microstructure and jc improvements in overpressure

University of Wisconsin Applied Superconductivity Center

Microstructure and Jc improvements in overpressure processed Ag-sheathed

Bi-2223 tapesYongwen Yuan, Jianyi Jiang, Xueyu Cai, Satyabrata Patnaik, Matt

Feldmann, Anatoly Polyanskii, Eric Hellstrom and David Larbalestier

University of Wisconsin–Madison

Yibing Huang

American Superconductor Corporation

Bob Williams

Oak Ridge National Laboratory

Support from DOE-EERE and partial facilities support from NSF-MRSEC

ASC 2002, Houston, TX


Overview

What is overpressure (OP) processing and why use it?OP improves microstructure OP increases JcSummary


Density varies through multi-step 2223 process

Density decreases as 2223 formsRolling increases densityCore can dedensify in final heat treatment10-30% porosity exists in best multifilament tapes

Cracks caused by IR never completely heal

tHT1 IR FHT

Rolli

ng

GT

4.4 5.6 4.65.6

Jiang et al. SuST 2001

Full density 6.3 g/cc

T


What is overpressure (OP) processing?

Applies isostatic pressure to compress samplesOP processing is a variant of hot isostatic pressing (HIP)Mixture of inert gas and O2

Inert gas applies pressure <200 atmpO2 sets thermodynamic condition needed to form 2223 pO2 = 0.075 atm

Ultimate Goal: 1 deformation/sinter (1DS) process


Overview of ORNL static OP system

Easy to change initial Ptotal and pO2, but gas is not replaced, Ptotal and pO2 change during run

Regulator

Shut Off Valve

Flow control valve

Reaction Chamber

Thermocouples

O2 and Ar Tanks

Pressure Gauge

Sample FurnaceClosure


Overview of UW flow OP system

Gas continuously replaced during run; Ptotal and pO2 remain constant

Relief ValveRegulators

Shut Off Valve

Flow Meter

High Pressure Mixed Ar/O2 Tank (400 bar) 500 ppm O2

Regulator

Shut Off Valve

Flow control valve

Reaction Chamber

Thermocouples

Pressure Gauge

Sample FurnaceClosure


UW flow OP system, 900°C, 200 atm1.3 m


Where to begin with OP?

Tmax, Ptotal, and pO2 are the most important OP parameters

• Use simple HT schedule to optimize Tmax

• Modify J.Jiang’s 1 atm processing schedule for OP

• Address pO2 uncertainty in OP gas mixture pO2 = 0.075 to 0.10 atm at 148 atm


Simple heat treatment to optimize Tmax

Ptotal=148atm, pO2=0.077atm (design)

Tmax


Microstructure as a function of Tmax, 148atm804C

812C

808C

816C 820C

812C

824C


Ic varies with temperature – OP 148 atm

804 808 812 816 820 824

25

30

35

40Ic

(A)

T ( 0C)

Green tape


OP thermal process

Begin with 1 atm HT developed by Jiang et al.

Ptotal=148 atm, pO2=0.077 atm (design)

Tmax

Simplified the 1 atm HT for OP processing

See Jiang – 2MM04 Tuesday 4:00pm


At Ptotal = 1 atm, pO2 has small influence on HT1 and IR, some on GT,

0.075 0.080 0.085 0.090 0.095 0.100 0.10510

20

30

40

50

60

GT

HT1

IR

Ic (A

)

pO2 (atm)


OP improves microstructure

Densifies filamentsRemoves porosityHeals deformation cracks


OP removes porosity and heals cracks

1atm, multifilamentary IR tape Jc = 33.5 kA/cm2

148atm, multifilamentary IR tape Jc = 58.7 kA/cm2


OP densifies BSCCO filaments

X Data

0

20

40

60

80

100

1atm OP

Rel

ativ

e m

ass d

ensi

ty (%

) 92%

Mass density

�A = 0%-11% -16%

0.0000

0.0004

0.0008

0.0012

0.0016

0.0020

Cro

ss s

ectio

n ar

ea (c

m2 )

IR + OP

HT1 +

OP

1 atm

-FHT

Core cross section area

78%


AFM micrographs show lower porosity in OP tape

148 atm

1 atm AFMSEM


OP increases Jc

OP drives Jc up by Densifing coreReducing 2212Improving connectivity


OP increases Jc

FHT

Jc(SF) of multifilamentary samples

0

10

20

30

40

50

60

AMSCFHTJ c

(kA/

cm2 ) 33.5

1 atm

58.7

48

31

IR+OPHT1+OPGT+OP

(0T, 77K)kA/cm2

GT HT1 IRThermomechanical Processing of Bi2223


0 50 100 150 2000

10

20

30

40

50

60

1 atm

OP

Jc (k

A/cm

2 )

�0H (mT)

Field dependence of 1 atm and OP multifilament tape

0 50 100 150 2000

10

20

30

40

50

60

1 atm

OP

Jc (k

A/cm

2 )

�0H (mT)

0.1T

See Chandler – 2ME07 Tuesday 1:00pm


OP increases critical current density

FHTGT HT1 IRThermomechanical Processing of Bi2223

Multifilamentary samples

0

10

20

30

40

50

60

AMSCFHTJ c

(kA/

cm2 ) 33.5

1 atm

58.7

48

31

IR+OPHT1+OPGT+OP

Jc (0T, 77K)kA/cm2

HT1+OP IR+OP

18.222.4

1atm

12.3

0

10

20

2010

0

Jc (0.1T, 77K)kA/cm2

J c(k

A/cm

2 )

New record Jc !


OP succeeds in other tapes too

46.829Multi - 2 – HT1

48.541Multi – 2 – IR

4838.7Mono – IR

58.733.5Multi - 1 - IR

OP (kA/cm2)

1atm(kA/cm2)

Samples


Jc increase in OP due to more than densifying the core

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7

J ckA

/cm

2

1 atm FHT

Additional Jcfrom OP

processing

Calculated �Jcfrom densifying

core

OP+HT1 OP+IR


Jc increases as 2212 decreases

�M2212/�MT

0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66J c

(kA

/cm

2 , 77K

)10

15

20

25

30

35

TA-1 TA-2TA-3TA-4TA-5TA-6

More Bi-2212Less Bi-2212

Less 2212 More 2212

-1

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

00 20 40 60 80 100 120

Temperature (K)

emu/

emu

(6K

)

45kA/cm2 48kA/cm2

34kA/cm2 40kA/cm2

A

B

See Jiang – 2MM04 Tuesday 4:00pm &See Huang – 2MM10 Tuesday 5:30pm


OP reduces 2212

T (K)0 20 40 60 80 100 120 140

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2Rolled TapesRed ction = 30%

Nor

mal

ized

mom

ent a

t 6K)

Magnetization after crushing

1 atmOP

u


MO-CR shows OP improves connectivity

1 atm Magneto-

opticCurrent-

Reconstruction OP

See Patnaik – 2MA04 Tuesday 11:00am &Cai – 2ME09 Tuesday 1:00pm

Mono, 46mT, 77K


Direct comparison of connectivity

0

� XkA/cm2

OP1 atmX = 50 kA/cm2

Transport Jc: 1 atm 39 kA/cm2, OP 48 kA/cm2


What next?

Optimize OP processingCombine OP with other novel processesOP processing at low pressure


Nonsuperconducting 2nd phases and microporosity still in OP tape

Jc = 58.7 kA/cm2 , 148 atm

Ag

Ag

Ag


How low can Ptotal go?

Ptotal (atm)0 20 40 60 80 100 120 140 160

Rel

ativ

e de

nsity

(%)

75

80

85

90

95

100


SummaryOP improves Bi2223 microstructure by densifying filaments – remove porosity, heal cracksOP increases Jc by several mechanisms

OP increase Jc by ~20-70% wrt 1 atm processing, new record value of 22.4 kA/cm2 at 77K,0.1TOP increases Jc by densifying core, reducing 2212, improving connectivity

OP needs to be optimized – eliminate 2nd

phases and microporosityOP densifies tape at 65 atm, lower pressure experiments are underway

microstructure and jc improvements in overpressure

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