Smaller and larger magnets constructed in the lab, demonstrating how the manufacturing method can be upscaled. Image: U.S. Department of Energy Ames National Laboratory.
Smaller and larger magnets constructed in the lab, demonstrating how the manufacturing method can be upscaled. Image: U.S. Department of Energy Ames National Laboratory.

Researchers from the US Department of Energy (DOE)’s Critical Materials Institute (CMI) and Ames National Laboratory have improved the properties of a rare-earth-free permanent magnetic material and demonstrated that this improvement method can be upscaled for manufacturing.

The researchers developed a new method for manufacturing manganese bismuth (MnBi) magnets based on microstructure engineering. This advance represents a step towards making compact, energy-efficient electric motors without the use of rare earths. The researchers report the method in a paper in the Journal of Magnetism and Magnetic Materials.

High-power permanent magnets are increasingly important for a variety of renewable energy technologies, including wind turbines and electric cars. According to Wei Tang, CMI researcher and Ames Lab scientist, these magnets are currently constructed from rare-earth elements such as neodymium and dysprosium. However, these elements are currently low-stock and high-demand, leading to an unreliable supply chain and high prices. One solution to this problem is for scientists to find rare-earth-free magnetic materials, such as MnBi.

The permanent magnets required for electric motors require high energy density, or high levels of magnetism and coercivity. Coercivity refers to a magnet’s ability to maintain its current level of magnetism when exposed to high heat and outside influences that could demagnetize it.

“If we use high-power-density magnets, we can reduce the motor size and make a more compact motor,” said Tang. “Right now, it is very important that we can make some devices smaller and more compact, more energy-efficient.”

The challenge with MnBi is that traditional manufacturing methods require high heat to transform the individual materials into a large magnet, and this heat reduces the energy density of the magnet. To address this problem, the team developed an alternative manufacturing process.

The researchers started with a very fine powder for each of the materials in MnBi, as this increases the starting magnetic energy level. Next, they used a warm heating method rather than a high-temperature method to form the magnet. But the key to their new process was to add a non-magnetic component that would prevent grain particles from touching each other. This additional element, called a grain boundary phase, provides more structure to the magnet, and keeps the magnetism that runs through individual particles/grains from affecting other particles/grains.

“It is like the structural material,” explained Tang. “It’s like if we use concrete to build a wall. With just the concrete itself, it’s weak, but if we put a steel rebar inside first then pour the concrete, it’s going to be several dozen times stronger.”

The effect of the warm temperature on the magnetic properties of MnBi is unique. The researchers expected the coercivity and magnetism to decrease with increasing temperature, which is what happens with most magnetic materials. For MnBi, however, the warm temperature increased the coercivity and decreased the magnetization. This increased coercivity helps to keep the magnet more stable at elevated temperatures than other known magnets.

The team also focused on making magnets that were larger than the typically small magnets developed in labs. Upsizing the magnets in this way helps to demonstrate to manufacturing companies that they can build large magnets on a commercial scale.

“If we cannot make the larger one, we cannot use it for any application,” Tang said. “We need a big magnet, and we need to make it into whatever the shape needed. Also, we need to be able to mass produce at a low cost. This is important for future applications.”

The team is currently working with PowderMet Inc on using their patent-pending techniques to pursue mass production of the MnBi magnets for use in novel electric motors. That project is funded by the DOE Small Business Innovation Research program and has already entered phase II, which means it has been proven feasible and additional funding has been awarded to further develop and demonstrate the technology.

This story is adapted from material from Ames National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.