~ Solid State Communications, vol. 77, NO. 2 pp. 121-124, 1991. 0038-1098/9153.00+.00 Printed in Great Britain Pergamon Press plc

THE EXTENT OF CRITICAL TEMPERATURE VARIATIONS OF THE SUPERCONDUCTING PHASES IN THE Bi-Pb-Sr-Ca-Cu-O SYSTEM

Asok K. Sarkar and I. Maa_rtense

University of Dayton, Research Institute, 300 College Park, Dayton, Ohio 45469-0170 USA

(Received 22 August 1990 by M. F. Collins)

Bulk specimens having nominal composition of Bi2_xPbxSr2Ca2Cu3Oy (x=0, 0.2, and 0.4) were subjected to various annealing and post-sinter cooling treatn~nts to alter the oxygen stoichiometries and the granular critical temperature (To) values of the superconducting phases. Tc was found to vary from 53 K to 93 K in the 2212 phase and from 100 K to 108 K in the 2223 phase, depending on the processing. The shifts in the Tc values of the two phases were not always in opposite directions as reported by others. Inferior intergranular coupling resulted when Ca2PbO4 precipitated into the grain boundaries during oxygen cooling, if the lead content was high (x--0.4). The implications of these processing parameters on the bulk superconductive properties of this system are discussed.

1. Introduction

The two important superconducting phases in the Bi-Sr-Ca-Cu-O system are the Low-T¢ (-80 K), Bi2Sr2CaCu2Os+8 (2212) phase and the High-To (~110 K), Bi2Sr'2Ca2C'u3Ol0+8 (2223) phase. I A very significant role is played by oxygen stoiehiometry in changing the intrinsic critical temperatures, Tc, of these phases. 2 However, it is not fully understood why the Tc's of these two phases are affected in opposite ways by the oxygen stoichiomelry. Whereas T¢ of the 2212 phase is highest in the presence of an oxygen deficiency, T¢ of the 2223 phase shifts downwards as a result of this. 3 Moreover, T e of the 2212 phase can be widely varied by adjusting the oxygen stoichiometry through various preparation techniques. 4 Fortunately, this has not been the case for the 2223 phase. Although Tc of the 2223 phase can be lowered by as much as 15 K by oxygen deficiency, in non-stoichiometric batch compositions, 3,5,6 in most cases this value has been reported to vary by only -5 K. 2,7 If it be assumed that a high Tc is desirable in the present system, the fact that the 2223 phase resists Tc variation while the Te of the 2212 phase can be raised close to 90 K may be of considerable practical importance for processing these superconductors. These variations in Tc have been observed also in Pb/Bi- substituted compositions of this system and this makes it more likely that the granular variations can be imparted to bulk sintered materials, since lead-containing preparations are known to have enhanced bulk properties. 8

A number of methods for altering the Tc values of these two superconductors has been reported in the literature. 9 In this paper, we present our results showing the extent by which their granular 're values can he shifted in bulk samples of a fixed nominal composition. The shifts resulted from subjecting the samples to different processing conditions which are assumed to lead to variations in the oxygen stoichiometry.

2. Experimental

The samples examined during this study had a nominal composition of Bi2.xPbxSr2Ca2Cu3Oy (x=0, 0.2, and 0.4) and were prepared from powders synthesized by

solid state reactions of reagent grade Bi203, PbO, SrCO3, CaCO3, and CuO. The details of the powder synthesis are gtven elsewhere. 8 Approximately 5 g of the individual powders were uniaxially pressed at 310 MPa (45 kpsi) into pellets -20 mm in diameter and -3 mm thick.

In most cases, the pellets were placed on open alumina dishes in air inside a programmable tube furnace. Following the sintering with a specific time/temperature schedule, changes in the oxygen stoichiometry of the specimens were accomplished either by slow cooling of the furnace at a programmed rate of 2°C/h or by fast quenching of the pellets in tetrachloroethylene or in air. A few samples were also heat treated in air for a fixed time and then slowly cooled in flowing oxygen or in nitrogen. However, to avoid melting of the specimen, the nia'ogen flow was not turned on until the temperature reached 750°C during the cool-down period.

In order to study the effect of a very low partial pressure of oxygen, pellets of each composition were also vacuum annealed by either an open or a sealed method.

In the open method the slowly cooled pellets, after being sintered at 870°C for 40 h, were re.annealed fast at 350°C for 11 h and then at 500°C for an additional 20 h while a mechanical vacuum pump was connected to the tube furnace. In the sealed method, the freshly pressed pellets were sealed inside silica glass tubes under a vacuum - I -2 mTorr. The samples were then sintered at 760°C for 40 h and slowly cooled inside the furnace.

The various sample compositions and processing parameters used during the present study are summarized in Table 1. The superconductive properties were characterized by means of AC magnetic susceptibility measurements using samples taken from inside each pellet. The phase identifications were performed using standard powder x-ray diffraction (XRD) techniques with CuKct radiation.

3. Results and Discussion

From the susceptibility measurements it is possible to determine the granular as well as intergranular (bulk) superconducting temperatures. The data collected from these measurements are shown in Table 1. In most cases both the 2212 and 2223 superconducting granular phases were detected, the first with a low Tc (<95 K) and the other

121

122 SUPI.'RCONI)UCT[NC ]'}IASES IN TIlE B i - l ' b - S r - C a - C u - O SYSTEM V01. 77, NO. 2

with a high Tc (>I(X) K). However, when the composition did not contain any lead (x=0), the amount of the 2223 phase was very small (-2% of the full diamagnetic susceptibility). The quantity of the 2223 phase, in general, increased with the amount of lead (x) in the starting compositions. This was the case for all samples sintered in air and cooled in air or oxygen. The 2223 phase was not detected in leaded samples cooled in nitrogen, since this phase is unstable in a pure nitrogen atmosphere at high temperatures.10 Also, the amount of the 2223 phase was much diminished in samples treated by the open vacuum method.

Table 1 reveals some interesting facts about the granular Tc values of the superconducting phases. It is seen that Tc can be varied over a wide range (40 K) in the 2212 phase and over a smaller range (-6 K) in the 2223 phase. We observe that cooling in oxygen can in fact slightly lower the T c of the 2223 phase, contrary to the observations reported earlier that oxygen loading enhances its To. It should be remarked that all these observed variations are assumed to be due to changes in the oxygen stoichiometry of the phases resulting from the sample treatments. We have no independent information on the oxygen content either of the individual phases or of the total sample. It appears that processing the Bi-superconductors in air is the most effective way to obtain the highest Tc in the 2223 phase. Liquid quenching and the open vacuum treatments lowered Tc of this phase to -101 K, whereas the sealed vacuum method did not seem to affect To. In the latter case,

Table I. Superconducting critical temperatures of samples with nominal composition Bi2.xPbxSr2Ca2Cu3Oi0+8 after various heat ireatments

To, K Sintering Lead Temp- Post-Sinter Content, 2223 2212 Tnne Treatment x phase phase (°C-h)

Slow 0 107" 65 cooling in 0.2 107 60 air 0.4 107 65

87(I-40

Bulk

65 1 (12 100

870-40 Liquid 0 --- 89 30 quenched 0.2 101" 86 83

0.4 101 85 82

each phase had close to its highest Tc; we believe this to be the result of a partitioning of the available oxygen between the two superconducting phases, with the 2223 phase taking up the most oxygen.

Although the different oxygen partial pressure environments had a relatively small influence on the Tc of the 2223 granular phase, this was not the case for the 2212 phase. As can be seen in Table 1, Tc of the 2212 phase can vary from 53 K to 93 K and is highest at the lowest oxygen partial pressure, i.e. when there is an oxygen deficiency. This is the first time we have observed a Tc as low as 53 K in the oxygen-rich 2212 phase. We can not yet say why this happens with this specific sample composition (x=0.2); the presence of Pb is suspected to play a pan in this. Since the low-Tc phase is partially obscured in the x(T) data of the bulk superconducting sample, its Tc was determined from the susceptibility data on a powdered sample, shown in Fig. 1, where it is confirmed to be 53 K.

The superconducting transition temperature of bulk samples, as defined by the low-current zero-resistivity onset is also strongly affected by the processing variables. The bulk Tc depends not only on the relative volume fractions of the superconducting phases but also on the quality of their intergranular coupling. Generally, the presence of a large volume fraction of the high-Tc phase increases the bulk transition temperature of the samples. The coupling is affected by the presence of impurity phases in the grain boundaries and also by microcracking introduced in the sample by fast quenching. For this reason, the liquid- quenched samples have lower bulk-Tc values than the slowly cooled samples.

The samples all show a strong dependence of z'(T) and if(T) on the AC field strength, below the bulk transition temperature. This behavior is typical of the bulk superconductivity in these ceramic oxides, and is evidence of the low critical current density of these materials. We have reported many data of this kind relating to the Bi-based superconductors, 1,6,8,9,11 but we show in Fig. 2 the deleterious effect of slow cooling in oxygen on the leaded composition with x=0.4. Although the bulk Tc (102 K) is close to that of the air-cooled sample, the multi-component structure of the AC loss data indicates that several distinct grain-boundary precipitates are present.

The identification of the superconducting phases was also can'ied out with the help of powder XRD in order to corroborate the susceptibility data. The presence of a small amount of the 2223 phase, seen in those data, was not always detected by XRD, especially if its volume fraction

.870-40 Air 0 --- 75 61 quenched 0.2 1(16 65 90

0.4 106 67 102

870-20 Slow 0 I (17 60 cooling in (1.2 104 53 ox~,~en 0.4 105 63

870-40 Slow 0.2 --- 93 cooling in nitroten

870-40 Open 0 --- 86 vacuum 0.2 100" 86 method 0.4 101* 86

760-40

53 96 102

60

66 81 82

Sealed 0 --- 82 35 vacuum 0.2 106 82 83 method 0.4 108 85 90

* Present in small amount

-0.2

m

-04

t,u -0.6

n- -08

-1,0

Fig. 1

j j "

/ /

/ /

/ f /

h = 3 6 Oe

20 40 60 80 1 O0 120

TEMPERATURE (K)

AC susceptibility of a powdered sample of nominal composition Bi i.sPbo.2Sr2Ca2Cu3010+8 sintered at 870°C and slowly cooled in oxygen.

Vol. 77,

0

.0.2

0 -0 4

E o

-~ . 0 6 ' <

-0.8

-1,0

0

No. 2 SUPERCONDUCTIN(; P t l A S E S IN TIlE Bi-Pb-Sr-Ca-Cu-O SYSTEH 123

C c . o o o . lo o.

,;; ///'~/ // ' /

(a)

20 40 60 80 100 120

TEMPERATURE (K)

0.15

~ , z.jT~x" (b)

" 005 , / , .

0

o 2o 4o 6o so too 12o TEMPERKtURE (K)

Fig. 2 (a) The AC susceptibility, f(T), and (b) the AC loss, f '(T), of a bulk sample of nominal composition Bil.rPb0.4Sr2Ca2Cu3Ot0di sintered at 870°C and slowly cooled in oxygen. The applied AC fields are listed in (a).

was below -5%. Otherwise, the attribution of granular Tc values to specific superconducting phases was conf'trmed by the XRD patterns of the samples. No shifting of diffraction peaks, depicting variations in the lattice parameters was observed for samples containing similar phases with widely different granular Tc values.

The impurities detected were Ca2CuO3 and Ca2PbO4 (for leaded samples). It is interesting to note that the amount of the Ca2PbO4 phase increased for samples cooled in oxygen. This observation is consistent with the behavior described above, relating to Fig. 2.

4. Summary and Conclusions

The present investigation shows that the processing atmosphere can directly affect the quality of the

superconductors in the Pb-substituted Bi-Sr-Ca-Cu-O system. In addition to the common problems (such as impurities, low density, etc.) from which a sample in this system may suffer, the overall bulk properties are also influenced by the granular Tc values of the two superconducting phases. Although these values are known to vary with the oxygen stoichiometry of the 2212 and 2223 phases, it appears that addition of lead to the system can increase the extent of the Tc variations.

It has been demonstrated that Tc in the bismuth cuprates is depressed when the optimum value of the hole density in the CuO2 layers is exceeded 4,12 and that this condition occurs in the 2212 phase when it is oxygen rich. The presence of lead may allow a further increase in the oxygen content and hole density.

Although we have consistently found that the 2212 phase in unleaded ceramics has Tc - 65 K, after slow cooling in air, It our overall results show that the lowest values of Tc usually occur when the starting composition has a lead content of x = 0.2. In the case of slow cooling in oxygen, we then find that Tc = 53 K. Whether this can be attributed directly to the incorporation of lead into the structure is not clear. When the nominal Pb content is raised to x = 0.4, we see in many cases that Tc is lowered to a lesser extent. We plan to investigate the correlation of this trend with physical properties such as grain size, and Pb chemistry such as plumbate formation.

Based on our overall observations in this work the following new conclusions can be listed, in addition to our previously published resuhs.8,9, I I

!. The critical temperature of the 2212 phase can be easily varied from 53 K to 93 K, by changing the method of cooling the samples after annealing at high temperatures.

2. Oxygen can partition between the two superconducting phases present in a sample when it is heat treated in a sealed container, causing their critical temperatures to vary in the same direction.

3. The presence of excess oxygen is detrimental to this system since it promotes the formation of Ca2PbO4 in the grain boundaries, harming the intergrantdar coupling and the critical current density in the bulk material.

4. Careful lowering of the oxygen partial pressure during cooling can enhance the Tc of the 2212 phase to 77 K. Such processing may improve the bulk properties of the leaded Bi-Sr-Ca-Cu-O systems at liquid nitrogen temperature.

A c k n o w l e d g e m e n t s - We are grateful for the support and advice given by P. M. Hemenger and T. L. Peterson at the Air Force Materials Laboratory, Wright Research and Development Center, Wright-Patterson Air Force Base, Ohio, where we performed the magnetic and structural characterizations and which sponsored the work of 1. M. under Contract No. F33615-88-C-5423.

References

1 .

2.

3 .

4.

A. K. Sarkar, B. Kumar, I. Maartense, and T. L. Peterson, J. Appl. Phys. 65, 2392 (1989). J. L. Tallon, R. G. Buckley, P. W. Gilberd, M. R. R'esland, I. W. M. Brown, M. E. Bowden, L. A. Christian and R. Goguel, Nature 333, 153 (1988). J. Zhao, M. Wu, W. Abdul-Razzaq and M. S. Seehra, Physica C 165, 135 (1990). W. A. Groen, D. M. de Leeuw, and L. F. Feiner, Physica C 165. 55 (1990).

5 .

6.

7.

8.

M. A. Dinia, O. Pena, C. Pen-in, and M. Sergent, Solid State Commun. 73, 715 (1990). A. K. Sarkar and I. Maartense, Mat. Res. Bull. 25, 1065 (1990). D. E. Morris, C. T. Hultgren, A. M. Markeh, J. Y. T. Wei, N. G. Asmar and J. H. Nickel, Phys. Rev. B39, 6612 (1989). A. K. Sarkar, 1. Maartense, T. L. Peterson and B. Kumar, J. Appl. Phys. 66, 3717 (1989).

SUPERCONDUCTING P}~SES IN THE Bi-Pb-Sr-Ca-Cu-O SYSTEM Vol. 77, NO. 124

9. A.K. Sarkar and 1. Maartense, Physica C 168, 591 (1990).

10. N.M. Hwang, G. W. Bahng, H. G. Moon, and J. C. Park, Appi. Phys. Lett. 54, 1588 (1989).

11. A.K. Sarkar, 1. Maartense, B. Kumar, and T. L. Peterson, Supercond. Sci. Technol. 3, 199 (1990).

12. D. B. Mitzi, L. W. Lombardo, A. Kapitulnik, S. S. Laderman, and R. D. Jacowitz, Phys. Rev. B 41, 6564 (1990).