Improvement of Rare Earth Free Magnet by Microstructural Engineering

Ames Laboratory - Researchers from the Key Materials Institute (CMI) of the US Department of Energy and Ames National Laboratory improved the performance of a rare earth free permanent magnetic material and proved that the process can expand the production scale. Researchers have proposed a new method for manufacturing manganese bismuth (MnBi) magnets based on microstructure engineering. This process has taken a step towards the direction of manufacturing compact, energy saving motors that do not use rare earths.

High power permanent magnets are becoming increasingly important for various renewable energy technologies, including wind turbines and electric vehicles. According to Tang Wei, a CMI researcher and Ames Laboratory scientist, these magnets are currently composed of neodymium, dysprosium and other rare earth elements. However, he explained that the low inventory and high demand of these elements led to unreliable supply chain and high prices. One solution to this problem is that scientists find alternative materials, such as MnBi used in this study.

Permanent magnets for motors require high energy density, or high levels of magnetism and coercivity. Coercivity is the ability of a magnet to maintain its current magnetic level, although exposure to high heat and external influences may cause it to lose magnetism.

"If we use high power density magnets, we can reduce the size of the motor and make it more compact," Tang said. "It is very important now that we can make some equipment smaller, more compact and more energy efficient."

The challenge for MnBi is that traditional manufacturing methods require high temperatures to convert a single material into a large magnet. The necessary heat reduces the energy density of the magnet. To solve this problem, the team developed an alternative process.

Tang said that they started with very fine powder of each material, which increased the initial magnetic energy level. Next, they used a warm heating method rather than a high temperature method to form magnets. Finally, the key to their new process is to add a non-magnetic component to prevent particles from contacting each other. This additional element is called the grain boundary phase, which provides more structure for the magnet and makes the magnetism flow through a single particle/grain without mutual influence.

"It's like a structural material," Tang said. "This is like building a wall with concrete. The concrete itself is very fragile, but if we put a steel bar in it first, and then pour concrete, its strength will be dozens of times that of concrete."

The influence of temperature on MnBi magnetic properties is unique. The researchers predict that the coercivity and magnetism will decrease with the increase of temperature, which is correct for most magnetic materials. For MnBi, the increase of temperature can increase the coercivity and reduce the magnetization. This increased coercivity helps to make magnets more stable at high temperatures than other known magnets.

The team also focused on making larger magnets, rather than smaller magnets that are usually developed in the laboratory. Enlarging the size of the magnet helps to prove to manufacturing companies that they can manufacture large magnets on a commercial scale.

"If we can't make it bigger, we can't use it for any purpose," Tang said. "We need a big magnet, and we need to make it into any shape we want. In addition, we need to be able to mass produce at low cost. This is very important for future applications."

The team is currently working with PowderMet to pursue mass production of MnBi magnets for new motors using the technology they are applying for patents. The project is funded by the Small Business Innovation Research Program of the US Department of Energy. The project has entered the second phase, which means that the project has been proved feasible, and additional funds have been awarded to further develop and demonstrate the technology.