Improving rare-earth-free magnets through microstructure engineering

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Newswise – Researchers from the Department of Energy’s Critical Materials Institute (CMI) and Ames National Laboratory have improved the properties of a rare-earth-free permanent magnet material and demonstrated that the process can be scaled up for manufacturing. Researchers have developed a new method for manufacturing manganese-bismuth (MnBi) magnets based on microstructure engineering. This process is a step towards manufacturing compact and energy efficient motors without the use of rare earths.

High-powered permanent magnets are increasingly important for a variety of renewable energy technologies, including wind turbines and electric cars. According to Wei Tang, a researcher at the CMI and a scientist at the Ames laboratory, these magnets are currently constructed from rare earth elements such as neodymium and dysprosium. However, he explained that these items are low in stock and high in demand, resulting in an unreliable supply chain and high prices. One solution to this problem is for scientists to find alternative materials, such as the MnBi used in this research.

Permanent magnets used for motors require high energy density or high levels of magnetism and coercivity. Coercivity is the ability of a magnet to maintain its current level of magnetism, despite exposure 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,” Tang said. “At present, 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 turn the individual materials into a large magnet. The heat required reduces the energy density of the magnet. To solve this problem, the team developed an alternative process.

Tang said they started with very fine powder for each of the materials, which increases the starting magnetic energy level. Then they used a hot heating method rather than a high temperature method to form the magnet. Finally, the key to their new process was to add a non-magnetic component that would prevent the grain particles from touching each other. This extra element, called the grain boundary phase, provides more structure to the magnet and prevents the magnetism passing through the individual particles/grains from affecting each other.

“It’s like the structural material,” Tang said. “It’s like using concrete to build a wall. With just the concrete itself it’s weak, but if we first put a steel rebar inside and then pour the concrete, it’s going to be dozens of times stronger.

The effect of hot temperature on the magnetic properties of MnBi is unique. The researchers expected coercivity and magnetism to decrease with increasing temperature, which is true for most magnetic materials. However, for MnBi, hot temperature increased coercivity and decreased magnetization. This increased coercivity helps keep the magnet more stable at high temperatures than other known magnets.

The team also focused on making larger magnets, compared to the typically small magnets developed in the lab. Increasing the size of the magnets helps demonstrate to manufacturing companies that they can build large magnets on a commercial scale.

“If we can’t make the bigger one, we can’t use it for any application,” Tang said. “We need a big magnet and we have to give it the shape we need. In addition, we must be able to mass produce at a lower cost. This is important for future applications.

The team is currently working with PowderMet Inc., using their patent-pending techniques to pursue mass production of MnBi magnets for use in new electric motors. This project is funded by the DOE’s Small Business Innovation Research Program. The project has already entered Phase II, which means the project has proven feasible and additional funding has been granted to further develop and demonstrate the technology.

This research is discussed in more detail in the article “Engineered microstructure to improve coercivity of bulk MnBi magnet”, written by Wei Tang, Gaoyuan Ouyang, Xubo Liu, Jing Wang, Baozhi Cui and Jun Cui, and published in the Journal of Magnetism and Magnetic Materials.

The Institute of Critical Materials is a Department of Energy Innovation Center led by the U.S. Department of Energy’s Ames National Laboratory and supported by the Advanced Manufacturing Office of the Office of Energy Efficiency and Renewable Energy. innovations, collaborations, research and development, technical assistance and workforce training. CMI seeks ways to eliminate and reduce dependence on rare earth metals and other materials critical to the success of clean energy technologies.

Ames National Laboratory is a US Department of Energy Science Office National laboratory operated by Iowa State University. Ames Laboratory creates innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and cross-disciplinary collaborations to solve global problems.

Ames National Laboratory is supported by the US Department of Energy’s Office of Science. The Office of Science is the largest supporter of basic physical science research in the United States and works to address some of the most pressing challenges of our time. For more information, please visit https://energy.gov/science.

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