- Email: [email protected]
Progress in research and development on long length coated conductors in Fujikura
Physica C 469 (2009) 1290–1293
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Progress in research and development on long length coated conductors in Fujikura H. Kutami *, T. Hayashida, S. Hanyu, C. Tashita, M. Igarashi, H. Fuji, Y. Hanada, K. Kakimoto, Y. Iijima, T. Saitoh Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan
a r t i c l e
i n f o
Article history: Available online 30 May 2009 PACS: 68.55.Jk 74.76.Bz 81.05.Je 81.15.Jj Keywords: RE-123 coated conductors Ion beam assisted deposition (IBAD) Pulsed laser deposition (PLD)
a b s t r a c t This paper describes a process for the fabrication of coated conductor (CC) wires, which is developed by Fujikura, and discusses their properties. These wires are composed of a buffer layer, which is formed by the ion beam assisted deposition (IBAD) technique, and a superconducting layer, which is formed by the pulsed laser deposition (PLD) process. In order to increase the Ic L which means the product of critical current (Ic) and the length (L) of the CC wires the Ic value and Ic distribution along the length of CC wires have been improved by using a new PLD pilot plant. The Ic value increases approximately linearly with the thickness of the superconducting layer, and it continues to increase up to a thickness of approximately 4 lm or 5 lm. The Ic L value of our fabricated CC wire is 176,023 Am. The Ic value of a short sample is 972 A/cm. The process rate of IBAD is enhanced up to 100 m/h and that of PLD is enhanced up to 20 m/h. A 40-m long CC tape, which has an average Ic of 300 A, is fabricated by the improved IBAD and PLD techniques. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction In 1991, Fujikura developed a method to fabricate an in-plane textured buffer layer on a non-textured metal substrate by assisted ion irradiation. Since then, Fujikura has intensively improved the process of fabricating a buffer layer by developing a technique termed ion beam assisted deposition (IBAD). IBAD is the most promising technique for the fabrication of an in-plane textured buffer layer. A superconducting layer is formed on the buffer layer. Fujikura has synthesized a superconducting layer of a CC wire by a pulsed laser deposition (PLD) process. This process was used because Fujikura had proved its potential in the synthesis of a long superconducting layer [1,2]. Since a metal substrate has high mechanical strength, the CC wires are reliable under tensile stress. From the significant results of research and development on coated conductor (CC) wires, it is found that the critical current (Ic) value of CC wires increases over several hundred amperes, and their length increases to 100 m or more. The length and Ic value of CC wires are considered to be sufficient parameters for performing a practical proof test. Fujikura has been providing CC wires for the national project and contributing in conducting field tests of various devices such as cables and fault current limiters (FCL) [3,4]. This paper de* Corresponding author. Address: 1440, Mutsuzaki, Sakura, Chiba 285-8550, Japan. Tel.: +81 43 484 2476; fax: +81 43 484 2472. E-mail address: [email protected] (H. Kutami). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.05.135
scribes a process used for the fabrication of CC wires and discusses their properties; this process has been developed by Fujikura. 2. Long RE-123 CC wire with high Ic 2.1. Process for the fabrication of long RE-123 CC wire with high Ic Rolled Hastelloy tape (500 m) was used as a substrate for the fabrication of a CC tape. The thickness of the tape was 0.1 mm and its width was 10 mm. The surface of the Hastelloy tape was polished until its surface roughness (Ra) value was below 10 nm. An in-plane textured Gd2Zr2O7 (GZO) buffer layer was formed on the polished surface of the Hastelloy tape by the IBAD technique. The equipment used in the IBAD technique was developed to fabricate a long in-plane textured GZO layer. The equipment comprises a radio frequency discharged ion source which acts as an assisting ion beam source; it has an area of 110 cm 15 cm [5]. The in-plane textured buffer layer was grown in a vacuum chamber, which has a wide film formation area of 100 cm 30 cm. The Argon ions were used in the assisting ion and the sputtering sources of the GZO target. The incident angle of the assisting ion beam was 55° to the substrate normal. CeO2 secondary buffer layers and GdBa2Cu3Ox (GdBCO) layers were formed on the GZO film by the PLD process. Then, a silver film with a thickness of 15 lm was deposited on the GdBCO film by a reel-to-reel RF magnetron sputtering technique. In order to
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improve the superconducting properties of the as-prepared CC tape, it was annealed at 500 °C in 0.1 MPa oxygen atmospheres. The CC tape was laminated to a metal tape so that it could be used as a CC wire, and subsequently, it was warped with insulating tapes. Ic of the 500-m long wire was measured at 77 K and 0 T using a continuous reel-to-reel measurement system. The distance between the electrodes was 70 cm. Ic was determined at 10 6 V/ cm. In this study, crystalline alignment of the films was measured by X-ray diffraction pole figure, and the thickness of the films was measured using a stylus-type contacting profilometer.
Fig. 2. Cross-sectional view of coated conductor wires: (a) Cu tape laminated coated conductor wire, (b) Ni–Cr alloy tape laminated coated conductor wire.
5
2.2. Results and discussion of long RE-123 CC wire with high Ic
Mar.
Feb.
Jan.
Dec.
Oct.
Nov.
Sep.
Aug.
Jul.
Amount of supply [km]
0
Fig. 3. A amount of 5-mm-wide insulated coated conductor wires, supplied for national project.
ductors is 4.7 km. This length includes 1.8 km of CC tapes that were supplied from the Nagoya coated conductor center of SRLISTEC. Ni–Cr laminated conductors with Ic ranging from 60 A to 70 A were supplied for manufacturing three-phase test modules of super-to-normal transition type FCL. The total amount of these conductors is 1.4 km. 2.4. Further improvement in Ic characteristics In order to improve the Ic L value of a CC wire, the Ic value and Ic distribution along the length of a CC tape have been improved by using a new PLD pilot plant. Fig. 4 shows the schematic illustration of the vapor chamber of the improved PLD pilot plant. An important feature of this plant is that its film formation area is surrounded by heaters. This structure is termed ‘‘hot wall”. Since crystals are grown under small
Large deposition area IBAD template
500
Critical current (A/cm)
1
Jun.
A metal tape laminated with a CC was chosen on the basis of its application. Fig. 2 shows the cross-sectional view of the CC wires. The wires shown in Fig. 2a were used in the manufacture of a coil of a motor. A copper tape, which has high thermal conductivity and low electrical resistance, was chosen to avoid an accident resulting from the generation of critical hot spots. On the other hand, for an application such as a FCL, which is based on quenching phenomena, a Ni–Cr alloy tape having high electrical resistance was chosen. The cross-sectional view of the Ni–Cr tape laminated wire is shown in Fig. 2b. Fig. 3 shows the number of CC wires that are supplied for the national project. In 2007, 5-mm wide insulated CC wires were delivered. Cu-laminated conductors with Ic ranging from 50 A to 150 A were delivered for manufacturing prototypes of a ship propulsion motor, and transformers. The total amount of these con-
2
May.
2.3. Records of CC wires supplied for the national project
3
Apr.
Before the formation of the 500-m long superconducting layer on the buffer layer, the characteristics of the in-plane textured buffer layer of both the ends of the tapes were evaluated by determining the full width at half maximum (FWHM) of u-scans of GZO (2 2 2) and the characteristics of the cap layer were evaluated by determining the FWHM of CeO2 (2 2 0). The FWHM of CeO2 (2 2 0) was less than 5°. This value was sufficiently small for the formation of a superconducting layer having a critical current density (Jc) greater than 2 MA/cm2. Ic measured along the length of the CC wire is shown in Fig. 1. The maximum Ic was 400 A/cm. Some low-Ic points were also observed in this figure. These low-Ic points were formed by some defects such as delamination, cracks, and dust particles. However, Ic along approximately the entire length of the CC wire was greater than 350 A/cm; a 503.5-m long tape with Ic of 349.6 A/cm was successfully fabricated. In terms of Ic L, a CC wire with an Ic L of 176,023 A m/cm was fabricated.
Total Cu-laminated NiCr-laminated
4
400
300 200
Hot-wall heating
100
100 0
100
200
300
400
YBCO target
500
position(m) Fig. 1. Ic value distribution along the length of the 500-m-long coated conductor tape.
Laser beam scanning Fig. 4. Schematic illustration of the inside of the hot-wall-type PLD.
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H. Kutami et al. / Physica C 469 (2009) 1290–1293
350 300
c
I (77 K) (A)
250 200 150 100 50 0 0
40
20
60
80
100
Position (m) Fig. 5. Ic distribution along the length of a coated conductor which is fabricated using hot-wall-type PLD.
1000
Fig. 7. Cross-sectional TEM images of coated conductors with (1 1 1) MgO buffer layer (a), and ordinary GZO (b).
800
Ic (A)
600
400
200
0 0
1
2
3
4
5
6
7
Thickness of superconductor layer(mm) Fig. 6. Dependence of Ic value on the thickness of the superconducting layer.
temperature distribution, Ic distribution along the length of the CC tape is expected to improve. In addition, since the growth temperature of the crystal is not affected by the thickness of the grown superconducting layer, the Jc value is expected to be constant. Fig. 5 shows the Ic distribution along the length of the CC tape. At approximately 70 m, both the Ic value and Ic distribution decreased significantly. Fig. 6 shows the relationship between the Ic value and the thickness of the superconducting layer. The Ic value increases approximately linearly with the thickness of the superconducting layer, and this relationship continues to exist up to thickness of approximately 4 lm or 5 lm. The Ic value of a short sample of CC wires which was fabricated by PLD was 972 A.
cess rate [6,7]. Fig. 7a shows the TEM cross-sectional image of a CC tape which is fabricated by a simple IBAD technique. First a (1 1 1) GZO buffer layer is grown on the metal substrate. When the thickness of the GZO crystals increases to 800 nm, the (1 0 0) GZO buffer layer starts growing. It is well known that the secondary buffer layer of CeO2 is grown on the (1 0 0) GZO crystal surface. In order to use this buffer layer as a substrate of the secondary buffer layer of CeO2, the thickness is required more than 800 nm. Two types of buffer layer structures have been examined to reduce the thickness of a buffer layer. Fig. 7b shows the cross-sectional TEM image of one of the improved structure type. A GZO in-plane textured buffer layer was deposited on a MgO in-plane textured buffer layer [8]. Both the in-plane textured buffer layers were fabricated by the IBAD technique. The (1 1 1) MgO buffer layer was parallel to the surface of metal substrate. The (1 1 1) GZO layer was parallel to the surface of the (1 1 1) MgO buffer layer. The thickness of the (1 1 1) GZO buffer layer was reduced to 140 nm and that the thickness of the (1 1 1) MgO buffer layer was 60 nm. Therefore, the thickness of the buffer layers, which were used as a substrate of the (1 0 0) GZO, is reduced from 800 nm to 200 nm. From the abovementioned result, it is found that (1 1 1) MgO crystals can be used effectively reduce the thickness of the (1 1 1) GZO buffer layer. The other structure type is the MgO buffer layer without the GZO buffer layer. The (1 0 0) MgO buffer layer is parallel to the surface of the metal substrate. Fig. 8 shows the TEM cross-sectional image of the CC tape, which comprises this buffer layer. The
3. Development of lower cost CC wires Facility installation cost and labor cost form a large part of the cost of RE-123 wires. Therefore Fujikura has attempted to significantly improve the time required to fabricate an in-plane textured buffer layers, a cap layer, and superconducting layer, since these layers are fabricated using expensive machines. 3.1. Process rate improvement of an IBAD process A reduction in the thickness of a buffer layer, which is fabricated by the IBAD technique, reflects an improvement in the IBAD pro-
Fig. 8. Cross-sectional TEM image of coated conductors with (1 0 0) MgO buffer layer.
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H. Kutami et al. / Physica C 469 (2009) 1290–1293 Table 1 (1 1 1) MgO buffer layer and (1 0 0) MgO buffer layer. Calculated process rate (m/h)
350
Length of sample
Ic (A/cm w)
Jc (MA/cm2)
(1 1 1)
20
Short 20 m
550 380
2.5 1.9
(1 0 0)
100
Short 10 m
550 400
2.7 2.0
300
I c (77 K) (A)
Miller index of MgO crystal
400
250 200 150 100 50 0
0
1000
20
30
40
Position (m) Fig. 10. Ic distribution along the length of the coated conductor, the IBAD process rate was 100 m/h and the PLD process rate of superconducting layer was 40 m/h.
800
superconducting layer and the Ic value of the CC tape. The Ic value of a short sample of the CC tape, which was fabricated at a process rate of 40 m/h, reached 380 A/cm w. Fig. 10 shows the Ic distribution along the length of the CC tape. The process rate of the inplane textured buffer layer of the CC wire was 100 m/h and that of the superconducting layer of the CC wire was 20 m/h. A rapid drop in the Ic value was observed however, a 40-m long CC tape, which has an average Ic value of 300 A, was fabricated.
600
Ic (A)
10
400
200
0 0
1
2
3
4
5
6
7
Thickness of SC layer ( µm) Fig. 9. Relationship between Ic value and the thickness of the superconducting layer.
thickness of the (1 0 0) MgO layer is approximately 10 nm, and the secondary buffer layer was formed on the (1 0 0) MgO. The thickness of texture buffer layer which is used as a substrate of the secondary buffer layer is reduced to 10 nm. Both new type of buffer layers (a (1 1 1) MgO with a (1 1 1) GZO, and a (1 0 0) MgO) are formed on a ceramic thin film which is deposited on the surface of the metal substrate. Therefore, in order to fabricate a MgO buffer layer, it is essential to incorporate a new process for the formation of a ceramic thin film. However, the cost impact of incorporating this additional process is expected to be small. Since the ceramic film is fabricated by a simple sputter process without assisted ion irradiation, it is apparent that the process rate of the sputter process is higher than that of the IBAD technique. Table 1 lists the calculated process rates, which are based on the thickness of the buffer layer, and the superconducting properties of these CC tapes. The Ic and Jc values of the CC tapes are equivalent to those of ordinary CC tapes.
4. Conclusions Efforts to improve the Ic L value and the process rate of the CC wires have been made by Fujikura. A long CC wire was successfully fabricated by a process that included the vacuum process of IBAD and PLD. The length of the wire was 503.6 m, the width of the tape was 10 mm, and the Ic L value of the wire was 176,023 A m/cm. An improvement in the process rate has also been achieved. A process rate of 100 m/h was proved to be suitable for the fabrication of an in-plane textured buffer layer by the IBAD technique, a process rate of 60 m/h was proved to be suitable for the fabrication of a CeO2 cap layer by the PLD process, and a process rate of 40 m/h was proved to be suitable for the fabrication of a GdBCO superconducting layer. Fujikura intends to enhance the process rate and the Ic L value in order to fabricate a kilometer long CC wire with an Ic value of several hundred amperes or more. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References
3.2. Process rate improvement of PLD process Using the ordinary PLD pilot plant, the substrate of CC tape passed through the deposition area only once. Since the number of pass lines in the PLD pilot plant were increased from a single line to multi lines, the length of substrate which runs the area of film formation were increased. This was possible by scanning the ablation laser light. The power of the laser light was also increased to enhance the process rate of PLD. As a result, in 2008, the process rate of a CeO2 cap layer was increased to 60 m/h. Fig. 9 shows the relationship between the PLD process rate of the GdBCO
[1] Y. Iijima, N. Tanabe, O. Kohno, Y. Ikeno, Appl. Phys. Lett. 60 (1992) 769. [2] Y. Iijima, K. Kakimoto, Y. Yamada, T. Izumi, T. Saitoh, Y. Shiohara, MRS Bull. 29 (2004) 564. [3] V. Selvamanickam, Y. Chen, X. Xiong, Y.Y. Xie, J.L. Reeves, X. Zhang, Y. Qiao, K.P. Lenseth, R.M. Schmidt, A. Rar, D.W. Hazelton, K. Tekletsadik, IEEE Trans. Appl. Supercond. 17 (2007) 3231. [4] R. Teranishi, T. Izumi, Y. Shiohara, Supercond. Sci. Technol. 19 (2006) S4. [5] S. Hanyu, Y. Iijima, H. Fuji, K. Kakimoto, T. Saitoh, Physica C 463 (2007) 568. [6] C.P. Wang, K.B. Do, M.R. Beasley, T.H. Geballe, R.H. Hammond, Appl. Phys. Lett. 71 (1997) 2955. [7] P.N. Arendt, S.R. Foltyn, MRS Bull. 29 (2004) 543. [8] S. Hanyu, T. Miura, Y. Iijima, Y. Hanada, H. Fuji, K. Kakimoto, T. Saitoh, Abst. CSJ Conf. 76 (2007) 22 (in Japanese).
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Progress in research and development on long length coated conductors in Fujikura H. Kutami *, T. Hayashida, S. Hanyu, C. Tashita, M. Igarashi, H. Fuji, Y. Hanada, K. Kakimoto, Y. Iijima, T. Saitoh Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan
a r t i c l e
i n f o
Article history: Available online 30 May 2009 PACS: 68.55.Jk 74.76.Bz 81.05.Je 81.15.Jj Keywords: RE-123 coated conductors Ion beam assisted deposition (IBAD) Pulsed laser deposition (PLD)
a b s t r a c t This paper describes a process for the fabrication of coated conductor (CC) wires, which is developed by Fujikura, and discusses their properties. These wires are composed of a buffer layer, which is formed by the ion beam assisted deposition (IBAD) technique, and a superconducting layer, which is formed by the pulsed laser deposition (PLD) process. In order to increase the Ic L which means the product of critical current (Ic) and the length (L) of the CC wires the Ic value and Ic distribution along the length of CC wires have been improved by using a new PLD pilot plant. The Ic value increases approximately linearly with the thickness of the superconducting layer, and it continues to increase up to a thickness of approximately 4 lm or 5 lm. The Ic L value of our fabricated CC wire is 176,023 Am. The Ic value of a short sample is 972 A/cm. The process rate of IBAD is enhanced up to 100 m/h and that of PLD is enhanced up to 20 m/h. A 40-m long CC tape, which has an average Ic of 300 A, is fabricated by the improved IBAD and PLD techniques. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction In 1991, Fujikura developed a method to fabricate an in-plane textured buffer layer on a non-textured metal substrate by assisted ion irradiation. Since then, Fujikura has intensively improved the process of fabricating a buffer layer by developing a technique termed ion beam assisted deposition (IBAD). IBAD is the most promising technique for the fabrication of an in-plane textured buffer layer. A superconducting layer is formed on the buffer layer. Fujikura has synthesized a superconducting layer of a CC wire by a pulsed laser deposition (PLD) process. This process was used because Fujikura had proved its potential in the synthesis of a long superconducting layer [1,2]. Since a metal substrate has high mechanical strength, the CC wires are reliable under tensile stress. From the significant results of research and development on coated conductor (CC) wires, it is found that the critical current (Ic) value of CC wires increases over several hundred amperes, and their length increases to 100 m or more. The length and Ic value of CC wires are considered to be sufficient parameters for performing a practical proof test. Fujikura has been providing CC wires for the national project and contributing in conducting field tests of various devices such as cables and fault current limiters (FCL) [3,4]. This paper de* Corresponding author. Address: 1440, Mutsuzaki, Sakura, Chiba 285-8550, Japan. Tel.: +81 43 484 2476; fax: +81 43 484 2472. E-mail address: [email protected] (H. Kutami). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.05.135
scribes a process used for the fabrication of CC wires and discusses their properties; this process has been developed by Fujikura. 2. Long RE-123 CC wire with high Ic 2.1. Process for the fabrication of long RE-123 CC wire with high Ic Rolled Hastelloy tape (500 m) was used as a substrate for the fabrication of a CC tape. The thickness of the tape was 0.1 mm and its width was 10 mm. The surface of the Hastelloy tape was polished until its surface roughness (Ra) value was below 10 nm. An in-plane textured Gd2Zr2O7 (GZO) buffer layer was formed on the polished surface of the Hastelloy tape by the IBAD technique. The equipment used in the IBAD technique was developed to fabricate a long in-plane textured GZO layer. The equipment comprises a radio frequency discharged ion source which acts as an assisting ion beam source; it has an area of 110 cm 15 cm [5]. The in-plane textured buffer layer was grown in a vacuum chamber, which has a wide film formation area of 100 cm 30 cm. The Argon ions were used in the assisting ion and the sputtering sources of the GZO target. The incident angle of the assisting ion beam was 55° to the substrate normal. CeO2 secondary buffer layers and GdBa2Cu3Ox (GdBCO) layers were formed on the GZO film by the PLD process. Then, a silver film with a thickness of 15 lm was deposited on the GdBCO film by a reel-to-reel RF magnetron sputtering technique. In order to
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H. Kutami et al. / Physica C 469 (2009) 1290–1293
improve the superconducting properties of the as-prepared CC tape, it was annealed at 500 °C in 0.1 MPa oxygen atmospheres. The CC tape was laminated to a metal tape so that it could be used as a CC wire, and subsequently, it was warped with insulating tapes. Ic of the 500-m long wire was measured at 77 K and 0 T using a continuous reel-to-reel measurement system. The distance between the electrodes was 70 cm. Ic was determined at 10 6 V/ cm. In this study, crystalline alignment of the films was measured by X-ray diffraction pole figure, and the thickness of the films was measured using a stylus-type contacting profilometer.
Fig. 2. Cross-sectional view of coated conductor wires: (a) Cu tape laminated coated conductor wire, (b) Ni–Cr alloy tape laminated coated conductor wire.
5
2.2. Results and discussion of long RE-123 CC wire with high Ic
Mar.
Feb.
Jan.
Dec.
Oct.
Nov.
Sep.
Aug.
Jul.
Amount of supply [km]
0
Fig. 3. A amount of 5-mm-wide insulated coated conductor wires, supplied for national project.
ductors is 4.7 km. This length includes 1.8 km of CC tapes that were supplied from the Nagoya coated conductor center of SRLISTEC. Ni–Cr laminated conductors with Ic ranging from 60 A to 70 A were supplied for manufacturing three-phase test modules of super-to-normal transition type FCL. The total amount of these conductors is 1.4 km. 2.4. Further improvement in Ic characteristics In order to improve the Ic L value of a CC wire, the Ic value and Ic distribution along the length of a CC tape have been improved by using a new PLD pilot plant. Fig. 4 shows the schematic illustration of the vapor chamber of the improved PLD pilot plant. An important feature of this plant is that its film formation area is surrounded by heaters. This structure is termed ‘‘hot wall”. Since crystals are grown under small
Large deposition area IBAD template
500
Critical current (A/cm)
1
Jun.
A metal tape laminated with a CC was chosen on the basis of its application. Fig. 2 shows the cross-sectional view of the CC wires. The wires shown in Fig. 2a were used in the manufacture of a coil of a motor. A copper tape, which has high thermal conductivity and low electrical resistance, was chosen to avoid an accident resulting from the generation of critical hot spots. On the other hand, for an application such as a FCL, which is based on quenching phenomena, a Ni–Cr alloy tape having high electrical resistance was chosen. The cross-sectional view of the Ni–Cr tape laminated wire is shown in Fig. 2b. Fig. 3 shows the number of CC wires that are supplied for the national project. In 2007, 5-mm wide insulated CC wires were delivered. Cu-laminated conductors with Ic ranging from 50 A to 150 A were delivered for manufacturing prototypes of a ship propulsion motor, and transformers. The total amount of these con-
2
May.
2.3. Records of CC wires supplied for the national project
3
Apr.
Before the formation of the 500-m long superconducting layer on the buffer layer, the characteristics of the in-plane textured buffer layer of both the ends of the tapes were evaluated by determining the full width at half maximum (FWHM) of u-scans of GZO (2 2 2) and the characteristics of the cap layer were evaluated by determining the FWHM of CeO2 (2 2 0). The FWHM of CeO2 (2 2 0) was less than 5°. This value was sufficiently small for the formation of a superconducting layer having a critical current density (Jc) greater than 2 MA/cm2. Ic measured along the length of the CC wire is shown in Fig. 1. The maximum Ic was 400 A/cm. Some low-Ic points were also observed in this figure. These low-Ic points were formed by some defects such as delamination, cracks, and dust particles. However, Ic along approximately the entire length of the CC wire was greater than 350 A/cm; a 503.5-m long tape with Ic of 349.6 A/cm was successfully fabricated. In terms of Ic L, a CC wire with an Ic L of 176,023 A m/cm was fabricated.
Total Cu-laminated NiCr-laminated
4
400
300 200
Hot-wall heating
100
100 0
100
200
300
400
YBCO target
500
position(m) Fig. 1. Ic value distribution along the length of the 500-m-long coated conductor tape.
Laser beam scanning Fig. 4. Schematic illustration of the inside of the hot-wall-type PLD.
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H. Kutami et al. / Physica C 469 (2009) 1290–1293
350 300
c
I (77 K) (A)
250 200 150 100 50 0 0
40
20
60
80
100
Position (m) Fig. 5. Ic distribution along the length of a coated conductor which is fabricated using hot-wall-type PLD.
1000
Fig. 7. Cross-sectional TEM images of coated conductors with (1 1 1) MgO buffer layer (a), and ordinary GZO (b).
800
Ic (A)
600
400
200
0 0
1
2
3
4
5
6
7
Thickness of superconductor layer(mm) Fig. 6. Dependence of Ic value on the thickness of the superconducting layer.
temperature distribution, Ic distribution along the length of the CC tape is expected to improve. In addition, since the growth temperature of the crystal is not affected by the thickness of the grown superconducting layer, the Jc value is expected to be constant. Fig. 5 shows the Ic distribution along the length of the CC tape. At approximately 70 m, both the Ic value and Ic distribution decreased significantly. Fig. 6 shows the relationship between the Ic value and the thickness of the superconducting layer. The Ic value increases approximately linearly with the thickness of the superconducting layer, and this relationship continues to exist up to thickness of approximately 4 lm or 5 lm. The Ic value of a short sample of CC wires which was fabricated by PLD was 972 A.
cess rate [6,7]. Fig. 7a shows the TEM cross-sectional image of a CC tape which is fabricated by a simple IBAD technique. First a (1 1 1) GZO buffer layer is grown on the metal substrate. When the thickness of the GZO crystals increases to 800 nm, the (1 0 0) GZO buffer layer starts growing. It is well known that the secondary buffer layer of CeO2 is grown on the (1 0 0) GZO crystal surface. In order to use this buffer layer as a substrate of the secondary buffer layer of CeO2, the thickness is required more than 800 nm. Two types of buffer layer structures have been examined to reduce the thickness of a buffer layer. Fig. 7b shows the cross-sectional TEM image of one of the improved structure type. A GZO in-plane textured buffer layer was deposited on a MgO in-plane textured buffer layer [8]. Both the in-plane textured buffer layers were fabricated by the IBAD technique. The (1 1 1) MgO buffer layer was parallel to the surface of metal substrate. The (1 1 1) GZO layer was parallel to the surface of the (1 1 1) MgO buffer layer. The thickness of the (1 1 1) GZO buffer layer was reduced to 140 nm and that the thickness of the (1 1 1) MgO buffer layer was 60 nm. Therefore, the thickness of the buffer layers, which were used as a substrate of the (1 0 0) GZO, is reduced from 800 nm to 200 nm. From the abovementioned result, it is found that (1 1 1) MgO crystals can be used effectively reduce the thickness of the (1 1 1) GZO buffer layer. The other structure type is the MgO buffer layer without the GZO buffer layer. The (1 0 0) MgO buffer layer is parallel to the surface of the metal substrate. Fig. 8 shows the TEM cross-sectional image of the CC tape, which comprises this buffer layer. The
3. Development of lower cost CC wires Facility installation cost and labor cost form a large part of the cost of RE-123 wires. Therefore Fujikura has attempted to significantly improve the time required to fabricate an in-plane textured buffer layers, a cap layer, and superconducting layer, since these layers are fabricated using expensive machines. 3.1. Process rate improvement of an IBAD process A reduction in the thickness of a buffer layer, which is fabricated by the IBAD technique, reflects an improvement in the IBAD pro-
Fig. 8. Cross-sectional TEM image of coated conductors with (1 0 0) MgO buffer layer.
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H. Kutami et al. / Physica C 469 (2009) 1290–1293 Table 1 (1 1 1) MgO buffer layer and (1 0 0) MgO buffer layer. Calculated process rate (m/h)
350
Length of sample
Ic (A/cm w)
Jc (MA/cm2)
(1 1 1)
20
Short 20 m
550 380
2.5 1.9
(1 0 0)
100
Short 10 m
550 400
2.7 2.0
300
I c (77 K) (A)
Miller index of MgO crystal
400
250 200 150 100 50 0
0
1000
20
30
40
Position (m) Fig. 10. Ic distribution along the length of the coated conductor, the IBAD process rate was 100 m/h and the PLD process rate of superconducting layer was 40 m/h.
800
superconducting layer and the Ic value of the CC tape. The Ic value of a short sample of the CC tape, which was fabricated at a process rate of 40 m/h, reached 380 A/cm w. Fig. 10 shows the Ic distribution along the length of the CC tape. The process rate of the inplane textured buffer layer of the CC wire was 100 m/h and that of the superconducting layer of the CC wire was 20 m/h. A rapid drop in the Ic value was observed however, a 40-m long CC tape, which has an average Ic value of 300 A, was fabricated.
600
Ic (A)
10
400
200
0 0
1
2
3
4
5
6
7
Thickness of SC layer ( µm) Fig. 9. Relationship between Ic value and the thickness of the superconducting layer.
thickness of the (1 0 0) MgO layer is approximately 10 nm, and the secondary buffer layer was formed on the (1 0 0) MgO. The thickness of texture buffer layer which is used as a substrate of the secondary buffer layer is reduced to 10 nm. Both new type of buffer layers (a (1 1 1) MgO with a (1 1 1) GZO, and a (1 0 0) MgO) are formed on a ceramic thin film which is deposited on the surface of the metal substrate. Therefore, in order to fabricate a MgO buffer layer, it is essential to incorporate a new process for the formation of a ceramic thin film. However, the cost impact of incorporating this additional process is expected to be small. Since the ceramic film is fabricated by a simple sputter process without assisted ion irradiation, it is apparent that the process rate of the sputter process is higher than that of the IBAD technique. Table 1 lists the calculated process rates, which are based on the thickness of the buffer layer, and the superconducting properties of these CC tapes. The Ic and Jc values of the CC tapes are equivalent to those of ordinary CC tapes.
4. Conclusions Efforts to improve the Ic L value and the process rate of the CC wires have been made by Fujikura. A long CC wire was successfully fabricated by a process that included the vacuum process of IBAD and PLD. The length of the wire was 503.6 m, the width of the tape was 10 mm, and the Ic L value of the wire was 176,023 A m/cm. An improvement in the process rate has also been achieved. A process rate of 100 m/h was proved to be suitable for the fabrication of an in-plane textured buffer layer by the IBAD technique, a process rate of 60 m/h was proved to be suitable for the fabrication of a CeO2 cap layer by the PLD process, and a process rate of 40 m/h was proved to be suitable for the fabrication of a GdBCO superconducting layer. Fujikura intends to enhance the process rate and the Ic L value in order to fabricate a kilometer long CC wire with an Ic value of several hundred amperes or more. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References
3.2. Process rate improvement of PLD process Using the ordinary PLD pilot plant, the substrate of CC tape passed through the deposition area only once. Since the number of pass lines in the PLD pilot plant were increased from a single line to multi lines, the length of substrate which runs the area of film formation were increased. This was possible by scanning the ablation laser light. The power of the laser light was also increased to enhance the process rate of PLD. As a result, in 2008, the process rate of a CeO2 cap layer was increased to 60 m/h. Fig. 9 shows the relationship between the PLD process rate of the GdBCO
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