Control of contact resistivity for REBCO NI magnets

Jun Lu, Jeremy Levitan, Yan Xin, Kyle Radcliff, and Eric Lochner

National High Magnetic Field laboratory,

Tallahassee, Florida 32310, USA

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Fri-Mo-Or27-03:

The National High Magnetic Field Laboratory is supported by the US National Science Foundation through NSF/DMR-1644779 and the State of Florida.

Outline

• Introduction

• REBCO Cu oxidation

• Stainless steel oxidation• XPS studies

• TEM studies

• Contact resistivity tests.

• Summary

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Introduction • For NI REBCO magnets, contact resistivity between adjacent turns (rc)

is a critical parameter. Low rc causes high current and high stress during a quench*, high ramp losses, and significant charging delays.

• The desirable rc varies depending on the details of the coil design, so it is highly desirable to be able to control rc in a wide range.

• Two basic approaches:1. Oxidizing REBCO Cu surface.

2. Oxidizing SS co-wind.

• Our goal is to be able to control rc between 1 – 1000 mW-cm2.

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* W Denis Markiewicz et al, 2019 Supercond. Sci. Technol. 32 105010

Reel-to-reel REBCO Cu oxidation

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• Ebonol C special: H2O = 1:4 at 98 C.• Reel-to-reel process 0.5 m/min (~40 sec in solution)• Oxide layer is ~ 0.5 mm thick, controllable by time

and temperature

Total of 4 piece lengths of 450 m SuperPowerREBCO tapes have been oxidized

Support by Florida State University GAP commercialization fund.

Process parameters:

Ebonol C oxidation process

• A surface chemical solution treatment process.• REBCO tape is treated in solution, rinsed, and dried.

rc measurement

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• The device used in either LN2 or LHe.• Can be adapted to a MTS machine for fast pressure cycles.

Pressure

REBCO REBCO25.4 mm

I+ V+ V- I-

w/ or w/o an interlayer

J. Lu, et al., SUST 2017, 045005

25 mmMeasurement current 1 A

Uniformity of rc from end to end

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• The scatter was significantly reduced after system modification• Further improvements are possible.

Modification of the system• Better speed control• Better temperature control

Significantly improved the uniformity.0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5

rc

at 2

5M

Pa a

t 7

7 K

(mW

-cm

2)

REBCO piece

115 m

120 m

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5

rc

at 2

5M

Pa 7

7 K

(mW

-cm

2)

REBCO pieces

19 m pilot

115 m

120 m

200 m

After system improvement

• Samples cut from the front and back end of each piece were measured at 77 K.

• Large scatter in rc

350C 400C 500C 600C

Heated in air for 60 seconds

316 SS tape: full-hard 50 mm, used in the 32 T project

Oxidizing SS co-wind tape

rc of SS at 77 K

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0

20

40

60

80

100

120

140

160

0 100 200 300 400 500 600 700

Rc

at 2

5 M

Pa

(mW

-cm

2)

Heat treatment temperature ( °C)

HT in air for 1 min.

Large scatter, but the effect of oxidation is consistent.

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400

No

rmal

ized

pea

k in

ten

sity

Ar milling time (min.)

Cr-2pFe-2pO-1s

Surface chemistry by x-ray photoelectron spectroscopy (XPS)

9Heat treated SS has much thicker oxide layer, and has FeOx on surface instead of CrOx

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400

No

rnal

ized

pea

k in

ten

sity

Ar milling time (min.)

Cr-2p

Fe-2p

O-1s

FeOx/CrOxCrOx

Depth profile by Ar millingAs etched 300 C /8 min

TEM of SS cross-section

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0 . 5 µ m

• TEM samples were prepared by FIB. • A layer of Pt was deposited to protect the thin

oxide layer during the ion cutting process.

10 mm

Pt

Stainless steel

Oxide layer

TEM of SS cross-section

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30 nm

O

30 nm

O

Fe

Cr

300 C /8 min 600 C /1 min

~10 nm oxide~30 nm oxide

1

10

100

1000

10000

100000

1000000

1 10 100 1000 10000 100000

rc

at 2

5 M

pa

(mW

-cm

2)

Number of 2.5 - 25 MPa cycles

77 K

Effect of contact pressure cycling

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1

10

100

1000

10000

100000

1000000

1 10 100 1000 10000 100000

rc

at 2

5 M

pa

(mW

-cm

2)

Number of 2.5 - 25 MPa cycles

77 K

4.2 K

• Considerable rc decrease after pressure cycle at 4.2 K• This behavior is consistent with coil test results (Dixon, et al. Tue-Af-Po2.14-10)

Sample oxidized 300 C/ 8 min

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1

10

100

1000

10000

100000

1000000

1 10 100 1000 10000 100000

rc

at 2

5 M

Pa (mW

-cm

2)

Number of 2.5 – 25 MPa cycles

As received

300 C/ 8 min

T = 4.2 K1

10

100

1000

10000

100000

1000000

1 10 100 1000 10000 100000

rc

at m

ax p

ress

ure

(mW

-cm

2)

Number of cycles

25 MPa

10 MPa

6 MPa

T = 4.2 K

Effect of contact pressure cycling

As received SS has greater rc decrease The effect is less for lower peak pressure.

Influence of different surface treatments

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1

10

100

1000

10000

100000

1000000

1 10 100 1000 10000 100000

rc

at 2

5 M

pa

(mW

-cm

2)

Number of 2.5 - 25 MPa cycles

300 C/8 min

600 C/1 min

Oxidized Cu

T = 4.2 K

1

10

100

1000

10000

100000

1000000

1 10 100 1000 10000 100000

rc

at 2

5 M

pa

(mW

-cm

2)

Number of 2.5 - 25 MPa cycles

300 C/8 min

600 C/1 min

T = 4.2 K

Thicker or/and different surface chemistry seems to make the layer more robust.

REBCO Cu oxidized layer (~0.5 mm) is more robust against cycling.

Summary

• In order to control rc, we experimented with REBCO Cu oxidization and stainless steel co-wind oxidation.

• A reel-to-reel REBCO oxidation capability is developed.

• Stainless steel samples with 10 -30 nm oxide layer are made.

• TEM and XPS were used to analyze the oxide layers.

• Drastic rc decrease in some oxidized SS samples was observed after pressure cycles at 4.2 K.

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Development for rc control continues …