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As an organic binder for bonded Nd-Fe-B magnets, epoxy resin (EP) has poor heat resistance but good moisture resistance, while sodium silicate (SS) has poor moisture absorption but better heat resistance and corrosion resistance. In order to improve high temperature stability and decrease moisture absorption of bonded Nd-Fe-B magnets, EP/SS composites were applied as the binder to prepare bonded Nd-Fe-B magnets. The magnetic properties, moisture absorption, corrosion resistance, compressive strength and microstructure of composite bonded magnets were investigated. The results show that EP/SS bonded magnets can obtain excellent magnetic properties at room temperature, and even useable magnetic properties a thigh temperature environments at 200 °C. EP/SS composite binder effectively improves heat resistance and corrosion resistance of bonded Nd-Fe-B magnets, and reduces the hygroscopic properties. The molecule of sodium silicateis rigid and keeps it original shape at high temperature environments. In addition, SS in composite binder improves the mobility of the magnetic powders during the pre-pressing process, which makes the magnetic powders attain a more regular structure. These two factors will increase the mechanical properties. Moreover, sodium silicate in the composite binder can also cover the surfaces protecting the magnetic powders from oxidation and corrosion. EP in composite binder can cover SS surface to reduce the water absorption of SS as epoxy is a hydrophobic material. The EDX analysis shows that the composite binder has accumulated in the gaps of the magnet powders, which not only improves heat resistance and corrosion resistance, but also increases the mechanical properties. Therefore, EP/SS composite binder endows bonded Nd-Fe-B magnets excellent comprehensive properties.

Bonded neodymium iron boron (Nd-Fe-B) magnets with excellent properties were prepared by using waterborne-epoxy resin/sodium silicate composite binder (W-EP/SS). The results show that this composite binder can endow the bonded magnets with high temperature resistance as well as strong corrosion resistance. In addition, the compressive strength of W-EP/SS bonded magnet is higher than other types of bonded magnets. W-EP/SS composite binder has excellent homogeneity, which can coat the magnetic powders uniformly during mixing and curing, and therefore forms a protective thin film on the surface of the magnet to prevent thermal oxidation under high temperature conditions as well as corrosion during chemical treatments. Moreover, the W-EP/SS composite binder, which combines the toughness of epoxy resin with the rigidity of sodium silicate, endows bonded magnets better mechanical properties. It is attributed to the co-existence of Si-O-Si and C-O-C network structure in the W-EP/SS. Therefore, it is expected that W-EP/SS composite binder will provide a broader application prospects for future bonded magnets.

In present study, sodium silicate, a kind of heat-resistant binder, was used to prepare bonded Nd-Fe-B magnets with improved thermal stability and mechanical strength. Effect of curing temperature and curing time of the new binder to the magnetic properties, microstructure, and mechanical strength of the magnets was systematically investigated. Fracture surface morphology observation show that sodium silicate in bonded magnets could completely be cured at 175 °C for 40 min, and the magnets prepared under this condition exhibit optimal properties. They exhibit usable magnetic properties of B r of 4.66 kGs, H cj of 4.84 kOe, and (BH) max of 4.06 MGOe at 200 °C. Moreover, the magnets possess high compressive strength of 63 MPa.