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New Nb3Al-based A15 multifilamentary wires with high Jc in high fields
Cryogenics 40 (2000) 345±348
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Research and technical note
New Nb3Al-based A15 multi®lamentary wires with high Jc in high ®elds Y. Iijima *, A. Kikuchi, K. Inoue National Research Institute for Metals, Sengen 1-2-1, Tsukuba-shi, Ibaraki 305-0047, Japan Received 13 April 2000; accepted 10 June 2000
Abstract When Nb/Al±X (X Ge or Cu) microcomposite precursor wires were rapidly heated and quenched, disordered Nb3 Al-based A15-phase ®laments formed. With annealing at 750±800°C, their superconducting properties improved drastically. The Nb3 Albased A15 wire, with Ge or Cu added, showed a maximum value of Tc 19.4 or 18.2 K, and that of Hc2 (4.2 K) 39.5 or 28.7 T, respectively, which are higher than the maximum values of Tc 17.9 K and Hc2 (4.2 K) 25.8 T for the rapid heating, quenching and transforming (RHQT)-processed Nb3 Al wire without additional elements. In addition, the new superconductors showed higher Jc than that of the RHQT-processed Nb3 Al multi®lamentary wire in high ®elds. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nb3 Al-based high-®eld conductor; Cu or Ge addition; Rapid heating and quenching
1. Introduction Stoichiometric Nb3 Al shows higher values of Tc and Hc2 than commercialized Nb3 Sn superconducting wire. However, stoichiometric Nb3 Al is unstable except at high temperatures near 2000°C, while Nb-rich (o-stoichiometric) Nb3 Al, showing relatively low values of Tc and Hc2 , is stable at low temperatures as shown in the Nb±Al binary phase diagram [1]. In addition, Nb3 Al formation rate through solid-state diusion reaction between Nb and Al is very slow. In order to overcome the slow diusion reaction, various processes have been developed to increase the density of Nb/Al diusion couples in microcomposite wire, such as a jelly-roll process [2], a powder metallurgy process [3], a rod-in-tube process [4] and a cladchip extrusion process [5]. The combination of low temperature diusion reaction and Nb/Al microcomposite wire produced an Nb3 Al multi®lamentary wire having excellent strain tolerance and large Jc in low ®elds. However, the wire has o-stoichiometric Nb3 Al ®laments, of which Tc and Hc2 (4.2 K) are relatively low.
*
Coresponding author. Tel.: +81-2-98-59-2645; fax: +81-2-98-592601. E-mail address: [email protected] (Y. Iijima).
Six years ago, we proposed a rapid heating, quenching and transforming (RQHT) process to fabricate Nb3 Al multi®lamentary wires with near-stoichiometric composition [6]. In this process, an Nb/Al multi®lamentary microcomposite wire was rapidly heated continuously up to about 2000°C by resistive heating and then quenched into a molten Ga bath to form Nb±Al supersaturated bcc ®laments in the Nb matrix. The supersaturated bcc phase is metastable and ductile at room temperature and transformed into near-stoichiometric Nb3 Al phase with additional annealing at about 800°C. The resulting Nb3 Al multi®lamentary wire shows not only an excellent strain tolerance [7] but also 2±5 times larger Jc than that of the bronze-processed (Nb,Ti)3 Sn multi®lamentary wire, which is a commercialized standard high-®eld superconductor [8]. In addition, the RHQT-processed Nb3 Al wire shows maximum values of Tc 17.9 K and Hc2 (4.2 K) 25.8 T, which are almost the same as those of the bronze-processed (Nb, Ti)3 Sn multi®lamentary wires. On the way to investigating the eects of additional elements on the RHQT-processed Nb3 Al multi®lamentary wire, we found another route to obtain the Nb3 Albased A15 multi®lamentary wires with high Jc in high ®elds. Nb/Al±X (X Ge or Cu) multi®lamentary microcomposite wires were heat-treated by using the rapid heating and quenching (RHQ) apparatus. Due to the treatment, disordered A15 ®laments were directly
0011-2275/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 1 - 2 2 7 5 ( 0 0 ) 0 0 0 3 8 - 2
346
Y. Iijima et al. / Cryogenics 40 (2000) 345±348
Table 1 Speci®cations of Nb/Al±Ge and Nb/Al±Cu precursor wires Sample A Composition of Al±X alloy core Average diameter of Al±X alloy core (lm) Number of ®rst stacked composites Nb/Al±X alloy volume ratio Number of A15 ®laments (second stacked composites) Number of total stacked composites Matrix of the multi®lamentary wire Average diameter of A15 ®laments (lm) Nb-matrix/A15-®laments ratio
Sample B
Al±2 at.%Cu 1.5 0.6 121 121 3.079 2.852 121 121 14,641 102,487 Nb (99.8%) 33 33 3.16 3.16
obtained instead of the supersaturated bcc ®laments in the ordinary RHQT-process without Ge or Cu addition [9]. With annealing at 750±800°C, their superconducting properties improved drastically. The Nb3 Al-based A15 wire with Ge or Cu added showed maximum values of Tc 19.4 or 18.2 K and those of Hc2 (4.2 K) 39.5 or 28.7 T, respectively, which are higher than the maximum values of Tc 17.9 K and Hc2 (4.2 K) 25.8 T for the RHQT-processed Nb3 Al wire without additional elements. These values are relatively close to those reported for arc-melted Nb3 Al1ÿx Gex [10,11]. In addition, the new wires showed higher Jc (4.2 K) in high ®elds than RHQT-processed Nb3 Al wire. We call the new process the rapid heating, quenching and ordering (RHQO) process.
2. Experimental procedures and results The Nb/Al±X (X Ge or Cu) precursor wires were fabricated by the rod-in-tube process. Al±20 at.% Ge and Al±2 at.% Cu alloy rods were inserted into Nb pipes and then cold-drawn into Nb/Al±Ge and Nb/Al±Cu mono-core composite wires with an Nb/Al±X ratio of about 3. The mono-core composite wires were cut into short pieces and packed into Nb pipes. The ®rst stacked billets were cold-drawn into Nb/Al±Ge and Nb/Al±Cu multi-core composite wires, cut into short pieces and packed into Nb pipes again. The second stacked billets were cold-drawn into precursor wires with diameter of 0.5±0.74 mm. Detailed speci®cations of the precursor wires are shown in Table 1. The precursor wires were rapidly heated continuously up to about 2000°C by resistive heating, then quenched into a molten Ga bath in a vacuum chamber. After the coated Ga on the as-quenched wire was removed by chemical etching in hydrochloric acid, the wire was cut into short samples, enclosed into quartz capsules with vacuum and heat-treated additionally at 750±800°C. Finally, the edges of samples were Cu-plated electrically and soldered with the current and the potential leads. Tc was measured by the 4-probe resistive method and de®ned as the temperature where the sample shows half
Sample C
Sample D
Al±20 at.%Ge 1.5 0.3 121 317 3.079 3.079 121 361 14,641 114,437 33 3.16
10 5.78
the value of normal-state resistance. Hc2 (4.2 K) was estimated by extrapolation from Kramer plots [9]. Ic was de®ned as the current where the sample showed a potential drop of 100 lV/m in perpendicular ®elds. Jc was de®ned as Ic /S, where S is the total cross-sectional area of Nb/Al±X microcomposite ®laments, which is expected to be the maximum total cross-sectional A15phase area formed in the sample. According to the X-ray diraction patterns of the asquenched Nb/Al and Nb/Al±X (X Ge and Cu) wires, disordered A15-phase ®laments are formed directly in the Nb/Al±Ge and Nb/Al±Cu composite wires by the RHQ treatment, while supersaturated bcc-phase ®laments are formed in the Nb/Al composite wire by the RHQ treatment. The as-quenched Nb/Al±X (X Ge or Cu) wires show Tc of 15 or 12 K, respectively. With annealing at 750±800°C, Tc values of the Geadded Nb3 Al wire and the Cu-added one increased up to 19.4 and 18.2 K, respectively, as shown in Fig. 1. The annealing increased Tc through the improvement of long-range ordering in the A15 crystal structures. Jc (4.2 K) vs. B curves for the RHQO-processed Nb/Al±X (X Ge or Cu) wires and the RHQT-processed Nb/Al wire are shown in Fig. 2, as a parameter of Al±X alloy
Fig. 1. Tc versus additional annealing time curves for Nb/Al, Nb/Al± 20 at.% Ge and Nb/Al±2 at.% Cu wires, after RHQ treatment.
Y. Iijima et al. / Cryogenics 40 (2000) 345±348
Fig. 2. Jc versus magnetic ®eld B curves for the RHQT-processed Nb/ Al wire annealed at 800°C for 12 h, the RHQO-processed Nb/Al±20 at.% Ge wire annealed at 800°C for 24 h and the RHQO-processed Nb/ Al±2 at.% Cu wire annealed at 750°C for 96 h. Average diameters of the Al±X alloy cores or thickness of the Al ®lms are shown in parentheses. Non-Cu overall Jc versus B curve for the bronze processed (Nb,Ti)3 Sn wire is shown for comparison.
core diameter in the precursor wires. In the case of the RHQT-processed Nb/Al wire made by the jelly-roll process, the thickness of Al ®lms is shown in parentheses. The non-Cu overall Jc (4.2 K) vs. B curve for the bronzeprocessed (Nb,Ti)3 Sn wire is also shown for comparison. With the reduction of Al-alloy core diameter, the Jc of the RHQO-processed Nb3 Al-based A15 wires increases drastically, which may be caused by the increase of the A15 phase volume ratio in the ®laments. Jc (4.2 K) above 20 T for the Nb/Al±Ge wire with core size of 0.3 lm and for the Nb/Al±Cu wire with core size of 0.6 lm is the highest value among those for all the metallic multi®lamentary superconductors reported up to date.
3. Discussion Non-Cu overall Jc is very important for practical applications and depends on the cross-sectional con®guration of the superconducting wire. We have not yet optimized the cross-sectional con®guration of the RHQO-processed Nb3 Al wire. The Nb-matrix/A15 ratio can be reduced experimentally to 0.5 without any serious problems for the RHQT-process. If the Nb matrix ratio of RHQO-processed wire can also be reduced to 0.5, Jc /1.5 is the attainable non-Cu overall Jc . In this case, non-Cu overall Jc (4.2 K) above 150 A/mm2 is attainable in ®elds up to 23 T for the RHQO-processed wires. The bronze-processed (Nb,Ti)3 Sn multi®lamentary wire shows non-Cu overall Jc (4.2 K) above 150 A/
347
mm2 in ®elds below 18±19 T, which is the upper limit ®eld generated by using the (Nb,Ti)3 Sn wire at 4.2 K. Therefore, we will be able to make a superconducting magnet generating ®elds up to 23 T at 4.2 K and 26 T at 1.8 K by using the RHQO-processed wire in the near future. Because the disordered A15 phase in the asquenched RHQO-processed wire is brittle, the wind and react method does not seem to be convenient for coil winding. However, the react and wind method should be useful for the RHQO-processed wire due to the excellent strain tolerance of Nb3 Al [7]. The direct formation of A15 phase in the RHQOprocess may be caused by insucient cooling rate during the RHQ treatment. The unstable supersaturated bcc phase seems to transform into A15 phase during cooling. For the melt-quenched Nb±Al±Ge alloys, the supersaturated Nb±Al±Ge bcc phase was formed with extremely high quenching rate, although the A15 phase was formed directly with the ordinary quenching rate [12]. 4. Conclusions The RHQO-processed Nb3 Al-based conductors show the highest Jc (4.2 K) value above 20 T among all the metallic multi®lamentary superconductors reported up to date. Today, the high Tc superconducting wires, Bi2223 and Bi-2212 wires, are also known to have high Jc in high ®elds at cryogenic temperatures below 20 K. However, the regions of desirable conditions for fabricating Bi-system high Tc superconducting wires with high Jc are much narrower than those of commercialized metallic superconducting wires. For example, a large deviation of 25°C from the best heating temperature is negligible for the commercialized metallic superconductor; on the other hand a small deviation of 5°C causes a severe Jc degradation in the Bi-system superconductor. Therefore, the Bi-system high Tc superconducting wires must be fabricated under conditions controlled exactly, and then become very expensive or lacking in reliability. High cost or unreliability has prevented the use of high Tc superconductors in largescaled applications in the last decade. On the other hand, the fabrication cost of the RHQO-processed wire is comparable to that of commercialized metallic superconductors, which are used for various kinds of large-scale applications. Therefore, the new Nb3 Albased multi®lamentary superconductors seem to be very promising as the next-generation superconductors. Acknowledgements The present authors would like to thank the operators of Tsukuba Magnet Laboratory of NRIM for their help in measuring Ic in high ®elds.
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Y. Iijima et al. / Cryogenics 40 (2000) 345±348
References [1] Jorda JL, Fl ukige R, Muller J. J Less-Common Met 1980;75:227. [2] Ceresara S, Ricci MV, Sacchetti N, Sacerdoti G. IEEE Trans Magn 1977;MAG-11:465. [3] Akihama R, Murphy RJ, Foner S. Appl Phys Lett 1980;37:1107. [4] Inoue K, Iijima Y, Takeuchi T. Appl Phys Lett 1988;53:1724. [5] Saito S, Ikeda S, Ikeda K, Hanada S. J Jpn Inst Metals 1990;54:737. [6] Iijima Y, Kosuge M, Takeuchi T, Inoue K. Adv Cryogenic Eng Mater 1994;40:899.
[7] Takeuchi T, Iijima Y, Inoue K, Wada H, ten Haken B, ten Kate HHJ, et al. Appl Phys Lett 1997;71:122. [8] Kamata K, Moriai H, Tada N, Fujinaga T, Itoh K, Tachikawa K. IEEE Trans Magn 1985;MAG-21:277. [9] Iijima Y, Kikuchi A, Inoue K, Takeuchi T. IEEE Trans Supercond 1998;9:2696. [10] Foner S, McNi Jr EJ, Geballe TH, Willen RH, Bueler E. Physica 1971;55:534. [11] Foner S, McNi Jr EJ, Matthias BT, Geballe TH, Willens RH, Corenzwit E. Phys Lett 1970;31A:349. [12] Togano K, Takeuchi K, Tachikawa K. Appl Phys Lett 1982;41:199.
www.elsevier.com/locate/cryogenics
Research and technical note
New Nb3Al-based A15 multi®lamentary wires with high Jc in high ®elds Y. Iijima *, A. Kikuchi, K. Inoue National Research Institute for Metals, Sengen 1-2-1, Tsukuba-shi, Ibaraki 305-0047, Japan Received 13 April 2000; accepted 10 June 2000
Abstract When Nb/Al±X (X Ge or Cu) microcomposite precursor wires were rapidly heated and quenched, disordered Nb3 Al-based A15-phase ®laments formed. With annealing at 750±800°C, their superconducting properties improved drastically. The Nb3 Albased A15 wire, with Ge or Cu added, showed a maximum value of Tc 19.4 or 18.2 K, and that of Hc2 (4.2 K) 39.5 or 28.7 T, respectively, which are higher than the maximum values of Tc 17.9 K and Hc2 (4.2 K) 25.8 T for the rapid heating, quenching and transforming (RHQT)-processed Nb3 Al wire without additional elements. In addition, the new superconductors showed higher Jc than that of the RHQT-processed Nb3 Al multi®lamentary wire in high ®elds. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Nb3 Al-based high-®eld conductor; Cu or Ge addition; Rapid heating and quenching
1. Introduction Stoichiometric Nb3 Al shows higher values of Tc and Hc2 than commercialized Nb3 Sn superconducting wire. However, stoichiometric Nb3 Al is unstable except at high temperatures near 2000°C, while Nb-rich (o-stoichiometric) Nb3 Al, showing relatively low values of Tc and Hc2 , is stable at low temperatures as shown in the Nb±Al binary phase diagram [1]. In addition, Nb3 Al formation rate through solid-state diusion reaction between Nb and Al is very slow. In order to overcome the slow diusion reaction, various processes have been developed to increase the density of Nb/Al diusion couples in microcomposite wire, such as a jelly-roll process [2], a powder metallurgy process [3], a rod-in-tube process [4] and a cladchip extrusion process [5]. The combination of low temperature diusion reaction and Nb/Al microcomposite wire produced an Nb3 Al multi®lamentary wire having excellent strain tolerance and large Jc in low ®elds. However, the wire has o-stoichiometric Nb3 Al ®laments, of which Tc and Hc2 (4.2 K) are relatively low.
*
Coresponding author. Tel.: +81-2-98-59-2645; fax: +81-2-98-592601. E-mail address: [email protected] (Y. Iijima).
Six years ago, we proposed a rapid heating, quenching and transforming (RQHT) process to fabricate Nb3 Al multi®lamentary wires with near-stoichiometric composition [6]. In this process, an Nb/Al multi®lamentary microcomposite wire was rapidly heated continuously up to about 2000°C by resistive heating and then quenched into a molten Ga bath to form Nb±Al supersaturated bcc ®laments in the Nb matrix. The supersaturated bcc phase is metastable and ductile at room temperature and transformed into near-stoichiometric Nb3 Al phase with additional annealing at about 800°C. The resulting Nb3 Al multi®lamentary wire shows not only an excellent strain tolerance [7] but also 2±5 times larger Jc than that of the bronze-processed (Nb,Ti)3 Sn multi®lamentary wire, which is a commercialized standard high-®eld superconductor [8]. In addition, the RHQT-processed Nb3 Al wire shows maximum values of Tc 17.9 K and Hc2 (4.2 K) 25.8 T, which are almost the same as those of the bronze-processed (Nb, Ti)3 Sn multi®lamentary wires. On the way to investigating the eects of additional elements on the RHQT-processed Nb3 Al multi®lamentary wire, we found another route to obtain the Nb3 Albased A15 multi®lamentary wires with high Jc in high ®elds. Nb/Al±X (X Ge or Cu) multi®lamentary microcomposite wires were heat-treated by using the rapid heating and quenching (RHQ) apparatus. Due to the treatment, disordered A15 ®laments were directly
0011-2275/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 1 - 2 2 7 5 ( 0 0 ) 0 0 0 3 8 - 2
346
Y. Iijima et al. / Cryogenics 40 (2000) 345±348
Table 1 Speci®cations of Nb/Al±Ge and Nb/Al±Cu precursor wires Sample A Composition of Al±X alloy core Average diameter of Al±X alloy core (lm) Number of ®rst stacked composites Nb/Al±X alloy volume ratio Number of A15 ®laments (second stacked composites) Number of total stacked composites Matrix of the multi®lamentary wire Average diameter of A15 ®laments (lm) Nb-matrix/A15-®laments ratio
Sample B
Al±2 at.%Cu 1.5 0.6 121 121 3.079 2.852 121 121 14,641 102,487 Nb (99.8%) 33 33 3.16 3.16
obtained instead of the supersaturated bcc ®laments in the ordinary RHQT-process without Ge or Cu addition [9]. With annealing at 750±800°C, their superconducting properties improved drastically. The Nb3 Al-based A15 wire with Ge or Cu added showed maximum values of Tc 19.4 or 18.2 K and those of Hc2 (4.2 K) 39.5 or 28.7 T, respectively, which are higher than the maximum values of Tc 17.9 K and Hc2 (4.2 K) 25.8 T for the RHQT-processed Nb3 Al wire without additional elements. These values are relatively close to those reported for arc-melted Nb3 Al1ÿx Gex [10,11]. In addition, the new wires showed higher Jc (4.2 K) in high ®elds than RHQT-processed Nb3 Al wire. We call the new process the rapid heating, quenching and ordering (RHQO) process.
2. Experimental procedures and results The Nb/Al±X (X Ge or Cu) precursor wires were fabricated by the rod-in-tube process. Al±20 at.% Ge and Al±2 at.% Cu alloy rods were inserted into Nb pipes and then cold-drawn into Nb/Al±Ge and Nb/Al±Cu mono-core composite wires with an Nb/Al±X ratio of about 3. The mono-core composite wires were cut into short pieces and packed into Nb pipes. The ®rst stacked billets were cold-drawn into Nb/Al±Ge and Nb/Al±Cu multi-core composite wires, cut into short pieces and packed into Nb pipes again. The second stacked billets were cold-drawn into precursor wires with diameter of 0.5±0.74 mm. Detailed speci®cations of the precursor wires are shown in Table 1. The precursor wires were rapidly heated continuously up to about 2000°C by resistive heating, then quenched into a molten Ga bath in a vacuum chamber. After the coated Ga on the as-quenched wire was removed by chemical etching in hydrochloric acid, the wire was cut into short samples, enclosed into quartz capsules with vacuum and heat-treated additionally at 750±800°C. Finally, the edges of samples were Cu-plated electrically and soldered with the current and the potential leads. Tc was measured by the 4-probe resistive method and de®ned as the temperature where the sample shows half
Sample C
Sample D
Al±20 at.%Ge 1.5 0.3 121 317 3.079 3.079 121 361 14,641 114,437 33 3.16
10 5.78
the value of normal-state resistance. Hc2 (4.2 K) was estimated by extrapolation from Kramer plots [9]. Ic was de®ned as the current where the sample showed a potential drop of 100 lV/m in perpendicular ®elds. Jc was de®ned as Ic /S, where S is the total cross-sectional area of Nb/Al±X microcomposite ®laments, which is expected to be the maximum total cross-sectional A15phase area formed in the sample. According to the X-ray diraction patterns of the asquenched Nb/Al and Nb/Al±X (X Ge and Cu) wires, disordered A15-phase ®laments are formed directly in the Nb/Al±Ge and Nb/Al±Cu composite wires by the RHQ treatment, while supersaturated bcc-phase ®laments are formed in the Nb/Al composite wire by the RHQ treatment. The as-quenched Nb/Al±X (X Ge or Cu) wires show Tc of 15 or 12 K, respectively. With annealing at 750±800°C, Tc values of the Geadded Nb3 Al wire and the Cu-added one increased up to 19.4 and 18.2 K, respectively, as shown in Fig. 1. The annealing increased Tc through the improvement of long-range ordering in the A15 crystal structures. Jc (4.2 K) vs. B curves for the RHQO-processed Nb/Al±X (X Ge or Cu) wires and the RHQT-processed Nb/Al wire are shown in Fig. 2, as a parameter of Al±X alloy
Fig. 1. Tc versus additional annealing time curves for Nb/Al, Nb/Al± 20 at.% Ge and Nb/Al±2 at.% Cu wires, after RHQ treatment.
Y. Iijima et al. / Cryogenics 40 (2000) 345±348
Fig. 2. Jc versus magnetic ®eld B curves for the RHQT-processed Nb/ Al wire annealed at 800°C for 12 h, the RHQO-processed Nb/Al±20 at.% Ge wire annealed at 800°C for 24 h and the RHQO-processed Nb/ Al±2 at.% Cu wire annealed at 750°C for 96 h. Average diameters of the Al±X alloy cores or thickness of the Al ®lms are shown in parentheses. Non-Cu overall Jc versus B curve for the bronze processed (Nb,Ti)3 Sn wire is shown for comparison.
core diameter in the precursor wires. In the case of the RHQT-processed Nb/Al wire made by the jelly-roll process, the thickness of Al ®lms is shown in parentheses. The non-Cu overall Jc (4.2 K) vs. B curve for the bronzeprocessed (Nb,Ti)3 Sn wire is also shown for comparison. With the reduction of Al-alloy core diameter, the Jc of the RHQO-processed Nb3 Al-based A15 wires increases drastically, which may be caused by the increase of the A15 phase volume ratio in the ®laments. Jc (4.2 K) above 20 T for the Nb/Al±Ge wire with core size of 0.3 lm and for the Nb/Al±Cu wire with core size of 0.6 lm is the highest value among those for all the metallic multi®lamentary superconductors reported up to date.
3. Discussion Non-Cu overall Jc is very important for practical applications and depends on the cross-sectional con®guration of the superconducting wire. We have not yet optimized the cross-sectional con®guration of the RHQO-processed Nb3 Al wire. The Nb-matrix/A15 ratio can be reduced experimentally to 0.5 without any serious problems for the RHQT-process. If the Nb matrix ratio of RHQO-processed wire can also be reduced to 0.5, Jc /1.5 is the attainable non-Cu overall Jc . In this case, non-Cu overall Jc (4.2 K) above 150 A/mm2 is attainable in ®elds up to 23 T for the RHQO-processed wires. The bronze-processed (Nb,Ti)3 Sn multi®lamentary wire shows non-Cu overall Jc (4.2 K) above 150 A/
347
mm2 in ®elds below 18±19 T, which is the upper limit ®eld generated by using the (Nb,Ti)3 Sn wire at 4.2 K. Therefore, we will be able to make a superconducting magnet generating ®elds up to 23 T at 4.2 K and 26 T at 1.8 K by using the RHQO-processed wire in the near future. Because the disordered A15 phase in the asquenched RHQO-processed wire is brittle, the wind and react method does not seem to be convenient for coil winding. However, the react and wind method should be useful for the RHQO-processed wire due to the excellent strain tolerance of Nb3 Al [7]. The direct formation of A15 phase in the RHQOprocess may be caused by insucient cooling rate during the RHQ treatment. The unstable supersaturated bcc phase seems to transform into A15 phase during cooling. For the melt-quenched Nb±Al±Ge alloys, the supersaturated Nb±Al±Ge bcc phase was formed with extremely high quenching rate, although the A15 phase was formed directly with the ordinary quenching rate [12]. 4. Conclusions The RHQO-processed Nb3 Al-based conductors show the highest Jc (4.2 K) value above 20 T among all the metallic multi®lamentary superconductors reported up to date. Today, the high Tc superconducting wires, Bi2223 and Bi-2212 wires, are also known to have high Jc in high ®elds at cryogenic temperatures below 20 K. However, the regions of desirable conditions for fabricating Bi-system high Tc superconducting wires with high Jc are much narrower than those of commercialized metallic superconducting wires. For example, a large deviation of 25°C from the best heating temperature is negligible for the commercialized metallic superconductor; on the other hand a small deviation of 5°C causes a severe Jc degradation in the Bi-system superconductor. Therefore, the Bi-system high Tc superconducting wires must be fabricated under conditions controlled exactly, and then become very expensive or lacking in reliability. High cost or unreliability has prevented the use of high Tc superconductors in largescaled applications in the last decade. On the other hand, the fabrication cost of the RHQO-processed wire is comparable to that of commercialized metallic superconductors, which are used for various kinds of large-scale applications. Therefore, the new Nb3 Albased multi®lamentary superconductors seem to be very promising as the next-generation superconductors. Acknowledgements The present authors would like to thank the operators of Tsukuba Magnet Laboratory of NRIM for their help in measuring Ic in high ®elds.
348
Y. Iijima et al. / Cryogenics 40 (2000) 345±348
References [1] Jorda JL, Fl ukige R, Muller J. J Less-Common Met 1980;75:227. [2] Ceresara S, Ricci MV, Sacchetti N, Sacerdoti G. IEEE Trans Magn 1977;MAG-11:465. [3] Akihama R, Murphy RJ, Foner S. Appl Phys Lett 1980;37:1107. [4] Inoue K, Iijima Y, Takeuchi T. Appl Phys Lett 1988;53:1724. [5] Saito S, Ikeda S, Ikeda K, Hanada S. J Jpn Inst Metals 1990;54:737. [6] Iijima Y, Kosuge M, Takeuchi T, Inoue K. Adv Cryogenic Eng Mater 1994;40:899.
[7] Takeuchi T, Iijima Y, Inoue K, Wada H, ten Haken B, ten Kate HHJ, et al. Appl Phys Lett 1997;71:122. [8] Kamata K, Moriai H, Tada N, Fujinaga T, Itoh K, Tachikawa K. IEEE Trans Magn 1985;MAG-21:277. [9] Iijima Y, Kikuchi A, Inoue K, Takeuchi T. IEEE Trans Supercond 1998;9:2696. [10] Foner S, McNi Jr EJ, Geballe TH, Willen RH, Bueler E. Physica 1971;55:534. [11] Foner S, McNi Jr EJ, Matthias BT, Geballe TH, Willens RH, Corenzwit E. Phys Lett 1970;31A:349. [12] Togano K, Takeuchi K, Tachikawa K. Appl Phys Lett 1982;41:199.