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The grain alignment of Bi2223, Bi2212 and Bi2223 + Bi2212 phases in mechanical deformation and annealing processes
Physica C 279 ( 19971265-276
The grain alignment of Bi2223, Bi2212 and Bi2223 + Bi2212 phases in mechanical deformation and annealing processes S. Li ‘, Q.Y. Hu b, H.K. Liu b, S.X. Dou b and W. Gao av* ’ Department of Chemical and Materials Engineering The University of Auckland, Auckland, New Zealand b Centrefor Superconductingand Electronic Materials University of Wollongong, Wolkmgong. NSW 2522, Australia
Received 26 September 19%; revised 31 March 1!37
Abstract It is believed that the grain alignment of oxide superconductors plays an important role in improving the critical current density. Although mechanical deformation and thermal treatment are widely used to produce gram alignment in the BSCCO system, the alignment mechanisms have not been well understood. The present work studies the grain alignment behaviours of Bi2223, Bi2212 and Bi2223 + Bi2212 compounds by using X-ray diffraction. Results indicate that the effects of mechanical deformation on the grain alignment of Bi2212 and Bi2223 phases during the fabricating process of Bi2223 superconductors are quite different. Heavy deformation produces a high degree of grain alignment for Bi2212 in the early stage of Bi2223 fabrication but not for the Bi2223 phase ln the later processing stages. The subsequent annealing clearly improves the grain allgmnent for Bl2223. Bi2212 and Bi2223 + Bi2212. Based on these experhnental results, optimal mechanical-thermal processes were suggested for BSCCO superconductor fabrication. 0 1997 Elsevier Science B.V.
1. Introduction The powder-in-tube processes have been widely used to fabricate the high T, BSCCO superconduc-
tors for power applications [1,2]. The grain alignment of the superconducting oxides is believed to have a strong effect on the critical current density of the polycrystalline BSCCO superconductors [3,4,5,6,7]. It has been observed that the BSCCO/Ag superconducting materials develop a favourable grain alignment as a result of mechanical deformation [8],
’ Comspondmg author.
which has been most successfully used for producing grain aligned BSCCO/Ag wires and ribbons. Thermal treatments have also been associated with the processing of high T, superconductors. In BSCCO processing, controlled annealing has been used to obtain the desired phase, i.e. (Bi,Pb),Sr,-Ca,Cu,0 I0+X phase (Bi2223). and to heal the micro-cracks in superconductor grains which were caused by mechanical deformation [9]. In a previous paper [lo], we reported that the grain alignment and grain size of Bi2223 phase have been significantly affected by the recrystallisation annealing. Bi2223 and (Bi,Pb)zSr,CaCu,O,+X (Bi2212) are the main superconducting phases in the BSCCO
0921-4534/97/$17.00 Copyright Q 1997 Elsevier Science B.V. All rights reserved PII SO921-4534(97)00133-O
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system. They are used separately as well as in mixture to form superconductors in various forms. Phase transformation from Bi2212 to Bi2223 often takes place during thermal-mechanical treatments. Together with recrystallisation and growth, this phase transformation made the thermo-mechanical processes of BSCCO superconductors very complicated. Although there were reports on the effects of mechanical deformation and annealing on the texture degrees of BSCCO superconductors [ 11,121, the grain alignment behaviours of Bi2223 and Bi2212 phases have not been clearly distinguished. Samples containing mainly Bi2223 or Bi2212 phases have been treated similarly. On the whole, lack of understanding of the grain alignment mechanisms in the BSCCO system is one of the major hurdles standing in the way of improving microstructure and critical current density in high T, superconductors. The present paper reports a detailed study to compare the grain alignment behaviours of Bi2223, Bi2212 and Bi2223 + Bi22 12 phases through mechanical deformation and thermal treatment processes. In order to understand the grain alignment behaviours of Bi2212 and Bi2223 oxides in the real
C 279 (1997) 265-276
fabrication procedures and in the different processing stages, three processing routes were designed.
2. Experiments BSCCO/Ag specimens were fabricated using the established powder-in-tube (PIT) processes [ 131. The calcined BSCCO powders have a nominal cation ratio of Bi:Pb:Sr:Ca:Cu= 1.84:0.34:1.91: 2.04: 3.06 and contain mainly (Bi,Pb),Sr,CaCu,0 ,o+Y, Ca,PbO,, CuO and (Bi,Pb),Sr,CuOz phases, which were packed into a pure silver tube. The silver tubes were drawn into wires of 01 mm in external diameter. With the starting Bi2212 powder-in-tube wires, three sets of specimens have been prepared. Identical deformations and annealing conditions were applied for all the samples to avoid unwanted effects. The detailed processing methods are described as follows: (A) To establish a similar starting condition to the wires in other routes, the original Bi2212 wires were annealed at 800°C in air for 24 hours. The
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2 Theta Fig.
I.
X-ray spectra of powder specimen of Bi2212
+ Bi2223 mixture phases which was prepared with Route C.
S. Li er al./Physica
phase abundance after this annealing did not change because of the relatively low annealing temperature. X-ray diffraction of the powder specimens indicated that the main phase was Bi2212, plus small amounts of Ca,PbO, + CuO + (Bi,Pb),Sr,CuO,. The specimens were then subjected to rolling and further annealing. Bi2223 phase was formed after annealing at 833°C for 60 hours in air. This route simulates the early processing stage of BSCCO wires. Grain alignment degrees were measured before and after annealing. (B) In order to start with a pure Bi2223 phase, the original wires were annealed at 833°C for 60 hours in air and at 850°C for 60 hours in an atmosphere containing 10% 0, + 90% Ar. Powder X-ray diffraction indicated that this heat treatment produced a reasonably pure Bi2223 phase [lo]. The specimens were then subjected to rolling and annealing. Annealing was also performed at 833°C for 60 hours in air. No phase change was observed after this annealing. This simulates the final processing stage in the fabrication of Bi2223 tapes. Grain alignment was also measured before and after annealing. (C> The starting Bi2212 wires were annealed at 833°C in air for 60 hours. A mixture of Bi2212 + Bi2223 phases was developed in this process (Fig. 1). The percentage of Bi2223 phase in the phase mixture, XBizzz3, was defined by the equation [ 141:
are the integrated intensiwhere I~$‘~~ and Ziyt,2”2”{2 ties of X-ray diffraction (0010) of Bi2223 phase and (008) of Bi2212 phase, respectively. The obtained result indicated that there is - 60% Bi2223 phase formed after the heat treatment described above. The specimens were then subjected to rolling and annealing. Annealing at 833°C for 60 hours in air converted the rest of the Bi2212 phase to the Bi2223 phase. Recrystallisation also took place in the deformed Bi2223 phase. This route simulates the middle stages in the Bi2223 fabrication processes. Grain alignment was measured before and after annealing. The above described experimental routes has been summarised with the following flowchart.
267
C 279 (1997) 265-276 Route B
Route A
Route C
Mechanical deformation was conducted with multi-step rolling, which produced tapes with various thicknesses. The deformation ratio, R, is defined as the thickness reduction of the tape:
R = (fo-
r,>/t,
(2)
where t, and t, are the original and final thickness of the superconductor tapes, respectively. In order to study the surface grain alignment and microstructure, the silver sheaths on top of the superconductor tapes were removed by using a water solution containing 50 vol.%. of 32% NH, and 50 vol.% of 27% H,O, at room temperature. X-ray diffraction and SEM morphology observations after the removal of the silver sheaths indicated that there were no reaction products due to this etching treatment. The Lotgering method with X-ray diffraction was employed to measure the degree of c-axis orientation alignment in the polycrystalline BSCCO superconductors before and after annealing. The detailed method and Lotgering Factor calculation were described in the previous papers [10,12]. The Lotgering Factor, F, varies from 0 for an entirely nonoriented specimen to 1 for a completely oriented specimen.
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X-ray diffraction was performed in a Philips 1050 Diffractometer, using Co K, X-ray at 35 kV and 40 mA with a step of 0.01” and a scanning rate of O.O02”/second. These scanning parameters can provide accurate integral areas of diffraction peaks for calculating the Lotgering Factors.
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3. Results and discussion Table 1 summaries the measured Lotgering Factors together with the processing parameters. The grain alignment behaviours are descried below separately for the different routes:
Table 1 The processing parameters and measured results Route (A)
Route (B)
Route CC>
Preannealing
annealing at 800°C in air for 24 hours
Deformation ratio Lotgering Factors before annealing
24% 0.24 (Bi2212)
annealing at 833°C in air and 850°C in 10% 0, for 60 hours to form Bi2223 81% 22% 0.76 0.81 (Bi2223) (Bi2223)
annealing at 833°C in air for 60 hours to form Bi2223 + Bi2212 22% 81% 0.88 0.67 (mixture) (mixture)
Annealing Lotgering Factors after annealing
at 833°C in air for 60h 0.70 0.95 (Bi2223) (Bi2223)
at 833°C in air for 60h 0.92 0.82 (Bi2223) (Bi2223)
at 833°C in air for 60h 0.94 0.93 (Bi2223) (Bi2223)
80% 0.53 (Bi2212)
3.1. Grain alignment with mechanical deformation Bi2212 and phase Bi2223 (Route A)
transformation
from
Bi2212
of to
Experimental Route (A) conducted deformation on Bi2212 phase, followed by phase transformation annealing. The X-ray spectra of the Bi2212 tapes, after mechanical deformation with 24% and 80% thickness reduction and before annealing, are shown in Figs. 2 and 3. The Lotgering Factors of the Bi2212 specimens after 24% and 80% deformations were 0.24 and 0.53, respectively. It can be clearly seen that in the Bi2212 tape, the relative intensities of (001) peaks in the specimen with 80% deformation are much stronger, and the intensities of fhkl) peaks including (113), (1151, (1 l7), (200), (2010) and (1115) are much weaker compared with ze specimenof 24% reduction. Figs. 4 and 5 show the X-ray spectra after annealing and Bi2223 phase formation. The (hkl) peaks including (015), (019), (I lo), (Olll), (0119), (1118) and (0121) from Bi2223 which existed in the specimen with 24% deformation disap-
peared in the specimen with 80% deformation. It is obvious that the high deformation ratio promoted the (001) grain alignment in the Bi2212 specimens treated with Route (A). Fig. 6 plots the Lotgering Factors as a function of mechanical deformation for Route (A). The results indicate that: (1) The grain alignment extent on the surface of the Bi2212 tapes (before annealing) increases with increasing reduction ratio. (2) The grain alignment on the surface of Bi2223 (after annealing) also increases with increasing reduction, although the increase is saturated when the reduction ratio becomes high (70%-90%). (3) The phase transformation (from Bi2212 to Bi2223) improves the grain alignment significantly. Lotgering Factor of Bi2223 reached - 0.95 with 80% mechanical deformation after phase transformation sintering. Further increase of deformation to 90% did not produce any better alignment, probably because the grain alignment may be damaged by serious “sausage” formed with the heavy mechanical deformation. (4) The Bi2212 phase with better grain alignment leads to a higher degree
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C 279 (1997) 265-276
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2 Theta Fig. 2. X-ray spectra of Bi2212
tape prepared with Route A after 24% mechanical deformation before annealing.
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2 Theta Fig. 3. X-ray spectra of Bi2212 tape prepared with Route A after 80% mechanical deformation before annealing
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s
$
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+
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2 Theta Fig. 4. X-ray spectra of Bi2212 tape prepared with Route A with 24% mechanical
a 0
deformation
after annealing.
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2 Theta Fig. 5. X-ray spectra of Bi2212 tape prepared with Route A with 80% mechanical
deformation
after annealing.
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C 279 (1997) 265-276
1 0.9 --
0.8 -_
t&
0.7 --
Lotgering Factors of 812223 phase of
- .& L Lotgering Factors of 812212phase of
i5
asdeformed tape
IT
..A---..
:’
-‘._A
I
0.2
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1 100
Mechanical Deformation Ratio, % Fig. 6. Lotgering
Factors versus mechanical
deformation
for the specimens
prepared with Route A.
1
. 0.9 L-
--__ ----._ --__
0.8
??
-c__
--__ ---+-_-___+__.--~
5
0.7
z d g
0.6
'E B 3
0.5
- + - Lotgering Factors of Bi2223 Before Recrystallisation Annealing
04
-
Lotgering Factors of Bi2223 After Rectystallisation Annealing
0.31 --
0.2
10
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Mechanical Deformation Ratio, % Fig. 7. Lotgering
Factors versus mechanical
deformation
for Bi2223 tapes prepared with Route B.
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S. Li et al. / Physica C 279 (1997) 265-276
Fig. 8. SEM micrographs showing the surface morphology of Bi2223 wire before mechanical deformation. The Ag sheath was removed by etching.
of grain alignment in Bi2223 after annealing, revealing a relationship between the microstructures before and after phase transformation. 3.2. Grain alignment with mechanical deformation of Bi2223 and recrystallisation phase (Route B)
annealing
of Bi2223
This route started with the fully annealed superconducting wire which contains almost pure Bi2223 phase. Surface grain alignment has already existed
due to the phase transformation annealing. The mechanical deformation and subsequent annealing were all performed with Bi2223 phase so that no phase transformation was involved. In our previous work [lo], the X-ray spectra of Bi2223 tapes after 22% and 81% deformation before annealing show that the relative intensity of some (h/d) peaks such as (019), (011 l), (1114) and (0119) increased with increasing mechanicaldeformation,and some other (hkl) peaks including (015). (116), (1118) and (0121) appeared. Microstructural evidences indicated thatheavy mechanical deformation damages the (001) grain alignment in Bi2223 by breaking and rotating the plate-like grains away from the COOL) orientations [lo]. X-ray spectra of Bi2223 tapes with 22% and 81% deformation after show that some (hkl) peaks, e.g. (1118) peak, in the Bi2223 tape with 81% deformationdisappeared after annealing. This indicates that the recrystallisation annealing rotated or coalesced the crystals from other orientations back to the (001) orientation [lo]. In order to compare the effects of annealings on grain alignment in Routes (A) and (B), the Lotgering Factors of the Bi2223 tapes versus the mechanical deformation ratio before and after annealing were plotted in Fig. 7 with the same scale in Fig. 6. ‘Ihe results from Route B showed two clear
louo ..
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s
aoo.-
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2 Theta
Fig. 9. X-ray spectra of Bi2223 + Bi2212 tape (Route C). with 22% mechanical deformation before annealing.
S. Li et al./ Physica C 279 (1997) 265-276
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I
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M
~~1~~~ 55
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2ThMa
Fig. 10. X-ray spectra of Bi2223 + Bi2212 tape (Route C) with 91% mechanical
deformation
before annealing.
3500
I
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2%
c”l
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L
5wg 0
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/j 30
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zlmta
Fig. I I. X-ray spectra of Bi2223 + Bi2212 tape (Route C> with 22% mechanical
deformation
after annealing.
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S. Li et al. / Physica C 279 (1997) 265-276
274
tendencies: (1) Unlike Bi2212 shown in Fig. 6, the Lotgering Factors decrease with the increasing mechanical deformation for Bi2223 phase. (2) Similar to Bi2212 phase, the Lotgering Factors of Bi2223 phase after annealing are higher than before annealing. The maximum increase of Lotgering Factors after annealing is - 0.20 with - 50% mechanical deformation. The Lotgering Factors on the surface of the Bi2223 tapes decrease with increasing mechanical deformation, an observation seems against general thought that higher deformation should produce better grain alignment. It can be attributed to that the surface of the Bi2223 wire already had a good grain alignment resulted from the previous wiredrawing and phase transformation annealing (Fig. 8). Therefore, the Route B simulates the late stages of BSCCO fabricating processes where deformation and annealing are performed to the textured Bi2223 microstructures. 3.3. Grain alignment and annealing (Route C)
with mechanical deformation of Bi2212 + Bi2223 phase mixture
As described before, a phase mixture of - 60%Bi2223 + 40%Bi2212 was formed through
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annealing at 833°C for 60 hours. Deformation and annealing were performed with this phase mixture. The different grain alignment behaviours of the Bi2212 and Bi2223 phases in the same specimen can be compared, and the total effects of the Bi2212 and Bi2223 alignment can also be investigated. X-ray spectra after 21% and 91% mechanical deformations are shown in Figs. 9 and 10, respectively. With 2 50% deformation, the (O06)8i22,2 diffraction became visible and the relative intensities of (008)ai2212 and fO012)Bi2212increased. At the same time, the relativeintensities of (OO1JBi2223 diffractions such as (0010)Bi2223 and (O014)8i2223 decreased with the increasing deformations, as shown in Fig. 10. The intensity of the peak at 28 = 33.4” was contributed by (00g)BiZZ23 + f00]O)Bi2212 bebecame much stronger than cause the pool&,,,, before. Figs. 11 and 12 are the X-ray spectra of the Bi2212 + Bi2223 phase mixture with 21% and 91% deformations after annealing. It can be seen that the (006)ai22,2 and (hkl) B,2212peaks disappeared and the intensities of (008)BiZZ,2 and (O012)ai22,, were reduced after annealing. The intens& of (001),,2,,, increased substantially due to both phase transformation from Bi2212 to Bi2223 and grain alignment (of
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ZTheta
Fig. 12. X-ray spectra of Bi2223 + Bi2212 tape (Route C) with 91% mechanical
deformation
after annealing.
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S. Li et al./Phyysica C 279 (1997) 265-276
Bi2223). However, the specimens with 21% deformation (Fig. 11) showed an overall better grain alignment of Bi2223 than the specimens with 91% deformation (Fig. 12). Lotgering Factors of Bi2212 + Bi2223 phases (before annealing) were calculated separately, together with the Lotgering Factors of Bi2223 after annealing, plotted in Fig. 13. The results confirm the above discussion: (1) The effect of mechanical deformation on the grain alignment for the Bi2223 + Bi2212 mixture follows two ways. The Lotgering Factor of Bi2212 phase increases with the increasing mechanical deformation, while the Lotgering Factor of Bi2223 decreases with the deformation. This result agrees with the observations obtained from Routes (A) and (B), where the Bi2212 and Bi2223 phases showed different effects with increasing deformation. (2) The grain alignment (of Bi2223) after annealing was affected by the combination of the effects of Bi2212 and Bi2223 phases. The Lotgering Factor has less variations over a wide range of reduction ratio because the effect from Bi2212 offsets the effect from Bi2223. (3) The annealing process also improves the grain alignment significantly, especially around 6040% reduction, where the an-
275
nealing has the strongest effect on the grain alignment. The annealing-grain alignment processes in the different routes described above represent different mechanism. In Route (A), annealing of the deformed Bi2212 results in phase transformation (from Bi2212 to Bi2223). The grain alignment was greatly enhanced by this phase transformation process. In Route (B), there was no phase transformation during annealing. The deformed Bi2223 underwent recrystallisation and grain growth processes. Therefore, the effect of annealing on the grain alignment is not as strong as in Route (A). We also see the damaging effects to the grain alignment by heavy deformation after annealing. The annealing in Route (C) is a mixed process of phase transformation and recrystallisation/grain growth. Like in Route (A), the grain alignment is strongly promoted by the annealing process, indicating that the partial phase transformation can also enhance the grain alignment significantly. Summarising the above results and discussion, we see annealing always improves the grain alignment in BSCCO superconductors. This effect becomes stronger when Bi2212 to Bi2223 phase transforma-
0.9
0.8
&____-“_..-
04
Lotgering Factors of IX223 annealing
.._..
I.
Afler
- * - Lotgering Factors of Bi2223 Before Annealing - .L - Lotgering Factors of 812212 Before Annealing
03
0.2 10
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90
Mechanical Deformation Ratio, % Fig.
13.Lotgering
Factors versus mechanical deformation for the specimens prepared with Route C
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S. Li et al./Physica
tion takes place during the annealing. The mechanical deformation, however, has different effects on the grain alignment of Bi2212 and Bi2223 phases. In order to obtain good grain alignment, Bi2212 is in favour of high deformation ratio, while Bi2223 is in favour of low deformation ratio. Based on the above discussion, the processing principles for obtaining a high degree of grain aligned Bi2223 phase can be suggested as below: (1) Heavy mechanical deformation should be performed on the Bi2212 tapes at the early stage. (2) Light deformation should be conducted at the late stages when Bi2223 phase has already been formed. (3) Short annealings after deformation should be frequently used throughout the whole processes, and (4) Whenever possible, take the advantage of Bi2212 + Bi2223 phase transformation to align the grains more efficiently.
Conclusions The grain alignment behaviour of BSCCO/Ag superconductors in mechanical deformation and subsequent annealing processes have been studied. The degree of grain alignment in Bi2212 phase increases with the increasing mechanical deformation, while the degree of grain alignment in Bi2223 phase decrease with the increasing extent of mechanical deformation. The subsequent annealing improves the grain alignment of Bi2212, Bi2223 or Bi2212 + Bi2223 mixture specimens significantly. The grain alignment process can be further enhanced if the Bi2212 to Bi2223 phase transformation occurs during the annealing process. This understanding can be used to design and to optimise the mechanical-ther-
C 279 (1997) 265-276
mal processes for the BSCCO superconductor fabrication.
Acknowledgments This work is partially supported by an Auckland University Postgraduate Research Grant. Two of the authors (WG and SL) would like to thank the technical staff in the Chemical and Materials Engineering Department and Surface and Materials Research Centre for various assistance.
References t11 R. Hawsey and J. Daley, JOM August (1995) 56.
121U. Balachandran, A.N. Iyer, R. Jammy, P. Haldar and M. Suenaga, Cryogenic Engineering 42 (1995). [31 Q.Y. Hu, H.W. Weber, H.K. Liu, S.X. Dou and H.W. Neumiller, Physica C 252 ( 1995) 211. [41 Q.Y. Hu, R.M. Schalk, H.W. Weber, H.K. Liu, R.K. Wang, C. Czurda and S.X. Dou, J. Appl. Phys. 78 (1995) 1123. [51 B. Hensel, G. Grasso and R. Fliikiger; Physical Review B 51, No. 21 (1995) 15456. [61 L.N. Bulaevskii, L.L. Daemen, M.P. Maley and J.Y. Coulter, Physical Review B 48, No. 18, (1993) 13798. [71 W. Gao and J.B. Vander Sande, Physica C 181 (1991) 105. Is1 A. Perin, G. Grasso, M. Daumling, B. Hensel, E. Walker and R. Flttkiger, Physica C 216 (1993) 339. [91 Q. Li, K. Brodersen, H.A. Hjuler and T. Freltoft, Physica C 217 (1993) 360. [lOI S. Li, M. BredehBft, Q.Y. Hu, H.K. Liu, S.X. Dou and W. Gao, Physica C275 (1997). [Ill J.M. Yoo, K. Mukherjee, Physica C 222 (1994) 241. 1121 W. Gao and J.B. Vander Sande; Supercon. Sci. Technol. 5, (1992) 318. 1131 S.X. Dou and H.K. Liu, Advances in High-T, Superconductors; Materials Science Forum vol. 137- 139 (1993) 65 1. 1141 Q.Y. Hu, H.K. Liu, S.X. Dou, Physica C 250 (1995) 7.
The grain alignment of Bi2223, Bi2212 and Bi2223 + Bi2212 phases in mechanical deformation and annealing processes S. Li ‘, Q.Y. Hu b, H.K. Liu b, S.X. Dou b and W. Gao av* ’ Department of Chemical and Materials Engineering The University of Auckland, Auckland, New Zealand b Centrefor Superconductingand Electronic Materials University of Wollongong, Wolkmgong. NSW 2522, Australia
Received 26 September 19%; revised 31 March 1!37
Abstract It is believed that the grain alignment of oxide superconductors plays an important role in improving the critical current density. Although mechanical deformation and thermal treatment are widely used to produce gram alignment in the BSCCO system, the alignment mechanisms have not been well understood. The present work studies the grain alignment behaviours of Bi2223, Bi2212 and Bi2223 + Bi2212 compounds by using X-ray diffraction. Results indicate that the effects of mechanical deformation on the grain alignment of Bi2212 and Bi2223 phases during the fabricating process of Bi2223 superconductors are quite different. Heavy deformation produces a high degree of grain alignment for Bi2212 in the early stage of Bi2223 fabrication but not for the Bi2223 phase ln the later processing stages. The subsequent annealing clearly improves the grain allgmnent for Bl2223. Bi2212 and Bi2223 + Bi2212. Based on these experhnental results, optimal mechanical-thermal processes were suggested for BSCCO superconductor fabrication. 0 1997 Elsevier Science B.V.
1. Introduction The powder-in-tube processes have been widely used to fabricate the high T, BSCCO superconduc-
tors for power applications [1,2]. The grain alignment of the superconducting oxides is believed to have a strong effect on the critical current density of the polycrystalline BSCCO superconductors [3,4,5,6,7]. It has been observed that the BSCCO/Ag superconducting materials develop a favourable grain alignment as a result of mechanical deformation [8],
’ Comspondmg author.
which has been most successfully used for producing grain aligned BSCCO/Ag wires and ribbons. Thermal treatments have also been associated with the processing of high T, superconductors. In BSCCO processing, controlled annealing has been used to obtain the desired phase, i.e. (Bi,Pb),Sr,-Ca,Cu,0 I0+X phase (Bi2223). and to heal the micro-cracks in superconductor grains which were caused by mechanical deformation [9]. In a previous paper [lo], we reported that the grain alignment and grain size of Bi2223 phase have been significantly affected by the recrystallisation annealing. Bi2223 and (Bi,Pb)zSr,CaCu,O,+X (Bi2212) are the main superconducting phases in the BSCCO
0921-4534/97/$17.00 Copyright Q 1997 Elsevier Science B.V. All rights reserved PII SO921-4534(97)00133-O
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system. They are used separately as well as in mixture to form superconductors in various forms. Phase transformation from Bi2212 to Bi2223 often takes place during thermal-mechanical treatments. Together with recrystallisation and growth, this phase transformation made the thermo-mechanical processes of BSCCO superconductors very complicated. Although there were reports on the effects of mechanical deformation and annealing on the texture degrees of BSCCO superconductors [ 11,121, the grain alignment behaviours of Bi2223 and Bi2212 phases have not been clearly distinguished. Samples containing mainly Bi2223 or Bi2212 phases have been treated similarly. On the whole, lack of understanding of the grain alignment mechanisms in the BSCCO system is one of the major hurdles standing in the way of improving microstructure and critical current density in high T, superconductors. The present paper reports a detailed study to compare the grain alignment behaviours of Bi2223, Bi2212 and Bi2223 + Bi22 12 phases through mechanical deformation and thermal treatment processes. In order to understand the grain alignment behaviours of Bi2212 and Bi2223 oxides in the real
C 279 (1997) 265-276
fabrication procedures and in the different processing stages, three processing routes were designed.
2. Experiments BSCCO/Ag specimens were fabricated using the established powder-in-tube (PIT) processes [ 131. The calcined BSCCO powders have a nominal cation ratio of Bi:Pb:Sr:Ca:Cu= 1.84:0.34:1.91: 2.04: 3.06 and contain mainly (Bi,Pb),Sr,CaCu,0 ,o+Y, Ca,PbO,, CuO and (Bi,Pb),Sr,CuOz phases, which were packed into a pure silver tube. The silver tubes were drawn into wires of 01 mm in external diameter. With the starting Bi2212 powder-in-tube wires, three sets of specimens have been prepared. Identical deformations and annealing conditions were applied for all the samples to avoid unwanted effects. The detailed processing methods are described as follows: (A) To establish a similar starting condition to the wires in other routes, the original Bi2212 wires were annealed at 800°C in air for 24 hours. The
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2 Theta Fig.
I.
X-ray spectra of powder specimen of Bi2212
+ Bi2223 mixture phases which was prepared with Route C.
S. Li er al./Physica
phase abundance after this annealing did not change because of the relatively low annealing temperature. X-ray diffraction of the powder specimens indicated that the main phase was Bi2212, plus small amounts of Ca,PbO, + CuO + (Bi,Pb),Sr,CuO,. The specimens were then subjected to rolling and further annealing. Bi2223 phase was formed after annealing at 833°C for 60 hours in air. This route simulates the early processing stage of BSCCO wires. Grain alignment degrees were measured before and after annealing. (B) In order to start with a pure Bi2223 phase, the original wires were annealed at 833°C for 60 hours in air and at 850°C for 60 hours in an atmosphere containing 10% 0, + 90% Ar. Powder X-ray diffraction indicated that this heat treatment produced a reasonably pure Bi2223 phase [lo]. The specimens were then subjected to rolling and annealing. Annealing was also performed at 833°C for 60 hours in air. No phase change was observed after this annealing. This simulates the final processing stage in the fabrication of Bi2223 tapes. Grain alignment was also measured before and after annealing. (C> The starting Bi2212 wires were annealed at 833°C in air for 60 hours. A mixture of Bi2212 + Bi2223 phases was developed in this process (Fig. 1). The percentage of Bi2223 phase in the phase mixture, XBizzz3, was defined by the equation [ 141:
are the integrated intensiwhere I~$‘~~ and Ziyt,2”2”{2 ties of X-ray diffraction (0010) of Bi2223 phase and (008) of Bi2212 phase, respectively. The obtained result indicated that there is - 60% Bi2223 phase formed after the heat treatment described above. The specimens were then subjected to rolling and annealing. Annealing at 833°C for 60 hours in air converted the rest of the Bi2212 phase to the Bi2223 phase. Recrystallisation also took place in the deformed Bi2223 phase. This route simulates the middle stages in the Bi2223 fabrication processes. Grain alignment was measured before and after annealing. The above described experimental routes has been summarised with the following flowchart.
267
C 279 (1997) 265-276 Route B
Route A
Route C
Mechanical deformation was conducted with multi-step rolling, which produced tapes with various thicknesses. The deformation ratio, R, is defined as the thickness reduction of the tape:
R = (fo-
r,>/t,
(2)
where t, and t, are the original and final thickness of the superconductor tapes, respectively. In order to study the surface grain alignment and microstructure, the silver sheaths on top of the superconductor tapes were removed by using a water solution containing 50 vol.%. of 32% NH, and 50 vol.% of 27% H,O, at room temperature. X-ray diffraction and SEM morphology observations after the removal of the silver sheaths indicated that there were no reaction products due to this etching treatment. The Lotgering method with X-ray diffraction was employed to measure the degree of c-axis orientation alignment in the polycrystalline BSCCO superconductors before and after annealing. The detailed method and Lotgering Factor calculation were described in the previous papers [10,12]. The Lotgering Factor, F, varies from 0 for an entirely nonoriented specimen to 1 for a completely oriented specimen.
S. Li et al./Physica
268
X-ray diffraction was performed in a Philips 1050 Diffractometer, using Co K, X-ray at 35 kV and 40 mA with a step of 0.01” and a scanning rate of O.O02”/second. These scanning parameters can provide accurate integral areas of diffraction peaks for calculating the Lotgering Factors.
C 279 (1997) 265-276
3. Results and discussion Table 1 summaries the measured Lotgering Factors together with the processing parameters. The grain alignment behaviours are descried below separately for the different routes:
Table 1 The processing parameters and measured results Route (A)
Route (B)
Route CC>
Preannealing
annealing at 800°C in air for 24 hours
Deformation ratio Lotgering Factors before annealing
24% 0.24 (Bi2212)
annealing at 833°C in air and 850°C in 10% 0, for 60 hours to form Bi2223 81% 22% 0.76 0.81 (Bi2223) (Bi2223)
annealing at 833°C in air for 60 hours to form Bi2223 + Bi2212 22% 81% 0.88 0.67 (mixture) (mixture)
Annealing Lotgering Factors after annealing
at 833°C in air for 60h 0.70 0.95 (Bi2223) (Bi2223)
at 833°C in air for 60h 0.92 0.82 (Bi2223) (Bi2223)
at 833°C in air for 60h 0.94 0.93 (Bi2223) (Bi2223)
80% 0.53 (Bi2212)
3.1. Grain alignment with mechanical deformation Bi2212 and phase Bi2223 (Route A)
transformation
from
Bi2212
of to
Experimental Route (A) conducted deformation on Bi2212 phase, followed by phase transformation annealing. The X-ray spectra of the Bi2212 tapes, after mechanical deformation with 24% and 80% thickness reduction and before annealing, are shown in Figs. 2 and 3. The Lotgering Factors of the Bi2212 specimens after 24% and 80% deformations were 0.24 and 0.53, respectively. It can be clearly seen that in the Bi2212 tape, the relative intensities of (001) peaks in the specimen with 80% deformation are much stronger, and the intensities of fhkl) peaks including (113), (1151, (1 l7), (200), (2010) and (1115) are much weaker compared with ze specimenof 24% reduction. Figs. 4 and 5 show the X-ray spectra after annealing and Bi2223 phase formation. The (hkl) peaks including (015), (019), (I lo), (Olll), (0119), (1118) and (0121) from Bi2223 which existed in the specimen with 24% deformation disap-
peared in the specimen with 80% deformation. It is obvious that the high deformation ratio promoted the (001) grain alignment in the Bi2212 specimens treated with Route (A). Fig. 6 plots the Lotgering Factors as a function of mechanical deformation for Route (A). The results indicate that: (1) The grain alignment extent on the surface of the Bi2212 tapes (before annealing) increases with increasing reduction ratio. (2) The grain alignment on the surface of Bi2223 (after annealing) also increases with increasing reduction, although the increase is saturated when the reduction ratio becomes high (70%-90%). (3) The phase transformation (from Bi2212 to Bi2223) improves the grain alignment significantly. Lotgering Factor of Bi2223 reached - 0.95 with 80% mechanical deformation after phase transformation sintering. Further increase of deformation to 90% did not produce any better alignment, probably because the grain alignment may be damaged by serious “sausage” formed with the heavy mechanical deformation. (4) The Bi2212 phase with better grain alignment leads to a higher degree
S. Li et al./Physica
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C 279 (1997) 265-276
200
50
4 20
25
30
35
40
45
50
,
55
60
65
70
2 Theta Fig. 2. X-ray spectra of Bi2212
tape prepared with Route A after 24% mechanical deformation before annealing.
300
250 Y Ei 200 8 150
100
50
0 20
25
30
35
40
45
50
55
60
65
70
2 Theta Fig. 3. X-ray spectra of Bi2212 tape prepared with Route A after 80% mechanical deformation before annealing
S. Li er al./Physica
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C 279 (1997) 265-276
s
$
1200
1000
600
s
600
”
400
B
s
L
200
g
N
Gi
+
0 20
25
30
35
40
45
50
55
60
65
70
2 Theta Fig. 4. X-ray spectra of Bi2212 tape prepared with Route A with 24% mechanical
a 0
deformation
after annealing.
6000
w 20
25
30
35
40
45
50
55
60
65
2 Theta Fig. 5. X-ray spectra of Bi2212 tape prepared with Route A with 80% mechanical
deformation
after annealing.
70
S. Li et al./Physica
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C 279 (1997) 265-276
1 0.9 --
0.8 -_
t&
0.7 --
Lotgering Factors of 812223 phase of
- .& L Lotgering Factors of 812212phase of
i5
asdeformed tape
IT
..A---..
:’
-‘._A
I
0.2
4 10
20
40
30
50
60
70
80
90
1 100
Mechanical Deformation Ratio, % Fig. 6. Lotgering
Factors versus mechanical
deformation
for the specimens
prepared with Route A.
1
. 0.9 L-
--__ ----._ --__
0.8
??
-c__
--__ ---+-_-___+__.--~
5
0.7
z d g
0.6
'E B 3
0.5
- + - Lotgering Factors of Bi2223 Before Recrystallisation Annealing
04
-
Lotgering Factors of Bi2223 After Rectystallisation Annealing
0.31 --
0.2
10
20
30
40
50
60
70
80
90
Mechanical Deformation Ratio, % Fig. 7. Lotgering
Factors versus mechanical
deformation
for Bi2223 tapes prepared with Route B.
100
S. Li et al. / Physica C 279 (1997) 265-276
Fig. 8. SEM micrographs showing the surface morphology of Bi2223 wire before mechanical deformation. The Ag sheath was removed by etching.
of grain alignment in Bi2223 after annealing, revealing a relationship between the microstructures before and after phase transformation. 3.2. Grain alignment with mechanical deformation of Bi2223 and recrystallisation phase (Route B)
annealing
of Bi2223
This route started with the fully annealed superconducting wire which contains almost pure Bi2223 phase. Surface grain alignment has already existed
due to the phase transformation annealing. The mechanical deformation and subsequent annealing were all performed with Bi2223 phase so that no phase transformation was involved. In our previous work [lo], the X-ray spectra of Bi2223 tapes after 22% and 81% deformation before annealing show that the relative intensity of some (h/d) peaks such as (019), (011 l), (1114) and (0119) increased with increasing mechanicaldeformation,and some other (hkl) peaks including (015). (116), (1118) and (0121) appeared. Microstructural evidences indicated thatheavy mechanical deformation damages the (001) grain alignment in Bi2223 by breaking and rotating the plate-like grains away from the COOL) orientations [lo]. X-ray spectra of Bi2223 tapes with 22% and 81% deformation after show that some (hkl) peaks, e.g. (1118) peak, in the Bi2223 tape with 81% deformationdisappeared after annealing. This indicates that the recrystallisation annealing rotated or coalesced the crystals from other orientations back to the (001) orientation [lo]. In order to compare the effects of annealings on grain alignment in Routes (A) and (B), the Lotgering Factors of the Bi2223 tapes versus the mechanical deformation ratio before and after annealing were plotted in Fig. 7 with the same scale in Fig. 6. ‘Ihe results from Route B showed two clear
louo ..
800 -.
E
s
aoo.-
8
400 --
20
25
30
35
40
45
50
55
60
05
70
2 Theta
Fig. 9. X-ray spectra of Bi2223 + Bi2212 tape (Route C). with 22% mechanical deformation before annealing.
S. Li et al./ Physica C 279 (1997) 265-276
273
I
”
20
25
30
35
40
45
M
~~1~~~ 55
50
I~ 65
70
2ThMa
Fig. 10. X-ray spectra of Bi2223 + Bi2212 tape (Route C) with 91% mechanical
deformation
before annealing.
3500
I
3ow
2%
c”l
2000
&
s s s 1500
1000
t
L
5wg 0
20
25
/j 30
35
40
45
50
55
80
65
zlmta
Fig. I I. X-ray spectra of Bi2223 + Bi2212 tape (Route C> with 22% mechanical
deformation
after annealing.
70
S. Li et al. / Physica C 279 (1997) 265-276
274
tendencies: (1) Unlike Bi2212 shown in Fig. 6, the Lotgering Factors decrease with the increasing mechanical deformation for Bi2223 phase. (2) Similar to Bi2212 phase, the Lotgering Factors of Bi2223 phase after annealing are higher than before annealing. The maximum increase of Lotgering Factors after annealing is - 0.20 with - 50% mechanical deformation. The Lotgering Factors on the surface of the Bi2223 tapes decrease with increasing mechanical deformation, an observation seems against general thought that higher deformation should produce better grain alignment. It can be attributed to that the surface of the Bi2223 wire already had a good grain alignment resulted from the previous wiredrawing and phase transformation annealing (Fig. 8). Therefore, the Route B simulates the late stages of BSCCO fabricating processes where deformation and annealing are performed to the textured Bi2223 microstructures. 3.3. Grain alignment and annealing (Route C)
with mechanical deformation of Bi2212 + Bi2223 phase mixture
As described before, a phase mixture of - 60%Bi2223 + 40%Bi2212 was formed through
20
25
30
35
40
annealing at 833°C for 60 hours. Deformation and annealing were performed with this phase mixture. The different grain alignment behaviours of the Bi2212 and Bi2223 phases in the same specimen can be compared, and the total effects of the Bi2212 and Bi2223 alignment can also be investigated. X-ray spectra after 21% and 91% mechanical deformations are shown in Figs. 9 and 10, respectively. With 2 50% deformation, the (O06)8i22,2 diffraction became visible and the relative intensities of (008)ai2212 and fO012)Bi2212increased. At the same time, the relativeintensities of (OO1JBi2223 diffractions such as (0010)Bi2223 and (O014)8i2223 decreased with the increasing deformations, as shown in Fig. 10. The intensity of the peak at 28 = 33.4” was contributed by (00g)BiZZ23 + f00]O)Bi2212 bebecame much stronger than cause the pool&,,,, before. Figs. 11 and 12 are the X-ray spectra of the Bi2212 + Bi2223 phase mixture with 21% and 91% deformations after annealing. It can be seen that the (006)ai22,2 and (hkl) B,2212peaks disappeared and the intensities of (008)BiZZ,2 and (O012)ai22,, were reduced after annealing. The intens& of (001),,2,,, increased substantially due to both phase transformation from Bi2212 to Bi2223 and grain alignment (of
45
50
55
60
65
ZTheta
Fig. 12. X-ray spectra of Bi2223 + Bi2212 tape (Route C) with 91% mechanical
deformation
after annealing.
70
S. Li et al./Phyysica C 279 (1997) 265-276
Bi2223). However, the specimens with 21% deformation (Fig. 11) showed an overall better grain alignment of Bi2223 than the specimens with 91% deformation (Fig. 12). Lotgering Factors of Bi2212 + Bi2223 phases (before annealing) were calculated separately, together with the Lotgering Factors of Bi2223 after annealing, plotted in Fig. 13. The results confirm the above discussion: (1) The effect of mechanical deformation on the grain alignment for the Bi2223 + Bi2212 mixture follows two ways. The Lotgering Factor of Bi2212 phase increases with the increasing mechanical deformation, while the Lotgering Factor of Bi2223 decreases with the deformation. This result agrees with the observations obtained from Routes (A) and (B), where the Bi2212 and Bi2223 phases showed different effects with increasing deformation. (2) The grain alignment (of Bi2223) after annealing was affected by the combination of the effects of Bi2212 and Bi2223 phases. The Lotgering Factor has less variations over a wide range of reduction ratio because the effect from Bi2212 offsets the effect from Bi2223. (3) The annealing process also improves the grain alignment significantly, especially around 6040% reduction, where the an-
275
nealing has the strongest effect on the grain alignment. The annealing-grain alignment processes in the different routes described above represent different mechanism. In Route (A), annealing of the deformed Bi2212 results in phase transformation (from Bi2212 to Bi2223). The grain alignment was greatly enhanced by this phase transformation process. In Route (B), there was no phase transformation during annealing. The deformed Bi2223 underwent recrystallisation and grain growth processes. Therefore, the effect of annealing on the grain alignment is not as strong as in Route (A). We also see the damaging effects to the grain alignment by heavy deformation after annealing. The annealing in Route (C) is a mixed process of phase transformation and recrystallisation/grain growth. Like in Route (A), the grain alignment is strongly promoted by the annealing process, indicating that the partial phase transformation can also enhance the grain alignment significantly. Summarising the above results and discussion, we see annealing always improves the grain alignment in BSCCO superconductors. This effect becomes stronger when Bi2212 to Bi2223 phase transforma-
0.9
0.8
&____-“_..-
04
Lotgering Factors of IX223 annealing
.._..
I.
Afler
- * - Lotgering Factors of Bi2223 Before Annealing - .L - Lotgering Factors of 812212 Before Annealing
03
0.2 10
20
30
40
50
60
70
60
90
Mechanical Deformation Ratio, % Fig.
13.Lotgering
Factors versus mechanical deformation for the specimens prepared with Route C
100
276
S. Li et al./Physica
tion takes place during the annealing. The mechanical deformation, however, has different effects on the grain alignment of Bi2212 and Bi2223 phases. In order to obtain good grain alignment, Bi2212 is in favour of high deformation ratio, while Bi2223 is in favour of low deformation ratio. Based on the above discussion, the processing principles for obtaining a high degree of grain aligned Bi2223 phase can be suggested as below: (1) Heavy mechanical deformation should be performed on the Bi2212 tapes at the early stage. (2) Light deformation should be conducted at the late stages when Bi2223 phase has already been formed. (3) Short annealings after deformation should be frequently used throughout the whole processes, and (4) Whenever possible, take the advantage of Bi2212 + Bi2223 phase transformation to align the grains more efficiently.
Conclusions The grain alignment behaviour of BSCCO/Ag superconductors in mechanical deformation and subsequent annealing processes have been studied. The degree of grain alignment in Bi2212 phase increases with the increasing mechanical deformation, while the degree of grain alignment in Bi2223 phase decrease with the increasing extent of mechanical deformation. The subsequent annealing improves the grain alignment of Bi2212, Bi2223 or Bi2212 + Bi2223 mixture specimens significantly. The grain alignment process can be further enhanced if the Bi2212 to Bi2223 phase transformation occurs during the annealing process. This understanding can be used to design and to optimise the mechanical-ther-
C 279 (1997) 265-276
mal processes for the BSCCO superconductor fabrication.
Acknowledgments This work is partially supported by an Auckland University Postgraduate Research Grant. Two of the authors (WG and SL) would like to thank the technical staff in the Chemical and Materials Engineering Department and Surface and Materials Research Centre for various assistance.
References t11 R. Hawsey and J. Daley, JOM August (1995) 56.
121U. Balachandran, A.N. Iyer, R. Jammy, P. Haldar and M. Suenaga, Cryogenic Engineering 42 (1995). [31 Q.Y. Hu, H.W. Weber, H.K. Liu, S.X. Dou and H.W. Neumiller, Physica C 252 ( 1995) 211. [41 Q.Y. Hu, R.M. Schalk, H.W. Weber, H.K. Liu, R.K. Wang, C. Czurda and S.X. Dou, J. Appl. Phys. 78 (1995) 1123. [51 B. Hensel, G. Grasso and R. Fliikiger; Physical Review B 51, No. 21 (1995) 15456. [61 L.N. Bulaevskii, L.L. Daemen, M.P. Maley and J.Y. Coulter, Physical Review B 48, No. 18, (1993) 13798. [71 W. Gao and J.B. Vander Sande, Physica C 181 (1991) 105. Is1 A. Perin, G. Grasso, M. Daumling, B. Hensel, E. Walker and R. Flttkiger, Physica C 216 (1993) 339. [91 Q. Li, K. Brodersen, H.A. Hjuler and T. Freltoft, Physica C 217 (1993) 360. [lOI S. Li, M. BredehBft, Q.Y. Hu, H.K. Liu, S.X. Dou and W. Gao, Physica C275 (1997). [Ill J.M. Yoo, K. Mukherjee, Physica C 222 (1994) 241. 1121 W. Gao and J.B. Vander Sande; Supercon. Sci. Technol. 5, (1992) 318. 1131 S.X. Dou and H.K. Liu, Advances in High-T, Superconductors; Materials Science Forum vol. 137- 139 (1993) 65 1. 1141 Q.Y. Hu, H.K. Liu, S.X. Dou, Physica C 250 (1995) 7.