Consider the energy product, or (BH)max, the “figure of merit” for permanent magnets. It is based on a ferromagnetic material’s magnetization and coercivity, the material’s resistance to demagnetization. During the past 100 years, the energy product of permanent magnets has risen from less than 1 MGOe (megagauss-oersted) to more than 50 MGOe, but only through use of rare earth materials. For example, today’s neodymium magnets achieve roughly 56 MGOe, compared to around 10 for the best non-rare-earth magnets, and less than 5 for typical refrigerator magnets. The discovery of Nd2Fe14B in the early 1980s — the major phase of the strongest permanent magnets known so far — was the last giant step in the advancement of magnet strength to date.
Now, based on more than 20 years of research, exchange-coupled nanocomposite magnets are widely regarded as the next generation of permanent magnetic materials. That said, many technical hurdles remain with regard to understanding the inter-phase exchange interactions, not to mention actually processing bulk nanocomposite magnets that retain enhanced energy products. Much research is taking place globally to solve these technical issues and bring advanced magnet materials into widespread production one day.
The goal is to make magnets that are both stronger and less expensive than today’s varieties, which are heavy on the use of rare-earth elements such as neodymium and dysprosium. The new magnets wouldn’t abandon the costly rare earths altogether, but would use less of them, along with more of the cheaper ingredients such as iron. Beyond price, using fewer rare earth materials will help avoid sourcing challenges, such as China’s recent restrictions on exports. China now mines and processes more than 97% of the world’s rare earth materials, though production has recently started up again in the U.S. and a few other countries.
Several institutions in the U.S. are working on nanocomposite magnet research projects, and the DOE’s Advanced Research Projects Agency — Energy (ARPA-E) is funding two of them. One involves GE Global Research, Niskayuna, N.Y., which received roughly $2.25 million from ARPA-E. The idea is to employ nanomaterials technology to develop bulk magnets with finely tuned structures using iron-based mixtures that contain 80% fewer rare earth materials than traditional magnets. If successful, GE’s design would create sophisticated magnets that enable advanced energy technologies such as electric vehicles and wind turbines (which GE makes) to produce more power at a lower cost.
The other ARPA-E magnet project involves a consortium led by the University of Delaware, with partner organizations including Ames Laboratory, University of Nebraska, Virginia Commonwealth University, Northeastern University, and Electron Energy. ARPA-E provided nearly $4.5 million in funding for this project, which aims to develop permanent magnets that contain less rare earth material and produce twice the energy of the strongest rare earth magnets currently available.
To do this, the idea is to mix the more expensive magnet materials with those that are more abundant, like iron. Both materials are prepared in the form of nanoparticles by using techniques such as wet chemistry and ball milling. The nanoparticles must then be assembled in a 3D array and consolidated at low temperatures to form a magnet. With small particles and good contact between the two materials, the best qualities of each should allow for development of exceptionally strong composite magnets.
To create a successful nanocomposite magnet, at least four requirements must be met: The magnet grains must be tiny (10 nm or smaller), have the correct crystal structure and aligned magnetic directions, and be closely packed together.
One of the lead researchers in all of these difficult areas is Ping Liu, a physicist at the University of Texas at Arlington. In 2006, Liu developed a unique ball milling method that uses steel balls to crush magnetic material with the right crystal structure within a solution featuring detergents. Incorporating soap enables production of tiny grains that don’t stick together, but still keep their magnetic properties. The method is being widely employed, including its use by the University of Delaware team.
Although it’s now possible for researchers to make the magnetic nanoparticles in small batches in the lab, one of the biggest remaining challenges is to make a bulk magnet out of the microscopic grains. Liu’s team is making some progress using “bottom-up” approaches to nanomagnet fabrication. That’s because for exchange-coupled magnets to work effectively, the grain size of the hard and soft magnet phases must be strictly controlled at the nanoscale, which is extremely difficult using traditional top-down fabrication methods.
“When I began my independent research program as an assistant professor in 2000, I believed that I should start a novel approach to fabricate nanocompsosite magnets. The disadvantage of top-down approaches is obvious: They do not lead to a homogenous nanostructure, which is essential for nanocomposite magnets. The bottom-up approach based on nanoparticles overcomes this hurdle,” explains Liu.
Besides the surfactant-assisted ball milling technique previously described, Liu and his team have also used salt-matrix annealing (using ordinary table salt as the annealing matrix) to prepare nanostructured powder particles, such as Fe-Pt and Fe-Co based materials. Recently, the team has discovered another mechanism in which severe plastic deformation of composite materials may result in a homogenous nanoscale composite, with the relatively soft phase broken into nanoparticles during the extensive deformation. According to Liu, the process is somehow like making Ramen — the Chinese noodles made by hand extension.
“One of the things we’ve paid special attention to is that our processing techniques will be easy for future scaling up to industrial production,” says Liu. “People often think that nanotechnology is expensive, but our methods make it very affordable.”
Liu is now also experimenting with warm compaction and explosive compaction techniques to produce bulk nanomagnets. He reports that strong magnetic exchange coupling has been achieved in the compacted samples, making the bottom-up approaches appear promising for large-scale magnet production. However, alignment issues linger on. Liu’s team is working on ideas that include putting the magnetic material through a second slow-compaction procedure, but they have not had much success yet. He hopes to discover a winning method for producing bulk exchange-coupled nanocomposite permanent magnets before he reaches retirement age, a goal no doubt shared by magnet researchers everywhere.