- Email: [email protected]
Optimization on processing parameters for Bi(Pb)-2223 superconducting tapes
Physica C 299 Ž1998. 315–326
Optimization on processing parameters for Bi žPb /-2223 superconducting tapes N.V. Vo b
a,)
, J.O. Willis a , D.E. Peterson a , H.K. Liu b, S.X. Dou
b
a SuperconductiÕity Technology Center, MS-G755, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Centre for Superconducting and Electronic Materials, Institute of Materials Technology and Manufacturing, UniÕersity of Wollongong, Northfields AÕe., Wollongong, NSW 2522, Australia
Received 8 September 1997; accepted 8 December 1997
Abstract Although there exists a large variety of thermo-mechanical procedures for preparing BiŽPb.-2223rAg tapes, primarily because of variation in precursor characteristics, the fabrication of long length BiŽPb.-2223rAg high-temperature-superconductors ŽHTSC. whether single- ŽSF. or multi-filamentary ŽMF. tapes, still requires the finesse of an optimizational approach on processing parameters, such as heat treatment, mechanical deformation, filament design, cladding materials and so on, to maximize the engineering critical current density Ž Je . and ultimately improve high field performance for applications. An outline for the small-scale processing of short- and long-length HTSC SF and MF BiŽPb.-2223rAg tapes addresses these issues. q 1998 Elsevier Science B.V. Keywords: BiŽPb.-2223 superconducting tapes; Optimization; Processing parameters
1. Introduction There has been rapid progress in numerous engineering applications of the so-called ‘high-Tc superconductors’ since their discovery in 1986 w1x. Among these ŽBi, Pb. 2 Sr2 Ca 2 Cu 3 O 10 Žor BiŽPb.-2223. has proven to be a strong contender in the development of composite tapes Ždue to its flake-like grains. over an extended temperature range under high fields w2–6x. Besides the manipulation of the microstructure Žgrain size, boundaries, orientation, etc.., achieving high critical current densities Ž Jc . from high-quality precursor powders requires a combination of good material science and process control. This nor)
Corresponding author.
mally involves the monitoring andror optimization of process parameters such as heat treatment, mechanical deformation, filament design, cladding materials, etc. This paper reports some results obtained for several process-related parameters from work performed on multiple batches of short and long composite BiŽPb.-2223rAg tapes.
2. Experimental details The samples were prepared by using methods of oxide-powder-in-tube ŽOPIT. and continuous-tubeforming-filling ŽCTFF.. Details of sample preparation have been published elsewhere ŽVo et al., w7–10x. for both methods. Batches of short unreacted composite tapes each containing between 6 and 12 sam-
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 8 5 - 6
N.V. Vo et al.r Physica C 299 (1998) 315–326
316
Table 1 Precursor powder phase composition and particle characteristics Powder ID
S
T
D
U
B
Initial 2212 phase Initial content of Ca 2 PbO4 Unreacted phases Dw4, 3x ) Ž m m. Span@ Uniformity a
high low low 20.82 2.58 0.82
high very low high 29.00 2.51 0.80
high moderate high 11.02 1.57 0.48
high moderate low 5.09 1.53 0.47
high very low low 8.76 3.18 0.91
)
@
Volume weighted mean diameter.
Width of distribution, independent of median sizes.
ples were obtained from longer lengths produced by flat-rolling. Precursor powder characteristics are tabulated in Table 1. These include initial phase assemblage Žfrom X-ray diffractometry shown in Fig. 1. and particle distribution and size Žfrom the MasterSizer particle size analyser ŽPSA... The BiŽPb.-2223 compositional ratios for the precursor powders designated as S Žsolution route., T Žtwo-powder process., D Ždry-mixing., U Ždry-mixing. and B Žspray pyrolysis. are: Bi:Pb:Sr:Ca:Cuf 1.84:0.35:1.91:2.05:3.05 for S, T and D, f 1.84:0.35:1.91:2.05:3.06 for U, and f 1.80:0.33:1.87:2.00:3.00 for B.
a
Absolute deviation from the median.
The heat treatment of tapes was conducted in tubular and muffle furnaces. Optimization on sintering temperature and duration for each type of powder was determined based on differential thermal analysis and thermogravimetric data ŽFig. 2., as well as from the electrical transport measurements of the samples after successive sintering steps. The scanning electron microscopy ŽSEM. system used for microstructural studies of specimens of BiŽPb.-2223 was the Leica Cambridge Stereoscan S440, equipped with X-ray analytical capabilities. The topographic Žtypically in secondary electron or SE mode., crystal-
Fig. 1. X-ray diffraction patterns of precursor powders S, T, D, U and B.
N.V. Vo et al.r Physica C 299 (1998) 315–326
317
Fig. 2. Differential thermal analysis patterns of precursor powders S, T, D, U and B.
lographic or atomic variation Žbest with backscattered electron or BSE mode. and compositional information can thus be obtained simultaneously from the same area of a given specimen. For optical imaging, the MD-20 optical image analyzer from the Flinders University of South Australia was used. SEM specimen preparation for initial precursor powders of BiŽPb.-2223 involved sprinkling of some of the powder onto a thin layer of silver paste applied on the surface of the aluminium stub. The preparation of SEM tape involved mounting ; 5–10 mm long pieces of tape in epoxy resin mounts. The bottom surface Žwith the tape’s cross-section exposed. of the mount was then polished using the various grades of SiC grinding papers. The specimens were then polished on a 6 m m and a 1 m m diamond embedded pads Žfrom Struers. for up to 20 min each, with a final 0.06 m m finishing touch on a Buehler micro-cloth with colloidal silica used as a lubricant. The polished specimens were then glued onto Al stub holders. Prior to SEM examination, the polished specimens would normally be sputtered with gold or alternatively, amorphous carbon coated by vapour deposition if EDS analysis was to be performed, since the characteristic peaks of C are posi-
tioned well away Žof lower energy. from those of the elements contained in BiŽPb.-2223.
3. Results and discussion The granulometric study performed using SEM has demonstrated that the particles exhibited a size distribution with a majority of the particles below ; 1 m m, but also showing the presence of larger aggregates, ranging from ; 5 to 15 m m, depending on the powder type. The secondary electron ŽSE. images of the precursor powders of S, T, D, and B are shown in Fig. 3, agreeing well in particle size characteristics with the results obtained from PSA. The morphology of the unreacted BiŽPb.-2223 precursor powders observed in these SEM images shows the appropriate characteristics of large aggregates of 2212 platelet type grains, surrounded by smaller CaCuO 2 particles. The presence of smaller secondary phase particles such as CaCuO 2 and CuO not only enhanced the reaction kinetics for 2223 phase formation, but also improved the phase uniformity due to an increase in the contact surface between 2212 grains and secondary phases.
318
N.V. Vo et al.r Physica C 299 (1998) 315–326
Fig. 3. Secondary electron SEM micrographs of the unreacted precursor powders S, T, D, and B of BiŽPb.-2223 used for the fabrication of the composite tapes.
After determining the effect of sintering periods on the microstructure, phase development and electrical properties of BiŽPb.-2223rAg tapes w11,12x, a stepwise reduction of sintering temperature during heat treatment was studied. It was found that for these powders, a significant improvement in the electrical transport properties and 2223 phase transformation rate could be achieved when a reduced sintering temperature Žof DT amount. was made during the second heat treatment. Details on the DT drop with sintering temperature and duration used for each type of powder are given in Table 2. The temperature reduction was found to depend on the characteristics of the precursor powder. The extent of the drop appeared to be larger for precursor powders with a relatively higher sintering temperature. This correlation can further be linked to the initial amount of plumbate phase present in the precursor powder,
since the results strongly suggest that increasing amount of Ca 2 PbO4 tends to result in a lower optimal sintering temperature Žas indicated in Fig. 1, Tables 1 and 2.. The appropriate time during the second heat treatment at which to make the temperature drop was
Table 2 Sintering temperatures ŽT . and dwelling time before Ž P1 . and after Ž P2 . a temperature drop Ž DT . for different tapes Powder type
S
T
D
U
B
T Ž"18C. P1 Žh. P2 Žh. DT Ž"18C. )
834 ; 25 ; 40 ;15
838 ; 25 ; 40 ;15
832 Sa S ;10
832 S S ;5
841 ; 25 ; 40 ;15
)
DT s 08C ´ optimal period of ;60 h w10x. h.
S s P1 q P2 f60
a
N.V. Vo et al.r Physica C 299 (1998) 315–326
also investigated, with P1 and P2 chosen to represent the sintering duration Žor dwelling time. before and after the drop, respectively. For powders of S, T and B, the respective dwelling time of P1 and P2 has been determined to be ; 25 and 40 h. Altering P1 and keeping P2 constant Žand vice versa. degrades the Jc of composite tapes Žin particular MF tapes. when durations other than the aforementioned were used. For composite tapes prepared from powders of D and U Žnamely by dry-mixing., the dwelling time of P1 and P2 have not been investigated, but is expected to be such that their sum equals ; 60 h as indicated in Table 2. It should be pointed out for completeness of the discussion that the sintering period chosen here to be ; 60 h was made based on earlier work reported by Vo et al., w11,12x. Since the objective was to produce long lengths of composite tapes for the construction of coils and magnets w13x, flat rolling was employed as the intermediate deformation ŽID. technique between sintering stages. The degree of ID was found to be crucial in determining the performance of the final tape, and indeed it has been shown that the evolution and development of the 2223 phase, as well as the transport properties of BiŽPb.-2223rAg SF tapes, are governed by the degree of deformation induced w14,15x. Based on these findings, it was necessary to determine approximately the range or window of amount of ID within which an optimal value of Jc could be achieved. This required a systematic study of short samples from numerous batches of MF tapes, and was carried out for samples prepared from both methods of OPIT and CTFF Žall unreacted tapes had an initial thickness of ; 0.32 mm.. The ID window, or more informatively, the ‘critical-intermediate-rolling-reduction-zone’ ŽCIRRZ. for each batch of MF BiŽPb.-2223rAg and Ag-alloy Žviz. Ag-7 at.% Cu. is shown in Table 3 as reduction percentages of the tapes’ thickness. The shaded regions indicate maximal Jc attained after the subsequent sintering period for the indicated percentage reduction carried out during intermediate deformation of that batch of tapes. Note the gradual reduction in percentage thickness deformation required for each subsequent sintering stage. This reflects the complexity involved in controlling processing parameters, such as intermediate deformation and filamentary design and preparation. This also means that
319
Table 3 Critical–intermediate-rolling-reduction zone ŽCIRRZ as%. for multifilamentary BiŽPb.-2223rAg tapes Filaments
1st CIRRZ
2nd CIRRZ
3rd CIRRZ
60q3=Ž80. h 9 ŽCTFF. 12 ŽCTFF. 14 ŽCTFF. 36 ŽCTFF. 37 ŽCTFF. 36Cu ŽCTFF. 31 ŽOPIT.
16–30 ) 27–30 ) 12 27) 30 ) 1315–30 )
1716–21 18–25 1510 815–23
3.587.57–8) 5.55.53–5
100q3=Ž80. h 9 ŽCTFF. 12 ŽCTFF. 14 ŽCTFF. 36 ŽCTFF. 36Cu ŽCTFF.
16 16 37) 15–33 23-
10.513–13.5 11.7–13.7 11.6–12.5 10 -
97–9) 6.5–8.5 6.4 3-
as the reaction proceeds, progressively more Bi-2223 phase and less liquid is present. Large deformations early in the thermomechanical deformation process are necessary to consolidate the core that has expanded from reaction to Bi-2223 phase and anisotropic growth. As the growth slows with time, there is less need for large deformations. Small deformations increase the density to allow further reaction without generating too many cracks; large deformations at this time often do more harm than good, by introducing macrocracks that cannot be healed by the increasingly smaller supply of liquids. In general, to achieve a high degree of grain alignment in BiŽPb.-2223 tapes, the oxide core of these tapes should be thin Ž; 40–50 m m or less., which requires the initial powder to have a small particle size. Tapes with ceramic cores F 40 m m thick often exhibit sausaging. This phenomenon occurs primarily due to the variation in mechanical attributes of the composite material being processed. Thus, to reduce this effect, the powder should ideally contain a single phase, so that each grain has the same mechanical properties and the powder deforms more uniformly. However, precursor powders must contain second phases to allow reactions to form the 2223 phase. Practically speaking, the best powders should contain large flaky 2212 grains to allow rotation of the grains during deformation to produce good texture and second-phase particles as small as
N.V. Vo et al.r Physica C 299 (1998) 315–326
320
Fig. 4. Highest Jc values achieved, obtained with single- ŽSF. and multi-filamentary tapes ŽMT. using various processing techniques for the corresponding powders of BiŽPb.-2223. ŽThe S powder used at the time had a volume weighted mean diameter of ; 5 m m..
possible both to inhibit the rotation of the 2212 grains as little as possible and also for good chemical reactivity. More details on the bulk processing of 2223 BSCCO powders for tape rolling can be found elsewhere w16x. Equally important, the mismatch in mechanical properties between sheath and ceramic core also dictates the tape’s final uniformity w17,18x. To give an indication of the Jc values obtained with various powders used, Fig. 4 shows roughly the highest Jc achieved, obtained with single- ŽSF. and multi-filamentary tapes ŽMT. using several process-
ing techniques for the corresponding powders of BiŽPb.-2223. Optical imaging is used for calculating the transverse cross-sectional area of a composite tape for the determination of its fill factor Žratio of superconductor core to total conductor cross-section., Jc , Je , and for qualitative analysis of the spatial distribution and uniformity of the filaments. Fig. 5 shows optical cross-sections of tubes and groove-rolled wires prepared from the methods of PIT ŽMT19 and MT31. and CTFF ŽMT9, MT12, MT14, MT36, MT36c,
Table 4 Corresponding dimensions of cross sections shown in Fig. 5 MF
9-CTFF 12-CTFF 14-CTFF 19-OPIT-vib ) 31-OPIT-ham@ 36-CTFF 37-CTFF 50-CTFF & 36Cu% -CTFF 41-CTFF & a &
Tube
Wire
1st, 2nd Stacking Žo.d.ri.d. - mm.
Filament size Žgroove-rolled.a
Wire size Žgroove-rolled.
Fill factor, l
6.5r5.5 6.5r4.5 6.5r5.5 6.5r4.5, 6.5r5.5 6.5r4.5, 10r8 10r8 10r8 10r8, 10r8 10r8 6.5r5.5, 10r8
after 13th grv. after 13th grv. after 17th grv. after 17th grv. after 14th grv. after 17th grv. after 17th grv. 12thr7th groove after 17th grv. 12thr10th groove
after 13th grv. after 13th grv. after 13th grv. after 13th grv. after 13th grv. after 12th grv. after 12th grv. after 5th grv. after 12th grv. after 9th grv.
0.233 0.218 0.197 0.297 0.257 0.210 0.184 0.106 0.154 0.142
The actual groove sizes are given in Table 5. ) Vibrator used for PIT. @ Hammer and rod used for PIT. Double-stacked. % Pure Ag cladding for filaments, Stirling Ag ŽAg:Cuf 93:7. for outer sheath.
N.V. Vo et al.r Physica C 299 (1998) 315–326
321
Fig. 5. Optical images of cross sections of BiŽPb.-2223 MF CTFF Ž9, 12, 14, 36, 36Cu, 37, 50, and 41 filaments. and OPIT Ž19, and 31. composites, before Žfilaments stacked in tube. and after Žwire forming. groove-rolling.
MT37, MT40, and MT50.. The corresponding dimensions of the tubes and wires are listed in Table 4. Table 5 shows the groove-size parameters for the dual-rolling machine used for deformation of wires and tapes. Fig. 6 shows some optical images of the transverse and longitudinal cross-sections of BiŽPb.-
2223 unreacted Žgreen. MF flat-rolled tapes prior to sintering. As can be seen from the cross-section of MT9 of Fig. 6, the filaments can sometimes be displaced during wire processing, which results in an uneven distribution of the filaments throughout the composite tape. With pure Ag as a cladding material,
N.V. Vo et al.r Physica C 299 (1998) 315–326
322
the distribution of the filaments in general was found to be fairly uniform. This spatial distribution can be further improved by the use of various alloys of Ag. To arrange the position of the filaments closer to the edge and evenly across the tape for uniform cooling of the filaments, several tubes of various sizes would be needed for the preparation of wires. For MT36c, the outer layer Žor strand. matrix was that of Ag–Cu alloy, with pure Ag used as the cladding material for the filaments. The initial ideas behind this attempt were to achieve better uniformity, and to provide extra reinforcement in mechanical strength for the conductor, at the same time eliminate the possibility of Cu reacting with the oxide core at elevated temperature. As a result, the Jc was found to increase by twofold Žcf. between MT36 and MT36c. and an improvement in strength Žthrough Vickers microhardness tests. was also achieved. Added problems include the formation of ‘lobes’ during tape processing, whereby the filaments towards the center of the tape received the greatest amount of vertical strain causing those near the edges to bulge outwards Žas can be seen with MT36c in Fig. 6.. This was attributable to the added mismatch of mechanical properties between Ag–Cu and Ag, and secondly, the oxidation of Cu at elevated
temperature Židentified by EDS on SEM image of MT36c, as shown in Fig. 7. which has been shown to increase the matrix’s electrical resistivity above that of pure Ag, in particular, for the oxidation of Mg w19x. This in effect would have an adverse influence on the stability of the tape with respect to quenching because of the matrix’s lower thermal conductivity. The backscattered electron images of the longitudinal Žleft column. and transverse Žright column. cross-sections of BiŽPb.-2223 composite PIT and CTFF red tapes obtained after the final sintering step shown in Fig. 7 are the products of the wires shown in Fig. 5 and green tapes shown in Fig. 6. Apart from the problem of sausaging, prominent in filaments of thickness less than ; 30 m m, the undesirable presence of the current-shunting ‘needle-like’ Žactually 2D plates. intergrowth between filaments was often encountered. This inter-filamentary phenomenon is sometimes referred to as ‘bridging’. The manifestation of bridging is demonstrated in samples MT19 and MT31 of Fig. 7. Nevertheless, good grain connectivity and alignment has generally been observed with increase in core density and texture following the final heat treatment. Most of the platelike grains were observed to be well aligned longitu-
Table 5 Approximate groove dimensions on the dual-rolling machine Groove no.
Long-oppositeside distance Žmm.
Short-oppositeside distance Žmm.
Approximate area of groove Žmm2 .
Approximate area reduction between grooves Ž%.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
5.71 4.62 3.95 3.55 3.18 2.84 2.55 2.27 2.06 1.80 1.57 1.48 1.33 1.24 1.13 1.04 0.98
5.78 5.06 4.24 4.00 3.48 3.13 2.83 2.47 2.24 2.04 1.87 1.68 1.59 1.52 1.39 1.31 1.25
27.83 17.53 12.34 9.60 7.45 5.70 4.39 3.29 2.52 1.73 1.03 0.95 0.70 0.53 0.31 0.24 0.19
– 37.01 29.60 22.22 22.43 23.42 23.05 24.93 23.60 31.43 40.05 7.83 26.95 23.75 41.36 22.34 22.06
N.V. Vo et al.r Physica C 299 (1998) 315–326
323
Fig. 6. Optical images of transverse and longitudinal cross-sections of BiŽPb.-2223 composite green tapes prior to sintering. These samples have been flat-rolled from groove-rolled wires prepared using the methods of PIT and CTFF Žshown in Fig. 5. with a final ‘as-rolled’ thickness of ; 0.32 mm.
324
N.V. Vo et al.r Physica C 299 (1998) 315–326
Fig. 7. Backscattered electron images of longitudinal Žleft column. and transverse Žright column. cross-sections of BiŽPb.-2223 composite red tapes obtained after the final sintering step, predecessors of those shown in Fig. 2.
N.V. Vo et al.r Physica C 299 (1998) 315–326
325
Fig. 7 Žcontinued..
dinally Žalong the tape length or rolling direction. parallel to the wide surface of tape. The high mass density and high degree of grain alignment in the silver-clad tapes are believed to be responsible for the enhancement of Jc and Jc –magnetic field characteristic of these tape conductors. The identification of different phases present was made from direct SEM observations supplemented with qualitative EDS microanalyses. Apart from the superconducting platelets of low- and high-Tc phases of 2212 Žlight grey. and 2223 Ždark grey., large black regions w ith m ixtures of oxides, Ca 1y x Sr x CuO 3 Žup to ; 25 m m. were observed, as well as the very fine plates of Ca 2 PbO4 Ž; 0.1–0.3 m m thick. which cannot be seen directly with the
contrast and magnification used in the microimages shown in Fig. 7. It is possible that these non-superconducting Ca–Cu–O phases consolidated into larger grains through sintering of the smaller CaCuO 2 grains, which originally were part of the agglomeration present in the precursor powder. These black regions can be understood to have a negative effect on the tape’s electrical, as well as mechanical properties, attributed to the low plasticity of these secondary phases, which tend to accumulate in specific areas of the core. As a consequence, these are the culprits in the diminishing of grain alignment of the superconducting phases during intermediate mechanical deformation, and are also the cause of multiple ruptures often found in the relatively softer and
326
N.V. Vo et al.r Physica C 299 (1998) 315–326
ductile silver sheath. At the same time, these large Ca–Cu–O phases can lead to large transversal cracks while bending a tape.
ment. Some of this work was supported in part by the Australian Research Council and the Energy Research and Development Corporation. We thank E.W. Collings for making available long lengths of CTFF BiŽPb.-2223rAg wire.
4. Summary Apart from the conclusive existence of an optimal sintering period, the precursor powders of BiŽPb.2223 investigated have been shown to react much quicker when a temperature drop is incorporated into the second sintering period, with the extent of the drop being dependent on the characteristics of the precursor powder, especially the amount of plumbate phase. The results presented here are still inconclusive as to the effects of the precursor powder characteristics on the phase development and transport properties of BiŽPb.-2223rAg composite tapes. However, it is expected to be more beneficial to have the powder initially as fine and homogeneous as possible to increase its reactivity, which would expedite the phase transformation process and to give a more uniform microstructure, although it is speculative that for precursor powders with the average weighted mean diameter in particle sizes of 1 m m or less, other problems might be encountered, such as too high a reactivity and contamination potential, etc. With flat-rolling as the intermediate mechanical deformation process between sintering periods, it has been conclusively shown that there exists a criticalintermediate-rolling-reduction-zone within which superior performance of BiŽPb.-2223rAg and alloy of Ag composite tapes can be produced. This reduction zone is found to depend on filamentary characteristics and sample preparation methods.
Acknowledgements Work at Los Alamos National Laboratory was performed under the auspices of the United States Department of Energy, Office of Energy Manage-
References w1x J.G. Bednorz, K.A. Muller, Z. Phys. B 64 Ž1986. 189–193. ¨ w2x K. Sato, T. Hikata, Y. Iwasa, Appl. Phys. Lett. 57 Ž1990. 1928. w3x T. Hikata, M. Ueyama, H. Mukai, K.-I. Sato, Cryogenics Nov. 30, 1990, pp. 924–929. w4x D.P. Hampshire, S.S. Oh, K. Osamura, D.C. Larbalestier, Supercond. Sci. Technol. 3 Ž1990. 560–563. w5x M. Daumling, G. Grasso, A. Perin, R. Flukiger, Supercond. Sci. Technol. 7 Ž1994. 926–930. w6x G. Grasso, R. Flukiger, Supercond. Sci. Technol. 10 Ž1997. ¨ 223–226. w7x N.V. Vo, S.X. Dou, H.K. Liu, Physica C 250 Ž1995. 139–152. w8x N.V. Vo, H.K. Liu, S.X. Dou, E.W. Collings, Proc. 2nd PACRIM, Cairns, Australia, July 15–17, 1996. w9x N.V. Vo, H.K. Liu, S.X. Dou, E.W. Collings, Proc. 16th ICECrICMC, Kitakyushu, Japan, May 20–24, 1996. w10x N.V. Vo, S.X. Dou, H.K. Liu, IEEE Trans. Appl. Supercond. 6 Ž2. Ž1996. 102–105. w11x N.V. Vo, H.K. Liu, S.X. Dou, J. Mater. Res. 11 Ž5. Ž1996. 1101–1107. w12x N.V. Vo, H.K. Liu, S.X. Dou, Supercond. Sci. Technol. 9 Ž1996. 104–112. w13x N.V. Vo, H.K. Liu, S.X. Dou, IEEE Trans. Appl. Supercond., 1997, in press. w14x N.V. Vo, H.K. Liu, S.X. Dou, I. Kusevic, Mater. Sci. Eng. B, 1996, pp. 217–223. w15x Z. Han, P. Skov-Hansen, T. Freltoft, Supercond. Sci. Technol. 10 Ž1997. 371–387. w16x S.E. Schoenfeld, S. Ahzi, R.J. Asaro et al., Philos. Mag. A 73 Ž1996. 1591. w17x J.O. Willis, R.D. Ray II, T.G. Holesinger et al., Proc. 7th US-Japan Workshop on High Tc Superconductors, Tsukuba, Japan, 24–25 Oct 1995, K. Tachikawa ŽEd.., New Superconducing Materials Forum, The Society of Non-Traditional Technology, Tokyo, p. 96, 1996. w18x B.N. Hubert, R. Zhou, T.G. Holesinger et al., J. Elec. Mater. 24 Ž12. Ž1995. 1869–1872. w19x O. Dur, S. Nakamae, J. Schwartz, Y.S. Hascicek, presented at ASC96, Pittsburgh, PA, USA, 25–30 Aug., 1996.
Optimization on processing parameters for Bi žPb /-2223 superconducting tapes N.V. Vo b
a,)
, J.O. Willis a , D.E. Peterson a , H.K. Liu b, S.X. Dou
b
a SuperconductiÕity Technology Center, MS-G755, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Centre for Superconducting and Electronic Materials, Institute of Materials Technology and Manufacturing, UniÕersity of Wollongong, Northfields AÕe., Wollongong, NSW 2522, Australia
Received 8 September 1997; accepted 8 December 1997
Abstract Although there exists a large variety of thermo-mechanical procedures for preparing BiŽPb.-2223rAg tapes, primarily because of variation in precursor characteristics, the fabrication of long length BiŽPb.-2223rAg high-temperature-superconductors ŽHTSC. whether single- ŽSF. or multi-filamentary ŽMF. tapes, still requires the finesse of an optimizational approach on processing parameters, such as heat treatment, mechanical deformation, filament design, cladding materials and so on, to maximize the engineering critical current density Ž Je . and ultimately improve high field performance for applications. An outline for the small-scale processing of short- and long-length HTSC SF and MF BiŽPb.-2223rAg tapes addresses these issues. q 1998 Elsevier Science B.V. Keywords: BiŽPb.-2223 superconducting tapes; Optimization; Processing parameters
1. Introduction There has been rapid progress in numerous engineering applications of the so-called ‘high-Tc superconductors’ since their discovery in 1986 w1x. Among these ŽBi, Pb. 2 Sr2 Ca 2 Cu 3 O 10 Žor BiŽPb.-2223. has proven to be a strong contender in the development of composite tapes Ždue to its flake-like grains. over an extended temperature range under high fields w2–6x. Besides the manipulation of the microstructure Žgrain size, boundaries, orientation, etc.., achieving high critical current densities Ž Jc . from high-quality precursor powders requires a combination of good material science and process control. This nor)
Corresponding author.
mally involves the monitoring andror optimization of process parameters such as heat treatment, mechanical deformation, filament design, cladding materials, etc. This paper reports some results obtained for several process-related parameters from work performed on multiple batches of short and long composite BiŽPb.-2223rAg tapes.
2. Experimental details The samples were prepared by using methods of oxide-powder-in-tube ŽOPIT. and continuous-tubeforming-filling ŽCTFF.. Details of sample preparation have been published elsewhere ŽVo et al., w7–10x. for both methods. Batches of short unreacted composite tapes each containing between 6 and 12 sam-
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 8 8 5 - 6
N.V. Vo et al.r Physica C 299 (1998) 315–326
316
Table 1 Precursor powder phase composition and particle characteristics Powder ID
S
T
D
U
B
Initial 2212 phase Initial content of Ca 2 PbO4 Unreacted phases Dw4, 3x ) Ž m m. Span@ Uniformity a
high low low 20.82 2.58 0.82
high very low high 29.00 2.51 0.80
high moderate high 11.02 1.57 0.48
high moderate low 5.09 1.53 0.47
high very low low 8.76 3.18 0.91
)
@
Volume weighted mean diameter.
Width of distribution, independent of median sizes.
ples were obtained from longer lengths produced by flat-rolling. Precursor powder characteristics are tabulated in Table 1. These include initial phase assemblage Žfrom X-ray diffractometry shown in Fig. 1. and particle distribution and size Žfrom the MasterSizer particle size analyser ŽPSA... The BiŽPb.-2223 compositional ratios for the precursor powders designated as S Žsolution route., T Žtwo-powder process., D Ždry-mixing., U Ždry-mixing. and B Žspray pyrolysis. are: Bi:Pb:Sr:Ca:Cuf 1.84:0.35:1.91:2.05:3.05 for S, T and D, f 1.84:0.35:1.91:2.05:3.06 for U, and f 1.80:0.33:1.87:2.00:3.00 for B.
a
Absolute deviation from the median.
The heat treatment of tapes was conducted in tubular and muffle furnaces. Optimization on sintering temperature and duration for each type of powder was determined based on differential thermal analysis and thermogravimetric data ŽFig. 2., as well as from the electrical transport measurements of the samples after successive sintering steps. The scanning electron microscopy ŽSEM. system used for microstructural studies of specimens of BiŽPb.-2223 was the Leica Cambridge Stereoscan S440, equipped with X-ray analytical capabilities. The topographic Žtypically in secondary electron or SE mode., crystal-
Fig. 1. X-ray diffraction patterns of precursor powders S, T, D, U and B.
N.V. Vo et al.r Physica C 299 (1998) 315–326
317
Fig. 2. Differential thermal analysis patterns of precursor powders S, T, D, U and B.
lographic or atomic variation Žbest with backscattered electron or BSE mode. and compositional information can thus be obtained simultaneously from the same area of a given specimen. For optical imaging, the MD-20 optical image analyzer from the Flinders University of South Australia was used. SEM specimen preparation for initial precursor powders of BiŽPb.-2223 involved sprinkling of some of the powder onto a thin layer of silver paste applied on the surface of the aluminium stub. The preparation of SEM tape involved mounting ; 5–10 mm long pieces of tape in epoxy resin mounts. The bottom surface Žwith the tape’s cross-section exposed. of the mount was then polished using the various grades of SiC grinding papers. The specimens were then polished on a 6 m m and a 1 m m diamond embedded pads Žfrom Struers. for up to 20 min each, with a final 0.06 m m finishing touch on a Buehler micro-cloth with colloidal silica used as a lubricant. The polished specimens were then glued onto Al stub holders. Prior to SEM examination, the polished specimens would normally be sputtered with gold or alternatively, amorphous carbon coated by vapour deposition if EDS analysis was to be performed, since the characteristic peaks of C are posi-
tioned well away Žof lower energy. from those of the elements contained in BiŽPb.-2223.
3. Results and discussion The granulometric study performed using SEM has demonstrated that the particles exhibited a size distribution with a majority of the particles below ; 1 m m, but also showing the presence of larger aggregates, ranging from ; 5 to 15 m m, depending on the powder type. The secondary electron ŽSE. images of the precursor powders of S, T, D, and B are shown in Fig. 3, agreeing well in particle size characteristics with the results obtained from PSA. The morphology of the unreacted BiŽPb.-2223 precursor powders observed in these SEM images shows the appropriate characteristics of large aggregates of 2212 platelet type grains, surrounded by smaller CaCuO 2 particles. The presence of smaller secondary phase particles such as CaCuO 2 and CuO not only enhanced the reaction kinetics for 2223 phase formation, but also improved the phase uniformity due to an increase in the contact surface between 2212 grains and secondary phases.
318
N.V. Vo et al.r Physica C 299 (1998) 315–326
Fig. 3. Secondary electron SEM micrographs of the unreacted precursor powders S, T, D, and B of BiŽPb.-2223 used for the fabrication of the composite tapes.
After determining the effect of sintering periods on the microstructure, phase development and electrical properties of BiŽPb.-2223rAg tapes w11,12x, a stepwise reduction of sintering temperature during heat treatment was studied. It was found that for these powders, a significant improvement in the electrical transport properties and 2223 phase transformation rate could be achieved when a reduced sintering temperature Žof DT amount. was made during the second heat treatment. Details on the DT drop with sintering temperature and duration used for each type of powder are given in Table 2. The temperature reduction was found to depend on the characteristics of the precursor powder. The extent of the drop appeared to be larger for precursor powders with a relatively higher sintering temperature. This correlation can further be linked to the initial amount of plumbate phase present in the precursor powder,
since the results strongly suggest that increasing amount of Ca 2 PbO4 tends to result in a lower optimal sintering temperature Žas indicated in Fig. 1, Tables 1 and 2.. The appropriate time during the second heat treatment at which to make the temperature drop was
Table 2 Sintering temperatures ŽT . and dwelling time before Ž P1 . and after Ž P2 . a temperature drop Ž DT . for different tapes Powder type
S
T
D
U
B
T Ž"18C. P1 Žh. P2 Žh. DT Ž"18C. )
834 ; 25 ; 40 ;15
838 ; 25 ; 40 ;15
832 Sa S ;10
832 S S ;5
841 ; 25 ; 40 ;15
)
DT s 08C ´ optimal period of ;60 h w10x. h.
S s P1 q P2 f60
a
N.V. Vo et al.r Physica C 299 (1998) 315–326
also investigated, with P1 and P2 chosen to represent the sintering duration Žor dwelling time. before and after the drop, respectively. For powders of S, T and B, the respective dwelling time of P1 and P2 has been determined to be ; 25 and 40 h. Altering P1 and keeping P2 constant Žand vice versa. degrades the Jc of composite tapes Žin particular MF tapes. when durations other than the aforementioned were used. For composite tapes prepared from powders of D and U Žnamely by dry-mixing., the dwelling time of P1 and P2 have not been investigated, but is expected to be such that their sum equals ; 60 h as indicated in Table 2. It should be pointed out for completeness of the discussion that the sintering period chosen here to be ; 60 h was made based on earlier work reported by Vo et al., w11,12x. Since the objective was to produce long lengths of composite tapes for the construction of coils and magnets w13x, flat rolling was employed as the intermediate deformation ŽID. technique between sintering stages. The degree of ID was found to be crucial in determining the performance of the final tape, and indeed it has been shown that the evolution and development of the 2223 phase, as well as the transport properties of BiŽPb.-2223rAg SF tapes, are governed by the degree of deformation induced w14,15x. Based on these findings, it was necessary to determine approximately the range or window of amount of ID within which an optimal value of Jc could be achieved. This required a systematic study of short samples from numerous batches of MF tapes, and was carried out for samples prepared from both methods of OPIT and CTFF Žall unreacted tapes had an initial thickness of ; 0.32 mm.. The ID window, or more informatively, the ‘critical-intermediate-rolling-reduction-zone’ ŽCIRRZ. for each batch of MF BiŽPb.-2223rAg and Ag-alloy Žviz. Ag-7 at.% Cu. is shown in Table 3 as reduction percentages of the tapes’ thickness. The shaded regions indicate maximal Jc attained after the subsequent sintering period for the indicated percentage reduction carried out during intermediate deformation of that batch of tapes. Note the gradual reduction in percentage thickness deformation required for each subsequent sintering stage. This reflects the complexity involved in controlling processing parameters, such as intermediate deformation and filamentary design and preparation. This also means that
319
Table 3 Critical–intermediate-rolling-reduction zone ŽCIRRZ as%. for multifilamentary BiŽPb.-2223rAg tapes Filaments
1st CIRRZ
2nd CIRRZ
3rd CIRRZ
60q3=Ž80. h 9 ŽCTFF. 12 ŽCTFF. 14 ŽCTFF. 36 ŽCTFF. 37 ŽCTFF. 36Cu ŽCTFF. 31 ŽOPIT.
16–30 ) 27–30 ) 12 27) 30 ) 1315–30 )
1716–21 18–25 1510 815–23
3.587.57–8) 5.55.53–5
100q3=Ž80. h 9 ŽCTFF. 12 ŽCTFF. 14 ŽCTFF. 36 ŽCTFF. 36Cu ŽCTFF.
16 16 37) 15–33 23-
10.513–13.5 11.7–13.7 11.6–12.5 10 -
97–9) 6.5–8.5 6.4 3-
as the reaction proceeds, progressively more Bi-2223 phase and less liquid is present. Large deformations early in the thermomechanical deformation process are necessary to consolidate the core that has expanded from reaction to Bi-2223 phase and anisotropic growth. As the growth slows with time, there is less need for large deformations. Small deformations increase the density to allow further reaction without generating too many cracks; large deformations at this time often do more harm than good, by introducing macrocracks that cannot be healed by the increasingly smaller supply of liquids. In general, to achieve a high degree of grain alignment in BiŽPb.-2223 tapes, the oxide core of these tapes should be thin Ž; 40–50 m m or less., which requires the initial powder to have a small particle size. Tapes with ceramic cores F 40 m m thick often exhibit sausaging. This phenomenon occurs primarily due to the variation in mechanical attributes of the composite material being processed. Thus, to reduce this effect, the powder should ideally contain a single phase, so that each grain has the same mechanical properties and the powder deforms more uniformly. However, precursor powders must contain second phases to allow reactions to form the 2223 phase. Practically speaking, the best powders should contain large flaky 2212 grains to allow rotation of the grains during deformation to produce good texture and second-phase particles as small as
N.V. Vo et al.r Physica C 299 (1998) 315–326
320
Fig. 4. Highest Jc values achieved, obtained with single- ŽSF. and multi-filamentary tapes ŽMT. using various processing techniques for the corresponding powders of BiŽPb.-2223. ŽThe S powder used at the time had a volume weighted mean diameter of ; 5 m m..
possible both to inhibit the rotation of the 2212 grains as little as possible and also for good chemical reactivity. More details on the bulk processing of 2223 BSCCO powders for tape rolling can be found elsewhere w16x. Equally important, the mismatch in mechanical properties between sheath and ceramic core also dictates the tape’s final uniformity w17,18x. To give an indication of the Jc values obtained with various powders used, Fig. 4 shows roughly the highest Jc achieved, obtained with single- ŽSF. and multi-filamentary tapes ŽMT. using several process-
ing techniques for the corresponding powders of BiŽPb.-2223. Optical imaging is used for calculating the transverse cross-sectional area of a composite tape for the determination of its fill factor Žratio of superconductor core to total conductor cross-section., Jc , Je , and for qualitative analysis of the spatial distribution and uniformity of the filaments. Fig. 5 shows optical cross-sections of tubes and groove-rolled wires prepared from the methods of PIT ŽMT19 and MT31. and CTFF ŽMT9, MT12, MT14, MT36, MT36c,
Table 4 Corresponding dimensions of cross sections shown in Fig. 5 MF
9-CTFF 12-CTFF 14-CTFF 19-OPIT-vib ) 31-OPIT-ham@ 36-CTFF 37-CTFF 50-CTFF & 36Cu% -CTFF 41-CTFF & a &
Tube
Wire
1st, 2nd Stacking Žo.d.ri.d. - mm.
Filament size Žgroove-rolled.a
Wire size Žgroove-rolled.
Fill factor, l
6.5r5.5 6.5r4.5 6.5r5.5 6.5r4.5, 6.5r5.5 6.5r4.5, 10r8 10r8 10r8 10r8, 10r8 10r8 6.5r5.5, 10r8
after 13th grv. after 13th grv. after 17th grv. after 17th grv. after 14th grv. after 17th grv. after 17th grv. 12thr7th groove after 17th grv. 12thr10th groove
after 13th grv. after 13th grv. after 13th grv. after 13th grv. after 13th grv. after 12th grv. after 12th grv. after 5th grv. after 12th grv. after 9th grv.
0.233 0.218 0.197 0.297 0.257 0.210 0.184 0.106 0.154 0.142
The actual groove sizes are given in Table 5. ) Vibrator used for PIT. @ Hammer and rod used for PIT. Double-stacked. % Pure Ag cladding for filaments, Stirling Ag ŽAg:Cuf 93:7. for outer sheath.
N.V. Vo et al.r Physica C 299 (1998) 315–326
321
Fig. 5. Optical images of cross sections of BiŽPb.-2223 MF CTFF Ž9, 12, 14, 36, 36Cu, 37, 50, and 41 filaments. and OPIT Ž19, and 31. composites, before Žfilaments stacked in tube. and after Žwire forming. groove-rolling.
MT37, MT40, and MT50.. The corresponding dimensions of the tubes and wires are listed in Table 4. Table 5 shows the groove-size parameters for the dual-rolling machine used for deformation of wires and tapes. Fig. 6 shows some optical images of the transverse and longitudinal cross-sections of BiŽPb.-
2223 unreacted Žgreen. MF flat-rolled tapes prior to sintering. As can be seen from the cross-section of MT9 of Fig. 6, the filaments can sometimes be displaced during wire processing, which results in an uneven distribution of the filaments throughout the composite tape. With pure Ag as a cladding material,
N.V. Vo et al.r Physica C 299 (1998) 315–326
322
the distribution of the filaments in general was found to be fairly uniform. This spatial distribution can be further improved by the use of various alloys of Ag. To arrange the position of the filaments closer to the edge and evenly across the tape for uniform cooling of the filaments, several tubes of various sizes would be needed for the preparation of wires. For MT36c, the outer layer Žor strand. matrix was that of Ag–Cu alloy, with pure Ag used as the cladding material for the filaments. The initial ideas behind this attempt were to achieve better uniformity, and to provide extra reinforcement in mechanical strength for the conductor, at the same time eliminate the possibility of Cu reacting with the oxide core at elevated temperature. As a result, the Jc was found to increase by twofold Žcf. between MT36 and MT36c. and an improvement in strength Žthrough Vickers microhardness tests. was also achieved. Added problems include the formation of ‘lobes’ during tape processing, whereby the filaments towards the center of the tape received the greatest amount of vertical strain causing those near the edges to bulge outwards Žas can be seen with MT36c in Fig. 6.. This was attributable to the added mismatch of mechanical properties between Ag–Cu and Ag, and secondly, the oxidation of Cu at elevated
temperature Židentified by EDS on SEM image of MT36c, as shown in Fig. 7. which has been shown to increase the matrix’s electrical resistivity above that of pure Ag, in particular, for the oxidation of Mg w19x. This in effect would have an adverse influence on the stability of the tape with respect to quenching because of the matrix’s lower thermal conductivity. The backscattered electron images of the longitudinal Žleft column. and transverse Žright column. cross-sections of BiŽPb.-2223 composite PIT and CTFF red tapes obtained after the final sintering step shown in Fig. 7 are the products of the wires shown in Fig. 5 and green tapes shown in Fig. 6. Apart from the problem of sausaging, prominent in filaments of thickness less than ; 30 m m, the undesirable presence of the current-shunting ‘needle-like’ Žactually 2D plates. intergrowth between filaments was often encountered. This inter-filamentary phenomenon is sometimes referred to as ‘bridging’. The manifestation of bridging is demonstrated in samples MT19 and MT31 of Fig. 7. Nevertheless, good grain connectivity and alignment has generally been observed with increase in core density and texture following the final heat treatment. Most of the platelike grains were observed to be well aligned longitu-
Table 5 Approximate groove dimensions on the dual-rolling machine Groove no.
Long-oppositeside distance Žmm.
Short-oppositeside distance Žmm.
Approximate area of groove Žmm2 .
Approximate area reduction between grooves Ž%.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
5.71 4.62 3.95 3.55 3.18 2.84 2.55 2.27 2.06 1.80 1.57 1.48 1.33 1.24 1.13 1.04 0.98
5.78 5.06 4.24 4.00 3.48 3.13 2.83 2.47 2.24 2.04 1.87 1.68 1.59 1.52 1.39 1.31 1.25
27.83 17.53 12.34 9.60 7.45 5.70 4.39 3.29 2.52 1.73 1.03 0.95 0.70 0.53 0.31 0.24 0.19
– 37.01 29.60 22.22 22.43 23.42 23.05 24.93 23.60 31.43 40.05 7.83 26.95 23.75 41.36 22.34 22.06
N.V. Vo et al.r Physica C 299 (1998) 315–326
323
Fig. 6. Optical images of transverse and longitudinal cross-sections of BiŽPb.-2223 composite green tapes prior to sintering. These samples have been flat-rolled from groove-rolled wires prepared using the methods of PIT and CTFF Žshown in Fig. 5. with a final ‘as-rolled’ thickness of ; 0.32 mm.
324
N.V. Vo et al.r Physica C 299 (1998) 315–326
Fig. 7. Backscattered electron images of longitudinal Žleft column. and transverse Žright column. cross-sections of BiŽPb.-2223 composite red tapes obtained after the final sintering step, predecessors of those shown in Fig. 2.
N.V. Vo et al.r Physica C 299 (1998) 315–326
325
Fig. 7 Žcontinued..
dinally Žalong the tape length or rolling direction. parallel to the wide surface of tape. The high mass density and high degree of grain alignment in the silver-clad tapes are believed to be responsible for the enhancement of Jc and Jc –magnetic field characteristic of these tape conductors. The identification of different phases present was made from direct SEM observations supplemented with qualitative EDS microanalyses. Apart from the superconducting platelets of low- and high-Tc phases of 2212 Žlight grey. and 2223 Ždark grey., large black regions w ith m ixtures of oxides, Ca 1y x Sr x CuO 3 Žup to ; 25 m m. were observed, as well as the very fine plates of Ca 2 PbO4 Ž; 0.1–0.3 m m thick. which cannot be seen directly with the
contrast and magnification used in the microimages shown in Fig. 7. It is possible that these non-superconducting Ca–Cu–O phases consolidated into larger grains through sintering of the smaller CaCuO 2 grains, which originally were part of the agglomeration present in the precursor powder. These black regions can be understood to have a negative effect on the tape’s electrical, as well as mechanical properties, attributed to the low plasticity of these secondary phases, which tend to accumulate in specific areas of the core. As a consequence, these are the culprits in the diminishing of grain alignment of the superconducting phases during intermediate mechanical deformation, and are also the cause of multiple ruptures often found in the relatively softer and
326
N.V. Vo et al.r Physica C 299 (1998) 315–326
ductile silver sheath. At the same time, these large Ca–Cu–O phases can lead to large transversal cracks while bending a tape.
ment. Some of this work was supported in part by the Australian Research Council and the Energy Research and Development Corporation. We thank E.W. Collings for making available long lengths of CTFF BiŽPb.-2223rAg wire.
4. Summary Apart from the conclusive existence of an optimal sintering period, the precursor powders of BiŽPb.2223 investigated have been shown to react much quicker when a temperature drop is incorporated into the second sintering period, with the extent of the drop being dependent on the characteristics of the precursor powder, especially the amount of plumbate phase. The results presented here are still inconclusive as to the effects of the precursor powder characteristics on the phase development and transport properties of BiŽPb.-2223rAg composite tapes. However, it is expected to be more beneficial to have the powder initially as fine and homogeneous as possible to increase its reactivity, which would expedite the phase transformation process and to give a more uniform microstructure, although it is speculative that for precursor powders with the average weighted mean diameter in particle sizes of 1 m m or less, other problems might be encountered, such as too high a reactivity and contamination potential, etc. With flat-rolling as the intermediate mechanical deformation process between sintering periods, it has been conclusively shown that there exists a criticalintermediate-rolling-reduction-zone within which superior performance of BiŽPb.-2223rAg and alloy of Ag composite tapes can be produced. This reduction zone is found to depend on filamentary characteristics and sample preparation methods.
Acknowledgements Work at Los Alamos National Laboratory was performed under the auspices of the United States Department of Energy, Office of Energy Manage-
References w1x J.G. Bednorz, K.A. Muller, Z. Phys. B 64 Ž1986. 189–193. ¨ w2x K. Sato, T. Hikata, Y. Iwasa, Appl. Phys. Lett. 57 Ž1990. 1928. w3x T. Hikata, M. Ueyama, H. Mukai, K.-I. Sato, Cryogenics Nov. 30, 1990, pp. 924–929. w4x D.P. Hampshire, S.S. Oh, K. Osamura, D.C. Larbalestier, Supercond. Sci. Technol. 3 Ž1990. 560–563. w5x M. Daumling, G. Grasso, A. Perin, R. Flukiger, Supercond. Sci. Technol. 7 Ž1994. 926–930. w6x G. Grasso, R. Flukiger, Supercond. Sci. Technol. 10 Ž1997. ¨ 223–226. w7x N.V. Vo, S.X. Dou, H.K. Liu, Physica C 250 Ž1995. 139–152. w8x N.V. Vo, H.K. Liu, S.X. Dou, E.W. Collings, Proc. 2nd PACRIM, Cairns, Australia, July 15–17, 1996. w9x N.V. Vo, H.K. Liu, S.X. Dou, E.W. Collings, Proc. 16th ICECrICMC, Kitakyushu, Japan, May 20–24, 1996. w10x N.V. Vo, S.X. Dou, H.K. Liu, IEEE Trans. Appl. Supercond. 6 Ž2. Ž1996. 102–105. w11x N.V. Vo, H.K. Liu, S.X. Dou, J. Mater. Res. 11 Ž5. Ž1996. 1101–1107. w12x N.V. Vo, H.K. Liu, S.X. Dou, Supercond. Sci. Technol. 9 Ž1996. 104–112. w13x N.V. Vo, H.K. Liu, S.X. Dou, IEEE Trans. Appl. Supercond., 1997, in press. w14x N.V. Vo, H.K. Liu, S.X. Dou, I. Kusevic, Mater. Sci. Eng. B, 1996, pp. 217–223. w15x Z. Han, P. Skov-Hansen, T. Freltoft, Supercond. Sci. Technol. 10 Ž1997. 371–387. w16x S.E. Schoenfeld, S. Ahzi, R.J. Asaro et al., Philos. Mag. A 73 Ž1996. 1591. w17x J.O. Willis, R.D. Ray II, T.G. Holesinger et al., Proc. 7th US-Japan Workshop on High Tc Superconductors, Tsukuba, Japan, 24–25 Oct 1995, K. Tachikawa ŽEd.., New Superconducing Materials Forum, The Society of Non-Traditional Technology, Tokyo, p. 96, 1996. w18x B.N. Hubert, R. Zhou, T.G. Holesinger et al., J. Elec. Mater. 24 Ž12. Ž1995. 1869–1872. w19x O. Dur, S. Nakamae, J. Schwartz, Y.S. Hascicek, presented at ASC96, Pittsburgh, PA, USA, 25–30 Aug., 1996.