Preparation and Characterization of Phenol Formaldehyde Bonded Nd–Fe–B Magnets With High Strength and Heat Resistance

The phenol formaldehyde (PF), a kind of heat-resistant binder, was used to prepare bonded Nd-Fe-B magnets via mold pressing. The magnetic properties, compressive strength, and microstructure of the magnets were systematically investigated. The magnetic measurements show that PF binder endows Nd-Fe-B magnets excellent magnetic properties. Moreover, the magnets can keep the original shape and exhibit usable magnetic properties at 200 °C. With increasing binder content, the compressive strength of the magnets reach 253.6 MPa with 4 wt.% PF binder, which is 14 times higher than that of epoxy resin bonded Nd-Fe-B magnets. In addition, the reason of high compressive strength was investigated. The main chain of PF binder molecule contains rigid structure like benzene rings, which can form hard cross-linking network structure in the gaps of magnetic powders making the bonded magnets with high strength. Further fracture surface morphology observation and energy dispersive X-ray analysis reveal that liquid PF binder can permeate into gaps between the magnetic powders during the process of warm-molding and exert excellent bonding effect. Two mechanisms of action can endow PF/Nd-Fe-B magnets excellent mechanical strength and heat resistance.