Superconducting Joints for Magnet Applications

Susie Speller Greg Brittles, Tayebeh Mousavi, Canan Aksoy, Chris Grovenor

Centre for Applied Superconductivity

University of Oxford

Superconducting Joints

Superconducting magnets for MRI machines require ultra-low resistance joints (<10-12W) to enable them to operate in persistent mode. Exceptional field stability →decay time ~10,000 years

Superconducting magnets require at least one joint to create a closed coil. The resistance of the joints influences the field decay of the entire magnet.

𝐵 𝑡 = 𝐵𝑜 exp −𝑡

𝜏,

𝜏 =𝐿𝐶𝑜𝑖𝑙𝑅𝐽𝑜𝑖𝑛𝑡

Early HTS joints in Oxford

“Friday afternoon” experiment carried out by Harry Jones and Chris Grovenor in 1997. 14 turn coil wound from dip-coated Bi-2212/Ag tape made in the Materials Department. Joints were designed by “trial and error” using the bridge method followed by melt processing .

Bi-2212 coil

Ryan, Jones, Morgan and Grovenor, Proc. of 15th international conference on Magnet Technology, Beijing 1997

Early HTS joints in Oxford

• Joints were tested at 77K, 64.5K and 4.2K • Decay measurements gave resistance values of ~3x10-12W at 4.2K • Persistent current operation achieved, even at 77K.

77K

NbTi Joints

• Magnet industry typically uses Pb-Bi solder. • Health and safety legislation is likely to restrict the use of

Pb-based solders for the magnet industry in the near future.

• New materials / jointing methods need to be developed.

Pb-free solder Spot welding Cold-pressing

Literature search

Solder TC (K) BC2 (T)

Pb60Bi40 8.4 1.77

Sn50In50 6.5 0.64

Sn57Bi43 4.2 -

Sn66Cd34 4.8 -

Sn85Zn15 4.4 -

Sn95Sb5 3.8 0.04

Sn43Bi57 2.3 0.04

Sb29.6Ti30.4 4.3 -

In78Bi22 4.2 -

26Sn-54Bi-20Cd 3.7 0.06

28Bi-19Cd-35In 5.9 0.21

95.5Sn-3.5Ag-1 Cd 3.7 0.02

In the 1960s a wide range of low melting point alloys were investigated for their solder properties. Pb-Bi joints routinely carry 1000A at 4.2K and 1T. Sn-In is the most promising Pb-free binary system. Can we improve the properties of Sn-In?

Brittles et al, Supercond. Sci. Technol. 28 (2015) 093001

In-Sn binary system

𝜷 𝜸

Tc=6.5 K Tc= 4.7 K 𝜷 + 𝜸

Eutectic temperature: 120⁰C

B1 B2 B3 B4 B5

Chuang et al, Phys Rev C, 414, 175, 2002

In-Sn binary system

β phase γ phase

25 μm 25 μm 25 μm

B4 B5

25 μm

B2

25 μm

B3 B1

T (K)

B2 B2

Increasing Sn content in alloy

In-Sn binary system

Quenched in liquid nitrogen

T c(K

)

Solute content (Sn) in β-phase

BC

2 (

T) a

t 4

.2K

Increasing Sn content in b phase

Increasing the solute content in the b phase improves the superconducting properties

Ageing effects

25 μm 25 μm

Quenched in Lq. N2

25 μm

Aged 1 week

Sample Air-cooled Quenched Aged 1 week (room temp)

Tc (K) 5.6

6.7 6.1

Bc2 (T) at 4.2 K 0.04

0.15

0.08

JC (107A/m2) at T=4.2 K, B=0.03T

3.1

12.2

8.5

Air-cooled

In-Sn-Bi system

P. Villars, A. Prince, H. Okamoto, Handbook of ternary alloy phase diagrams, ASM International, 1995.

Four different alloy compositions were selected to engineer different microstructures.

In-Sn-Bi system

In-rich b

Sn-rich g

BiIn2

BiIn

Solder Properties

Magnetic Field (T)

J C (

10

6A

/m2)

PbBi

T2

B2 T4

T3 at 4.2 K

Highest JC in binary alloys

Highest JC in ternary alloys

Spot-welded Joints

Spot-welded joints were manufactured by etching in HNO3 to remove the Cu-matrix, braiding the filaments from the two wires and spot- welding between Cu plates.

Spot-welded Joints

40µm Nb-42wt%Ti

Nb-50wt%Ti

2.5µm 10 µm

2.5µm

Cold-pressed Joints

Joint manufacture • Cu matrix removed by etching in HNO3. • NbTi filaments inserted into Nb crimp . • Cold-pressed at pressures up to ~600MPa.

Oxide layer and residual Cu found at interface between NbTi filaments → No metallurgical bond

Cold-pressed Joints

Improved interface properties achieved using standard tinning method to prevent filament oxidation. Filaments coated in solder were twisted, inserted into Nb crimp and cold-pressed. → Good metallurgical interface between NbTi filaments.

Testing Joints

Simple method developed in-house for rapid testing of joint resistance and Ic(B,T) using a commercial SQUID MPMS instrument.

Brittles GD et al 2014, Rapid characterisation of persistent current joints by SQUID

magnetometry, Supercond. Sci. Technol. 27122002

Joint Properties

8K

Joint Resistance

Resistance (R(I,B,T)) was measured from the time decay of the moment.

Resistances are within the persistent mode regime and grow sharply as Ic is approached.

Summary

• Ultra-low resistance joints are essential components of persistent mode magnets.

• New lead-free joint technologies are needed to replace PbBi solder in LTS magnets.

• Research programme underway investigating new superconducting solder alloys.

• Spot-welding can be used to form good metallurgical joints, but the carefully engineered microstructure of NbTi wire is ruined by local melting.

• Cold-pressing technologies are promising, provided oxidation of the filaments is prevented.

Acknowledgements

Department of Materials Chris Grovenor Tayebeh Mousavi Greg Brittles Canan Aksoy Siemens Magnet Technology M’hamed Lakrimi Adrian Thomas STFC Tom Bradshaw Victoria Bayliss