Application problem of flat multifilamentary Nb3Sn Superconductors

Applied Superronductirity Vol. 2, No. 5. pp. 321 336, 1994

Pergamon

Copyright cl 1994 Elsevier Science Ltd Prmted in Great Britain. All rights reserved

0964-1807(94)00024-7

0964-l X07194$7.00 + 0.00

APPLICATION

PROBLEM OF FLAT MULTIFILAMENTARY Nb3Sn SUPERCONDUCTORS

P. KovAC’, L. CESNAK’, H. FIKIS’, G. HILSCHER’ and H. KIRCHMAYR’ 1Institute of Elecrical Engineering, Slovak Academy of Sciences, Dubravska cesta, 84239 Bratislava, Slovakia

and ’ Institute

of Experimental

(Received

Physics, Technical University, Wien, Austria

I2 April 1993; in revised form

Wiedner

Hauptstrasse

8-10, A-1040

9 April 1994)

Abstract--Results are presented concerning the superconducting properties of flat multifilamentary Nb,Sn conductors fabricated by the bronze route. We investigated degradation caused by winding the prereacted conductor on relatively small diameters, the influence of Zr and Ti doping of the Nb filaments and the anisotropic behaviour of critical current for the two principal orientations of the flat conductor with respect to the magnetic field. The performance of flat conductors in high field insert coils wound after the conductor has been heat treated (the R&W technique) is discussed.

1. INTRODUCTION

Superconductors with rectangular cross section are used to wind magnets after the conductor has been heat treated. The strain in the outside filaments can be held below the critical value of =0.5% by winding the high aspect ratio flat conductor on its broad side. The rectangular cross section ensures a better filling and mechanical stiffness of the winding, as well. Winding the conductor after reaction requires, no doubt, special attention during the winding procedure, but it mades it possible to insulate the conductor by only a thin varnish layer and to enhance further the filling factor. On the other hand, the flat conductor has an essential disadvantage: the magnitude of the transport critical current is not a constant but is different for the two principal orientations of the conductor in the transverse magnetic field. The critical current is lower for the conductor in the parallel orientation of the broad side relative to the magnetic field than for the conductor in the perpendicular orientation, and it is also lower than the critical current of an equivalent cylindrical conductor. The parallel orientation is in practice the prevailing situation of the conductor in a coil. In the literature, Adam et al. (the Airco group) [l] reported various flat filamentary bronze-route conductors which exhibit a minimum safe bend radius of approximately 82 times the conductor thickness. An insert coil having a clear bore of 36 mm has been wound with a reacted 3553 filament Nb,Sn conductor (0.2 x 2.1 mm). It quenched at 9.5 T in a background field of 7.7 T at a current density in the conductor of about 269 A/mm’, a value similar to the value of the short sample critical current density measured in a background field of 7.7 T. Hillman et al. (the Vakuumschmelze-Siemens group) [2-4] developed various types of flat multifilamentary Nb,,Sn conductors, among them a 9882 filamentary bronze-route conductor with a cross-section of 0.35 x 1 mm. This conductor was used in a 2-4-fold combination soldered between two 50-urn thick copper tapes. The top coil wound with this type of conductor [4] produced a field of 14.2 T with a background field of 8.5 T in a clear bore of 51 mm at a current density of 151 A/mm2 in the conductor. The addition of a third chemical element to the filaments or into the bronze matrix of bronze processed multifilamentary Nb,Sn superconductors affects the diffusion process and improves the current carrying capacity in high magnetic fields [S]. The performance of pure Nb,Sn wires appears to be limited to magnetic fields lower than 14 T at 4.2 K because of the rapidly decreasing critical current density. The addition of third elements increases the growth rate of the Nb,Sn layer, prevents grain coarsening and increases the value of the upper critical magnetic field Bc,. The latter is attributed to an increase of the residual resistivity [6]. The increased growth rate allows the heat treatment time to be shortened. The addition of third elements also reduces the strain sensitivity as was shown by Ekin et al. [7] and Sekine et al. [S] 327

P. KovAC: et al.

328

as a consequence of the work hardening and increased strength of the doped filaments. The work hardening of the doped Nb filaments is important with respect to the reduction of the filament diameters to several urn [9]. For example, the work hardening in Nb filaments containing Zr concentrations greater than 0.8 wt% is larger than the work hardening in pure Nb filaments or in filaments with lower Zr content. All the above mentioned effects have to be taken into account when selecting the alloying elements. After an optimization procedure with Zr, Ti and Ta as the doping elements in single filament wires produced by the bronze-route [9], we established the optimal concentrations to be 0.25 wt% Zr, 1 wt% Ti and 7.3 wt% Ta. The present investigation was undertaken to explore the effect of Zr and Ti doping upon the I, anisotropy and the performance of flat multifilamentary Nb,Sn conductors in coils wound according to the react and wind (R&W) technique.

2. EXPERIMENTAL

RESULTS

2.1. Basic conductor properties and experimental arrangement We examined flat multifilamentary conductors. A conductor consists of a Cu 12.5 wt% Sn matrix containing pure or doped Nb filaments and surrounded by a Cu envelope. The envelope is separated from the matrix by a Ta diffusion barrier. The conductor was flattened by a three-step rolling procedure starting from a 0.78 mm diameter round cross section. The original volume ratio of bronze matrix plus filaments to envelope of 45: 55 was slightly reduced during the rolling procedure to a statistically averaged value of 43:57. The main parameters of the tested conductors are listed in Table 1. The Zr-doped conductor was twisted before rolling with a twist pitch of approximately 10 mm. The other conductors have been fabricated without twisting. The superconducting properties of the conductors have been investigated in two ways: as short (z 10 cm long) straight samples and as single layer coils wound after heat treatment. The four-probe technique was applied to monitor the voltage-current characteristics in the voltage range 0.1-10 f.tV/cm. The short samples have been measured in transverse magnetic fields ranging up to 6 T in a split-pair, superconducting magnet at boiling He temperature. The sample was fixed in a groove of a brass holder by lead-tin solder. The sample holder could be rotated in the magnetic field in order to study the conductor in various orientations of the broad side relative to the magnetic field. Some short samples were heat treated in U-form in order to test them in a cylindrical-bore magnet with the broad side perpendicular to the magnetic field. The single layer coils have been wound after reaction onto a 42 mm diameter and 40 mm long glass-fibre cylinder which was glued between two copper rings serving as current leads. A varnish-insulated copper wire was wound in parallel with the uninsulated superconductor in order to guarantee a well defined displacement of the neighbouring turns. During winding, the conductor was only subjected to a slight stress, ensuring the correct manipulation. Some strain arises in the conductor due to its straightening from the stock mandrel (0 90 mm) and the following bending onto the test mandrel. We estimate this strain to be -0.206% at straightening and +0.236% after subsequent bending into the final state.

Table Conductor

dimensions

Starting structure (% of cross section) CuSn : Nb ratio Number of filaments Filament diameter (pm) Addition to the filaments

1. Conductor

parameters

(mm)

0.3 x 1.5 CuSn + Nb Ta cu

: C

45 15 40 2.5 1560 ~6.5 None 0.25 wt% Zr 1 wt% Ti

Flat multifilamentary

0

2

Nb,Sn

4 MAGNETIC

Fig. 1. Conductor

resistance

6

329

superconductors

8

10

12

FIELD, T

per unit length as a function of the magnetic 7OO’C: (a) 0 min. (b) 30 min.

field. Heat treatment

period

at

The test coils were measured in a coaxial arrangement with respect to a cylindrical superconducting magnet (clear bore of 50 mm) providing a background field up to 12 T in the centre. The coincidence of the background field and the coil self field causes an additional electrodynamic stress in the conductor. The outer surfaces of the test coils were in direct contact with the boiling liquid helium. Some coils were also tested at lower temperatures ranging down to 2.5 K. The test samples were heat treated at 700°C for various times selected from the following: 0, 5 and 30 min and 4.5, 15,48 and 50 h. In the following sections, we will characterize the heat treatment procedure by these time intervals. The corresponding short samples were heat treated together with the samples for the test coils (several meters long). We determined the critical current using the 1 uV/cm criterion and the n-exponent (or n factor) from the corresponding I/ - I” characteristic and we present a comparison of the short sample values with those of the single layer coils. The background magnetic field at the centre of the background coil has always been used as the nominal main parameter. Two additional effects have been neglected: Firstly, the contribution to the total magnetic field of the coil self field lowers the critical current as it is in the same direction as the background field. Secondly, the coil windings are situated in a somewhat higher field than the field in the centre because of the finite magnet height which further reduces the critical current. The contribution of the self field is estimated to be 0.066 T per 100 A in the central plane. The radial inhomogeneity enhances the field by approximately 0.6% at the position of the test coil. The effect of heat treatment upon the normal resistance of the conductor (which reflects mainly the preserved purity of the copper) was checked in low fields (t&l T by “residual” resistance measurecharacterments at z 19 K and in high fields (up to 12 T) by recording the complete voltage-current istic including the flux-flow region, the transition into the normal state and the recovery branch. The latter measurements could be performed only at low transport currents with short time annealed samples because of the self-heating effect at high currents. The results are shown in Fig. 1. For low magnetic fields, the “residual” resistance measurements showed only a slight variation with the different heat treatment times. In the flux flow region, the resistance values are somewhat reduced after the heat treatment of 30 min. This reduction in resistance can be attributed to the reduced APSUP2/5-B

330

P. KovAC et al.

0,Ol

0.1

1

100

10

HEAT TREATMENT

TIME, h

Fig. 2. Critical currents of single layer test coils as a function of the heat treatment time at 700°C. Additions: (a) Nb, (b) Nb + 0.25 wt% Zr, (c) Nb + 1 wt% Ti, magnetic field: (A) 6 T, (B) 9 T, (C) 12 T.

tin content in the bronze matrix and to structural changes in the copper produced by annealing. The comparison of the room temperature and 19 K zero field resistance yields a ratio of 187.5. 2.2. Superconducting properties Figure 2 shows the critical currents (I,) of the various single layer test coils as a function of the heat treatment time. The I, values increase with the heat treatment time as a consequence of the growth of the Nb,Sn layer. This growth is faster when the filaments are doped with Zr and/or (at a lower rate) with Ti in comparison to the case of undoped Nb filaments. The data show that the Zr addition is beneficial for the critical currents in the low field region, while the Ti addition enhances the critical currents in the high field regime (> 12 T). The tendency of the critical currents to saturate at long heat treatment times shows that the Ti doped filaments tend to be reacted to saturation at 48 h, while the undoped Nb filaments may possibly reach even higher currents at longer heat treatment time. The superior result for the Ti-doped conductor at 12 T can be explained by the increase in higher B,, that results from the doping, as will be shown later. The critical currents of the single layer coils are compared numerically with those of short samples in Table 2. In spite of the unfavourable circumstances mentioned above (self field, inhomogeneity of the background field), the critical currents of the single layer coils are slightly higher than those Table

2. Critical curents at 1 pV/cm of single layer coils at 6 T (I,--), of short samples oriented parallel to the magentic field at 6 T (Ic,,) and their ratio R = ICC/I,,, for varius heat treatment times (h.t.t.) at 700 “C

h.t.t.

0 min 5 min 30 min 4.5 h 15 h 48 h 50 h

Nb + 0.25 wt% Zr

Pure Nb k&A)

135.3 191.3 209.4 215

MA)

187.7 200 207.8

R(%)

101.9 104.7 103.5

IDAA)

h(A)

Nb+ R(%)

59.5 113 177 200

58 110 175 214

102.6 102.7 101.1 93.5

226

222

101.8

IcdA)

147.5 187.7 196.1 197.3

wt%Ti WV

176.6 183.5 192

R(%)

106.2 106.9 102.8

Flat multifilamentary

Nb,Sn

superconductors

331

Table 3. The rate of decrease of critical current with increasing temperature (- AIc/AT at 1 uV/cm in A/K) between 4.2-2.5 K in single layer coils (h.t. 700 “C/48 h) 6

B(T) Pure Nb Nb + 1 wt% Ti

19 14.25

8

10

12

13.15 11.75

11.25 10

IO 8.5

of the corresponding short samples measured in a comparable configuration, namely with the broad side of the conductor parallel to the magnetic field. This is probably because the axial electrodynamic stress which strains the conductor when tested in a coincident background and self field mode. Roughly speaking, the observed “upgradation ” is highest in the Ti doped conductor, while it is lowest in the Zr doped conductor. This may suggest that the Ti doped conductor is less affected by the winding manipulation due to its good filament homogeneity. Indeed, microscopic studies of Zr doped conductor cross-sections show, a less perfect grain uniformity than the others. Furthermore, the Zr doped conductor seems to be more sensitive to bending due to the twisting of filaments. Except for the Zr doped wires, the results compiled in Table 2 show that the winding procedure of the single layer coils did not degrade I, of the tested conductors even after long heat treatment periods where 90% of the filaments cross-section can be assumed to be prereacted. The rate of decrease of critical current with increasing temperature (-AIc/AT) of two single layer coils wound with the conductors heat treated 7Oo”C/48 h and measured at lowered temperatures in the interval 4.2 - 2.5 K, are listed in Table 3. Given that the critical currents of the two conductors compared are approximately the same, the lower rate of the Ti-doped conductor indicates that its critical temperatures T,(B) are somewhat higher than those of the conductor containing pure Nb filaments. Figure 3 shows the n factors of the single layer coils. Their high values indicate that the conductors

60

I

I

I

I

I

50

6

8 BACKGROUND

10

12

FIELD, T

Fig. 3. The n factor of single layer test coils as a function of the magnetic field. Additions: (a) Nb, (b) Nb + 0.25 wt% Zr, (c) Nb + 1 wt% Ti, heat treatment period at 700°C: (----) 4.5 h, (-) 48 h (exception: 50 h for case (b)).

332

P. KovAC et al.

6

8

10

12

BACKGROUND Fig. 4. Kramer’s

14

16

18 20

FIELD, T

plot of the Zr doped conductor as a function of the magnetic field. Heat treatment (a) 0 min, (b) 5 min, (c) 30 min, (d) 4.5 h, (e) 50 h.

period:

endured the winding procedure well. They decrease with the magnetic field, as is usually observed in multifilamentary superconductors. An improvement of the current carrying layer can be qualitatively deduced from the observation of n-value increasing with the heat treatment time. The higher n factors of the parent and T&doped filaments reflect their higher microstructural uniformity as compared to those doped with Zr. The Kramer’s plots (the Zh’2B1’4(B) dependencies) for differently heat treated Zr-doped conductors exhibit a deviation from linearity down to lower Bc2 values in particular for those samples with a shorter heat treatment (see Fig. 4). The increase of B,, with the heat treatment time is significant. The Kramer’s plots of all extensively prereacted conductors exhibit a rather linear behaviour as shown in Fig. 5 and demonstrate the favourable influence of the Ti addition upon the Bc2 enhancement. The critical current of the Ti doped conductor is higher than that of other samples in fields above 10 T. The extrapolated Bc2 values can be used in a first approximation to estimate critical currents at high fields. Figure 6 shows the variation in I, anisotropy of the flat short sample conductors as a function of the heat treatment time. For all samples investigated, if the heat treatment time exceeds 30 min, the critical currents for samples in the parallel orientation are lower than for those in the perpendicular orientation. For short reaction times, however, the Zr doped conductor exhibits an anisotropy ratio (defined as ZcIi/Zcl) larger than 1. We observed a similar crossover of the Zc anisotropy annealing at temperatures lower than 700°C but associated with longer reaction times. The reason for this behaviour is not fully clarified yet. The higher Zc in comparison to Zc at short heat treatment could reflect the prevailing effect of the interface pinning between the flattened filaments and the matrix, as suggested by Bevk et al. [13] for in situ formed multifilamentary Nb,Sn wires and confirmed by Takeuchi et al. [14] in ultrafine Nb,Al superconductors. The contribution

Flat multifilamentary

Nb,Sn

superconductors

333

a

6

8

10

12

EXTERNAL

14

16

18

20

FIELD, T

Fig. 5. The I,-(B) characteristics and Kramer’s plot of the tested conductors heat treated at 7OO’C 48 h (or 50 h in the case of the Zr doped conductor). Additions: (a) pure Nb, (b) Nb + 0.25 wt% Zr, (c) Nb + 1 wt% Ti.

of grain boundary pinning appears to be more important after longer heat treatment. During the diffusion reaction columnar grains are growing preferentially at the Nb surface. This grain shape increases the pinning force in the perpendicular orientation due to the enhanced projection of the grain boundary area into the plane perpendicular to the Lorenz force. As the heat treatment is extended, the portion of equiaxial grains exceeds that of the columnar ones. Therefore, the columnar grains can no longer be regarded as the only reason for the increased pinning force in the perpendicular orientation. Microscopic investigations (TEM) of the Nb,Sn layers [9] showed that the boundaries between grains growing perpendicularly to the Nb surface are much straighter and smoother than those oriented parallel to the filament surface. This observation leads to a possible explanation to the effect that the smoothness of the perpendicular grain boundaries is responsible for the crossover of Zc,, by I,, at rising heat treatment times. Furthermore, Fig. 6 shows that the critical currents of all conductors tend to saturation in the parallel orientation, while this is not the case for Icl. Longer heat treatment times promise even higher I, values. For the convenience of the reader the numerical values of the anisotropy ratio are collected in Table 4. The Zr doped conductors yield the lowest anisotropy, while those of the Ti doped ones are the highest. There may be two reasons for this: The Zr doped filaments undergo a less smooth deformation process than the others and, furthermore, the twisting lowers their anisotropy. Concerning the magnetic field dependence of the anisotropy, we cannot yet present complete results. The Ti doped conductor, heat treated at 700°C for 15 h, was measured in the perpendicular orientation as a U-shaped sample in the cylindrical magnet up to 12 T. The resulting I, values are summarized with those of single layer coils at the same field in Table 5. These values allow only a preliminary comparison because of the two different sample configurations. The effect of bending strain was omitted because its influence upon the critical currents of single layer coils was quite negligible (see Table 2). The anisotropy falls only slightly with the magnetic field. This tendency is in agreement with the results obtained for short samples. The properties of various forms of our Ti doped conductors are compared with the properties of the best conductors presented in the literature in Fig. 7. All conductors were fabricated by the bronze route. The current density is calculated with the filament plus bronze matrix cross sectional area.

P.

334

KovACet al.

300

50

0.01

0.1

1

HEAT TREATMENT

10

100

TIME, h

Fig. 6. Critical currents of short flat conductor samples as a function of the heat treatment period at 700°C in a magnetic field 6 T oriented parallel (+) and/or perpendicular (4) to the broad side of the conductor, (a) Nb, (b) Nb + 0.25 wt% Zr, (c) Nb + 1 wt% Ti.

The curves a, b and c show the difference between our round and flat conductors for the same heat treatment. The lower current density of the flat conductor in the parallel orientation and the high degree of anisotropy are obvious. The real matrix cross section was estimated to be 43% of the total conductor cross section. The literature data exceed the current densities found in our tests. The cited conductors differ from ours by the use of different addition (Ta) to the filaments [lo], or by Ti addition to the matrix [ll, 123. In references [lo] and [12] the conductors have thinner filaments which are favourable for the reaction process. Our experiments with a 0.5 mm diameter round wire of the same type as the 0.8 mm one, support a possible current density enhancement of 20% by this procedure. The relatively high current density in the perpendicular orientation is surprising and remains unexplained up to now. However, this fact suggests that it may be advantageous to use the flat conductor (or a combination of several conductors) wound on its narrow side onto an adequate diameter. Table 4. Anisotropy h.t.t. Pure Nb Nb + 0.25 wt% Zr Nb + wt%Ti

ratio A = l,,,/I,-, of short sample flat conductors with varius as a function of the heat treatment time (h.t.t.) at 700°C

additions

0 min

5 min

30 min

4.5 h

15 h

48 h 0.70

1.195

0.989

0.84 0.915 0.753

0.749

1.61

0.68

0.63

at 6 T

50 h

0.758

Flat multifilamentary

Nb,Sn

335

superconductors

Table 5. Critical currents of single layer coils (I,,) and U-formed short samples (I,:,) of the Ti-doped flat conductor heat treated at 700°C 15 h as the function of the magnetic field B(T)

6

8

10

I2

Ice@) MA) I,&”

196.1 261.2 0.752

139.3 187.4 0.743

101 137.6 0.717

73.4 102.3 0.717

3.

The central

field of a cylindrical

DISCUSSION

coil can be expressed

as

(see e.g. [15]), where a, is the coil inner radius, I the conductor current, A the averaged cross section of one turn in the winding (including insulation and free space filled by impregnation) and F(or, /?) is the form factor with respect to the overall coil dimensions (c( = outer/inner radius, fl = height/inner diameter). The relation of the central fields of two coils having the same inner diameter 2al, but wound with a round and/or a flat conductor, is then

where the symbol r and f denotes the round and the flat conductor, respectively. In the following we estimate the form factor ratio for two coils wound either with our round Iz, 0.8 mm glass fibre insulated conductor (A, = 0.81 mm’) employing the W&R method and/or with our flat 0.3 x 1.5 mm varnish insulated conductor (A, = 0.35 x 1.52 = 0.532 mm’) using the R&W method, respectively. In a field of 18 T, the corresponding conductor critical currents can

5

lo3 % ,E 6

5

-Z vl 2

: .,o

lo*

5 6

8

10

12

MAGNETIC

14

16

18

20

22

FIELD,T

Fig. 7. Comparison of critical current densities in various conductors as function of magnetic field. Conductor specifications: (a) flat 1560 Nb + 1 wt% Ti til. 0.3 x 1.5 mm, non-twisted, R&W single layer coil, h.t. 7OO”C/48 h, parallel orient; (b) as a, U-sample, perpendicular orient; (c) cylindrical 1560 Nb + 1 wt% Ti fil. @ 0.8 mm, non-twisted W&R thin single layer coil, h.t. 7Oo”C/48 h; (d) as (c), short sample, h.t. 7OO”C/75 h [9]; (e) cylindrical 13,000 Nb + 7 wt% Ta fil. Qr 1.02 mm, h.t. 7Oo”C/4C-140 h [lo]; (l) cylindrical 721 Nb fil. 0 0.3 mm, 3 mm twist, Cu+ 13.2 wt% Sn + 0.3 wt% Ti matrix [ll]; (g) cylindrical 3025 Nb fil. 0 0.6mm, Cuf 7.4 at% Sn + OS at% Ti matrix, h.t. 68O”C/200 h + 72O”C/6 h 1121.

336

P. KovAC et al.

be estimated to be Z, = 34 A and I, = 25 A. If both coils have to generate the same field, the corresponding coil form factor ratio is given by F(a,, PJIF@,, B,) = Z/$/Z,4

= 0.893.

The coil wound with the flat conductor exhibits a smaller outer diameter (CQ< LX,)if the coils are assumed to have the same height (/$ = 8,). Employing the R&W technique with the discussed flat conductor will result, in spite of its anisotropy coefficient lower than one, in a smaller coil volume than the W&R technique with the equivalent round conductor. This result does not necessarily mean that the R&W coil requires a smaller amount of conductor, because in such a coil the filling factor is clearly higher. If, however, the designed coils are considered as insert coils producing the same field increment in a background system, the R&W coil requires an outer magnet with a significantly smaller inner diameter. The R&W technique brings about savings in the total system. 4. CONCLUSIONS

A flat multifilamentary bronze-route copper stabilized Nb,Sn conductor with the aspect ratio 1:5 was subjected to detailed tests. The experimental results presented allow us to make the following conclusions. The comparison of critical currents of straight short samples and single layer coils wound after heat treatment proves that no degradation of the critical current occurs by the winding procedure (i.e. bending of the conductor), even after a heat treatment up to 50 h at 700°C and irrespective of the doping elements. The Zr and Ti additions accelerate the growth of the Nb,Sn layer as compared with the pure Nb filaments and offer the advantages of shorter heat treatment times. The Ti addition is favourable for the increase of critical currents in the high field region (> 12 T), but the higher anisotropy of critical currents represents a clear disadvantage. The anisotropy ratio (Zc,,/Zc,) is larger than 1 and depends significantly upon the heat treatment time for short annealing periods (< 30 min). It becomes lower than 1 at longer annealing periods and decreases slowly with rising magnetic fields. Comparing the suitability of our flat Ti-doped and prereacted conductor (R&W) and the equivalent round nonreacted one (W&R), we conclude that the R&W promises savings in the overall coil volume of the complete high-field magnet system. This can be attributed to the higher packing factor in the winding. However, the advantages are reduced by the Zc anisotropy. The Zc anisotropy problem-its causes and possible ways to overcome it-deserves further basic studies. Acknowledgements-The presented work has been sponsored by the East-West Programme Academy of Sciences and by the Grant Agency of the Slovak Academy of Sciences.

(OWP-35)

of the Austrian

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