Packing bimodal magnetic particles to fabricate highly dense anisotropic rare earth bonded permanent magnets

Abstract

Highly dense and magnetically anisotropic rare earth bonded magnets have been fabricated via packing bimodal magnetic particles using a batch extrusion process followed by compression molding technology. The bimodal feedstock was a 96 wt% magnet powder mixture, with 40% being anisotropic Sm-Fe-N (3 μm) and 60% being anisotropic Nd-Fe-B (100 μm) as fine and coarse particles, respectively; these were blended with a 4 wt% polyphenylene sulfide (PPS) polymer binder to fabricate the bonded magnets. The hybrid bonded magnet with an 81 vol% magnet loading yielded a density of 6.15 g cm⁻³ and a maximum energy product (BH)_m of 20.0 MGOe at 300 K. Scanning electron microscopy (SEM) indicated that the fine-sized Sm-Fe-N particles filled the gap between the large Nd-Fe-B particles. Rietveld analysis of the X-ray diffraction data showed that the relative contents of the Nd₂Fe₁₄B and Sm₂Fe₁₇N₃ phases were 61% and 39%, respectively, in the hybrid bonded magnet. The PPS binder coated most of the magnetic particles homogeneously. Compared with the magnetic properties of the initial Nd-Fe-B and Sm-Fe-N powders, the reduction in the remanence, from the demagnetization curve, is ascribed to the dilution effect of the binder, the non-perfect alignment, and the internal magnetic stray field.