Superconductor Tapes: A Solution to the Rare Earth Shortage Crisis
Rare-Earth materials are used to make permanent magnets, which are needed for motors in electric cars and wind turbines. However, China, which controls 97% of rare-earth production, recently restricted their export to rare-earths. So, there is a world-wide incentive to develop strong magnets containing no or very small amounts of rare-earth materials. I thought of a solution to this problem: Superconductors. Superconductor tapes are made by coating a thin film of superconductor on a metal tape. Only 2% of the tape contains the actual superconductor, and hence a very small amount of rare-earth: A possible solution for the crisis.
Link to Google Presentation:
- Why I chose this project: I was reading up on the rare-earth shortage crisis, and I started brainstorming ways to help. Superconductor pucks had already been experimented with, but not superconductor tapes. I was curious and wanted to find out!
- Problem/Hypothesis: I wanted to find out what arrangement of tapes would have the highest and uniform trapped magnetic field. I hypothesized that a criss-cross arrangement would satisfy this condition. I also wanted to find out how the tape trapped field would compare to that of the puck. I predicted that a stack of tapes half as high as the puck would trap the same magnetic field as the puck.
- Research: I researched the rare-earth crisis, how superconductors can become magnets, and superconductor tapes/pucks.
- Testing: I used RE-Ba-Cu-O (RE=rare earth) thin film superconductor tapes to trap the magnetic field, a 1.5 Tesla electromagnet to induce magnetic field into the tapes, liquid nitrogen to keep the tapes at a cold temperature (so the field would stay trapped), a hall probe to measure trapped field.
- What I discovered/Conclusion: I discovered that the criss-cross arrangement resulted in more uniform magnetic field than straight-arrranged tapes and the same level of trapped field as in pucks but with 42 times less rare earth content.
- Why it's useful: My findings can be used to develop magnets that are nearly rare-earth-free to help the crisis!
- Future: Investigations with higher-quality tapes, more tapes/layer, and with more layers can be conducted.
Hi! My name is Kavita Selva and I go to school at Westbrook Intermediate, Texas, USA. I love reading (non-fiction and fiction), science (especially problem solving and the human body), mathematics (again problem solving) and music (playing and listening). What I love most about science is the fact that there are so many unsolved problems and mysteries. This has influenced me to find possible solutions whenever I see a problem or question. For example, take my science fair project: I had heard about the problem with the rare-earths. China, which controls 97% of the rare-earth industry, restricted their export. Rare-earths are needed for electric motors, so they are extremely important. So, I focused on finding a soultion for this problem, which got me to where I am now. The scientists who I admire the most are the ones that found solutions for the biggest problems: Edison, who gaves us light without using candles; Benz, who invented a faster, more modern way to get around; and Bell, who gave people the opportunity to talk to others over long distances. All of these inventions were invented long ago, but we can't imagine living without them today. I hope to become thought as, "She's that person who solved the rare-earth crisis!". In the future, I would like to become someone who can help others, like a scientist or a doctor. Winning this fair would mean that I had helped the world, and I have extra opportunities to continue my research on superconductors.
For my experiment, I have two purpose questions that go hand-in-hand with each other because I could test them out together in the same experiment (and make my results more useful).
Purpose Questions: How is the magnetic field trapped by a stack of thin film superconductor tapes influenced by the number and arrangement of tapes in the stack? How does the magnetic field trapped by a stack of superconductor tapes compare with the magnetic field trapped by a bulk monolithic superconductor (puck)?
Hypotheses: I expect that a criss-cross arrangement of a stack of superconductor tapes would result in a more uniform trapped field profile than a straight arrangement of a stack of superconductor tapes. Additionally, I expect that if a stack of thin film superconductor tapes is made half as thick as a bulk superconductor puck, then the tape stack would be able to trap as high a magnetic field as the puck.
Need to drastically reduce rare-earth content in permanent 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. Recently, there has been a world-wide supply problem with rare-earth supply after China which controls 97% of rare-earth production, restricted their export. Therefore, there is great incentive to develop strong magnets with no or miniscule amount of rare-earth materials.
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 (maximum current /cross section with zero resistance)
- Above a certain magnetic field, magnetic lines of force will penetrate a superconductor. As long as the superconductor is kept cold, the magnetic lines of force will be trapped in it. Now the superconductor becomes a magnet! It has been known that bulk, monolithic REBCO superconductors (pucks) can trap large magnetic fields and act a strong magnets.
Thin film superconductor tapes
- While the rare earth content in bulk superconductors is lower than that in permanent magnets, it is not insignificant.
- 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 superconductor tapes, it would lead to a new class of magnets that are near rare-earth free!
Video of Procedure
- Independent: Number/arrangment of tapes in the stack
- Criss-cross/straight arrangement
- 25/50/74 tapes
- Dependent: Trapped magnetic field characteristics
Experiment Location Information
- The experiment was conducted in a lab in the University of Houston
- The equipment was provided by University of Houston and SuperPower Inc.
- Wear safety goggles during the experiment
- Wear cryogenic gloves when transferring liquid nitrogen
Procedure (Basic steps from YouTube video. Specific steps listed on Google Presentation attached to the "Summary" section)
- Pack superconductor tapes (with specific arrangement (criss cross or straight) and number of layers) into sample holder.
- Place sample holder into metal cyrogen container.
- Place container between poles of electromagnet.
- Turn on electromagnet.
- Magnetic lines of force will now penetrate into the superconductor.
- Put on cryogenic gloves to protect against cold temperatures of liquid Nitrogen.
- Fill up two cryogen dewars with liquid nitrogen.
- Use one of the cryogen dewars to pour liquid nitrogen into the metal cryogen container (still in between the poles of the electromagnet). This cold temperature will make the tapes superconducting and will result in magnetic field to be trapped inside of the superconductor tapes.
- Turn off electromagnet.
- Take out metal cryogen container from its place between the electromagnet poles.
- Transfer metal cryogen container to a location closer to the testing area (near the x-y-z axis motion table, hall probe, and computer).
- Take the other (unused) cryogen dewar and pour the liquid Nitrogen from that into a styrofoam container (near the x-y-z motion table and under the hall probe).
- Quickly transfer the sample holder from the metal cryogen container to the styrofoam container to maintain the cold temperature (so that the magnetic field trapped inside of the Superconductor tapes does not escape).
- Position the sample holder inside of the styrofoam container in a way so that the hall probe is directly above the sample holder.
- Begin measuring the voltage for distances all across a 40 x 40 mm area in 1 mm steps along both x and y axes (convert voltage to value of trapped magnetic field using conversion factor (87.5 µV/Tesla).
- Use this information and examine the trapped field profile data to determine the (x, y) coordinates of the location of the maximum trapped field.
- Move the hall probe to that (x, y) location and move it down to make contact with the sample holder (1 mm from the tape stack surface).
- Move the hall probe up 10 mm along the z-axis in steps of 0.25 mm and record the voltage (and hence the trapped field) values.
- Repeat steps 1 - 18 with different arrangments of the tapes and number of tapes in the stack.
- Examine data and draw conclusions.
Link to Google Presentation of Data Figures (Which are explained in the "Explanation of Charts/Figures" section)
Explanation of Charts/FIgures
- From Figure 1, it is seen that a criss-cross arrangement of superconductor tapes results in a nine peaks in the trapped field profile corresponding to the nine overlapping regions formed by criss-crossing of three tapes in each layer. The valleys in the trapped-field profile correspond to the regions with overlapping edges of each tape.
- Figure 2 shows that the trapped field profile in a stack of superconductor tapes in a straight arrangement matches exactly with the tape arrangement. The valleys in the trapped-field profile are deep and match with the edges of each tape.
- The trapped field profile in a stack of 74 straight-arranged tapes crossed with 67 straight-arranged tapes presented in Figure 3 does not show the nine peaks seen in Figure 1 where every single layer was criss-crossed. The profile matches with the orientation of the straight-arranged 74 layers in the stack which were closer to the surface and the Hall probe. However, the valleys are lot less deep compared to that shown in Figure 2 indicating that 67 straight layers arranged cross wise to the 74 layers are contributing to the trapped-field profile.
- From Figure 5, it is seen that the trapped field values increase steadily with increasing number of layers of superconductor tapes in the stack from 25 to 50 to 74.
- It is observed in Figure 6 that a straight-arrangement of 74 layers of tapes results in a uniform field profile and higher trapped field values along the x-axis (direction of the tape) compared to criss-cross arrangement. But, as seen in Figure 7, the straight-arrangement results in extremely non-uniform trapped field profile with sharp valleys along the y-axis. The criss-cross arrangement results in very similar profiles along the x-axis and y-axis and the valleys are shallow.
- Figure 8 shows that both the maximum and average trapped magnetic field values increase linearly with increasing number of layers from 25 to 50 to 74.
- It is seen from Figure 8 that a straight arrangement of layers results in a higher maximum trapped field but slightly lower average trapped field compared to that of criss-cross-arranged layers.
- From Figure 8 it is observed that the maximum and average trapped field values in a stack of 141 layers (additional 67 layers arranged cross-wise to first 74 layers) are lower than that expected from a straight-line increase based on the trend up to 74 layers.
- It is observed from Figure 8 that while the puck, which is two times thicker than the thickest stack of superconductor tapes, can trap a higher magnetic field (0.173 T compared to 0.135 T), its average trapped magnetic field is about the same (0.076 T) as the tape stack.
- Figure 9 shows that the maximum trapped magnetic field decreases slower with increasing distance from the tape stack surface if the tape layers are criss-cross arranged rather than straight arranged.
- It has been found that the average trapped magnetic field in a 7.7 mm thick stack of 141 layers of superconductor tapes is same as in a 15 mm thick bulk superconductor puck. Hence, Hypothesis 1 has been proven to be correct.
- The magnitude of the trapped fields in criss-cross- and straight-arrangements are comparable. But, the 3D trapped-field profiles and 2D trapped-field profiles along the y-axis of the criss-cross-arranged stack of tapes are significantly more uniform than that of the straight-arranged stack. Also, the maximum trapped magnetic field decreases slower with increasing distance from the tape stack surface if the tape layers are criss-cross arranged rather than straight arranged. All these results confirm that criss-cross arrangement of a stack of superconductor tapes lead to a more uniform trapped field profile than a straight arrangement of a stack of superconductor tapes. Hence, Hypothesis 2 has been proven to be correct.
Applications of Results
- The finding in this project that a superconductor tape stack that is only about ½ as thick as a bulk superconductor puck traps the same level of average magnetic field strongly supports the reason to use thin film superconductor tapes as trapped-field magnets. The amount of rare-earth material in the 15 meters of superconductor tape used in this project is only 0.15 g. In comparison, the amount of rare-earth material in the one 25 mm diameter, 15 mm thick bulk superconductor puck used in this project is 6.4 g! That is a 42.6 times reduction in the amount of rare-earth used in the thin film superconductor tape stack! A Nd-Fe-B permanent magnet with the same size as the bulk superconductor puck contains 14.7g of the rare-earth neodymium. Therefore, 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 a criss cross arrangement of layers of thin film superconductor tapes results in more uniform trapped magnetic field will have a very beneficial impact on the design of superconductor tape stacks to make magnets with very uniform fields.
- The criss-cross arrangement of layers of superconductor tapes needs to be extended beyond 74 layers to determine how far the linear trend in increase in trapped magnetic fields will persist and to find the maximum possible trapped fields.
- The experiment needs to be repeated with higher performance superconductor tapes (with higher critical current density) and even better quality bulk superconductor pucks.
- Investigate stacks with more than three tapes in each layer so as to achieve more homogeneity in trapped magnetic field.
- Measure trapped field profiles at various distances from the superconductor tape stack.
Sources used in Research
M. Mukakami, “Melt Process, Flux Pinning, and Levitation.” Processing and Properties of High TC Superconductors. Ed. S. Jin. World Scientific. 1993. PP 248-250.
D. A. Cardwell, W. K. Yeoh, S. K. Pathak, Y-H Shi, A. R. Dennis, N. Hari Babu, and K. Iida, “The Generation of High Trapped Fields in Bulk (RE)BCO High Temperature Superconductors”, AIP Conference Proceedings 1219, pp. 397-406 (2010)
Test facility for this work and superconductor puck were provided by the University of Houston
- Specialized equipment includes:
- X-Y-Z (3 axis) motion system (Zaber Technologies T-LSR300B)
- High Linearity Hall Probe
- Keithley 2400 Sourcemeter
- 1.5 Tesla Electromagnet
- Frame to mount Hall Probe to 3-axis linear motion table
- Cables for communication between motion table and computer among Hall probe, multimeter and computer
Dr. Goran Majkic at University of Houston provided training on testing of superconductors.
- Dr. Majkic trained me on how to use the program Matlab
- He helped me with a couple test rounds
- The rest of the testing I did without help
- Dr. Majkic also trained me on the proper handling and precautions when dealing with the equipment (hall probe, liquid nitrogen)
Superconductor tape was provided by SuperPower Inc.