

(Received May 23, 1990; Communicated by W.B. White)

ABSTRACT

Changes in Tc resulting from variations in the post-sintering cooling rate, from liquid quenching to slow air cooling, were investigated for the 2223 phase in the Bi-Pb-Sr-Ca-Cu-O system with a nominal composition of Bil.6Pb0.4Sr2Ca3Cu4Ox. Complex ac susceptibility measurements were used to identify the critical tempera- tures of the high-To 2223 and the low-Tc, 2212 phases, as well as those of the bulk material. Tc of the granular 2223 phase decreased from 107 K to 93 K after fast quenching. Similar treatment, however, raised Tc of the 2212 phase from 63 K to 80 K. The opposing behaviors of these two superconducting phases, associated with variations in oxygen stoichiometry resulting from different cooling rates, and their effects on bulk Tc values are discussed.

MATERIALS INDEX: bismuth, euprates, superconductors

Introduction

The superconductive properties of the high-Tc cuprates can be very sensitive to their oxygen stoichiometries. Specifically, the "80 K" Bi2Sr2CaCu208+8 (2212) phase in the Bi-Sr-Ca-Cu-O system has its lowest Tc at the highest oxygen content. In other words, T c of this 2212 phase is optimized through oxygen deficiency. Depending on the oxygen stoichiometry, Tc of the 2212 phase can be varied by -40 K. The "110 K" Bi2Sr2Ca2Cu3Olo+8 (2223) phase in the Bi-system does not behave in this fashion at all. As a matter of fact, Tc of this phase has been reported by several investigators to be maximized by oxygen loading of its structure (1-3). The reason for the opposite movements of Tc in these phases is not yet known.

1065

1066 A,K, SARKAR, et al. Vol. 25, No. 8

The oxygen stoichiometry, and hence To of the superconducting 2212 phase can be altered by fast quenching from high temperatures (4). Interestingly, Tc of the Pb-stabilized 2223 phase was found to be only marginally affected (by ~6 K), when employing these quenching treatments, if the starting cation stoichiometry was Bi(Pb):Sr:Ca:Cu=2:2:2:3 (5). Zhao et al. (6) have reported that Tc of the Pb-doped 2223 phase can decrease from 107 K to 98 K due to oxygen deficiency resulting from ther- mai cycling of samples in a thermogravimetric analysis instrument. They did not observe any structural change in the 2223 phase of their samples. Their starting composition was Bil.6Pb0.4Sr2Ca3Cu4Ox and the final samples consisted of both 2223 and 2212 phases.

Here, we present our results showing that Tc of the 2223 phase can be widely varied and shifted in directions opposite to those found for the 2212 phase, by sub- jecting specimens containing these two phases to different post-sintering cooling protocols.

Experimental Method

Precursor powder with nominal composition (Bil.6Pb0.4)Sr2Ca3Cu4Oy was prepared in an alumina crucible by conventional solid-state reaction of stoichiomet- ric amounts of reagent grade Bi203, PbO, SrCO3, CaCO3 and CuO. The uniformly blended chemicals were calcined at ~865°C for 16 hours, ground, pelletized and then sintered at ~870°C for 60 h. The product was again ground and pelletized, and resintered at ~850°C for an additional 40 hours before it was thoroughly ground and sieved through a 200 mesh screen.

Three pellets, each -20 mm in diameter and ~3 mm thick were uniaxially pressed using ~5 gra of this powder and were sintered on alumina plates inside a tube furnace at a well-controlled temperature of 850°C for 44 hours in air. After sin- tering, the pellets were subjected to three types of cooling described elsewhere (4). These samples are designated as liquid quenched, air quenched and slowly cooled depending upon the type of cooling regimen applied to each pellet.

The superconductive properties of each sample were characterized by means of ac magnetic susceptibility measurements, and the various crystalline phases pre- sent in each sample were identified via powder X-ray diffraction (XRD); these meth- ods are described in earlier publications (2,4,7,8).

Results and Discussion

Magnetic Characterization

The temperature dependences of the real and imaginary parts of the complex ac magnetic susceptibility, x'(T) and X"(T), of the three specimens in various ac fields, h, are shown in FIGS. 1 to 3. Each of these figures displays the features common to sintered ceramic superconductors whose intergranular coupling con- sists of weak links formed by grain boundaries and nonsuperconducting phases. Descriptions and explanations of the complex susceptibility behavior of ceramic

Vol. 25, No. 8 CUPRATES 1067

O!

- 0 . 2

6-" E .~ -0.4

- o 6

- 0 . 8

- I . 0

- H=O

, ' " / ~ /CURVE I - - 0 . 0 4 / 2 / . . . . . i - 6~;6

/ / s - o . 4 o / / I ] ~ - ^

/" .I 4 - 1.00 / / 5 - 1.60

/ / " 6 - 2 4 0 7 - 3 .60

I i i i J 20 4 0 60 80

TEMPERATURE (K)

- - o . o ~

- - 0 . 02

~u

- - 0 . 03

- - 0 . 0 4 (a)

i - 0 . 0 5 I 0 0 120

FIG. 1

0.2

0.1

_ I I I

2

0 - I I I I I

0 20 40 60 80 I 0 0 TEMPERATURE (K)

Temperature dependence of the ac susceptibility in various ac fields, h, for Bil.6Pbo.4Sr2Ca3Cu4Ox quenched in tetrachloroethylene; (a) real part, Z'; (b) imaginary part (loss), Z". The Z' curves are expanded on the right-hand axis.

- -0 .2

E ~. - o . 4

- 0 . 6

- - 0 . 8

- - I . 0 r 0

- 0 . 2

E - -0 .4

- o . 6 k

- 0.8

- r

CURVE

20

H=O h in Oe

I -- 0 .04 2 - 0 .16 3 - 0 .40 4 - 1.00 5 - 1.60 6 - 2.40 7 - 3.60

4 0 60 80

TEMPERATURE (K)

i

H=O h in Oe

CURVE I - 0 .04 2 - 0 .16 3 - 0 .40 4 - 1.00 5 - 1.60 6 - 2 .40 7 - 3 .60 /

20 4 0 6 0

- - 0 . 01

- - 0 . 0 2

- - 0 . 03

- - - 0 . 04

I - - 0 .05 I00 120

o23

d" E 0.2 u

I= 0.~ ~t

-I.0 0

0

0 20 4 0 60 80 TEMPERATURE (K)

FIG. 2

S a m e as FIG. 1 but for the air-quenched sample.

i ~ , ~ 0.3 ~ ,-.-z" O.

i/// _ o

- - - 0 . 0 4

" Ca) i - 0 . 05

80 I 0 0 120

FIG. 3

6" E 0.2

0.1

TEMPERATURE (K)

0 i

0 20 4 0 60 80 TEMPERATURE (K)

Same as FIG. 1 but for the slowly cooled sample.

100

I00

120

120

120

1068 A.K. SARKAR, et al. Vol. 25, No. 8

superconductors in the Bi-Sr-Ca-Cu-O system, processed by various techniques can be found in our previous reports (2,4,7,8).

The susceptibility data show that all three samples consist of two superconducting granular components, which will be identified as the 2212 and 2223 phases. The effective volume fractions of the high-Tc phase are nearly identical in these samples, as estimated from the initial step in x'(T) below the onset of diamagnetism in FIGS. l(a)-3(a). Defining Tc of the granular phases as the onset temperature of these steps, it is found that Tc of the 2223 phase is ~93 K after liquid quenching, 105 K after air quenching, and 107 K after slow cooling.

In the liquid-quenched sample, the second granular component has Tc~80 K; following our earlier arguments (4), this transition is due to the 2212 phase. In the other samples, the low-Tc transition is partially obscured by the field-dependent bulk behavior (FIGS. 2 and 3), but it can be seen that Tc of the 2212 phase is 70 K and 63 K in the air-quenched and slowly cooled samples, respectively. The phase identifica- tions will be confirmed in the next section.

The changes in Tc of the 2212 phase are due mainly to oxygen stoichiometry variations, as is well known, and therefore they can be affected by the post-sintering cooling rates (4). Here we observe also, for the first time, that fast cooling can drasti- cally lower Tc in the 2223 phase while raising Tc in the 2212 phase. We believe that these Tc shifts are again related to changes in the oxygen stoichiometry of these phases. The low oxygen content established at the sintering temperature is frozen- in metastably during the fast cooling of the liquid-quenched sample. A lowering of Tc, from 107 K to -98 K, in the Pb-doped 2223 phase has also been reported by Zhao et al. (6), who used samples with the same nominal composition.

The different cooling procedures also cause large changes in the bulk behav- ior, in terms of intergranular coupling and Tc values. The bulk superconductive behavior is identified by the strong dependence of the susceptibility on ac field strength, h, in FIGS. 1-3; its onset coincides with the zero-resistance temperature found in electrical transport measurements (2,7). The liquid-quenched sample (FIG. 1) shows the worst intergranular coupling and the lowest bulk Tc, ~60 K; the bulk transition occurs far below the two granular transitions. We have attributed such inferior bulk properties to extensive microcracking in liquid-quenched samples not containing Pb, which were very brittle (4). The present leaded material is not as brittle, but the lack of good intergranular contact probably stems from the same structural deficiencies, although effects due to quenched-in compositional variations at the grain boundaries cannot be ruled out.

The air-quenched (FIG. 2) and slowly cooled samples (FIG. 3), on the other hand, both show properties which are similar to those of other Pb-stabilized, Bi- based superconductors (7,8). The bulk superconductive transition is the result of in- tergranular coupling entirely within the high-Tc 2223 phase. This direct intraphase connectivity occurs despite the fact that the 2223 fraction is not larger than it is in the liquid-quenched sample. The bulk Tc is 95 K in the air-quenched and 90 K in the slowly cooled samples.

Vol. 25, No. 8 CUPRATES 1069

XRD Characterization

The powder XRD patterns for the three specimens are shown in FIG. 4. Most of the peaks belong to the two superconducting 2223 and 2212 phases and are marked as H and L, respectively. Based on the intensity of the diffraction peaks, it is estimated that each specimen contains twice as much of the 2223 phase as of the 2212 phase. Such an estimate can be made from the bulk susceptibility curves only for the liquid-quenched sample, FIG. l(a), where it is in good agreement with the XRD results. Based on these data, we can state that in the liquid-quenched sample, the 2223 phase has Tc=93 K and the 2212 phase has Tc=80 K.

rY ,< ZZ I.--

n,-"

>- l . -

Z W I.-. Z

• = 2 2 2 3 • = 2 2 q 2 + =(SGCo)I4CU24041 o =(CO,SdzCuO 3 o :co2PbO 4

o =CuO ? =UNKNOWN

14 2 4

T

(o)

5 4

(b)

4 14 24 3 4 4 4

2 8 ( D E G R E E )

FIG. 4

Powder XRD patterns for Bil.6Pb0.4Sr2Ca3Cu4Ox samples; (a) quenched in tetrachloroethylene; (b) quenched in air; (c) slowly cooled in air.

1070 A.K. SARKAR, et al. Vol. 25, No. 8

The XRD patterns for all three samples are nearly identical in terms of phase constituents. In addition to the two superconducting phases, there is a host of im- purity phases, identified as insulating (Ca,Sr)2CuO3, Ca2PbO4 and CuO, and the lay- ered (Sr,Ca)14Cu24041 phase. The slowly cooled sample, however, did not contain any of the latter phase, as seen in FIG. 4(c). A few unidentified peaks are also marked in FIG. 4; they could not be matched with any known phase in this system, although a peak at 14.7• may signal the formation of an unknown layered compound in this complex system. No shifts in the peak positions of the superconducting phases are observed in these XRD patterns. Thus, any lattice parameter variations due to changing oxygen stoichiometry are very small.

Conclusions

It is shown that the post-sintering cooling rate can affect the Tc values of the superconducting 2212 and 2223 phases in the Pb-doped Bi-Sr-Ca-Cu-O system. The movement of Tc of these two phases in opposite directions is related to their oxygen stoichiometry. A faster cooling rate can stabilize an oxygen-deficient structure. The Tc of the 2212 phase in this study has been shown to vary from 80 K, when the structure is oxygen deficient to a value of 63 K, when the structure is oxygen loaded. At the same time, Tc of the 2223 phase is seen to vary from -93 K when the structure is oxygen deficient to a maximum value of 107 K when the structure is properly oxygenated by slow cooling.

Although these remarkable changes (of as much as 40 K) have been observed in the 2212 phase by many investigators (4), such has not been the case for the 2223 phase. We believe that the starting composition has a paradoxical effect on the structure of the 2223 phase. The starting composition for obtaining the ideal 2223 phase composition is Ca and Cu rich, and for this reason many Ca and Cu bearing impurity phases are formed. The presence of excess Ca and Pb may also alter the 2223 unit cell formula. This specific unit cell composition may in fact make Tc of the 2223 phase more sensitive to oxygen stoichiometry. The origin of the opposite behaviors of these two phases may lie in the extra square-planar Cu-O planes situ- ated between the two square pyramidal Cu-O planes in the 2223 crystal structure.

No variation in lattice parameters was found in any of the superconducting phases from x-ray measurements performed during this study.

Acknowledgements

We are grateful for the support and advice given by P. M. Hemenger and T. L. Peterson of the Air Force Materials Laboratory, Wright Research and Development Center, Wright-Patterson Air Force Base, OH where we performed the magnetic and structural characterizations. I. M. was supported under Contract No. F33615-88-C-5423.

Vol. 25, No. 8 CUPRATES 1071

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2.

3.

4. 5. 6. 7.

8.

References

J. L. Tallon, R. G. Buckley, P. W. Gilberd, M. R. Presland, I. W. M. Brown, M. E. Bowden, L. A. Christian, and R. Goguel, Nature 333, 153 (1988). A. K. Sarkar, B. Kumar, I. Maartense, and T. L. Peterson, J. Appl. Phys. ~ 2392 (1989). D. E. Morris, C. T. Hultgren, A. M. Markelz, J. Y. T. Wei, N. G. Asmar, and J. H. Nickel, Phys. Rev. B3__9,9 6612 (1989). A. K. Sarkar and I. Maartense, Physica C (in press). A. K. Sarkar and I. Maartense, unpublished work. J. Zhao, M. Wu, W. Abdul-Razzaq, and M. S. Seehra, Physica C 165, 135 (1990). A. K. Sarkar, I. Maartense, T. L. Peterson, and B. Kumar, J. Appl. Phys. 6_6_6_,6 3717 (1989). A. K. Sarkar, I. Maartense, B. Kumar, and T. L. Peterson, Supercond. Sci. Technol. 3 199 (1990).