Superconductors for magnets II
René Flükiger
CERN
TE-MSC
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Outline
I. Nb3Sn wires in magnets: mechanical stress effects
IA. Case study: NbTi and Nb3Sn wires
II. MgB2 wires for high current leads in LHC Upgrade
III. Other materials for round s.c. wires for future high
field accelerators
IIIA. Bi-2212 wires (HTS)
IIIB. Pnictides (?)
IV. YBaCuO tapes (Coated conductor HTS)
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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I. Wires in magnets:
mechanical stress effects
Question: What is the effect of the strong Lorentz forces at
high fields in large devices?
The 3D stress situation in a wire can be subdivided in two
cases:
Effect of uniaxial stress and
Effect of compressive stresses
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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A. Presence of axial strain:
Effects on Jc
• The effect of applied stress on Jc is mainly correlated to changes in the
phonon spectrum (Markiewicz et al.).
• Nb3Sn has the highest electronic densitiy of states N(EF) among all A15
type compounds, but this is thought to be of minor importance for
understanding the effects of strain.
• Jc is sensitive to hydrostatical pressure components, but even more to
nonhydrostatical ones.
* Various measuring devices:
Linear strain rig (J. Ekin)
Pacman (Univ. Twente)
Walters spiral (Univ. Geneva)
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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The mechanical precompression
in Nb3Sn wires
.004
1 – c/a
.003
a, c: tetragonal
lattice parameters
.002
.001
Degree of the elastic tetragonal
deformation of the A15 phase in
the wire when cooling
Thermal contraction:
0
100
300
500
T (K)
700
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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3D Stress Distribution in a Nb3Sn wire
Longitudial Compression (//) Wire Axis); Tetragonal distortion
Filament
orientation
c - axis
Neutron diffraction analysis
(ILL Grenoble)
473K
290K
10K
a - axis
-a
5.30
5.28
5.26
R.F., W. Schauer, W. Specking,
Adv. Cryo. Eng. 28(1982)361
5.26
5.28
5.30 å
-c
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The linear strain rig (KIT Karlsruhe)
Wire soldering
wire
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The Pacman strain device (Univ. Twente)
Pacman
wire
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Modified Walters Spiral (WASP)
for Jc(e) of Wires and Tapes
Rotation of the central axis of the spiral
uniaxial force on the wire
Wire
Max. current: 1’000 A
Wire length ≤ 0.8 m
Ic criterion: 0.01V/cm
Applied field: ≤ 21T
Tape
Modified Walters Spiral (WASP) for round wires and tapes
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Ic/Ico vs. uniaxial strain e
for various Nb3Sn wires
Stronger
precompression
in the bronze route
wire :
Effect
increases
with field
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
This is due to the
stronger Cu-Sn
matrix, due to a
higher Sn content
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Stronger Criterion (0.01 mV/cm) leads to early
detection of Nanocracks
Furukawa bronze
Nb3Sn wire for ITER,
dia. 0.8 mm; 4.2K/13T
Ic does not depend
on strain criterion up
to 0.6%/0.7%
0.1 V/cm
1 V/cm
After releasing the
strain, Ic depends
strongly on the
criterion
The detection of the irreversible strain limit (begin of nanocracks) depends on
the Ic criterion: advantage for the Walters spiral, with the longest wire length
D. Uglietti, V. Abächerli, R.Flükiger, 2004
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Example: Tensile stress on Coated Conductor
Tapes (AMSC 344 type)
Tape length: 0.8 m
em = 0.53 %
em = 0.53 %
4.2 K
Uniaxial stress effects are also effective in HTS superconductors!
D. Uglietti, B. Seeber, R. Flükiger, SuST
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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B. Transverse compressive stresses
Transverse stresses are particularly
important in the cables of large magnets,
due to the large Lorentz forces:
ITER TF model coil: cable
The knowledge about the effect of transverse
compressive stresses is decisive for the safe
magnet operation.
Examples:
* High field magnets (B > 20 T)
* Large magnets, for Fusion magnets
(Tokamak)
* Accelerators (LHC, LHC Upgrade)
Measuring devices:
* Pacman, Univ. Twente
* Inverse Walters Spiral (Univ. Geneva)
ø 40 mm, 1.5 mm thick steel
Rated current: 70 kA/11.8 T/4,6 K
1028 strands: Nb3Sn + 1/3 Cu
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The ITER Machine
(Tokamak)
LTS conductors
Fusion Power: 500 MW
Plasma Current: 15 MA
Plasma Volume: 837 m3
Torus Radius:
6.2 m
Plasma Radius: 2.0 m
Energy Amplification: Q=10
+ 13,5 T  - 12 T
12 T
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Inverse Walters spiral (Univ. Geneva)
Inverse spiral for application of compressive stresses
Stainless
steel anvils
Nb3Sn wire
Fixed part
Moving part
wire
Voltage taps
Inverse Walters spiral (Univ. Geneva): F = 5kN, I = 1000 A, H = 21 T
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Jc vs. transverse compressive stress st
Tensile
Nb3Sn
wire
stress sa
Effect of transverse
stresses (from 2 sides):
much stronger than that
of uniaxial stress:
st(Icm) << sa(Icm)
Reversibility:
st : only up to 20 MPa
sa: up to 150 MPa
Compressive
stress st
0
100
200
300
Stress s (MPa)
400
J. Ekin (1986); W. Specking, R.Flükiger, (1987)
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Effect of compressive stresses
Recently, Mondonico et al. have
applied compressive stresses on
Nb3Sn wires by pressing
simultaneously from 4 sides:
quasi - hydrostatical pressure.
G. Mondonico, PhD thesis ,Geneva, February 2013
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Compressive stresses in
accelerator magnets
Effects of
compressive
stresses
4 - wall
4 - wall + epoxy
(accelerators magnet)
G. Mondonico, PhD thesis 2012
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Compressive stresses in
various magnets
The effect of compressive stress was markedly lower than for the
above data (pressing from 2 sides).
ITER:
The 4 - side compression is now closer to the situation in ITER.
However, the wires in ITER cables are submitted to an even more
complicated mechanical load, comprising bending stresses, too.
Accelerator magnets:
The wires in accelerator magnets are impregnated, i.e. with epoxy.
It was found that the epoxy impregnation considerably reduces the
effect of transverse stresses.
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Conclusions : Nb3Sn wires
• Low and intermediate fields: Jc determined by flux pinning (grain size)
• At high fields, Jc is determined by the value of Bc2
• Industrial round wires for magnets up to 23.5 T (1 GHz): Nb3Sn
• The amount of Nb3Sn wire in a magnet increases strongly with the
produced field: at 20T, 5 times more Nb3Sn than for 12 T.
• Bronze route wires: best suited for «persistent mode» operation of NMR
magnets, in spite of their lower Jc value with respect to Internal Sn wires
• Internal Sn (RRP) and Powder-in-Tube (PIT) wires satisfy the
conditions for LHC Upgrade accelerator magnets: Jc = 1’500 A/mm2 at
4.2K/15T.
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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A case study
We have now finished the presentation of NbTi and Nb3Sn wires
and their properties. Before to move to the other superconductors,
we will briefly present a few subjects and data on NbTi and Nb3Sn
wires, in view of the « case study ».
The « Case study » will be performed later during this school.
(Examples, e.g. calculation of the magnet load line)
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Supplement for the case study
Cu/Superconductor ratio
In a strand, the copper to superconductor ratio is the ratio
between the area of Cu and the area of non-Cu material
Typical values
Note: strand is another
term for wire; both are used
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Supplement to the case study
Strand diameters
Typical values of
strand diameters
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Case study 1 solution
Margins (Summers formula)
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Case study 1 solution
Margins (Bottura’s formula)
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The MgB2 system
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II. MgB2 wires
MgB2 phase: Hexagonal lattice: a = 0.30834 nm, c = 0.35213 nm
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At.% Boron
MgB7
MgB2
MgB4
The binary Mg-B Phase Diagram
B
Binary Alloys Phase Diagrams, 2nd Edition, Ed. T. Massalski (A.S.M.International, 1990)
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Properties of MgB2
MgB2: behaves more like a LTS, but is anisotropic
Birr (10%Rn)
Bc2 (T)
Bc2
(T)
Bc2
Birr (Jc = 10A/cm2)
Irreversibility field Birr
D. Larbalestier et al., 2002
There is always
Birr < Bc2
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Determination of Bc2// and Bc2 in MgB2 wires
Bc2
Bc2//
Bc2
Birr
B//
Birr: Irreversibility field
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Upper critical field of MgB2: films and wires
Films:
60
50
Bulk
Field (T)
40
Bulk, wires: Bc2 = 30 - 40 T
Thin
MgB
films
2
MgB2
The reason for this
difference is still
unknown
30
20
NbTi
10
0
0
Bc2 = 60 T
Nb3Sn
10
20
30
40
Temperature (K)
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Possible applications of MgB2
* Level measurement of Liquid H2 containers
* Hydrogen cooled high current leads at T ≈ 20K (Russia)
* LINK project (CERN): 13’000A current leads at ≈10K
(> 10’000 km of MgB2 wire): under investigation
(talk at CAS, last day: Amalia Ballarino)
* Ignitor (fusion experiment under construction
(Russia/Italy)
* Wind generators ? First device under study
* Poloidal field coils for fusion reactors? The question is
discussed
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Preparation of MgB2 tapes
Powdermetallurgical approach
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Known MgB2 wire configurations
Ex situ
In situ
Tsukuba laboratory, Japan
IMD
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Limitation of Jc in MgB2 due to anisotropy
MgB2 wires
Strong anisotropy
B // surface: Jc
comparable
to that of Nb3Sn
B p surface: Jc much
smaller than Nb3Sn.
It is the limiting factor.
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Jc of densified MgB2 wires after densification
deformation
x8
x5
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Densification machine for long s.c. wires
Densification:
can be applied to any
kind of powder
metallurgical wire
process
Possible application of
densification:
Bi-2212, pnictides,....
Prototype machine for the densification of long wires;
(University of Geneva)
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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MgB2 wire: highest Jc at high fields
Wire B3 (highest value): Jc (eng.) = 1.67 x 104 A/cm2 at 10 T
Susner et al, 2012
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Conclusion: The system MgB2
Advantages:
Abundant constituents Mg and B
No chemical toxicity
Low cost material (comparable to NbTi)
Applicable at 4.2 ≤ T ≤ 25K
Mechanically stable
Magnets possible: up to 5.5 T at 20K
up to 2.5 T at 25K
Persistent mode operation up to 25K
Disadvantages: At 4.2K, only applicable up to ~ 11 T
Lower pinning force compared to Nb3Sn at 4.2K
(average) requires larger magnets for the same
field
higher costs!
Thermal stability: sufficient, but less stable than
NbTi or Nb3Sn
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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IIIA: The system Bi-2212
Bi-2212 is the only HTS superconductor allowing the
fabrication of round wires.
Already performed:
Fully superconducting system: 22.5 T magnet at 4.2K (20T+2.5T insert)
Combined with polyhelix:
> 30 T
Future:
Accelerator magnets at B >> 15T ??
Main research efforts: * D. Larbalestier et al., Florida State University
* Oxford Instruments (OST)
Example of a round Bi-2212 wires (OST)
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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III. Other materials for round superconducting
wires for future high field accelerators
The system Bi-2212
15 T
Je: overall critical current densities
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Comparison between
various superconductors
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Bi-2212: Problem: large bubbles form
on melting and holding at Tmax
Bi-2212
reaction
scheme
T
t
Malagoli et al, SuST, 24, 075016 (2011), Kametani et al, SuST 24, 075009 (2011),
Jiang et al. SuST 24 082001 (2011), Scheuerlein eta al, SuST 24, 115004 (2011).
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Solution of the bubble problem: enhancement
of Jc by compaction and slow heating
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Conclusion: Round Bi-2212 wires
Advantages:
* It is the only HTS materials available in round wires
* Multifilamentary configuration: OK
* Excellent thermal stability
* At 4.2K, very high Jc values up to fields > 25 T
* Jc values: close to level required by LHC Upgrade
Improvements expected.
Disadvantages: * Important costs due to processing and Ag sheath
* Bubble problem: is in principle solved, but
industrial tests on long wire lengths are still going
on.
* Poor mechanical stability: no good solution yet for
improving the mechanical reinforcement
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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III B. Jc in Pnictide wires and tapes
Pnictides: compounds based on FeAs or FeSe
Examples: Wires based on (Ba,K)Fe2As2 and (Sr,K)Fe2As2,
Commonly called “122” compounds
Tc = 35K, Bc2 > 50 T: promising for future high field applications
Very recent data on wires and tapes:
• J. Weiss, M. Hannion, E. Hellstrom, J. Jiang, F. Kametani, D. Larbalestier, A.
Polyanskii, and C. Tarantini, FSU, 2012
• Z. Gao, L. Wang, C. Yao, Y. Qi, C. Wang, X. Zhang, C. Wang, Y.W. Ma, ArXiv:1110.
5784,2012;
• YW. Ma, ICSM2012
• I. Pallecchi, M. Tropeano, G. Lamura, M. Pani, M. Palombo, A. Palenzona, M.
Putti, to be published in Physica C
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SEM and TEM on 122 pnictides
TEM shows K-doped 122 has small grains, contains only little
amounts of nonsuperconducting material, and has many clean
grain boundaries.
F. Kametani et al., FSU, ASC 2012
J. Weiss, M. Hannion, E. Hellstrom, J. Jiang, F. Kametani, D. Larbalestier, A. Polyanskii, C. Tarantini,
Arkhiv, 2012
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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New synthesis method for 122 wires
Highest critical currents in 122 wires known so far (2013)
J. Weiss , E. Hellstrom et al.,2012
Z. Gao et al., ArXiv 1110.5784
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Conclusion: Pnictides
Advantages
* Abundant basis materials
* Low costs of constituents and wires
* Possibility to fabricate round wires
* Lower anisotropy than in HTS: higher current densities
are possible, even in polycrystalline wires.
* Wire fabrication methods: close to those of MgB2
* Very high values of Bc2 and also Birr: there is hope that
pnictides will produce magnets with high magnetic
fields (30 T and more)
Disadvantages * Toxicity of As and Se
* Metallurgic homogeneity is difficult to achieve
* Thermal stability: no data yet
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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IV. HTS Coated Conductors
The systems Y-123 or R.E.-123 («Coated Conductors»)
YBa2Cu3O7 - d : Superconductor of the
Future?
Levitating YBaCuO sample at 77K
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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HTS Coated Conductors
The layered oxide structure causes a strong anisotropy in Bc2, Jc,… .
This induces a layered conductor configuration: Tapes
(also called «Coated Conductors»)
Typical shaping of
a HTS Coated
Conductor with the
structure
YBa2Cu3O7.
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Two main deposition techniques
(many variations used by various manufaturers)
Rolling Assisted,
biaxially textured substrate
RABITS
IonBeam Assisted
Deposition
IBAD
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Typical «Coated Conductor» tape architectures
REBaCuO tape of AMSC
Other manufacturers: Sunam, S. Corea;
Sumitomo, Japan; BEST, Germany,…..
REBaCuO tape Fujikura, Japan
of SuperPower
M. Rupich et al., AMSC, 2008
Note: only 1% of the total cross
section is superconducting!
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
Selvamanickam et al, 2008
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Requirements to a REBaCuO tape
Current density * Carry optimized current in REBaCuO (dopants)
Mechanical
* Substrate strong enough at high temperature
to stand the formation of REBaCuO
* Tape as a whole strong and flexible enough
to be wound into cable and coils at 300 K
* Tape must withstand longitudinal and
transverse stresses during operation
Electrical stability * Carry excess current in Ag layer and in
in Al, Cu,……. outer layers
Thermal stability * Enable heat transfer to the coolant
AC losses
* Modify architecture to minimize AC losses
(Roebel, striations)
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
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Towards higher Jcw values: thicker HTS layer
Unit for describing the
critical current density
in Coated Conductors:
[A/cm]
Jcw = Jc/cm2 x 1 cm
= A/cm
is normalized for a tape
width of 10 mm
0
Flükiger, CAS, Erice (I) 25 April - 4 May 2013
0.5
1
1.5
2
2.5
HTS film thickness (m)
3
3
55
EXAMPLE: Ic-B-T property for 600A class C.C.
Measured by Prof. Kiss group of Kyushu University, in collaboration
with Florida State Univ. & Tohoku Univ.
2.5 m-thick-GdBCO/IBAD CC by Fujikura Co.
1,000
414A/cm
@31T
4K
20K
100
c
I [A/cm-width]
~1,120A/cm @5T
Field
50K
Current
65K
B//c
77K
10
0
5
10
15
20
25
30
35
magnetic field [T]
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Highest Jcw known so far: > 1’000 A/cm !
Enhanced layer
thickness:
Fujikura reports
6 mm thick layer with
1’040 A/cm-w
(Deposition time not reported)
M. Igarishi et al., EUCAS 2009 (Fujikura)
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Reduction of AC losses in tapes and cables
* Striations:
on tapes (6 - 10 by “channels”, by laser)
* Roebel technique:
for cables
* Roebel + striations:
for cables
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What are the tasks to be accomplished
by Coated Conductors ?
•
•
•
•
•
•
Higher homogeneity of Jc over whole tape length
Thicker HTS layer Ic values
Reproducible production of > 1 km lengths
Enhanced pinning by nano-additives
Reduced anisotropy by nano-additives
Reduced costs
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Thank you for your attention!
For additional references or details:
Rene.flukiger@cern.ch
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