Effect of the layer thickness on Nb3Al superconducting wires

Materials Science and Engmeermg, B7 (1990) 31-36

31

Effect of the Layer Thickness on Nb3A! Superconducting Wires S. SAITO and S. HANADA

lnstttute for Matertals Research, Tohoku Umverstty, Sendat 980 (Japan) K. IKEDA

Department of Matertals Processmg, Tohoku Umversity, Sendat 980 (Japan) A. NAGATA

Department of Matertalsfor Engmeering, Aktta Umversity, Aktta 101 (Japan) K. NOTO

Department of Electrical Engmeermg, lwate Umverstty, Mortoka 020 (Japan) (Received January 19, 1990)

Abstract

The Nb/Al composite wires with vartous average thicknesses of niobium layer, from 43 to 300 nm, were fabricated by the method using aluminium-clad niobium foils as a starting material and the dependency of the critical current density Jc on the niobium layer thickness was studied. Even at a thickness of 43 nm, Jc gave no tendency for saturation. In contrast, the transition temperature was saturated at about 17K. Although the obtained Jc values exceeded the best ones for the Nb/Al composite wires fabricated by the powder metallurgy process and niobium-tube method, they were still far from the results for stoichiometric Nb3AI. The Jc dependency on the niobium layer thickness was compared with the data in other methods and discussed in detail.

I. Introduction

Recent progress in studies on the fabrication process of Nb3AI superconducting wires has been remarkable. Following the pioneering study on the powder metallurgy process by Thieme et al. [1], the niobium tube method by Inoue et al. [2] and our clad-chip extrusion (CCE) technique [3] have successively realized critical current densities of more than 10 s A m-2 at 18 T, which are comparable with those of (Nb,Ti)3Sn multifilamentary wires. In addition, it has been reported that the Nb3A1 wire shows strain tolerance [2, 4] superior to NbaSn and low a.c. loss [5, 6], indicating that the Nb3A1 cable is very promising for 0921-5107/90/$3.50

high field applications in the near future. However, it is also true that there is still a margin for improvement, because the superconducting properties of the Nb3AI wires are considerably lower than the values for stoichiometric Nb3AI

[7, 8]. The extent of Nb3AI formation by the solid state reaction at low temperature (less than 1300 K) is predominantly dependent on the layer thickness of niobium and aluminium. The study on sputter-deposited Nb/AI multilayers by Bormann et al. [9] indicated that the reaction giving A15 formation went to completion only at a niobium layer thickness of 30 nm or less. In fact, the CCE-processed Nb/A1 composite wire included a small amount of unreacted niobium and gave no indication of Jc saturation even at an average niobium layer thickness of 80 nm [10]. Similar results have been reported in the P/M process, although the average niobium layer thickness was not less than 100 nm [1, 11]. It is of crucial importance basically as well as practically to grasp the limit of superconducting properties attainable by the interdiffusion reaction at low temperatures. The Nb/A1 composite wire fabricated by the method using aluminiumclad niobium, including the CCE technique, has good workability. Moreover, the total amount of deformation throughout the process can be considerably reduced by using clad foils as thin as possible. Such a method is also favourable for investigating the effect of the layer thickness on the superconducting properties because the composite wire has a Nb/Al-multilayered structure © Elsevier Sequoia/Printed m The Netherlands

32

from the start. Therefore two kinds of Nb/AI composite wires were fabricated using aluminium-clad niobium foils with thicknesses of 15 and 100 /~m, and the dependency of the Jc properties on the niobium layer thickness was studied.

(a) CT Specimen

(b) CL Sp. . . . . .

\Cu Alloy

2. Experimental procedures A niobium sheet, 1 mm thick in which the oxygen content was less than 100 ppm and an aluminium foil, 0.14 mm thick, were used as starting materials. The deformation process in the "clad method", including the CCE technique for fabricating composite wires, consists of plane strain deformation in the clad-rolling process and axisymmetric deformation in the following mechanical working processes. An areal reduction ratio of 108 is required to reduce 1 mm thick mobium to 0.1 /~m only by axisymmetric deformation, while that for plane strain deformation alone is 104 . Appropriate comparison of the amount of deformation between different deformation modes must be made not using the areal reduction ratio R but using the equivalent strain eeq. An areal reduction of 108 in axisymmetric deformation corresponds to eeq= 18.4, while R = 104 in plane strain deformation corresponds to e~q= 10.6. Therefore the larger the amount of deformation in the clad-rolling process, the smaller the total plastic strain through the whole fabrication process. This is the reason why thin Nb/AI multilayers can be obtained at relatively small strains, despite using thick niobium sheet as the starting material. At first, 15 and 100/~m thick aluminium-clad niobium foils were prepared by rolling niobium sheet sandwiched between aluminium foil. Circular foils with a diameter of 30 mm were blanked out from 10 ~m thick ahiminium-clad niobium, and stacked into a copper alloy sheath, as shown m Fig. l(a). After evacuation, the billet was extruded with a reduction ratio of five and followed by rod-rolling and drawing. After etching away the sheath material, 19 strands of the Nb/AI composite were bundled in a double sheath consisting of niobium on the inside and copper alloy on the outside and re-extruded to achieve a strong bond between them. Then the extrudate was rod rolled and drawn to give fine wires. Alternatively, the 15 /~m clad foil was tightly wound around a niobium wire with a diameter of 1.3 mm in a jelly roll fashion and inserted into a

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double sheath consisting of niobium on the inside and copper alloy on the outside, as shown in Fig. l(b). After evacuation, the billet was extruded and followed by rod rolling and drawing. The two kinds of specimens shown in Fig. l(a) and Fig. l(b) are given the sample names of "CT" and "CU' respectively. In the CCE method, small square chips of aluminium-clad niobium are filled up randomly into a sheath material, as mentioned in proceeding papers [3]. The initial arrangement of niobium and aluminium layers may have some effect on the resulting composite wires. To investigate this, billets in which aluminium-clad niobium foil was stacked in two different ways were prepared.

3. Experimental results and discussion Both CL and CT specimens were processed thoroughly to an average niobium layer thickness of about 70 nm, and the bare Nb/AI composite wires after etching away the sheath material were very ductile. However, they lost ductility partially when they were drawn to a niobium layer thickness less than 70 nm. Nevertheless, a large portion of the wire was sound. The CCE-processed wire, made of 0.2 mm thick aluminium-clad niobium, also exhibited breaks at a niobium layer thickness of less than about 70 nm during wire drawing. Since the total equivalent strain required to yield a niobium thickness of 70 nm is largely different among the three kinds of materials, it is

33

without doubt that the working limit is decided not by the degree of deformation but by a critical niobium layer thickness. It is well known that niobium continues to work harden linearly up to very large areal reductions [12]. Continuing work hardening results in raising the flow stress ratio of niobium to aluminium; however, it retards the occurrence of plastic instability which is one of the main reasons for the failure of metal-metal composite materials. It is certain that the excellent workability of the Nb/AI composite wire made of aluminium-clad niobium is largely due to the work-hardening characteristics of niobium. Macroscopic shear banding, resulting from the plastic instability, was always observed in composite wires which lost ductility. It is probable that work hardenability of niobium disappears owing to some microscopic structure changes below a critical thickness. Analytical electron microscopic observation will provide an answer to this problem. Figure 2 shows variations of J~ at 14.4 T and 4.2 K as a function of the average niobium layer thickness for the CL and CT specimens. They were heat treated at 1273 K for 120 s followed by 1023 K for 4 days. The thickness dependence of Jc is quite similar in spite of large differences in the initial thickness and stacking manner of the aluminium-clad niobium foil and also the amount of deformation. J¢ continues to increase with decreasing average niobium layer thickness, presenting no evidence of saturation even at a

thickness of 43 nm. It appears that Jc continues to rise until the critical niobium layer thickness of 30 nm, predicted from the multilayer study [9], is reached. However, the Jc value extrapolated from the curve in Fig. 2 seems to be appropriately low as compared with the one reported for nearly stoichiometric N b 3 A l [8]. As mentioned after, T~ also remained at a lower value. The multilayers produced by mechanical working would not fully react with N-baAls even if the average niobium layer thickness was reduced to the critical thickness and the sporadic agglomerations of niobium layers probably produce an excessively thick layer. Figure 3 shows a typical result for the overall current density vs. applied field for the CT specimen with an average niobium layer thickness of 50 nm and heat treated at 1273 K for 120 s followed by 1023 K for 4 days. For comparison, the results for three kinds of Nb/Al composite wires fabricated by the P/M process [1], niobium tube method [2] and CCE method [3] are included in this figure. The present curve indicates higher magnetic field performance by about 1 T than the others, giving Jc= 1 x 108 A m--' at 19.6 T and Be2*= 24.1 T at 4.2 K. However, the transition temperature Tc remained at 17 K as the value

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34 reached by the CCE-processed wires. No increase in T~ means that J~ of the specimen is enhanced only through increased formation of the AI5 phase and/or slightly increased B~2. The P/M-processed wires are also in the same situation [13], and T~ above 17 K has never been reported with regard to Nb3AI formed by the solid state reaction at low temperatures. According to the relation between the transition temperature and aluminium content in the A15 phase by Jorda et aL [14] and Moehlecke et al. [15], a Tc value of 17 K corresponds to about 22 at.% AI which is the maximum solubility at the temperatures below 1300 K on the equilibrium phase diagram. This suggests that the A15 phase containing more than 22 at.% A1 may not be formed by the solid state reaction, even when the niobium layer thickness is sufficiently small. Bormann et al. [9] reported that the T~ of the A15 phase formed from the Nb/Al-multilayered structure reached only 16 K even for samples which had reacted perfectly to form Nb3AI. If this is true, it is likely that the superconducting properties of the metastable Nb3A1, created by the interdiffusion of Nb/AI multilayers, may differ essentially from those of bulk Nb3AI. In either way, a substantial improvement in Jc can hardly be expected to be realized without further increasing T~. Figure 4 shows the variation in J~ at 14.4 T and 4.2 K as a function of the average niobium layer thickness for the samples heat treated at

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1373 K for 90 s followed by 1003 K for 4 days. In contrast to Fig. 2, the increase in reaction temperature to 1373 K not only lowers the J~ values as a whole, but also gives a Jc peak at an average niobium layer thickness of about 70 nm. The thickness giving the Jc peak happens to coincide with one at which the workability of the Nb/AI composite wire begins to deteriorate. Although the reproducibility of the results shown in Figs. 2 and 4 was checked, almost the same data were obtained and the Jc peak at the average niobium layer thickness of about 70 nm was never seen for the sample heat treated at 1273 K. Therefore a drop in Jc at a niobium layer thickness of less than 70 nm is believed to have no connection with the degradation of workability. A similar phenomenon has been reported for the Nb3AI wire by the niobium tube method [2] and the other method [16]. A detailed study of the degradation of Jc below a critical niobium layer thickness is now in progress. The dependency of Jc on the niobium layer thickness in the present study is compared with those for the wires fabricated by the P/M process and the CCE method as shown in Fig. 5. J~ values for the P/M-processed wires are higher at every niobium layer thickness although the results are distributed over a wide range, depending on the initial powder size, aluminium content, areal reduction ratio, heat treatment etc. However, with decreasing niobium layer thickness the three curves become closer together and are likely to converge towards the end value. The niobium in the P/M-processed composite is fibre like and surrounded by a thin aluminium layer, while the wire fabricated by the clad method has a multilayered structure. It is doubtless that fibre-like

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35 niobium has the advantage over planar niobium because of the increased production of the A15 phase with a given diffusion distance. The curve for the CCE-processed wire is located in the middle of the results for the P/M-processed and present wires, indicating that using chipped clad foils is preferable to a continuously layered structure as the method adopted in present study or the jelly roll technique. Both CT and CL specimens presented substantially the same dependency of J¢ on the niobium layer thickness, probably because of the initial perfect layer structure. In contrast, the P/M-processed wires give different J¢ values at a given niobium layer thickness, depending on the initial niobium powder size. The average niobium layer thickness at which a J¢ ~ 2 x 108 A m - 2 at 14 T is obtained becomes thicker with decreasing the starting powder size as indicated by the open circles in Fig. 6. It is well known that niobium filaments change gradually from a rod-like morphology to a ribbon-like morphology with increasing deformation. With a larger initial powder size, the resulting structure may need a thinner layer thickness to guarantee the same extent of A15 formation. As indicated by the filled circles, the clad method requires a niobium layer thickness less than that required by the P/M process. However, it should be noted that the amount of deformation to obtain a given niobium layer thickness can be reduced by using thinner clad foils.

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A Jc level of about 1.5 × 108 A m - 2 at 18 T has been realized by three different techniques; the powder metallurgy process [1], the niobium tube method [2] and the CCE method [3]. In the jelly roll process, only J~ values from 6.5 to 11 T were reported by Bruzzese et al. [17]. Their data were slightly higher in this range than those for the CCE-processed wire. If measurements were performed up to sufficiently high magnetic fields, a Jc value greater than 108 A m-2 might be obtained. Since the above-mentioned methods bring about a different morphology of the Nb/AI composite, the Jc dependency on the niobium layer thickness cannot be compared simply. Nevertheless, a J~ at this level is also likely to be attained for thicker niobium layers with a smaller starting size of niobium, as can be seen in Fig. 6. The above-mentioned results show clearly that using niobium with a starting size as small as possible is favourable for attaining high J~ properties with a small amount of deformation. However, areal reductions around 10 3 w e r e the working limits when niobium powder with a starting size of about 40/~m was used [18]. Also, in the jelly roll process [17], the composite wire was impossible to draw to areal reductions of more than 2 x 10 4, when a niobium foil, 20 g m thick, containing oxygen at about 440 ppm was used. It is difficult to achieve a sufficient amount of deformation to realize excellent J¢ properties unless fine niobium powder or a thin foil with a low oxygen content are available. In the P/M process or the jelly roll method, the starting size of niobium must be determined by a compromise with workability. However, the Nb/AI composite, fabricated by the niobium tube method and the CCE method, shows good workability and high J~ properties are sufficiently attainable before the working limit is reached.

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The critical current density for the Nb3AI wire formed by the solid state reaction is strongly dependent on the layer thickness of niobium and aluminium. An attempt was made to fabricate the Nb/Al composite wire with a niobium layer as thin as possible by the method using aluminium clad niobium foils as a starting material. Consequently, a minimum average thickness of 43 nm could be reached. Even at this average layer thickness, Jc gave no indication of saturation. However, T~ remained at 17 K. It is more prob-

36 able that the increase in Jc for the wires with a niobium layer thickness less than the critical value at which Tc is saturated is caused by the increased formation of the A 1 5 phase a n d / o r slightly enhanced Bc2. However, since Bc2 increases essentially in p r o p o r t i o n to To, a substantial i m p r o v e m e n t in the Jc properties at high magnetic fields can hardly be expected without enhancement of To. T h e search for an effective way to i m p r o v e T~ should be made. Jc at a given niobium layer thickness was always higher for P/M-processed wires than for the present clad-processed wires, although the difference between them tended to be smaller with decreasing niobium layer thickness. In the P/M-processed wire, fibre-like niobium is surrounded by a thin aluminium layer. Such a structure is possibly preferable to the N b / A I multilayered structure, because of increased formation of A 1 5 phase with a given diffusion distance. T h e effect of initial thickness of niobium on J~ properties was also studied. It was confirmed that use of thinner niobium as a starting material yielded high J~ at a thicker niobium layer.

Acknowledgments We are indebted to H. Tokuno, S. Watanabe and K. O b a r a for preparing clad foils and to the m e m b e r s of the High Field L a b o r a t o r y for Superconducting Materials, T o h o k u University, for giving us the chance to m e a s u r e the superconducting properties. This w o r k was s u p p o r t e d by a G r a n t in Aid for Special Research for Fusion f r o m the Ministry of Education, Science and Culture, Japan.

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