The occurrence, composition, and melting behavior of lead-rich phases detected during the thermal processing of Ag/Bi-2223 composite conductors have been investigated by a combination of transmission electron microscopy (TEM), energy dispersive analysis (EDS), x-ray diffraction, differential thermal analysis, and Raman spectroscopy. A (Bi,Pb)-Sr-Ca-Cu-O composition with {open_quotes}4112{close_quotes} metal atom stoichiometry and Pb{approx_equal}Bi was detected during TEM/EDS examinations of partially processed/quenched Ag/Bi-2223 composites. An as-synthesized oxide mix with this stoichiometry was found to melt at a temperature <700{degrees}C. The lead-rich {open_quotes}3221{close_quotes} phase was found to have a wide range of compositional stability in terms of Pb/Bi ratio and Bi + Pb stoichiometry. Evidence was seen for the co-existence of this phase with (Ca,Sr){sub 2}PbO{sub 4} in partially processed Ag/Bi-2223 composites that were cooled slowly through the 800 to 600{degrees}C range

Luo, J. S.

Maroni, V. A.

Merchant, N.

UNT Digital Library

. Q. LEAD-RICH PHASES IN PARTIALLY PROCESSED Ag/Bi-2223 COMPOSITE CONDUCTORS* J.S. Luo, N. Merchant, V.A. hiaroni, and M. Hash Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 M. Rupich American Superconductor Corporation Two Technology Drive Westborough, MA 0 158 1 February 1996 Prepared for publication in the Proceedings of the Symposium on High-Temperature Superconductors: Synthesis, Processing, and Large Scale Applications, held at the TMS Annual Meeting in Anaheim, CA, February 4-8, 1996 The submitted manuscript has been authored by a contractor of the U.S. Government under contract No. W- 3 1-109-ENG-38. Accordingly, the U.S. Government retains nonexclusive, royalty-fiee license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. . * Work at Argonne National Laboratory was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy as part of a DOE program to develop electric power technology, under Contract W-3 I-109-ENG-38. T LEAD-RICH PHASES IN PARTIALLY PROCESSED Ag/Bi-2223 COMPOSITE CONDUCTORS* J.S. Luo', N. Merchant', V.A. Maronit, M. Hasht, andM. Rupich' 'Argonne National Laboratory 9700 South Cass Avenue Argonne, IL 60439 'American Superconductor Corporation Two Technology Drive Westborough, MA 01581 Abstract The occurrence, composition, and melting behavior of lead-rich phases detected during the thermal processing of Ao i -2223  composite conductors have been investigated by a combination of transmission electron microscopy (TEM), energy dispersive analysis (EDS), x- ray diffraction, differential thermal analysis, and Raman spectroscopy. A (Bi ,Pb)-Sr-Ca-Cu-0 composition with "41 12" metal atom stoichiometry and Pb-Bi was detected during TEM/EDS examinations of partially processedquenched Ag/Bi-2223 composites. An as-synthesized oxide mix with this stoichiometry was found to melt at a temperature <700"C. The lead-rich "3221" phase was found to have a wide range of compositional stability in terms of PbBi ratio and Bi + Pb stoichiometry. Evidence was seen for the co-existence of this phase with (Ca,Sr),PbO, in partially processed Ag/Bi-2223 composites that were cooled slowly through the 800 to 600°C range. 4 m a +- * Work at Argonne National Laboratory was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, as part of a DOE program to develop electric power technology, under Contract W-3 1- 109-ENG-38. r, 0 Introduction There is a considerable body of evidence('-'') suggesting that lead-rich phases play a significant role in the phase evolution and microstructure development that occurs during thermomechanical processing of ~ilver-sheathed/(Bi,Pb)~Sr,Ca&u~O, (AgBi-2223) composite conductors. Whereas the occurrence of plumbates, i.e., (Ca,Sr),PbO,, is well established, two other phases -- a lead-rich "liquid" phase''") and a lead-rich (Bi,Pb),Sr,Ca;?CuO,, (3221) phase(s.l 1-15) _ _  have been implicated but not thoroughly characterized. In a prior pape? we reported the observation by transmission electron microscopy (TEM) of a lead-rich amorphous phase in Ami-2223 composite specimens that were quenched during the early stages (I 1500 minutes) of heat treatment. Calibrated energy dispersive spectroscopy (EDS), done in conjunction with the TEiM measurements, was used to determine the approximate composition of this amorphous phase, but its melting temperature was not determined. We have now completed a more detailed study of this amorphous phase and, in addition, have investigated by x-ray diffraction (XRD) and Raman spectroscopy the lead-rich "3221" and plumbate phases that appear during processing of Ag/I3i-2223 composites. Experimental Procedures The procedures employed to fabricate and heat treat the Ami-2223 composite conductor examined in this study have been reported elsewhere.'2*1"18) The TEM/EDS measurements were performed using a Philips CM-30 (operated at 300 kV) equipped with a Noran EDS detector. The DTA scans were made with a Netzsch STA 409. XRD patterns were recorded on peeled specimens using a Philips XRG-3 100 with a Cu Ka source. Raman spectra were obtained with a Renishaw System 2000 instrument using 633 nm radiation from an He-Ne laser. The methodology used to prepare samples for the Raman measurements is described in a recently submitted paper by Wu et The "41 12" comDosition Results and Discussion In two previous we reported the observation of amorphous phases in Ag/Bi-2223 composite specimens that were heat treated at near-optimum conditions of temperature (-825 "C) and oxygen pressure (0.075 atm 0,) for periods ranging from 10 to 1500 minutes, then rapidly quenched to ambient temperature by dropping them into an oil bath. By quenching these samples in a partially processed state, we were able to lock in the phases present during the early to middle stages of the Bi-2223 formation reaction. It is in these stages that one expects to find the ubiquitous "liquid" phase whose demonstrable presence was highlighted in the work of Morgan et al.") Typical examples of the manifestations of this "liquid" phase in our prior work on partially processed AgBi-2223 composites are shown in Fig. 1 for a monofilament specimen that was heat treated at 825°C in 0.075 atm 0, for 1500 minutes prior to quenching."' At this stage, where the Bi-2223 formation reaction is at most 50% complete, there are numerous evidences in our TEM results for pockets of amorphous phase (as in Fig. la) and streaks of amorphous material (as in Fig. lb). We presumed at the time of this earlier work that the amorphous phase had been a liquid at the heat treatment temperature and that the streaks in Fig. 1 b were caused by wetting-induced flow of amorphous pockets like the one in Fig. la. An EDS examination of the amorphous patch in 4 cn * r c 0 a 2/ 1 . . .  Figure 1. Transniisjic:-. electron micrographs of an Ag/Bi-2233 composite heat treated at 525  'C i:i (S 111-5 atni 0- for 1500 minutes then quenched. Image (a) shows amorphous I 'liquid") pocket in rnultiphase grain cluster. Image (b) shows xiiorpho~:s 1 ::L;uid") streaks betneen Bi-3212 planes. w Fig. l a  (calibrated against an analyzed Bi-2212 specimen) gave a metal atom composition (normalized to Cu = 2.0) of Biz.,Pb,~,Sr,.,Ca,,cu2.0. (Hereafter we shall refer to this composition as "41 12".) Similar pockets with much the same metal atom stoichiometry were detected in other regions of the same specimen and in other partially reacted AgBi-2223 composite specimens as well. Therefore, it seemed appropriate to examine the melting characteristics of an oxide mix having this same stoichiometry. To explore this "4 1 12" composition further, a Bi,,2Pb,,6Sr,,3Ca,,o~U2~o~~ mix was prepared from an aqueous nitrate precursor solution by aerosol spray pyrolysis. Figure 2 shows the results of a DTA measurement on this as-synthesized "41 12" powder composition. A melting endotherm is observed starting near 650°C. Examination of the powder specimen after a similar DTA scan terminated at 750°C showed that it had fused together almost completely, indicating melting of nearly all of the sample. Repeated cycling between 500°C and 800°C in the DTA apparatus eventually caused the endotherm in Fig. 2 to broaden out and disappear. Also, the endotherm shown in Fig. 2 was the one obtained for the second cycle, meaning that the sample had already been to 800°C at least once. We believe this result constitutes convincing evidence that there are compositions in the lead-rich region of the Bi-Pb-Sr-Ca-Cu-0 phase field with melting points below 700°C. It is not tenuous to surmise from this result that a much broader range of liquidus compositions is possible at temperatures above 800 "C. Time (minutes) 4 v, . r C 0 ID r+ P, P . Figure 2. DTA curve for Bi2,2Pbl~6Srl ,3Ca, , ,~U2,0~~ powder mix after first cycling to 800°C then back to 500°C. The "322 1 'I Phase The numerous observations".' '-I5) of a lead-rich "322 1 'I phase, particularly prevalent in A@i- 2223 composites cooled slowly from typical heat treatment temperatures, deserves further exploration. Four sampIes of this phase with varying Pb/Bi ratios (2 to 8) and Bi + Pb stoichiometries (2.5 to 3 . 3 ,  prepared by other groups of investigators, were characterized by XRD and Raman spectroscopy. The four compositions were: (1) Bi,~~b,,oSr,,oCa2,0Cul,oOx (Bi + Pb = 3.0, PbBi = 2) (2) Bio.,~b2.0Sr,.1Ca,,,Cu,,0, (Bi + Pb = 2.5, PbBi = 4) (3) Bio~,Pb,,Sr,,oCa~.OCul,oOx (Bi + Pb = 2.9, PbBi = 6) (4) Bio.,Pb3,,Sr2.,Ca2.0Cul.~O~ (Bi + Pb = 3.5, PbBi = 8) Composition (1) was prepared at Los Alamos National Laboratory, composition (2) at American Superconductor Corporation, and compositions (3) and (4) at Argonne National Laboratory. XRD patterns of two of the four compositions are shown in Fig. 3. These two patterns are, for all practical purposes, indistinguishable from one another and from the pattern of the other two compositions (not shown). In essence, all four patterns look the same. Figure 4 shows Raman spectra of the same four compositions, and here again the differences are minor. From these XRD and Raman data we have concluded that the "3221" phase exists over a wide range of PbBi values (possibly wider than the 2 to 8 range of the four compositions studied here) and a sizable range of Pb + Bi values, i.e., "(2.5)221" to "(3.5)221." DTA measurements on these four compositions indicated a melting point 2830°C in all cases, which is well above the <70O0C value for the "41 12" composition, but not much above typical Ag/Bi- 2223 heat treatment temperatures. We have also observed the "322 1 I' phase in Ami-2223 composite specimens that were furnace cooled (-5"C/min) from heat treatment temperatures >800°C and in other specimens that were slowly heated (first cycle) from room temperature to temperatures in the 650 to 750°C range, then quenched. (Raman microspectroscopy was particularly helpful in making these observations.) We have deduced from these and other related studies that the conditions for prevalence of the "3221" phase are nominal temperatures of 650 to 750°C (possibly even to 600°C on the lower end and to 790°C on the upper end) and nominal oxygen pressures 10.08 atm. We have also noted that it is possible to misinterpret (Ca,Sr),PbO, for the "3221" phase (and vice versa) from XRD data, as will be discussed below. The Plumbate Phase The plumbate phase in calcined Bi-2223 precursor powders, usually described as being Ca2Pb0,, is most probably a partially strontium-substituted phase, which we shall refer to as (Ca,Sr),PbO,. It is almost always present in Bi-2223 precursor powders (even after calcination) and often persists during part of the Bi-2223 conversion process. Higher oxygen pressures (0.20 atm and above) tend to stabilize (Ca,Sr),PbO, relative to the lead-doped (orthorhombic) Bi-22 12 phase, quite possibly because the higher oxygen pressures tend to favor the Pb4+ state 4 En . z 0 (D ct I 3500 3000 2500 cn 3 E 2000 s 1500 1000 500 0. r. 10 20 30 40 50 60 2 -Theta 4 v) * . F 0 Figure 3. XRD patterns for two "3221" compositions as indicated in the figure. v) +-r C 3 0 0 u7 w e 3 0 0 # w C 3 0 0 v) +J e I 0 0 23000 I 3 0 0 0  1 5 U O O  t 4000 12000 IJ000 3000 50 00 40 00 2 0 0 0  0 30 00 0 25000 20000 1 so00 10000 suo0 0 J J U U U  30000 e m 0 0  2 0 0 0 0  15000 10000 5000 0 25000 20000 15000 10000 5000 ---Bi-322 1 ! 4 v1 . I 5 0 200 300 400 500 600 700 800 Wavenumbers Figure 4. Raman spectra of four "322 1 'I compositions as indicated in the figure. relative to the Pb2+ state. It is also interesting to note that a one mole plus one mole combination of the Bi-2201 phase and C+PbO, (1020) would produce a lead-rich "3221" of the type discussed above. Since Bi-220 1 and CqPbO, are commonly observed constituents of calcined Bi-2223 precursor powders, the possibility exists that reaction of Bi-2201 with CqPbO, is one of the pathways to "322 1 I' phase formation. The XRD results presented in Fig. 5a represent what we have interpreted as evidence for the co- existence of (Ca,Sr)2Pb0, and the "3221" phase in an AgBi-2223 composite that was heated to 700°C and quenched under conditions that should stabilize C%PbO,. The Cu Ka-based XRD line used by most investigators to establish the presence of C%PbO, is the one at a 28 value of 17.79". This same diffraction line is located at 16.84' in Sr2Pb0,, and Kitaguchi et a1.(21) have shown that there is a monotonic shift of this line between the two values as Sr substitutes for Ca. There is also a prominent "3221" line in this same 28 range at 17.87'. Since we know that the latter line is relatively insensitive to composition changes (e.g., Fig. 3), we interpret that the two lines in Fig. 5a are due to a slightly Sr-doped C+PbOi and "3221", which is consistent with the fact that the sample in Fig. 5a was processed to stabilized CqPbO, and the lead-free (tetragonal) form of Bi-2212 in accord with the method of Jeremie et al.(*) 550 500 450 - 400 a & 350 I a 5 300 250 20a v) c c. v) C - 150 100 2 I 1 I I 16.5 17 17.5 1 I 18 18.5 19 19.5 2-Theta Figure 5. XRD patterns for Bi-2223 precursor powders containing (a) tetragonal Bi-2212 and (b) orthorhombic Bi-22 12. (1) = Ca,PbO,, (2) = Sr2Pb0,, and (3) = "322 1 'I. 4 tn e e 9 0 (P R In contrast to Fig. 5a, the sample in Fig. 5b was processed to stabilize the lead-doped (orthorhombic) form of Bi-2212 and destabilize C%PbO,. In the case of Fig. 5b, only the "3221" phase appears to be present. These conclusions were confirmed by Raman spectroscopy, as will be reported in a subsequent publi~ation.('~) Some caution needs to be exercised in interpreting XRD data for Bi-2223 samples that spent any significant time in the 600 to 800°C temperature range prior to cooling to room temperature. Even 20 to 30 minutes in this range might be enough time to generate detectable quantities of "3221" and/or (Ca,Sr),PbO,, especially if some "liquid" phase was still present when cool down started. Acknowledgments The authors are grateful to T.G. Holesinger of Los Alamos National Laboratory and S.E. Doms of Argonne National Laboratory for providing three of the "3221" samples examined in this work, to B.S. Tani of Argonne National Laboratory for help with the XRD measurements, and to A.K. Fischer for assistance in the processing of the Raman data. References 1. P.E.D. Morgan, J.D. Pich6, and R.M. Housley, Physica C 191, 179 (1992). 2. 63,690 (1993). J.S. Luo, N. Merchant, V.A. Maroni, G.N. Riley, Jr., and W.L. Carter, Appl. Phys. Lett 3. E.E. Hellstrom, Mater. Res. SOC. Bull. 17(8), 45 (1992). 4. J.-C. Grivel, A. Jeremie, B. Hensel, and R. Fliikiger, Supercond. Sci. Technol. 6, 725 (1993); A. Jeremie, K. Alami-Yadri, J.-C. Grivel, and R. Fliikiger, Supercond. Sci. Technol 6, 730 (1993). 5. A. Jeremie, J.-C. Grivel, and R. Fliikiger, Physica C 235-240, 943 (1994). 6. P. Majewski, S. Kaesche, H.-L. Su, and F. Aldinger, Physica C 221,295 (1994). 7. S.E. Dorris, B.C. Prorok, M.T. Lanagan, N.B. Browning, M.R. Hagen, J.A. Parnell, Y. Feng, A. Umezawa, and D.C. Largalestier, Physica C 223, 163 (1994); S.E. Dorris, B.C. Proprok, M.T. Lanagan, S. Sinha, and R.B. Poeppel, Physica C 212,66 (1993). 8. S.S. Oh and K. Osamura, Supercond. Sci. Technol. 4,239 (1991). 9. S.R. Su, M. O'Conner, and P.G. Rossoni, Physica C 198,95 (1992). 10. M.G. Smith, D.S. Phillips, D.E. Peterson, and J.O. Willis, Physica C 224, 168 (1994). 11. S.X. Dou, H.K. Liu, Y.L. Zhang, and W.M. Blant, Supercond. 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The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ... . . .  

Lead-rich phases in partially processed Ag/Bi-2223 composite conductors

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Lead-rich phases in partially processed Ag/Bi-2223 composite conductors

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