Effects of intergranular additions of oxides on the coercivity, thermal stability and microstructure of Nd-Fe-B magnets

Twenty kinds of oxides (Al2O3, CaO, Co2O3, Cr2O3, CuO, Fe2O3, GeO2, MgO, MnO2, MoO3, Nd2O3, P2O5, PbO, SiO2, SnO2, Ta2O3, TiO2, V2O5, ZnO and ZrO2) were added into the intergranular regions of Nd22Fe71B7 magnets and their effects on the coercivity, thermal stability and interganular microstructure were investigated. It was found that (1) the coercivity is considerably enhanced by additions of MgO, Al2O3, Cr2O3, ZnO and CaO, but greatly decreased by ZrO2, TiO2, P2O5 and CuO; (2) the open circuit flux loss from room temperature to 200 degrees C is decreased by additions of MgO and ZnO, slightly increased by CaO, Cr2O3 and Al2O3, and greatly increased by MnO2, ZrO2, TiO2 and P2O5. Microstructural studies reveal that a new intergranular Nd-O-Fe-M phase with composition close to 70Nd-220-5Fe-(1-3M) (M = Mg, Zn, Ca, Mn, Zr, etc.) appears in the oxide-doped magnets. The variations in the magnetic proper-ties arise from the occurrence of this Nd-O-Fe-M phase. By comparing the effects of the oxide additives with those of the element additives, it was also found that a small amount of oxygen in the intergranular regions is helpful in improving the coercivity and the thermal stability of Nd-Fe-B magnets.