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Jointing of Bi-2223 sintered bulk using superconductor paste
Physica C 356 (2001) 23±30
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Jointing of Bi-2223 sintered bulk using superconductor paste Shuetsu Haseyama a,*, Noriyasu Fujinaka b, Shuji Yoshizawa c, Hiroshi Nakane b a
Superconductivity Development Center, Dowa Mining Co. Ltd., Hachioji, Tokyo 192-0001, Japan b Department of Electric Engineering, Kogakuin University, Shinjuku, Tokyo 163-8677, Japan c Advanced Materials R&D Center, Meisei University, Hino, Tokyo 191-8506, Japan
Received 11 September 2000; received in revised form 27 December 2000; accepted 11 January 2001
Abstract In order to fabricate large sized superconductor bulk, small pieces of sintered bulks were jointed using superconductor paste prepared with mixture of calcined powder and organic vehicle. The superconductor paste was sprayed several times on the surface of the bulks. Four Bi-2223 bulks, 20 10 2 mm3 , were stuck to 20 mm square plate and 4 mm thick. This sample was sintered at 840°C for 50 h and then, intermediately subjected to CIP and sintered again. After three times of CIP and sintering, the superconducting properties of Tc 104 K and Jc 7000 A/cm2 were obtained for the sample, which were almost the same as those of the sintered bulks. The magnetic property was measured at 77 K. When a uniform magnetic ®eld was applied perpendicular to the plate surface, two-dimensional distribution of the magnetic ¯ux density over the sample surface was measured with a Hall probe in the direction perpendicular to the sample surface. The ¯ux leak of density 2 mT was observed at the center of the jointed sample where an external magnetic ®eld of 15 mT was applied. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bi-2223; Paste material; Magnetic shield; Superconducting joint
1. Introduction High Tc superconductors of various shapes are needed for magnetic shields [1±3] and fault current limiters [4] for large size, for wires [5], cables [6,7] and current leads [8] for long size, for accelerator cavities [9] and superconductor switches [10] for complicated shapes. A plasma spray method and pasting method have been used to produce largesized superconducting thick ®lm on the metal base body. Large-sized cylindrical magnetic shields in * Corresponding author. Address: Toshima Mfg. Co. Ltd., 1414 Shimonomoto, Higashimatuyama, Saitama 355-0036, Japan. Tel.: +81-493-24-6774; fax: +81-492-24-6715. E-mail address: [email protected] (S. Haseyama).
the order of meter length were reported [1±3] to measure feeble magnetic ®elds emanating from human brain. A large apparatus for the molding and sintering process is needed to prepare largesized superconducting bulk. Particular care must be taken in handling the green bulk specimen. In order to fabricate the large-sized and the complicated-structured bulks and the long-sized wires, jointing small/short pieces of the superconductors were devised to large, complicated and long ones. Jointing Y-123 melt processed bulks [11] and jointing dip-coated Bi-2212/Ag tapes [12] were reported. Thus, the superconducting joint was formed because the melt-processed method could be applied in Y-123 system and Bi-2212 system. In the case of Bi-2223 system, it is dicult
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to apply the melt-processed method from the standpoint of the phase diagram. Tc of 68.6 K was recognized in the jointing part of Bi-2223 sintered bulk by laser irradiation [13,14]. It is important to approximate the superconductivity of the jointing part to the superconductivity of the jointed bulks and wires for practical purposes. Superconducting paste was used for jointing Bi-2223 sintered bulk pieces by sintering method. In the previous paper [15], large sized plate of 20 mm square and 4 mm thick was fabricated by jointing 13 pieces of Bi-2223 bulk with superconductor paste. When magnetic ®eld of 10 mT was applied to the sample, it was reduced to 1 mT at the center region of the sample. These results indicated possibility of fabricating larger sized superconducting bulk shield. In this paper, the eect of sintering time and intermediate CIP on the superconductivity of the jointed samples and the magnetic characteristics of the jointing part are studied, and grain alignment in the jointing part was observed using a scanning electron microscopy (SEM). 2. Experimental 2.1. Jointing superconducting bulks Superconductor paste was made by mixing acrylic resin as an organic vehicle to calcined powder of Bi1:85 Pb0:35 Sr1:90 Ca2:05 Cu3:05 Ox . The paste was diluted with butyl acetate as an organic solvent to the concentration where this paste could be sprayed. Superconductor paste was sprayed several times on the surface of a MgO substrate and sintered at 840°C for 50 h. Finally superconducting thick ®lm was obtained. The calcined powder used for the paste was molded with metal dice using coaxial pressing equipment. The molded bulk was sintered on the condition of an atmospheric 850°C for 50 h. Then, the sintered bulk taken out from an electric furnace was pressed intermediately by CIP and sintered again at 850°C for 50 h. After repeating this process three times, a sintered bulk of 20 20 2 mm3 was obtained [16±18]. Two plates of 20 10 2 mm3 were cut from the plate of 20 20 2 mm3 . Then, three and four
Fig. 1. (a) Sample A is stuck with superconducting joint for Tc and Jc measurement. (b) Schematic illustration of four-probe method.
plates of 20 10 2 mm3 were stuck with superconductor paste with spray method as shown in Figs. 1(a) and 2(c), respectively. Pressure of 0.4 kg/ cm2 was added during sintering at 840°C for 50 h for raising adhesion force of the superconducting joint. Then, the sintered bulk taken out from an electric furnace was subjected to CIP and sintered again at 840°C for 50 h. After repeating this process three times, jointed samples A and D were obtained. 2.2. Measurement Superconducting properties, Tc and Jc , of the jointed samples were measured with four-probe method at 77 K as illustrated in Fig. 1(b). The sample was cut into 1 mm wide slices from the sample A. Silver electrodes were painted for the
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Fig. 3. Schematic illustration of measuring apparatus for magnetic properties.
Fig. 2. (a) Sample B is stuck with two plates without joint. (b) Sample C is stacked four plates without joint. (c) Sample D is stack with superconducting joint.
measurement. The cross-section of the jointed part for the sample A was observed using a SEM. Magnetic shielding properties were compared among three types of samples B, C and D as illustrated in Fig. 2(a)±(c), respectively; (a) two plates of 20 20 2 mm3 were stuck without paste, (b) four plates of 20 10 2 mm3 were stuck without paste, and (c) four plates of 20 10 2 mm3 were stuck with superconducting joint. Fig. 3 is schematic illustration of measuring apparatus for magnetic properties at 77 K. A uniform magnetic ®eld of up to 15 mT was applied
perpendicular to the plate surface with a Helmholtz typed coil magnet. The magnetic ¯ux density over the sample surface was measured with a Hall probe in the direction perpendicular to the sample surface. The distance between the sample surface and a support plate, on which the Hall sensor is mounted, was 0.5 mm [15]. A two-dimensional distribution of the magnetic ¯ux density was plotted in the region of x 20 mm and y 20 mm where the x-axis and the y-axis were taken in the direction parallel to the sample surface.
3. Results and discussion 3.1. Property of thick ®lm First in order to optimize preparation condition of the thick ®lm for jointing superconducting sintered bulks, superconducting property of the thick ®lm fabricated on MgO substrate was measured with four-probe method. Fig. 4 shows the sintering time dependence of Tc and Jc of the thick ®lms sintered at 840°C. The Jc of the sample increases with increasing the sintering time, and in sintering
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Fig. 5. XRD pattern of (a) the thick ®lm sintered at 840°C for 50 h and (b) the bulk sintered at 850°C for 100 h. Fig. 4. Sintering time dependence of Tc and Jc of the thick ®lm on MgO substrate at 77 K: ( ) Tc , ( ) Jc . Inserted ®gure: sintering time dependence of Tc of the sintered bulk.
time for 30±50 h the highest value of 1500 A/cm2 is obtained. As the sintering time increases from 50 h, the Jc decreases. It is suggested that Mg atom diusion into the thick ®lm from the MgO substrate deteriorates the Jc of the thick ®lm. The Tc increases with increasing the sintering time, and after 30 h sintering the Tc is maintained to be 104 K. In the case of the sintered bulk as shown in the inserted ®gure, Tc is observed to be 80 K at 25 h of the sintering time, and the Tc increases with increasing the sintering time. After sintering time of 100 h the Tc is maintained to be 105 K. Thus, in order to study on the dierence of reactivity into Bi-2223 phase between the thick ®lm and the bulk, XRD of them was measured. Fig. 5 shows the XRD patterns of the thick ®lm sintered at 840°C for 50 h and the bulk sintered at 850°C for 100 h. These main peaks are assigned to Bi-2223 phase. These are observed some peaks of Bi-2212 phase in the XRD pattern of the bulk. As regards the peak strength of (0 0 `) of Bi-2223 phase, it was found that the (0 0 `) peaks of the thick ®lm are stronger than those of the sintered bulk. It is due to the fact that when the paste was painted to MgO substrate, the plate-like grains, which are characteristic of Bi system superconductor, are aligned along the surface of the substrate, and superconducting pass formed easily with the short sintering time. Therefore, the Tc above 100 K could be attained in
a shorter time with the thick ®lm than the sintered bulk. To improve the superconductivity of the thick ®lm, it is necessary to increase the density of the grain structure. That is why CIP was applied to the thick ®lm as an intermediate pressing. In the case of sintered bulk, sintering time over 40 h after the CIP recovers the grain structure destroyed by the CIP [16±18]. The Jc changes of the thick ®lm on MgO substrate and the sintered bulk due to the number of intermediate pressing and the succeeding sintering are shown in Fig. 6, which are compared with that of the jointed sample. The number of CIP by which Jc becomes a peak values is two times, and this is less than that, four times, in the sintered bulk [18]. Bi-2223 phase in the thick ®lm grows more rapidly than that in the sintered bulk. Once grains of Bi-2223 phase are destroyed by CIP, it is dicult to re-connect superconductingly fracture surfaces of the grains. And it is suggested that Mg molecules diuses into the superconductor in the process of the sintering after CIP. This must be the cause Jc decreases after three times of CIP and sintering. 3.2. Jointing sintered bulks The Jc of jointed sample was measured and the result compared with Jc of the sintered bulk is shown in Fig. 6. After three times of CIP and the succeeding sintering, the Jc values of the jointed sample and the sintered bulk increase to be 7000
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value of the jointed sample is con®ned in 80% of the sintered bulk in the magnetic ®eld of 0 mT, but it is 40% of the bulk in the magnetic ®eld of 20 mT. Fig. 8 shows the SEM photograph and schematics of the cross-section of the jointed part of the sample. In Fig. 8(b), high degree of alignment of the plate-like grains and the dense structure in the sintered bulk can be observed. The jointing part made from superconductor paste between upper and lower bulks in the photograph is not clearly distinguished, because jointing part and
Fig. 6. Sintering time dependence of Jc of the thick ®lm on MgO substrate, the jointed sample and the sintered bulk at 77 K and self-®eld: ( ) thick ®lm on MgO substrate, ( ) jointed sample, ( ) sintered bulk.
and 8000 A/cm2 , respectively. This result means that Jc of the jointed part made from superconductor paste approximates to that of sintered bulk and reaches to be 7000 A/cm2 . The dependence of Jc of the jointed sample and the sintered bulk on the applied magnetic ®eld is shown in Fig. 7. As the applied magnetic ®eld increases, both of the Jc decreases rapidly. The Jc
Fig. 7. The dependence of critical current density of the jointed sample and the sintered bulk on the applied magnetic ®eld at 77 K: ( ) jointed sample, ( ) sintered bulk.
Fig. 8. Image of the jointed part: (a) illustration of the jointed sample, (b) SEM photograph of cross-section of the jointed part, (c) schematic illustration of alignment of plate-like grains in the jointing part.
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bulk grow to be monolithic body. It is observed that plate-like grains, in jointing part between left and right bulks in the upper half part of the photograph, aligns along the jointed surface of the bulks, where the orientation of the jointing part is dierent around 90° from that of the jointed bulks. When CIP as intermediate pressing is applied to the sample, pressure is exerted not only along the vertical direction but also along the horizontal direction in the photograph. Thus, the plate-like grains grow along the surface of the bulks. And small gap is also observed between jointing part and jointed bulks, because jointed part was exfoliated partly at the time of sample cutting for SEM observation. This means that adhesion force of joining part with jointed bulks is weak where the orientation of plate-like grains diers each other. From the standpoint of superconducting current, as it is expected that electrical connectivity among the grains depends on the adhesion force among grains, it is suggested that superconducting pass is formed through the joining part whose orientation accords with the orientation of the jointed bulks. 3.3. Magnetic property of jointing part Two-dimensional distribution of the magnetic ¯ux density over the surface of the samples B, C and D was measured at 77 K. In Fig. 9 the distribution of the ¯ux density over the sample B
Fig. 9. Two-dimensional distribution of the magnetic ¯ux density over the surface of the sample B measured in magnetic ®eld of 15 mT at 77 K.
Fig. 10. Two-dimensional distribution of the magnetic ¯ux density over the surface of the sample C measured in magnetic ®eld of 5 mT at 77 K.
under magnetic ®eld of 15 mT shows a uniform one. The black part corresponds to the higher ¯ux density, while the white part corresponds to the lower ¯ux density. In the sample C in Fig. 10, when 5 mT is applied, the magnetic ¯ux leaks into the gap among the plates and penetrates in the center of the sample. The uniform magnetic distribution was observed in the superconductivity jointed sample D up to 10 mT of applied external ®eld as illustrated in Fig. 11(a) and (b). With applying 15 mT in Fig. 11(c) the two-dimensional distribution is observed to be uneven over the sample D. Line distribution of the magnetic ¯ux density along the centerline of the sample is plotted and shown in Fig. 12(a)±(c). In the sample B, magnetic ¯ux permeates from the edge toward the center of the sample. There exists a magnetic ¯ux density from 1 to 2 mT at the center, whose distribution can be explained by the Bean model. The magnetic ¯ux at the center part of the sample C increases with increasing the external magnetic ®eld. This result means that the magnetic ¯ux penetrates through the gap among the plates. When magnetic ®eld of 10 mT was applied to the sample D, the magnetic ¯ux at the center part reaches to 1.5 mT, which is almost same as the value of the sample B. With the external magnetic ®eld of 15 mT, the magnetic gradient decreases. The magnetic ¯ux penetrates the sample D, and the leak ¯ux density of 4 mT is observed at the center. The Jc value of the jointing part is much lower than that of the
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Fig. 11. Two-dimensional distribution of the magnetic ¯ux density over the surface of the sample D at 77 K: (a) measured in magnetic ®eld of 5 mT, (b) measured in 10 mT, (c) measured in 15 mT.
sintered bulk in the higher magnetic ®eld as shown in Fig. 7, where magnetic property of the jointed
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Fig. 12. A line distribution of magnetic ¯ux density in the centerline of the sample at 77 K: (a) measured in magnetic ®eld of 5 mT, (b) measured in 10 mT, (c) measured in 15 mT. ( ) sample B, ( ) sample C and ( ) sample D.
sample is inferior to the single plate. It is mentioned that Jc dierence between the sintered bulk and the jointed sample in¯uences the magnetic
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gradient in Fig. 12. This performance of the jointed sample can be applied for measurement of biomagnetic ®eld as a magnetic shield, because the outside magnetic ®eld is under several mT. 4. Conclusion 20 mm square plates with the thickness of 4 mm were made by jointing superconductingly Bi-2223 four-cornered bulks using superconductor paste. SEM photograph of the cross-section of the jointing part showed that the sintered bulk and the superconductor paste grow to be monolithic body when the paste was sandwiched with paralleloriented plate-like grains. When magnetic ®eld of 10 mT was applied to the sample, the magnetic shielding eect was observed uniformly. The external magnetic ®eld was reduced to be 1.5 mT at the center region of the sample. Using this method, it is possible to scale up to large sized superconductor bulk required for the practical applications. The Jc value of the jointed sample was reached to 80% of the sintered bulk in the magnetic ®eld of 0 mT by optimizing jointing condition, but it was 40% of the bulk in the magnetic ®eld of 20 mT. References [1] H. Matuba, Cryogenic Engineering 30 (1995) 2 (in Japanese). [2] S. Haseyama, M. Kojima, S. Yoshizawa, S. Mukoyama, N. Ishikawa, S. Shibuya, Advances in Superconductivity IX, Springer, Tokyo, 1996, p. 1447.
[3] H. Ohta, et al., IEEE Trans. Appl. Supercond. 9 (1999) 4073. [4] M. Ichikawa, M. Okazaki, IEEE Trans. Appl. Supercond. 5 (1995) 1067. [5] S. Inagaki, et al., Jpn. J. Appl. Phys. 36 (1997) 3478. [6] K. Sato, K. Hayashi, K. Ohmatsu, J. Fujikami, N. Saga, T. Shibata, S. Isojima, IEEE Trans. Appl. Supercond. 7 (1997) 345. [7] S. Mukoyama, K. Miyoshi, H. Tsubouti, M. Miura, N. Uno, N. Ichiyanagi, M. Ikeda, IEEE Trans. Appl. Supercond. 7 (1997) 1069. [8] T. Honjo, S. Miyake, T. Hasegawa, Advances in Superconductivity X, Springer, Tokyo, 1997, p. 933. [9] K. Miyoshi, H. Tsubouchi, S. Nagaya, Advances in Superconductivity X, Springer, Tokyo, 1997, p. 1255. [10] M. Mimura, H. Ii, K. Kosugi, Y. Tanaka, N. Uno, K. Satou, Advances in Superconductivity VIII, Springer, Tokyo, 1995, p. 859. [11] N. Sakai, D.N. Matthews, R. Heddericch, H. Takaichi, M. Murakami, Advances in Superconductivity VI, Springer, Tokyo, 1993, p. 803. [12] T. Muroga, J. Sato, H. Kitaguchi, H. Kumakura, K. Togano, M. Okada, Advances in Superconductivity X, Springer, Tokyo, 1997, p. 837. [13] K. Hotta, H. Miyazawa, Y. Ogawa, K. Funato, M. Murakawa, H. Hirose, Physica C 235±240 (1994) 3429. [14] K. Hotta, H. Miyazawa, Y. Ogawa, M. Murakawa, H. Hirose, Advances in Superconductivity IX, Springer, Tokyo, 1996, p. 935. [15] S. Haseyama, N. Fujinaka, S. Yoshizawa, H. Nakane, Physica C, accepted for publication. [16] M. Satoh, A. Murata, S. Haseyama, M. Kojima, S. Yoshizawa, M. Fujisawa, T. Negishi, Advances in Superconductivity IX, Springer, Tokyo, 1996, p. 883. [17] M. Satoh, S. Haseyama, M. Kojima, A. Murata, S. Yoshizawa, I. Tanaka, Advances in Superconductivity X, Springer, Tokyo, 1997, p. 857. [18] M. Satoh, S. Haseyama, M. Kojima, A. Murata, S. Yoshizawa, T. Negishi, I. Tanaka, Advances in Cryogenic Engineering, vol. 44, Plenum Press, New York, 1998, p. 405.
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Jointing of Bi-2223 sintered bulk using superconductor paste Shuetsu Haseyama a,*, Noriyasu Fujinaka b, Shuji Yoshizawa c, Hiroshi Nakane b a
Superconductivity Development Center, Dowa Mining Co. Ltd., Hachioji, Tokyo 192-0001, Japan b Department of Electric Engineering, Kogakuin University, Shinjuku, Tokyo 163-8677, Japan c Advanced Materials R&D Center, Meisei University, Hino, Tokyo 191-8506, Japan
Received 11 September 2000; received in revised form 27 December 2000; accepted 11 January 2001
Abstract In order to fabricate large sized superconductor bulk, small pieces of sintered bulks were jointed using superconductor paste prepared with mixture of calcined powder and organic vehicle. The superconductor paste was sprayed several times on the surface of the bulks. Four Bi-2223 bulks, 20 10 2 mm3 , were stuck to 20 mm square plate and 4 mm thick. This sample was sintered at 840°C for 50 h and then, intermediately subjected to CIP and sintered again. After three times of CIP and sintering, the superconducting properties of Tc 104 K and Jc 7000 A/cm2 were obtained for the sample, which were almost the same as those of the sintered bulks. The magnetic property was measured at 77 K. When a uniform magnetic ®eld was applied perpendicular to the plate surface, two-dimensional distribution of the magnetic ¯ux density over the sample surface was measured with a Hall probe in the direction perpendicular to the sample surface. The ¯ux leak of density 2 mT was observed at the center of the jointed sample where an external magnetic ®eld of 15 mT was applied. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bi-2223; Paste material; Magnetic shield; Superconducting joint
1. Introduction High Tc superconductors of various shapes are needed for magnetic shields [1±3] and fault current limiters [4] for large size, for wires [5], cables [6,7] and current leads [8] for long size, for accelerator cavities [9] and superconductor switches [10] for complicated shapes. A plasma spray method and pasting method have been used to produce largesized superconducting thick ®lm on the metal base body. Large-sized cylindrical magnetic shields in * Corresponding author. Address: Toshima Mfg. Co. Ltd., 1414 Shimonomoto, Higashimatuyama, Saitama 355-0036, Japan. Tel.: +81-493-24-6774; fax: +81-492-24-6715. E-mail address: [email protected] (S. Haseyama).
the order of meter length were reported [1±3] to measure feeble magnetic ®elds emanating from human brain. A large apparatus for the molding and sintering process is needed to prepare largesized superconducting bulk. Particular care must be taken in handling the green bulk specimen. In order to fabricate the large-sized and the complicated-structured bulks and the long-sized wires, jointing small/short pieces of the superconductors were devised to large, complicated and long ones. Jointing Y-123 melt processed bulks [11] and jointing dip-coated Bi-2212/Ag tapes [12] were reported. Thus, the superconducting joint was formed because the melt-processed method could be applied in Y-123 system and Bi-2212 system. In the case of Bi-2223 system, it is dicult
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to apply the melt-processed method from the standpoint of the phase diagram. Tc of 68.6 K was recognized in the jointing part of Bi-2223 sintered bulk by laser irradiation [13,14]. It is important to approximate the superconductivity of the jointing part to the superconductivity of the jointed bulks and wires for practical purposes. Superconducting paste was used for jointing Bi-2223 sintered bulk pieces by sintering method. In the previous paper [15], large sized plate of 20 mm square and 4 mm thick was fabricated by jointing 13 pieces of Bi-2223 bulk with superconductor paste. When magnetic ®eld of 10 mT was applied to the sample, it was reduced to 1 mT at the center region of the sample. These results indicated possibility of fabricating larger sized superconducting bulk shield. In this paper, the eect of sintering time and intermediate CIP on the superconductivity of the jointed samples and the magnetic characteristics of the jointing part are studied, and grain alignment in the jointing part was observed using a scanning electron microscopy (SEM). 2. Experimental 2.1. Jointing superconducting bulks Superconductor paste was made by mixing acrylic resin as an organic vehicle to calcined powder of Bi1:85 Pb0:35 Sr1:90 Ca2:05 Cu3:05 Ox . The paste was diluted with butyl acetate as an organic solvent to the concentration where this paste could be sprayed. Superconductor paste was sprayed several times on the surface of a MgO substrate and sintered at 840°C for 50 h. Finally superconducting thick ®lm was obtained. The calcined powder used for the paste was molded with metal dice using coaxial pressing equipment. The molded bulk was sintered on the condition of an atmospheric 850°C for 50 h. Then, the sintered bulk taken out from an electric furnace was pressed intermediately by CIP and sintered again at 850°C for 50 h. After repeating this process three times, a sintered bulk of 20 20 2 mm3 was obtained [16±18]. Two plates of 20 10 2 mm3 were cut from the plate of 20 20 2 mm3 . Then, three and four
Fig. 1. (a) Sample A is stuck with superconducting joint for Tc and Jc measurement. (b) Schematic illustration of four-probe method.
plates of 20 10 2 mm3 were stuck with superconductor paste with spray method as shown in Figs. 1(a) and 2(c), respectively. Pressure of 0.4 kg/ cm2 was added during sintering at 840°C for 50 h for raising adhesion force of the superconducting joint. Then, the sintered bulk taken out from an electric furnace was subjected to CIP and sintered again at 840°C for 50 h. After repeating this process three times, jointed samples A and D were obtained. 2.2. Measurement Superconducting properties, Tc and Jc , of the jointed samples were measured with four-probe method at 77 K as illustrated in Fig. 1(b). The sample was cut into 1 mm wide slices from the sample A. Silver electrodes were painted for the
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Fig. 3. Schematic illustration of measuring apparatus for magnetic properties.
Fig. 2. (a) Sample B is stuck with two plates without joint. (b) Sample C is stacked four plates without joint. (c) Sample D is stack with superconducting joint.
measurement. The cross-section of the jointed part for the sample A was observed using a SEM. Magnetic shielding properties were compared among three types of samples B, C and D as illustrated in Fig. 2(a)±(c), respectively; (a) two plates of 20 20 2 mm3 were stuck without paste, (b) four plates of 20 10 2 mm3 were stuck without paste, and (c) four plates of 20 10 2 mm3 were stuck with superconducting joint. Fig. 3 is schematic illustration of measuring apparatus for magnetic properties at 77 K. A uniform magnetic ®eld of up to 15 mT was applied
perpendicular to the plate surface with a Helmholtz typed coil magnet. The magnetic ¯ux density over the sample surface was measured with a Hall probe in the direction perpendicular to the sample surface. The distance between the sample surface and a support plate, on which the Hall sensor is mounted, was 0.5 mm [15]. A two-dimensional distribution of the magnetic ¯ux density was plotted in the region of x 20 mm and y 20 mm where the x-axis and the y-axis were taken in the direction parallel to the sample surface.
3. Results and discussion 3.1. Property of thick ®lm First in order to optimize preparation condition of the thick ®lm for jointing superconducting sintered bulks, superconducting property of the thick ®lm fabricated on MgO substrate was measured with four-probe method. Fig. 4 shows the sintering time dependence of Tc and Jc of the thick ®lms sintered at 840°C. The Jc of the sample increases with increasing the sintering time, and in sintering
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Fig. 5. XRD pattern of (a) the thick ®lm sintered at 840°C for 50 h and (b) the bulk sintered at 850°C for 100 h. Fig. 4. Sintering time dependence of Tc and Jc of the thick ®lm on MgO substrate at 77 K: ( ) Tc , ( ) Jc . Inserted ®gure: sintering time dependence of Tc of the sintered bulk.
time for 30±50 h the highest value of 1500 A/cm2 is obtained. As the sintering time increases from 50 h, the Jc decreases. It is suggested that Mg atom diusion into the thick ®lm from the MgO substrate deteriorates the Jc of the thick ®lm. The Tc increases with increasing the sintering time, and after 30 h sintering the Tc is maintained to be 104 K. In the case of the sintered bulk as shown in the inserted ®gure, Tc is observed to be 80 K at 25 h of the sintering time, and the Tc increases with increasing the sintering time. After sintering time of 100 h the Tc is maintained to be 105 K. Thus, in order to study on the dierence of reactivity into Bi-2223 phase between the thick ®lm and the bulk, XRD of them was measured. Fig. 5 shows the XRD patterns of the thick ®lm sintered at 840°C for 50 h and the bulk sintered at 850°C for 100 h. These main peaks are assigned to Bi-2223 phase. These are observed some peaks of Bi-2212 phase in the XRD pattern of the bulk. As regards the peak strength of (0 0 `) of Bi-2223 phase, it was found that the (0 0 `) peaks of the thick ®lm are stronger than those of the sintered bulk. It is due to the fact that when the paste was painted to MgO substrate, the plate-like grains, which are characteristic of Bi system superconductor, are aligned along the surface of the substrate, and superconducting pass formed easily with the short sintering time. Therefore, the Tc above 100 K could be attained in
a shorter time with the thick ®lm than the sintered bulk. To improve the superconductivity of the thick ®lm, it is necessary to increase the density of the grain structure. That is why CIP was applied to the thick ®lm as an intermediate pressing. In the case of sintered bulk, sintering time over 40 h after the CIP recovers the grain structure destroyed by the CIP [16±18]. The Jc changes of the thick ®lm on MgO substrate and the sintered bulk due to the number of intermediate pressing and the succeeding sintering are shown in Fig. 6, which are compared with that of the jointed sample. The number of CIP by which Jc becomes a peak values is two times, and this is less than that, four times, in the sintered bulk [18]. Bi-2223 phase in the thick ®lm grows more rapidly than that in the sintered bulk. Once grains of Bi-2223 phase are destroyed by CIP, it is dicult to re-connect superconductingly fracture surfaces of the grains. And it is suggested that Mg molecules diuses into the superconductor in the process of the sintering after CIP. This must be the cause Jc decreases after three times of CIP and sintering. 3.2. Jointing sintered bulks The Jc of jointed sample was measured and the result compared with Jc of the sintered bulk is shown in Fig. 6. After three times of CIP and the succeeding sintering, the Jc values of the jointed sample and the sintered bulk increase to be 7000
S. Haseyama et al. / Physica C 356 (2001) 23±30
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value of the jointed sample is con®ned in 80% of the sintered bulk in the magnetic ®eld of 0 mT, but it is 40% of the bulk in the magnetic ®eld of 20 mT. Fig. 8 shows the SEM photograph and schematics of the cross-section of the jointed part of the sample. In Fig. 8(b), high degree of alignment of the plate-like grains and the dense structure in the sintered bulk can be observed. The jointing part made from superconductor paste between upper and lower bulks in the photograph is not clearly distinguished, because jointing part and
Fig. 6. Sintering time dependence of Jc of the thick ®lm on MgO substrate, the jointed sample and the sintered bulk at 77 K and self-®eld: ( ) thick ®lm on MgO substrate, ( ) jointed sample, ( ) sintered bulk.
and 8000 A/cm2 , respectively. This result means that Jc of the jointed part made from superconductor paste approximates to that of sintered bulk and reaches to be 7000 A/cm2 . The dependence of Jc of the jointed sample and the sintered bulk on the applied magnetic ®eld is shown in Fig. 7. As the applied magnetic ®eld increases, both of the Jc decreases rapidly. The Jc
Fig. 7. The dependence of critical current density of the jointed sample and the sintered bulk on the applied magnetic ®eld at 77 K: ( ) jointed sample, ( ) sintered bulk.
Fig. 8. Image of the jointed part: (a) illustration of the jointed sample, (b) SEM photograph of cross-section of the jointed part, (c) schematic illustration of alignment of plate-like grains in the jointing part.
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bulk grow to be monolithic body. It is observed that plate-like grains, in jointing part between left and right bulks in the upper half part of the photograph, aligns along the jointed surface of the bulks, where the orientation of the jointing part is dierent around 90° from that of the jointed bulks. When CIP as intermediate pressing is applied to the sample, pressure is exerted not only along the vertical direction but also along the horizontal direction in the photograph. Thus, the plate-like grains grow along the surface of the bulks. And small gap is also observed between jointing part and jointed bulks, because jointed part was exfoliated partly at the time of sample cutting for SEM observation. This means that adhesion force of joining part with jointed bulks is weak where the orientation of plate-like grains diers each other. From the standpoint of superconducting current, as it is expected that electrical connectivity among the grains depends on the adhesion force among grains, it is suggested that superconducting pass is formed through the joining part whose orientation accords with the orientation of the jointed bulks. 3.3. Magnetic property of jointing part Two-dimensional distribution of the magnetic ¯ux density over the surface of the samples B, C and D was measured at 77 K. In Fig. 9 the distribution of the ¯ux density over the sample B
Fig. 9. Two-dimensional distribution of the magnetic ¯ux density over the surface of the sample B measured in magnetic ®eld of 15 mT at 77 K.
Fig. 10. Two-dimensional distribution of the magnetic ¯ux density over the surface of the sample C measured in magnetic ®eld of 5 mT at 77 K.
under magnetic ®eld of 15 mT shows a uniform one. The black part corresponds to the higher ¯ux density, while the white part corresponds to the lower ¯ux density. In the sample C in Fig. 10, when 5 mT is applied, the magnetic ¯ux leaks into the gap among the plates and penetrates in the center of the sample. The uniform magnetic distribution was observed in the superconductivity jointed sample D up to 10 mT of applied external ®eld as illustrated in Fig. 11(a) and (b). With applying 15 mT in Fig. 11(c) the two-dimensional distribution is observed to be uneven over the sample D. Line distribution of the magnetic ¯ux density along the centerline of the sample is plotted and shown in Fig. 12(a)±(c). In the sample B, magnetic ¯ux permeates from the edge toward the center of the sample. There exists a magnetic ¯ux density from 1 to 2 mT at the center, whose distribution can be explained by the Bean model. The magnetic ¯ux at the center part of the sample C increases with increasing the external magnetic ®eld. This result means that the magnetic ¯ux penetrates through the gap among the plates. When magnetic ®eld of 10 mT was applied to the sample D, the magnetic ¯ux at the center part reaches to 1.5 mT, which is almost same as the value of the sample B. With the external magnetic ®eld of 15 mT, the magnetic gradient decreases. The magnetic ¯ux penetrates the sample D, and the leak ¯ux density of 4 mT is observed at the center. The Jc value of the jointing part is much lower than that of the
S. Haseyama et al. / Physica C 356 (2001) 23±30
Fig. 11. Two-dimensional distribution of the magnetic ¯ux density over the surface of the sample D at 77 K: (a) measured in magnetic ®eld of 5 mT, (b) measured in 10 mT, (c) measured in 15 mT.
sintered bulk in the higher magnetic ®eld as shown in Fig. 7, where magnetic property of the jointed
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Fig. 12. A line distribution of magnetic ¯ux density in the centerline of the sample at 77 K: (a) measured in magnetic ®eld of 5 mT, (b) measured in 10 mT, (c) measured in 15 mT. ( ) sample B, ( ) sample C and ( ) sample D.
sample is inferior to the single plate. It is mentioned that Jc dierence between the sintered bulk and the jointed sample in¯uences the magnetic
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gradient in Fig. 12. This performance of the jointed sample can be applied for measurement of biomagnetic ®eld as a magnetic shield, because the outside magnetic ®eld is under several mT. 4. Conclusion 20 mm square plates with the thickness of 4 mm were made by jointing superconductingly Bi-2223 four-cornered bulks using superconductor paste. SEM photograph of the cross-section of the jointing part showed that the sintered bulk and the superconductor paste grow to be monolithic body when the paste was sandwiched with paralleloriented plate-like grains. When magnetic ®eld of 10 mT was applied to the sample, the magnetic shielding eect was observed uniformly. The external magnetic ®eld was reduced to be 1.5 mT at the center region of the sample. Using this method, it is possible to scale up to large sized superconductor bulk required for the practical applications. The Jc value of the jointed sample was reached to 80% of the sintered bulk in the magnetic ®eld of 0 mT by optimizing jointing condition, but it was 40% of the bulk in the magnetic ®eld of 20 mT. References [1] H. Matuba, Cryogenic Engineering 30 (1995) 2 (in Japanese). [2] S. Haseyama, M. Kojima, S. Yoshizawa, S. Mukoyama, N. Ishikawa, S. Shibuya, Advances in Superconductivity IX, Springer, Tokyo, 1996, p. 1447.
[3] H. Ohta, et al., IEEE Trans. Appl. Supercond. 9 (1999) 4073. [4] M. Ichikawa, M. Okazaki, IEEE Trans. Appl. Supercond. 5 (1995) 1067. [5] S. Inagaki, et al., Jpn. J. Appl. Phys. 36 (1997) 3478. [6] K. Sato, K. Hayashi, K. Ohmatsu, J. Fujikami, N. Saga, T. Shibata, S. Isojima, IEEE Trans. Appl. Supercond. 7 (1997) 345. [7] S. Mukoyama, K. Miyoshi, H. Tsubouti, M. Miura, N. Uno, N. Ichiyanagi, M. Ikeda, IEEE Trans. Appl. Supercond. 7 (1997) 1069. [8] T. Honjo, S. Miyake, T. Hasegawa, Advances in Superconductivity X, Springer, Tokyo, 1997, p. 933. [9] K. Miyoshi, H. Tsubouchi, S. Nagaya, Advances in Superconductivity X, Springer, Tokyo, 1997, p. 1255. [10] M. Mimura, H. Ii, K. Kosugi, Y. Tanaka, N. Uno, K. Satou, Advances in Superconductivity VIII, Springer, Tokyo, 1995, p. 859. [11] N. Sakai, D.N. Matthews, R. Heddericch, H. Takaichi, M. Murakami, Advances in Superconductivity VI, Springer, Tokyo, 1993, p. 803. [12] T. Muroga, J. Sato, H. Kitaguchi, H. Kumakura, K. Togano, M. Okada, Advances in Superconductivity X, Springer, Tokyo, 1997, p. 837. [13] K. Hotta, H. Miyazawa, Y. Ogawa, K. Funato, M. Murakawa, H. Hirose, Physica C 235±240 (1994) 3429. [14] K. Hotta, H. Miyazawa, Y. Ogawa, M. Murakawa, H. Hirose, Advances in Superconductivity IX, Springer, Tokyo, 1996, p. 935. [15] S. Haseyama, N. Fujinaka, S. Yoshizawa, H. Nakane, Physica C, accepted for publication. [16] M. Satoh, A. Murata, S. Haseyama, M. Kojima, S. Yoshizawa, M. Fujisawa, T. Negishi, Advances in Superconductivity IX, Springer, Tokyo, 1996, p. 883. [17] M. Satoh, S. Haseyama, M. Kojima, A. Murata, S. Yoshizawa, I. Tanaka, Advances in Superconductivity X, Springer, Tokyo, 1997, p. 857. [18] M. Satoh, S. Haseyama, M. Kojima, A. Murata, S. Yoshizawa, T. Negishi, I. Tanaka, Advances in Cryogenic Engineering, vol. 44, Plenum Press, New York, 1998, p. 405.