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