Researchers have discovered a potential new method for making the high-performance magnets used in wind turbines and electric cars without the need for rare-earth elements, which come almost exclusively from China.
A team from the University of Cambridge, in collaboration with Austrian colleagues, has found a new way to possibly replace rare-earth magnets: tetrataenite, a “cosmic magnet” that takes millions of years to develop naturally in meteorites.
Previous attempts to make tetrataenite in the laboratory relied on extreme impractical methods. But the addition of a common element, phosphorus, could mean it’s possible to make tetrataenite artificially and on a large scale, without specialized processing or expensive techniques.
The results are published in the journal Advanced sciences. A patent application on the technology has been filed by Cambridge Enterprise, the university’s commercialization arm, and the Austrian Academy of Sciences.
High-performance magnets are a vital technology for building a carbon-free economy, and the best permanent magnets available today contain rare earth elements. Despite their name, rare earths are abundant in the earth’s crust. However, China holds a virtual monopoly on global production: in 2017, 81% of rare earths in the world came from China. Other countries, such as Australia, also mine these elements, but as geopolitical tensions with China increase, there are fears that the supply of rare earths may be at risk.
“Rare earth deposits exist elsewhere, but mining operations are very disruptive: you have to extract a huge amount of material to get a small volume of rare earths,” said Professor Lindsay Greer of the Department of Materials Science and Metallurgy. of Cambridge, who conducted the study. to research. “Between the environmental impacts and the heavy reliance on China, there has been an urgent search for alternative materials that do not require rare earths.”
Tetrataenite, an iron-nickel alloy with a particular ordered atomic structure, is one of the most promising of these alternatives. Tetrataenite forms over millions of years as a meteorite slowly cools, giving the iron and nickel atoms enough time to arrange themselves into a particular stacking sequence within the crystal structure, resulting ultimately to a material with magnetic properties close to those of rare-earth magnets.
In the 1960s, scientists were able to artificially form tetrataenite by bombarding iron-nickel alloys with neutrons, allowing the atoms to form the desired ordered packing, but this technique is not suitable for mass production.
“Since then, scientists have been fascinated with getting this orderly structure, but it always seemed like something very far off,” Greer said. Despite many attempts over the years, it has not yet been possible to manufacture tetrataenite on a scale approaching an industrial scale.
Now Greer and his colleagues at the Austrian Academy of Sciences and Montanuniversität Leoben have found a possible alternative that doesn’t require millions of years of neutron cooling or irradiation.
The team was studying the mechanical properties of iron-nickel alloys containing small amounts of phosphorus, an element also present in meteorites. The phase diagram within these materials showed the expected tree-like growth structure called dendrites.
“For most people it would have ended there: nothing interesting to see in the dendrites, but when I looked closer I saw an interesting diffraction pattern indicating an ordered atomic structure,” said said first author Dr. Yurii Ivanov, who completed the work. at Cambridge and is now based at the Italian Institute of Technology in Genoa.
At first glance, the diffraction pattern of tetrataenite resembles that of the structure expected for iron-nickel alloys, namely a disordered crystal of no interest as a performing magnet. It took closer examination by Ivanov to identify the tetrataenite, but Greer says it’s odd no one has noticed it before.
The researchers say that phosphorus, which is present in meteorites, allows iron and nickel atoms to move faster, allowing them to form the necessary ordered stacking without waiting millions of years. By mixing iron, nickel, and phosphorus in the right amounts, they were able to accelerate the formation of tetratainite by 11 to 15 orders of magnitude, so that it forms in seconds in a single pour.
“What was so amazing was that no special treatment was needed: we just melted the alloy, poured it into a mold, and we had tetrataenite,” Greer said. “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 shift in the way we think about this material.”
Although researchers have found a promising method for producing tetrataenite, more work is needed to determine if it will be suitable for high-performance magnets. The team hopes to work on this with major magnet manufacturers.
The work may also force a revision of views on whether tetrataenite formation in meteorites really takes millions of years.
Direct formation of hard magnetic tetrataenite in massive alloy castings, Advanced sciences (2022). DOI: 10.1002/advs.202204315
Provided by the University of Cambridge
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