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Ag tapes
Physica C 314 Ž1999. 139–146
Much enhanced Jc by intermediate rolling in just-rolled Tl-1223rAg tapes D.Y. Jeong a
a,)
, H.K. Kim a , Y.C. Kim
b
Korea Electrotechnology Research Institute, Changwon 641-120, South Korea b Pusan National UniÕersity, Pusan 609-735, South Korea
Received 7 December 1998; revised 25 December 1998; accepted 27 January 1999
Abstract Tl-1223rAg tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2 Ca 2.2 Cu 3 Oz nominal composition were prepared using the powder-in-tube method incorporating an in situ reaction method. Connectivity and directional alignment of plate-like Tl-1223 grains were much enhanced by incorporating an intermediate rolling during the final heat-treatment process. As a result, high Jc values near 2.5 y 10 4 Arcm2 at 77 K and 0 T with an excellent reproducibility were obtained in just-rolled tapes. The high Jc’s seem to result from relatively easy recovery of excellent grain-connectivity during the final heat-treatment after intermediate rolling, probably due to retarded Tl evaporation and excessive Ca content in the present composition. The strong field dependence of Jc even in low fields, however, indicated that there still existed significant weak-links and the degree of directional grain-alignment was far from the desired one. The high Jc is ascribed to enhanced contact areas between Tl-1223 grains rather than enhanced directional grain-alignment. The intergranular binding in the tapes seemed to be mainly dominated by an SIS junctional type. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Tl-1223rAg tapes; High Jc ; Directional grain-alignment; Powder-in-tube method
1. Introduction It is known that Tl-1223 superconductor has an irreversibility line which is located at a relatively high position in H–T space, compared to those of other high-Tc superconductors w1x. Therefore, it has been expected that Tl-1223 wires with large currentcarrying capacities at relatively high fields and temperatures will be able to develop and realize 40 K technology instead of 20 K in the case of Bi-based superconducting wires. )
Corresponding author. Tel.: q82-551-80-1608; Fax: q82551-80-1696; E-mail: [email protected]
Different from remarkable advances w2x in aspects of Jc and preparation costs in Tl-1223 film-type tapes prepared by so-called open methods, the progress in preparation of Tl-1223 tapes with high Jc ’s by using the powder-in-tube ŽPIT. method has been slowed down. Tl-1223 tapes prepared by the PIT method have revealed relatively low Jc ’s due to poor grain-connectivity and little directional grainalignment w1,3x, although various chemical compositions and preparation methods have been tried w3–9x. The Jc values were primarily dependent on the nominal compositions used for the tape cores. 2.1 = 10 4 Arcm2 was obtained in Tl 0.93 Bi 0.22 Sr1.6 Ba 0.4Ca 2 Cu 3 Oz composition w7x as a highest reproducible
0921-4534r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 1 4 7 - 1
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Jc at 77 K and 0 T among pressed tapes, and 1.8 = 10 4 Arcm2 in Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2Cu 3 Oz w9x among just-rolled tapes, which is more important in practical applications. The present authors have also tried several chemical compositions to overcome the poor grain-connectivity and the weak tendency of directional grainalignment in Tl-1223 tapes w9–11x. As a result of such efforts, they could prepare just-rolled Tl1223rAg tapes with much enhanced grain-connectivity and directional grain-alignment, and thus high Jc ’s near 2.5 = 10 4 Arcm2 at 77 K and 0 T by using a Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2 Ca 2.2 Cu 3 Oz nominal composition. In the present study, the grain morphology, the changes in morphology and Jc with the thermomechanical treatment ŽTMT. history, the field dependence of Jc and the nature of intergranular bonding in the tapes are presented.
X-ray diffractometer ŽXRD. using a Cu K a target. The XRD measurements were performed after pealing off the Ag-sheath on one side using a blade. The microstructure of the tapes was examined using a Hitachi S-2700 scanning electron microscope ŽSEM.. The microstructures of core surfaces of the tapes were examined after pealing off the Ag-sheath on one side using a blade. Magnetization was measured in fields up to 6 T parallel to the tape surface, by using a Quantum Design model MPMS superconducting quantum interference device ŽSQUID. magnetometer. The field dependence of transport Jc was evaluated in fields up to 1 T perpendicular to the tape surface and the current flow direction, while the tape was immersed into liquid nitrogen. The field was applied using a Lakeshore EM4-HV electromagnet. ac susceptibilities were measured using Lakeshore 7000 ac susceptometer in ac fields of 1 kHz in a range of 0.01–19 Oe parallel to the tape surface.
2. Experimental Tl-1223rAg tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2Ca 2.2 Cu 3 Oz were prepared using the power-in-tube method incorporating an in situ reaction method as follows. A mixture of Sr–Ba–Ca–Cu–O prepowder and Tl 2 O 3 , PbO and Bi 2 O 3 powders in a stoichiometric ratio of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2 Ca 2.2 Cu 3 Oz was filled into a Ag-tube. Here 0.1 mol of excessive Tl 2 O 3 was added to compensate Tl loss during the tape preparation process, which was estimated by an ion-coupled plasma analysis. The Ag-tubes were drawn to a wire ; 1.1 mm in diameter and then rolled to tapes 0.5 mm in thickness using a twin roller 150 mm in diameter. The tape was cut to tapes 1 m long and heat-treated in 820–8408C for 10–30 min. Then, the tape was rolled to a tape 0.095–0.115 mm thick and cut to pieces ; 5 cm long. The tape pieces were heat-treated in 820–8508C up to 20 h. In some of the tapes, an intermediate rolling process was incorporated during the final heat-treatment. The details on preparation of the tapes are given elsewhere w9x. Jc ’s of the tapes were evaluated using dc fourprobe electrical measurements after immersing them into liquid nitrogen. The phases present in the tape cores were investigated using a Philips PW 1710
3. Results and discussion Table 1 shows variations of Ic and Jc of as-prepared tapes with the TMT history. The tape which was heat-treated at 8408C for 20 min, rolled and heat-treated at 8408C for 7 h, showed a Jc comparable to the highest Jc in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6Ba 0.4 Ca 2 Cu 3 Oz tapes w9x. The Jc decreased with increasing the heat-treatment time. By the way, it is worthwhile to note in Table 1 that upon incorporating an intermediate rolling during final heat-treatment for 14 h, the Jc did not severely drop, but more or less increased. This behavior is quite different from that in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes w9x. So, the change in Jc with the TMT history when an intermediate rolling during the final heattreatment was incorporated, was investigated. The results are shown in Table 2. High Jc ’s near 2.5 = 10 4 Arcm2 at 77 K and 0 T were obtained in two tapes shortly heat-treated, rolled, heat-treated at 8408C for 4 h, rolled and heat-treated again for 4 h. Then to check out the reproducibility of the Jc value, two more tapes were prepared again using the same preparation procedure, and their Jc ’s were compared. The Jc values of four tapes were consistent within an uncertainty of 2000 Arcm2 , as shown in Table 3.
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Table 1 Variations of Ic and Jc at 77 K and 0 T with the history of the thermo-mechanical treatment in as-prepared tapes Thermo-mechanical treatment history
Thickness Žmm.
Ic ŽA.
Jc ŽArcm2 .
8208Cr20 min q R q 8308Cr7 h q R q 8308Cr7 h 8308Cr20 min q R q 8208Cr7 h 8308Cr20 min q R q 8308Cr7 h 8308Cr20 min q R q 8308Cr7 h q R q 8308Cr7 h 8308Cr20 min q R q 8408Cr7 h 8308Cr20 min q R q 8458Cr7 h 8308Cr20 min q R q 8508Cr7 h 8408Cr20 min q R q 8208Cr7 h 8408Cr20 min q R q 8308Cr7 h 8408Cr20 min q R q 8308Cr7 h q R q 8308Cr7 h 8408Cr20 min q R q 8308Cr7 h q R q 8408Cr7 h 8408Cr20 min q R q 8408Cr4 h 8408Cr20 min q R q 8408Cr6 h 8408Cr20 min q R q 8408Cr7 h 8408Cr20 min q R q 8408Cr8 h 8408Cr20 min q R q 8408Cr10 h 8408Cr20 min q R q 8408Cr15 h 8408Cr20 min q R q 8408Cr20 h 8408Cr20 min q R q 8458Cr7 h 8408Cr20 min q R q 8508Cr7 h
0.113 0.102 0.102 0.108 0.125 0.115 0.119 0.108 0.112 0.109 0.098 0.125 0.114 0.112 0.119 0.139 0.151 0.141 0.114 0.131
16.19 14.30 17.04 11.12 14.28 14.60 12.65 15.52 16.67 14.40 18.79 12.35 16.45 16.92 17.19 13.74 11.61 13.08 16.72 14.87
15 900 15 300 17 700 11 300 12 800 14 200 12 000 15 600 16 600 14 900 20 900 11 800 15 900 16 700 15 900 11 900 9300 11 100 16 300 12 500
Fig. 1 shows XRD patterns taken in Ža. a tape with Jc of 16 600 Arcm2 , which was heat-treated at 8408C for 20 min, rolled and heat-treated at 8308C for 7 h, Žb. a tape with Jc of 20 900 Arcm2 , which was prepared by rolling the tape Ža. and heat-treating again at 8408C for 7 h, and Žc. a tape with Jc of 25 200 Arcm2 , which was heat-treated at 8408C for 20 min, rolled, heat-treated at 8408C for 4 h, rolled and heat-treated again for 4 h. Most peaks in Fig. 1
correspond to Tl-1223 phase. The distinctions of Fig. 1 from XRD patterns in tapes of other compositions are as follows: First, the incorporation of the intermediate rolling resulted in a significant increase in intensities of Ž00 l . peaks, as recognized by comparing Fig. 1Ža. to Žb,c.. Such prominent Ž00 l . peaks shown in Fig. 1Žb,c. indicate that Tl-1223 grains in the tapes are aligned in a preferred orientation in a certain degree, and were not reported in just-rolled
Table 2 Variations of Ic and Jc at 77 K and 0 T with the history of the thermo-mechanical treatment in the tapes, which were prepared by incorporating an intermediate rolling during final heat-treatment Thermo-mechanical treatment history
Thickness Žmm.
Ic ŽA.
Jc ŽArcm2 .
8408Cr20 min q R q 8408Cr1 h q R q 8408Cr6 h 8408Cr20 min q R q 8408Cr2 h q R q 8408Cr5 h 8408Cr20 min q R q 8408Cr3 h q R q 8408Cr3 h 8408Cr20 min q R q 8408Cr3 h q R q 8408Cr4 h 8408Cr20 min q R q 8408Cr3 h q R q 8408Cr5 h 8408Cr20 min q R q 8408Cr3 h q R q 8408Cr6 h 8408Cr20 min q R q 8408Cr4 h q R q 8408Cr3 h 8408Cr20 min q R q 8408Cr4 h q R q 8408Cr4 h 8408Cr20 min q R q 8408Cr4 h q R q 8408Cr5 h 8408Cr20 min q R q 8408Cr4 h q R q 8408Cr6 h
0.112 0.098 0.109 0.106 0.112 0.112 0.092 0.090 0.101 0.098
9.12 15.50 13.20 18.67 10.94 12.87 19.10 21.23 18.54 17.47
8900 17 000 13 100 18 600 10 800 12 500 22 300 25 200 19 200 18 400
D.Y. Jeong et al.r Physica C 314 (1999) 139–146
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Table 3 Reproducibility of Ic , Jc and engineering critical current density Ž Je . obtained in four tapes prepared by using the same preparation method Trial no.
Thickness Žmm.
Ic ŽA.
Jc ŽArcm2 .
Je ŽArcm2 .
1
0.090 0.095
21.81 21.70
24 900 23 500
6200 5900
2
0.090 0.096
21.23 20.90
25 200 23 400
6300 5900
Each time, two tapes were prepared. The tapes were heat-treated at 8408C for 20 min, rolled, heattreated at 8408C for 4 h, rolled and heat-treated again at 8408C for 4 h.
Tl-1223 tapes of other compositions. They were only observed in a polycrystalline bulk of the Tl 0.93Bi 0.22 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz composition pelletized using a very high pressure and then heat-treated w12x. It is estimated that a much lower pressure was applied during rolling in the present study. Therefore, Fig. 1 implies that the intermediate rolling is very effective in directional grain-alignment in the present composition. Second, ŽSr,Ca.14 Cu 24 O x peak, which appeared strongly as an strong evidence of decomposition of Tl-1223 phase resulting from severe Tl evaporation due to relatively long heat-treatment w9,13x, appeared with much reduced intensity in Fig. 1Žb.. The reduced intensity of ŽSr,Ca.14 Cu 24 O x peak in Fig. 1Žb. may indicate that the decomposition of Tl-1223 phase is less severe in the present composition, and be a strong evidence for Jc increase in the tape heat-treated for 14 h with an intermediate rolling shown in Table 1, as will be discussed later. Third, BaPbO 3 peak, which appeared strongly in tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 O z w9x, appeared with a noticeably reduced intensity in Fig. 1Ža,c.. It indicates that the formation and growth mechanism of Tl-1223 phase in the present composition may be different from that in Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2Cu 3 Oz . Fig. 2 shows SEM images taken in polished longitudinal surfaces of Ža. a tape heat-treated at 8408C for 20 min, rolled and heat-treated at 8408C for 4 h, and of Žb. a tape with Jc of 22 300 Arcm2 prepared by rolling the tape Ža. and heat-treating at 8408C for 3 h. In Fig. 2, plate-like Tl-1223 grains tend to be
directionally aligned, at least in local regions, different from the cases of other compositions w3,6,8,9x. It can be recognized that degrees of directional grainalignment and densification were much enhanced upon incorporating an intermediate rolling during the final heat-treatment. The visible enhancement of directional grain-alignment shown in Fig. 2Žb. supports the enhanced intensities of Ž00 l . peaks in Fig. 1Žb.. Fig. 2Žb. also shows a possibility that Jc and its field dependence can be much enhanced provided that a proper thermo-mechanical treatment, which results in directional grain-alignment in a long range order, is found. Fig. 3Ža. shows magnetic hysteresis loops measured in the tape with Jc of 24 900 Arcm2 Ž Ic s 21.81 A. at 77 K and 0 T at selected temperatures. Fig. 3Žb. shows the field dependence of magnetiza-
Fig. 1. XRD patterns taken in Ža. a tape with Jc of 16 600 Arcm2 heat-treated at 8408C for 20 min, rolled and heat-treated at 8308C for 7 h, Žb. a tape with Jc of 20 900 Arcm2 prepared by rolling the tape Ža. and heat-treating again at 8408C for 7 h, and Žc. a tape with Jc of 25 200 Arcm 2 heat-treated at 8408C for 20 min, rolled, heat-treated at 8408C for 4 h, rolled and heat-treated again at 8408C for 4 h.
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Oe, and is saturated near 3000 Oe. Jc at 77 K and 1 T is estimated to be 350 Arcm2 , which is lower by two orders of magnitude than the intragranular Jc Ž Jc,m .. Such a great difference between intergranular and intragranular Jc ’s and the strong field dependence of Jc indicate that there exist significant weak-links in the tape, despite of such a high Jc at 77 K and 0 T, and the degree of directional grainalignment observed is far from a desired one. Based on the report of Nabatame et al. w14x, in which a great difference between intergranular and intragranular Jc ’s by two orders of magnitude started to appear in Tl-1223 thin film when the angles of tilt grain boundaries began to be greater than 108, the
Fig. 2. SEM images taken in polished longitudinal surfaces of Ža. a tape heat-treated at 8408C for 20 min, rolled and heat-treated at 8408C for 4 h, and Žb. a tape with Jc of 22 300 Arcm2 prepared by rolling the tape Ža. and heat-treating at 8408C for 3 h.
tion Jc Ž Jc,m . calculated using the Bean formula Jc,m s 20D Mrd from the magnetization hysteresis shown in Fig. 3Ža.. Here D M is the magnitude of magnetization hysteresis and d the average grain size, which is estimated to 7 mm. Jc,m at 77 K and 1 T of the tape is estimated to 50 000 Arcm2 , which is higher by twice than that in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6Ba 0.4 Ca 2 Cu 3 Oz tape w9x. However, the dependence of Jc,m on temperature and field at the high temperature and high field region appeared to be stronger in the present tapes than in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6Ba 0.4 Ca 2 Cu 3 Oz tapes, indicating that the irreversible line of the present tapes at the high temperature region may be located in a lower position than that of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes. Fig. 4 shows the field dependence of transport Jc measured at 77 K in a tape with Jc of 24 900 Arcm2 . The Jc starts to rapidly decrease near 50
Fig. 3. Ža. Magnetic hysteresis loops measured at selected temperatures in the tape with a transport Jc of 24 900 Arcm2 Ž Ic s 21.81 A.. Žb. Field dependence of magnetization Jc at selected temperatures calculated using the Bean formula Jc,m s 20 D Mr d from the magnetization hysteresis.
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Fig. 4. Field dependence of Jc measured at 77 K in a tape with Jc of 24 900 Arcm2 .
transport property in the present tape seems to be dominated by large-angle grain boundaries. However, Jc of the present tapes start to drop at a little higher field and is saturated to a little higher value than that of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz w9x. This indicates that a little larger portion of intergranular contact survives as current paths in high fields in the present tapes than in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6Ba 0.4 Ca 2 Cu 3 Oz tapes. Fig. 5Ža. shows the temperature dependence of real Ž x X . and imaginary Ž x Y . components of ac susceptibilities measured at ac magnetic fields Ž Hm . of 0.01–19 Oe in the tape with a Jc of 24 900 Arcm2 . In the framework of a ‘finite temperature’ critical state model for a granular superconductor suggested by Muller et al. w15–17x, x Y in Fig. 5Ža. represents the intergranular bulk pinning loss. The temperature ŽTp . in which maximum x Y appears under a specific magnitude of the applied ac field means a temperature in which the applied magnetic flux fully penetrates to the center of the specimen, i.e., a critical state is established in the specimen. Therefore, Jc ’s at Tp can be calculated by using the critical state formula Jc A Hm ra, where 2 a is the superconducting core thickness of the tape w18x. Then their logarithmic values were plotted to lnŽ1 y TprTc . in Fig. 5Žb. to evaluate the junctional characteristic of the intergranular contact by estimating the b value in Jc ŽT . A Ž1 y TrTc . b w19,20x. Here, Tc is the critical transition temperature, which is estimated to 119.0 K. As b is estimated to be 1.1 from Fig. 5Žb., the intergranular contacts in the superconduct-
ing core seem to be an SIS junction type. Also the Jc at 77 K and 0 T is estimated to 26 200 Arcm2 from Fig. 5Žb., and is consistent with the measured transport Jc . Tl-1223 grains in the present tapes are plate-like and directionally aligned, at least local regions, different from those in tapes of other compositions. Furthermore, incorporation of the intermediate rolling in the final heat-treatment process enhanced degrees of densification and directional grain-alignment, and thus Jc . By the way, such an Jc enhancement by incorporating the intermediate rolling has not been observed in just-rolled Tl-1223 tapes. In the case of Tl 0.93 Bi 0.22 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes, Jc could be enhanced by intermediate pressing, not intermediate rolling w7x. In the case of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4-
Fig. 5. Ža. Temperature dependence of ac susceptibilities measured at selected fields in the tape with Jc of 24 900 Arcm2 and Žb. Ž1yTp r Tc . dependence of Jc calculated by applying Ža. to the critical state formula Jc f Hm r a.
D.Y. Jeong et al.r Physica C 314 (1999) 139–146
Ca 2 Cu 3 Oz tapes w9x, Jc severely dropped upon incorporating the intermediate rolling during the final heat-treatment for 14 h. The severe Jc decrease upon the intermediate rolling was ascribed to the significant decomposition of Tl-1223 phase resulting from severe Tl evaporation during the relatively long heat-treatment w9x. In the present tapes heat-treated for around 14 h, however, the Jc value increased by more than twice upon incorporating the intermediate rolling, as shown in Table 1. The Jc enhancement by incorporating the intermediate rolling is attributed to relatively facilitated grain-reconnection and microcrack heal-out during the final heat-treatment due to suppressed evaporation of Tl and excessive Ca content in the present composition. The possible explanations are as follows. Incorporation of the intermediate rolling during the final heat-treatment enhances degree of densification in the superconducting core, as well as directional grain-alignment in the present case. However, the intermediate rolling is also a process of grain fragmentation and microcrack generation. To get an excellent grain-connectivity and thus high Jc , the broken grains must be reconnected and the microcracks must be healed out during subsequent heattreatment. By the way, it is thought that reconnection of the broken grains and healing out of the microcracks are accommodated by formation and growth of new Tl-1223 phase from an unreacted mixture of Tl-1212 and ŽCa,Cu.-rich liquid phase w21x. It is expected, however, that severe Tl evaporation during the subsequent heat-treatment causes already-made Tl-1223 phase to decompose and new Tl-1223 phase to hardly form and grow by changing Tl content in the unreacted mixture, resulting in poor grain-connectivity and thus low Jc , as observed in Tl 0.8Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes w9x. However, in the present case where Tl evaporation is retarded, excellent grain-connectivity and thus high Jc can be achieved provided that formation and growth of new Tl-1223 phase participating in reconnection of broken grains is well controlled. Furthermore, as the present composition contains excessive Ca over the stoichiometric ratio of Tl-1223 phase, it will be beneficial to supply sufficient Ca necessary for the formation and growth of new Tl-1223, which is expected to slowly occur due to low mobility of Ca atoms.
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The reason why the intermediate pressing is effective in enhancing Jc and the intermediate rolling is not in other compositions seems to be as follows: It is well-known that rolling results in a higher degree of fragmentation and less densification and alignment than pressing does w21x, due to its lower magnitude of pressure and more complicated stress distribution. This means that rolling offers a higher density of channels for Tl evaporation and necessitates formation of a larger amount of new Tl-1223 for grain-reconnection and crack-healing during final heat-treatment. Therefore, it is expected that Tl-1223 grains after the intermediate rolling hardly reconnect each other, because a relatively small amount of unreacted mixture is available at that stage and severe Tl evaporation results in a shortage of Tl necessary for formation of new Tl-1223 and even decompose already-formed Tl-1223. By the way, the reason why the decomposition of Tl-1223 phase is suppressed in the present composition is left to be unknown at present. The b value Ž b s 1.1. of the present tape obtained from the ac susceptibility study appeared to be lower than that Ž b s 1.37, Ref. w22x. in Tl 0.8 Pb 0.2Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes, even though Jc value in the former tape is higher than that of the latter. This fact may mean that the latter tape has a larger fraction of SNS intergranular junction than the former, and thus the Jc value in Tl-1223 tapes depends on grain-connectivity rather than intergranular bonding nature. 4. Conclusion In the present study, the grain morphology, the changes in morphology and Jc with the TMT history, the field dependence of Jc and the nature of intergranular bonding were studied in Tl-1223rAg tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2 Ca 2.2 Cu 3 Oz nominal composition prepared using the powder-in-tube method incorporating an in situ reaction method. The tape core showed the morphology in which plate-like Tl-1223 grains tend to be aligned in preferred orientations, at least in local regions. Furthermore, connectivity and directional alignment of the grains were much enhanced by incorporating an intermediate rolling during the final heat-treatment process. As a result, high Jc values near 2.5 y 10 4 Arcm2 at 77
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K and 0 T with an excellent reproducibility were obtained in just-rolled tapes. The strong field dependence of Jc even in low fields, however, indicated that there still existed significant weak-links and the degree of directional grain-alignment was far from the desired one. The intergranular binding in the tapes seemed to be mainly dominated by an SIS junctional type. The high Jc ’s seem to result from relatively easy recovery of the excellent grain-connectivity during the final heat-treatment after the intermediate rolling, probably due to retarded Tl evaporation and excessive Ca content in the present composition, and are attributed to enhanced contact areas between Tl-1223 grains rather than enhanced directional grain-alignment. The present study shows a possibility that Jc and its field dependence can be enhanced provided that a proper thermo-mechanical treatment, which results in directional grain-alignment in a long range order, is found. Acknowledgements The authors express their gratitude to the Korean Ministry of Science and Technology for their financial support. References w1x M. Jergel, A. Conde Gallardo, C. Falcony Guajardo, V. Strbik, Supercond. Sci. Technol. 9 Ž1996. 427. w2x National Renewable Energy Lab. Report, Superconductor Week, Vol. 12, No. 9, 1, 1998.
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Much enhanced Jc by intermediate rolling in just-rolled Tl-1223rAg tapes D.Y. Jeong a
a,)
, H.K. Kim a , Y.C. Kim
b
Korea Electrotechnology Research Institute, Changwon 641-120, South Korea b Pusan National UniÕersity, Pusan 609-735, South Korea
Received 7 December 1998; revised 25 December 1998; accepted 27 January 1999
Abstract Tl-1223rAg tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2 Ca 2.2 Cu 3 Oz nominal composition were prepared using the powder-in-tube method incorporating an in situ reaction method. Connectivity and directional alignment of plate-like Tl-1223 grains were much enhanced by incorporating an intermediate rolling during the final heat-treatment process. As a result, high Jc values near 2.5 y 10 4 Arcm2 at 77 K and 0 T with an excellent reproducibility were obtained in just-rolled tapes. The high Jc’s seem to result from relatively easy recovery of excellent grain-connectivity during the final heat-treatment after intermediate rolling, probably due to retarded Tl evaporation and excessive Ca content in the present composition. The strong field dependence of Jc even in low fields, however, indicated that there still existed significant weak-links and the degree of directional grain-alignment was far from the desired one. The high Jc is ascribed to enhanced contact areas between Tl-1223 grains rather than enhanced directional grain-alignment. The intergranular binding in the tapes seemed to be mainly dominated by an SIS junctional type. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Tl-1223rAg tapes; High Jc ; Directional grain-alignment; Powder-in-tube method
1. Introduction It is known that Tl-1223 superconductor has an irreversibility line which is located at a relatively high position in H–T space, compared to those of other high-Tc superconductors w1x. Therefore, it has been expected that Tl-1223 wires with large currentcarrying capacities at relatively high fields and temperatures will be able to develop and realize 40 K technology instead of 20 K in the case of Bi-based superconducting wires. )
Corresponding author. Tel.: q82-551-80-1608; Fax: q82551-80-1696; E-mail: [email protected]
Different from remarkable advances w2x in aspects of Jc and preparation costs in Tl-1223 film-type tapes prepared by so-called open methods, the progress in preparation of Tl-1223 tapes with high Jc ’s by using the powder-in-tube ŽPIT. method has been slowed down. Tl-1223 tapes prepared by the PIT method have revealed relatively low Jc ’s due to poor grain-connectivity and little directional grainalignment w1,3x, although various chemical compositions and preparation methods have been tried w3–9x. The Jc values were primarily dependent on the nominal compositions used for the tape cores. 2.1 = 10 4 Arcm2 was obtained in Tl 0.93 Bi 0.22 Sr1.6 Ba 0.4Ca 2 Cu 3 Oz composition w7x as a highest reproducible
0921-4534r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 1 4 7 - 1
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Jc at 77 K and 0 T among pressed tapes, and 1.8 = 10 4 Arcm2 in Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2Cu 3 Oz w9x among just-rolled tapes, which is more important in practical applications. The present authors have also tried several chemical compositions to overcome the poor grain-connectivity and the weak tendency of directional grainalignment in Tl-1223 tapes w9–11x. As a result of such efforts, they could prepare just-rolled Tl1223rAg tapes with much enhanced grain-connectivity and directional grain-alignment, and thus high Jc ’s near 2.5 = 10 4 Arcm2 at 77 K and 0 T by using a Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2 Ca 2.2 Cu 3 Oz nominal composition. In the present study, the grain morphology, the changes in morphology and Jc with the thermomechanical treatment ŽTMT. history, the field dependence of Jc and the nature of intergranular bonding in the tapes are presented.
X-ray diffractometer ŽXRD. using a Cu K a target. The XRD measurements were performed after pealing off the Ag-sheath on one side using a blade. The microstructure of the tapes was examined using a Hitachi S-2700 scanning electron microscope ŽSEM.. The microstructures of core surfaces of the tapes were examined after pealing off the Ag-sheath on one side using a blade. Magnetization was measured in fields up to 6 T parallel to the tape surface, by using a Quantum Design model MPMS superconducting quantum interference device ŽSQUID. magnetometer. The field dependence of transport Jc was evaluated in fields up to 1 T perpendicular to the tape surface and the current flow direction, while the tape was immersed into liquid nitrogen. The field was applied using a Lakeshore EM4-HV electromagnet. ac susceptibilities were measured using Lakeshore 7000 ac susceptometer in ac fields of 1 kHz in a range of 0.01–19 Oe parallel to the tape surface.
2. Experimental Tl-1223rAg tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2Ca 2.2 Cu 3 Oz were prepared using the power-in-tube method incorporating an in situ reaction method as follows. A mixture of Sr–Ba–Ca–Cu–O prepowder and Tl 2 O 3 , PbO and Bi 2 O 3 powders in a stoichiometric ratio of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2 Ca 2.2 Cu 3 Oz was filled into a Ag-tube. Here 0.1 mol of excessive Tl 2 O 3 was added to compensate Tl loss during the tape preparation process, which was estimated by an ion-coupled plasma analysis. The Ag-tubes were drawn to a wire ; 1.1 mm in diameter and then rolled to tapes 0.5 mm in thickness using a twin roller 150 mm in diameter. The tape was cut to tapes 1 m long and heat-treated in 820–8408C for 10–30 min. Then, the tape was rolled to a tape 0.095–0.115 mm thick and cut to pieces ; 5 cm long. The tape pieces were heat-treated in 820–8508C up to 20 h. In some of the tapes, an intermediate rolling process was incorporated during the final heat-treatment. The details on preparation of the tapes are given elsewhere w9x. Jc ’s of the tapes were evaluated using dc fourprobe electrical measurements after immersing them into liquid nitrogen. The phases present in the tape cores were investigated using a Philips PW 1710
3. Results and discussion Table 1 shows variations of Ic and Jc of as-prepared tapes with the TMT history. The tape which was heat-treated at 8408C for 20 min, rolled and heat-treated at 8408C for 7 h, showed a Jc comparable to the highest Jc in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6Ba 0.4 Ca 2 Cu 3 Oz tapes w9x. The Jc decreased with increasing the heat-treatment time. By the way, it is worthwhile to note in Table 1 that upon incorporating an intermediate rolling during final heat-treatment for 14 h, the Jc did not severely drop, but more or less increased. This behavior is quite different from that in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes w9x. So, the change in Jc with the TMT history when an intermediate rolling during the final heattreatment was incorporated, was investigated. The results are shown in Table 2. High Jc ’s near 2.5 = 10 4 Arcm2 at 77 K and 0 T were obtained in two tapes shortly heat-treated, rolled, heat-treated at 8408C for 4 h, rolled and heat-treated again for 4 h. Then to check out the reproducibility of the Jc value, two more tapes were prepared again using the same preparation procedure, and their Jc ’s were compared. The Jc values of four tapes were consistent within an uncertainty of 2000 Arcm2 , as shown in Table 3.
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Table 1 Variations of Ic and Jc at 77 K and 0 T with the history of the thermo-mechanical treatment in as-prepared tapes Thermo-mechanical treatment history
Thickness Žmm.
Ic ŽA.
Jc ŽArcm2 .
8208Cr20 min q R q 8308Cr7 h q R q 8308Cr7 h 8308Cr20 min q R q 8208Cr7 h 8308Cr20 min q R q 8308Cr7 h 8308Cr20 min q R q 8308Cr7 h q R q 8308Cr7 h 8308Cr20 min q R q 8408Cr7 h 8308Cr20 min q R q 8458Cr7 h 8308Cr20 min q R q 8508Cr7 h 8408Cr20 min q R q 8208Cr7 h 8408Cr20 min q R q 8308Cr7 h 8408Cr20 min q R q 8308Cr7 h q R q 8308Cr7 h 8408Cr20 min q R q 8308Cr7 h q R q 8408Cr7 h 8408Cr20 min q R q 8408Cr4 h 8408Cr20 min q R q 8408Cr6 h 8408Cr20 min q R q 8408Cr7 h 8408Cr20 min q R q 8408Cr8 h 8408Cr20 min q R q 8408Cr10 h 8408Cr20 min q R q 8408Cr15 h 8408Cr20 min q R q 8408Cr20 h 8408Cr20 min q R q 8458Cr7 h 8408Cr20 min q R q 8508Cr7 h
0.113 0.102 0.102 0.108 0.125 0.115 0.119 0.108 0.112 0.109 0.098 0.125 0.114 0.112 0.119 0.139 0.151 0.141 0.114 0.131
16.19 14.30 17.04 11.12 14.28 14.60 12.65 15.52 16.67 14.40 18.79 12.35 16.45 16.92 17.19 13.74 11.61 13.08 16.72 14.87
15 900 15 300 17 700 11 300 12 800 14 200 12 000 15 600 16 600 14 900 20 900 11 800 15 900 16 700 15 900 11 900 9300 11 100 16 300 12 500
Fig. 1 shows XRD patterns taken in Ža. a tape with Jc of 16 600 Arcm2 , which was heat-treated at 8408C for 20 min, rolled and heat-treated at 8308C for 7 h, Žb. a tape with Jc of 20 900 Arcm2 , which was prepared by rolling the tape Ža. and heat-treating again at 8408C for 7 h, and Žc. a tape with Jc of 25 200 Arcm2 , which was heat-treated at 8408C for 20 min, rolled, heat-treated at 8408C for 4 h, rolled and heat-treated again for 4 h. Most peaks in Fig. 1
correspond to Tl-1223 phase. The distinctions of Fig. 1 from XRD patterns in tapes of other compositions are as follows: First, the incorporation of the intermediate rolling resulted in a significant increase in intensities of Ž00 l . peaks, as recognized by comparing Fig. 1Ža. to Žb,c.. Such prominent Ž00 l . peaks shown in Fig. 1Žb,c. indicate that Tl-1223 grains in the tapes are aligned in a preferred orientation in a certain degree, and were not reported in just-rolled
Table 2 Variations of Ic and Jc at 77 K and 0 T with the history of the thermo-mechanical treatment in the tapes, which were prepared by incorporating an intermediate rolling during final heat-treatment Thermo-mechanical treatment history
Thickness Žmm.
Ic ŽA.
Jc ŽArcm2 .
8408Cr20 min q R q 8408Cr1 h q R q 8408Cr6 h 8408Cr20 min q R q 8408Cr2 h q R q 8408Cr5 h 8408Cr20 min q R q 8408Cr3 h q R q 8408Cr3 h 8408Cr20 min q R q 8408Cr3 h q R q 8408Cr4 h 8408Cr20 min q R q 8408Cr3 h q R q 8408Cr5 h 8408Cr20 min q R q 8408Cr3 h q R q 8408Cr6 h 8408Cr20 min q R q 8408Cr4 h q R q 8408Cr3 h 8408Cr20 min q R q 8408Cr4 h q R q 8408Cr4 h 8408Cr20 min q R q 8408Cr4 h q R q 8408Cr5 h 8408Cr20 min q R q 8408Cr4 h q R q 8408Cr6 h
0.112 0.098 0.109 0.106 0.112 0.112 0.092 0.090 0.101 0.098
9.12 15.50 13.20 18.67 10.94 12.87 19.10 21.23 18.54 17.47
8900 17 000 13 100 18 600 10 800 12 500 22 300 25 200 19 200 18 400
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Table 3 Reproducibility of Ic , Jc and engineering critical current density Ž Je . obtained in four tapes prepared by using the same preparation method Trial no.
Thickness Žmm.
Ic ŽA.
Jc ŽArcm2 .
Je ŽArcm2 .
1
0.090 0.095
21.81 21.70
24 900 23 500
6200 5900
2
0.090 0.096
21.23 20.90
25 200 23 400
6300 5900
Each time, two tapes were prepared. The tapes were heat-treated at 8408C for 20 min, rolled, heattreated at 8408C for 4 h, rolled and heat-treated again at 8408C for 4 h.
Tl-1223 tapes of other compositions. They were only observed in a polycrystalline bulk of the Tl 0.93Bi 0.22 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz composition pelletized using a very high pressure and then heat-treated w12x. It is estimated that a much lower pressure was applied during rolling in the present study. Therefore, Fig. 1 implies that the intermediate rolling is very effective in directional grain-alignment in the present composition. Second, ŽSr,Ca.14 Cu 24 O x peak, which appeared strongly as an strong evidence of decomposition of Tl-1223 phase resulting from severe Tl evaporation due to relatively long heat-treatment w9,13x, appeared with much reduced intensity in Fig. 1Žb.. The reduced intensity of ŽSr,Ca.14 Cu 24 O x peak in Fig. 1Žb. may indicate that the decomposition of Tl-1223 phase is less severe in the present composition, and be a strong evidence for Jc increase in the tape heat-treated for 14 h with an intermediate rolling shown in Table 1, as will be discussed later. Third, BaPbO 3 peak, which appeared strongly in tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 O z w9x, appeared with a noticeably reduced intensity in Fig. 1Ža,c.. It indicates that the formation and growth mechanism of Tl-1223 phase in the present composition may be different from that in Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2Cu 3 Oz . Fig. 2 shows SEM images taken in polished longitudinal surfaces of Ža. a tape heat-treated at 8408C for 20 min, rolled and heat-treated at 8408C for 4 h, and of Žb. a tape with Jc of 22 300 Arcm2 prepared by rolling the tape Ža. and heat-treating at 8408C for 3 h. In Fig. 2, plate-like Tl-1223 grains tend to be
directionally aligned, at least in local regions, different from the cases of other compositions w3,6,8,9x. It can be recognized that degrees of directional grainalignment and densification were much enhanced upon incorporating an intermediate rolling during the final heat-treatment. The visible enhancement of directional grain-alignment shown in Fig. 2Žb. supports the enhanced intensities of Ž00 l . peaks in Fig. 1Žb.. Fig. 2Žb. also shows a possibility that Jc and its field dependence can be much enhanced provided that a proper thermo-mechanical treatment, which results in directional grain-alignment in a long range order, is found. Fig. 3Ža. shows magnetic hysteresis loops measured in the tape with Jc of 24 900 Arcm2 Ž Ic s 21.81 A. at 77 K and 0 T at selected temperatures. Fig. 3Žb. shows the field dependence of magnetiza-
Fig. 1. XRD patterns taken in Ža. a tape with Jc of 16 600 Arcm2 heat-treated at 8408C for 20 min, rolled and heat-treated at 8308C for 7 h, Žb. a tape with Jc of 20 900 Arcm2 prepared by rolling the tape Ža. and heat-treating again at 8408C for 7 h, and Žc. a tape with Jc of 25 200 Arcm 2 heat-treated at 8408C for 20 min, rolled, heat-treated at 8408C for 4 h, rolled and heat-treated again at 8408C for 4 h.
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Oe, and is saturated near 3000 Oe. Jc at 77 K and 1 T is estimated to be 350 Arcm2 , which is lower by two orders of magnitude than the intragranular Jc Ž Jc,m .. Such a great difference between intergranular and intragranular Jc ’s and the strong field dependence of Jc indicate that there exist significant weak-links in the tape, despite of such a high Jc at 77 K and 0 T, and the degree of directional grainalignment observed is far from a desired one. Based on the report of Nabatame et al. w14x, in which a great difference between intergranular and intragranular Jc ’s by two orders of magnitude started to appear in Tl-1223 thin film when the angles of tilt grain boundaries began to be greater than 108, the
Fig. 2. SEM images taken in polished longitudinal surfaces of Ža. a tape heat-treated at 8408C for 20 min, rolled and heat-treated at 8408C for 4 h, and Žb. a tape with Jc of 22 300 Arcm2 prepared by rolling the tape Ža. and heat-treating at 8408C for 3 h.
tion Jc Ž Jc,m . calculated using the Bean formula Jc,m s 20D Mrd from the magnetization hysteresis shown in Fig. 3Ža.. Here D M is the magnitude of magnetization hysteresis and d the average grain size, which is estimated to 7 mm. Jc,m at 77 K and 1 T of the tape is estimated to 50 000 Arcm2 , which is higher by twice than that in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6Ba 0.4 Ca 2 Cu 3 Oz tape w9x. However, the dependence of Jc,m on temperature and field at the high temperature and high field region appeared to be stronger in the present tapes than in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6Ba 0.4 Ca 2 Cu 3 Oz tapes, indicating that the irreversible line of the present tapes at the high temperature region may be located in a lower position than that of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes. Fig. 4 shows the field dependence of transport Jc measured at 77 K in a tape with Jc of 24 900 Arcm2 . The Jc starts to rapidly decrease near 50
Fig. 3. Ža. Magnetic hysteresis loops measured at selected temperatures in the tape with a transport Jc of 24 900 Arcm2 Ž Ic s 21.81 A.. Žb. Field dependence of magnetization Jc at selected temperatures calculated using the Bean formula Jc,m s 20 D Mr d from the magnetization hysteresis.
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Fig. 4. Field dependence of Jc measured at 77 K in a tape with Jc of 24 900 Arcm2 .
transport property in the present tape seems to be dominated by large-angle grain boundaries. However, Jc of the present tapes start to drop at a little higher field and is saturated to a little higher value than that of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz w9x. This indicates that a little larger portion of intergranular contact survives as current paths in high fields in the present tapes than in the Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6Ba 0.4 Ca 2 Cu 3 Oz tapes. Fig. 5Ža. shows the temperature dependence of real Ž x X . and imaginary Ž x Y . components of ac susceptibilities measured at ac magnetic fields Ž Hm . of 0.01–19 Oe in the tape with a Jc of 24 900 Arcm2 . In the framework of a ‘finite temperature’ critical state model for a granular superconductor suggested by Muller et al. w15–17x, x Y in Fig. 5Ža. represents the intergranular bulk pinning loss. The temperature ŽTp . in which maximum x Y appears under a specific magnitude of the applied ac field means a temperature in which the applied magnetic flux fully penetrates to the center of the specimen, i.e., a critical state is established in the specimen. Therefore, Jc ’s at Tp can be calculated by using the critical state formula Jc A Hm ra, where 2 a is the superconducting core thickness of the tape w18x. Then their logarithmic values were plotted to lnŽ1 y TprTc . in Fig. 5Žb. to evaluate the junctional characteristic of the intergranular contact by estimating the b value in Jc ŽT . A Ž1 y TrTc . b w19,20x. Here, Tc is the critical transition temperature, which is estimated to 119.0 K. As b is estimated to be 1.1 from Fig. 5Žb., the intergranular contacts in the superconduct-
ing core seem to be an SIS junction type. Also the Jc at 77 K and 0 T is estimated to 26 200 Arcm2 from Fig. 5Žb., and is consistent with the measured transport Jc . Tl-1223 grains in the present tapes are plate-like and directionally aligned, at least local regions, different from those in tapes of other compositions. Furthermore, incorporation of the intermediate rolling in the final heat-treatment process enhanced degrees of densification and directional grain-alignment, and thus Jc . By the way, such an Jc enhancement by incorporating the intermediate rolling has not been observed in just-rolled Tl-1223 tapes. In the case of Tl 0.93 Bi 0.22 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes, Jc could be enhanced by intermediate pressing, not intermediate rolling w7x. In the case of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4-
Fig. 5. Ža. Temperature dependence of ac susceptibilities measured at selected fields in the tape with Jc of 24 900 Arcm2 and Žb. Ž1yTp r Tc . dependence of Jc calculated by applying Ža. to the critical state formula Jc f Hm r a.
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Ca 2 Cu 3 Oz tapes w9x, Jc severely dropped upon incorporating the intermediate rolling during the final heat-treatment for 14 h. The severe Jc decrease upon the intermediate rolling was ascribed to the significant decomposition of Tl-1223 phase resulting from severe Tl evaporation during the relatively long heat-treatment w9x. In the present tapes heat-treated for around 14 h, however, the Jc value increased by more than twice upon incorporating the intermediate rolling, as shown in Table 1. The Jc enhancement by incorporating the intermediate rolling is attributed to relatively facilitated grain-reconnection and microcrack heal-out during the final heat-treatment due to suppressed evaporation of Tl and excessive Ca content in the present composition. The possible explanations are as follows. Incorporation of the intermediate rolling during the final heat-treatment enhances degree of densification in the superconducting core, as well as directional grain-alignment in the present case. However, the intermediate rolling is also a process of grain fragmentation and microcrack generation. To get an excellent grain-connectivity and thus high Jc , the broken grains must be reconnected and the microcracks must be healed out during subsequent heattreatment. By the way, it is thought that reconnection of the broken grains and healing out of the microcracks are accommodated by formation and growth of new Tl-1223 phase from an unreacted mixture of Tl-1212 and ŽCa,Cu.-rich liquid phase w21x. It is expected, however, that severe Tl evaporation during the subsequent heat-treatment causes already-made Tl-1223 phase to decompose and new Tl-1223 phase to hardly form and grow by changing Tl content in the unreacted mixture, resulting in poor grain-connectivity and thus low Jc , as observed in Tl 0.8Pb 0.2 Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes w9x. However, in the present case where Tl evaporation is retarded, excellent grain-connectivity and thus high Jc can be achieved provided that formation and growth of new Tl-1223 phase participating in reconnection of broken grains is well controlled. Furthermore, as the present composition contains excessive Ca over the stoichiometric ratio of Tl-1223 phase, it will be beneficial to supply sufficient Ca necessary for the formation and growth of new Tl-1223, which is expected to slowly occur due to low mobility of Ca atoms.
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The reason why the intermediate pressing is effective in enhancing Jc and the intermediate rolling is not in other compositions seems to be as follows: It is well-known that rolling results in a higher degree of fragmentation and less densification and alignment than pressing does w21x, due to its lower magnitude of pressure and more complicated stress distribution. This means that rolling offers a higher density of channels for Tl evaporation and necessitates formation of a larger amount of new Tl-1223 for grain-reconnection and crack-healing during final heat-treatment. Therefore, it is expected that Tl-1223 grains after the intermediate rolling hardly reconnect each other, because a relatively small amount of unreacted mixture is available at that stage and severe Tl evaporation results in a shortage of Tl necessary for formation of new Tl-1223 and even decompose already-formed Tl-1223. By the way, the reason why the decomposition of Tl-1223 phase is suppressed in the present composition is left to be unknown at present. The b value Ž b s 1.1. of the present tape obtained from the ac susceptibility study appeared to be lower than that Ž b s 1.37, Ref. w22x. in Tl 0.8 Pb 0.2Bi 0.2 Sr1.6 Ba 0.4 Ca 2 Cu 3 Oz tapes, even though Jc value in the former tape is higher than that of the latter. This fact may mean that the latter tape has a larger fraction of SNS intergranular junction than the former, and thus the Jc value in Tl-1223 tapes depends on grain-connectivity rather than intergranular bonding nature. 4. Conclusion In the present study, the grain morphology, the changes in morphology and Jc with the TMT history, the field dependence of Jc and the nature of intergranular bonding were studied in Tl-1223rAg tapes of Tl 0.8 Pb 0.2 Bi 0.2 Sr1.8 Ba 0.2 Ca 2.2 Cu 3 Oz nominal composition prepared using the powder-in-tube method incorporating an in situ reaction method. The tape core showed the morphology in which plate-like Tl-1223 grains tend to be aligned in preferred orientations, at least in local regions. Furthermore, connectivity and directional alignment of the grains were much enhanced by incorporating an intermediate rolling during the final heat-treatment process. As a result, high Jc values near 2.5 y 10 4 Arcm2 at 77
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K and 0 T with an excellent reproducibility were obtained in just-rolled tapes. The strong field dependence of Jc even in low fields, however, indicated that there still existed significant weak-links and the degree of directional grain-alignment was far from the desired one. The intergranular binding in the tapes seemed to be mainly dominated by an SIS junctional type. The high Jc ’s seem to result from relatively easy recovery of the excellent grain-connectivity during the final heat-treatment after the intermediate rolling, probably due to retarded Tl evaporation and excessive Ca content in the present composition, and are attributed to enhanced contact areas between Tl-1223 grains rather than enhanced directional grain-alignment. The present study shows a possibility that Jc and its field dependence can be enhanced provided that a proper thermo-mechanical treatment, which results in directional grain-alignment in a long range order, is found. Acknowledgements The authors express their gratitude to the Korean Ministry of Science and Technology for their financial support. References w1x M. Jergel, A. Conde Gallardo, C. Falcony Guajardo, V. Strbik, Supercond. Sci. Technol. 9 Ž1996. 427. w2x National Renewable Energy Lab. Report, Superconductor Week, Vol. 12, No. 9, 1, 1998.
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