Researchers at the University of Pittsburgh (Pitt) Swanson School of Engineering are working with Powdermet Inc., a nanomaterials and advanced materials research and development company in Ohio, to develop a rare-earth-mineral-free electric motor.

Most electric motors for electric vehicles rely on permanent magnets made with rare-earth metals, which are a limited resource. In addition to their scarcity, extracting and processing these materials has environmental consequences, leaving behind a significant amount of toxic waste. Because China accounts for the vast majority of rare-earth production, price volatility is another concern. To meet the needs of a growing market, designing electric motors without rare-earth metals is a crucial step, especially for sustainable supply chains.

The Powdermet-led project hopes to create an electric machine that uses permanent magnets made of more abundant metals instead of rare-earth metals. The project recently received $200,000 in funding from the US Department of Energy (DOE) that will allow Powdermet to commercialize MnBi-based (manganese-bismuth) permanent magnetic materials developed at the US Department of Energy Ames Laboratory Critical Materials Institute (CMI).

In a 2019 paper published in Acta Materialia, Ames researchers described their efforts in optimizing the composition of the MnBi material:

MnBi is an attractive rare-earth-free permanent magnetic material due to its low materials cost, high magnetocrystalline anisotropy (1.6 × 10⁶ J m⁻³), and good magnetization (81 emu g⁻¹) at room temperature. Although the theoretical maximum energy product (BH)_max of 20 MGOe is lower than that of NdFeB-based magnets, the low temperature phase (LTP) of MnBi has a positive temperature coefficient of coercivity, up to 200 °C, which makes it a potential candidate for high temperature applications such as permanent magnet motors.

However, the oxygen sensitivity of the MnBi compound and the peritectic reaction between Mn and Bi make it difficult to synthesize into a material with high purity. This challenge is partly offset by adding excess Mn to the alloy, with composition close to Mn₅₅Bi₄₅ resulting in the highest saturation magnetization after common processing techniques such as arc melting, casting, melt spinning, and ball milling.

Here we report a systematic process which reduces the amount of excessive Mn, while simultaneously providing a large saturation magnetization (MS) of 79 emu g⁻¹ at 300 K in the annealed Mn₅₂Bi₄₈ ribbons. We also report excellent magnetic properties in the ball powders, resulting in 0.5–5 µm particles with MS of 75.5 emu g⁻¹, coercivity H_ci of 10.8 kOe, and (BH)_max of 13 MGOe using 9 T applied field at 300 K. A secondary annealing treatment on various ball milled powders increased Hci by up to 21%, and also resulted in an increase in MS up to 78.8 emu g⁻¹.

At Pitt, this work will be led by Paul Ohodnicki, associate professor of mechanical engineering and materials science, and Brandon Grainger, Eaton Faculty Fellow and assistant professor of electrical and computer engineering. Together, the Pitt team will use ANSYS MotorCAD to benchmark an electric motor design that takes advantage of the novel magnetic materials.

Permanent magnets are used in electric motors because they can produce and maintain a strong magnetic field, even in the presence of an opposing magnetic field, as opposed to electromagnets, which require an electric current. Using alternative materials such as MnBi-based permanent magnets, developed at the Ames Laboratory, to create a permanent magnet instead of rare-earth metals like neodymium and dysprosium would make electric vehicles more affordable, accessible, and sustainable, and would help the US become a leader in the EV market.

—Paul Ohodnicki

Powdermet is an industry participant of the Advanced Magnetics for Power & Energy Development (AMPED) Consortium, a research consortium led by director Ohodnicki and co-director Grainger at the University of Pittsburgh. AMPED includes several schools at Pitt, Carnegie Mellon University, North Carolina State University, national labs, and industry partners, bringing together an interdisciplinary skillset well-suited to the research and development of magnetic materials for power electronics and power conversion systems.

Resources

• Brandt A. Jensen, Wei Tang, Xubo Liu, Alexandra I. Nolte, Gaoyuan Ouyang, Kevin W. Dennis, Jun Cui, (2019) “Optimizing composition in MnBi permanent magnet alloys,” Acta Materialia, Volume 181, Pages 595-602 doi: 10.1016/j.actamat.2019.10.003

• Cui, J.; Choi, J. P.; Li, G.; Polikarpov, E.; Darsell, J.; Kramer, Matthew J.; Zarkevich, Nikolai A.; Wang, Linlin; Johnson, Duane D.; Marinescu, M.; Huang, Q. Z.; Vuong, N. V.; and Liu, J. Ping, (2014) “Development of MnBi permanent magnet: Neutron diffraction of MnBi powder” Ames Laboratory Publications. 244. https://lib.dr.iastate.edu/ameslab_pubs/244