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Outline •  Superconductor lap splices electrical resistance overview •  Set-­‐up for resistance vs transverse compressive stress measurements •  Electrical resistance of soldered Bi-­‐2223 and REBCO tape lap splices as a func6on of transverse compressive stress •  Electrical resistance of unsoldered REBCO tape lap splices as a func6on of transverse pressure • Conclusion

C. Scheuerlein, MEM2016, 23 March 2016

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Lap splice resistance overview Superconductor R-­‐77 K (nΩ.cm) R-­‐4.2 K (nΩ.cm) Nb-­‐Ti (US welded) -­‐ 3.4±0.8 Nb-­‐Ti (soldered) -­‐ 25±7.3 Nb3Sn RRP (welded by EMPT and reacted) -­‐ 5.5±2.0 Nb3Sn RRP (reacted and soldered) -­‐ 36±10 Bi-­‐2223 SEI type HT-­‐CA 84.0±13.2 66 REBCO SuperPower SCS4050 (a) SC-­‐SC 148±4.1 150 (b) SC-­‐substrate 2550±466 n.m. (c) substrate-­‐substrate 3760±265 n.m. REBCO AMSC 8700 (a) SC-­‐SC 546±91.8 368 (b) SC-­‐substrate 5800±3400 n.m. (c) substrate-­‐substrate 14700±8600 n.m. MgB2 ex situ Columbus -­‐ 3100 MgB2 in situ Hypertech -­‐ 3300

C. Scheuerlein, MEM2016, 23 March 2016

Set-­‐up for 77 K resistance vs transverse compression experiments

•  Universal test machine (UTM) with a 5 kN load cell from Hegewald & Peschke MPT GmbH.

•  UTM has been equipped with a reverse load frame, such that instrumented samples and current leads can be immersed in liquid nitrogen.

•  For t ransverse compress ion experiments samples are mounted on a flat stainless steel plate that is connected to the bodom of the reverse load frame.

7

(a) UTM with liquid nitrogen cryostat. (b) Sample holder and pressing tool for resistance vs transverse stress measurements.

(b) (a)

C. Scheuerlein, MEM2016, 23 March 2016

Sample holder and pressing tool •  Tape is posi6oned on the stainless steel sample holder and insulated with Polyimide tape. •  Flat stainless steel pressing tool. •  Maximum stress that can be applied on a 20 mm2 splice in combina6on with a 5 kN load cell is 250 MPa. •  Alignment with pressure sensi6ve tape at RT.

Soldered REBCO tape lap splice with 3.5 mm overlap length mounted on the stainless steel sample holder for 77 K electrical resistance measurements as a func@on of transverse compressive stress.

C. Scheuerlein, MEM2016, 23 March 2016

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Soldered AMSC REBCO lap splice resistance as a funcFon of transverse compressive stress

•  No resistance and Ic change up to 400 MPa. •  Stresses above 400 MPa were achieved using a 1 mm-­‐wide pressing tool. •  A resistance increase from 1.3 to 2.2 μΩ occurs at 460 MPa, without a strong Ic reduc6on. This possibly indicates par6al delamina6on of the REBCO layer inside the tape.

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120

Volta

ge (µ

V)

Current (A)

0 MPa400 MPa460 MPA

V-­‐I curves of the AMSC tape SC-­‐SC lap splice with 3.5 mm overlap length measured at zero stress and at a transverse compressive stress of 400 MPa and 460 MPa.

C. Scheuerlein, MEM2016, 23 March 2016

Soldered SuperPower REBCO lap splice resistance as a funcFon of transverse compressive stress

•  No resistance and Ic change up to the maximum stress that could be applied on the splices with 20 mm2 overlap area. •  Ic of the substrate-­‐substrate splice is lower than the 77 K Ic of the tape, presumably because of hea6ng at the high resistance splice. 0

100

200

300

400

500

600

700

800

900

1000

0 20 40 60 80 100 120 140

Volta

ge (µ

V)

Current (A)

0 MPa220 MPa0 MPa260 MPa

SC-­‐SC 0.35 µΩ

V-­‐I curves of the SuperPower tape SC-­‐SC and substrate-­‐substrate lap splices with 5 mm overlap length at zero stress and at a transverse compressive stress of 260 MPa and 220 MPa, respec@vely.

C. Scheuerlein, MEM2016, 23 March 2016

Unsoldered REBCO lap splice resistance at different transverse pressures

•  In order to determine the Cu stabiliser contact resistance at the crossovers of the tapes in Roebel cables, the 77 K resistance of a pressed unsoldered lap splice was measured as a func6on of transverse compressive stress. •  Before moun6ng the tapes on the stainless steel sample holder they were cleaned with Scotch-­‐Brite and ethyl alcohol. •  The tapes were fixated and insulated from the sample holder and pressing tool by means of adhesive polyimide tape.

-­‐250

250

750

1250

1750

2250

2750

0 20 40 60 80

Volta

ge (µ

V)

Current (A)

V-­‐I curves of the unsolder SuperPower SC-­‐SC lap splice with 3 mm overlap length at different transverse pressures.

C. Scheuerlein, MEM2016, 23 March 2016

Unsoldered SuperPower REBCO SC-­‐SC lap splice resistance vs transverse compressive stress

100

1000

10000

100000

0 50 100 150 200 250

Resistance (nΩ.cm)

Stress (MPa)

5 mm (2)5 mm (2) unloading3 mm3 mm unloading10 mm10 mm unloading5 mm5 mm unloading

SC-­‐SC soldered R=148 nΩ.cm

SC-­‐substrate soldered R=2550 nΩ.cm

At 12.5 MPa the contact resistances of the four unsoldered SuperPower SC-­‐SC splices differ by about one order of magnitude. At a pressure of 100 MPa the average unsoldered SC-­‐SC plice resistance is R1cm=1490±965 nΩ.cm, which is lower than the resistance of soldered SC-­‐substrate splices.

R1cm of unsoldered SuperPower SC-­‐SC splices. The empty symbols represent the resistance values aNer par@al unloading to 12.5 MPa. The resistance of the soldered SC-­‐SC and SC-­‐substrate splices is shown for comparison.

Unsoldered AMSC REBCO SC-­‐SC lap splice resistance as a funcFon of transverse pressure

100

1000

10000

100000

0 50 100 150 200 250

Resistance (nΩ.cm)

Stress (MPa)

AMSC unsoldered (1)AMSC unsoldered (1) unloadingAMSC unsoldered (2)AMSC unsoldered (2) unloading

AMSC SC-­‐SC, R=546 nΩ.cm

AMSC SC-­‐substrate, R=5800 nΩ.cm

At 100 MPa the contact resistance between the clean stabiliser surfaces is about 2000 nΩ.cm.

R1cm of unsoldered AMSC SC-­‐SC splices. The empty symbols represent the resistance values aNer par@al unloading to 12.5 MPa.

Lap splice resistance overview Superconductor R-­‐77 K (nΩ.cm) R-­‐4.2 K (nΩ.cm) Nb-­‐Ti (US welded) -­‐ 3.4±0.8 Nb-­‐Ti (soldered) -­‐ 25±7.3 Nb3Sn RRP (welded by EMPT and reacted) -­‐ 5.5±2.0 Nb3Sn RRP (reacted and soldered) -­‐ 36±10 Bi-­‐2223 SEI type HT-­‐CA 84.0±13.2 66 REBCO SuperPower SCS4050 (a) SC-­‐SC 148±4.1 150 (b) SC-­‐substrate 2550±466 n.m. (c) substrate-­‐substrate 3760±265 n.m. (d) SC-­‐SC unsoldered at 100 MPa pressure 1490±965 n.m. REBCO AMSC 8700 (a) SC-­‐SC 546±91.8 368 (b) SC-­‐substrate 5800±3400 n.m. (c) substrate-­‐substrate 14700±8600 n.m. (d) SC-­‐SC unsoldered at 100 MPa pressure 2400 n.m. MgB2 ex situ Columbus -­‐ 3100 MgB2 in situ Hypertech -­‐ 3300

C. Scheuerlein, MEM2016, 23 March 2016

Conclusion • Applying during the resistance measurements transverse compressive stress up to 260 MPa and 400 MPa did not change the resistance and Ic of the SuperPower and AMSC splices, respec6vely. •  The contact resistance between the opposing stabiliser surfaces depends strongly on the transverse pressure. • At 100 MPa the contact resistance between the about 4 mm wide unsoldered clean stabiliser surfaces of the SuperPower and AMSC tapes is in the order of 2000 nΩ.cm.

C. Scheuerlein, MEM2016, 23 March 2016

Back up slides

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Electrical resisFvity ρ of the different materials present in the superconducFng wire and tape splices

Material ρ at RT (nΩ.m) ρ at 77 K (nΩ.m) ρ at 4.2 K (nΩ.m) Cu (RRR=100) [] 17.2 2 0.17 Nb (RRR=200) [] 158 Es6mated 16 0.8 Ag (RRR=200) [] 16 2.7 0.08

Ag alloy [] 38 Es6mated 10 Ag-­‐Au5.4wt% [] 58 Es6mated 30

Monel [] 500 Es6mated 250 Es6mated 230 Hastelloy C-­‐276 [] 1050 1030 Es6mated 1000

Ni5at%W [] 320 258 Es6mated 240 Brass (AMSC) [] 45 24 Es6mated 22

Cu alloy (Bi-­‐2223 HT) [] 25 7 Sn60Pb40 [] 140 Es6mated 40 3 Sn96Ag4 [] 120 Es6mated 20 1

C. Scheuerlein, MEM2016, 23 March 2016

Decay constant measurements with test loops with known inductance

y = 1.0006xR² = 0.996

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7 8

R = 98

8 nH

/τ (n

Ω)

R4-­‐point (nΩ)

C. Scheuerlein, MEM2016, 23 March 2016

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