Investigations on the formation mechanism of the Bi(2223) phase in bulk samples and Ag-sheathed tapes

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Investigations on the formation mechanism of the Bi(2223) phase in bulk samples and Ag-sheathed tapes

GRIVEL, Jean-Claude, et al.

Abstract

We present observations performed on ceramic and Ag-sheathed samples. A model for the formation mechanism of the (Bi,Pb)2Sr2Ca2Cu30Y (Bi(2223)) phase is proposed and the processing of tapes is discussed in relation with this model.

GRIVEL, Jean-Claude,

. Investigations on the formation mechanism of the Bi(2223) phase in bulk samples and Ag-sheathed tapes. In:

Superconductivity

. [S.l.] : [s.n.], 1998. p. 50-53

Available at:

http://archive-ouverte.unige.ch/unige:106533

Disclaimer: layout of this document may differ from the published version.

THE 1998 INTERNATIONAL WORKSHOP ON SUPERCONDUCTIVITY

July 12-15, 1998 Okinawa, Japan

lf#i

INVESTIGATIONS ON THE FORMATION MECHANISM OF THE BIC2223) PHASE IN BULK SAMPLES AND AG-SHEATHED TAPES

J.-C. Grivel^1•2, H.F. Poulsen^2, L.G. Andersen^2, T. Frello^2, N.H. Andersen2, E. Giannini3, D.P. Grindatto⁴ and R. Flilkiger

1Nordic Superconductor Technologies, Priorparken 848, DK-2605 Brnndby

2Risa National Laboratory, DK-4000 Roskilde

3University of Geneva, Dpt of Condensed Matter Physics, CH-1211.Geneve 4

4Laboratorium ftir Festkorperphysik, ETH Zilrich, CH-8093 Zilrich, Switzerland

ABSTRACT

We present observations performed on ceramic and Ag-sheathed samples. A model for the formation mechanism of the (Bi,Pb)2^Sr2^C~Cu3^0Y (Bi(2223)) phase is proposed and the processing of tapes is discussed in relation with this model.

INTRODUCTION

Owing to its high potential for technical use in the form of Ag-sheathed tapes, the Bi(2223) phase has been the object of numerous studies aiming at a thorough understanding of its formation mechanism from calcined precursor powders [1 and references therein]. Several models have been proposed for explaining the formation of this compound. Contrary to the case of bulk samples where the evolution of individual grains can be followed during the reaction, at least on the surface of ceramic pellets, Ag-sheathed tapes can only be investigated through statistical methods. Consequently, the interpretation of results is quite difficult and numerous features can be explained within the frame of several different models.

In this contribution, we review some previous results and present new investigations performed on bulk, Ag-free samples and on Ag-sheathed tapes. The results are interpreted within the frame of a reaction model and some consequences of the Bi (2223) phase formation mechanism on the processing of Ag-sheathed tapes are discussed.

EXPERIMENTS

Simultaneous DTNTG measurements were performed in a SETARAM TAG24 thermal analyser, using calcined Al₂0₃ powder as reference, in flowing synthetic air with a heating rate ^o[ 2°C min·'. Scanning Elcctr0i1 Iviicroscope (SEM) observations were performecl in a Stereoscan 360 from Cambridge Instruments, equipped with a Si(Li) x-ray detector for microprobe analysis. Transmission Electron Microscope (TEM) investigations were carried out on longitudinal cross-sections of tapes. Room temperature x-ray diffraction (XRD) patterns were recorded in a 8-28 diffractometer using Ni filtered Cu Kn radiation. In situ, high temperature studies were performed at the BW5 synchrotron beamline at HASYLAB, Hamburg. Details about the experimental setup and data analysis have been published else\vhere [2].

RESULTS

50

The formation mechanism of the Bi(2223) phase can basically be studied in 2 different situations, i.e. in Ag-sheathed tapes or in bulk samples. The same precursor powders can be used in both cases and the transformation mechanism is not expected to vary significantly.

There are however some important differences that have to be taken into account. First, the presence of the Ag sheath can influence the process for various reasons, of which the most obvious are: a slight decrease of the reaction temperature and a reduced diffusion capability of some gaseous species. Second, a significant preferential orientation of the Bi(2212) and Bi(2223) plate shaped crystallites is found to occur inside the sheath.

Furthermore, in tapes, the grain size of the precursor powders is significantly reduGed during the mechanical processing. The mean grain size, which initially lies around 1 µm is reduced to a few tens of nm at the end of the tape forming process. It is therefore suitable to study the Bi(2223) formation mechanism not only in bulk, Ag-free samples, which enable more direct observations owing to the absence of a surrounding material, but in tapes as well in order to get insights into the possible influence of the sheath material on the involved reactions.

The difference in powder grain size resulting from the initial mechanical processing of tapes does not seem to have a direct influence on the formation mechanism, since the precursor powder grains grow up to a size comparable with that they initially had, before the Bi(2223) phase starts to form. SEM observations performed on tapes quenched along a heating ramp similar to the one used during the thermal processing of tapes revealed that the size of the various grains present inside the Ag sheath drastically increases starting from temperatures as low as 750°C [3]. Due to the 2 dimensional growth of the Bi(2212) platelets, this effect results in the formation of porosity inside the ceramic core of the tapes and is also likely to play a role in the slight increase of thickness of the tapes. As a consequence, it appears that at the onset of the Bi(2223) formation, the precursor powder inside the Ag sheath has recovered its original morphology, except for a significant preferential orientation of the Bi(2212) grains.

During the initial temperature increase, other interesting phenomena occur in the precursor powders. TG measurements showed that the mass qf the tape samples increases between 500°C and 650°C. XRD patterns recorded on powders extracted from tapes quenched along a heating ramp similar to the one used in the DT AITG measurements enabled us to understand the mass increase as being due to an oxygen uptake. The mechanism is the following. The Bi(2212) phase present in the precursor powders almost always contains a small amount of Pb, as a result of the high temperature calcination performed during the precursor powders preparation. The solubility limit of Pb however decreases with temperature [4] and the Pb doped Bi(2212) phase present in the precursor powders should be considered as metastable.

Consequently, during the slow heating of the green tapes, Pb is released from the Bi(22 l 2) phase, resulting in the formation of the Pb₃Sr_2.5Bi_0.5Ca₂Cu0Y (3321) phase. The oxidation state of Pb being different in those 2 phases (2+ and 4+ in Bi(2212) and 3321 respectively), oxygen is absorbed from the ambient atmosphere and results in a net weight increase [5]. This feature was also observed in Ag free samples. Pb-free Bi(2223) precursor powders were also investigated and showed no weight gain in this temperature interval, sustaining the above interpretation.

At higher temperatures (above 700°C), Pb is again incorporated in Bi(2212). This Pb uptake occurs from 2 different sources: from the 3321 phase formed during the first part of the heating ramp and I or from an interaction with the Ca₂Pb0₄ phase \Vhen this compound ts present in the powders.

..

,,

I

In ceramic, Ag-free samples, it has been found that the Bi(2212) phase becomes thermodynamically unstable as a result of the increase of its Pb content. Consequently, the Bi(2212) grains progressively decompose, giving rise to a liquid phase that can lead to the growth of the Bi(2223) phase after suitable changes in its composition. The decomposition of the Pb enriched Bi(2212) phase as weU as the growth of Bi(2223) platelets was directly observed at the free surface of bulk samples by use of SEM. Consequently, a model was proposed to describe the formation mechanism of the Bi(2223) phase, that might be summarised in the following way:

C<tzPb04

+

+

•

CaO

+

Bi-rich liquid+ alkaline earth cuprate phases ~ liquid' liquid' ~ Bi,Pb(2223) +secondary phases

Similar observations performed under a reduced oxygen partial pressure (p₀₂ = 7.5%) instead of air showed that this reaction scheme is also suited for conditions slightly different from those under which the it was first deduced. The composition of the alkaline earth cuprate phases is however likely to be different in lower p_{02 ,} due to differences in the equilibrium phase assembly.

Owing to the presence of the Ag sheath, the direct observation of a progressive decomposition of Bi(2212) grains is not possible in tapes. The question thus arises whether the same reaction scheme can be used to describe the Bi(2223) formation mechanism inside the Ag sheath of tapes or not.

In situ, high temperature investigations are possible by use of synchrotron radiation, e.g. at HASYLAB [2]. This non destructive method enables to get information on the transformations occurring in the ceramic core through the Ag sheath. Of special interest is also the ability to avoid the problem of crystallisation of phases during the cooling (quenching) of samples, that might result in a misinterpretation of the data. It was possible to show that the Bi(2201) phase is not present in detectable amounts at the reaction temperature (835°C in air) but readily formed upon cooling below 810°C. An analysis of the Bi(2223) formation kinetics by use of the A vrarni model yielded results similar to those found in bulk samples. Although such analyses do probably not really provide insights into the microscopic mechanisms of the phase formation, this result indicates that the overall transformation kinetics are not drastically affected by the Ag sheath.

TEM investigations have also been performed on quenched samples. Numerous stacking faults were observed along the c-axis of the Bi(2223) grains, but no evidence for the formation of an intermediate Bi(3345) phase was found [6]. Furthermore, strong indications for the presence of a Bi-rich liquid phase during the first stages of the transformation have been found, in agreement with the above proposed model.

We could check that the grains containing a majority of Bi(2223) layers tend to be systematically thinner than the thicker initial Bi(22 l 2) grains. This observation is not in

52

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·, ...

l

agreement with an intercalation mechanism, since in such_ a case, the thickness of the crystallites should increase owing to the longer c-axis parameter of the Bi(2223) phase. One might nevertheless also explain this feature by a splitting of the Bi(2212) crystals along the Bi-0 planes owing to the tension forces exerted during an intercalation. It should however be stressed that only very few Bi(2223) half layer interrupted within a Bi(2212) matrix have been observed, even for sintering times corresponding to the highest transformation rate.

DISCUSSION AND CONCLUSION

If the transformations were occurring through an intercalation mechanism, it would appear that the texture of the Bi(2212) phase plays a very important role in the final performances of tapes, since the Bi(2223) phase should inherit this pre-existing texture. A key step in tapes processing would then be to obtain the best possible Bi(2212) texture before converting the precursor powders into the Bi(2223) phase. A transformation mechanism including a progressive decomposition of the Bi(2212) phase as well as a nucleation and growth of the new Bi(2223) phase grains, does however not imply basic changes in the approach of this problem. The texture of the Bi(2212) crystallites can still be transmitted to the growing Bi(2223) platelets, provided the new phase grows onto the free surfaces of some not yet decomposed Bi(2212). platelets or earlier grown Bi(2223) crystallites. Nevertheless, this texture transmission mechanism is only possible as long as the sintering temperature is not too high. It is inferred that the higher the temperature, the faster the decomposition rate of the Pb enriched Bi(2212). If the Bi(2212) grains disappear too rapidly, the Bi(2223) grains will not find suitable substrates for their growth, and their texture will be determined by other factors.

Furthermore, the use of a too high temperature will preclude the formation of the Bi(2223) phase owing to the small size of the stability domain of this compound.

ACKNOWLEDGEMENTS

Support for this work was provided by the Swiss National Foundation (PNR 30), the Priority Program "Materials Research and Engineering" (PPM) the Brite Euram II project No BRE2 CT92 0229 (OFES BR60), the Danish Research Councils through DanSync and IVC, the Danish Energy Agency and by the companies NST, ELSAM and ELKRAFT.

REFERENCES

[1] Grivel J.-C. and FlUkiger R., Supercond. Sci. Technol. 11 (1998) 288 [2] Poulsen H.F et al., to appear in Physica C

[3] Grivcl J.-C. cl al., in Proc of the ICM/1.S-93, Paris, Eds. Etoumcw.u ct al. I.I.T.T. 1993 359 [4] Majewski P., Kaesche S., Su H.-L. and Aldinger F., Physica C 221(1994)295

[5] Grivel J.-C., Kube! F. and F!Ukiger R., J. Thermal Anal. 48 (1997) 665 [6] Grindatto D.P et al., to appear in Physica C.

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