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Superconducting joint of Y–Ba–Cu–O superconductors using Er–Ba–Cu–O solder
Physica C 370 (2002) 53–58 www.elsevier.com/locate/physc
Superconducting joint of Y–Ba–Cu–O superconductors using Er–Ba–Cu–O solder K. Iida *, J. Yoshioka, N. Sakai, M. Murakami Superconductivity Research Laboratory, International Superconductivity Technology Center, Division III, 3-35-2 Iioka-Shinden, Morioka, Iwate 020-0852, Japan Received 19 July 2001; received in revised form 9 August 2001; accepted 27 August 2001
Abstract Two melt-textured Y–Ba–Cu–O superconductor blocks were successfully welded with Er–Ba–Cu–O solder having a lower peritectic temperature than that of Y–Ba–Cu–O. The key to a strongly coupled joint was a combination of the control of crystal growth along h1 1 0i direction and the employment of a highly dense sintered Er–Ba–Cu–O bar as a solder material. Microstructural analyses revealed that no segregation of secondary phase and/or residual liquid phase was observed at the joint interface. Furthermore, it was found through magneto-optical observations that magnetic field could not penetrate into the joint, indicating that the two superconductors were strongly connected with this method. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Superconducting joint; Y–Ba–Cu–O; Er–Ba–Cu–O; Welding; Solder
1. Introduction Large grain Y–Ba–Cu–O superconductors have significant potential for industrial applications such as a levitation device and a quasi-permanent magnet, since they can trap large fields greater than conventional permanent magnet [1]. It is known that the field trapping capability of bulk superconductor is dependent on the critical current density ðJc Þ and the size of the superconductor without weak links [1,2]. Thus an enlargement of grain size is essentially important along
*
Corresponding author. Tel.: +81-19-635-9015; fax: +81-19635-9017. E-mail address: [email protected] (K. Iida).
with an increase in Jc values. Recent advances in processing techniques for bulk Y–Ba–Cu–O enabled the production of single-grain materials about 10 cm in diameter [3], which was produced with a top-seeded melt-growth process. However, the production of large-grain materials is extremely difficult especially when the size exceeds 10 cm. Hence several novel techniques have been developed for the enlargement of an effective grain size of bulk Y–Ba–Cu–O. Those are multi-seeded melt growth (MSMG) process [4–6] and welding or joining techniques [7–9]. All the MSMGprocessed samples however contain some grain boundaries that act as weak links. The welding technique has been successful in making superconducting joint for small samples. For joining two Y–Ba–Cu–O monoliths it is common to use
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 1 0 8 2 - 6
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K. Iida et al. / Physica C 370 (2002) 53–58
Yb–Ba–Cu–O [8,10–13], Tm–Ba–Cu–O [14,15] and Er–Ba–Cu–O [16,17] having a lower peritectic temperature than that of Y–Ba–Cu–O as a welding reagent. However, conventional welding techniques had problems that the secondary phase, residual liquid phase and oxygen bubbles were segregated at the final solidification front, which led to the interruption of supercurrent flow at the joint [11]. In this paper, we show that such segregation can be greatly reduced by controlling crystal growth direction using sintered bars as welding reagents, leading to the production of strongly coupled superconducting joints.
2. Experimental Rectangular specimens with dimensions of 3 mm 4 mm 5 mm were cut from commercial single-domain Y–Ba–Cu–O bulk (Nippon Steel Corporation) such that the joint surfaces were perpendicular to h1 0 0i and h1 1 0i directions, as shown in Fig. 1. Er–Ba–Cu–O solder was prepared from Er2 O3 , BaCO3 and CuO powders, which were weighed in a nominal composition of 0.75Er123 þ 0:25Er211 and mixed with an agate mortar and pestle for 3 h. Well-mixed powders were calcined at 890 °C in pure O2 for 24 h and pulverized, which was repeated for four times. In the final stage, 0.5 wt.% of Pt was added to the powders. Er–Ba–Cu–O precursor powders were pressed into rectangular bars 9 mm 5 mm 19 mm in size with uniaxial pressing, and then consolidated with cold isostatic pressing under a pressure of 2000 kg/cm2 . The sample was sintered at 980 °C for 10 h, which was then sliced into the plates of 0.5 mm thickness. A sliced sintered Er–Ba–Cu–O bar was sandwiched by two Y–Ba–Cu–O blocks with the surfaces perpendicular to h1 1 0i direction (denoted as a (1 1 0)/(1 1 0) joint hereafter). For comparative study, Y–Ba–Cu–O blocks were also welded with the surfaces perpendicular to h1 0 0i direction (denoted as a (1 0 0)/(1 0 0) joint hereafter). They were placed on a MgO single crystal substrate and inserted into a muffle furnace. The samples were heated to 995 °C for 3 h, kept for 1 h, slowly cooled to 945 °C for 100 h and finally cooled to
Fig. 1. Schematic illustration of how the Y–Ba–Cu–O blocks were cut from the massive bulk pellet.
room temperature. The whole process was conducted in air. Small specimens with dimensions of 2 mm 2:2 mm 0:5 mm were cut from the joint and subjected to oxygen annealing in flowing pure oxygen at 520 °C for 150 h. The microstructures of the samples were observed and analyzed with an electron probe microanalyzer (EPMA) and an optical microscope. DC magnetization measurements were performed with a SQUID magnetometer to determine Tc and Jc values for fields parallel to the c-axis. The Jc values were determined using the extended Bean’s model [18]. The field penetration along the supercon-
K. Iida et al. / Physica C 370 (2002) 53–58
ducting joint was observed with a magneto-optical (MO) technique.
3. Results and discussion Fig. 2 shows back-scattered electron compositional images (BEI-COMPO) and characteristic X-ray images of the joined samples. In the BEICOMPO, dark regions are Y–Ba–Cu–O and slightly light regions are Er–Ba–Cu–O, and white contrasted particles in the joint area are Er211
55
particles. In both (1 1 0)/(1 1 0) and (1 0 0)/(1 0 0) joints, no voids were observed in the whole area of the joint, while voids were often observed in the joint made by conventional methods [11]. Consequently, the employment of sintered Er–Ba–Cu–O bar as a solder was effective in reducing the formation of voids during the solidification. In the case of the (1 0 0)/(1 0 0) joint, however Er211 particles were segregated at the center of the joint. Furthermore, copper-rich and/or barium deficient phases, which were associated with the residual liquid phases, were also observed throughout the
Fig. 2. Results of electron probe microanalyses and characteristic X-ray images of the (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples.
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K. Iida et al. / Physica C 370 (2002) 53–58
Fig. 3. Temperature dependence of normalized v for (1 0 0)/ (1 0 0) and (1 1 0)/(1 1 0) joint samples.
joint interface. In contrast, for the (1 1 0)/(1 1 0) joint, no segregation of secondary phase and/or the residual liquid phase was observed at the joint. Fig. 3 shows the temperature dependence of the magnetization for (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples. The field of 10 Oe was applied parallel to the c-axis. The critical temperature Tc of around 91 K was obtained for both samples, showing that the welding process had no appreciable effect on Tc . Fig. 4 shows the field dependence of Jc (77 K and H kc-axis) for the joined samples. One can see that (1 1 0)/(1 1 0) joint sam-
ple has larger Jc and higher irreversibility field than those of (1 0 0)/(1 0 0) joint. Fig. 5 shows the MO images of the flux penetration at 77 K for (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples. In the case of (1 0 0)/(1 0 0) joint, magnetic field preferentially penetrated through the joint even at 200 Oe, which indicates that two monoliths of Y–Ba–Cu–O were weakly jointed. In a great contrast, no preferential penetration of field was observed along the (1 1 0)/(1 1 0) joint even at 1000 Oe, showing that two Y–Ba–Cu–O blocks were strongly connected. Strong coupling at the (1 1 0)/(1 1 0) joint can be explained in terms of a crystal growth mode in the Er–Ba–Cu–O joint. Fig. 6 shows the crosssectional view of Er–Ba–Cu–O grown with a (1 1 0)-oriented Y–Ba–Cu–O crystal on top. This configuration can simulate the conditions for the crystal growth during the joining experiment. One can see that triangular Er–Ba–Cu–O crystal was grown from the seed with the facet surface tilted about 45° from the seed surface. Likewise, when the joining is performed along the (1 1 0) plane, the growth front is not parallel to the surface but tilted about 45°, and thereby the residual liquid phase and small 211 particles are pushed away from the interface. On the other hand, the growth front for the (1 0 0)/(1 0 0) joint is always parallel to the joint surface so that the residual liquid phase and small 211 particles are pushed toward the center of the joint, leading to the formation of weak links in the joined region. Although we first speculated that the temperature gradient would be necessary in order effectively to remove the residual liquid from the interface, the present results showed that a clean interface could be obtained without temperature gradient. Thus we believe that the present technique can be applied to larger samples.
4. Summary
Fig. 4. Field dependence of Jc (77 K, H kc) for (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples.
We have succeeded in producing a strongly coupled superconducting Y–Ba–Cu–O joint by combining the control of crystal growth along h1 1 0i direction and the employment of a highly
K. Iida et al. / Physica C 370 (2002) 53–58
57
Fig. 5. MO images showing the flux penetration (77 K, Hkc) for (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples.
dense Er–Ba–Cu–O sintered solder. No segregation of the secondary phase and/or the residual liquid
was observed at the joint, leading to the formation of strongly connected superconducting joint.
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K. Iida et al. / Physica C 370 (2002) 53–58
Fig. 6. The cross-sectional view of Er–Ba–Cu–O grown with Y–Ba–Cu–O crystal with (1 1 0) surface on top. The sample was quenched on the way of crystallization.
Acknowledgements This work was supported by New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technology for Superconductivity Applications. Two of the authors (K.I. and J.Y.) would like to thank the Iwate Industrial Promotion Center for their financial support.
References [1] M. Murakami (Ed.), Melt Processed High-Temperature Superconductors, World Scientific, Singapore, 1992. [2] Y. Iwasa, H. Lee, Cryogenics 37 (1997) 807. [3] T. Fujimoto, M. Morita, N. Masahashi, T. Kaneko, Proceedings of EUCAS’99, Balcelona, 1999. [4] P. Sch€ atzle, G. Krabbes, G. St€ over, G. Fuchs, D. Schl€afer, Supercond. Sci. Technol. 12 (1999) 69. [5] Y.A. Jee, C.-J. Kim, T.-H. Sung, G.-W. Hong, Supercond. Sci. Technol. 13 (2000) 195. [6] C.-J. Kim, H.-J. Kim, Y.A. Jee, G.-W. Hong, J.-H. Joo, S.-C. Han, Y.-H. Han, T.-H. Sung, S.-J. Kim, Physica C 338 (2000) 205.
[7] K. Salama, V. Selvamanickam, Appl. Phys. Lett. 60 (1992) 898. [8] K. Kimura, K. Miyamoto, M. Hashimoto, Advances in Superconductivity VII, Springer, Tokyo, 1995, p. 681. [9] N. Sakai, D.N. Matthews, R. Hedderich, H. Takaichi, M. Murakami, Advances in Superconductivity VI, Springer, Tokyo, 1994, p. 803. [10] G.J. Schmitz, A. Tigges, J.C. Schmidt, Supercond. Sci. Technol. 11 (1998) 73. [11] J. Maeda, S. Seiki, M. Kambara, Y. Nakamura, T. Izumi, Y. Shiohara, Advances in Superconductivity XII, Springer, Tokyo, 2000, p. 449. [12] S.M. Mukhopadhyay, N. Mahadev, S. Sengupta, Physica C 329 (2000) 95. [13] M.P. Delamare, H. Walter, B. Bringmann, A. Leenders, H.C. Freyhardt, Physica C 329 (2000) 166. [14] H. Zheng, M. Jiang, R. Nikolova, U. Welp, A.P. Paulikas, Yi. Huang, G.W. Crabtree, B.W. Veal, Physica C 322 (1999) 1. [15] B.W. Veal, H. Zheng, H. Claus, L. Chen, A.P. Paulikas, A. Koshelev, G.W. Crabtree, Extended Abstracts of the 2000 International Workshop on Superconductivity, 2000, p. 211. [16] S.J. Manton, C. Beduz, Y. Yang, IEEE Trans. Appl. Supercond. 9 (2) (1999) 2089. [17] M.P. Delamare, C. Jooss, B. Bringmann, H. Walter, A. Leenders, H.C. Freyhardt, IEEE Trans. Appl. Supercond. 11 (1) (2001) 3788. [18] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250.
Superconducting joint of Y–Ba–Cu–O superconductors using Er–Ba–Cu–O solder K. Iida *, J. Yoshioka, N. Sakai, M. Murakami Superconductivity Research Laboratory, International Superconductivity Technology Center, Division III, 3-35-2 Iioka-Shinden, Morioka, Iwate 020-0852, Japan Received 19 July 2001; received in revised form 9 August 2001; accepted 27 August 2001
Abstract Two melt-textured Y–Ba–Cu–O superconductor blocks were successfully welded with Er–Ba–Cu–O solder having a lower peritectic temperature than that of Y–Ba–Cu–O. The key to a strongly coupled joint was a combination of the control of crystal growth along h1 1 0i direction and the employment of a highly dense sintered Er–Ba–Cu–O bar as a solder material. Microstructural analyses revealed that no segregation of secondary phase and/or residual liquid phase was observed at the joint interface. Furthermore, it was found through magneto-optical observations that magnetic field could not penetrate into the joint, indicating that the two superconductors were strongly connected with this method. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Superconducting joint; Y–Ba–Cu–O; Er–Ba–Cu–O; Welding; Solder
1. Introduction Large grain Y–Ba–Cu–O superconductors have significant potential for industrial applications such as a levitation device and a quasi-permanent magnet, since they can trap large fields greater than conventional permanent magnet [1]. It is known that the field trapping capability of bulk superconductor is dependent on the critical current density ðJc Þ and the size of the superconductor without weak links [1,2]. Thus an enlargement of grain size is essentially important along
*
Corresponding author. Tel.: +81-19-635-9015; fax: +81-19635-9017. E-mail address: [email protected] (K. Iida).
with an increase in Jc values. Recent advances in processing techniques for bulk Y–Ba–Cu–O enabled the production of single-grain materials about 10 cm in diameter [3], which was produced with a top-seeded melt-growth process. However, the production of large-grain materials is extremely difficult especially when the size exceeds 10 cm. Hence several novel techniques have been developed for the enlargement of an effective grain size of bulk Y–Ba–Cu–O. Those are multi-seeded melt growth (MSMG) process [4–6] and welding or joining techniques [7–9]. All the MSMGprocessed samples however contain some grain boundaries that act as weak links. The welding technique has been successful in making superconducting joint for small samples. For joining two Y–Ba–Cu–O monoliths it is common to use
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 1 0 8 2 - 6
54
K. Iida et al. / Physica C 370 (2002) 53–58
Yb–Ba–Cu–O [8,10–13], Tm–Ba–Cu–O [14,15] and Er–Ba–Cu–O [16,17] having a lower peritectic temperature than that of Y–Ba–Cu–O as a welding reagent. However, conventional welding techniques had problems that the secondary phase, residual liquid phase and oxygen bubbles were segregated at the final solidification front, which led to the interruption of supercurrent flow at the joint [11]. In this paper, we show that such segregation can be greatly reduced by controlling crystal growth direction using sintered bars as welding reagents, leading to the production of strongly coupled superconducting joints.
2. Experimental Rectangular specimens with dimensions of 3 mm 4 mm 5 mm were cut from commercial single-domain Y–Ba–Cu–O bulk (Nippon Steel Corporation) such that the joint surfaces were perpendicular to h1 0 0i and h1 1 0i directions, as shown in Fig. 1. Er–Ba–Cu–O solder was prepared from Er2 O3 , BaCO3 and CuO powders, which were weighed in a nominal composition of 0.75Er123 þ 0:25Er211 and mixed with an agate mortar and pestle for 3 h. Well-mixed powders were calcined at 890 °C in pure O2 for 24 h and pulverized, which was repeated for four times. In the final stage, 0.5 wt.% of Pt was added to the powders. Er–Ba–Cu–O precursor powders were pressed into rectangular bars 9 mm 5 mm 19 mm in size with uniaxial pressing, and then consolidated with cold isostatic pressing under a pressure of 2000 kg/cm2 . The sample was sintered at 980 °C for 10 h, which was then sliced into the plates of 0.5 mm thickness. A sliced sintered Er–Ba–Cu–O bar was sandwiched by two Y–Ba–Cu–O blocks with the surfaces perpendicular to h1 1 0i direction (denoted as a (1 1 0)/(1 1 0) joint hereafter). For comparative study, Y–Ba–Cu–O blocks were also welded with the surfaces perpendicular to h1 0 0i direction (denoted as a (1 0 0)/(1 0 0) joint hereafter). They were placed on a MgO single crystal substrate and inserted into a muffle furnace. The samples were heated to 995 °C for 3 h, kept for 1 h, slowly cooled to 945 °C for 100 h and finally cooled to
Fig. 1. Schematic illustration of how the Y–Ba–Cu–O blocks were cut from the massive bulk pellet.
room temperature. The whole process was conducted in air. Small specimens with dimensions of 2 mm 2:2 mm 0:5 mm were cut from the joint and subjected to oxygen annealing in flowing pure oxygen at 520 °C for 150 h. The microstructures of the samples were observed and analyzed with an electron probe microanalyzer (EPMA) and an optical microscope. DC magnetization measurements were performed with a SQUID magnetometer to determine Tc and Jc values for fields parallel to the c-axis. The Jc values were determined using the extended Bean’s model [18]. The field penetration along the supercon-
K. Iida et al. / Physica C 370 (2002) 53–58
ducting joint was observed with a magneto-optical (MO) technique.
3. Results and discussion Fig. 2 shows back-scattered electron compositional images (BEI-COMPO) and characteristic X-ray images of the joined samples. In the BEICOMPO, dark regions are Y–Ba–Cu–O and slightly light regions are Er–Ba–Cu–O, and white contrasted particles in the joint area are Er211
55
particles. In both (1 1 0)/(1 1 0) and (1 0 0)/(1 0 0) joints, no voids were observed in the whole area of the joint, while voids were often observed in the joint made by conventional methods [11]. Consequently, the employment of sintered Er–Ba–Cu–O bar as a solder was effective in reducing the formation of voids during the solidification. In the case of the (1 0 0)/(1 0 0) joint, however Er211 particles were segregated at the center of the joint. Furthermore, copper-rich and/or barium deficient phases, which were associated with the residual liquid phases, were also observed throughout the
Fig. 2. Results of electron probe microanalyses and characteristic X-ray images of the (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples.
56
K. Iida et al. / Physica C 370 (2002) 53–58
Fig. 3. Temperature dependence of normalized v for (1 0 0)/ (1 0 0) and (1 1 0)/(1 1 0) joint samples.
joint interface. In contrast, for the (1 1 0)/(1 1 0) joint, no segregation of secondary phase and/or the residual liquid phase was observed at the joint. Fig. 3 shows the temperature dependence of the magnetization for (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples. The field of 10 Oe was applied parallel to the c-axis. The critical temperature Tc of around 91 K was obtained for both samples, showing that the welding process had no appreciable effect on Tc . Fig. 4 shows the field dependence of Jc (77 K and H kc-axis) for the joined samples. One can see that (1 1 0)/(1 1 0) joint sam-
ple has larger Jc and higher irreversibility field than those of (1 0 0)/(1 0 0) joint. Fig. 5 shows the MO images of the flux penetration at 77 K for (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples. In the case of (1 0 0)/(1 0 0) joint, magnetic field preferentially penetrated through the joint even at 200 Oe, which indicates that two monoliths of Y–Ba–Cu–O were weakly jointed. In a great contrast, no preferential penetration of field was observed along the (1 1 0)/(1 1 0) joint even at 1000 Oe, showing that two Y–Ba–Cu–O blocks were strongly connected. Strong coupling at the (1 1 0)/(1 1 0) joint can be explained in terms of a crystal growth mode in the Er–Ba–Cu–O joint. Fig. 6 shows the crosssectional view of Er–Ba–Cu–O grown with a (1 1 0)-oriented Y–Ba–Cu–O crystal on top. This configuration can simulate the conditions for the crystal growth during the joining experiment. One can see that triangular Er–Ba–Cu–O crystal was grown from the seed with the facet surface tilted about 45° from the seed surface. Likewise, when the joining is performed along the (1 1 0) plane, the growth front is not parallel to the surface but tilted about 45°, and thereby the residual liquid phase and small 211 particles are pushed away from the interface. On the other hand, the growth front for the (1 0 0)/(1 0 0) joint is always parallel to the joint surface so that the residual liquid phase and small 211 particles are pushed toward the center of the joint, leading to the formation of weak links in the joined region. Although we first speculated that the temperature gradient would be necessary in order effectively to remove the residual liquid from the interface, the present results showed that a clean interface could be obtained without temperature gradient. Thus we believe that the present technique can be applied to larger samples.
4. Summary
Fig. 4. Field dependence of Jc (77 K, H kc) for (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples.
We have succeeded in producing a strongly coupled superconducting Y–Ba–Cu–O joint by combining the control of crystal growth along h1 1 0i direction and the employment of a highly
K. Iida et al. / Physica C 370 (2002) 53–58
57
Fig. 5. MO images showing the flux penetration (77 K, Hkc) for (1 0 0)/(1 0 0) and (1 1 0)/(1 1 0) joint samples.
dense Er–Ba–Cu–O sintered solder. No segregation of the secondary phase and/or the residual liquid
was observed at the joint, leading to the formation of strongly connected superconducting joint.
58
K. Iida et al. / Physica C 370 (2002) 53–58
Fig. 6. The cross-sectional view of Er–Ba–Cu–O grown with Y–Ba–Cu–O crystal with (1 1 0) surface on top. The sample was quenched on the way of crystallization.
Acknowledgements This work was supported by New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technology for Superconductivity Applications. Two of the authors (K.I. and J.Y.) would like to thank the Iwate Industrial Promotion Center for their financial support.
References [1] M. Murakami (Ed.), Melt Processed High-Temperature Superconductors, World Scientific, Singapore, 1992. [2] Y. Iwasa, H. Lee, Cryogenics 37 (1997) 807. [3] T. Fujimoto, M. Morita, N. Masahashi, T. Kaneko, Proceedings of EUCAS’99, Balcelona, 1999. [4] P. Sch€ atzle, G. Krabbes, G. St€ over, G. Fuchs, D. Schl€afer, Supercond. Sci. Technol. 12 (1999) 69. [5] Y.A. Jee, C.-J. Kim, T.-H. Sung, G.-W. Hong, Supercond. Sci. Technol. 13 (2000) 195. [6] C.-J. Kim, H.-J. Kim, Y.A. Jee, G.-W. Hong, J.-H. Joo, S.-C. Han, Y.-H. Han, T.-H. Sung, S.-J. Kim, Physica C 338 (2000) 205.
[7] K. Salama, V. Selvamanickam, Appl. Phys. Lett. 60 (1992) 898. [8] K. Kimura, K. Miyamoto, M. Hashimoto, Advances in Superconductivity VII, Springer, Tokyo, 1995, p. 681. [9] N. Sakai, D.N. Matthews, R. Hedderich, H. Takaichi, M. Murakami, Advances in Superconductivity VI, Springer, Tokyo, 1994, p. 803. [10] G.J. Schmitz, A. Tigges, J.C. Schmidt, Supercond. Sci. Technol. 11 (1998) 73. [11] J. Maeda, S. Seiki, M. Kambara, Y. Nakamura, T. Izumi, Y. Shiohara, Advances in Superconductivity XII, Springer, Tokyo, 2000, p. 449. [12] S.M. Mukhopadhyay, N. Mahadev, S. Sengupta, Physica C 329 (2000) 95. [13] M.P. Delamare, H. Walter, B. Bringmann, A. Leenders, H.C. Freyhardt, Physica C 329 (2000) 166. [14] H. Zheng, M. Jiang, R. Nikolova, U. Welp, A.P. Paulikas, Yi. Huang, G.W. Crabtree, B.W. Veal, Physica C 322 (1999) 1. [15] B.W. Veal, H. Zheng, H. Claus, L. Chen, A.P. Paulikas, A. Koshelev, G.W. Crabtree, Extended Abstracts of the 2000 International Workshop on Superconductivity, 2000, p. 211. [16] S.J. Manton, C. Beduz, Y. Yang, IEEE Trans. Appl. Supercond. 9 (2) (1999) 2089. [17] M.P. Delamare, C. Jooss, B. Bringmann, H. Walter, A. Leenders, H.C. Freyhardt, IEEE Trans. Appl. Supercond. 11 (1) (2001) 3788. [18] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250.