trapped field superconducting magnets
TRANSCRIPT
Trapped Field Magnets Using Thin Film
Superconductor TapesKavita Selva
Clear Lake High SchoolHouston, TX, USA
22
Background InformationNeed to drastically reduce rare-earth content in magnets Permanent magnets are used widely in a variety of application
including electric motors in vehicles and wind turbine generators. Rare earth materials such as neodymium and dysprosium are used in these magnets. About 3600 kg of neodymium is used in a 6 MW wind turbine! Recently, there has been a world-wide supply problem with rare-earth supply. Therefore, there is great incentive to develop strong magnets with no or miniscule amount of rare-earth materials.
“Critical Materials Strategy”, US Department of Energy, 2011 http://energy.gov/sites/prod/files/DOE_CMS2011_FINAL_Full.pdf
6 MW Wind Turbine using Permanent
Magnets“Permanent Magnet GeneratorsFor Wind Turbines: Status and Outlook”, Siemens 2014
3.5 MW Permanent Magnet Wind
Generatorhttp://www.terramagnetica.com/
33
Superconductors as trapped-field magnets Rare earth-Barium-Copper-Oxide (REBCO) superconductors are
superconducting above the boiling temperature of liquid nitrogen (77 K). They have zero resistance to the flow of current i.e. very high critical current density
Above a certain magnetic field, magnetic lines of force will penetrate a superconductor. As long as the superconductor is kept cold, the magnetic lines will be trapped in it. Now the superconductor becomes a magnet!
Superconductor suspended below magnet because of magnetic flux trapped in the superconductor(http://www.imagesco.com/articles/supercond/07.html)
Magnetic flux that penetrated into a superconductor pinned at nanoscale defects
superconductor
Magnetic field
Defect pinning flux line
(K. Matsumoto and Mele, Superconductor Science & Technology, 23, 014001 (2009))
44
Thin film superconductor tapes Superconductor tapes are made by coating a thin film of
superconductor on a metal tape. Only about 2% of the tape contains the superconductor and hence a negligible amount of rare-earth.
While the amount of superconductor in the tape is small, its critical current density is very high.
If large magnetic fields can be trapped within these tapes, it would lead to a new class of magnets that are near rare-earth free!
The objective of this work is to determine the factors that result in the highest trapped magnetic field and the least decay in the field with time
Permanent magnet
Bulk superconductor puck (15 mm thick)
Thin film superconductor
tapes (each 0.055 mm thick)
100 nm
c-axis
BaZrO3 nanoscale defects in thin film superconductor tape
55
Purpose Questions How is the magnitude of magnetic field trapped by a stack of
superconductor tapes influenced by i) the stacking configuration, ii) nanoscale defect density in tapes, iii) number of layers of stacked tapes, iv) number density of the tapes and v) the operating temperature?
How is the time-dependent decay of magnetic field trapped by a stack of superconductor tapes influenced by the number of layers of stacked tapes?Hypotheses
• The magnitude of the trapped magnetic field will increase if i) a crisscross stacking of tapes is used ii) the nanoscale defect density in the tapes is increased iii) number of layers of stacked tapes is increased iv) the number density of tapes is increased v) the operating temperature is decreased
• If the number of layers of stacked tapes is increased, then the rate of decay of trapped magnetic field will remain unchanged.
66
Materials List RE-Ba-Cu-O thin film superconductor tapes
(12 mm wide, 0.055 mm with 0%, 7.5% and 15% Zr, 0.025 mm thick with 7.5% Zr)
Trapped-field measurement system 3-axis linear motion table (Zaber Technologies T-
LSR300B) High Linearity Hall Probe Frame to mount Hall Probe to 3-axis linear
motion table Keithley 2400 Sourcemeter Cables for communication between motion table
and computer and among Hall probe, multimeter and computer
Sample holder with a volume of 36 mm × 36 mm × 15 mm deep to hold stack of superconductor tapes
1.5 Tesla Electromagnet Metal and Styrofoam cryogen containers Liquid nitrogen Critical current measurement system
Cryostat Sample test probe 1 Tesla Electromagnet High current power supply Keithley nanovoltmeter
z-axis
y-axis
x-axis
Frame to hold hall probe
Hall probe
77
Cryogen container
Sample holder
Procedure: Preparation of superconductor tape stack
Cut the superconductor tape into 35 mm long segments
Pack the tapes in the sample holder, three tapes across.
Pack the next layer of three tapes perpendicular to the first layer. Repeat the crisscross alternating sequence
Seal the sample holder and insert it in the metal cryogen container
88
Electromagnet
Pole pieces
Place the cryogen container between the pole pieces of the 1.5 Tesla electromagnetTurn the electromagnet to full power to reach 1.5 Tesla. Magnetic field will now penetrate into the superconductor tapes
Fill dewar with liquid nitrogen
Pour liquid nitrogen into cryogen container in magnet to cool the superconductor tapes to make them superconductingAfter 6 minutes of cooling the sample holder, turn magnet off. Magnetic field will now be trapped in the superconductor tapes.
Procedure: Trapping magnetic field in tape stack
99
Fill a styrofoam container with liquid nitrogen
Quickly transfer the sample holder from the metal cryogen container to the styrofoam container
Align the sample holder directly under the Hall probe
Procedure: Trapped field measurements on tape stack
Begin trapped field measurements over the entire tape stack area and as a function of time elapsed since turning off the electromagnet.
1010
Influence of tape stacking configurations on trapped field profiles
Crisscross stacking of tapes
Straight stacking of tapes
• Trapped field profile of crisscross stacked tapes shows nine peaks corresponding to locations of overlap of tapes and four valleys at the gaps between tapes
• Trapped field profile of straight stacked tapes show three flat-topped peaks and two deep valleys at the gap between tapes
1111
Crisscross configuration yields a more uniform trapped field profile
● A straight-arrangement of 74 layers shows a uniform field profile and higher trapped field values along the x-axis (direction of the tape) compared to crisscross arrangement.
● The straight-arrangement results in extremely non-uniform trapped field profile with sharp valleys along the y-axis. The crisscross arrangement results in very similar profiles along the x-axis and y-axis and the valleys are shallow.
12
100 nm 100 nm 100 nm
Microstructure of REBCO superconducting tapes with 0%, 7.5% and 15% zirconium addition. The density of nanoscale defects in the superconductor film is seen to increase with the increasing zirconium addition.
0% Zr 7.5% Zr
15% Zr
Investigation of nanoscale defect density on trapped field
1313
Influence of nanoscale defect density on trapped field
-21-18-15-12 -9 -6 -3 0 3 6 9 12 15 18 210.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16 0% Zr 7.5% Zr 15% Zr
Horizontal distance from center of stacked tapes (mm)
Trap
ped
Mag
netic
Fie
ld*
(Tes
la)
*at a distance of 3 mm from tape stack
While the nanoscale defect density is the maximum in the 15%Zr-added tapes, the 7.5% Zr added tapes exhibited the highest trapped field.
The trapped field is better correlated to critical current of the tapes at 77 K in a 1 T magnetic field
0% 5% 10% 15%0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0
20
40
60
80
100
120
140
Max trapped fieldAverage trapped fieldCritical current
Zirconium Content in Tape
Trap
ped
Mag
netic
Fie
ld (T
)
Criti
cal C
urre
nt (A
)
1414
Influence of number of layers of tapes in stack on trapped field
Trapped magnetic field increases linearly with number of layers in tape stack up to 236 layers
0 2 4 6 8 10 12 140.00
0.04
0.08
0.12
0.16
0.20
0.24
0.28 Tapes criss cross - Max FieldLinear (Tapes criss cross - Max Field)Tapes criss cross - Av-erage FieldLinear (Tapes criss cross - Average Field)
Total thickness of tape stack (mm)
Trap
ped
Mag
netic
Fie
ld*
(Tes
la)
* at a distance of 3 mm from tape surface
1515
Influence of number of layers of tapes in stack on trapped field
Logarithmic decay in trapped magnetic field with time in all tape stacks consistent with the phenomenon of thermally-activated flux creep
Time-dependent decay of trapped field decreases with increasing number of layers of tapes
tdU
kT
M
dMln
0
M. Murakami et al. “Flux Creep in YBa2Cu3O7 Crystals Jpn. J. Appl. Phys. 28, L1754 (1989)
M = magnetization at time ‘t’M0 = magnetization at t=0T = temperatureU = pinning potentialk = Boltzmann’s constant
0 20 40 60 80 1000.900
0.920
0.940
0.960
0.980
1.000
f(x) = − 0.0357704117331971 ln(x) + 1.05333332988444R² = 0.945244258915845
f(x) = − 0.028822826302205 ln(x) + 1.04449341512678R² = 0.968039505872638
f(x) = − 0.0182872272023306 ln(x) + 1.03401125907582R² = 0.981763689584658
236 layersLogarithmic (236 layers)120 layers
Time (minutes)
Trap
ped
Mag
netic
Fie
ld (t
) /
Tr
appe
d M
agne
tic F
ield
(t =
6 m
in)
1616
36 mm
12 mmCurrent density
Magnetic field profile in superconductor when an external field is applied.
Magnetic field profile in superconductor after the external field is removed.
Magnetic field profile (above) and current density distribution (below) in a single tape
Simulation of trapped field profiles using COMSOL
Odd layer magnetization
Even layer magnetization
Current density distribution in one layer of three tapes adjacent to each other
1717
Current density distribution in two layers of crisscross-arranged tapes.
Net current density direction from COMSOL model of two layers of crisscross-arranged tapes (because of symmetry, only ¼ of the stack is modeled)
Simulation of trapped field profiles using COMSOL
0 1 2 3 4 5 6 7 8 9 10 -
50,000
100,000
150,000
200,000
250,000
300,000
350,000
400,000
450,000
500,000
77 K 65 K
50 K 40 K
30 K
Magnetic Field (T)
Engi
neer
ing
Curr
ent D
ensi
ty
(A/c
m2)
Engineering current density (critical current/ cross sectional area of tape) in increasing magnetic field at different temperatures. This data was used in the COMSOL simulation to calculate the trapped magnetic field values
1818
COMSOL model to simulate trapped magnetic profile in crisscross-arranged stack of superconductor tapes; mesh used (because of symmetry, only ¼ of the stack is modeled); trapped field profile obtained
Simulation of trapped field profiles using COMSOL
1919
Trapped field from COMSOL model of a stack of 128 layers (because of symmetry, only ¼ of the stack is modeled)
Trapped field profile from experimental measurements of a stack of 128 layers of crisscross-arranged tapes
Good match between trapped fields from simulation and experiments
0 5 10 15 20 250
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Simulation - 12.98 mm thick stack
Experimental - 12.98 mm thick stack
Simulation - 19.25 mm thick stack
Distance from tape stack (mm)
Max
imum
trap
ped
field
(T)
77 K, 55 µm thick tape
Increasing the tape stack thickness from 12.98 to 19.25 mm does not lead to significantly higher trapped field. So, with 55 µm thick tapes, no benefit making the tape stack thicker than 12.98 mm
2020
0 5 10 15 200
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
30 K40 K50 K65 K77 K
Tape stack thickness (mm)
Max
imum
trap
ped
field
* (T
) Tape thickness = 55 µm
* 3 mm from stack surface
Influence of temperature on trapped magnetic field
The maximum trapped field values increase with decreasing temperature from 77 K to 30 K.
At all temperatures, the trapped field values increase with increasing number of layers of 55 µm thick tapes
Unlike the experimental data, the simulation data shows a non-linear (logarithmic) increase in trapped field values with increasing number of layers of tapes in the stack.
2121
Influence of number density (tape thickness) on trapped field
The maximum trapped field values increase with decreasing tape thickness from 55 µm to 20 µm i.e. with increasing number density of tapes in the stack.
In tapes of all thickness, the maximum trapped field values increase with increasing number of layers of tapes.
0 5 10 15 200
0.5
1
1.5
2
2.5
3
3.5
0.055mm
0.035mm
0.02mm
Tape stack thickness (mm)
Max
imum
trap
ped
field
* (T
)30 K
* 3 mm from stack surface
2222
Unexpected sharp increase in trapped field at 77 K at thickness below 35 µm
A sharp rise in trapped field beyond a certain number of layers. This number of layers at which this sharp transition occurs reduces with decreasing tape thickness.
The sharp transition to high trapped field values at 77 K occurs around a tape thickness of 30 µm in a 19.25 mm thick tape stack. Maximum trapped field values exceed 1 T in stacks with 25 µm thick tapes.
0 5 10 15 20 250
0.2
0.4
0.6
0.8
1
1.2
1.4
0.02mm
0.25mm
0.028mm
0.03mm
0.035mm
0.055mm
Tape stack thickness (mm)
Max
imum
trap
ped
field
* (T
) 77 K
* 3 mm from stack surface0 0.01 0.02 0.03 0.04 0.05 0.06
00.20.40.60.8
11.21.41.61.8
2
Maximum field
Average field
Tape thickness (mm)Tr
appe
d m
agne
tic fi
eld*
(T)
77 K, 19.25 mm tape stack* 3 mm from stack surface
2323
Much higher trapped fields can be obtained at 77 K with 20 µm thick tapes, reaching nearly 2 Tesla, 1 mm from the stack surface, which is well above the capability of permanent magnets.
0 5 10 15 20 250
0.5
1
1.5
2
2.5
Simulation - 0.055 mm tape
Simulation - 0.02 mm tape
Distance from tape stack (mm)
Max
imum
trap
ped
field
(T) 77 K, 12.98 mm thick tape stack
Influence of number density (tape thickness) on trapped field
2424
Experimental verification of simulation results on influence of number density
25 µm thick tape made by novel method to verify simulation results. Experimental data with 25 µm thick tapes shows 60% higher trapped field
compared to 55 µm thick tapes for the same number of layers in the stacks. The higher trapped field with thinner tapes is consistent with the simulation results.
The increase in trapped field with 25 µm thick tapes becomes steeper with shorter distance to the tape stack. This result is also consistent with simulation results.
20 40 60 800.00
0.04
0.08
0.12
0.1625 µm tapeLinear (25 µm tape)
Number of layers in tape stack
Max
Tra
pped
Mag
netic
Fie
ld*
(Tes
la)
* at a distance of 3 mm from tape surface
0 2 4 6 8 100.00
0.05
0.10
0.15
0.20
0.25 25 layers, 55 µm tape50 layers, 55 µm tape74 layers, 55 µm tape25 layers, 25 µm tape50 layers, 25 µm tape74 layers, 25 µm tape
Distance from tape stack surface (mm)M
ax T
rapp
ed M
agne
tic F
ield
(Tes
la)
2525
Applications of Results
The amount of rare-earth material in magnets can be drastically reduced by using a stack of thin film superconductor tapes. Such superconductor tape magnets can have a very favorable impact in applications where rare-earths are abundantly used such as wind generators and electric motors such as those in electric and hybrid cars.
The finding in this project that the trapped magnetic field increases with increasing number density (reducing tape thickness) and increases with decreasing temperature can enable superconducting magnets with even higher trapped magnetic fields. Especially, tapes thinner less than 0.03 mm could yield sharp increase in trapped field values, to levels of 1 - 2 Tesla at 77 K. So, strong trapped-field magnets can be made with fewer thin tapes, which will then greatly reduce the cost of these magnets.
Also, the finding that tape stacks with more number of layers will exhibit a reduced rate of decay of trapped magnetic field can be greatly beneficial for applications.
Amount of rare-earth material in the 25 meters of 0.055 mm thick superconductor tape used for 12.98 mm tape stack
0.25 g
Amount of rare-earth material in one Nd-Fe-B permanent magnet of the same volume as the tape stack
30 g
Reduction of rare-earth material in superconductor tape magnets
120 times!
2626
Conclusions It was found that crisscross stacking configuration results in a lower
trapped field and the increasing the nanodefect density beyond 7.5% Zr does not lead to an increased trapped field. So, this part of Hypothesis 1 has been proven to be incorrect. It has been however found that a crisscross arrangement of tapes results in a more uniform trapped field profile (important for large area magnets).
It has been found that the trapped magnetic field values increase with increasing number of layers of 0.055 mm tapes, increase with increasing number density of tapes (decreasing tape thickness) and increase with decreasing temperature from 77 K to 30 K. Hence, this part of Hypothesis 1 has been proven to be correct.
The rate of decay of trapped magnetic field decreased with increasing number of layers in the tape stack. Hence, Hypothesis 2 has been proven to be incorrect.
Future Work Experimental work with more layers of 25 µm thick tapes to confirm
findings from simulation; in particular, to verify the sharp rise in trapped field values in tape thickness below 30 µm in thicker tape stacks.
Experimental work at temperatures lower than 77 K with normal (55 µm) and thinner (25 µm) tapes to confirm findings from simulation.
Refine COMSOL model. Add flux creep analysis.
2727
The test facility for this work, superconductor tapes and microstructure images were provided by the University of Houston
Dr. Xiao-Fen Li at University of Houston provided training on testing of superconductors. Dr. Philippe Masson provided training on COMSOL modeling.
Acknowledgments