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Highlights in R&D for coated conductors in Japan
Physica C 445–448 (2006) 496–503 www.elsevier.com/locate/physc
Highlights in R&D for coated conductors in Japan Yuh Shiohara *, Yutaka Kitoh, Teruo Izumi Superconductivity Research Laboratory, ISTEC, 1-10-13, Shinonome, Koto-Ku, Tokyo 135-0062, Japan Available online 30 June 2006
Abstract The current 5-year national project since 2003 for development of coated conductors (CC) using Y-system superconductors has passed for almost a half term and has achieved satisfactory results. In this paper, the current status and the future prospect are reviewed. The group of Fujikura Ltd. and SRL-ISTEC has worked on the long tape with high performance in the PLD-YBCO superconducting tapes on the IBAD-Gd2Zr2O7 buffered substrates. The highest value on the product of Ic · L in the world was marked by the result which were 51,940 A m (212 m · 245 A) by the SRL group. Fujikura Ltd. also realized the longest tape of 200 m with a reasonable high Ic value of 100 A. The values have been steadily improved and the trend is going to be continued, since the large equipments for both IBAD and PLD have been installed, and ready to work on large tapes with a high production rate. In another group, the long tape processing has been developed focusing on lowering the production cost. The extremely high Ic value of 470 A was obtained in the film by the TFA-MOD method on CeO2 (PLD)/GZO(IBAD)/hastelloy substrate. In the efforts for the long tape in the process, a 25 m long tape with its Ic value of 100 A was realized by a continuous reel-to-reel system. Additionally, 100 m class long tapes were also obtained by the MOCVD and PLD-HoBCO processes. Both groups are aiming to achieve the final goals of 500 m long tapes with the high Ic value of 300 A/cm-w by the production rate of 5 m/h. Furthermore, the feasibility study for applications using coated conductors has been already started due to the above-mentioned success of long tape production. Several kinds of coils using long coated conductors such as a solenoid and a pancake coils and the spiral shaped conductors for cable applications were firstly made. Reasonable high performances were confirmed in the trials. For the future plans of coated conductor applications, the following power devices using coated conductors have been proposed; (1) power cable, (2) transformer, (3) motor, (4) fault current limiter, (5) cryocooler, and so on. 2006 Elsevier B.V. All rights reserved. PACS: 81.15. z; 74.72.Bk; 81.10.Jt Keywords: Coated conductors; Long tape; Ic · L; REBa2Cu3O7 d; YBCO; PLD; IBAD; TFA-MOD; MOCVD
1. Introduction Since the discovery of oxide superconductors in 1986 [1], research in material science fields has proceeded and developments for applications have been carried out extensively. Silver sheathed wires for the Bi-system superconductors such as Bi2Sr2Ca2Cu3Oy (Bi2223) have preceded in the development of the long length oxide superconducting wires and tapes. However, there are several weak points
*
Corresponding author. Tel.: +81 3 3536 5711; fax: +81 3 3536 5717. E-mail address: [email protected] (Y. Shiohara).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.04.043
in the Bi-system, including low critical current density (Jc) under the high magnetic fields in the range of the liquid nitrogen temperatures, which limits the usage at about 20 K or lower in many practical applications. Additionally, the limited future cost reduction due to its relatively high Ag volumetric ratio is also one of the difficulties for applications. Development of coated conductors with REBa2Cu3O7 d (RE123: RE is rare earth elements) system superconductors are expected to overcome these weak points, since the RE123 superconductors essentially have a high Jc potential at the liquid nitrogen temperature even in high external magnetic fields. Additionally, it is possible to make a low cost wire compared with that of the
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Bi-system due to the low silver ratio as well as high Jc values. In the Y-system, however, the in-plane crystal grain alignment is required in order to attain full material characteristics of RE123. This requirement has to be satisfied in long tapes for applications of coated conductors. In order to control crystal grain alignments of the superconducting layer, two distinct technologies exist, including fabrication of textured metallic substrates such as RABiTSTM [2,3] and crystal grain alignment of a buffer layer on a non-textured high strength metallic substrate by IBAD [4,5] and/or ISD [6,7] which were both originally developed in Japan. These coated conductors have a multi-layered film structure, that is fabricated an RE123 superconducting layer on a metal substrate with some buffer layers. The metal substrate gives a mechanical strength and the crystal grain alignment in some cases, and the roles of the buffer layers are the crystal grain alignment, suppression of chemical reaction between a superconducting layer and a metal substrate and crack generation. Fabrication of a highly oriented thick superconducting layer is also important for obtaining a high performance, high Ic in coated conductors. In order to solve this problem, various processes such as PLD (pulsed laser deposition) [8], MOD (metal organic deposition) using TFA (trifluoroacetates) [9] and MOCVD (metal organic chemical vapor deposition) [10] have been investigated to obtain the superconducting layers. Recently, the long tape processing for both buffer and superconducting layers with good crystal alignment and high crystallinity shows a rapid progress and enough to start R&D for power device applications in Japan. 2. National project in Japan In the R&D of HTS (high temperature superconductivity), several projects have been conducted as a national
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project by the Ministry of Economics, Trade and Industry (METI) in the style of collaborative research of private companies and universities. The new national project, which has been started at the end of May 2003, consists of four sub-groups as shown in Table 1. First two groups were responsible for the process development. The selection of the processes in the groups were based on the results in the previous national project. In this project, combination of the buffer layer by IBAD and superconducting one by PLD realized the most excellent result on the length with high performance [11–13]. In this type of the tape, the high performance even in the magnetic fields and the suppression of the Ic reduction in a long tape could be achieved due to its fine grain size and the high grain texturing so that this group is selected as ‘‘High Performance Long Tape Development Group’’. In this group, development of stable production for a long tape, improvement of its production rate, achievement of high Ic performance and reduction of the production cost were the main objectives and the targeted goals were listed in Table 2. Fujikura Ltd. and SRL-Nagoya Coated Conductor Center (NCCC) are the players in this group. On the other hand, there might be many different kinds of tapes for different expected applications from an industrial engineering point of view. The lower cost is preferable to the performance in the magnetic fields in some cases. The second processing group was organized in order to respond these requirements. R&D of several different techniques for substrates, buffer layers and superconducting layers aiming at lower cost, which is called as ‘‘Low Cost Long Tape Process Development Group’’. Concerning the substrates, processing for bi-axially textured metal substrates has been considered to be a low cost texturing technique because of its simplicity. The MOD process is one of the promising processes for the low cost production since a
Table 1 Sub-group and their aim in the new national project National Project of C.C. in Japan (2003–2007) Group
Theme
High performance long tape processing Low cost long tape processing Characterizations Material improvement
IBAD (buffer) + PLD (superconducting layer) long tape, stable process conditions, width control, etc. MOD, MOCVD, RABiTS, SOE, PLD long tape, high performance, low cost, etc. Continuous Ic measurement, Jc–B–T–h, AC-losses, Jc distribution, thermal properties, mechanical properties, etc. Material designing (RE123, O-control, etc.) and grain boundary
Table 2 Goals in the present national project in Japan Specifications
Length (m) Ic(@77 K, 0 T) Ic(@77 K, 3 T) Production rate Cost(@77 K, 0 T)
Interim targets (2005)
Final targets (2007)
High performance type
Low cost type
High performance type
Low cost type
P200 m Ic P 200 A/cm-w Ic P 20 A/cm-w – –
P200 m Ic P 200 A/cm-w – – –
P500 m Ic P 300 A/cm-w Ic P 30 A/cm-w P5 m/h 612 Yen/A m
P500 m Ic P 300 A/cm-w – P5 m/h 68 Yen/A m
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large equipment with a high vacuuming system is not required and high yields of production could be expected. A high superconducting performance was already confirmed in the films fabricated by the MOD process using TFA salts [14–20]. Furthermore MOCVD [10] and the combination of PLD and MOD processes have been also worked as a low cost process for a superconducting layer and obtained the results of a long tape fabricated [21]. Sumitomo Electric Industry has aimed at realization of higher Je (engineering Jc) tapes using HoBCO superconducting materials by the combination of PLD and MOD processes. The final goals of this group are also listed in Table 2. The objectives of both the third and the fourth R&D group are basically to support the development in the above-mentioned two process development groups for realizing their R&D targets. The third one includes development of several characterization methods not only Tc, Ic, Jc, Jc–B measurements but of AC loss, quench behavior non-destructive measurements of Ic and Jc, etc. Material designing, including other RE123 and O-control, etc., and the control of grain boundary characteristics have been investigated in the fourth group. All efforts in the national project have been combined and a steady progress has been attained. Then, the recent progress in the above mentioned two process development groups and the future plans for coated conductor development in Japan are reviewed.
the low growth rate, was developed in the NCCC group. They found that the in-plane grain alignment rapidly improved by deposition of thick CeO2 layers by the PLD process without any assistance such as Ar+ ion like the IBAD, which called self-epitaxy, on the IBAD buffered tapes [23]. As a result, the in-plane grain alignment of the buffer layer was achieved; D/ value of 2.6 in CeO2 with a 0.6 lm thick film on the underlying Zr2Gd2O7 buffer with that of 12. This phenomenon can be also applied from the IBAD layer with lower degree of grain alignment as shown in Fig. 1. The growth rate of the CeO2 film is 10 times higher than that of the Zr2Gd2O7 buffer by the IBAD process. Consequently, the total production rate to achieve high texturing including IBAD and CeO2 depositions can be much higher using thin IBAD buffer layer as shown in Fig. 2. As concerning a superconducting layer by the PLD process, there have been several problems to be solved, from the engineering point of view, such as a slow deposition rate, difficulty to fabricate thicker films with maintaining high Jc and low material yields, etc. When the YBCO film thickness increases more than about 1.5 lm, the Jc value drastically decreases in general. This suggests that improve-
3. Progress in R&D for YBCO coated conductors 3.1. High performance long tape processing NCCC (Nagoya Coated Conductor Center, SRLISTEC) and Fujikura Ltd. are the players in this group. They are focusing on the IBAD and the PLD methods for the YBCO coated conductor development. Their R&D objectives are to improve IBAD and PLD methods for longer tapes and rapid production. In the case of NCCC, they also make long IBAD buffered substrate tapes to supply for other research groups to study other superconducting layer deposition processes in the second group. The IBAD method is a process for deposition of an inplane crystal grain aligned buffer layer by applying an additional assist-ion-beam from an appropriate direction during the deposition of the buffer layer. This IBAD process for depositing the in-plane aligned YSZ buffer layer was originally established by the research team of Fujikura Ltd. in 1991. The IBAD-YSZ process gives an excellent crystal grain alignment with a small grain size, which is suitable for maintaining high performance in a long tape. However, the slow deposition rate was a problem from the engineering point of view. The high production rate of 1 m/h was achieved by the development of large-scale IBAD equipment and the discovery of a much better buffer layer material, Zr2Gd2O7 [12,13,22]. The new process to overcome the disadvantage of the IBAD process, which is
Fig. 1. In-plane crystal grain alignment improvement by CeO2 cap layer deposition on IBAD-GZO layer.
Fig. 2. Deposition time dependences of in-plane alignment in IBADZr2Gd2O7/hastelloy tapes and CeO2 film on them by the PLD process.
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ment of Ic by simply increasing the film thickness is difficult to realize in this system. One of the reasons for the Jc degradation in a thick PLD-YBCO film is the increase of the volume fraction of a-axis grains in a thick film. As a cause of the increase in thick YBCO films, it was explained that the surface temperatures of the YBCO layer were considered to be lowered with increasing the number of deposition cycles since the emissivity of the film increases with increment of thickness. Then, the NCCC group investigated the suitable YBCO deposition conditions with raising the heater temperature to maintain the constant film surface temperature even with thickening the film. It was found to be effective to suppress the a-axis oriented grain growth. As a result, the high Ic values were obtained even in a thick YBCO film [24], as shown in Fig. 3, and an extremely high Ic value of 480 A/cm-w was finally obtained by optimizing the deposition temperatures [25]. In order to trace the consideration for a-axis grain suppression for long tape processing, a multi-deposition system is convenient to apply the optimum temperature control dependent of the YBCO film thickness since we can select and control the appropriate temperature for each deposition cycle. Then, a new PLD apparatus, which is shown in Fig. 4, with a multi-plume and multi-turn system for YBCO layers has been used for long tape fabrication, and an extremely high Ic performance with reasonably high end-to-end Ic value of 245 A/cm in 212 m-long YBCO tape, which corresponds to 51,940 A m as a product of Ic · L, has been attained, as shown in Fig. 5 [26]. Fujikura Ltd. has also recently fabricated a 105 m long coated conductor [27] with a reasonable end-to-end Ic value of 126 A/cm was realized. The uniform critical current distribution was obtained through its entire length compared with the result in 2003, which is the endto-end Ic value of 38 A in the 100 m long YBCO tape. They have constructed a large-scale equipment for 500 m-long tape, which was shown in Fig. 6. Finally, a 217 m long YBCO tape with end-to-end Ic value of 88 A, which corre-
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Fig. 4. Photograph of the multi-plume zone in a PLD machine. Large area deposition of YBCO with multi-layered deposition enables to change heater temperature for thickening and to homogenize low Ic region.
Fig. 5. Photograph of a 212 m YBCO tape fabricated by PLD process on a CeO2-buffered IBAD-Gd2Zr2O7/hastelloy substrate.
Fig. 6. Large scale IBAD-equipment for 500 m tapes.
Fig. 3. Dependence of Ic values on numbers of deposition cycle for 10 cm long YBCO layer deposited at various temperatures. All YBCO layers, except the highest Ic one, were deposited on the PLD-CeO2 cap layers with D/ of around 8. The highest Ic ones PLD-CeO2 cap layer was with D/ of 5.
sponds to 19,100 A m as a product of Ic · L, was obtained [28]. The R&D race for the higher products of Ic · L values on RE123 tape in the world was listed in Table 3. As seen in the table, R&D for long coated conductors in the Japan national project proceed steadily.
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Table 3 R&D race for higher Ic · L 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th
Ic · L (A m)
Organization
Ic (A/cm-w)
L (m)
Processing
51,940 22,055 19,100 15,120 (12,870) 9400 8550 6200 2500 2230
SRL-ISTEC IGC-Super Power Fujikura AMSC Sumitomo EHTS Chubu SWCC SRL-ISTEC Goettingen
245 107 88 168 110 (Ic, min) 235 95 155 100 223
223 207 217 90 117 40 90 40 25 10
YBCO(PLD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(MOCVD)/STO/MgO(Cap)/MgO(IBAD)/Y2O3/Al2O3/hatelloy YBCO(PLD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(TFA-MOD)/CeO2/YSZ/Y2O3/Ni-W HoBCO(PLD)/CeO2/YSZ/CeO2/Ni-alloy YBCO(HR-PLD)/CeO2/YSZ/S.S. YBCO(MOCVD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(TFA-MOD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(TFA-MOD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(PLD)/YSZ(IBAD)/S.S.
In addition, improvement of Jc–B properties is required as well, since most cases of the coated conductor applications for electric power devices have expected under the external magnetic fields. One of the promising procedures to solve the Jc degradation in high magnetic fields is introduction of artificial pinning centers in the superconducting layer. Yamada et al. fabricated YBCO films by the PLD method using a target containing the YSZ particles. As a result, the Jc values of the YBCO film deposited by the YSZ mixed target were improved to be higher than that of the conventional PLD-YBCO film, as shown in Fig. 7 [29]. Cross-sectional TEM observations were carried out for the YBCO films deposited by the YSZ/YBCO mixed target. As seen in Fig. 8, the YSZ phase particles were formed nano-epitaxy grain alignments during the YBCO phase crystal growth, which looks like ‘‘bamboo structure’’. These YSZ phase structures have been considered to be the magnetic flux pinning sites. 3.2. Low cost long tape processing Metal organic deposition (MOD) of precursor solutions containing metal trifluoroacetates (TFA) is an attractive process to fabricate YBCO films since it is relatively easy to attain high Jc performance by a simple process in a non-vacuum low cost manner [17,30–33]. By the SRL-
Fig. 7. Magnetic field dependence of the Jc values.
Fig. 8. Cross-sectional TEM image of the PLD-YBCO film deposited by using the YSZ mixed target.
Tokyo group, the optimization of the growth conditions both for the calcination and the crystallization steps has been studied to obtain higher Ic performance. Through the investigations, it was found that the low heating rate and the higher water vapor inlet temperature in the calcination step and the low heating rate and the high water vapor pressure in the crystallization step were effective for higher Ic performance [34–37]. Consequently, the high Ic value of 470 A/cm-w was achieved by combination of the above optimized growth conditions with use of highly textured CeO2 cap layered IBAD-GZO substrates, as shown in Fig. 9. For the long tape processing, the reel-to-reel (RTR) system is applied for both the calcination and the crystallization steps in the SRL group. In this system, it is important to obtain the uniform reaction along the long direction. A stagnant gas region in the furnace could be another reason not only for the low growth rate but for the long transient in the initial and final reaction regions, which lead to the non-uniformity in the long direction. The computer simulation of the crystal growth and the gas flow in the furnace was studied to solve the problem of the gas stagnant region. From the calculations, the several possibilities as the solutions were suggested. A higher gas flow condition during the growth of the YBCO layer was applied in this study. Relatively uniform performance
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Fig. 9. Ic values dependence on YBCO film thickness for different in-plane grain alignments (D/) of CeO2 buffer layers.
was obtained in a 4 m-long YBCO tape with the high Ic value of about 250 A. Furthermore, a 25 m long YBCO tape with the end-to-end Ic value of 100 A/cm was fabricated as shown in Fig. 10, which corresponds to 2500 A m as a product of Ic · L values [36]. The SWCC (Showa Wires and Cable Comp.) is also focusing on the TFA-MOD method for the YBCO layer and has worked on the heat treatment in the batch system. The concept of the batch type apparatus was schematically shown in Fig. 11. They obtained a 6 m-long YBCO tape with the end-to-end Ic value of 69 A/cm-w. They also optimized the heat-treatment conditions for the batch system which is able to fabricate a 200 m-class YBCO tape. Recently, they have fabricated a 40 m-long YBCO tape with the end-to-end Ic of 155 A/cm-w. In addition, the
Fig. 10. Ic distribution in the 25 m-long YBCO superconducting tape fabricated by TFA-MOD process using the reel-to-reel system.
Fig. 11. Schematic image of the basic concept for the butch system furnace in the crystallization step.
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SWCC has worked on the ‘‘All-MOD’’ process, which means that not only a superconducting layer but buffer layers are fabricated by the MOD process as well, and is expected to be the lowest cost process. They already found the suitable materials of Ce–Gd–O for the MOD buffer on a textured metal substrate without cracking [38]. However, the superconducting performance was still in a low Jc level of 105 A/cm2 due to the contamination/diffusion of the substrate elements into the superconducting phase. This problem was solved by the doping of Nb into the buffer layer materials and the Tc value was improved [39]. Concerning the development of the MOCVD process, Chubu Electric Power Company deposited the YBCO phase layer on a PLD-CeO2/IBAD-G2Zr2O7/hastelloy substrate [10,40]. The high Jc value of 2 MA/cm2 at 77 K was confirmed in a short sample. Recently, they have obtained a 92 m YBCO tape with the end-to-end Ic value of 96 A/ cm, which corresponds to 8832 A m as a product of Ic · L values, by the reel-to-reel system. The Sumitomo group has aimed at realization of higher Je tapes using HoBCO superconducting materials by the combination of PLD and MOD processes [41]. They successfully fabricated a 117 m-long HoBCO tape by PLD process with the Ic value of over 110 A/cm. 4. Future plans for R&D of coated conductors YBCO coated conductors have several merits such as lower cost, higher performance in high magnetic fields, mechanical strength, and simplicity of post-treatments compare to the Bi-system for lower AC losses. For example, a multi-filamental structure for AC loss reduction has been successfully demonstrated by both YAG laser scribing and chemical etching on YBCO coated conductors. Additionally, several preliminary results for the coated conductor applications have been recently reported. The Fujikura Ltd. group investigated the bending strain properties for the PLD-YBCO tapes fabricated on the IBAD substrate. Furthermore, they reported the demonstration of a solenoid type coil using long IBAD/PLDYBCO tape [29]. The Super-GM reported the HTS power cable application as well. For the future plans for coated conductor applications, we have proposed the following themes using coated conductors: (1) power cable, (2) transformer, (3) motor, (4) fault current limiter, (5) cryocooler. As concerning proposal (1), it is expected that the installation cost must be lowered drastically by using coated conductor HTS power cable compare to the current cables. An appropriate architecture for lower AC losses, over-current measurement, cable-joints and a proto-type test are included in this proposal. To develop HTS electric power machines and devices applicable to practical power grid; (i) low ac loss, (ii) large current capacity, (iii) stability and protection, (iv) insulation layer are also included. In the case of the development for HTS motor, a full superconducting synchronous motor has been proposed, which means that both
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rotors and stators without iron cores are constructed by HTS coils. The requirements of conductors for a fault current limiter are mainly the operation technology under LN2 temperature and high resistance in the current limiting process. On the proposal (5), development of highly efficient cooling design/manufacture technology which is suitable for YBCO power devices has been proposed. 5. Summary In this paper, the recent progress and the future plans for coated conductor development in Japan were reviewed. At present, the highest value on the product of Ic · L in the world was marked by the result which was 51,940 A m (212 m · 245 A) by the SRL group. Long tape processing with high performance, especially in the PLD-YBCO system, has been developed in recent years. Not only the development of longer tapes with higher Ic values, but the processing for lower cost, higher production rate and high yields are required for the future coated conductor application. Then, now is the time that the R&D stage could be shifted on to the next coated conductor applications, since 100 m-100 A/cm-w class tapes could be repeatedly fabricated and supplied for R&D in applications. Acknowledgements The authors sincerely thank to all scientists who kindly allowed us to use their results in this paper, including scientists of the Furukawa Electric Company, Fujikura, Sumitomo Electric Industries, Showa Electric Wire and Cable Company and Chubu Electric Power Company. 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 [1] J.G. Bednorz, K.A. Mu¨ller, Phys. B: Condens. Matter 64 (1986) 189. [2] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, O. He, B. Saffian, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hatfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [3] A. Goyal, D.F. Lee, F.A. List, E.D. Specht, R. Feenstra, M. Paranthaman, X. Cui, S.W. Lu, P.M. Martin, D.M. Kroeger, D.K. Christen, B.W. Kang, D.P. Norton, C. Park, D.T. Verebelyi, J.R. Thompson, R.K. Williams, T. Aytug, C. Cantoni, Physica C 357–360 (2001) 903. [4] Y. Iijima, K. Onabe, N. Futaki, N. Tanabe, N. Sadakata, O. Kohno, Y. Ikeno, in: H. Hayakawa, N. Koshizuka (Eds.), Advances in Superconductivity IV, Springer-Verlag, Tokyo, 1992, p. 517. [5] Y. Iijima, K. Kakimoto, K. Takeda, Physica C 357–360 (2001) 952. [6] K. Hasegawa, K. Fujino, H. Mukai, M. Konishi, K. Hayashi, K. Sato, S. Honjo, Y. Sato, H. Ishii, Y. Iwata, Appl. Supercond. 4 (1996) 487. [7] K. Ohmatsu, K. Murakami, S. Hahakura, T. Taneda, K. Fujino, H. Takai, Y. Sato, K. Matsuo, Y. Takahashi, Physica C 357–360 (2001) 946.
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Highlights in R&D for coated conductors in Japan Yuh Shiohara *, Yutaka Kitoh, Teruo Izumi Superconductivity Research Laboratory, ISTEC, 1-10-13, Shinonome, Koto-Ku, Tokyo 135-0062, Japan Available online 30 June 2006
Abstract The current 5-year national project since 2003 for development of coated conductors (CC) using Y-system superconductors has passed for almost a half term and has achieved satisfactory results. In this paper, the current status and the future prospect are reviewed. The group of Fujikura Ltd. and SRL-ISTEC has worked on the long tape with high performance in the PLD-YBCO superconducting tapes on the IBAD-Gd2Zr2O7 buffered substrates. The highest value on the product of Ic · L in the world was marked by the result which were 51,940 A m (212 m · 245 A) by the SRL group. Fujikura Ltd. also realized the longest tape of 200 m with a reasonable high Ic value of 100 A. The values have been steadily improved and the trend is going to be continued, since the large equipments for both IBAD and PLD have been installed, and ready to work on large tapes with a high production rate. In another group, the long tape processing has been developed focusing on lowering the production cost. The extremely high Ic value of 470 A was obtained in the film by the TFA-MOD method on CeO2 (PLD)/GZO(IBAD)/hastelloy substrate. In the efforts for the long tape in the process, a 25 m long tape with its Ic value of 100 A was realized by a continuous reel-to-reel system. Additionally, 100 m class long tapes were also obtained by the MOCVD and PLD-HoBCO processes. Both groups are aiming to achieve the final goals of 500 m long tapes with the high Ic value of 300 A/cm-w by the production rate of 5 m/h. Furthermore, the feasibility study for applications using coated conductors has been already started due to the above-mentioned success of long tape production. Several kinds of coils using long coated conductors such as a solenoid and a pancake coils and the spiral shaped conductors for cable applications were firstly made. Reasonable high performances were confirmed in the trials. For the future plans of coated conductor applications, the following power devices using coated conductors have been proposed; (1) power cable, (2) transformer, (3) motor, (4) fault current limiter, (5) cryocooler, and so on. 2006 Elsevier B.V. All rights reserved. PACS: 81.15. z; 74.72.Bk; 81.10.Jt Keywords: Coated conductors; Long tape; Ic · L; REBa2Cu3O7 d; YBCO; PLD; IBAD; TFA-MOD; MOCVD
1. Introduction Since the discovery of oxide superconductors in 1986 [1], research in material science fields has proceeded and developments for applications have been carried out extensively. Silver sheathed wires for the Bi-system superconductors such as Bi2Sr2Ca2Cu3Oy (Bi2223) have preceded in the development of the long length oxide superconducting wires and tapes. However, there are several weak points
*
Corresponding author. Tel.: +81 3 3536 5711; fax: +81 3 3536 5717. E-mail address: [email protected] (Y. Shiohara).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.04.043
in the Bi-system, including low critical current density (Jc) under the high magnetic fields in the range of the liquid nitrogen temperatures, which limits the usage at about 20 K or lower in many practical applications. Additionally, the limited future cost reduction due to its relatively high Ag volumetric ratio is also one of the difficulties for applications. Development of coated conductors with REBa2Cu3O7 d (RE123: RE is rare earth elements) system superconductors are expected to overcome these weak points, since the RE123 superconductors essentially have a high Jc potential at the liquid nitrogen temperature even in high external magnetic fields. Additionally, it is possible to make a low cost wire compared with that of the
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Bi-system due to the low silver ratio as well as high Jc values. In the Y-system, however, the in-plane crystal grain alignment is required in order to attain full material characteristics of RE123. This requirement has to be satisfied in long tapes for applications of coated conductors. In order to control crystal grain alignments of the superconducting layer, two distinct technologies exist, including fabrication of textured metallic substrates such as RABiTSTM [2,3] and crystal grain alignment of a buffer layer on a non-textured high strength metallic substrate by IBAD [4,5] and/or ISD [6,7] which were both originally developed in Japan. These coated conductors have a multi-layered film structure, that is fabricated an RE123 superconducting layer on a metal substrate with some buffer layers. The metal substrate gives a mechanical strength and the crystal grain alignment in some cases, and the roles of the buffer layers are the crystal grain alignment, suppression of chemical reaction between a superconducting layer and a metal substrate and crack generation. Fabrication of a highly oriented thick superconducting layer is also important for obtaining a high performance, high Ic in coated conductors. In order to solve this problem, various processes such as PLD (pulsed laser deposition) [8], MOD (metal organic deposition) using TFA (trifluoroacetates) [9] and MOCVD (metal organic chemical vapor deposition) [10] have been investigated to obtain the superconducting layers. Recently, the long tape processing for both buffer and superconducting layers with good crystal alignment and high crystallinity shows a rapid progress and enough to start R&D for power device applications in Japan. 2. National project in Japan In the R&D of HTS (high temperature superconductivity), several projects have been conducted as a national
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project by the Ministry of Economics, Trade and Industry (METI) in the style of collaborative research of private companies and universities. The new national project, which has been started at the end of May 2003, consists of four sub-groups as shown in Table 1. First two groups were responsible for the process development. The selection of the processes in the groups were based on the results in the previous national project. In this project, combination of the buffer layer by IBAD and superconducting one by PLD realized the most excellent result on the length with high performance [11–13]. In this type of the tape, the high performance even in the magnetic fields and the suppression of the Ic reduction in a long tape could be achieved due to its fine grain size and the high grain texturing so that this group is selected as ‘‘High Performance Long Tape Development Group’’. In this group, development of stable production for a long tape, improvement of its production rate, achievement of high Ic performance and reduction of the production cost were the main objectives and the targeted goals were listed in Table 2. Fujikura Ltd. and SRL-Nagoya Coated Conductor Center (NCCC) are the players in this group. On the other hand, there might be many different kinds of tapes for different expected applications from an industrial engineering point of view. The lower cost is preferable to the performance in the magnetic fields in some cases. The second processing group was organized in order to respond these requirements. R&D of several different techniques for substrates, buffer layers and superconducting layers aiming at lower cost, which is called as ‘‘Low Cost Long Tape Process Development Group’’. Concerning the substrates, processing for bi-axially textured metal substrates has been considered to be a low cost texturing technique because of its simplicity. The MOD process is one of the promising processes for the low cost production since a
Table 1 Sub-group and their aim in the new national project National Project of C.C. in Japan (2003–2007) Group
Theme
High performance long tape processing Low cost long tape processing Characterizations Material improvement
IBAD (buffer) + PLD (superconducting layer) long tape, stable process conditions, width control, etc. MOD, MOCVD, RABiTS, SOE, PLD long tape, high performance, low cost, etc. Continuous Ic measurement, Jc–B–T–h, AC-losses, Jc distribution, thermal properties, mechanical properties, etc. Material designing (RE123, O-control, etc.) and grain boundary
Table 2 Goals in the present national project in Japan Specifications
Length (m) Ic(@77 K, 0 T) Ic(@77 K, 3 T) Production rate Cost(@77 K, 0 T)
Interim targets (2005)
Final targets (2007)
High performance type
Low cost type
High performance type
Low cost type
P200 m Ic P 200 A/cm-w Ic P 20 A/cm-w – –
P200 m Ic P 200 A/cm-w – – –
P500 m Ic P 300 A/cm-w Ic P 30 A/cm-w P5 m/h 612 Yen/A m
P500 m Ic P 300 A/cm-w – P5 m/h 68 Yen/A m
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large equipment with a high vacuuming system is not required and high yields of production could be expected. A high superconducting performance was already confirmed in the films fabricated by the MOD process using TFA salts [14–20]. Furthermore MOCVD [10] and the combination of PLD and MOD processes have been also worked as a low cost process for a superconducting layer and obtained the results of a long tape fabricated [21]. Sumitomo Electric Industry has aimed at realization of higher Je (engineering Jc) tapes using HoBCO superconducting materials by the combination of PLD and MOD processes. The final goals of this group are also listed in Table 2. The objectives of both the third and the fourth R&D group are basically to support the development in the above-mentioned two process development groups for realizing their R&D targets. The third one includes development of several characterization methods not only Tc, Ic, Jc, Jc–B measurements but of AC loss, quench behavior non-destructive measurements of Ic and Jc, etc. Material designing, including other RE123 and O-control, etc., and the control of grain boundary characteristics have been investigated in the fourth group. All efforts in the national project have been combined and a steady progress has been attained. Then, the recent progress in the above mentioned two process development groups and the future plans for coated conductor development in Japan are reviewed.
the low growth rate, was developed in the NCCC group. They found that the in-plane grain alignment rapidly improved by deposition of thick CeO2 layers by the PLD process without any assistance such as Ar+ ion like the IBAD, which called self-epitaxy, on the IBAD buffered tapes [23]. As a result, the in-plane grain alignment of the buffer layer was achieved; D/ value of 2.6 in CeO2 with a 0.6 lm thick film on the underlying Zr2Gd2O7 buffer with that of 12. This phenomenon can be also applied from the IBAD layer with lower degree of grain alignment as shown in Fig. 1. The growth rate of the CeO2 film is 10 times higher than that of the Zr2Gd2O7 buffer by the IBAD process. Consequently, the total production rate to achieve high texturing including IBAD and CeO2 depositions can be much higher using thin IBAD buffer layer as shown in Fig. 2. As concerning a superconducting layer by the PLD process, there have been several problems to be solved, from the engineering point of view, such as a slow deposition rate, difficulty to fabricate thicker films with maintaining high Jc and low material yields, etc. When the YBCO film thickness increases more than about 1.5 lm, the Jc value drastically decreases in general. This suggests that improve-
3. Progress in R&D for YBCO coated conductors 3.1. High performance long tape processing NCCC (Nagoya Coated Conductor Center, SRLISTEC) and Fujikura Ltd. are the players in this group. They are focusing on the IBAD and the PLD methods for the YBCO coated conductor development. Their R&D objectives are to improve IBAD and PLD methods for longer tapes and rapid production. In the case of NCCC, they also make long IBAD buffered substrate tapes to supply for other research groups to study other superconducting layer deposition processes in the second group. The IBAD method is a process for deposition of an inplane crystal grain aligned buffer layer by applying an additional assist-ion-beam from an appropriate direction during the deposition of the buffer layer. This IBAD process for depositing the in-plane aligned YSZ buffer layer was originally established by the research team of Fujikura Ltd. in 1991. The IBAD-YSZ process gives an excellent crystal grain alignment with a small grain size, which is suitable for maintaining high performance in a long tape. However, the slow deposition rate was a problem from the engineering point of view. The high production rate of 1 m/h was achieved by the development of large-scale IBAD equipment and the discovery of a much better buffer layer material, Zr2Gd2O7 [12,13,22]. The new process to overcome the disadvantage of the IBAD process, which is
Fig. 1. In-plane crystal grain alignment improvement by CeO2 cap layer deposition on IBAD-GZO layer.
Fig. 2. Deposition time dependences of in-plane alignment in IBADZr2Gd2O7/hastelloy tapes and CeO2 film on them by the PLD process.
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ment of Ic by simply increasing the film thickness is difficult to realize in this system. One of the reasons for the Jc degradation in a thick PLD-YBCO film is the increase of the volume fraction of a-axis grains in a thick film. As a cause of the increase in thick YBCO films, it was explained that the surface temperatures of the YBCO layer were considered to be lowered with increasing the number of deposition cycles since the emissivity of the film increases with increment of thickness. Then, the NCCC group investigated the suitable YBCO deposition conditions with raising the heater temperature to maintain the constant film surface temperature even with thickening the film. It was found to be effective to suppress the a-axis oriented grain growth. As a result, the high Ic values were obtained even in a thick YBCO film [24], as shown in Fig. 3, and an extremely high Ic value of 480 A/cm-w was finally obtained by optimizing the deposition temperatures [25]. In order to trace the consideration for a-axis grain suppression for long tape processing, a multi-deposition system is convenient to apply the optimum temperature control dependent of the YBCO film thickness since we can select and control the appropriate temperature for each deposition cycle. Then, a new PLD apparatus, which is shown in Fig. 4, with a multi-plume and multi-turn system for YBCO layers has been used for long tape fabrication, and an extremely high Ic performance with reasonably high end-to-end Ic value of 245 A/cm in 212 m-long YBCO tape, which corresponds to 51,940 A m as a product of Ic · L, has been attained, as shown in Fig. 5 [26]. Fujikura Ltd. has also recently fabricated a 105 m long coated conductor [27] with a reasonable end-to-end Ic value of 126 A/cm was realized. The uniform critical current distribution was obtained through its entire length compared with the result in 2003, which is the endto-end Ic value of 38 A in the 100 m long YBCO tape. They have constructed a large-scale equipment for 500 m-long tape, which was shown in Fig. 6. Finally, a 217 m long YBCO tape with end-to-end Ic value of 88 A, which corre-
499
Fig. 4. Photograph of the multi-plume zone in a PLD machine. Large area deposition of YBCO with multi-layered deposition enables to change heater temperature for thickening and to homogenize low Ic region.
Fig. 5. Photograph of a 212 m YBCO tape fabricated by PLD process on a CeO2-buffered IBAD-Gd2Zr2O7/hastelloy substrate.
Fig. 6. Large scale IBAD-equipment for 500 m tapes.
Fig. 3. Dependence of Ic values on numbers of deposition cycle for 10 cm long YBCO layer deposited at various temperatures. All YBCO layers, except the highest Ic one, were deposited on the PLD-CeO2 cap layers with D/ of around 8. The highest Ic ones PLD-CeO2 cap layer was with D/ of 5.
sponds to 19,100 A m as a product of Ic · L, was obtained [28]. The R&D race for the higher products of Ic · L values on RE123 tape in the world was listed in Table 3. As seen in the table, R&D for long coated conductors in the Japan national project proceed steadily.
500
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Table 3 R&D race for higher Ic · L 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th
Ic · L (A m)
Organization
Ic (A/cm-w)
L (m)
Processing
51,940 22,055 19,100 15,120 (12,870) 9400 8550 6200 2500 2230
SRL-ISTEC IGC-Super Power Fujikura AMSC Sumitomo EHTS Chubu SWCC SRL-ISTEC Goettingen
245 107 88 168 110 (Ic, min) 235 95 155 100 223
223 207 217 90 117 40 90 40 25 10
YBCO(PLD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(MOCVD)/STO/MgO(Cap)/MgO(IBAD)/Y2O3/Al2O3/hatelloy YBCO(PLD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(TFA-MOD)/CeO2/YSZ/Y2O3/Ni-W HoBCO(PLD)/CeO2/YSZ/CeO2/Ni-alloy YBCO(HR-PLD)/CeO2/YSZ/S.S. YBCO(MOCVD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(TFA-MOD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(TFA-MOD)/CeO2(PLD)/GZO(IBAD)/hastelloy YBCO(PLD)/YSZ(IBAD)/S.S.
In addition, improvement of Jc–B properties is required as well, since most cases of the coated conductor applications for electric power devices have expected under the external magnetic fields. One of the promising procedures to solve the Jc degradation in high magnetic fields is introduction of artificial pinning centers in the superconducting layer. Yamada et al. fabricated YBCO films by the PLD method using a target containing the YSZ particles. As a result, the Jc values of the YBCO film deposited by the YSZ mixed target were improved to be higher than that of the conventional PLD-YBCO film, as shown in Fig. 7 [29]. Cross-sectional TEM observations were carried out for the YBCO films deposited by the YSZ/YBCO mixed target. As seen in Fig. 8, the YSZ phase particles were formed nano-epitaxy grain alignments during the YBCO phase crystal growth, which looks like ‘‘bamboo structure’’. These YSZ phase structures have been considered to be the magnetic flux pinning sites. 3.2. Low cost long tape processing Metal organic deposition (MOD) of precursor solutions containing metal trifluoroacetates (TFA) is an attractive process to fabricate YBCO films since it is relatively easy to attain high Jc performance by a simple process in a non-vacuum low cost manner [17,30–33]. By the SRL-
Fig. 7. Magnetic field dependence of the Jc values.
Fig. 8. Cross-sectional TEM image of the PLD-YBCO film deposited by using the YSZ mixed target.
Tokyo group, the optimization of the growth conditions both for the calcination and the crystallization steps has been studied to obtain higher Ic performance. Through the investigations, it was found that the low heating rate and the higher water vapor inlet temperature in the calcination step and the low heating rate and the high water vapor pressure in the crystallization step were effective for higher Ic performance [34–37]. Consequently, the high Ic value of 470 A/cm-w was achieved by combination of the above optimized growth conditions with use of highly textured CeO2 cap layered IBAD-GZO substrates, as shown in Fig. 9. For the long tape processing, the reel-to-reel (RTR) system is applied for both the calcination and the crystallization steps in the SRL group. In this system, it is important to obtain the uniform reaction along the long direction. A stagnant gas region in the furnace could be another reason not only for the low growth rate but for the long transient in the initial and final reaction regions, which lead to the non-uniformity in the long direction. The computer simulation of the crystal growth and the gas flow in the furnace was studied to solve the problem of the gas stagnant region. From the calculations, the several possibilities as the solutions were suggested. A higher gas flow condition during the growth of the YBCO layer was applied in this study. Relatively uniform performance
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Fig. 9. Ic values dependence on YBCO film thickness for different in-plane grain alignments (D/) of CeO2 buffer layers.
was obtained in a 4 m-long YBCO tape with the high Ic value of about 250 A. Furthermore, a 25 m long YBCO tape with the end-to-end Ic value of 100 A/cm was fabricated as shown in Fig. 10, which corresponds to 2500 A m as a product of Ic · L values [36]. The SWCC (Showa Wires and Cable Comp.) is also focusing on the TFA-MOD method for the YBCO layer and has worked on the heat treatment in the batch system. The concept of the batch type apparatus was schematically shown in Fig. 11. They obtained a 6 m-long YBCO tape with the end-to-end Ic value of 69 A/cm-w. They also optimized the heat-treatment conditions for the batch system which is able to fabricate a 200 m-class YBCO tape. Recently, they have fabricated a 40 m-long YBCO tape with the end-to-end Ic of 155 A/cm-w. In addition, the
Fig. 10. Ic distribution in the 25 m-long YBCO superconducting tape fabricated by TFA-MOD process using the reel-to-reel system.
Fig. 11. Schematic image of the basic concept for the butch system furnace in the crystallization step.
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SWCC has worked on the ‘‘All-MOD’’ process, which means that not only a superconducting layer but buffer layers are fabricated by the MOD process as well, and is expected to be the lowest cost process. They already found the suitable materials of Ce–Gd–O for the MOD buffer on a textured metal substrate without cracking [38]. However, the superconducting performance was still in a low Jc level of 105 A/cm2 due to the contamination/diffusion of the substrate elements into the superconducting phase. This problem was solved by the doping of Nb into the buffer layer materials and the Tc value was improved [39]. Concerning the development of the MOCVD process, Chubu Electric Power Company deposited the YBCO phase layer on a PLD-CeO2/IBAD-G2Zr2O7/hastelloy substrate [10,40]. The high Jc value of 2 MA/cm2 at 77 K was confirmed in a short sample. Recently, they have obtained a 92 m YBCO tape with the end-to-end Ic value of 96 A/ cm, which corresponds to 8832 A m as a product of Ic · L values, by the reel-to-reel system. The Sumitomo group has aimed at realization of higher Je tapes using HoBCO superconducting materials by the combination of PLD and MOD processes [41]. They successfully fabricated a 117 m-long HoBCO tape by PLD process with the Ic value of over 110 A/cm. 4. Future plans for R&D of coated conductors YBCO coated conductors have several merits such as lower cost, higher performance in high magnetic fields, mechanical strength, and simplicity of post-treatments compare to the Bi-system for lower AC losses. For example, a multi-filamental structure for AC loss reduction has been successfully demonstrated by both YAG laser scribing and chemical etching on YBCO coated conductors. Additionally, several preliminary results for the coated conductor applications have been recently reported. The Fujikura Ltd. group investigated the bending strain properties for the PLD-YBCO tapes fabricated on the IBAD substrate. Furthermore, they reported the demonstration of a solenoid type coil using long IBAD/PLDYBCO tape [29]. The Super-GM reported the HTS power cable application as well. For the future plans for coated conductor applications, we have proposed the following themes using coated conductors: (1) power cable, (2) transformer, (3) motor, (4) fault current limiter, (5) cryocooler. As concerning proposal (1), it is expected that the installation cost must be lowered drastically by using coated conductor HTS power cable compare to the current cables. An appropriate architecture for lower AC losses, over-current measurement, cable-joints and a proto-type test are included in this proposal. To develop HTS electric power machines and devices applicable to practical power grid; (i) low ac loss, (ii) large current capacity, (iii) stability and protection, (iv) insulation layer are also included. In the case of the development for HTS motor, a full superconducting synchronous motor has been proposed, which means that both
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rotors and stators without iron cores are constructed by HTS coils. The requirements of conductors for a fault current limiter are mainly the operation technology under LN2 temperature and high resistance in the current limiting process. On the proposal (5), development of highly efficient cooling design/manufacture technology which is suitable for YBCO power devices has been proposed. 5. Summary In this paper, the recent progress and the future plans for coated conductor development in Japan were reviewed. At present, the highest value on the product of Ic · L in the world was marked by the result which was 51,940 A m (212 m · 245 A) by the SRL group. Long tape processing with high performance, especially in the PLD-YBCO system, has been developed in recent years. Not only the development of longer tapes with higher Ic values, but the processing for lower cost, higher production rate and high yields are required for the future coated conductor application. Then, now is the time that the R&D stage could be shifted on to the next coated conductor applications, since 100 m-100 A/cm-w class tapes could be repeatedly fabricated and supplied for R&D in applications. Acknowledgements The authors sincerely thank to all scientists who kindly allowed us to use their results in this paper, including scientists of the Furukawa Electric Company, Fujikura, Sumitomo Electric Industries, Showa Electric Wire and Cable Company and Chubu Electric Power Company. 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 [1] J.G. Bednorz, K.A. Mu¨ller, Phys. B: Condens. Matter 64 (1986) 189. [2] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, O. He, B. Saffian, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hatfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [3] A. Goyal, D.F. Lee, F.A. List, E.D. Specht, R. Feenstra, M. Paranthaman, X. Cui, S.W. Lu, P.M. Martin, D.M. Kroeger, D.K. Christen, B.W. Kang, D.P. Norton, C. Park, D.T. Verebelyi, J.R. Thompson, R.K. Williams, T. Aytug, C. Cantoni, Physica C 357–360 (2001) 903. [4] Y. Iijima, K. Onabe, N. Futaki, N. Tanabe, N. Sadakata, O. Kohno, Y. Ikeno, in: H. Hayakawa, N. Koshizuka (Eds.), Advances in Superconductivity IV, Springer-Verlag, Tokyo, 1992, p. 517. [5] Y. Iijima, K. Kakimoto, K. Takeda, Physica C 357–360 (2001) 952. [6] K. Hasegawa, K. Fujino, H. Mukai, M. Konishi, K. Hayashi, K. Sato, S. Honjo, Y. Sato, H. Ishii, Y. Iwata, Appl. Supercond. 4 (1996) 487. [7] K. Ohmatsu, K. Murakami, S. Hahakura, T. Taneda, K. Fujino, H. Takai, Y. Sato, K. Matsuo, Y. Takahashi, Physica C 357–360 (2001) 946.
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