- Email: [email protected]
Relationship between Bi2Sr2CaCu2Ox layer thickness and Jc enhancement by pre-annealing and intermediate rolling process
Physica C 320 Ž1999. 71–76
Relationship between Bi 2 Sr2 CaCu 2 O x layer thickness and Jc enhancement by pre-annealing and intermediate rolling process H. Kitaguchi a b
a,)
, H. Miao
a,b
, H. Kumakura a , K. Togano
a,b
National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan CREST, Japan Science and Technology, 2-1-6 Sengen, Tsukuba 305-0047, Japan
Received 13 April 1999; received in revised form 12 May 1999; accepted 24 May 1999
Abstract Pre-annealing and intermediate rolling ŽPAIR. process enables one to fabricate Bi 2 Sr2 CaCu 2 O x ŽBi-2212.rAg tapes with high transport Jc ) 5.0 = 10 5 Arcm2 at 4.2 K, 10 T. We study the relationship between thickness of Bi-2212 layer and Jc enhancement by PAIR process for monolayer Bi-2212rAg tapes. Jc enhancement by PAIR process is more significant for thinner Bi-2212 layer. For Bi-2212 layer less than 10 mm thick, oxide Jc Ž4.2 K, 10 T. around 4.0 and 1.6 = 10 5 Arcm2 are obtained for PAIR and no-PAIR samples, respectively, however, a large variation of Jc is seen for both series of samples in this thickness range. Good reproducibility of Jc is seen for the samples with the Bi-2212 layer around 20 mm with oxide Jc Ž4.2 K, 10 T. around 3.2 = 10 5 Arcm2. PAIR samples show smaller Jc variation than no-PAIR samples. Jc enhancement by PAIR process diminishes with increasing Bi-2212 layer thickness and disappears at the thickness around 50 mm. PAIR process produces Bi-2212 layer with small variation of thickness Ž2–8% along the tape length and 2–19% across it., however, large variation of thickness Ž4–24% along the tape length and 30–80% across it. remains in no-PAIR samples. PAIR process has a large effect to improve the macroscopic homogeneity of Bi-2212 layer. This is supposed to be one of the reasons for the large Jc enhancement achieved by PAIR process. q 1999 Elsevier Science B.V. All rights reserved. PACS: 74.72.Hs ŽBi-based.; 85.25.K ŽSuperconducting tape. Keywords: Bi-2212; Tape; PAIR process; Critical current density; Thickness
1. Introduction Bi–Sr–Ca–Cu–O ŽBSCCO. high transition temperature ŽTc . superconductors ŽHTS. are promising and expected for practical applications such as magnets, cables, etc. In most cases, the conductors are fabricated in the form of BSCCOrAg composite
)
Corresponding author. Tel.: q81-298-59-2329; fax: q81298-59-2301; e-mail: [email protected]
tapes to obtain uniform and long conductors with high critical current Ž Ic .. Among BSCCOrAg conductors, a Bi 2 Sr2 CaCu 2 O x ŽBi-2212.rAg tape is one of the promising materials for magnet applications at low temperatures due to its high critical current density Ž Jc . in high magnetic fields and simpler fabricating process in spite of lower Tc than Bi 2 Sr2 Ca 2 Cu 2 O x ŽBi-2223. w1–8x. In the past several years, the considerable progress has been achieved in the fabrication of Bi-2212rAg tapes. One of the most effective techniques is the melt-so-
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 3 2 7 - 5
72
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
lidification method, obtaining highly-oriented Bi2212 grains with oxide Jc Ž Icrcross-section of the oxide part. values of order of 10 5 Arcm2 at 4.2 K in high magnetic fields above 20 T w9–12x. In Bi2212rAg multifilamentary tapes fabricated by using powder-in-tube ŽPIT. method and melt-solidification process, oxide Jc of 2.5 = 10 5 Arcm2 at 4.2 K, 10 T has been achieved in short samples w2x. As an alternative fabrication process, we developed a lamination method to fabricate long multilayer Bi-2212rAg tapes w13,14x, and oxide Jc of about 10 5 Arcm2 Ž4.2 K, 10 T. was obtained. In recent works w15–18x, we have developed the ‘‘pre-annealing and intermediate rolling’’ ŽPAIR. process to improve significantly Bi-2212 grain alignment in Bi-2212rAg tapes. PAIR process doubles the oxide Jc of dip-coated monolayer Bi-2212rAg tapes w15x. Oxide Jc Ž4.2 K, 10 T. ) 5 = 10 5 Arcm2 was obtained for Bi-2212rAg multilayer tapes w17,18x. Optimal processing parameters for the PAIR process such as pre-annealing temperature and deformation have been reported so far w16x. In the long tape fabrication in an industrial scale, another important parameter is the optimal Bi-2212 layer thickness. Thinner Bi-2212 layer gives higher oxide Jc of the oxide layer, however, it reduces superconductor ŽSC.rAg ratio and results in no significant increase of engineering Jc Ž Icrwhole cross-section of the conductor including non-SC part such as silver.. In practical applications, the most important property is not oxide Jc but engineering Jc . Another important factor in the industrial field is a reproducibility Žor reliability. of long tape fabrication. It is difficult to fabricate conductors with very thin Bi-2212 layer in the whole length of long tapes with a good homogeneity and reproducibility. In this paper, we investigate the relationship between thickness of Bi-2212 layer and Jc enhancement by PAIR process for monolayer Bi-2212rAg tapes in order to optimize Bi-2212 layer thickness in long tape fabrication.
2. Experimental Fig. 1 shows the sample preparation procedure. Bi-2212 powder prepared by a conventional ceramics technique with the nominal composition of
Fig. 1. The fabrication procedure of samples.
Bi 2.00 Sr2.05 Ca 0.95 Cu 2.00 O x was mixed with an organic solvent, a dispersant, and a binder to obtain paste. Pure silver tapes of 50 mm thick and 200 mm long were coated in the width of 2.5–3.5 mm with the paste through a painting process. Several kinds of tapes with different coated layer thickness ranging 25–200 mm in green state were prepared. Silver part of the tapes without paste coating was turned over at the edge of the coating in order to fold the oxide layer. After this folding procedure, the tapes were cut into two pieces. One of them was processed with PAIR technique of pre-annealing in pure oxygen gas at 8408C for 1 h and subsequent one-path cold-rolling along the longitudinal direction of the tapes with 25% deformation. Melt-solidification process in the flowing oxygen atmosphere has been performed as follows: the tapes with PAIR process ŽPAIR samples. and without it Žno-PAIR samples. were cut into pieces and heated up to 8888C. After keeping 10 min at the highest temperature, the samples were cooled to 8358C at the cooling rate of 58Crh, and then rapidly cooled to room temperature. Transport Ic was measured by a four-probe method at 4.2 K in a magnetic field of 10 T parallel to the tape surface. The value of Ic was determined
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
by using 1 mVrcm electric field criterion. The thickness and the width of the oxide layer were determined by microscope observation for transverse cross-section Žacross the tape length.. Oxide Jc was calculated by using Ic , the thickness, and the width. Thickness in longitudinal cross-section Žalong the tape length. was also investigated. Thickness distribution across and along the tape length was examined with measuring the thickness at every 0.4 and 0.8 mm, respectively.
3. Results and discussion Fig. 2 shows the relationship between Bi-2212 layer thickness and oxide Jc at 4.2 K and 10 T for the samples prepared with and without PAIR process. Oxide Jc decreases with increasing the oxide layer thickness in both cases. Jc enhancement by PAIR process is more significant for thinner oxide layer. For the oxide layer less than 10 mm thick, oxide Jc Ž4.2 K, 10 T. around 4.0 and 1.6 = 10 5
Fig. 2. Relationship between Bi-2212 layer thickness and oxide Jc .
73
Arcm2 are obtained for PAIR and no-PAIR samples, respectively. However, a large variation of Jc is observed for both series of samples in this thickness range. For example, oxide Jc of 1.0–4.4 and 0.8–3.0 = 10 5 Arcm2 are obtained for PAIR and no-PAIR samples, respectively. This supports that there still remains the difficulty to achieve a good homogeneity of very thin oxide layer even with performing PAIR process. Good reproducibility of Jc is seen for PAIR samples with thickness around andror more than 20 mm except for one sample with Jc of 1.8 = 10 5 Arcm2 at 23 mm in thickness. For example, oxide Jc of 3.2 " 0.4 = 10 5 Arcm2 were obtained for PAIR processed Bi-2212 layer with thickness of 19–22 mm. Low Jc value of 1.8 = 10 5 Arcm2 at 23 mm in thickness supposed not to be an essential Jc variation for PAIR samples. It is supposed to originate from sample processing such as green coating, intermediate rolling andror heat treatment. Further optimization of processing parameters will lead to the better Jc reproducibility. It is notable that PAIR samples show higher Jc with smaller Jc variation than no-PAIR samples. Jc enhancement by PAIR process diminishes with increasing the oxide layer thickness and disappears at the thickness around 50 mm. These results indicate that PAIR process is more effective for thinner oxide layer. Thickness variation in the oxide layer is shown in Fig. 3a and b for PAIR and no-PAIR samples, respectively. Average value of measured thickness in longitudinal cross-section Žalong the tape length. of the oxide layer is plotted against that in transverse cross-section Žacross the tape length. for each sample. Error bars represent the standard deviation of measured values. Coefficient of deviation ŽCOV, standard deviationraverage. is shown in Fig. 4a and b for the longitudinal cross-section and the transverse one, respectively. Thickness variation is much smaller in PAIR sample than in no-PAIR sample in each cross-section. PAIR process enables one to obtain Bi-2212 oxide layer with small thickness variation ŽCOV around 2–8% in rolling direction and 2–19% across the tape length., however, large variation of thickness ŽCOV around 4–24% in rolling direction and 30–80% across the tape length. is observed for no-PAIR samples. PAIR process has a large effect to improve the macroscopic homogeneity
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H. Kitaguchi et al.r Physica C 320 (1999) 71–76
Bi-2212 ™ liquid q Bi-free solid phase q Cu-free solid phase. This results in Bi-2212 grain growth in random orientation in the solidification reaction. Degradation of Bi-2212 grain alignment also originates in random crystal growth at the surface of voids. By performing PAIR process, the oxide layer is densified in the intermediate rolling stage. The amount of voids is decreased to almost zero and grain alignment of Bi-2212 particles before melting is improved w17,18x. PAIR process suppresses random nucleation and growth of Bi-2212 crystals in the solidification reaction and enhances grain alignment in the final product. Typical microstructures in
Fig. 3. Thickness variation in Bi-2212 layer for Ža. PAIR and Žb. no-PAIR samples.
of Bi-2212 layer. This is supposed to be one of the reasons for the large Jc enhancement achieved by PAIR process. Fig. 5 illustrates schematically, a possible explanation for this thickness variation. In Bi-2212 fabrication by using a paste or slurry, green coating includes a large amount of organic stuff such as solvent, dispersant, and binder. In calcination or pre-annealing stage to remove them, a large amount of pore orrand void is produced and remains in melting process. These defects prohibit a homogeneous progress of the melt-solidification reaction and degrade Bi-2212 grain alignment in the final product. Random orientation of calcined Bi-2212 powders leads to a random network of solid particles which are produced in the incongruent melting reaction,
Fig. 4. Thickness variation in Bi-2212 layer in Ža. longitudinal cross-section and Žb. transverse one.
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
75
Fig. 5. Schematic illustration of a possible explanation for the difference in thickness variation between PAIR and no-PAIR samples.
the final products with and without PAIR process have been already reported w15–18x.
4. Conclusion The relationship between thickness of the Bi-2212 layer and Jc enhancement by PAIR process for monolayer Bi-2212rAg tapes was studied in order to optimize Bi-2212 layer thickness in long tape fabrication. Thickness variation in Bi-2212 layer was also investigated in detail to show the effect of PAIR process in macroscopic geometry. We obtained conclusions as follows: Ž1. Jc enhancement by PAIR process is more significant for thinner Bi-2212 layer. For Bi-2212 layer less than 10 mm thick, oxide Jc Ž4.2 K, 10 T. around 4.0 and 1.6 = 10 5 Arcm2 are obtained for PAIR and no-PAIR samples, respectively, however, a large variation of Jc is seen for
both series of samples in this thickness range. Ž2. Good reproducibility of Jc is observed for PAIR samples with the Bi-2212 layer thicker than 20 mm. PAIR samples show smaller Jc variation than noPAIR samples. Ž3. Jc enhancement by PAIR process diminishes with increasing Bi-2212 layer thickness and disappears at the thickness around 50 mm. Ž4. PAIR process produces Bi-2212 layer with small variation of thickness Ž2–8% along the tape length and 2–19% across it., however, large variation of thickness Ž4–24% along the tape length and 30–80% across it. remains in no-PAIR samples. PAIR process has a large effect to improve the macroscopic homogeneity of Bi-2212 layer. This is supposed to be one of the reasons for the large Jc enhancement achieved by PAIR process. Ž5. Optimal thickness of Bi-2212 layer is around 20 mm considering Jc reproducibility and macroscopic homogeneity. The combination of Bi-2212 layer around 20 mm in
76
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
thickness and PAIR process enables one to obtain superconducting tapes with oxide Jc Ž4.2 K, 10 T. of 3.2 " 0.4 = 10 5 Arcm2 and small variation in thickness.
Acknowledgements The authors would like to thank Dr. T. Hasegawa ŽShowa Electric Wire and Cable. for her contribution in the sample preparation.
References w1x N. Tomita, M. Arai, E. Yanagisawa, T. Morimoto, H. Fujii, H. Kitaguchi, H. Kumakura, K. Inoue, K. Togano, H. Maeda, K. Nomura, Appl. Phys. Lett. 65 Ž1994. 898. w2x M. Okada, K. Tanaka, K. Fukushima, J. Sato, K. Higashiyama, Physica B 216 Ž1996. 248. w3x M. Mimura, T. Kinoshita, N. Uno, Y. Tanaka, K. Doi, Adv. Supercond. V Ž1993. 693. w4x T. Hase, K. Shibutani, S. Hayashi, R. Ogawa, Y. Kawate, Cryogenics 35 Ž1995. 127. w5x M. Yoshida, A. Ando, N. Hara, Jpn. J. Appl. Phys. 33 Ž1994. L421.
w6x J. Tenbrink, H. Krauth, Adv. Cryog. Eng. 40 Ž1994. 305. w7x J.W. Lue, S.W. Schwenterly, M.S. Lubell, J.N. Luton, M.D. Manlief, C.H. Joshi, E.R. Podtburg, L.J. Masur, Adv. Cryog. Eng. 40 Ž1994. 327. w8x K.R. Marken, W. Dai, L. Cowey, S. Ting, S. Hong, IEEE Trans. Appl. Supercond. 7 Ž1997. 2211. w9x J. Kase, T. Morimoto, K. Togano, H. Kumakura, D.R. Dietderich, H. Maeda, IEEE Trans. Magn. 27 Ž1991. 1254. w10x H. Kumakura, K. Togano, D.R. Dietderich, H. Maeda, J. Kase, T. Morimoto, IEEE Trans. Magn. 27 Ž1991. 1250. w11x H. Kumakura, H. Kitaguchi, K. Togano, H. Maeda, J. Shimoyama, T. Morimoto, K. Nomura, M. Seido, Adv. Supercond. IV Ž1992. 547. w12x H. Kumakura, H. Kitaguchi, K. Togano, N. Sugiyama, J. Appl. Phys. 80 Ž1996. 5162. w13x T. Hasegawa, Y. Hikichi, T. Koizumi, A. Imai, H. Kumakura, H. Kitaguchi, K. Togano, IEEE Trans. Supercond. 7 Ž1997. 1703. w14x T. Hasegawa, Y. Hikichi, H. Kumakura, H. Kitaguchi, K. Togano, Jpn. J. Appl. Phys. 34 Ž1995. L1638. w15x H. Miao, H. Kitaguchi, H. Kumakura, K. Togano, Cryogenics 38 Ž1998. 257. w16x H. Miao, H. Kitaguchi, H. Kumakura, K. Togano, T. Hasegawa, Physica C 301 Ž1998. 116. w17x H. Miao, H. Kitaguchi, H. Kumakura, K. Togano, T. Hasegawa, T. Koizumi, Physica C 303 Ž1998. 81. w18x H. Kitaguchi, H. Miao, H. Kumakura, K. Togano, T. Hasegawa, T. Koizumi, IEEE Trans. Supercond. 8 Ž1999. to be published.
Relationship between Bi 2 Sr2 CaCu 2 O x layer thickness and Jc enhancement by pre-annealing and intermediate rolling process H. Kitaguchi a b
a,)
, H. Miao
a,b
, H. Kumakura a , K. Togano
a,b
National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan CREST, Japan Science and Technology, 2-1-6 Sengen, Tsukuba 305-0047, Japan
Received 13 April 1999; received in revised form 12 May 1999; accepted 24 May 1999
Abstract Pre-annealing and intermediate rolling ŽPAIR. process enables one to fabricate Bi 2 Sr2 CaCu 2 O x ŽBi-2212.rAg tapes with high transport Jc ) 5.0 = 10 5 Arcm2 at 4.2 K, 10 T. We study the relationship between thickness of Bi-2212 layer and Jc enhancement by PAIR process for monolayer Bi-2212rAg tapes. Jc enhancement by PAIR process is more significant for thinner Bi-2212 layer. For Bi-2212 layer less than 10 mm thick, oxide Jc Ž4.2 K, 10 T. around 4.0 and 1.6 = 10 5 Arcm2 are obtained for PAIR and no-PAIR samples, respectively, however, a large variation of Jc is seen for both series of samples in this thickness range. Good reproducibility of Jc is seen for the samples with the Bi-2212 layer around 20 mm with oxide Jc Ž4.2 K, 10 T. around 3.2 = 10 5 Arcm2. PAIR samples show smaller Jc variation than no-PAIR samples. Jc enhancement by PAIR process diminishes with increasing Bi-2212 layer thickness and disappears at the thickness around 50 mm. PAIR process produces Bi-2212 layer with small variation of thickness Ž2–8% along the tape length and 2–19% across it., however, large variation of thickness Ž4–24% along the tape length and 30–80% across it. remains in no-PAIR samples. PAIR process has a large effect to improve the macroscopic homogeneity of Bi-2212 layer. This is supposed to be one of the reasons for the large Jc enhancement achieved by PAIR process. q 1999 Elsevier Science B.V. All rights reserved. PACS: 74.72.Hs ŽBi-based.; 85.25.K ŽSuperconducting tape. Keywords: Bi-2212; Tape; PAIR process; Critical current density; Thickness
1. Introduction Bi–Sr–Ca–Cu–O ŽBSCCO. high transition temperature ŽTc . superconductors ŽHTS. are promising and expected for practical applications such as magnets, cables, etc. In most cases, the conductors are fabricated in the form of BSCCOrAg composite
)
Corresponding author. Tel.: q81-298-59-2329; fax: q81298-59-2301; e-mail: [email protected]
tapes to obtain uniform and long conductors with high critical current Ž Ic .. Among BSCCOrAg conductors, a Bi 2 Sr2 CaCu 2 O x ŽBi-2212.rAg tape is one of the promising materials for magnet applications at low temperatures due to its high critical current density Ž Jc . in high magnetic fields and simpler fabricating process in spite of lower Tc than Bi 2 Sr2 Ca 2 Cu 2 O x ŽBi-2223. w1–8x. In the past several years, the considerable progress has been achieved in the fabrication of Bi-2212rAg tapes. One of the most effective techniques is the melt-so-
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 3 2 7 - 5
72
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
lidification method, obtaining highly-oriented Bi2212 grains with oxide Jc Ž Icrcross-section of the oxide part. values of order of 10 5 Arcm2 at 4.2 K in high magnetic fields above 20 T w9–12x. In Bi2212rAg multifilamentary tapes fabricated by using powder-in-tube ŽPIT. method and melt-solidification process, oxide Jc of 2.5 = 10 5 Arcm2 at 4.2 K, 10 T has been achieved in short samples w2x. As an alternative fabrication process, we developed a lamination method to fabricate long multilayer Bi-2212rAg tapes w13,14x, and oxide Jc of about 10 5 Arcm2 Ž4.2 K, 10 T. was obtained. In recent works w15–18x, we have developed the ‘‘pre-annealing and intermediate rolling’’ ŽPAIR. process to improve significantly Bi-2212 grain alignment in Bi-2212rAg tapes. PAIR process doubles the oxide Jc of dip-coated monolayer Bi-2212rAg tapes w15x. Oxide Jc Ž4.2 K, 10 T. ) 5 = 10 5 Arcm2 was obtained for Bi-2212rAg multilayer tapes w17,18x. Optimal processing parameters for the PAIR process such as pre-annealing temperature and deformation have been reported so far w16x. In the long tape fabrication in an industrial scale, another important parameter is the optimal Bi-2212 layer thickness. Thinner Bi-2212 layer gives higher oxide Jc of the oxide layer, however, it reduces superconductor ŽSC.rAg ratio and results in no significant increase of engineering Jc Ž Icrwhole cross-section of the conductor including non-SC part such as silver.. In practical applications, the most important property is not oxide Jc but engineering Jc . Another important factor in the industrial field is a reproducibility Žor reliability. of long tape fabrication. It is difficult to fabricate conductors with very thin Bi-2212 layer in the whole length of long tapes with a good homogeneity and reproducibility. In this paper, we investigate the relationship between thickness of Bi-2212 layer and Jc enhancement by PAIR process for monolayer Bi-2212rAg tapes in order to optimize Bi-2212 layer thickness in long tape fabrication.
2. Experimental Fig. 1 shows the sample preparation procedure. Bi-2212 powder prepared by a conventional ceramics technique with the nominal composition of
Fig. 1. The fabrication procedure of samples.
Bi 2.00 Sr2.05 Ca 0.95 Cu 2.00 O x was mixed with an organic solvent, a dispersant, and a binder to obtain paste. Pure silver tapes of 50 mm thick and 200 mm long were coated in the width of 2.5–3.5 mm with the paste through a painting process. Several kinds of tapes with different coated layer thickness ranging 25–200 mm in green state were prepared. Silver part of the tapes without paste coating was turned over at the edge of the coating in order to fold the oxide layer. After this folding procedure, the tapes were cut into two pieces. One of them was processed with PAIR technique of pre-annealing in pure oxygen gas at 8408C for 1 h and subsequent one-path cold-rolling along the longitudinal direction of the tapes with 25% deformation. Melt-solidification process in the flowing oxygen atmosphere has been performed as follows: the tapes with PAIR process ŽPAIR samples. and without it Žno-PAIR samples. were cut into pieces and heated up to 8888C. After keeping 10 min at the highest temperature, the samples were cooled to 8358C at the cooling rate of 58Crh, and then rapidly cooled to room temperature. Transport Ic was measured by a four-probe method at 4.2 K in a magnetic field of 10 T parallel to the tape surface. The value of Ic was determined
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
by using 1 mVrcm electric field criterion. The thickness and the width of the oxide layer were determined by microscope observation for transverse cross-section Žacross the tape length.. Oxide Jc was calculated by using Ic , the thickness, and the width. Thickness in longitudinal cross-section Žalong the tape length. was also investigated. Thickness distribution across and along the tape length was examined with measuring the thickness at every 0.4 and 0.8 mm, respectively.
3. Results and discussion Fig. 2 shows the relationship between Bi-2212 layer thickness and oxide Jc at 4.2 K and 10 T for the samples prepared with and without PAIR process. Oxide Jc decreases with increasing the oxide layer thickness in both cases. Jc enhancement by PAIR process is more significant for thinner oxide layer. For the oxide layer less than 10 mm thick, oxide Jc Ž4.2 K, 10 T. around 4.0 and 1.6 = 10 5
Fig. 2. Relationship between Bi-2212 layer thickness and oxide Jc .
73
Arcm2 are obtained for PAIR and no-PAIR samples, respectively. However, a large variation of Jc is observed for both series of samples in this thickness range. For example, oxide Jc of 1.0–4.4 and 0.8–3.0 = 10 5 Arcm2 are obtained for PAIR and no-PAIR samples, respectively. This supports that there still remains the difficulty to achieve a good homogeneity of very thin oxide layer even with performing PAIR process. Good reproducibility of Jc is seen for PAIR samples with thickness around andror more than 20 mm except for one sample with Jc of 1.8 = 10 5 Arcm2 at 23 mm in thickness. For example, oxide Jc of 3.2 " 0.4 = 10 5 Arcm2 were obtained for PAIR processed Bi-2212 layer with thickness of 19–22 mm. Low Jc value of 1.8 = 10 5 Arcm2 at 23 mm in thickness supposed not to be an essential Jc variation for PAIR samples. It is supposed to originate from sample processing such as green coating, intermediate rolling andror heat treatment. Further optimization of processing parameters will lead to the better Jc reproducibility. It is notable that PAIR samples show higher Jc with smaller Jc variation than no-PAIR samples. Jc enhancement by PAIR process diminishes with increasing the oxide layer thickness and disappears at the thickness around 50 mm. These results indicate that PAIR process is more effective for thinner oxide layer. Thickness variation in the oxide layer is shown in Fig. 3a and b for PAIR and no-PAIR samples, respectively. Average value of measured thickness in longitudinal cross-section Žalong the tape length. of the oxide layer is plotted against that in transverse cross-section Žacross the tape length. for each sample. Error bars represent the standard deviation of measured values. Coefficient of deviation ŽCOV, standard deviationraverage. is shown in Fig. 4a and b for the longitudinal cross-section and the transverse one, respectively. Thickness variation is much smaller in PAIR sample than in no-PAIR sample in each cross-section. PAIR process enables one to obtain Bi-2212 oxide layer with small thickness variation ŽCOV around 2–8% in rolling direction and 2–19% across the tape length., however, large variation of thickness ŽCOV around 4–24% in rolling direction and 30–80% across the tape length. is observed for no-PAIR samples. PAIR process has a large effect to improve the macroscopic homogeneity
74
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
Bi-2212 ™ liquid q Bi-free solid phase q Cu-free solid phase. This results in Bi-2212 grain growth in random orientation in the solidification reaction. Degradation of Bi-2212 grain alignment also originates in random crystal growth at the surface of voids. By performing PAIR process, the oxide layer is densified in the intermediate rolling stage. The amount of voids is decreased to almost zero and grain alignment of Bi-2212 particles before melting is improved w17,18x. PAIR process suppresses random nucleation and growth of Bi-2212 crystals in the solidification reaction and enhances grain alignment in the final product. Typical microstructures in
Fig. 3. Thickness variation in Bi-2212 layer for Ža. PAIR and Žb. no-PAIR samples.
of Bi-2212 layer. This is supposed to be one of the reasons for the large Jc enhancement achieved by PAIR process. Fig. 5 illustrates schematically, a possible explanation for this thickness variation. In Bi-2212 fabrication by using a paste or slurry, green coating includes a large amount of organic stuff such as solvent, dispersant, and binder. In calcination or pre-annealing stage to remove them, a large amount of pore orrand void is produced and remains in melting process. These defects prohibit a homogeneous progress of the melt-solidification reaction and degrade Bi-2212 grain alignment in the final product. Random orientation of calcined Bi-2212 powders leads to a random network of solid particles which are produced in the incongruent melting reaction,
Fig. 4. Thickness variation in Bi-2212 layer in Ža. longitudinal cross-section and Žb. transverse one.
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
75
Fig. 5. Schematic illustration of a possible explanation for the difference in thickness variation between PAIR and no-PAIR samples.
the final products with and without PAIR process have been already reported w15–18x.
4. Conclusion The relationship between thickness of the Bi-2212 layer and Jc enhancement by PAIR process for monolayer Bi-2212rAg tapes was studied in order to optimize Bi-2212 layer thickness in long tape fabrication. Thickness variation in Bi-2212 layer was also investigated in detail to show the effect of PAIR process in macroscopic geometry. We obtained conclusions as follows: Ž1. Jc enhancement by PAIR process is more significant for thinner Bi-2212 layer. For Bi-2212 layer less than 10 mm thick, oxide Jc Ž4.2 K, 10 T. around 4.0 and 1.6 = 10 5 Arcm2 are obtained for PAIR and no-PAIR samples, respectively, however, a large variation of Jc is seen for
both series of samples in this thickness range. Ž2. Good reproducibility of Jc is observed for PAIR samples with the Bi-2212 layer thicker than 20 mm. PAIR samples show smaller Jc variation than noPAIR samples. Ž3. Jc enhancement by PAIR process diminishes with increasing Bi-2212 layer thickness and disappears at the thickness around 50 mm. Ž4. PAIR process produces Bi-2212 layer with small variation of thickness Ž2–8% along the tape length and 2–19% across it., however, large variation of thickness Ž4–24% along the tape length and 30–80% across it. remains in no-PAIR samples. PAIR process has a large effect to improve the macroscopic homogeneity of Bi-2212 layer. This is supposed to be one of the reasons for the large Jc enhancement achieved by PAIR process. Ž5. Optimal thickness of Bi-2212 layer is around 20 mm considering Jc reproducibility and macroscopic homogeneity. The combination of Bi-2212 layer around 20 mm in
76
H. Kitaguchi et al.r Physica C 320 (1999) 71–76
thickness and PAIR process enables one to obtain superconducting tapes with oxide Jc Ž4.2 K, 10 T. of 3.2 " 0.4 = 10 5 Arcm2 and small variation in thickness.
Acknowledgements The authors would like to thank Dr. T. Hasegawa ŽShowa Electric Wire and Cable. for her contribution in the sample preparation.
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