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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 …