O. Miura¹, D. Ito¹, P. J. Lee², and D. C. Larbalestier²

Transcription

1 Paper M-R-02 Presented at CEC-ICMC 2003, Anchorage, Alaska, September 2003, accepted for publication in Advances in Cryogenic Engineering. FLUX PINNING PROPERTIES IN Nb-Ti COMPOSITES HAVING Nb AND Ti MIXED ARTIFICIAL PINS O. Miura¹, D. Ito¹, P. J. Lee², and D. C. Larbalestier² Department of Electrical Engineering Tokyo Metropolitan University -Minami-Osawa Hachioji, Tokyo , JAPAN 2 Applied Superconductivity Center, University of Wisconsin-Madison 500 Engineering Drive, Madison WI , USA ABSTRACT Flux pinning and superconducting properties for multifilamentary Nb-Ti composites having different kinds of artificial pins, Nb and Ti mixed pins were studied by magnetization measurements. It is known that Nb pins act as repulsive ones, making them well suited for low-field applications, while Ti pins act as attractive ones having maximum flux pinning strength at 5 T for optimized Nb-Ti wires. However, for the mixed pins, the flux pinning force density F p was less than half of that for pure Nb pins. Moreover, the saturation tendency of F p was observed with further decreasing the pin size. This suggests that an offset of elementary pinning force takes place between repulsive Nb and attractive Ti pins. We discuss these flux pinning properties concerned with other superconducting properties and nanostructure of pins. INTRODUCTION We have been studying magnetic flux pinning mechanisms in superconductors using artificial pinning center (APC) technology applying to traditional Nb-Ti multifilamentary wires [,2]. One of the important results is that heavily folded nanometer-thick Nb artificial pins act as mighty repulsive pinning centers due to the strong proximity effect, and they have remarkably enhanced critical current densities in relatively low magnetic fields under 5 T [3,4]. On the other hand, it is known that normal conductive Ti pins act as attractive ones having maximum flux pinning strength at 5 T and making good high-field performance for optimized Nb-Ti wires [5]. Therefore, if superposition principle holds on such flux pinning systems, Nb and Ti mixed artificial pins can be utilized for developing new APC composites having excellent critical current densities in the whole magnetic field range. Moreover, if the mixing effect, i.e. the apparent alloying effect between pins and Nb

2 Ti matrices occurs [6], it is expected that the degradation of superconductivity, such as T c and H c2 due to the proximity effect is well depressed. By the way, a pronounced peak effect often occurs in RE-23 superconductors such as Nd-23 bulk superconductors, and it is believed that some superconducting regions with lower T c, such as substituted or oxygen deficient regions, cause the peak effect [7]. Recently in Nd-23 superconductors, Mochida et al found that peak effect at medium fields decreased on addition of Nd-422 normal conductive particles [8]. Matsushita discusses that this may be caused by the interference between repulsive pinning by the lower T c regions and attractive pinning by normal particles [9]. It is considered that either case of superposition or interference may take place, depending on the various flux pinning parameters, such as flux line lattice properties and microstructure of pins. In this study, to solve these interesting subjects, we have fabricated new APC Nb-Ti composites with Nb and Ti mixed pins having the same volume fraction and shape of pins at the design stage, and investigated their superconducting properties and flux pinning characteristics. EXPERIMENTAL Nb-Ti multifilamentary composites having Nb and Ti mixed pins were fabricated by a conventional double stacking method. Ten Ti pin and nine Nb pin rods with four nines were randomly introduced into a Nb-50wt.%Ti filament with the triangular arrangement and the total volume fraction of 7 %. To avoid interfilamentary-proximity coupling, Cu-Ni alloy was used as the matrix material. The extrusion temperature was carefully controlled. The final wires were drawn down to a proper diameter where the pin size was comparable to the flux line lattice at high magnetic fields. Pure Nb APC wires with the same design were also prepared for comparison. The specification of the wire specimens is shown in TABLE. It is noticed that the pin diameter for specimens means the nominal values. FIGURE a and b show cross sections for Nb-Ti filaments with APC s imaged with a high-resolution field emission scanning electron microscope (FESEM). It is observed that TABLE. Specification of Nb and Nb/Ti mixed APC composites. Nb/Ti mixed APC Nb APC Wire dia. D (mm) Filament dia. d f (µm) Pin dia. d P (nm) Pin spacing d ps (nm)

3 FIGURE. a) A high resolution back-scattered electron image of a cross section for Nb-Ti filament of 20.4 µm with mixed Nb (white) and Ti (black) artificial pins. The nominal diameter of pins is.9 µm. b) A secondary electron image for Nb-Ti filaments of 0.9 µm with the nominal pin diameter of 84 nm. It should be noted that both pins are much folded with thin ribbon sheets less than ten nanometers in thickness produced by the heavy wire drawing. both Nb and Ti round pins are gradually deformed to ribbon shapes, as in the case of α-ti, by heavy cold drawing. It should be noted that they are much folded, producing nanometer thick ribbons in the final wire specimens. Magnetizations were measured by a SQUID magnetometer in magnetic fields up to 7 T at 4.2 K. The magnetic field dependence of flux pinning force density, F p, was evaluated from the magnetization data using the critical state model. T c was defined as the temperature at which the magnetization at mt changes from a diamagnetic to a paramagnetic state. The magnetic field dependence of critical temperature T c (B) and irreversibility temperature T i (B) was evaluated from measuring the temperature dependence of magnetization under zero-field-cooled and field-cooled conditions. Then these two data were converted to the temperature dependence of upper critical field B c2 (T) and irreversibility field B i (T), respectively. RESULTS AND DISCUSSION FIGURE 2 shows F p -B properties at 4.2 K for Nb APC specimens with the different nominal pin size d p. F p value increases quickly at the whole field range up to 7 T and the position of peak F p shifts to high field side with reducing d p. The maximum F p reaches 29 GN/m³ at 2 T for the specimen with d p of 37.7 nm, and no saturation of F p is observed until this range of d p. For the specimen with d p of 46.4 nm, the maximum F p value is about three times larger than that for a no-apc specimen with the same filament diameter. FIGURE 3 shows F p -B properties at 4.2 K for Nb and Ti mixed APC specimens. As d p is reduced, F p values increase gradually at the whole magnetic fields and the peak F p slightly shifts to higher field, similar to Nb APC specimens. However, the strength of F p remains less than half of that for Nb APC, and the saturation of F p is observed from the specimen with d p of 47.5 nm. In the case that the size of pins is larger than the flux line spacing and the linear summation model is applicable due to the strong pinning limit, F p is inversely proportional to the size of pins. The d p dependence of F p at 2 T and 4 T for both APC specimens is indicated in FIGURE 4. F p values for Nb APC specimens are almost inversely proportional 3

4 F [N/m 3 ] P d P = 37.7 [nm] d P = 46.4 [nm] d P =68.5 [nm] d P = 84.3 [nm] d P = 04 [nm] B[T] FIGURE 2. F p -B properties at 4.2 K for Nb APC specimens with the different nominal pin size. to d p, according to the theory. However, those for the mixed APC are unconformable to the linear summation model with further reducing d p, and the saturation tendency of F p is pronounced under d p of 56 nm. This suggests that the interference between repulsive pinning by Nb pins and attractive pinning by Ti pins takes place, as both pins are heavily deformed into ribbon shape and interaction between both pins and fluxoids stands out by drawing. Since the longitudinal correlation length l 44 evaluated from the F p values for the mixed APC specimen with d p of 47.5 nm is larger than the filament diameter of 0.53 µm, each fluxoid overlaps both Nb and Ti pins. When the fluxoid moves across the pins, the difference of the condensation energy of the fluxoid is much lower than that in the case of one kind of pins. This is thought to be the main reason why the saturation of F p occurs with d P =38.0[nm] d P =47.5[nm] d P =7.5[nm] d P =84.0[nm] d P =26[nm] F p [N/m³] B[T] FIGURE 3. F p -B properties at 4.2 K for Nb and Ti mixed APC specimens with the different nominal pin size.

5 Pinning Force Density F p [GN/m³] 00 0 Nb <2T> Nb <4T> Ti/Nb <2T> Ti/Nb <4T> Pin Diameter d p [nm] FIGURE 4. The nominal pin size dependence of F p at 2 T and 4 T for both APC specimens. The dotted line shows that F p is inversely proportional to the size of pins. reducing pin size. We begin more elaborate studies about this subject by a computer simulation. Another reason for the saturation of F p for the mixed pins is the degradation of superconductivity by the proximity effect. FIGURE 5 shows d p dependence of T c for both APC specimens. For Nb APC specimens T c gradually decreases until d p of 50 nm and then becomes the steady value of 9.5 K. This suggests that the saturation of the proximity, i.e. homogeneity of order parameters between Nb-Ti and Nb pins occurs in this level. On the other hand, for the mixed APC, degradation of T c is getting pronounced with reducing d p. 9.4 Critical Temperature T c [K] Nb Ti/Nb Pin Diameter d p [nm] FIGURE 5. The nominal pin size dependence of T c for both APC specimens. 5

6 B c2 (T), B i (T) B c2 (Nb/Ti) B i (Nb/Ti) B c2 (Nb) B i (Nb/Ti) T(K) FIGURE 6. Temperature dependence of B c2 and B i for both APC specimens with d p of 56 nm. For the specimen with d p of 38 nm, T c drops to 8.6 K due to the proximity effect to Ti pins. This means that the material mixing effect does not occur between Nb and Ti pins, unfortunately. Assuming that T c is proportional to thermodynamic critical field H c, the difference of the condensation energy density between Nb APC and mixed APC specimens with d p of 38 nm is : Hence it is considered that the proximity effect does not so much affect the deterioration of F p for the mixed APC specimens. FIGURE 6 shows the temperature dependence of B c2 and B i for both APC specimens with d p of 56 nm. The degradation of B c2 for the mixed APC specimen comes from the proximity effect to Ti pins, similar to the case of T c. The difference between B c2 and B i is observed for both specimens, especially for the mixed pin specimen. This is caused by the flux creep, related to the filament size and the flux pinning strength [0]. The large difference between B c2 and B i for the mixed pins is thought to be caused by the depression of F p due to the interference between Nb and Ti pins. CONCLUSION In order to study flux pinning properties for the different kinds of pins, repulsive and attractive mixed pinning system, we have fabricated new APC Nb-Ti composites having Nb and Ti mixed pins with the same volume fraction and shape each other, and investigated their superconducting properties and flux pinning characteristics. For the mixed pins, the flux pinning force density F p remains less than half of that for Nb pins and the saturation of F p was observed with decreasing the pin size. Prospective restoration of T c and H c2 due to the mixing effect was not observed. This suggests that an offset of elementary pinning force f p takes place between repulsive Nb and attractive Ti pins due to increasing interaction volume between both pins and fluxoids. 6

7 ACKNOWLEDGEMENTS We would like to thank Dr. Yun Zhu for technical assistance. The work of Lee and Larbalestier was supported by U.S. Department of Energy, Department of High Energy Physics under Grant DE-FG02-9ER REFERENCES. Miura, O., Zhu, Y, Okubo, T., Ito, D., Endo, S., IEEE Trans. Applied Superconductivity, 9(2), pp (999). 2. Zhu, Y., Miura, O., Hayakawa, K., Ito, D., IEEE Trans. Applied Superconductivity, (), pp (200). 3. Miura, O., Inoue, I., Suzuki, T., Matsumoto, K., Tanaka, Y., Yamafuji, K., Funaki, K., Iwakuma, M. and Matsushita, T., Cryogenics 35, pp (995). 4. Matsushita, T., Iwakuma, M., Funaki, K., Yamafuji, K., Matsumoto, K., Miura, O., and Tanaka, Y., Adv. Cryog. Eng. (Materials), 42A, edited by L. T. Summers, Plenum, New York, 996, pp Lee, P. J., McKinnell, J. C. and Larbalestier, D.C., Adv. Cryog. Eng. (Materials), 36A, edited by R. P. Reed and F. R. Fickett, Plenum, New York, 990, pp Cooley, L. D. and Motowidlo, L. R., Supercond. Sci. Technol., 2 R35-R5 (999). 7. Daeumling, M., Seuntjens J. M. and Larbalestier, D. C., Nature, 346(6282), pp (990). 8. Mochida, T., Chikumoto, N., Higuchi, T. and Murakami, M., Advances in Superconductivity X Proc. Of the 0 th International Symposium, edited by K. Osamura and I Hirabayashi, Springer Verlag Tokyo, Japan 998, pp Matsushita, T., Supercond. Sci. Technol., 3, pp (2000). 0. Zhu, Y., Miura, O., and Ito, D., IEEE Trans. Applied Superconductivity, 2(), pp (2002). 7