11

Flux pinning study of YBa2Cu3O7- coated conductors – ideas

for performance enhancement

Aixia XuDepartment of Mechanical Engineering

Applied Superconductivity Center

National High Magnetic Field Laboratory

Florida State University

22

Outline

• Background and Motivation • Experimental methods• Preliminary results• Future work

33

YBCO is the only superconductor for application above 77K.

Highest Hirr

Larbalestier et al., 2001 Nature 414, 368

YBCO is the best material for high field magnet applications

YBCO superconductor brings up the application of >30T all-superconducting magnet.

Highest Jc

Courtesy of P. Lee at ASC @ MagLab

44

YBCO coated conductor

RABiTS

Rolling assisted biaxially textured substrate

Provided by AMSC

http://www.superpower-inc.com/system/files/

Ion beam assisted deposition

Provided by SuperPower

<0.1mm

http://www.amsc.com/products/htswire/index.html

5

1994 1996 1998 2000 2002 2004 2006 2008 20100

4

8

12

16

20

24

28

32

36

Mag

netic

fie

ld (

T)

Year

High field magnet application of HTS

Greg Boebinger presented at 2009 user summer school

Sumitomo/MITHitachi/NIMS

NHMFL/OST

Yamada

BSCCO

YBCO

NHMFL 33.8T

SuperPower

26.8T

66

High Ic requirement of high field magnet

B// c

I c

Angle

B//ab-plane

Ic is strongly anisotropic in background magnetic fields

YBCO layer

H

Although magnetic field is generally in ab-plane, it is tilted at the ends of the magnet. This limits its performance.

High Ic is important in the whole range!

77

How to increase Ic

twJAJI ccc Thickness (t)

Width (w)

Length (l)

Ic

Approach I: Enhance Jc

Flux pinning study BJF cp

Approach II: Increase thickness of YBCO layer

Thickness dependence of Jc

8

YBCO+BCO Feldmann 2009

4.9

YBCO+BZO Feldmann 2009

-20 0 20 40 60 80 100

0.4

0.8

1.2

1.6

2.0

2.4

Jc (

MA

/cm

2 )

(Deg)

Pure YBCO Harrington 2009

YBCO+YbTaO Harrington 2009

//ab//c

YBCO+BYNO Feldmann 2009

11

Jc is enhanced significantly by second phase addition.

Different doping results in different Jc angular dependence

Approach I-Jc enhancement at high temperature and low field

Harrington et al., 2009 Supercond. Sci. Technol. 22, 022001

Feldmann et al., HTS peer review 2009, August 4-6, Alexandria VA

77K/75.5K 1T

99

Motivation I

Jc() at low temperature and high fields

What is Jc() of YBCO coated conductors

High field magnet application

It is important for high field magnet application to cool YBCO coated conductor down to low temperature

YBCO CC work in high background magnetic field

Jc() is an key parameter for magnet design

Flux pinning study

Jc() is a powerful tool for flux pinning study

Theoretical and experimental work has shown that pinning mechanism is significantly different from that at high temperature

At very high field, the vortex density may be higher than the density of strong pinning centers.

what are effective pins at low temperature and high magnetic fields?

How to modulate Jc() to meet the requirement of high field magnet?

Increase of thermal fluctuation

Gurevich, Supercond. Sci. Technol. 2007 S128 Gutierrez J et al 2007 Nature Mat. 6, 367

10

However, some recent data show almost thickness independent Jc

10

0.0 0.5 1.0 1.5 2.0 2.5 3.00

1

2

3

4

5

6

7

Jc (

MA

/cm

2)

t (m)

77K self field Usually, Jc decreases with

thickness in YBCO coated conductor.

Some data show t -1/2 like dependence Jc(t).

Approach II-thick film growth

1111

Motivation II

Jc(t) study

Can we suppress thickness dependence of Jc? Indeed, there is theoretical model predicting t-independent Jc in the 3D strong pinning regime.

Gurevich et al., HTS peer review 2004, July 27-29, Washington DC

How can we obtain effective pins to keep high Jc through thickness in thick films?

1212

Outline

• Background and Motivations • Experimental methods• Preliminary results• Discussion• Future work

1313

Thin film growth and

coated conductors

Jc(t) study Flux pinning study

Jc (H, T, ) measurement

Ion milling

Ic<0.1AIc>0.1A

Microstructure analysis

Sample growth

Jc (H, ) measurement at high temperature

Jc (H, ) measurement at low temperature and

very high fields

1414

Outline

• Background and Motivations • Experimental methods• Preliminary results

– MOD RABiTS from AMSC – mainly Jc(t)

– Many SuperPower IBAD-MOCVD – mainly Jc(H,) at high and low T and high and low H

– PLD, ASC-grown thin films to address intrinsic pinning issue

• Discussion• Future work

1515

sample t (m) Tc (K) Jc (MA/cm2)77K, sf

Hirr (T) Fp max (GN/m3) Substrate

FSU003  1.4  93.8  2.66 9.2 5.8 RABiTS

Preliminary result- Jc (t) study of MOD-RABiTS

Jc is independent on thickness except close to buffer

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

50

100

150

200

250

300

0.0

0.5

1.0

1.5

2.0

2.5

Jc(t)

Ic (

A)

t (m)

Jc

(M

A/c

m2 )I

c(t)

Jc is independent on thickness except close to buffer

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.690.0

90.5

91.0

91.5

92.0

92.5

93.0

93.5

94.0

0

2

4

6

8

10

Tc

(K

)t (nm)

Tc(t)

90 91 92 93 945

6

7

8

9

10

H

irr

(T)

Tc (K)

H

irr

(T)

Hirr

(t)

Tc and Hirr is independent on thickness except close to buffer

1616

TEM image of MOD/RABiTS YBCO sample

Surface roughnessassociated with the MOD film process. (rms = 84nm)

Interfacial roughness due tolocalized reactions with theCeO2 (BaCeO3). (rms = 17 nm)

Surface roughness + Voids + Interfacial roughness uncertainty of thin layer thickness

TEM by Terry Holesinger

voidscontaining in the sample through thickness.

1717

Strong flux pinning centers MOD/RABiTS YBCO sample

1. High density of RE2O3 precipitates are strong 3D effective pinning centers pushing Jc into thickness independent regime.

2. Voids, threading dislocations are effective pinning centers for Jc enhancement.

3. high density stacking faults are major correlative pinning centers that responsible to Jc enhancement along ab-plane

Void

SF (black line)

(Y,Dy)2O3

Threading dislocation

TEM by T. Kametani

1818

Summary of Jc(t) MOD-RABiTS study

Jc is independent of thickness except close to the buffer layer. Tc and Hirr show the same thickness dependence as Jc

The high density of RE2O3 precipitates is expected as the source of strong 3D pinning which makes the high thickness-independent Jc for the top YBCO layer

The degradation of Jc near the interface is not fully understood. In former times, MOD-RABiTS had the inverse of this behavior, good near the bottom and degrading near the top. The conversion process for the MOD-TFA route is complex and kept private from us.

• The key point is that it IS possible to have a high thickness-independent Jc

The data is consistent with strong 3D pinning models

Alex Gurevich et al., HTS peer review 2004, July 27-29, Washington DC

Gurevich, Supercond. Sci. Technol. 2007 S128

1919

Sample detailsSample details ThicknessThickness((m)m)

TcTc(K)(K)

Jc (MA/cmJc (MA/cm22) ) 77K, sf77K, sf

Hirr (T)Hirr (T)77K, //c77K, //c

commentcomment

Standard-2.1m (Y,Gd)BCO 2.13 90.4 1.47 7.78  RE2O3 and SFs

BZO-1.5m YBCO/BZO 1.55 90.3 1.40 8.6 RE2O3 and SFsBZO nanorods

Double layer 2.15  90.5 1.85  8.8

BZO-0.9 mM3-687-2 MS

(Y,Gd)BCO/BZO

0.939 89.9 2.46 9

Standard-1.2mM3-674 FS

GdBCO 1.177 93.3 2.51 9.7

PLD-LTG YBCO/STO 0.5 90.3 4.43 7.07 SF only as visible pins

PLD-HTG YBCO/STO 0.35 89.4 1.69 7.75 Only ppts as visible pins

Jc study at low T and high HSuperPower versus my PLD samples

Goal: contrast SP samples with many stacking faults with SF-free samples to better understand ab-plane peak

Representative samples

20

double layer

20

0 30 60 900.0

0.2

0.4

0.6

0.8

1.0

standard-1.2m

Jc

(M

A/c

m2 )

(deg)

standard-2.1m

77K, 1T

BZO-0.9m

BZO-1.5m

Jc () at 77 K 1 T of SP samples

Different samples shows different Jc() at high temperature and low field. Strong pins dominate the high temperature pinning because strong thermal fluctuations are present. Slight rewording

BZO-containing samples show high Jc around c-axis .

2121

Jc (H) of SP at 4.2K

0 5 10 15 20 25 301

10

H//c

Jc (

MA

/cm

2 )

0H (T)

standard-2.1um standard-1.2um BZO-1.5um BZO-0.9um double layer-2.1um

H//ab

4.2K30

Jc is almost independent on the field when field is in ab-plane

Jc is suppressed for H > 20T because of LHe levitation in a strong field gradient.

Jc is thickness dependent. Thin film shows higher Jc.

0 5 10 15 20 25 30

100

1000

H//c

I c (A

/4m

m-w

idth

)

0H (T)

standard-2.1um standard-1.2um BZO-1.5um BZO-0.9um double layer-2.1um

H//ab

4.2K

2000

The in-plane Ic at 20 T for 4 mm wide CCs is 1.2kA and 1.4kA for double layer and standard-2.1m sample, respectively, values quite high enough for magnet

applications.

Sample BZO-0.9m shows highest Ic below 4T even though its thickness is only 1 m.

BZO sample decrease faster with the increasing of field.

2222

Jc () of SP at 4.2K

0 30 60 908

10

12

14

16

18

0 30 60 902

4

6

8

10

12

14

16

18

0 30 60 90

2

4

6

8

10

12

14

16

0 30 60 90

2

4

6

8

10

standard double layer

Jc (

MA

/cm

2 )

1 T

standard double layer

5 T

standard BZO double layer

Jc (

MA

/cm

2 )

(deg)

15 T

standard BZO double layer

(deg)

30 T

At 1T, both samples show broad maximum when magnetic field is around ab-plane.

Raising the field from <5 T to 30 T at 4 K causes a marked transition from a broad maximum to a marked cusp-like behavior.

No measureable c-axis peak is observed

BZO-1.5m show broader Jc() around ab-plane

Xu et al., Supercond. Sci. Technol. January 2010

2323

HfJHJ cc ,

222 sincos f

is the effective electron mass anisotropy parameter

Jc() at 1 T follows very well the G-L model. Random pins are dominant.

At higher fields, Jc() is failed to fit GL scaling, which strongly suggests that correlative pinning is dominant at high fields around ab-plane.

Question I What controls the Jc() at low temperature-GL scaling

2424

Where produces the correlated pinning effects in SP samples?

BZO-1.5m

1. RE2O3 precipitate arrays

2. Stacking faults

3. Intrinsic pinning

Standard-2.1m

RE2O3 precipitate

Stacking faults Stacking

faults

BZO nanorods

RE2O3 precipitate

Possible sources

TEM by T. Kametani TEM by T. Kametani

2525

1. Stacking faults or precipitate arrays? Compare PLD samples………….

Stacking faults

PLD-LTG

No stacking faults or threading dislocations

Y2O3

PLD-HTG

SFs are the only visible pinning centers

Ppts are the only visible pinning centers

TEM by T. KametaniTEM by F. Kametani

2626

Stacking faults are effective pinning centers both at low temperature and high temperature.

0 30 60 900

10

20

30

40

(deg)

10K

0

7

14

21

30K

0

3

6

9

50K

0.0

0.3

0.6

0.9

J c (M

A/c

m2 )

77KPLD-LTG

(a)

Mainly stacking-faults

Black 1T

Red 4T

Green 9T

0 30 60 90

10

20

30

400

7

14

21

0

3

6

90.0

0.3

0.6

0.9

10K

30K

(Deg)

50K

PLD-HTG77K(b)

Mainly precipitates

Precipitates are effective pins along c-axis especially at high temperature

Due to high Lorentz force

2727

For YBCO, the separation s of the CuO layers is around 0.4nm, while the coherence lengths are c = 0.3 nm, ab= 1.6nm at T = 0, Tc=92K and

Thus coherence length becomes shorter than separation of the CuO layers

c = 0.4nm = s when T 40K

R. M. Schalk et. Al., Cryogenics 1993 vol.33 No. 3 371

cT

Tt

tT

1

)0()(

2. Intrinsic pinning?

Bc axis

x

z (//c axis)

B

y

J

ab plane

x

z (//c axis)

B

y

J

ab plane

z (//c axis)

B

y

J

z (//c axis)

B

y

J

B

y

J

ab plane

• Kes law

It states that only the c-axis component of the applied

magnetic field affects the critical current density.

• T-T model

2/1cos

)0(

)90(

)(

c

c

c J

J

ofsmallerJ

)0,,cos(),,( TBJTBJ cc

28

T-T and Kes model fits for Turbo (double layer SP)

4

8

12

16

20

0 30 60 90

4

8

12

16

20

0 30 60 90

1T

5T

15T

(Deg)

Jc

(M

A/c

m2)

(Deg)

30T

Red:T-T modelGreen: Kes law

Jc

(M

A/c

m2)

Intrinsic pinning is not dominant at low field

Kes model predicts a higher ab-peak beyond real data. At high fields, He gas bubble heating may explain greater deviation

T-T model is a good fit at high fields

2929

Question II: Can we make Jc () broader?

0 30 60 900

5

10

15

20

25

30

10T5T

4T

Jc

(M

A/c

m2 )

Angle (deg)

BZO-0.9m

1T

SP sample with BZO nanorods

0 30 60 900

5

10

15

20

25

30

10T5T

4T

Jc

MA

/cm

2 )

Angle (deg)

standard-1.2m

1T

SP sample without BZO

They show similar Jc() to previous three samples.

BZO-containing sample show broader maximum around ab-plane.

No c-axis peak is observed even at low fields.

30

0

0.5

1.0

0 30 60 90

Angle

Ic (

norm

aliz

ed)

30

BZO-containing sample has broader Jc() at 10T and 4.2K

Blue BZO-0.9m

Green: standard-2.1m

Magenta: double layer-2.1m

3131

Summary for Jc() study

Jc() at low temperature and high background magnetic fields All samples show similar Jc() even if they are very different at high temperature

(77K) and low field (1T). At low fields ( 1T), there is a broad maximum around ab-plane. The broad maximum evolves to a cusp which becomes sharper with increasing

magnetic field. No measureable c-axis maximum is observed regardless of sample and fields.

GL fitting of the angular dependence of Jc At low fields, random pinning centers are dominant in the all angle range. At high fields, correlative pinning along ab-plane takes over.

Jc(H) at low temperature (4.2 K) Jc is almost magnetic field-independent along ab-plane. Jc decreases significantly with increasing field along c-axis.

TEM images show that SP CCs have SFs, RE2O3 precipitate arrays and intrinsic pinning as potential ab-plane correlated pinning centers Stacking faults are effective pins from 4.2K to 77K and fields below 9T. Intrinsic pinning is negligible above 30K but becomes stronger at lower temperature. Jc can be greatly enhanced by precipitates except around ab-plane.

BZO-containing samples have reduced anisotropy and broader peak around ab-plane

3232

What we have done Can we eliminate thickness dependence of Jc in sample with strong 3D

pinning? Thickness independent Jc is obtained in Dy-doped MOD/RABiTS coated conductor except the

significantly degradation of Jc near buffer layer. Dy2O3 nanoparticls is attributed to the strong 3D pins for the t-independent Jc

Uncertainty of current-carrying cross-section due to the roughness of thin YBCO layer is the possible reason for lower Jc near buffer layer.

Jc() Study at low temperature and very high magnetic fields What is Jc() at low temperature at background magnetic fields?

At low field, Jc() is GL-like regardless of sample.

GL-like Jc() evolves to cusp-like with the increasing of magnetic fields.

What are effective pins at low temperature? 3D random pins, atomic disorder are dominant at low field Correlative pins, RE2O3 precipitate arrays, stacking faults and intrinsic pinning are dominant at

higher fields around ab-plane Stacking faults are effective pinning centers around ab-plane at temperature regime from 77K to 10K below

9T Precipitate arrays enhance Jc except around ab-plane at temperature regime from 77K to 10K below 9T

Pinning effect from intrinsic pinning become evidence at low temperature

Can we modulated Jc() at low temperature and high magnetic fields? It is possible to obtain high Jc along ab-plane by enhance the density of stacking faults

BZO-containing sample make Jc() broader

3333

What is our next work Systematic study of Jc(t)

Understand what cause Jc degradation near the buffer layer

Set of samples, grown by different process with various additions on multiple substrates will be studied .

Systematic study of Jc() Perform Jc() measurement at low temperature and very high fields above 9T.

Samples with stacking faults only Samples with precipitates arrays only Samples with BZO nanorods

Intrinsic pinning Extend the measurement regime of temperature and external magnetic fields. Span the sample set with different pins

YBCO thin film growth Facility

1kA capability probe for high field and high Ic measurement; 100A rotator for small sample angular dependence Jc measurement at low temperature and very high fields;

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Jc (MA/cm2)

t (

m)

3434

What we want to know

What is the general Jc(t)? Does it dependent on the process, second-phase addition or substrates?

Is there other effective strong pins to eliminate thickness dependence of Jc. RE2O3 nanoparticls

2D correlative pins, for example, BZO nanorods, are potential effective pins based on Feldmann’s work.

How can we keep high Jc through thickness by a homogenous microstructure with a high density effective strong pins?

What are effective pins at very high magnetic fields? Are stacking faults effective at very high magnetic fields? What role does intrinsic pinning play at very high magnetic fields? Does BZO nanorods affect Jc() at very high fields?

How the correlative pins modulated Jc() and why? Where is the pinning effect from? the second phases themselves, the strain corresponding to

nanoparticles or other defects ? Is there other pins exist to modulate Jc()?

What is the relation between Jc at different temperature and different fields regime?

Jc() at high temperature and low fields, like 77K, 1T and Jc() at low temperature and very high magnetic fields

Jc (H) and Jc()

35

Thanks for your attention

3636

Fields

Samples

1T 3T 4T 5T 10T 15T 20T 25T 30T

Standard 33.5 14.4 16.3 12.2 13.6 13.9 13.4

BZO 12.7 13.3 21.1

Double layer 46.7 24.8 17.6 10.9 6.2 6.8 8.7 7.6

10% Zr doping 49

M3-674 FS 40 16.5 13 10.9

M3-687-2 MS 73.2 40 39.4 17.8

FWHM values of samples measured at 4.2K

Samples Standard BZO Double layer

10% Zr M3-674 FS M3-687-2 MS

BZO nanorod

NO Yes NO Yes NO Yes

BZO-containing sample has higher FWHM at low field regime low temperature