Pinning force densities of continuous ultra-fine Nb3Al multifilamentary conductor

Pinning force densities of continuous ultra-fine Nb3AI multifilamentary conductor* K. I n o u e , T. T a k e u c h i , Y. l i j i m a a n d M . K o s u g e National Research Institute for Metals, Sengen 1-2-1, Tsukuba-shi, Ibaraki 305, Japan Dependence of Tc, #0Hc2 and Jc on filament size have been studied on a Nb3AI multifilamentary wire fabricated by a newly developed composite diffusion process, in which a multicore composite consisting of a niobium matrix and a large number of aluminium-based alloy cores is cold-drawn into a wire with continuous ultra-fine aluminium-based alloy filaments below 300 nm, and then heat treated to form Nb3AI filaments by the diffusion reaction. When the aluminium-alloy core size is 30-90 nm, the wire shows excellent superconducting properties, e.g. Tc of 1 5-1 7 K, #oHc2(4.2 K) of 21-25 T and J~ (4.2 K, 10 T) of 1-1.5 x 109 A m -2, which are comparable to those of the commercially available Nb3Sn multifilamentary wires. A significant enhancement in pinning force densities caused by reducing aluminium-alloy core size indicates that the Nb3A1 -matrix and the Nb3AI-core interfaces act as dominant pinning centres of fluxoids. An anisotropy observed in Jc of a rolled Nb3AI multifilamentary tape also supports the same pinning mechanism.

Keywords: critical currents; high Tc superconductivity; Nb3AI; ultra-fine M F wire; pinning mechanism

Only two kinds of superconductors, Nb3Sn and V3Ga, are currently used for the high-field superconducting magnets, although there are many AI5 superconductors with high Tc and high H~2, such as Nb3Ge, Nb3Ga and Nb3AI. Among them, Nb3A1 is considered as the most promising alternative to Nb3Sn and V3Ga, since high Jc has been demonstrated in the diffusion processed Nb3AI. The diffusion process is the most suitable method for producing a multifilamentary conductor commercially, and fabrication of a NbaAI wire has been investigated by two different methods. One of them is the powdermetallurgy process 1, in which a noibium and aluminium powder mixture was packed into a copper alloy sheath, cold-worked into a wire, and finally heat treated to form Nb3A1 filaments. Another method is the so-called jelly-roll process 2, in which two thin foils of niobium and aluminium were superimposed, wound around a small copper cylinder, inserted into a copper sheath, coldworked into a wire, and finally heat treated to form Nb3AI layers. To obtain high Jc in these methods, it is necessary to form Nb3A1 layers by the short-distance Nb/AI diffusion reaction. However, it has been very difficult to realize the short-distance diffusion reaction in a practical large-scale multifilamentary conductor due to the poor cold-workability of the Nb/A1 composite. We have been successful in realizing the short-distance Nb/A1 diffusion reaction and moreover have made a *Paperpresentedat the InternationalConferenceon Critical Currents in High-Temperature Superconductors, Snowmass Village, Colorado, USA, 16-19 August 1988 0011-2275/89/030361-06 $03.00 ~ 1989 Butterworth & Co (Publishers) Lid

muttifilamentary conductor with ultra-fine continuous Nb3A1 filaments using a newly-developed composite diffusion process 3. Alloying the aluminium-core with magnesium, silver, copper and/or zinc diminishes the difference in hardness between the niobium matrix and aluminium-core, and improves the workability of the Nb/A1 multifilamentary composite, resulting in success in fabricating a wire with continuous ultra-fine aluminium filaments embedded in the niobium matrix 3'4. The wires reacting at 700-1000°C show excellent superconducting properties, involving Jc over 1.5 x 10 9 A m - 2 at 4.2 K and 10 T. Among many Nb/Al-alloy composites studied, the Nb/A1-2at%Cu and the Nb/AI-7at%Mg composites show the best cold-workability, although the Nb/A15at%Mg composites show the best superconductivity. Therefore, in this study, the effects of the aluminium-alloy filament size on Jc and pinning force density have been studied in detail for the Nb/A1-2at%Cu and the Nb/AI5at%Mg composites, in order to clarify the pinning mechanism in the conductor with continuous ultra-fine Nb3A1 filaments. The results obtained indicate that the Nb3Al-matrix and Nb3Al-core interfaces act as dominant pinning centres of fluxoids.

Experimental procedures The large difference in hardness between pure niobium and pure aluminium makes it difficult to obtain a very short distance in Nb/AI diffusion couple through a Nb/AI composite process, resulting in breaking of the composite

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Pinning force densities of Nb aAI multifilamentary conductor." K. Inoue et a l. during wire-drawing. We have overcome the problem by hardening the aluminium cores through alloying with magnesium, silver, copper and/or zinc. In the proper composition range, these aluminium-based alloys exhibit similar hardness to that of the pure niobium, when either annealed or cold-worked. Particularly the Nb/AI2at%Cu and Nb/A1-7at%Mg composition show splendid cold-workability. The Nb3AI multililamentary wires were fabricated by the following procedure 3. The Al-(5-10)at%Mg, AI3at%Ag, AI-2at%Cu and A1-5at%Zn alloy ingots were made using a Tammann furnace under argon atmosphere and swaged into rods of 6.9 mm diameter. These rods were inserted into the niobium tubes of 7 mm i.d. and 14 mm o.d. The single-core Nb/Al-alloy composites were cold-drawn into wires of 1.14 mm diameter and cut into short pieces. The 121 short single-core wires were packed into a niobium pipe of 14 mm i.d. and 20 mm o.d. The resulting 12l-core Nb/Al-alloy composites were colddrawn into wires of 1.14 mm diameter and cut into short pieces, again. Similar processes were repeated three times and, finally 1.7 million (121 x 121 x 121)-core Nb/A1alloy composite wires of 0.2-3 mm diameter were fabricated. No intermediate annealing is necessary in the procedure, indicating that the fabrication process is very economical. The cross sectional optical m i c r o g r a p h s - o f the single-, 121-, 121 x 121-, and 121 x 121 x 121-core Nb/ A1-7at %Mg composite wires are shown in Figure 1. Figure 2 shows the scanning electron micrographs of the cross section of the 121 x 121 x 121-core Nb/AI-7at%Mg composite wire of 0.7 mm diameter. These micrographs reveal that the cold-workability of the Nb/Al-7at%Mg composite is excellent. Ultra-fine aluminium-alloy cores with diameters smaller than I00 nm are observed to be clearly separated from each other, and the cylindrical

shape of the aluminium-alloy cores remained remarkably unchanged in contrast with the ribbon-like filaments observed in the powder-metallurgy processed Nb/AI compositeL The composite wires with filaments of 25 to 2300 nm in diameter were heat treated above 675°C to form the Nb3A1 filaments through the diffusion reaction between the niobium matrix and the aluminium-alloy cores, and then coated with copper by electroplating. The current and potential copper leads were soldered on them in order to measure I~ and T~ by the four-probe resistive method. Tcs are defined as the temperatures where the samples show half of their normal-state resistances, los are defined as the currents where the samples show the maximum values of d2V/dI z in steady magnetic fields applied perpendicular to the central axes of the wires, where V is a voltage induced along the wire. The d 2 V / d I 2 gives the distribution of critical current in the multifilamentary wire 6. /toHc2 was determined by the extrapolation of Kramer plot. J¢ is defined as IJS, where S is the total cross sectional area of the AI5 phase calculated on the assumption that all A1-X alloy cores react to form the Nb3(AI, X) filaments. Some aluminium-poor X-alloys, however, are expected to remain at the centres of the filaments after heat treatment. Moreover intermediate phases other than Nb3AI, such as NbzA1 and NbAI 3, may be formed during the reaction v. Therefore real J~ of the Nb3AI layer seems to be much higher than the defined J~. If the cross-sectional configuration of the composite wire is ideally optimized, the J~ defined in this paper is expected to be nearly equal to the non-copper overall Jc. Recently, Nb3A1 multifilamentary wire embedded in a copper stabilizer matrix has been fabricated elsewhere by using this method 8 and nearly identical non-copper overall J¢ were demonstrated as those in the present paper, which indicated that our estimation is correct.

Results and discussions Jc versus B curves are in shown in Figure 3 for the Nb3A1 multifilamentary wires with several kinds of aluminium-alloy cores. The Nb/A1-5at%Mg composite wires show the best Jc, when reacted at a fixed temperature below 900°C; Jc(4.2 K)'s of 1.5 x 10 9 A m -2 at 10T and 1 x l0 s A m -z at 17.5T, #oHc2(4.2 K) of 21.8T and T~ of 15.7 K are obtained for the composite wire. The two-stage reactions process, which consists of a first reaction at a temperature higher than 950°C to form enough thick Nb3AI layers, and a subsequent second reaction at a temperature around 700°C to obtain a highly-ordered A15 crystal structure, has been found to improve To, #oHc2 and Jc in high magnetic fields; T~ of 16.4 K,/zoHc2 (4.2 K) of 22.7 T and Jc (4.2 K, 18.5 T) of l x l0 s A m -2 are achieved for the Nb/A1-5at%Mg composite, when the first reaction is carried out using the infrared image furnance 9. First reaction for a very short time at a high temperature above 1200°C is very effective to improve both T~ and #oHm2. T~ of 17. 4 K and tLoHc2 (4.2 K) of 25.4 T are obtained for the samples directly heated in liquid nitrogen by transmitting short pulsed current, although the heating conditions have not yet been optimized. Because these high values of J~ and izoHc2 are Typical

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362

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Pinning force densities of Nb3AI multifilamentary conductor. K. Inoue et al.

Figure 2 Scanning electron micrographs of transverse cross section for unreacted 121 x 121 x 121 .-core N b / A I - 7 a t % M g composite wire of 0.7 mm in diameter

comparable to those of the commercial Nb3Sn multifilamentary wires, the Nb3A1 multifilamentary wire is very promising as a practical high-field superconducting cable. Stabilized multifilamentary conductor embedded in a copper matrix can easily be fabricated by surrounding the niobium tubes with copper tubes in the present process. Furthermore, the low a.c. loss, expected from the ultrafine diameters of Nb3A1 filaments, seems to make the Nb3AI conductor attractive as an a.c. superconductor 1°. The electric coupling between filaments due to

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the proximity effect and the multi-connection of deformed filaments, which is one of the severe problems for a.c. superconducting cable 11, would be reduced desirably if the niobium tubes in the present process are inserted in tubes of materials such as Cu-Ni, C u - M n and so on, in addition to the optimization of wire configuration. Figure 4 shows the dependence of J~ (4.2 K) at 1 - 12.5 T on the aluminium-alloy core diameter for the Nb/AI2at%Cu and Nb/A1-5at%Mg composites heat treated at 750°C for 24 h. As the aluminium-alloy core diameter decreases, Jc initially increases, reaches a peak and then decreases. The aluminium-alloy diameter at which J~ reaches a peak is shifted towards smaller diameters with reducing magnetic field. In high fields the peaks for Nb/A1-5at%Mg composite appear at a slightly larger core diameter than those for Nb/AI-2at%Cu. The improvement of J~ with reducing aluminium-filament size was also reported in the powder-metallurgy processed Nb3A1 wire, where aluminium-filament size was con-

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Pinning force densities of Nb 3 AI multifilamentary conductor. K. Inoue et al.

trolled by varying the initial aluminium powder size or reduction ratio, R, of the Nb/A1 powder composite. Jc was increased with increasing R and did not saturate, at least up to R = 1400, for the composite with an initial aluminium powder size of 9 #m (Ref. 5). The final filament size is about 240 nm estimated from the formula of(initial powder size)/R ~ which is consistent with the results in Figure 4. For the large diameter aluminium-alloy core sample, the formation of Nb3A1 seems to be very small, because the amount ofaluminium supplied at the diffusion reaction is too much and NbA13 is the dominant form ~2. However, with decreasing aluminium-al!oy filament size, the intermediate compounds of NbA13 and Nb2A1, become more unstable than Nb3AI owing to the poor aluminium supply to the diffusion reaction, resulting in an increase in Nb3A1 formation and the improvement in J~. The Jc degradation for the sample with very fine aluminium-alloy filaments seems to be caused by the proximity effect described later. Similar aluminium-alloy filament size dependence are also observed for T~ and #oHm2, as shown in Figure 5. Maximum T~ and #oHm2 (4.2 K) values are obtained at a filament size of about 70 nm for both the Nb/AI2at%Cu and the Nb/A1-5at%Mg composites, although the core-size dependence of T~ and #oH,2 are very weak for the Nb/Al-5at% composite compared with those for the Nb/Al-2at% Cu composite. According to scanning electron microscopy the shapes of A1-5at%Mg alloy cases in the composite are more irregular than those of the A1-2at%Cu alloy cores, which may cause the weak size-dependence of Tc and #oHm2 (4.2 K) for the Nb/A15at% Mg composite. The aluminium-alloy core diameter at which T¢ and #oHm2 show peaks is apparently larger than those corresponding to the J¢ peak at 1-8 T in Figure 4. The optimized T~, by varying reaction time at a given aluminium-alloy core diameter, also shows a peak at a core diameter of 70 nm. Degradation of T~ at both large filament sizes and small ones can be explained by the proximity effect z3, because the thickness of Nb3A! layer formed by the diffusion reaction seems to be very thin in

both cases; excess alum,inium from the thick filament makes NbAI 3 and Nb2A1 stable and suppresses the growth of the Nb3AI layer lz, whilst very thin filaments, can never produce a thick Nb 3AI layer. When the filament size becomes less than 16 x ~s(~, coherence length), the critical parameters of the superconducting filaments decrease rapidly with decreasing filament size by the proximity effect. For the 38 nm A1-2at%Cu core sample, ~, is 4.2nm estimated from using the formula of (hc/ 4ne/z 0 He2) 1/2, where h is Plank's constant, c is the velocity of light, e is the electric charge of electron, and #0 He2 is 18.6 T as shown in Figure 5. In this case, the diameter of Nb3A1 formed around the core is at most 76 nm, which is comparable to 16 x ~s. In the pinning force density, Vp = "Jc × B, versus magnetic field curves at 4.2 K, a maximum Fp, which is denoted by Fp..... is observed at 3 4 T for the Nb/AI2at%Cu wire with #eric2 (4.2 K) of 19.3 T and for the Nb/A1-5at%Mg wire with #oHc2 (4.2 K) of 21.6T. As shown in Figure 6, there is a clear difference in the field dependence of pinning force density between the present Nb3A1 composites and a commercially available multifilamentary Nb3Sn wirer4; the normalized field showing Fp.... is 0.16 for the Nb3A1 wires while that of the Nb3Sn wire is 0.23. This difference in Fp versus h (normalized field) curves suggests that there are different dominant pinning mechanisms between the Nb3A1 and the Nb3Sn composites. It is well known that grain boundaries are major flux-pinning centres in the bronze-processed Nb3Sn composites. However, the Nb3Al-matrix and the Nb3AIcore interfaces seems to be the most important pinning 30

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h=H/Hc2 Figure 6 Pinning force density versus reduced magnetic field (H/Hc2) curves at 4.2 K for multifilamentary Nb3AI composites using AI-2at%Cu and AI-5at%Mg cores and commercially available multifilamentary Nb3Sn composite using Cu-7at%Sn matrix14: O, N b / A I - 5 M g at 750°C for 24h, filament diameter 0.05#m; O, Nb/AI-2 Cu at 750°C for 24h, filament diameter 0.04pm; x, Cu-7Sn/Nb at 650°C for 93 h, filament diameter 1 #m

Pinning force densities of Nb3AI multifilamentary conductor: K. Inoue centres in the present Nb3AI composite, because the grain size of Nb3AI would be at least comparable to the Nb3AI layer size. The average grain size of Nb3A1 in the powder-metallurgy processed Nb/AI composite reacted at 900°C has been reported to be 100 - 2 0 0 n m (Ref. 12). It should be noted that several tens of namometers is the smallest grain size of Nb3AI, reported up to date, which is obtained by transforming the supersaturated Nb-AI bcc solid solution into AI5 phase at 800°C 15. Therefore the number of grain boundaries existing in the direction perpendicular to the filament axis should be less than two. Futhermore, the elementary pinning force at the interface is expected to be much stronger than that at the grain boundary. Thus, by decreasing the Nb3AI layer size the interface pinning should become dominant. Dependence of Fp .... (4.2 K) on the aluminium-alloy filament size is also shown in Figure 5. Fp.... is much more sensitive to the aluminium-alloy filament size than T¢ and #oHm2 (4.2K). The highest F o.... (4.2K) of 1.6 x 10 t° N m -3 for the Nb/A1-2at%Cu composite and 2.6 x 10 ~° N m -3 for the Nb/A1-5at%Mg composite are obtained at an aluminium-alloy core diameter of about 40nm, which is much smaller than the core diameter (70 nm) at which the highest values of T¢ and #oHc2 (4.2 K) are shown. For the ultra-fine multifilamentary Nb-Ti conductor, Hl~isnik reported that Tc began to fall with decreasing Nb-Ti filament size from 135 nm, due to the proximity effect, while J~ continued to increase to 46 nm, reached a peak and then decreased rapidly 16. The phenomenon is explained by the increase of the surface current with decreasing filament size. The shift of Fp . . . . (4.2 K) peak toward smaller aluminium-alloy filament size can also be explained by the enhancement of Nb3AIinterface pinning caused by reducing the Nb3AI size. To verify the role of the interface pinning, a tape-formed composite of 0.16 mm thickness and 2.0 mm width was prepared by cold-rolling a 1.7 million core Nb/A12at%Cu composite wire of 0.7 mm diameter, because cold-rolling was expected to induce a morphology and configuration change of the Nb3A1 interface on which J~ should depend strongly. SEM observation revealed that the AI-atloy filaments were deformed into ribbon-like ones and aligned parallel to the rolling plane. If the Nb3AI

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interface acts as a dominant pinning centre, the strongest pinning should occur when the fluxoids are parallel to the tape surface, as is the case for the in situ processed Nb3Sn and V3Ga tape-formed conductors ~7. Experimentally, J¢ in the field parallel to the tape surface, J~ll, is larger than that in the field perpendicular to the surface, Jci, supporting the above speculation. As shown in Figure 7, the anisotropy factor of Jc, defined as Jcll/Jc±, increases with increasing magnetic field. ,ttoHc2 (4.2 K) is higher by about 1 T in the field parallel to the tape surface than that in the perpendicular field. The rapid increase in the anisotropy factor in high fields is caused by the anisotropy in ,uoHc2. On the other hand, tape-formed Nb3Sn multifilamentary conductors ~8, in which grain boundary pinning is believed to be dominant, show the opposite anisotropy; JcjJc I is below one as shown in Figure 7. Therefore the Jc anisotropy in the Nb3A1 multifilamentary tape also indicates that the Nb3Al-interfaces act as dominant pinning centres in the conductors. It is noticeable that such an anisotropy in J¢ is advantageous when the multifilamentary Nb3A1 tapeformed conductors with large aspect ratio are wound into coils, because the maximum field is applied parallel to the tape surface in the coil.

Conclusions The superconducting properties of the Nb3A1 multifilamentary wires fabricated by the newly developed composite-diffusion process have been studied. T~ and #oHc2 are found to depend on aluminium-alloy core diameter and show maximum values at a diameter of about 70 nm, while Fp.... (4.2 K) shows a maximum at 40 nm, in addition to its stronger dependence on the core diameter. The degradation of T¢, #oHc2 and Jc at extremely small filament sizes can be explained by the proximity effect. Moreover, the aluminium-alloy core diameters play important roles on varying both the amount of Nb3A1 formation and the surface current contribution. Ftuxoid pinning in the present Nb3AI multifilamentary wires is found to be dominantly caused by the Nb3Al-matrix and Nb3Al-core interfaces. The highest Jc (4.2 K)s of 1.5 x 109 Am-2atl0Tandi x 108Am-Zatl8.5Tareobtained, at least up to the present time hitherto, for the Nb/AI-5at%Mg composite wire with about 70 nm AI-Mg core reacted by the two-stage reaction. To understand the pinning mechanism in more detail, microstructural studies must be carried out using transmission electron microscopy and X-ray diffraction, which can also reveal the kinetics of the Nb3AI formation, its composition and its atomic long-range order parameter.

Acknowledgements _ ~

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B (T) Anisotropy in Jc (4.2 K), Jcil/Jc±, as a function of magnetic field for Nb/AI-2at%Cu (Nb3AI) tape reacted at 750°C for 24h, compared with that for Cu-7at%Sn-0.4at% Ti/Nb (Nb3Sn) tape TM

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The authors express their sincere thanks to Professor K. Noto, Dr K. Watanabe, and the staff of Institute for Materials Research, Tohoku University for measuring the critical currents in high fields by their 23 T hybrid magnet. References

! Thieme, C.L.H., Pourrahimi, S., Schwartz, B.B. and Foner, S. IEEE Trans Magn (1985) 21, 756 2 Bruzzese, R., Sacchetti, N., Spadoni, M., Barani, G., Donati, G. and Ceresara, S. IEEE Trans Magn (1987) 23, 653 3 lnoue, K., lijima, Y. and Takeuchi, T. Appl Phys Lett (1988) 52, 1724

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Pinning force densities of Nb 3 AI multifilamentary conductor: K. Inoue et al. 4 Takeuchi, T., iijima, Y. and lnoue, K. presented at the MRS Inter Meeting Adv Mater, (Tokyo, 1988) 5 Akihama, R., Murphy, R.J. and Foner, S. IEEE Trans Magn (1981) 17, 274 6 Warnes, W,H. and Larbalestier, D.C. Cryogenics (1986) 26, 643 7 Ludin, C.E. and Yamamoto, A.S. Trans Met Soc A IME (1966) 236, 863 8 Shimizu, H, and Tanaka, Y. Private communication (1988) 9 Inoue, K., lijima, Y. and Takeuchi, T. Cryogenics (1988) Submitted for publication I0 Hbisnik, 1. J Phys (1984) 45, C1-459 11 Kubota, Y. and Ogasawara T. IEEE Trans Magn (1987) 23, 1359

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12 im, Y., Johnson, P.E., McKnelly, L.T., Jr. and Morris, J.W., Jr. J Less-Common Met (1988) 139, 87 13 Yasohama, K., Morita, K. and Ogasawara, T. IEEE Trans Magn (1987) 23 1728 14 Ekin, J.W. Cryogenics (1980) 20. 611 15 Takeuehi, T., Togano, K. and Taehikawa, K. IEEE Trans Magn (1987) 23 965 16 Hl~isnik, I., Tak~ies, S., Burjak, V.P. et al. Cryogenics (1985) 25, 558 17 Takeuchi, T., Togano, K. and Tachikawa, K. J Mater Sci (1984) 19 2•72 18 Sekine, H., lijima, Y., ltoh, K. and Tachikawa, K. IEEE Trans Magn (1983) 19, 1429