Process-Property-Structure relationships in advanced rare earth magnet manufacturing: Towards enhanced performance and developing application

This research aims to study advanced rare earth magnet manufacturing, focusing on the structure-process-property relationships that govern their performance and applications. Rare earth minerals are classified as critical materials because they are essential in manufacturing products across numerous cutting-edge technologies including electric vehicles, renewable energy systems, and high-performance electronics.

Bonded magnets are composites with permanent magnet powder embedded in a polymer matrix. Finely powdered (3-300 microns) rare earth based intermetallics such as neodymium iron boron (NdFeB) and samarium iron nitride (SmFeN) are blended with engineering polymers such as epoxy, polyamides (PA6/PA12) and polyphenylene sulfide (PPS), followed by molding the compound to desired shape.

Addition of a polymer binder allows bonded magnets to be produced with low-cost manufacturing methods such as injection and compression molding, whereas sintered magnets are relatively more recourse and capital intensive. Bonded magnets also offer design flexibility and creation of complex shapes with tailored magnetic properties. Despite these manufacturability advantages, the performance of bonded rare earth magnets is limited due to challenges in optimizing the process parameters, material compositions and microstructural properties.

This research preliminarily investigated mechanical, magnetic and microstructural properties of NdFeB powders in a polycarbonate (PC) matrix using multi-phase compounding and compression molding. It is found that with optimum compaction pressure, high density compounds can be produced. 95% weight fraction magnetic material compounds offered highest maximum energy product [114 kJ/m³ (kiloJoules per cubic meters)], while 85% weight fraction magnetic material compounds offered highest tensile strength [59 megapascals].

Subsequently, hybrid neodymium-samarium powders with bimodal particle size distribution are studied in a polyphenylene sulfide (PPS) matrix using batch mixing and compression molding. It is found that use of bimodal PSD increases the particle packing density of the compound, thereby increasing the compound density (6.15 g/cc) and maximum energy product [159 kJ/m³].

Finally, small scale and large-scale additive manufacturing is studied using hybrid neodymium-samarium powders in a polyamide (PA12) matrix. It is shown that additive manufacturing is a competent alternative for bonded magnet manufacturing with compounds produced having maximum energy density of 121 kJ/m³.