G. Grasso, A. Malagoli, M. Modica, V. Braccini, A. Tumino, S. Roncallo, E. Bellingeri, C. Ferdeghini, A.S. Siri

INFM -LAMIA, Corso Perrone 24, 16152 Genova -Genova, Via Dodecaneso 33, 16146 Genova

Preparation and properties of MgB2 superconducting tapes

tube filling

PIT Processing routes for the fabrication of MgB2 wires

+B Mg

a)IN-SITU

b) +B Mg MgB2 EX-SITU

wire drawing

wire rolling

Cold Working

long lengths of tape

Cu-sheathed tape

transverse cross section

Tube filling with MgB2 reacted powders

Wire drawing and/or rolling

Flat rolling

Long lengths can be now fabricated

irregular cross section

No sintering is necessary!APL 79 (2001) 230

Alfa-Aesar or homemade precursors

ex-situ PIT processing

High critical currents are achieved even without any sintering process!

1 10 100

0

1x10-5

2x10-5

3x10-5

4x10-5

Ag f.f. 30% Cu f.f. 30% Ni f.f. 30%

Vol

tage

(V

)

Current (A)

I-V characteristics vs. sheath material

4.2 K

.25x3 mm2

strength

Harder sheath material larger jc

Less sausaging + higher powder compactionLow powder compaction => lower Ic and jc

240A 105 A/cm2

0.32 x 4 mm2

f. f. 17%

V-I characteristics at 4.2 K, self field

1 10 100 300

0

1x10-5

2x10-5

3x10-5

4x10-5

Ag f.f. 30% Cu f.f. 30% Ni f.f. 30% Ni f.f. 20%

Vol

tage

(V

)

Current (A)

strength

f.f.

Mechanical properties of unsintered tapes

Unsintered SUS316 MgB2 tapeKitaguchi et al. submitted to Phys. C

200 210 220 230 240 2500

2x10-5

4x10-5

6x10-5

8x10-5

diam. 30 mm

Vol

tage

[V]

I [A]

before bending after bending

Preliminary bending strain experiment

Tape double bent on diameter 30 mm No reduction of Ic

Mechanical properties are already sufficient for applications!

Transport jc(B,T) measurements on unsintered MgB2 tapes

0 2 4 6 8 100

2

4

6

8

10

20 K

15 K10 K

8 K

4.2 K

j c0.5 B

0.25

Magnetic Field [Tesla]

Kramer plot

Jc(B,T)0.5 B0.25 B

Thanks to ENEA-Frascati Lab, Rome, Italy (M. Spadoni, P. Gislon)

0 1 2 3 4 5 62

10

100

300

103

104

105

C

ritic

al c

urre

nt [A

]

Magnetic Field [T]

4.2 K 8 K 10 K 15 K 20 K

Critical current density [A

/cm2]

B//tape plane

Tc lower than expected for MgB2

Magnetoresistance measurements

Btape plane

0 10 20 30 40

0

2

4

6

8

10

Hirr [T

]

T [K]

Ni tape IL from R(B,T) Ni tape IL from I

c(B,T)

precursor MgB2 powders

IL of Ni-sheathed tape presents a larger slope than for MgB2 powders

Irreversibility lines from R , Ic(B,T)

Superconducting irreversibility line and XRD of unsintered MgB2 tapes

A new source of pinning centers is present in ex-situ MgB2 tapes

XRD peaks are broader in the Nickel-sheathed MgB2 filament; some level

of c-axis texture is also present

30 40 50 600

1x103

2x103

3x103

Ni-sheathedMgB

2 filament

MgB2

powders

Al

Al

Al

111

MgO 10

2110

002

101

100

001

Inte

nsity

[a.u

.]

2 [°]

XRD of Ni-sheathed tape

large lattice distortion: a/a = 0.3%

What if we heat?

0 10 20 30 400

50

100

150

200

250

0

3x104

6x104

9x104

1x105

I (A

)

T (K)

unsintered 400°C 700°C 800°C 900°C

jc (A/cm

2)

Treatment of 1 h in Ar

Too large Ic in low-T heated tapes quench

0 1 2 3 4 5

-400

-200

0

200

400

600°Cr

0=0.8 mm

d=100 m

M [e

mu/

cm3 ]

B [T]

5K 10K 20K 25K 30K

Magnetic measurement of MgB2 core

Assuming M=2/3 jc d

0 1 2 3 4

103

104

105

J C [A

/cm

2 ]

Magnetic Field [T]

5 K 10 K 20 K 25 K

0 10 20 30 400

50

100

150

200

250

300

0.0

3.0x104

6.0x104

9.0x104

1.2x105

1.5x105

600 Cmagnetic

700 Ctransport

400 Ctransport

I (A

)

T (K)

jc (A/cm

2)

Magnetic and transport jc coincide

Argon

0 2 4 6 8 10 121

10

100

1000

4

40

400

4000

B//tape

Btape

4.2 K

jc [A/m

m2]

Crit

ical

cur

rent

[A]

Magnetic Field [T]

Anisotropy of the critical current density at 4.2 K

Correlation between magnetic and transport jc => extrapolation of 20K behavior

1.0 1.5 2.0 2.5 3.0

10

100

1000

40

400

4000

jc [A/m

m2]

Htape

Transport 4.2K

Crit

ical

cur

rent

[A]

Magnetic Field [Tesla]

Inductive 5K 10K 15K 20K 25K

New transport Ic data:27 K, s. f. > 500A 1 T ~100 A 1.5 T ~ 20 A

20K, 1.5T

4.2K, 3 T

Tape dimensions: 3.5 mm x 0.35 mmFilling factor 20%

Treated at 900°C for 2 hours in Ar

Transport properties of ex-situ tapes reacted in Argon

atmosphere

0 10 20 30 40

0

3

6

9

unsintered Btape 800 C Btape 900 C Btape

unsintered B//tape 800 C B//tape 900 C B//tape

Hirr [

T]

Temperature [K]

IL of unsintered, 800ºC and 900°C tapes

10 20 30 40

0

2x10-5

4x10-5

6x10-5

Btape

R [

]

T [K]

0T 0.5T 1T 2T 3T 4T 5T 6T 8T 9T

10 20 30 40

0

2x10-5

4x10-5

6x10-5

8x10-5

B//tape

R [

]

T [K]

0T 0.5T 1T 2T 3T 4T 5T 6T 7T 8T 9T

Irreversibility lines from magnetoresistance measurements

At 800°C the initial Tc is recoveredPinning centers induced by cold working are partly removed at 900 °C

Neutron diffraction experimentsILL D1A facility – Grenoble (F)

MgB2MgO

In-situ analysis of Ni-sheathed tapes confirms:a) Lattice strain increases during cold workingb) Lattice strain progressively relaxes during the heat treatment

880°C

900°C

920°C

940°C

Reaction layer between Ni and MgB2

900°C

Ni

MgB2

MgB2Ni2.5

920°C

Ni

MgB2

MgB2Ni2.5

The reaction layer increases with increasing temperature

880 890 900 910 920 930 9400.0

5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

3.0x105

f.f. 10% f.f. 17%

5 K

j c at 1

Te

sla

Heat treatment temperature [°C]

0.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

f.f. 10% f.f. 17%

Magnetic jc

5 K

j c at 4

Te

sla

Optimal conditions for jc are different for low & high fields

Jc after heat treatment mainly results from the compromise between:

- MgB2 packing density

- residual lattice strain

- increase of Tc

- reaction layer with Ni sheath

Critical strain

4 5 6 7 8 9 10

10

100

NIST measurementsOctober 2000

4.2 KH//tape

tape section 0.9 mm2

fill factor 16%

Cri

tica

l cu

rre

nt [

A]

Magnetic field (T)

Transport jc and critical strain measurements (NIST)

0.00 0.05 0.10 0.15 0.20 0.25 0.300

10

20

30

40

50

60

4.2K, 6T

Crit

ical

cur

rent

(A

)

Applied strain, (%)

Tape dimensions: 3.5 mm x 0.315 mmFilling factor 16%

Superconducting cross section 0.176 mm2

0.00 0.05 0.10 0.15 0.20 0.25 0.300

10

20

30

40

50

60

4.2K, 6T

n-va

lue

Applied strain, (%)

First test of an MgB2 pancake

3.0 3.5 4.0 4.5 5.00

50

100

150

200

250

4.2 KC

ritic

al C

urr

en

t [A

]

Magnetic Field [T]

MgB2 pancake

BSCCO pancake

MgB2

BSCCO

Pancake coil

Ex-situ Nickel-sheathed MgB2 tapes have been fabricated by the Powder-In-Tube method in lengths up to 25 meters

Critical current densities in excess of 1000 A/mm2 have been achieved at 4.2 K and magnetic fields up to 3 Tesla

A direct correlation has been observed between MgB2 lattice strain induced by cold deformation and the irreversibility field

A relevant reaction layer between MgB2 and Nickel sheath has been observed

Conclusions