World Record, High Magnetic Fields from Bulk Superconductors

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/