World Record, High Magnetic Fields from Bulk Superconductors

Bulk Superconductivity Group, Department of Engineering

Dr Mark Ainslie Royal Academy of Engineering (UK) Research Fellow

CCD6-2015: The 6th Cryogenic Cluster Day, 23 September 2015

Bulk Superconductors

• Conventional magnets (NdFeB, SmCo) limited by material properties

• Magnetisation independent of sample volume

• Bulk HTS trap magnetic flux via macroscopic electrical currents

• Magnetisation increases with sample volume

• Trapped field given by

Btrap = k µ0 Jc R

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A large, single grain bulk superconductor

Bulk Superconductors

• Conventional magnets (NdFeB, SmCo) limited by material properties

• Magnetisation independent of sample volume

• Bulk HTS trap magnetic flux via macroscopic electrical currents

• Magnetisation increases with sample volume

• Trapped field given by

Btrap = k µ0 Jc R

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Typical trapped magnetic field profile of a

bulk superconductor

Bulk Superconductors

B S G Slide courtesy of Akiyasu Yamamoto, Tokyo University of Agriculture and Technology

Bulk Superconductors

• Btrap = k µ0 Jc R

• Candidate materials must be able to:

• Pin magnetic flux effectively

• Carry large current density, Jc, over large length scales

• Be insensitive to application of large magnetic fields, Jc(B)

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Example field dependence of critical current density, Jc(B), for bulk YBCO

Types of Bulk Superconductors

• (RE)BaCuO = Rare earth-based high temperature superconductors

• RE = Gd, Sm, Nd or Y

• Tc ≈ 90–94 K

• MgB2

• Discovered in 2001, Tc ≈ 39 K

• Cheap, light-weight, easy to manufacture

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Processing Bulk HTS

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Sintered YBCO 2 µm

• Simple sintering of (RE)BCO powder in bulks does not result in very good materials

• Early attempts at sintered bulk materials were disappointing • Low Jc

• Granularity is a problem & grain boundaries = ‘weak-links’

• Microcracking

Processing Bulk HTS

• Grain boundaries can be avoided using a seeded peritectic growth process

• All (RE)BCO melt processes are based on the following peritectic reaction that occurs around 1015 °C:

• 2(RE)Ba2Cu3O7-δ (RE)2BaCuO5 + (Ba3Cu5O8) (123) (211/422) Liquid

• Top Seeded Melt Growth (TSMG):

• Structurally compatible* seed with higher melting point usually used to seed large grain growth in top seeded melt growth (TSMG) initialises growth & controls orientation

• Tp(SmBCO) ~ 1070 °C, Tp(YBCO) ~ 1015 °C * same lattice structure

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Processing Bulk HTS – Top Seeded Melt Growth

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SmBCO seed

Pressed pellet

Tm

Tg1 0.2-0.8°C/h

1. Mixing 2. Pressing + Seeding 3. Melt-processing

Recommended composition: 70wt%Y-123 + 30wt%Y-211 + 0.1wt%Pt

Pressure applied: 2T – 5T for a pellet 20 – 40 mm in diameter

Tm = 1040°C Tg1 = 1000°C Tg2 = 960°C Cooling rate 0.4°C/min for a sample 20 mm in diameter

Tg2

Precursor powders of desired composition are mixed together using a mortar and a pestle. 2 hours are used for a mix of 200g.

The mixed precursor is weighed and poured into a die of desired dimensions. The powder inside the die is pressed using a press. A SmBCO seed is then placed on top of the surface of the pellet.

The pressed pellet with seed is then put into a furnace. The heating profile is as follows:

Bulk HTS – World Record Trapped Field

• Trapped field measurements tell us the potential of a sample as a strong, permanent magnet

• Demonstrated trapped fields over 17 T

• 17.24 T at 29 K 2 x 26.5 mm YBCO Tomita, Murakami Nature 2003

• 17.6 T at 26 K 2 x 25 mm GdBCO Durrell, Dennis, Jaroszynski, Ainslie et al. Supercond. Sci. Technol. 2014

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Stack of 2 x GdBCO samples that achieved 17.6 T at 26 K

Bulk HTS – World Record Trapped Field

• High JC is important, but not sufficient

• At ~17 T, internal stresses are ~90 MPa

• Stress scales as the square of field (B2)

• Leads to practical maximum trapped field of 7-9T in unreinforced samples as tensile strength can be < 10 MPa

• Very variable performance – worst crack problem

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Trapped field of a cracked YBCO sample

Bulk HTS – World Record Trapped Field

• What limits performance? • Common failure mode seems to be a

simple crack across sample

• How to overcome this?

• Add 15 wt% AgO – converted to Ag during processing, filling voids/cracks

• Can improve fracture strength by an order of magnitude (a few MPa 10 MPa)

• Shrink-fit stainless steel onto sample, achieves ~250 MPa interface pressure

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GdBCO sample

Stainless steel reinforcement ring

Bulk HTS – World Record Trapped Field

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-10 0 10

0

5

10

15

20 26 K (end of ramp) 26 K (after 160 min) 35 K 45 K 50 K

Fiel

d (T

)

Distance (mm)

(b)

Trapped field (17.6 T) after application of a 17.8 T background field

Bulk HTS at 77 K

• Significant potential at 77 K • Jc = up to 5 x 104 A/cm2 at 1 T

• Btrap up to 1 ~ 1.5 T for YBCO

• Btrap > 2 T for (RE)BCO

• Record trapped field = 3 T at 77 K

• 1 x 65 mm GdBCO

• Nariki, Sakai, Murakami Supercond. Sci. Technol. 2005

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Typical trapped field profile of GdBCO at 77 K

Bulk HTS Applications

• Two main classes:

• High field “permanent” magnet

• Self-stabilised levitation

• Challenges to practical applications:

• Cooling

• Charging Pulse charging system

Cryotel Cryocooler for Bulk Applications

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Portable High Magnetic Fields

• Modern cryocoolers allow bulks to be charged in a solenoid and then moved around in a portable system.

• Hitachi have demonstrated such a system with an eye on medical applications

Saho et al. Physica C 469 15-20 (2009) 1286-1289

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Thank you for listening

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Contact email: mark.ainslie@eng.cam.ac.uk Website: http://www.eng.cam.ac.uk/~mda36/