Scalable aerosol routes to enhance coercivity in α′′-Fe16N2 powders through α-Fe2O3 microstructure control

Abstract High-purity (≥ 90 wt%, up to ~ 95 wt%) and high-coercivity (up to 1.86 kOe) α′′-Fe 16 N 2 microparticles were synthesized by modifying the initial microstructure of α-Fe 2 O 3 precursors using aerosol processing. Three distinct α-Fe 2 O 3 powders were prepared through spray pyrolysis of Fe(NO 3 ) 3 (SP-nitrate) and spray drying, followed by calcination of Fe(NO 3 ) 3 (SD-nitrate) or Fe 2 (C 2 O 4 ) 3 (SD-oxalate). Following identical H 2 reduction and NH 3 nitridation, the resulting powders exhibited high α′′-Fe 16 N 2 phase fractions (90 to 95 wt%). Despite diverse oxide origins, the final nitride crystallite size converged to ~ 17–18 nm for all samples, while coercivity varied systematically: 1.14 kOe for SP-nitrate, 1.46 kOe for SD-nitrate, and 1.86 kOe for SD-oxalate. These differences align with the precursor-derived microstructure rather than modest variations in phase purity or crystallite size effects. The SD-oxalate route yielded the most intricate and porous particle morphology. Subsequent reduction and nitridation increased this porosity, generating numerous pore walls, edges, and rough surfaces that could serve as domain-wall pinning sites, consistent with the highest coercivity. These findings suggest that adjusting the aerosol route and precursor chemistry to regulate the precursor microstructure offers a practical and scalable approach to enhance the coercivity of rare-earth-free α′′-Fe 16 N 2 .