Delamination behavior study of REBCO coated conductor ...ASC2014+4MOr2A … · Delamination behavior study of REBCO ... • Dry-wound coil wet-wound coil A + wet-wound coil B (same

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Delamination behavior study of REBCO

coated conductor wires fabricated by

MOCVD on IBAD-MgO template Yifei Zhang, A. Knoll, A. Sundaram, D.W. Hazelton, R.B. McClure, and H. Sakamoto

2014 Applied Superconductivity Conference

August 10-15, 2014, Charlotte, NC, USA

2014 ASC Charlotte, NC Aug. 14, 2014 4MOr2A All Rights Reserved. Copyright SuperPower® Inc. 2014

Outline

• Delamination in coated conductor wire

• Performance of dry-wound and wet-wound coils

• Delamination behavior study using pin-pull test and anvil test

− Weibull analysis of test results

• Delamination behavior study using peel test

− Effect of peeling angle

− Peeling location

• Summary

2


Delamination in coated conductor wires • Multilayer structure – prone to delamination at an interface between two

layers or within a layer under c-axis tensile, peel, and/or cleavage stress

• Sources of the stresses – mechanical, thermal, and electromagnetic

• Delamination strength – important to coiling, bonding, cabling, splicing,

filamentization, cooling, and operation

3

Cu

REBCO

Buffer

Hastelloy

Cu

Cohesive delamination (within a layer)

Adhesive delamination (at an interface)

Mixed mode delamination (interlaminar)

Ag

Ag Transverse cross-section

• Wire delamination

recognized as one of the

possible causes for

degradation in impregnated

or wet-wound coils


Degradation of wet-wound coil – an example

4

• Stable performance in dry-wound coil (Kapton tape co-wound both sides)

through coiling/decoiling and multiple thermal cycles (3 windings by 3 TCs)

• Dry-wound coil wet-wound coil A + wet-wound coil B (same wire used for

the dry-wound coil) (Stycast 2850)

• Wet-wound coils, progressive decrease in Ic, through thermal cycles

Dry-wound coil Wet-wound coil A Wet-wound coil B

+


Failure analysis of degraded wet-wound coil

5

• Degraded wet-wound coil decomposed using a chemical solution

• Retrieved wire measured by Hall probe for Ic distribution (position of

lc degradation located)

• Peel strength measured for multiple samples within the ~1 meter

wire containing the Ic degradation

Ic (A)

Position (m)

7.3 7.4 7.5 7.6 7.7 7.8 7.9 8 8.1 8.2

Peel Strength (a.u.)


Investigation on delamination & coil degradation

6

• Investigation of coil degradation and development of advanced winding

techniques to prevent

− C. Barth, et al. SuST, 25(2013)055007

− H. Miyazaki, et al. IEEE TAS, 24(2014)4600905

− T. Takao, et al. IEEE TAS, 24(2014)8400205

− X. Jin, et al. IEEE TAS, 24(2014)4600104

• Development of testing methods to characterize wire delamination strength

− D.C. van der Laan, et al. SuST, 20(2007)765

− Y. Yanagisawa, et al. Physica C, 471(2011)480

− N. Sakai, et al. Physica C, 471(2011)1075

− Y. Zhang, et al. Physica C, 476(2012)41

− H.S. Shin, et al. SuST, 27(2014)025001

• Modeling and numerical studies

− J.R.C. Dizon, et al. SuST, 27(2014)055023

− Z. Jing, et al. J. Appl. Phys. 114(2013)033907

• Modification of processing to improve the delamination strength

− N. Sakai, et al. ISS2012, Tokyo, Japan, Presentation WTP-50

2014 ASC Charlotte, NC Aug. 14, 2014 4MOr2A All Rights Reserved. Copyright SuperPower® Inc. 2014 7

c-axis tensile test – pin-pull method (RT)

Chuck

Brass pin

Sample

X-Y stage

Force gauge

G10 base

Pin diameter=1.83mm

Sample preparation for pin-pull test

Brass pins

1-e-1


1-e-1

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c-axis tensile test – anvil method (RT and 77K) Upper anvil

Lower anvil

Contact area=9.14mm2

1-e-1

2014 ASC Charlotte, NC Aug. 14, 2014 4MOr2A All Rights Reserved. Copyright SuperPower® Inc. 2014 9

c-axis tensile test – Weibull analysis

• Weibull statistics provides a useful method to analyze c-axis tensile test data

which normally showed wide variation

• Relatively small Weibull moduli indicate a brittle material behavior in a

delamination process

• Estimation of stress limits by Weibull analysis provides a reference for

processing optimization and mechanical design of coils

𝑃 = 1 − 𝑒−()

Weibull distribution function is given by

− is the Weibull modulus

− is the characteristic stress

Pin-pull and anvil test results, Weibull 2-parameter analysis

TestContact Area

(mm2)

(Shape)

(Scale, MPa)

0.01

(for P=1%, MPa)

P

(for =20MPa)

Pin-Pull, RT 2.63 3.86 35 11 0.11

Anvil, RT 9.14 3.40 43 11 0.07

Anvil, 77K 9.14 3.64 62 18 0.02


Peel test

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90 Peel 180 Peel T-Peel

• Widely used for testing of peel resistance of adhesives, ASTM D-1876 (T-Peel)

• Steady state peeling process - reliable and reproducible results

• Peel strength (N/cm) determined from peeling load vs. displacement curve

• Test can be done at various peeling angles (change in stress state at peeling front), including 90, 180 peel, and T-peel


Peel strength changes with peeling angle • Peeling angle affects stress state at peeling front

• PS(180)>PS(90) normally observed

• Relationship between PS(T-peel) and PS(90-peel) varies with peel

strength level, possible effect from actual peeling angle variation

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• Peeling always occurs at the weakest location in a wire architecture

• Peeling locations frequently observed

− Interface between Ag/REBCO (adhesive peeling)

− Interface between REBCO/LMO (adhesive peeling)

− Within REBCO layer (cohesive peeling)

• Peeling at a specific location could be of either higher or lower peel strength

for different wires (i.e., they are same in where the weakest location is)

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Peeling location

At REBCOLMO interface

Lower arm surface

Higher peel strength

At Ag/REBCO interface

Upper arm surface

Higher peel strength

At Ag/REBCO interface

Upper arm surface

Lower peel strength


Peeling location – cont. • Peeling location depends on relative mechanical strengths of interfaces and

component layers, which are affected by conductor design and processing

• Most frequently observed peeling location is within REBCO layer (cohesive

peeling)

13

Peeled surface

Lower arm

OM image

Peeled surface

Upper arm

OM image

Peeling within REBCO

Schematic

REBCO


Peeling within REBCO layer

14

Upper arm

Lower arm

Peeled surface GDS spectra for Ba Profilometer scans at steps

on REBCO made by etching

Upper arm

Lower arm

• Exact peeling location verified using GDS and surface profilometer


Mechanical property study of REBCO layer • Mechanical strength of REBCO plays an important role in wire delamination

(relationship between mechanical property of REBCO layer and wire

delamination behavior needs investigation)

15

Sample A, Knoop, 50gf

Sample B, Knoop, 50gf

Sample A, Vickers, 50gf

Sample B, Vickers, 50gf

• Techniques that can be

used to study mechanical

behavior of thin REBCO

film: indentation test (micro-

or nano-), scratch test

(micro- or nano-)

• REBCO films showed

different responses to

microindentation, Knoop

and Vickers

• Preliminary nanoindentation

test on REBCO film showed

modulus=163.4GPa and

131.2GPa, hardness=6.8

GPa and 5.6GPa, for A and

B, respectively SEM images of indents on REBCO surfaces


Summary

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• HTS coated conductor wire delamination is investigated using pin-pull test,

anvil test, and peel test in combination with various microscopic analysis

techniques

• Measurement results from both pin-pull test and anvil test can be well

described with Weibull distribution

• Weibull analysis based on the result from anvil test at 77K indicated the c-axis

tensile strength (0.01) is about 18 MPa

• Peel test, featuring a steady state peeling process, is a useful method to study

delamination

• In peel tests, the most frequently observed peeling location is within REBCO

layer. Further investigation on mechanical property of REBCO film is needed

• “Degradation-free” coils can be made using coated conductor wires with

advanced winding techniques


Thank you for your attention!

17

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Delamination behavior study of REBCO coated conductor ...ASC2014+4MOr2A … · Delamination behavior study of REBCO ... • Dry-wound coil wet-wound coil A + wet-wound coil B (same