Influence of nanometer-scale copper filaments in niobium on mechanical and superconducting properties

NANoSTRUCTURED MATERIALS VOL. 2, PP. 73-80, 1993 COPYRIGHT©1993 PERGAMONPRESSLTD. ALL RIGHTSRESERVED.

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OF NANOMETER-SCALE COPPER FILAMENTS IN N I O B I U M ON M E C H A N I C A L AND SUPERCONDUCTING PROPERTIES

R. Zhou, S. Hong, B.H. Kear* and P. J. Lee** Oxford Instruments Inc. Superconducting Technology 600 Milik Street, Carteret, NJ 07008 * Department of Mechanics and Materials Science, Rutgers University, New Brunswick, NJ 08903 ** Applied Superconductivity Center, University of Wisconsin-Madison, Madison, WI 53706-1687

(Accepted January 1993) Abstract----Nanometer-scale copper filaments are introduced into niobium as artificial flux pinning centers (APC's) by multiple extrusion. A ribbon-like structure is observed in both niobium and copper at the final wire size. The composite experiences extensive work hardening during mechanical reduction. The pinningforce of niobium withAPC's is significantly enhanced relative to that of coM worked bulk niobium. INTRODUCTION In an applied magnetic field, a type II superconductor with high Ginzburg-Landau constant ~cis penetrated by quantized flux vortices, each carrying a quantum of flux ~o=2X10-15web. In the absence of any current flow, the flux will try to form into a triangular lattice with lattice spacing ao= 1.07X(~o/B)l/2, where B is the magnetic induction inside the superconductor. The passage of an electric current will set up a gradient in flux and impose on the flux line lattice a Lorentz force FL=JxB per unit volume. If the flux moves, an e.m.f, is generated and the current flow is no longer loss less. In order for the superconductor to have a high current density, the flux lattice must be pinned in place. This can be achieved by introducing microstructural defects, such as precipitates, grain boundaries and dislocations, which give rise to a pinning force. Critical current density is defined when the Lorentz force is just balanced by the pinning force, where JcxB=-Fp. Consequently, the stronger the pinning force the higher the critical current density. Theoretically, maximum pinning can be achieved at a specific magnetic field when the size of pinning centers is comparable to the coherence length of the superconductor, and their separation matches the lattice spacing. The coherence lengths of type II superconductors are on a nanometer-scale, and so are the lattice spacings at high magnetic fields. Therefore it is essential that the pinning centers be nanostructural defects. 73

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Figure la. Transverse cross-section of an APC conductor at lOOx.

Figure lb. An enlarged view of an effective filament after fourth extrusion.

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Figure 2. TEM photograph of effective filament at 1.83 mm wire diameter. Improvements in superconducting properties due to the presence of artificial flux pinning centers (APC's) in type II superconductors have been reported, (1-3). In an APC composite, pinning centers can be introduced in a controlled manner to obtain the desired high current density. Because of the complexity of many microstructures in commercially available superconductors, such as NbTi and Nb3Sn, it is very difficult to understand and study the pinning mechanism. Therefore, it is desirable to select a "model" system, where the microstructural defects and the superconducting properties are well defined. To study the pinning mechanism, and to establish the processing parameters for an APC composite, we selected the Nb-Cu system in which a nanometer-scale dispersion of copper filaments had been introduced by thermomechanical processing. COMPOSITE PROCESS Nanometer-scale artificial flux pinning centers in niobium, consisting of ultrafine copper ribbons, were formed by multiple extrusion. In the first step, a niobium ingot was inserted into a copper can, which was 12% of the total volume. After evacuation and electron beam welding, the

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Wire Diameter (mm) Figure 3. Microhardness of APC samples at different wire diameters. billet was extruded into a rod at 550°C. The rod was then processed to a certain size, deformed into a hexagonal shape and cut into uniform lengths. 475 of the hex rods were stacked into another copper can and processed by a second extrusion. After the second extrusion most of the copper from the can was removed by bar turning and chemical etching. Again the exlruded rod was fabricated into hexagonal rods, 187 of which were bundled and placed in a copper can for a third extrusion. The triple extruded material was then used as effective filaments for the fourth exlrusion. The cross section of the final conductor is shown in Figure la. It has 228 effective filaments each containing 88,825 sub-f'llaments. An enlarged view of an effective filament is shown in Figure lb. After the fourth extrusion, the composite was processed to different final wire sizes by conventional swaging and drawing at room temperature. The calculated values of niobium and copper APC's at different wire diameters are listed in Table 1. However, during mechanical processing each component may not be reduced evenly, so that the listed values in Table 1 are at best approximate.

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TABLE 1 Calculated APC Width and its Separation at Different Wire Diameters SamplelD WireSize (mm) #1 #2 #3 #4

1.83 1.02 0.65 0.41

SizeofCu SizeofNb (nm) (nm) 13.11 7.34 4.55 2.90

Area Reduction (%)

167.52 93.76 58.17 36.99

99.08 99.71 99.88 99.96

DISCUSSION OF RESULTS Structure As shown in Figure lb., the shapes of the copper and Nb filaments are distorted relative to the original hexagonal shapes. A transmission electron micrograph, Figure 2, reveals further distortion of the copper APC's into thin ribbons, when the composite is processed to 1.83 mm in diameter. The size of the copper ribbons is about 14 nm, which is close to the calculated value listed in Table 1. This distortion can be explained by the deformation mode in polycrystalline niobium first proposed by Hosford, (4). Copper has a fcc crystal SllUCture and is able to accommodate axisymmetric flow during the drawing process. However, niobium possesses a bcc crystal structure, which is known to develop a < 110 > fiber texture, where only two of the four < 111 > slip directions are oriented favorably to accommodate extension parallel to the wire axis. Consequently, further deformation produces plane strain rather than axisymmetric flow, resulting in a ribbon like cross section of the niobium filament, as can be seen in Figure 2. Since copper is much softer than niobium, therefore, as the niobium turns into ribbon shape, so does the copper. A similar phenomenon has been reported for a mechanically processed Cu-Nb3Sn superconductor, (5,6). A Cu-Nb ingot with 10-20 wt% Nb was cast and mechanically reduced to final wire size. The asdrawn niobium filaments changed from the original dendritic morphology to flat filaments, with an irregular rectangular cross section.

Microhardness The strength of a filamentary metal matrix composite can be predicted by the "rule of mixtures," which indicates that the strength of the composite should vary linearly with the volume fraction of reinforcing filaments. However, when the filaments are sufficiently small, the strength of the composite is found to be substantially higher than that predicted from the rule of mixtures, (7-9). The strength of in-situ formed Cu-Nb with 10-20 wt% Nb at an area reduction of 99.999% is reported to have a strength as high as that of copper whiskers, (10, 11). Extensive work hardening was observed in the mechanically processed APC conductor. Microhardness tests were performed on the effective filaments; results are plotted in Fig.3. At 14.7mm diameter (40% reduction in area after extrusion), the hardness of the filaments is about the same as that of cold worked niobium. When the wire is drawn down below 5mm in diameter, the microhardness increases sharply to about 310 kg/mm2.

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Area Reduction (1 - A f / A i ) Figure 4. Microhardness of APC samples processed after being annealed at 600°C for 2 hours compared with that of cold worked Nb samples. In order to pursue further mechanical deformation, the wire at 1.83 mm (99.08% area reduction) was subjected to an annealing heat treatment at 600°C for 2 hours. The wire was then further processed to 0.4 mm in diameter (99.96% area reduction) at room temperature. Figure 4 shows the microhardness of samples after annealing and further processing. After annealing the microhardness drops from 310 kg/mm2 to 235 kg/mm2, which is still higher than that of cold worked niobium. Then the microhardness increases steadily as the wire diameter decreases, up to a value of 310kg/mm 2 at 0.55 mm diameter (99.93% area reduction).

Superconducting Properties The pinning force, F=JcxB, is plotted as a function of reduced magnetic field, b=B/Bc2, in Figure 5 for various diameter APC samples, and compared with cold worked niobium. The maximum pinning force shifts from 0.4 reduced field for pure niobium to 0.5 reduced field for the APC samples, which is similar to well optimized NbTi, (12). This similarity in pinning

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Reduced Magnetic Field b (B/Be2) Figure 5. Pinning force F vs. reduced magnetic field b(B/Bc2) for APC samples and cold worked Nb sample. mechanism is atlributed to the fact that the shape and distribution of copper APC is very close to cc-Ti precipitates, which turn into thin ribbons and clusters during processing in the NbTi system, (13,14). It is also found that the pinning force increases with decreasing wire diameter,due to the increase in density of flux pinning sites and the improved upper critical field Be2. The latter increased steadily to about 1T as the wire diameter decreased. On the other hand, Be2 of the cold worked niobium saturated at about 0.5T. ACKNOWLEDGEMENTS This work was supported by the Office of Naval Research under contract #N00014-91-J1828. REFERENCES 1.

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