Ultrafine multifilamentary Nb  Ti wires with Cu  Si alloy matrix

Ultrafine multifilamentary N b - Ti wires with C u - Si alloy matrix S. Akita, S. Torii, H. Kasahara, K. M a t s u m o t o * , Y. Tanaka*, T. Ajiokat and K. Tachikawa t Central Research Institute of Komae-shi, Tokyo 201, Japan *Yokohama R&D Laboratories, Yokohama-shi, Kanagawa 220, *Faculty of Engineering, Tokai 259-12, Japan

Electric

Power Industry

(CRIEPI),

2-11-1

Iwatokita,

The Furukawa Electric Co. Ltd, 2 - 4 - 3 0 k a n o , Nishi-Ku, Japan University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa

Received 6 July 1992 The Cu - Si alloy has been proposed as a new matrix material for filamentary Nb - Ti wires in a.c. use. The C u - Si alloy shows appropriate mechanical and electrical properties, and is economically more favourable than the C u - Ni alloy matrix used currently. Moreover, the addition of Si to Cu prevents the formation of intermetallic compounds around the filaments. After extensive investigations on Cu-Si alloy as a matrix material, an ultrafine multifilamentary N b - T i / C u - 2 . 5 w t % Si composite wire has been successfully fabricated. The d.c. superconducting properties of the wire were adequate for use in electrical apparatus. A preliminary study has revealed that the a.c. loss of the new wire is equivalent to that of a typical ultrafine multifilamentary N b - T i / C u - Ni composite wire.

Keywords: Nb-Ti; multifilamentary wires; Cu-Si

If superconductors can be successfully applied to electric machines such as generators or transformers, substantial benefits will ensue because of the high efficiency and compactness of superconductive electric machines. Since most electric machines are operated in the 50 or 60 Hz a.c. current mode, a low a.c. loss superconductor is the most essential factor for the development of superconductive machines. To reduce the total a.c. loss at 50 or 60 Hz in a superconducting wire, it is necessary to reduce the diameter of the superconducting filaments to below 1 #m and to use a matrix with high electrical resistivity. Currently the combination of N b - T i filaments and a C u - N i matrix is applied to realize a low a.c. loss wire. In particular, a wire with low a.c. loss, compared to the Joule loss of a normal conductor, was successfully developed by applying C u - 3 0 wt% Ni alloy as the matrix'. Recently, C u - 0 . 5 wt% Mn alloy has also been proposed as a promising new matrix material 2. However, there are still a few problems with N b - T i / C u - N i composite a.c. wires. The first is that the overall critical current density is apt to decrease if the spacing between superconducting filaments is made wide to avoid superconductive current coupling between filaments, which causes excess hysteresis loss. The second is that the mechanical hardness of C u - 3 0 wt% Ni is too high for smooth drawing of the composite. The third problem is that the combination of

C u / C u - Ni and N b - T i is likely to produce intermetallic compounds on the surface of the NbTi filaments, which prevent successful fabrication of the filaments. We have chosen C u - S i alloy as a new matrix material instead of C u - N i alloy because of its appropriate hardness and electrical resistivity, as well as the possibility of reducing the cost of the matrix material 3. In this paper, we present the material characteristics of C u - S i alloy as the new matrix material for a.c. superconducting wires, the results of a preliminary fabrication of N b - T i / C u - S i multifilamentary wire and the superconducting characteristics of the wire.

Experimental procedures Matrix material preparation C u - S i alloy specimens containing 1 . 0 - 3 . 5 wt% Si were melted in a graphite crucible using 99.9% pure Cu and C u - 15 wt% Si mother alloy in open air, and cast into a metal mould. The rod-shaped cast specimens, 10 mm in diameter, were fabricated into wires 1 mm in diameter by grooved rolling and subsequent drawing at room temperature. As reference materials, C u - 10 and 30 wt% Ni alloy wires were also prepared by a similar procedure. The mechanical hardness of C u - S i alloy wires was measured using a micro-Vickers hardness tester. The electrical resistivity of the C u - S i and

OOl 1 - 2275/93/O2Ol 99 - 06 © 1993 Butterworth - Heinemann Ltd

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Multifilamentary Nb - Ti wires with Cu - S i matrix: S. Akita et al.

C u - N i alloy wires was measured using a conventional four probe method at 300, 77 and 4.2 K. Diffusion couples composed of pure Cu, C u - 10 wt% Ni or a C u - 3 . 5 wt% Si alloy matrix and a Nb-46.5 wt% Ti core were prepared in the following way. A 5 mm diameter N b - T i core was encased in a matrix tube with an outer diameter of 10 mm. The resulting composites were fabricated into a tape 0.3 mm in thickness and 5 mm in width, and then heat treated at temperatures between 500 and 700°C. The composition profile of a cross-section of the diffusion couples was studied using an electron probe microanalyser (EPMA).

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We used the C u - 2 . 5 wt% Si alloy matrix in a multifilamentary superconducting wire with Nb-46.5 wt% Ti fine filaments. In Table 1 the specifications of specimen wires are listed. The fabrication process for the multifilamentary wires was a conventional stacking technique. First, a C u - S i billet with 19 N b - T i cores was hot-extruded, formed into a hexagonal shape by wire drawing and then cut into 55 rods. These rods were encased in a C u - S i tube and then hot-extruded. The resulting composite was colddrawn down to the outer diameters of 0.85, 0.74, 0.64 and 0.522 ram, corresponding to samples A, B, C and D, respectively, described in Table 1. Part of the twice-extruded composite was again fabricated into a hexagonal shape and cut into 55 rods. These rods were restacked into a C u - S i tube and the obtained billet was then reduced to a wire with a diameter of 0.208 mm and with 57475 N b - T i filaments (sample E), by means of the same fabrication method. Here, annealing of the wire to obtain an appropriate level of hardness in the matrix was carried out for wire diameters 7.2 and 3.6 mm at 350°C for 0.33 h and 0.25 h, respectively. For a comparison between the present wires and a conventional one,, we prepared an a.c. superconducting wire with a C u - 10 wt% Ni matrix and a N b - T i filament diameter of 0.5 #m (sample F ) 4 as was used for the VAMAS project. Specifications of samples E and F are also shown in Table 1. Measurement methods

The critical temperature, Tc, the upper critical field, H~2, and the critical current density, Jc, of the fabricated N b - T i / C u - S i composite wires were measured by a conventional four probe method. He2 and J~ were measured at 4.2 K in transverse magnetic

Table 1

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Figure 1 Micro-Vickers hardness of C u - 2 . 5 (o) and 3.5 w t % Si ( • ) alloys (as cast) v e r s u s total deformation strain. O, Intermediate annealing at 3 8 0 ° C for 1 h; e , intermediate annealing at 4 0 0 ° C for 1 h

fields generated by a 13.5 T superconducting magnet. The critical current, Ic, was determined by the appearance of 1 /zV cm-~ of normal voltage across the specimen. Jc was calculated by dividing Ic by the crosssectional area of the N b - T i . Hysteresis loss of the wire was measured using a vibrating sample B - H curve tracer. Overall a.c. loss, including both hysteresis and coupling loss, was measured by a pick-up coil method in a 52.3 Hz a.c. magnetic field generated by an a.c. superconducting magnet. The maximum a.c. magnetic field was 1.7 T. The single-layer sample coil used for the pick-up coil method was 25 mm in diameter, and 40 mm in height. Results

and discussion

Material aspects of C u - Si alloy matrix

Figure 1 shows the Vickers hardness of C u - 2 . 5 and 3.5 wt% Si alloy specimens as a function of total deformation strain, e,, produced by fabrication, e, is defined as ln(Ao/A), where Ao and A are the crosssectional areas of the specimen before and after fabrication, respectively. The C u - S i alloy exhibits rather rapid work hardening, though it can be easily recovered by annealing at a relatively low temperature (380-400°C) for a short time. Figure 2 shows the Vickers hardness versus e, curves of C u - 3 . 5 wt% Si alloy specimens homogenized at 600-800°C for 1.5 h

Specifications of t h e wires

Sample

Diameter (mm)

Filament d i a m e t e r (#m)

N u m b e r of f i l a m e n t s

Matrix

A B C D E F

0.85 O. 7 4 0.64 0.522 0.208 0.14

12.16 10.36 9.15 7.46 0.36 0.50

1045 1045 1045 1045 57 4 7 5 14 2 8 0

Cu - Si Cu - Si Cu-Si Cu-Si Cu - Si Cu-Ni

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Multifilamentary N b - Ti wires with Cu - S i matrix: S. Akita et al.

before fabrication. The homogenization treatment appreciably suppresses work hardening in the C u - 3 . 5 wt% Si alloy and facilitates the reduction of area down to an e, value of 4.5 without intermediate annealing. Figure 3 shows the variation of electrical resistivity of C u - 1 . 0 , 2.5 and 3.5 wt% Si alloys, as well as that of C u - 1 0 and 30wt% Ni alloys as a function of temperature. The electrical resistivity of specimens was measured after 99% reduction of area. The resistivity of the C u - 2 . 5 wt% Si alloy exceeds that of the C u - 1 0 wt% Ni alloy at 4.2 K, while the resistivity of the C u - 3 . 5 wt% Si alloy is close to that of the C u - 3 0 wt% Ni alloy. These results imply that the C u - S i alloy has sufficient electrical resistivity at low temperatures to replace C u - N i alloy as a superconducting wire matrix for a.c. use. Furthermore, C u - S i alloy is economically more favourable than C u - N i alloy due to its lower cost. Figures 4 and 5 show EPMA composition mappings taken from a cross-section of diffusion couples composed of N b - 4 6 . 5 wt% Ti/Cu and N b - 4 6 . 5 wt% Ti/Cu-3.5 wt% Si, respectively. These diffusion couples were heat treated at 600°C for 100 h in a vacuum. In the N b - T i / C u composite, the formation of three intermetallic compound layers, i.e. Ti2CUT, (TixNbl_s)Cu and (TixNbl_~)2Cu, are observed. This result is consistent with that reported by Larbalestier et al. 5. In the N b - T i / C u - 1 0 wt% Ni composite, the formation of intermetallic compounds other than Ti2Cu7 is observed. The formation of these compound layers causes sausaging and breakage of N b - T i cores when they are reduced to the submicron diameter range. Meanwhile, no compound layer is formed in the diffusion couple composed of C u - 3 . 5 wt% Si alloy matrix and N b - T i core, as shown in Figure 5. This may be a key advantage of C u - S i alloy when it is used as the matrix of ultrafine filamentary N b - T i wires. A very thin layer of Si may cover the N b - T i core, which prevents diffusion against the matrix. The formation of a thin Si layer is observed around the N b - T i core when the N b - T i / C u - S i diffusion couple is heat treated at 700°C. Usually, N b - T i cores are enveloped by a Nb

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sheath to avoid the formation of compound layers when they are reduced to a submicron diameter. The C u - S i alloy matrix does not require the Nb envelope, which simplifies the fabrication procedure of filamentary N b - T i wires for a.c. use. D.c. s u p e r c o n d u c t i n g p r o p e r t i e s o f Nb - Ti/Cu - Si c o m p o s i t e wires

Measurements of To, He2 and J~. were performed on specimens A, B, C, D and E presented in Table 1. Figure 6 shows the variations in T~. and H~2 of specimens against the N b - T i filament diameter, i.e. the degree of cold reduction. The values of T~ and H~.2 of specimen E are appreciably smaller than those of other specimens. The depression of T, in the submicron filament wire may be associated with the proximity effect, which is brought about by the leaking of superconducting electrons from the filamentary N b - T i region into the C u - S i matrix. The depression of Tc with wire drawing has also been reported in wire with submicron N b - T i filaments and a Cu matrix 6, where the proximity effect at the interface between the N b - T i filament and Cu matrix is considered to be the major origin of this depression. The depression of He2 in specimen E may also be explained by the proximity effect, since H,.2 is proportional to T~, as mentioned below. The wire with a diameter of 0.522 mm (sample D) was heat treated at 380°C to investigate the effect of duration of heat treatment on Tc and H~.2. The results are shown in Figure 7. The observed He2 values show a monotonous decrease with heat treatment time, although the value of T~ remains nearly constant. Generally, He2 at absolute zero temperature is

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Multifilamentary Nb - Ti wires with Cu - S i matrix: S. Akita et al.

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Multifilamentary N b - 77 wires with C u - Si matrix: S. Akita et al.

Jc measurements were made for specimens D and E, and the results are shown in Figure 8. The observed values of J~ in specimen E are 11200 A mm -2 at 0.5 T and 6800 A mm-2 at 1 T. These values are comparable with those for the a.c. superconducting wires reported so far' and are high enough for practical use. The fact that the J~ value of specimen E in high fields is smaller than that of specimen D may be due to the depression of H~2, described above.

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The hysteresis loss of sample E per unit volume of the whole wire is shown in Figure 9. Measured losses are multiplied by 50 and shown in units of kW m -3 to approximate hysteresis losses at 50 Hz operation. The magnetic field dependence of hysteresis loss is quite similar to that of the ordinary N b - T i / C u - N i ultrafine filamentary superconducting wires reported previously 7. Hysteresis loss of the present samples, which have the same cross-sectional structure but different diameters, was also measured under a magnetic field of -4-1 T. The results were compared with values calculated from Equation (2) 8. From this comparison, we can obtain an effective filament diameter, d~ff, which gives the measured hysteresis loss in Equation (2)

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increases below d r = 0.51 #m. This characteristic implies the existence of electromagnetic coupling between N b - T i filaments due to the proximity effect. In the case of the C u - S i alloy matrix, the critical distance below which the proximity effect will occur is estimated to be 0.17 #m from its electrical resistivity. This value is near the critical distance of 0.18 #m in N b - T i / C u - 1 0 wt% Ni composite wire 9, while the spacing distance between filaments, d,, is about onethird of df in sample E. The value of d, below which the hysteresis loss increases is 0.17 #m and corresponds

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Cryogenics 1993 Vol 33, No 2

203

M u l t i f i l a m e n t a r y Nb - Ti wires w i t h Cu - Si matrix: S. A k i t a et al.

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E <

Conclusions The low cost C u - S i alloy has been chosen and tested as a new matrix material for ultrafine multifilamentary superconductors. No intermetallic compound is formed by diffusion between the N b - T i and C u - S i alloy. In addition, the mechanical and electrical properties of the C u - S i alloy make it appropriate for the matrix of a.c. superconductors. Based on these advantageous characteristics of the C u - S i alloy, we successfully fabricated an ultrafine multifilamentary N b - T i / C u - S i alloy composite. The a.c. loss of the preliminarily fabricated wire is as low as that of the well-developed N b - T i / C u - N i composite wire. Hereafter, optimization of the composition of the C u - Si alloy and the design of the wire configuration, as well as secondary fabrication of the wire, will be pursued.

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11 Overall a.c. losses normalized by overall critical current density. • , N b - T i / C u - S i c o m p o s i t e wire (sample E); 0 , N b - T i / C u - Ni c o m p o s i t e wire (sample F), as detailed in V A M A S project report 4. - • - , Cu, 10 A mm - 2 . (The losses at 4 . 2 K are normalized to room t e m p e r a t u r e using a refrigerator e f f i c i e n c y o f 1000)

with the critical distance mentioned above. This correspondence suggests that the main origin of enhancement of deff is the proximity effect. Figure 11, the a.c. loss of sample E is shown as a normalized value, which is obtained from the a.c. loss density divided by the overall critical current density shown in Figure 8. Below a magnetic field with a peak value of 1 T, the a.c. loss of the present preliminary N b - T i / C u - Si composite wire, including the refrigeration power of a liquid helium temperature refrigerator, is lower than the Joule loss in a normal copper conductor operated at a current density of l0 A mm -2. The a.c. loss of a typical N b - T i / C u - N i composite ultrafine filamentary superconductor (sample F of the intercomparison in the VAMAS project 4) is also shown in Figure 11. The values of normalized a.c. loss in both studies are comparable. A high possibility of fabricating low a.c. loss composite wires with C u - S i alloy may be predicted, because the preliminarily fabricated wire shows the same level of loss as the well-developed N b - Ti/Cu - Ni composite wire.

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References 1 Cave, J.R. Electromagnetic properties of ultrafine filamentary superconductors Cryogenics (1989) 29 304-31l 2 Gregory, E., Kreilick, T.S., Wong, J., Collings, E.W. et al. A conductor with uncoupled 2.5 #m diameter filaments designed for the outer cable of SSC dipole magnets IEEE Trans Magn (1989) MAG-25 1926-1929 3 Tachikawa, K., Ajioka, T., Akita, S., Matsumoto, K. et al. Development of NbTi fine multifilamentary superconducting wires with a new matrix material of C u - S i alloys Proc 43rd Meeting on Cryogenics and Superconductivity Cryogenic Society of Japan, Yokohama, Japan (1990) 118 (in Japanese) 4 Itoh, K., Akita, S., Tachikawa, K. et al. VAMAS 1: Inter comparison of AC loss measurement: Japanese results Adv Cryog Eng Mat (1990) 36 199-206 5 Larbalestier, D.C., Lee, P.J. and Samuel, R.W. The growth of intermetallic compounds at the copper-niobium-titanium interface Adv Cryog Eng Mat (1986) 32 715-722 6 Yasohama, K., Morita, K. and Ogasawara, T. Superconducting properties of Cu-NbTi composite wires with fine filaments IEEE Trans Magn (1987) M A G - 2 3 1728-1731 7 Sumiyoshi, F., Matsuyama, M., Noda, M., Matsushita, T. et al. Anomalous magnetic behaviour due to reversible fluxoid motion in superconducting multi-filamentary wires with very fine filaments Jpn J Appl Phys (1986) 25 L I 4 8 - L I 5 0 8 Wilson, M.N. Superconducting Magnets Clarendon Press, Oxford, UK (1983) 171 9 Matsumoto, K., Akita, S., Tanaka, Y. and Tsukamoto, O. Proximity coupling effect in N b - T i fine-multifilamentary superconductng composites Appl Phys Lett (1990) 57 (8) 816-818