English
Tesla’s investor day on 1 March began with a rambling, detailed discourse on energy and the environment before transitioning into a series of mostly predictable announcements and boasts. And then, out of nowhere, came an absolute bombshell: “We have designed our next drive unit, which uses a permanent-magnet motor, to not use any rare-earth elements at all,” declared Colin Campbell, Tesla’s director of power-train engineering.
It was a stunning disclosure that left most experts in permanent magnetism wary and perplexed. Alexander Gabay, a researcher at the University of Delaware, states flatly: “I am skeptical that any non-rare-earth permanent magnet could be used in a synchronous traction motor in the near future.” And at Uppsala University, in Sweden, Alena Vishina, a physicist, elaborates, “I’m not sure it’s possible to use only rare-earth-free materials to make a powerful and efficient motor.”
The problem here is physics, which not even Tesla can alter.
And at a recent magnetics conference Ping Liu, a professor at the University of Texas, in Arlington, asked other researchers what they thought of Tesla’s announcement. “No one fully understands this,” he reports. (Tesla did not respond to an e-mail asking for elaboration of Campbell’s comment.)
Tesla’s technical prowess should never be underestimated. But on the other hand, the company—and in particular, its CEO—has a history of making sporadic sensational claims that don’t pan out (we’re still waiting for that US $35,000 Model 3, for example).
The problem here is physics, which not even Tesla can alter. Permanent magnetism occurs in certain crystalline materials when the spins of electrons of some of the atoms in the crystal are forced to point in the same direction. The more of these aligned spins, the stronger the magnetism. For this, the ideal atoms are ones that have unpaired electrons swarming around the nucleus in what are known as 3d orbitals. Tops are iron, with four unpaired 3d electrons, and cobalt, with three.
But 3d electrons alone are not enough to make superstrong magnets. As researchers discovered decades ago, magnetic strength can be greatly improved by adding to the crystalline lattice atoms with unpaired electrons in the 4f orbital—notably the rare-earth elements neodymium, samarium, and dysprosium. These 4f electrons enhance a characteristic of the crystalline lattice called magnetic anisotropy—in effect, they promote adherence of the magnetic moments of the atoms to the specific directions in the crystal lattice. That, in turn, can be exploited to achieve high coercivity, the essential property that lets a permanent magnet stay magnetized. Also, through several complex physical mechanisms, the unpaired 4f electrons can amplify the magnetism of the crystal by coordinating and stabilizing the spin alignment of the 3d electrons in the lattice.
Since the 1980s, a permanent magnet based on a compound of neodymium, iron, and boron (NdFeB), has dominated high-performance applications, including motors, smartphones, loudspeakers, and wind-turbine generators. A 2019 study by Roskill Information Services, in London, found that more than 90 percent of the permanent magnets used in automotive traction motors were NdFeB.
So if not rare-earth permanent magnets for Tesla’s next motor, then what kind? Among experts willing to speculate, the choice was unanimous: ferrite magnets. Among the non-rare-earth permanent magnets invented so far, only two are in large-scale production: ferrites and another type called Alnico (aluminum nickel cobalt). Tesla isn’t going to use Alnico, a half-dozen experts contacted by IEEESpectrum insisted. These magnets are weak and, more important, the world supply of cobalt is so fraught that they make up less than 2 percent of the permanent-magnet market.
There are more than a score of permanent magnets that use no rare-earth elements, or don’t use much of them. But none of these have made an impact outside the laboratory.
Ferrite magnets, based on a form of iron oxide, are cheap and account for nearly 30 percent of the permanent-magnet market by sales. But they, too, are weak (one major use is holding refrigerator doors shut). A key performance indicator of a permanent magnet is its maximum energy product, measured in megagauss-oersteds (MGOe). It reflects both the strength of a magnet as well as its coercivity. For the type of NdFeB commonly used in automotive traction motors, this value is generally around 35 MGOe. For the best ferrite magnets, it is around 4.
“Even if you get the best-performance ferrite magnet, you will have performance about five to 10 times below neodymium-iron-boron,” says Daniel Salazar Jaramillo, a magnetics researcher at the Basque Center for Materials, Applications, and Nanostructures, in Spain. So compared to a synchronous motor built with NdFeB magnets, one based on ferrite magnets will be much larger and heavier, much weaker, or some combination of the two.
To be sure, there are more than a score of other permanent magnets that use no rare-earth elements or don’t use much of them. But none of these have made an impact outside the laboratory. The list of attributes needed for a commercially successful permanent magnet includes high field strength, high coercivity, tolerance of high temperatures, good mechanical strength, ease of manufacturing, and lack of reliance on elements that are scarce, toxic, or problematic for some other reason. All of the candidates today fail to tick one or more of these boxes.
A silver disk on a black background.Iron-nitride magnets, such as this one from startup Niron Magnetics, are among the most promising of an emerging crop of permanent magnets that do not use rare-earth elements.NIRON MAGNETICS
But give it a few more years, say some researchers, and one or two of these could very well break through. Among the most promising: iron nitride, Fe16N2. A Minneapolis startup, Niron Magnetics, is now commercializing technology that was pioneered with funding from ARPA-E by Jian Ping Wang at the University of Minnesota in the early 2000s, after earlier work at Hitachi. Niron’s executive vice president, Andy Blackburn, told Spectrum that the company intends to introduce its first product late in 2024. Blackburn says it will be a permanent magnet with an energy product above 10 MGOe, for which he anticipates applications in loudspeakers and sensors, among others. If it succeeds, it will be the first new commercial permanent magnet since NdFeB, 40 years ago, and the first commercial non-rare-earth permanent magnet since strontium ferrite, the best ferrite type, 60 years ago.
Niron’s first offering will be followed in 2025 by a magnet with an energy product above 30 MGOe, according to Blackburn. For this he makes a rather bold prediction: “It’ll have as good or better flux than neodymium. It’ll have the coercivity of a ferrite, and it’ll have the temperature coefficients of samarium cobalt”—better than NdFeB. If the magnet really manages to combine all those attributes (a big if), it would be very well suited for use in the traction motors of electric vehicles.
There will be more to come, Blackburn declares. “All these new nanoscale-engineering capabilities have allowed us to create materials that would have been impossible to make 20 years ago,” he says.
Source:https://spectrum.ieee.org/permanent-magnet-tesla
