Progress in second-generation HTS wire development and manufacturing

Physica C 468 (2008) 1504–1509

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Progress in second-generation HTS wire development and manufacturing V. Selvamanickam *, Y. Chen, X. Xiong, Y. Xie, X. Zhang, A. Rar, M. Martchevskii, R. Schmidt, K. Lenseth, J. Herrin SuperPower, Inc., 450 Duane Avenue, Schenectady, NY 12304, USA

a r t i c l e

i n f o

Article history: Available online 24 May 2008 PACS: 74.72.Bk 74.25.Sv 81.15.Gh 81.15.Jj Keywords: MOCVD IBAD Second-generation HTS

a b s t r a c t 2007 has marked yet another year of continued rapid progress in developing and manufacturing highperformance, long-length second-generation (2G) HTS wires at high speeds. Using ion beam assisted deposition (IBAD) MgO and associated buffer sputtering processes, SuperPower has now exceeded piece lengths of 1000 m of fully buffered tape reproducibly with excellent in-plane texture of 6–7 degrees and uniformity of about 2%. These kilometer lengths are produced at high speeds of about 350 m/h of 4 mm wide tape. In combination with metal organic chemical vapor deposition (MOCVD), 2G wires up to single piece lengths to 790 m with a minimum critical current value of 190 A/cm corresponding to a Critical current  Length performance of 150,100 Am have been achieved. Tape speeds up to 180 m/h have been reached MOCVD while maintaining critical currents above 200 A/cm in 100+ m lengths. Thick film MOCVD technology has been transitioned to Pilot manufacturing system where a minimum critical current of 320 A/cm has been demonstrated over a length of 155 m processed at a speed of 70 m/h in 4 mm width. Finally, nearly 10,000 m of 2G wire has been produced, exhaustively tested, and delivered to the Albany Cable project. The average minimum critical current of the wire delivered in 225 segments of 43–44 m is 70 A in 4 mm widths. A 30 m cable has been fabricated with this wire by Sumitomo Electric and has been installed in the power grid of National Grid in downtown Albany and is the world’s first 2G device installed in the grid. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Progress in scale up of second-generation (2G) high temperature superconductors (HTS) has been happening at a rapid pace in the last three years [1–3]. Critical current of 300 A/cm has been recently reported in a 368 m long tape produced by ion beam assisted deposition (IBAD) and pulsed laser deposition (PLD) process [3]. Tape lengths more than 400 m with critical currents (Ic) of about 200 A/cm have been produced at a high linear tape speed of 135 m/h of 4 mm wide tape [2]. In this case, high tape speed has been enabled by employing a combination of high-rate processes in all steps. High-rate processing has been achieved at SuperPower using IBAD MgO-based biaxially-textured templates [4] and metal organic chemical vapor deposition (MOCVD) for the HTS layer [5]. SuperPower had established Pilot-scale manufacturing of 2G HTS, which became fully operational in 2006 [6]. Since then, our program has been focused on achieving key metrics to improve the cost-performance characteristics of 2G wire. The key metrics that have been addressed are high currents, high throughput, and long lengths. Further, SuperPower has been working to * Corresponding author. Tel.: +1 518 346 1414; fax: +1 518 346 6080. E-mail address: [email protected] (V. Selvamanickam). 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.063

demonstrate the availability of large quantities of 2G wire for prototype projects, in particular for the Albany Cable project. In this paper, we report progress in 2007 in the 2G wire program at SuperPower specifically focusing on achieving key metrics as well as delivery of 2G wire for the Albany Cable project. 2. 2G Wire delivery for the albany cable project The Albany Cable project was successfully commissioned in July 2006 with a 350 m cable made with 1G wire and installed in the power grid of National Grid in downtown Albany [7]. The next step was to replace 30 m of the 1G cable section with an equal length of 3-phase, fully shielded 2G cable. The delivery requirements of 2G wire for the Albany Cable project were primarily 225 segments in 43 m piece lengths with a critical current value of 100 A/cm at 77 K for a total of 9700 m. Previously, we had reported that SuperPower had produced the wire for the Albany Cable project up to the level of electroplated copper stabilizer [2]. Since that report, we prepared 225 segments of 2G wires in piece lengths of 43–44 m and fully tested them for several mechanical and physical properties such as critical tensile stress, spiral winding, bend strain, thickness, width, and hermeticity. All tapes successfully passed and in fact exceeded requirements in several instances. Critical tensile

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60 50

# tapes

40 30 20 10 0

40 - 49 50 - 59 60 - 69 70 - 79 80 -89 90 - 99 100+

Minimum Ic over 42 to 44 m (A) Fig. 1. Distribution of minimum critical current measured every 5 m in 225 segments of 2G wire delivered in piece lengths of 43–44 m to the Albany Cable project.

is expected to commence in November 2007 followed by energization. This will mark the first 2G HTS device in the power grid anywhere in the world. 3. High critical currents In 2006, we reported a critical current value of 721 A/cm measured over entire tape width of 12 mm in a 3.5 lm thick MOCVD film produced in five passes on IBAD MgO buffered tape [2], which corresponds to a critical current density of 2.06 MA/cm2. Microstructural examination of thick films at various stages of processing (i.e.) after each pass, indicated that a-axis grain growth as well as compositional inhomogeneity increased with increasing film thickness. Hence in 2007, we focused on modifying the film composition in order to achieve higher current densities in thinner films. We changed the composition of the MOCVD films from (Y,Sm)–Ba–Cu–O to (Y,Gd)–Ba–Cu–O. With this composition, we were able to achieve critical current density (Jc) of 4.5 MA/cm2 corresponding to a critical current of 316 A/cm in 0.7 lm thick films. Next, we increased the film thickness in steps of 0.7 lm up to 2.8 lm by deposition of four HTS layers on atop each other in distinctly individual process runs. Results from this study are shown in Fig. 3 which includes comparison with data reported in 2006. As shown in the figure, a critical current of 740 A/cm was achieved in a 2.8 lm thick film of (Y,Gd)-Ba–Cu–O. This value corresponds to a critical current density of 2.65 MA/cm2, which is 30% higher than that previously achieved in the 3.5 lm film made with (Y,Sm)-Ba– Cu–O. Furthermore, this Jc is similar to the value previously achieved in a 2.1 lm film indicating that we have been able to extend the higher Jc to thicker film levels using the modified composition. In 2006, we had scaled up thick film MOCVD process to achieve 592 A/cm over 1 m, 486 A/cm over 10 m and 295 A/cm over 103 m. All these achievements were obtained in our Research MOCVD system. In the case of the 103 m result, an effective tape speed of 15 m/h of 4 mm wide tape was used. In 2007, we transitioned the thick film MOCVD process to our Pilot system. We produced a 155 m long tape in the Pilot MOCVD system at an effective speed of 70 m/h of 4 mm wide tape. Results from this tape are shown in Fig. 4. As shown in the figure, a minimum critical current of 320 A/

2006 Ic(Y, Sm)BCO 2007 Ic(Y, Gd)BCO 2007 Jc(Y, Sm)BCO 2007 Jc(Y, Gd)BCO

Critical current measured across entire tape width of 12 mm (no patterning) 800

Critical current (A/cm)

80 70

# tapes

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3

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10

2 0

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Minimum n-value over 42 to 44 m (A) Fig. 2. Distribution of minimum n-value every 5 m in 225 segments of 2G wire delivered in piece lengths of 43–44 m to the Albany Cable project.

Jc (MA/cm2)

stress levels were more than 550 MPa, minimum spiral winding twist pitch with no Ic degradation was less than 10 cm over 16 mm diameter mandrel, bend diameter without Ic degradation was less than 50 mm, thickness and width values met requirements when measured in 20 cm intervals and all 225 segments did not show any change in Ic and thickness when subjected to a hermeticity test of 10 atmospheres of liquid nitrogen for 24 h. Two hundred and twenty five segments of 2G wire were successfully delivered to Sumitomo Electric in December 2006 to construct the 30 m cable. Figs. 1 and 2 show histograms of number of 43–44 m long tape segments with distribution of minimum critical currents and minimum n-values respectively. Critical currents and n-values were measured every 5 m and the minimum data point is reported in Figs. 1 and 2 respectively. As shown in the figure, the average minimum Ic of 225 segments each 42.4–44 m long was 70 A. Over 56% of 225 segments were found to show a minimum Ic over 70 A. The average end-to-end Ic will be higher, about twice the spec of 40 A. The average minimum n-value of 225 segments was 24% and 56% of 225 segments have minimum n-value over 25. The nearly 10,000 m of 2G wire was successfully constructed into a 30 m, 3-phase cable by Sumitomo Electric and shipped to Albany in April 2007. The 30 m 2G cable was installed in the power grid of National Grid in August 2007 and cool down of the cable

0.5

1

1.5

2

2.5

3

3.5

4

MOCVD film thickness (microns) Fig. 3. Critical currents as a function of thickness of continuous, reel-to-reel processed MOCVD films on IBAD MgO buffered tapes produced in 2006 and 2007. All measurements were made in samples up to 10 cm in length and over entire width of 12 mm using continuous dc transport currents.

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400

Critical current (A/cm)

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95 10 5 11 5 12 5 13 5 14 5 15 5

85

75

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Position (m) Fig. 4. Critical current distribution over a 155 m long 2G wire made in Pilot MOCVD system at a speed of 70 m/h of 4 mm wide tape. A minimum critical current of 320 A/cm was measured over 155 m.

4. High throughput In 2006, we reported that the throughput of the IBAD MgO process was increased to 360 m/h of 4 mm wide tape [2]. In 2007, we were able to achieve higher tape speeds in our buffer deposition processes atop IBAD MgO. In cases of both homo-epi MgO and LMO buffers, we increased tape speed by employing 11 tape wraps in the helix tape handling system compared with 6 wraps last year. Hence, we were able to increase the homo-epi MgO and LMO process speeds to 345 m/h of 4 mm wide tape. In 2006, we reported increase in the speed of the MOCVD process to 135 m/h of 4 mm wide tape [2]. This year, we increased the precursor delivery rate in our Pilot MOCVD system and were able to achieve the same film thickness of 1 lm even at a speed of 180 m/h of 4 mm wide tape. By maintaining the same film thickness at the higher speed, we were able to achieve the same critical current of 285 A/cm at 180 m/h as shown in Fig. 5. Also, at 180 m/h in 2007, the film thickness is more than that achieved in October

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Ic : Aug. '06

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Ic : Oct. '06

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Ic : June '07

50 0 60

90

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*4 mm wide equivalent

Fig. 5. Critical currents measured in short samples of 2G wire made by MOCVD in 2006 and 2007. Increased precursor delivery over this period yielded HTS films with high currents even at high tape speeds up to 180 m/h.

250 Critical current (A/cm)

cm was achieved over 155 m with an uniformity of 2.5%. We plan to continue scaling up the thick film MOCVD process to longer lengths at the higher speeds.

200 150 100 Minimum Ic= 201 A/cm over 130 m

50

Uniformity= 3.5%

0 0

25

50 75 Position (m)

100

125

Fig. 6. Critical current distribution over a 130 m long 2G wire made with high speeds in all key processes. Tape speeds in 4 mm widths were 360 m/h for IBAD MgO, 345 m/h for homo-epi MgO and LMO and 180 m/h for MOCVD.

2006 (0.75 lm) and therefore a higher critical current was achieved. In order to fully verify the success of high throughput processing, high critical currents need to be demonstrated when all process steps are conducted at high tape speeds. We produced a 130 m long tape where the speeds of IBAD MgO, homo-epi MgO, LMO, and HTS layer were 360, 345, 345, and 180 m/h of 4 mm wide tape, respectively. Fig. 6 displays the critical current over the 130 m tape produced at high speeds in all key process steps. As shown in the figure a minimum critical current of 201 A/cm was achieved which is comparable to the critical current level achieved last year at lower tape speeds. This achievement verifies the ability to produce good quality 2G wire at high speeds in all key processes. The tape speeds indicated above correspond to about 1400 km/ year of production capacity for the IBAD MgO, homo-epi MgO, and LMO processes and 720 km/year for the MOCVD process, assuming 45% of the available time is used for deposition. 5. Long lengths A third and important metric to achieve low-cost 2G wire is the ability to produce long single piece lengths with good critical cur-

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ergy electron diffraction (RHEED) in our Pilot IBAD system was used to maintain a high quality of texture in the IBAD MgO uniformly over lengths greater than 1000 m. As of October 2007, we have produced 15 IBAD MgO tapes over 1000 in piece length. Next, complete 5-layer buffer stack was produced in lengths of up to 1400 m by stable deposition of homo-epi MgO and LMO. As of October 2007, over 10 tapes have been produced in lengths of more than 1000 m with a complete 5-layer buffer stack. In-plane texture measurements were conducted in a reel-to-reel mode continuously over the 10 long fully- buffered tapes and the results are shown in Fig. 8 and summarized in Table 1. As shown in the figure and table, in-plane texture values of 6–7 degrees were reproducibly and uniformly achieved over lengths of greater than 1000 m. The uniformity in texture over 1000+ m lengths was found to be about 2%. Using 1000 m long buffered tapes, we have been scaling up our MOCVD process to longer lengths. In September 2007, we were able to extend the tape length even further, to 790 m. This time, the speeds used in the various processes were 360 m/h for IBAD MgO, 345 m/h for homo-epi MgO, 345 m/h for LMO, and about 100 m/h in MOCVD of HTS layer, all values corresponding to 4 mm wide tape equivalent. Data obtained from critical current

rent. In 2006, we had demonstrated piece lengths up to 427 m with a minimum critical current of 191 A/cm which corresponds to a Ic  Length performance of 81,550 A m [2]. In January 2007, we successfully produced a 595 m long 2G wire at high tape speeds of 360 m/h for IBAD MgO, 213 m/h for homo-epi MgO, 360 m/h for LMO, and 135 m/h in MOCVD of HTS layer, all values corresponding to 4 mm wide tape equivalent. Critical current measurements done every 5 m of the 595 m long 2G wire yielded the data shown in Fig. 7. As shown in the figure, a minimum critical current of 173 A/cm was achieved over 595 m which corresponds to 102,935 A m and was the first demonstration of crossing the threshold of 100,000 A m. The uniformity of Ic over 595 m was 6.4%. In order to reach the important milestone of 1000 m long 2G wire, we scaled up our substrate polishing, IBAD MgO, homo-epi MgO and LMO processes to 1400 m lengths. After piece lengths of 1400 m of substrate material were obtained from our vendor, which is up from 600 m in 2006, we successfully scaled up our electropolishing operation to produce smooth substrates routinely over long lengths. As of October, we have successfully polished over 45 substrates over 1000 m in piece length. Next, we scaled up our IBAD MgO process up to 1400 m. On-line reflection high en-

Critical current (A/cm)

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Position (m) Fig. 7. A 595 m long 2G wire produced in January 2007. A minimum critical current of 173 A/cm was measured corresponding to 102,935 A m.

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Position (m) Fig. 8. In-plane texture data obtained over 10 tapes produced in lengths of more than 1000 m with a complete 5-layer buffer stack. A good in-plane texture of 6–7 degrees was reproducibly achieved with excellent uniformity.

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Table 1 Summary of in-plane texture measurements obtained over 10 tapes produced in lengths of over 1000 m with a complete 5-layer buffer stack

6.2% 3.3% 2.1% 2.2% 2.5% 2.1% 2.9% 1.7% 2.4% 1.2%

427 m

80.000

322 m

60.000 40.000

206 m 20.000 18 m 62 m 158 m 1m 97 m 0

Fig. 10. Progress in the Critical current  Length performance of IBAD-MOCVDbased 2G wires produced at SuperPower since June 2002.

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Title III goal June 2008

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Fig. 11. Status of critical current performance levels at various length scales and with different MOCVD film thickness.

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measurement performed over every 5 m is shown in Fig. 9. As shown in the figure, a minimum critical current of 190 A/cm was achieved over 790 m corresponding to 150,100 A m. Uniformity over 790 m was 9.7%, primarily affected by tape speed issues in the MOCVD process causing thickness fluctuations. This is the first demonstration of exceeding the milestone of 150,000 A m, to our knowledge. Furthermore, this achievement brings us a step close to producing 1000 m long 2G wires. The progress in SuperPower’s 2G wire program over the last 5 years is shown in Fig. 10. As seen in the figure, a tremendous progress has been made from 1 m in 2002 to 790 m in 2007. Furthermore, over the last 5 years, the critical current has been doubled and tape speed increased by a factor of more than 10. Finally, the status of SuperPower’s achievements in 2G HTS wires has been summarized in Fig. 11. As shown in the figure, high critical currents have been achieved in short lengths up to 10 m using HTS films up to 2.8 lm in thickness. Long lengths have been produced up to 790 m with critical currents of about 200 A/cm in our Pilot MOCVD production system with about 1 lm thick HTS layers. We have also begun transitioning thick film MOCVD technology to our Pilot system where with approximately 2.3 lm thick HTS layers, critical current of 320 A/cm and 269 A/cm have been achieved over 155 m and 318 m respectively. We plan to continue progress in our long-length scale up towards the goals of 1000 m and 500 A/cm.

595 m

100.000

100

7.84 7.16 7.35 6.68 7.14 7.09 7.79 7.12 7.13 6.26

Apr-07

6.20 5.80 6.00 5.83 6.23 5.80 6.66 6.30 5.47 5.95

10

6.79 6.33 6.85 6.20 6.58 6.59 7.09 6.81 6.33 6.18

120.000

Mar-03

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Critical Current * Length (A-m)

1001 1343 1346 1372 1375 1277 1346 1265 1246 1369

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0.1

1508

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Position (m) Fig. 9. A 790 m long 2G wire produced in September 2007. A minimum critical current of 190 A/cm was measured corresponding to 150,100 A m.

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6. Conclusions

Acknowledgments

Significant progress continues to be made against all metrics towards the goal of producing low-cost 2G wire. Critical current of 740 A/cm has been achieved in 2.8 lm thick films corresponding to a Jc of 2.65 MA/cm2. The thick film MOCVD process has been transitioned to our Pilot MOCVD system where a minimum critical current of 320 A/cm was achieved over 155 m at a tape speed of 70 m/h of 4 mm wide tape. Tape speeds in the IBAD MgO, homoepi MgO and LMO processes have been increased to about 350 m/h of 4 mm wide tape. At an increased tape speed of 180 m/h of 4 mm wide tape in the MOCVD process, critical currents over 200 A/cm have been achieved over a length of 130 m. Substrate polishing, IBAD MgO, homo-epi MgO, and LMO processes have been successfully scaled up to more than 1000 m lengths. Inplane texture values of 6–7 degrees have been achieved reproducibly over 10 tapes with a complete 5-layer buffer stack, with a uniformity of 2%. The complete 2G wire manufacturing process has been scaled up to 790 m with a minimum critical current of 190 A/cm corresponding to a record value of 150,100 A m. The tremendous progress in 2007 achieved in all key metrics continue to drive the cost of 2G wire down towards commercially acceptable levels.

This work was partially supported by the Title III office, US Department of Energy, Air Force Research Laboratory and the Air Force Office of Scientific Research. Part of the work was also done in a Cooperative Research and Development Agreements with Los Alamos National Laboratory, and Oak Ridge National Laboratory. References [1] Y. Yamada, T. Muroga, H. Iwai, T. Watanabe, S. Miyata, Y. Shiohara, Supercond. Sci. Technol. 17 (2004) 70. [2] V. Selvamanickam, Y. Chen, X. Xiong, Y. Xie, X. Zhang, Y. Qiao, J. Reeves, A. Rar, R. Schmidt, K. Lenseth, Physica C 463–465 (2007) 482. [3] T. Izumi, Presented at European Conf. Appl. Supercond. Brussels, Aug. 13–17, 2007. [4] J.R. Groves, P.N. Arendt, H. Kung, S.R. Foltyn, R.F. DePaula, L.A. Emmert, J.G. Stover, IEEE Trans. Appl. Supercond. 11 (2001) 2822. [5] V. Selvamanickam, Y. Xie, J. Reeves, Y. Chen, Mater. Res. Soc. Bull. 29 (2004) 579. [6] 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. [7] C.S. Weber, C.T. Reis, A. Dada, T. Masuda, J.C.S. Moscovic, IEEE Trans. Appl. Supercond. 15 (2005) 1793.