A new method to make high-performance magnets could minimize our reliance on rare earth elements
A team of researchers from the University of Cambridge, alongside colleagues in Austria, has discovered a potential replacement for the current method for making high-performance magnets without using rare earth elements.
These high-performance magnets, used in wind turbines and electric vehicles, are vital for building a zero-carbon economy. Currently, the best permanent magnets available require rare earth elements.
Even if the name "rare earth" sounds unpromising, these elements aren't quite as rare as they may sound. However, to date, China has a near monopoly on global production, cited a press release published by the University of Cambridge. For instance, in 2017, 81 percent of rare earth elements worldwide were sourced from China. And as geopolitical tensions with China increase, there are concerns that the rare earth supply could be at risk.
Rare earth elements are plentiful worldwide, but they're simply not easy to extract. On top of that, the mining extraction methods required are not very environmentally friendly.
An urgent need for alternative materials that do not require rare earth elements is needed, and this is where the team's research comes into play.
'Tetrataenite' and how it's made
The tetrataenite mineral is the rare earth element in question here. It is a ‘cosmic magnet,’ or an iron-nickel alloy with a particular ordered atomic structure, that takes millions of years to develop naturally in meteorites. This is what is required to make high-performance magnets.
Previous attempts to create artificial tetrataenite in labs have required extreme and impractical methods.
The team at Cambridge discovered that by adding a common element – phosphorus – tetrataenite could be made at scale and artificially without any extreme or costly methods. The team was studying the mechanical properties of iron-nickel alloys containing small amounts of phosphorus, an element that is also found in meteorites. The pattern of phases inside these materials showed the expected growth structure called dendrites.
"When I looked closer, I saw an interesting diffraction pattern indicating an ordered atomic structure,” said first author Dr. Yurii Ivanov, who completed the work while at Cambridge and is now based at the Italian Institute of Technology in Genoa.
Initially, the diffraction pattern of tetrataenite looked like that of the structure expected for iron-nickel alloys. But Dr. Ivanov’s closer look identified the tetrataenite. By mixing iron, nickel, and phosphorus in the right quantities, the team was able to speed up tetrataenite formation by between 11 and 15 orders of magnitude. It formed in mere seconds in simple casting.
“What was so astonishing was that no special treatment was needed: we just melted the alloy, poured it into a mould, and we had tetrataenite,” said Professor Lindsay Greer from Cambridge’s Department of Materials Science & Metallurgy, who led the research. “The previous view in the field was that you couldn’t get tetrataenite unless you did something extreme because otherwise, you’d have to wait millions of years for it to form. This result represents a total change in how we think about this material.”
More work needs to be carried out to determine whether this method will be suitable for high-performance magnets at a large scale. The team is hoping to work on this with major magnet manufacturers.
The team's findings were published in the journal Advanced Science.
Despite many attempts with diverse approaches, bulk synthesis of tetrataenite has not been reported. Here it is shown that with appropriate alloy compositions, bulk synthesis of tetrataenite is possible, even in conventional casting at cooling rates 11‒15 orders of magnitude higher than in meteorites. The barrier to obtaining tetrataenite (slow ordering from cubic close-packed to body-centered tetragonal) is circumvented, opening a processing window for potential rare-earth-free permanent magnets. The formation of tetrataenite on industrially practicable timescales also throws into question the interpretation of its formation in meteorites and their associated cooling rates.