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Effect of silver addition on the field trapping properties of Gd–Ba–Cu–O bulk superconductors
Physica C 378–381 (2002) 774–778 www.elsevier.com/locate/physc
Effect of silver addition on the field trapping properties of Gd–Ba–Cu–O bulk superconductors S. Nariki *, N. Sakai, M. Matsui, M. Murakami Superconductivity Research Laboratory, ISTEC, 1-16-25 Shibaura, Minato-ku, Tokyo 105-0023, Japan Received 27 September 2001; accepted 9 January 2002
Abstract The effect of Ag addition on the field trapping properties of Gd–Ba–Cu–O bulk samples has been studied. Gd–Ba– Cu–O bulk superconductors 32 mm in diameter were melt textured by varying the amount of Ag2 O in the range of 0–30 wt.%. The trapped magnetic field of Ag-free bulk sample was 1.3 T at 77 K. This value could be improved to 1.8–2.0 T with Ag addition of 10–20 wt.%. A further increase in Ag2 O content degraded the field trapping ability. Ag-free bulk Gd–Ba–Cu–O exhibited the trapped field of 3.0 T at 65 K, however, the bulk was fractured at lower temperature. On the other hand, Ag-added bulk Gd–Ba–Cu–O exhibited the high trapped field value of 6.7 T at 55 K. These results indicate that the mechanical strength of the bulk Gd–Ba–Cu–O could be improved with Ag addition. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.25.L; 74.80.B Keywords: Melt processing; Trapped magnetic field; Mechanical property; Ag; Gd–Ba–Cu–O
1. Introduction For the enhancement of trapped magnetic field of bulk superconductors, it is important to increase the critical current density (Jc ) and/or the size of single-domain. However, the defects such as cracks and high-angle grain boundaries deteriorate the field trapping ability. In addition, the maximum trapped-field is restricted by the mechanical strength when the sample traps high fields at low temperatures, since the cracks are formed with increasing electromagnetic force [1–5]. Therefore,
*
Corresponding author. Tel.: +81-3-3454-9284; fax: +81-33454-9287. E-mail address: [email protected] (S. Nariki).
the improvement in mechanical strength is critically important for achieving the high trapped magnetic field at low temperatures. We found that the Ag addition was very effective in reducing the cracks and thus in dramatically improving the mechanical properties of Gd–Ba–Cu–O bulk materials [6]. In this paper, we report the effect of Ag addition on the field trapping properties of Gd– Ba–Cu–O bulk superconductors.
2. Experimental Large single-grain Gd–Ba–Cu–O bulk samples 32 mm in diameter and 20 mm in thickness were prepared from the precursors with a molar ratio of GdBa2 Cu3 O7 (Gd123):Gd2 BaCuO5 (Gd211) ¼ 2:1
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 5 4 1 - 1
S. Nariki et al. / Physica C 378–381 (2002) 774–778
by the top-seeded melt growth technique. The size of Gd211 starting powder was 1.0 lm, which was prepared by the calcination of the powders of Gd2 O3 , BaO2 and CuO. 0.5 wt.% of Pt and 0–30 wt.% of Ag2 O were also added in the mixed powders. The melt processing was performed under controlled oxygen partial pressure of 1% O2 in Ar. The details of melt-processing conditions were described elsewhere [6,7]. The melt-textured sample was annealed at 400–450 °C for 300 h in flowing pure O2 gas. The small specimens with the dimensions of about 1:5 1:5 1:0 mm3 were cut from the bulk sample. Measurements of critical temperature (Tc ) and Jc were performed with a quantum design SQUID magnetometer [7]. The measurements of trapped magnetic fields were carried out by magnetizing the bulk samples using a 10 T superconducting magnet. The bulk samples were cooled to the liquid nitrogen temperature in the presence of the magnetic field 7 T applied parallel to the c-axis. After the removal of the external field, the profile of trapped magnetic flux density was measured by scanning Hall probe sensors [8]. The total gap between the top surface of the sample and the active area of the Hall sensor was adjusted to be 1.2 mm, which included the thickness of a mold of the sensor, 0.7 mm. For the trapped field measurements at low temperatures, the bulk sample was cooled by a refrigerator. The trapped field was recorded using a Hall sensor that was directly glued on the surface of a bulk superconductor.
3. Results and discussion Fig. 1 shows the trapped field distribution of bulk Gd–Ba–Cu–O samples with various Ag contents at 77 K. As indicated in Fig. 1(a), the trapped magnetic field of Ag-free bulk sample was 1.3 T at 77 K. The profile of this sample shows that the bulk was grown into a single-grain, however, its irregular shape suggests the presence of some weak-links. In contrast the profile of the Gd–Ba– Cu–O samples with 10–30 wt.% Ag2 O exhibited a circular shape with no irregularity. We reported that the amount of cracks in the Gd–Ba–Cu–O bulk samples is dramatically reduced with Ag ad-
775
dition [6]. The circular shape in the field distribution reflects a reduction of weak-links. The trapped fields of the bulk Gd–Ba–Cu–O samples with 10 and 20 wt.% Ag2 O exhibited high values of 1.8 and 2.0 T, respectively. However, the trapped field of the sample with 30 wt.% Ag2 O addition was lowered to 1.1 T. Fig. 2 shows Jc –B curves for the specimens with various Ag contents. The Jc values increased up to 20 wt.% of Ag2 O addition, which led to the enhancement in trapped magnetic field. As presented on Y–Ba–Cu–O [9], the Ag addition may enhance the Jc in the intermediate fields by the substitution of Ag with Cu. However, the Jc value greatly lowered with 30 wt.% Ag2 O addition. A large decrease in the volume fraction of the Gd123 superconducting phase may lead to a depression of the Jc values, as reported in Sm–Ba–Cu–O and Nd– Ba–Cu–O systems [10,11]. However, further investigation will be needed to understand the effect of Ag addition on the Jc –B properties. It is well known that the trapped fields of bulk superconductors increase dramatically with lowering temperature due to a drastic increase in Jc values [1–5,10–13]. Fig. 3 shows the temperature dependence of the trapped magnetic field for bulk Gd–Ba–Cu–O samples with 0 and 20 wt.% Ag2 O. In order to clarify the effect of Ag addition on mechanical properties, the measurements were performed on the as-grown bulk samples without reinforcement by the metal ring or epoxy resin. Ag-free Gd–Ba–Cu–O exhibited the trapped field of 3.0 T at 65 K, however, the sample was broken during the measurements at lower temperatures due to a electromagnetic force. In contrast, Agadded bulk Gd–Ba–Cu–O did not fracture and could trap the higher field of 6.7 T at 55 K. These results show the mechanical strength is improved with the addition of Ag. Fig. 4 shows the fracture surfaces of Ag-free and Ag-added bulk samples. In the fracture surface of Ag-free bulk, the flat mirror region indicating crack propagation was clearly observed. The site of crack initiation, in which the area with high concentration of Gd211 was visible and indicated by the arrow in this figure. In contrast, it was difficult to assess the fracture origin in Ag-added sample and small lamellar facets were observed everywhere. This suggests that the addition of Ag leads to the reduction of serious defects
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S. Nariki et al. / Physica C 378–381 (2002) 774–778
Fig. 1. Trapped field distributions of the Gd–Ba–Cu–O bulk samples 32 mm in diameter with various Ag contents at 77 K.
Fig. 2. Jc –B curves for Gd–Ba–Cu–O specimens with various Ag contents at 77 K.
as crack nucleation sites and/or the suppression of crack growth. Several researchers pointed out that the trapped field and the levitation force drop with increasing the number of thermal cycles between room temperature and liquid nitrogen temperature for the measurements [14–17]. This decrease is explained by the generation and propagation of cracks inside the melt-textured bulk [14,17]. Hence, we studied the influence of thermal cycles on the trapped field of Gd–Ba–Cu–O bulks with and without Ag addition. Ag-free bulk 32 mm in diameter and the larger bulk 48 mm in diameter with 10 wt.% Ag2 O were used in this experiment. The bulk samples were immediately immersed in liquid nitrogen in applied field of 7 T. After the trapped field measurement, the bulks were warmed up to room temperature using hot air. These measurements
S. Nariki et al. / Physica C 378–381 (2002) 774–778
Fig. 3. Temperature dependence of the trapped magnetic field measured on the surface of Ag-free and Ag-added Gd–Ba– Cu–O bulk samples using Hall sensor. The measurement was performed without reinforcement by the metal ring or resin.
777
Fig. 5. Relationship between the trapped field at 77 K for the Ag-free bulk 32 mm in diameter and the Ag-added bulk 48 mm in diameter and the number of measurement tests. The values of trapped field were normalized by the initial trapped field values of 1.3 T for Ag-free bulk and 2.0 T for Ag-added bulk, respectively.
were repeated five times. Fig. 5 shows the plots of the trapped field normalized by the initial trapped field versus the number of experiments. The trapped field of Ag-free bulk sample decreased with the number of measurement tests. On the other hand, the trapped field of Ag-added bulk was not degraded and stayed at a constant value regardless of thermal cycles. The enhancement of mechanical strength and thermal conductivity with Ag addition is very effective in depressing the deterioration of the trapped field by thermal cycles. 4. Summary
Fig. 4. Photographs showing the fracture surfaces of (a) Agfree and (b) Ag-added Gd–Ba–Cu–O bulk samples destroyed by the electromagnetic force in the trapped field measurement test. The arrow in figure (a) indicates the estimated site of crack initiation.
We studied the effect of Ag addition on the field trapping properties of OCMG-processed Gd–Ba– Cu–O bulk samples 32 mm in diameter. The trapped magnetic field of Ag-free bulk sample was 1.3 T at 77 K. This value could be improved with Ag addition. The trapped field of the bulk Gd–Ba– Cu–O samples with 10–20 wt.% Ag2 O exhibited high values of 1.8–2.0 T. The trapped fields at lower temperatures were also measured for the Agfree and 20 wt.% Ag2 O added bulk samples without reinforcement by the metal ring or resin. The
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S. Nariki et al. / Physica C 378–381 (2002) 774–778
Ag-free bulk exhibited the trapped field of 3.0 T at 65 K, however, the bulk was fractured at lower temperature due to an electromagnetic force. On the other hand, the Ag-added bulk exhibited the higher value of 6.7 T at 55 K. Furthermore, the trapped field of Ag-added sample did not deteriorate with thermal cycles.
Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as collaborative research and development of fundamental technologies for superconductivity applications.
References [1] Y. Ren, R. Weinstein, J. Liu, R.P. Sawh, C. Foster, Physica C 251 (1995) 15. [2] H. Ikuta, A. Mase, Y. Yanagi, M. Yoahikawa, Y. Itoh, T. Oka, U. Mizutani, Supercond. Sci. Technol. 11 (1998) 1345. [3] P. Sch€ atzle, G. Krabbes, S. Gruss, G. Fuchs, IEEE Trans. Appl. Supercond. 9 (1999) 2022.
[4] G. Krabbes, G. Fuchs, P. Sch€atzle, S. Gruss, J.W. Park, F. Hardinghaus, R. Hayn, S. Drechsler, Physica C 341–348 (2000) 2289. [5] S. Gruss, G. Fuchs, G. Krabbes, P. Verges, P. Sch€atzle, K.H. M€ uller, J. Fink, L. Schultz, IEEE Trans. Appl. Supercond. 11 (2001) 3720. [6] S. Nariki, N. Sakai, M. Tomita, M. Murakami, Physica C, in press. [7] S. Nariki, N. Sakai, M. Murakami, Physica C 341–348 (2000) 2409. [8] K. Nagashima, T. Higuchi, J. Sok, S.I. Yoo, H. Fujimoto, M. Murakami, Cryogenics 37 (1997) 577. [9] Y. Nakamura, K. Tachibana, T. Yoshida, H. Fujimoto, J. Jpn. Inst. Metals 63 (1999) 1089. [10] H. Ikuta, A. Mase, U. Mizutani, Y. Yanagi, M. Yoshikawa, Y. Itoh, T. Oka, IEEE Trans. Appl. Supercond. 9 (1999) 2219. [11] H. Ikuta, T. Hosokawa, M. Yoshikawa, U. Mizutani, Supercond. Sci. Technol. 13 (2000) 1559. [12] M. Morita, K. Nagashima, S. Takebayashi, M. Murakami, M. Sawamura, Mater. Sci. Eng. B 53 (1998) 159. [13] M. Matsui, T. Miyamoto, S. Nariki, N. Sakai, M. Murakami, Physica C 257–360 (2001) 694. [14] M. Ullrich, A. Leenders, H.C. Freyhardt, Appl. Phys. Lett. 68 (1996) 2735. [15] M. Tomita, M. Murakami, Supercond. Sci. Technol. 13 (2000) 722. [16] M. Tomita, M. Murakami, Physica C 341–348 (2000) 2443. [17] M. Tomita, M. Murakami, K. Sawa, Y. Tachi, Physica C 357–360 (2001) 690.
Effect of silver addition on the field trapping properties of Gd–Ba–Cu–O bulk superconductors S. Nariki *, N. Sakai, M. Matsui, M. Murakami Superconductivity Research Laboratory, ISTEC, 1-16-25 Shibaura, Minato-ku, Tokyo 105-0023, Japan Received 27 September 2001; accepted 9 January 2002
Abstract The effect of Ag addition on the field trapping properties of Gd–Ba–Cu–O bulk samples has been studied. Gd–Ba– Cu–O bulk superconductors 32 mm in diameter were melt textured by varying the amount of Ag2 O in the range of 0–30 wt.%. The trapped magnetic field of Ag-free bulk sample was 1.3 T at 77 K. This value could be improved to 1.8–2.0 T with Ag addition of 10–20 wt.%. A further increase in Ag2 O content degraded the field trapping ability. Ag-free bulk Gd–Ba–Cu–O exhibited the trapped field of 3.0 T at 65 K, however, the bulk was fractured at lower temperature. On the other hand, Ag-added bulk Gd–Ba–Cu–O exhibited the high trapped field value of 6.7 T at 55 K. These results indicate that the mechanical strength of the bulk Gd–Ba–Cu–O could be improved with Ag addition. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.25.L; 74.80.B Keywords: Melt processing; Trapped magnetic field; Mechanical property; Ag; Gd–Ba–Cu–O
1. Introduction For the enhancement of trapped magnetic field of bulk superconductors, it is important to increase the critical current density (Jc ) and/or the size of single-domain. However, the defects such as cracks and high-angle grain boundaries deteriorate the field trapping ability. In addition, the maximum trapped-field is restricted by the mechanical strength when the sample traps high fields at low temperatures, since the cracks are formed with increasing electromagnetic force [1–5]. Therefore,
*
Corresponding author. Tel.: +81-3-3454-9284; fax: +81-33454-9287. E-mail address: [email protected] (S. Nariki).
the improvement in mechanical strength is critically important for achieving the high trapped magnetic field at low temperatures. We found that the Ag addition was very effective in reducing the cracks and thus in dramatically improving the mechanical properties of Gd–Ba–Cu–O bulk materials [6]. In this paper, we report the effect of Ag addition on the field trapping properties of Gd– Ba–Cu–O bulk superconductors.
2. Experimental Large single-grain Gd–Ba–Cu–O bulk samples 32 mm in diameter and 20 mm in thickness were prepared from the precursors with a molar ratio of GdBa2 Cu3 O7 (Gd123):Gd2 BaCuO5 (Gd211) ¼ 2:1
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 5 4 1 - 1
S. Nariki et al. / Physica C 378–381 (2002) 774–778
by the top-seeded melt growth technique. The size of Gd211 starting powder was 1.0 lm, which was prepared by the calcination of the powders of Gd2 O3 , BaO2 and CuO. 0.5 wt.% of Pt and 0–30 wt.% of Ag2 O were also added in the mixed powders. The melt processing was performed under controlled oxygen partial pressure of 1% O2 in Ar. The details of melt-processing conditions were described elsewhere [6,7]. The melt-textured sample was annealed at 400–450 °C for 300 h in flowing pure O2 gas. The small specimens with the dimensions of about 1:5 1:5 1:0 mm3 were cut from the bulk sample. Measurements of critical temperature (Tc ) and Jc were performed with a quantum design SQUID magnetometer [7]. The measurements of trapped magnetic fields were carried out by magnetizing the bulk samples using a 10 T superconducting magnet. The bulk samples were cooled to the liquid nitrogen temperature in the presence of the magnetic field 7 T applied parallel to the c-axis. After the removal of the external field, the profile of trapped magnetic flux density was measured by scanning Hall probe sensors [8]. The total gap between the top surface of the sample and the active area of the Hall sensor was adjusted to be 1.2 mm, which included the thickness of a mold of the sensor, 0.7 mm. For the trapped field measurements at low temperatures, the bulk sample was cooled by a refrigerator. The trapped field was recorded using a Hall sensor that was directly glued on the surface of a bulk superconductor.
3. Results and discussion Fig. 1 shows the trapped field distribution of bulk Gd–Ba–Cu–O samples with various Ag contents at 77 K. As indicated in Fig. 1(a), the trapped magnetic field of Ag-free bulk sample was 1.3 T at 77 K. The profile of this sample shows that the bulk was grown into a single-grain, however, its irregular shape suggests the presence of some weak-links. In contrast the profile of the Gd–Ba– Cu–O samples with 10–30 wt.% Ag2 O exhibited a circular shape with no irregularity. We reported that the amount of cracks in the Gd–Ba–Cu–O bulk samples is dramatically reduced with Ag ad-
775
dition [6]. The circular shape in the field distribution reflects a reduction of weak-links. The trapped fields of the bulk Gd–Ba–Cu–O samples with 10 and 20 wt.% Ag2 O exhibited high values of 1.8 and 2.0 T, respectively. However, the trapped field of the sample with 30 wt.% Ag2 O addition was lowered to 1.1 T. Fig. 2 shows Jc –B curves for the specimens with various Ag contents. The Jc values increased up to 20 wt.% of Ag2 O addition, which led to the enhancement in trapped magnetic field. As presented on Y–Ba–Cu–O [9], the Ag addition may enhance the Jc in the intermediate fields by the substitution of Ag with Cu. However, the Jc value greatly lowered with 30 wt.% Ag2 O addition. A large decrease in the volume fraction of the Gd123 superconducting phase may lead to a depression of the Jc values, as reported in Sm–Ba–Cu–O and Nd– Ba–Cu–O systems [10,11]. However, further investigation will be needed to understand the effect of Ag addition on the Jc –B properties. It is well known that the trapped fields of bulk superconductors increase dramatically with lowering temperature due to a drastic increase in Jc values [1–5,10–13]. Fig. 3 shows the temperature dependence of the trapped magnetic field for bulk Gd–Ba–Cu–O samples with 0 and 20 wt.% Ag2 O. In order to clarify the effect of Ag addition on mechanical properties, the measurements were performed on the as-grown bulk samples without reinforcement by the metal ring or epoxy resin. Ag-free Gd–Ba–Cu–O exhibited the trapped field of 3.0 T at 65 K, however, the sample was broken during the measurements at lower temperatures due to a electromagnetic force. In contrast, Agadded bulk Gd–Ba–Cu–O did not fracture and could trap the higher field of 6.7 T at 55 K. These results show the mechanical strength is improved with the addition of Ag. Fig. 4 shows the fracture surfaces of Ag-free and Ag-added bulk samples. In the fracture surface of Ag-free bulk, the flat mirror region indicating crack propagation was clearly observed. The site of crack initiation, in which the area with high concentration of Gd211 was visible and indicated by the arrow in this figure. In contrast, it was difficult to assess the fracture origin in Ag-added sample and small lamellar facets were observed everywhere. This suggests that the addition of Ag leads to the reduction of serious defects
776
S. Nariki et al. / Physica C 378–381 (2002) 774–778
Fig. 1. Trapped field distributions of the Gd–Ba–Cu–O bulk samples 32 mm in diameter with various Ag contents at 77 K.
Fig. 2. Jc –B curves for Gd–Ba–Cu–O specimens with various Ag contents at 77 K.
as crack nucleation sites and/or the suppression of crack growth. Several researchers pointed out that the trapped field and the levitation force drop with increasing the number of thermal cycles between room temperature and liquid nitrogen temperature for the measurements [14–17]. This decrease is explained by the generation and propagation of cracks inside the melt-textured bulk [14,17]. Hence, we studied the influence of thermal cycles on the trapped field of Gd–Ba–Cu–O bulks with and without Ag addition. Ag-free bulk 32 mm in diameter and the larger bulk 48 mm in diameter with 10 wt.% Ag2 O were used in this experiment. The bulk samples were immediately immersed in liquid nitrogen in applied field of 7 T. After the trapped field measurement, the bulks were warmed up to room temperature using hot air. These measurements
S. Nariki et al. / Physica C 378–381 (2002) 774–778
Fig. 3. Temperature dependence of the trapped magnetic field measured on the surface of Ag-free and Ag-added Gd–Ba– Cu–O bulk samples using Hall sensor. The measurement was performed without reinforcement by the metal ring or resin.
777
Fig. 5. Relationship between the trapped field at 77 K for the Ag-free bulk 32 mm in diameter and the Ag-added bulk 48 mm in diameter and the number of measurement tests. The values of trapped field were normalized by the initial trapped field values of 1.3 T for Ag-free bulk and 2.0 T for Ag-added bulk, respectively.
were repeated five times. Fig. 5 shows the plots of the trapped field normalized by the initial trapped field versus the number of experiments. The trapped field of Ag-free bulk sample decreased with the number of measurement tests. On the other hand, the trapped field of Ag-added bulk was not degraded and stayed at a constant value regardless of thermal cycles. The enhancement of mechanical strength and thermal conductivity with Ag addition is very effective in depressing the deterioration of the trapped field by thermal cycles. 4. Summary
Fig. 4. Photographs showing the fracture surfaces of (a) Agfree and (b) Ag-added Gd–Ba–Cu–O bulk samples destroyed by the electromagnetic force in the trapped field measurement test. The arrow in figure (a) indicates the estimated site of crack initiation.
We studied the effect of Ag addition on the field trapping properties of OCMG-processed Gd–Ba– Cu–O bulk samples 32 mm in diameter. The trapped magnetic field of Ag-free bulk sample was 1.3 T at 77 K. This value could be improved with Ag addition. The trapped field of the bulk Gd–Ba– Cu–O samples with 10–20 wt.% Ag2 O exhibited high values of 1.8–2.0 T. The trapped fields at lower temperatures were also measured for the Agfree and 20 wt.% Ag2 O added bulk samples without reinforcement by the metal ring or resin. The
778
S. Nariki et al. / Physica C 378–381 (2002) 774–778
Ag-free bulk exhibited the trapped field of 3.0 T at 65 K, however, the bulk was fractured at lower temperature due to an electromagnetic force. On the other hand, the Ag-added bulk exhibited the higher value of 6.7 T at 55 K. Furthermore, the trapped field of Ag-added sample did not deteriorate with thermal cycles.
Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as collaborative research and development of fundamental technologies for superconductivity applications.
References [1] Y. Ren, R. Weinstein, J. Liu, R.P. Sawh, C. Foster, Physica C 251 (1995) 15. [2] H. Ikuta, A. Mase, Y. Yanagi, M. Yoahikawa, Y. Itoh, T. Oka, U. Mizutani, Supercond. Sci. Technol. 11 (1998) 1345. [3] P. Sch€ atzle, G. Krabbes, S. Gruss, G. Fuchs, IEEE Trans. Appl. Supercond. 9 (1999) 2022.
[4] G. Krabbes, G. Fuchs, P. Sch€atzle, S. Gruss, J.W. Park, F. Hardinghaus, R. Hayn, S. Drechsler, Physica C 341–348 (2000) 2289. [5] S. Gruss, G. Fuchs, G. Krabbes, P. Verges, P. Sch€atzle, K.H. M€ uller, J. Fink, L. Schultz, IEEE Trans. Appl. Supercond. 11 (2001) 3720. [6] S. Nariki, N. Sakai, M. Tomita, M. Murakami, Physica C, in press. [7] S. Nariki, N. Sakai, M. Murakami, Physica C 341–348 (2000) 2409. [8] K. Nagashima, T. Higuchi, J. Sok, S.I. Yoo, H. Fujimoto, M. Murakami, Cryogenics 37 (1997) 577. [9] Y. Nakamura, K. Tachibana, T. Yoshida, H. Fujimoto, J. Jpn. Inst. Metals 63 (1999) 1089. [10] H. Ikuta, A. Mase, U. Mizutani, Y. Yanagi, M. Yoshikawa, Y. Itoh, T. Oka, IEEE Trans. Appl. Supercond. 9 (1999) 2219. [11] H. Ikuta, T. Hosokawa, M. Yoshikawa, U. Mizutani, Supercond. Sci. Technol. 13 (2000) 1559. [12] M. Morita, K. Nagashima, S. Takebayashi, M. Murakami, M. Sawamura, Mater. Sci. Eng. B 53 (1998) 159. [13] M. Matsui, T. Miyamoto, S. Nariki, N. Sakai, M. Murakami, Physica C 257–360 (2001) 694. [14] M. Ullrich, A. Leenders, H.C. Freyhardt, Appl. Phys. Lett. 68 (1996) 2735. [15] M. Tomita, M. Murakami, Supercond. Sci. Technol. 13 (2000) 722. [16] M. Tomita, M. Murakami, Physica C 341–348 (2000) 2443. [17] M. Tomita, M. Murakami, K. Sawa, Y. Tachi, Physica C 357–360 (2001) 690.