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

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.

Additive Manufacturing of Isotropic NdFeB PPS Bonded Permanent Magnets.

Extrusion based additive manufacturing of polymer composite magnets can increase the solid loading volume fraction with greater mechanical force through the printing nozzle as compared to traditional injection molding process. About 63 vol% of isotropic NdFeB magnet powders were compounded with 37 vol% of polyphenylene sulfide and bonded permanent magnets were fabricated while using Big Area Additive Manufacturing without any degradation in magnetic properties. The polyphenylene sulfide bonded magnets have a tensile stress of 20 MPa, almost double than that of nylon bonded permanent magnets. Additively manufactured and surface-protective-resin coated bonded magnets meet the industrial stability criterion of up to 175 °C with a flux-loss of 2.35% over 1000 h. They also exhibit better corrosion resistance behavior when exposed to acidic (pH = 1.35) solution for 24 h and also annealed at 80 °C over 100 h (at 95% relative humidity) over without coated magnets. Thus, polyphenylene sulfide bonded, additively manufactured, protective resin coated bonded permanent magnets provide better thermal, mechanical, and magnetic properties.

Development of Mischmetal-Fe-Co-B Permanent Magnet Alloys via High-Throughput Methods.

Additive manufacturing synthesis using laser engineered net shaping (LENS) is utilized to rapidly print libraries of mischmetal (MM = La, Ce, Nd, and Pr) containing R2TM14B alloys (R = MM + separated Nd and TM = Fe and Co) enabling robust evaluation of physical properties over a wide composition range. High-throughput characterization of the magnetic and thermal properties are used to identify compositions for potential high-temperature, high-performance permanent magnets with reduced critical rare-earth elements. Improved Curie temperature (Tc ∼ 450 °C) is obtained with substitution of Fe by Co in pseudoternary R2TM14B alloys. Furthermore, a 4-fold decrease in the Nd content can be achieved through substitution with less critical Ce- and La-rich MM, while retaining high Tc. Guided by the properties of the LENS printed samples, selected compositions with and without TiC additions are synthesized via melt-spinning techniques to produce nanostructured ribbons. The maximum room temperature coercivity (Hc) and energy product ((BH)max) without TiC are found to be 5.8 kOe, 8.5 MGOe, respectively, while TiC additions as a grain refiner gave Hc and (BH)max of 4.9 kOe, 9.8 MGOe, respectively. Structural characterization of the melt-spun ribbons shows homogeneous grain refinement with TiC additions, which leads to an increase in the energy product.

Recycling of additively printed rare-earth bonded magnets.

In this work, we describe an efficient and environmentally benign method of recycling of additive printed Nd-Fe-B polymer bonded magnets. Rapid pulverization of bonded magnets into composite powder containing Nd-Fe-B particles and polymer binder was achieved by milling at cryogenic temperatures. The recycled bonded magnets fabricated by warm compaction of ground cryomilled coarse composite powders and nylon particles showed improved magnetic properties and density. Remanent magnetization and saturation magnetization increased by 4% and 6.5% respectively, due to enhanced density while coercivity and energy product were retained from the original additive printed bonded magnets. This study presents a facile method that enables the direct reuse of end-of-life bonded magnets for remaking new bonded magnets. In addition to magnetic properties, mechanical properties comparable to commercial products have been achieved. This research advances efforts to ensure sustainability in critical materials by forming close loop supply chain.

Synthesis and characterization of FeCo nanowires with high coercivity.

Ferromagnetic FeCo nanocrystals with high coercivity have been synthesized using a reductive decomposition method. The sizes and shapes of the nanocrystals were found to be dependent on reaction parameters such as the surfactant ratio, the precursor concentration and the heating rate. Synthesized nanocrystals have a body-centered cubic crystal structure for both particles and nanowires and the (110) crystalline direction is along the long axis of the nanowires. The coercivity and magnetization of the FeCo nanocrystals are found to be dependent on morphology. Nanowires of Fe60Co40 with saturation magnetization of 92 emu g(-1) and coercive force of 1.2 kOe have been obtained in this study.

High energy product developed from cobalt nanowires.

Cobalt nanowires with high aspect ratio have been synthesized via a solvothermal chemical process. Based on the shape anisotropy and orientation of the nanowire assemblies, a record high room-temperature coercivity of 10.6 kOe has been measured in Co nanowires with a diameter of about 15 nm and a mean length of 200 nm. As a result, energy product of the wires reaches 44 MGOe. It is discovered that the morphology uniformity of the nanowires is the key to achieving the high coercivity and high energy density. Nanowires of this type are ideal building blocks for future bonded, consolidated and thin film magnets with high energy density and high thermal stability.