Abstract
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.
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