Production and characterization of compacted Fe16N2 permanent magnets

Abstract

The permanent magnet industry has been in search of a rare-earth-free and cost-effective alternative to industry leader Nd-Fe-B magnet for over a decade. Thanks to its abundant and inexpensive constituent elements, Fe16N2, classified as a semi-hard magnet, is a promising candidate to fill the gap between ferrites and the Nd-Fe-B due to its giant magnetic saturation and relatively high magnetocrystalline anisotropy thus the expected high magnetic energy. Powder in flake form was prepared by surfactant-assisted ball milling of irregularly shaped Fe powder. As synthesized flakes were subjected to oxidation followed by reduction back to Fe to increase nitrogen uptake by having a porous morphology. Subsequently, the reduced powder was subjected to nitrogenization between 150 and 180 C up to 12 h. The resultant powder is transformed to 97% Fe16N2 with room temperature coercivity of up to 1050 Oe. Fe16N2 permanent magnets were then developed utilizing cold compaction at a pressure of 1 GPa. Further compaction by annealing at a temperature below 200 °C and under 2 GPa enabled consolidation of a 4 mm cylinder with ca 80% density while preserving the Fe16N2 phase.