Lawrence Berkeley National Laboratory Berkeley, CA, 94720, USA

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1 Paper M1-D-09 Presented at CEC-ICMC 2003, Anchorage, Alaska, September 2003, accepted for publication in Advances in Cryogenic Engineering. ATTEMPTS TO REDUCE A.C. LOSSES IN HIGH CURRENT DENSITY INTERNAL-TIN NB 3 SN E. Gregory 1, B. A. Zeitlin 1, M. Tomsic 2, T. Pyon 3, M. D. Sumption 4, E. W. Collings 4, E. Barzi 5, D. R. Dietderich 6, R. M. Scanlan 6, A. A. Polyanskii 7, and P. J. Lee 7 1 Supergenics LLC, Sarasota FL, 34242, USA 2 HyperTech Research Inc. Troy, OH, 45373, USA 3 Outokumpu Advanced Superconductors Waterbury, CT, 06704, USA 4 LASM, Ohio State University, Columbus OH, 43210, USA 5 Fermi National Accelerator Laboratory Batavia, IL, 60510, USA 6 Lawrence Berkeley National Laboratory Berkeley, CA, 94720, USA 7 University of Wisconsin-Madison, Applied Superconductivity Center Madison, WI, 53706, USA ABSTRACT High current density designs of internal-tin Nb 3 Sn employing a single barrier to separate the superconductor from the stabilizer have proved to be unstable at the desired sizes in some short sample tests. Bridging between subelements occurs and the effective filament diameter (d eff ) is considerably greater than the value of < 40 µm specified by the High Energy Physics (HEP) community. In this investigation we have explored the extent to which the other properties are changed when the losses are decreased by using multiple barriers to ensure the separation of the subelements. The effect of internal fins, of the type used in the Mono Element Internal Tin (MEIT) process [1], on the values for critical current density (J c ) and d eff of multiple barrier restacks, is also reported, as are some problems encountered by the addition of Ti and possibly Cu to the Sn core. 1

2 INTRODUCTION Conductors exhibiting non-cu J c values at 12 T of 3000 A/mm 2 have been reported [2] and efforts have concentrated recently on reducing hysteresis losses and costs and increasing piece length and reliability. One approach has been to adopt the Mono Element Internal Tin (MEIT) process [1]. This eliminates restacking of the subelements by drawing them down to either 0.25 mm Ø or 0.15 mm Ø and then cabling them with some Cu strands back up to a size suitable for Rutherford cabling. An alternative approach is described in this paper to avoid possible problems with handling fine wires and doubly cabled materials. Supergenics LLC, working with HyperTech Research Inc., (HRI), and Outokumpu Advanced Superconductors, (OKAS), has been exploring the potential of a multiple barrier approach. Most of the earlier designs of internal-tin Nb 3 Sn employed a single barrier to separate the stabilizing Cu from the superconductor. The advantage of this approach was that the amount of Cu could be varied over a wide range and its RRR maintained at a high level as the Ta barrier was thick and relatively resistant to fracture. The critical current could be kept high by reducing the amount of the stabilizing Cu. The principal disadvantages were a tendency to considerable flux jumping and high a.c. losses and values of d eff, as bridging could occur between the various subelements. In order to control these losses and approach the HEP d eff specification of < 40 µm, designs with multiple barriers, one around each subelement, have been explored. Although this approach makes it difficult to reduce the amount of stabilizing Cu below 50 % and therefore limits strand I c, it makes it easier to test and ensures that the ac losses are primarily controlled by the size of the subelement. Better stability and less flux jumping can be expected than was the case with the single barrier designs. The multiple barrier approach also allows the use of the more ductile Nb or Nb7.5wt.%Ta barrier in place of the Ta used previously suggesting improved piece lengths. Since a percentage of the barrier can be converted to Nb 3 Sn without adversely affecting the losses, high J c values can be expected, provided that sufficient tin is available. Subelements developed for the MEIT process, BAZ 6 and BAZ 7, described in a previous publication [3], were utilized in this work, as initially they were the only materials readily available. They both had a Nb barrier extruded as part of the subelement billet and BAZ 6 had two internal fins of NbTa60wt.% designed to divide the reacted and bridged filaments into two groups to reduce electrical losses. Both also had cores with Ti in the Sn, a practice used by IGC-AS in almost all internal-tin designs since the ITER-CSMC program, to improve the high field J c values and lower low field losses [4]. As the subelements were designed for the MEIT process they had a relatively large amount of Cu outside the barrier. This was not reduced before the restacks were made in this work. Possibly as a result, ductility problems were encountered when attempts were made to fabricate restacks containing larger numbers of subelements than 37 and therefore the property comparisons had to be made on 19 subelement restacks. These are reported and indications given on the probable reasons for the wire breakage and how to overcome them. The performance and effectiveness of the NbTa60wt.% fins were also explored and some of the preliminary results reported. FABRICATION AND RESULTS Fabrication Restacks of 18 subelements with a single Cu core were made at HRI and OKAS. These incorporated 3 different sizes of subelement in order to fill a round tube with as 2

3 (a) (b) FIGURE 1. (a) 36 subelement BAZ 7 at 0.84 mm Ø and (b) Small subelement at 2 o clock in FIG 1a. dense a packing of round subelements as possible. The restacks were drawn down to 0.84 mm, twisted and overdrawn to 0.8 mm. Both these materials were drawn to 0.5 mm Ø after twisting at 0.56 mm and overdrawing. These operations were carried out to give an indication of how the a.c. losses change with smaller subelements both split, (BAZ 6) and unsplit, (BAZ 7). In addition several attempts were made, at OKAS and HRI, to form 36 and 54 subelement restacks with both BAZ 6 and BAZ 7 material. While the BAZ 6 material, in the form of a 36 restack, was successfully drawn down to 0.8 mm, efforts to draw it to 0.5 mm Ø were unsuccessful. The BAZ 7 material was only reduced to 0.8 mm Ø by using cassette dies for the last few passes. Samples of the 18 and 37 restacks were sent to both Fermi National Accelerator Laboratory (FNAL) and the Lawrence Berkeley National Laboratory (LBNL) for J c testing and to Ohio State University (OSU) for magnetization testing. The results of these tests are reported and discussed in the section on Testing below. The wire breakage problem In the past, while ductility decreased when the number of restack subelements increased the severity of the problem was not as great with the single barrier materials as it has been with the multiple barrier restacks of BAZ 6 and BAZ 7. In an effort to determine the reason for this, an examination was made of the 36 restack of BAZ 7 that could only be reduced to 0.8 mm with the use of cassette dies. FIGURE 1a shows a cross section of this material at 0.84 mm Ø. While the wire did not break it is obvious that the smaller subelements are severely distorted and could be allowing Sn to leak out into the Cu stabilizer. Much of the distortion probably resulted from the large amount of Cu left on the subelement. This, in the restacks, reduced the extent to which neighboring subelements could support one another. FIGURE 1b shows a higher magnification backscatter SEM image of the small subelement shown at 2 o clock in FIGURE 1a. Two apparently hard phases, one dark (1) and one light (2) in the Sn core (3) appear to be causing severe distortions of the filament array. EDS analysis of the dark phase carried out at SMARTech LLC and by Lee at the University of Wisconsin-Madison (UW) showed it to be Ti 6 Sn 5 with a Sn value of 69 wt.%. The lighter phase is obviously lower in Ti. It appears to have a Sn value between 82 and 84 wt.% and is close to a composition of TiSn 2. No such low Ti phase appears to exist in the SnTi phase diagram [5]. Whatever this phase is, it greatly 3

4 FIGURE subelement strand at 0.17 mm Ø showing areas of η-phase in small subelements about to cause fracture. increases the total area of apparently hard phase. This is around 30 % in this particular subelement when the wire is 0.84 mm Ø. At this wire diameter the average subelement size is ~140 µm. To obtain more reliable 37 and 61 subelement restacks it was obviously desirable to remove Ti from the Sn cores. To confirm this we acquired a small amount of old Oxford/Wah Chang jellyroll subelement from Accelerator Technology Corp. (ATC). This had no Ti in the core but it had some in the Nb and there appears to be some Cu in the Sn core. The amount of Cu around each subelement in this jellyroll material was smaller than that in BAZ 6 and BAZ 7. This jellyroll subelement, which had a relatively thick barrier, was made into 37 and 61 restacks at HRI and the 61-restack material drew down to a wire diameter of mm without wire breakage, using the normal drawing dies. FIGURE 2 shows two of the most seriously distorted subelements at this wire size where the average subelement was around 17 µm in diameter. This would be expected to give a d eff of around 20 µm [6]. Once again the smaller subelements are beginning to show up as problem areas and the η-phase particles of Cu 6 Sn 5, which are hard relative to the Sn [7], appear to be contributing to the breakage of the filament array. This suggests that, for the best results it may be desirable to remove the Cu as well as the Ti from the Sn if we are to achieve d 0 < 40 µm reliably. While the ductility of the 61 subelement restack without Ti in the cores was adequate to enable the d eff of < 40 µm to be met, the wire had to be drawn to around 0.3 mm Ø. It is difficult to handle wires as small as a few fractions of a millimeter in diameter and the preferred diameter for Rutherford cabling is 0.8 mm. Three methods of achieving the desired properties at this diameter are being explored: 1. Restacking > 200 subelements. 2. Double restacking. 3. Making subelements with internal fins, which divide the bridged areas. In this work we have explored the last of the three approaches by examining the results obtained on BAZ 6 and BAZ 7 material. The heat treatments used are shown in TABLE 1. It will be noted that the strand diameters reported are after heat treatment and, in one case; the material was apparently not overdrawn although it was twisted. This BAZ 7 material was 0.85 mm in diameter and the BAZ 6 was mm. 4

5 TABLE 1. Heat Treatments LBNL FNAL 16 C/h to 210 C & hold for 100 h 25 C/h to 210 C & hold for 48 h 10 C/h to 340 C & hold for 48 h 50 C/h to 340 C & hold for 48 h 25 C/h to 650 C & hold for 96 h (0.8 mm), 48 h and 100 h (0.5 mm) 75 C/h to 700 C & hold for 20 h TABLE 2. Test Results J c(12tnon Cu) (A/mm 2 ) d eff. (µm) d o (µm) Design Dia. Condition* M 1/2 H 1/4 RRR mm (10 3 ka/m) 1/2 T 1/4 M(3π/4) (10 6 A/m) LBNL Heat Treatment, 18 subelements: BAZ G UR BAZ SB R BAZ G UR BAZ SB R Failed 90 FNAL Heat Treatment, 18 subelements: BAZ SB UR BAZ G R BAZ G UR BAZ SB R Failed 90 BAZ 6 36subs SB UR Testing *Condition: R = Appears to be completely reacted during the heat treatments. UR = Represents material that is not completely reacted. G = No broken subelements were detected by SEM #SB = The number, #, of broken or badly distorted subelements The results of the testing are shown in TABLE 2. The highest J c value was obtained for BAZ 7 at 0.85 mm (2664 A/mm 2 at 12 T in the non-cu) heat treated for 96 h at 650 C at LBNL. This can be compared with 2578 A/mm 2 at 12 T (4.2 K) in the non-cu at IGC-AS [8] single barrier material and 2850 A/mm 2 at 12 T (4.2 K) in the non-cu in similar material in the MEIT process [1,9]. In every case the J c values of the BAZ 6 samples are significantly lower than those of the equivalent BAZ 7 samples. In an effort to determine the reason for this, cross sections of many of the samples were examined in both the light microscope (LM) and in the scanning electron microscope (SEM). All the BAZ 6 samples appeared to be incompletely reacted. The degree of reaction seemed to vary from one side of the subelement to the other. FIGURE 3 clearly shows this and a different compound lines the inner row of filaments on the side where a Sn deficiency appears to exist. EDS showed the unknown compound to contain Nb, Sn, Cu and Ti. A similar compound has been found previously by a number of authors [9-11]. While reductions in M are seen for most of the split, BAZ 6, samples it is not fully clear if this is due to lower J c in the BAZ 6 subelements or lower effective subelement diameters, (splitting). The lower d eff values in some cases argue that it is the latter, although further experiments are needed to confirm this. A more detailed discussion of the data is given in another paper in this meeting [6]. Magneto-optical (MO) characterization was performed by Polyanskii at the UW using a 5 µm thick iron garnet indicator film with in-plane magnetization. The sample was placed on a cooling finger of a continuous flow optical cryostat located on the X-Y stage of a polarized optical microscope in reflective mode. The garnet indicator was placed on the 5

6 FIGURE 3. BAZ 6, mm Ø heat treated at LBNL for 96 h with EDS of compound layer inside filament array. polished surface of superconductors to register the normal components of the magnetic flux. The details of this method have been reported earlier [12]. THE EFFECTIVENESS OF THE FINS In order to determine how the NbTa60wt.% fins were functioning, x-ray mapping of the fin area was carried out, FIGURE 4, for Nb, Ta, Sn and Cu on the 0.5mm Ø BAZ 6 material after the FNAL heat treatment. In the Sn map the black line denoting the presence of the fin becomes wider and clearer as it approaches the Nb barrier. This suggests that the Sn is entering the fins principally in the area closest to the core. The quantitative extent to which this occurs is revealed in the EDS results listed in TABLE 3. Positions 8 to 4 show the Sn to increase slightly at first and then diminish as the Nb barrier is approached. The reason for the lower values for the Sn in the area adjacent to the core compared with the mid points has not been explained but the rest of the analysis confirms the indications of the x-ray maps and suggests again a Sn deficiency in the filament area of the BAZ 6. Analysis of points in the Nb barrier outside the bent NbTa60wt.% fin section confirmed the absence of Sn while positions 9 and 10 in the area unprotected by the fin show that a significant amount of the Nb is penetrated by the Sn. Analysis of the fin itself, positions 1 to 3, shows that in its circumferential area Sn is only detectable in the tip, position 1. This confirms the X-ray maps showing that the fins break the outer ring of Nb 3 Sn. High resolution Field Emission SEM (FESEM) of the microstructure of the fin area shows that, except for a thin sliver of material in the outer regions of the fins away from the Sn source, the material fractured in a brittle manner suggesting conversion to the brittle A15 phase [10]. The Sn had totally penetrated the alloy fin close to the Sn core. UW is also planning to analyze the grains and grain boundaries in this region. The MO examination of the 0.8mm Ø BAZ 6 and BAZ 7 after reaction heat treatment at FNAL showed a significant difference in appearance between the two materials, FIGURES 5a & b. At these very low fields and a relatively high temperature the fins break up the barrier of Nb 3 Sn and allow the magnetic flux to penetrate into the subelement in the case of BAZ 6, whereas the flux is prevented from entering the subelement in BAZ 7 that 6

7 FIGURE 4. SEM electron backscatter image (left) and X-ray (EDS) maps (right) of the fin area. TABLE 3. EDS analysis of the fin area in FIGURE 4. Position Nb (wt.%) Ta(wt.%) Sn(wt.%) 1 Nb Ta Very little 2 Nb Ta None 3 Nb Ta None Nb None None 10 Nb None Sn contains no fins. While this is an encouraging result, some more data proving that the fins are actually reducing d eff at higher fields and lower temperatures would be desirable [1]. SUMMARY It has been shown that the ac losses in multi barrier Nb 3 Sn materials are considerably reduced compared with those of single barrier material. A 12 T J c in the non-cu of 2664 A/mm 2 was obtained in the BAZ 7 restack but the BAZ 6 material appeared to have inadequate Sn and a lower J c. Fabrication problems in the 36 and 61 subelement restacks appeared to be due, at least in part, to Sn/Ti compound in the Sn core. When using old Oxford/TWCA jellyroll material, which contained no Ti in the core, breakage was not experienced until the wire had been reduced to a much smaller diameter. Then one problem at this very small subelement size appeared to be caused by the hard η-phase Cu 6 Sn 5. The TaNb40wt. % fins react with the Sn, particularly in the area close to the Sn core. The superconducting properties of the reacted fins are being investigated to determine their effectiveness. Magneto optical experiments appear to show that the fins are acting as expected at low fields and relatively high temperatures. Their effect on d eff at higher fields and lower temperatures is being explored further. ACKNOWLEDGEMENTS This work was supported by the US Department of Energy under SBIR Grant #DEFG0202ER83541 and additional work was supported at the University of Wisconsin- 7

8 a) b) FIGURE 5. MO Images at 11.6 K and B = 0.04 mt of a) 0.8 mm BAZ 6 showing flux penetration through the fins and b) 0.8mm BAZ 7 (no fins) showing shielded (solid black) cores. Madison Applied Superconductivity Center by the U.S. Dept. of Energy, Division of High Energy Physics (DE-FG02-91ER40643). REFERENCES 1. Zeitlin, B. A., Gregory, E., Pyon, T. and Scanlan, R. M., Results on the use of Internal Barriers to Reduce Magnetization on High Current Density Mono Element Internal Tin Conductors (MEIT), Paper M1-D-08, CEC/ICMC, Anchorage, AK, Sept Parrell, J. A., Field, M. B., Zhang, Y. and Hong, S., Nb 3 Sn Conductor Development for Fusion and Particle Accelerator Applications, Paper M1-M-04, CEC/ICMC, Anchorage, AK, Sept Zeitlin, B. A., Gregory, E., Pyon, T. and Scanlan, R. M., Continued Progress on a Low Cost High Current Density Mono Element Internal Tin Conductor (MEIT) with Integral Barriers, in IEEE Trans. on Appl. Superconductivity 13, No. 2, pp , 2003, 4. Gregory, E., Gulko, E., Pyon, T. and Goodrich, L. F., Improvements in the Properties of Internal-Tin Nb 3 Sn, in Proc. ICEC16/ICMC, edited by T. Haruyama et al., Elsevier Science Publishers, Amsterdam, The Netherlands, 1997, pp ASM Handbook Tenth Edition, Vol. 3, Alloy Phase Diagrams, Page 2 370, Murray. J. L., Sumption, M. D, Lee, E. Peng. X. Wu, X. Collings, E. W Gregory E., "Magnetization and Effective Filament Diameter in High Energy Physics Relevant Rod-In-Tube Type Nb 3 Sn Strands", Paper M3-G-03, CEC/ICMC, Anchorage, AK, Sept 22-26, Chromik, R. R., Vinci, R. P. Allen, S. L. and Notis, M. R., Measuring the Mechanical Properties of Pb- Free Solder and Sn-Based Intermetallics by Nanoindentation, in JOM, 5, No. 6, pp , Gregory, E. and Pyon, T., Internal-Tin Nb 3 Sn Conductor Development for High Energy Physics Applications, in Advances in Cryogenic Engineering 48B, edited by U. B. Balachandran et al., AIP Conference Proceedings, Vol. 614, New York, 2002, pp Zeitlin, B. A., Gregory, E., Pyon, T., and Scanlan, R. M., Update on a Low Cost High Current Density Mono Element Internal Tin Conductor (MEIT) with Integral Barriers and a Method to Lower D eff LTSW, Napa Valley Marriott, November 11-13, Lee, P. J. and Larbalestier, D.C., Advances in Superconducting Strands for Accelerator Magnet Application, to be published in the Proceedings of the 2003 Particle Accelerator Conference, IEEE, Paper TOAB003, PAC 03, Portland, OR, May Suenaga, M., Tin Diffusion Effects in Variable Diameter Internal Sn Conductors, LTSW, Napa Valley Marriott, November 11-13, Polyanskii, A. A., Cai, X. Y., Feldmann, D. M., and Larbalestier, D. C., in Nano-crystalline and Thin Film Magnetic Oxides (NATO Science Series 3. High Technology-Vol. 72), edited by I. Nedkov and M. Ausloos, Kluwer Academic Publishers, 1999, pp