Spark Plasma Sintering of permanent magnets for next-generation electric vehicle motors

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With the long-term future of fossil fuels in considerable doubt, Europe's roads are seeing the arrival of the first electric vehicles (EVs); the majority of which are hybrid EVs. Crucial to the widespread uptake of pure EVs is the development of superior next-generation electric motors; which will require improved, sustainable and economically viable permanent magnetic materials. The ideal magnet would retain a high level of flux at elevated temperatures and would be resistant to the reverse magnetic fields and corrosive atmospheres present in EV applications. Conventional rare-earth (RE) Nd-Fe-B permanent magnets do not perform well under these conditions and heavy-rare-earth (HRE) additives are currently employed to provide magnetic stability at elevated temperatures. However, following the establishment of a near-monopoly of RE production by Chinese firms, Europe is now facing a “Rare-Earth Crisis”; with the high price and unstable supply of REs now presenting one of the most significant barriers to the extensive adoption of pure EVs. MAG-DRIVE is an EU research initiative, aimed at addressing this issue; whose concept is based on the realisation that HREs, rather than REs in general, are the critical raw materials. Its' overall strategy is to re-think the conventional powder production and consolidation routes in order to yield finer microstructures in the finished Nd-Fe-B magnets. This should increase their coercivity (to retain high flux at elevated temperatures), thereby eliminating the need for HRE additions (reducing the price and improving the corrosion resistance). Towards this goal, a Spark Plasma Sintering (SPS) technique has been employed to consolidate both commercial and recycled nanostructured Nd-Fe-B powders. The rapid nature of this sintering technique limits the extent of microstructural coarsening during sintering. The sintering behaviour of the different starting materials has been investigated in order to understand and optimise the SPS process for the production of magnets with ideal microstructures and magnetic properties. It is found that the recycled material behaves very differently to the commercial powder, with much slower sintering kinetics, and this is attributed to differences in the composition and structure of the starting powder.