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

Abstract

Twenty kinds of oxides (Al 2O 3, CaO, Co 2O 3, Cr 2O 3, CuO, Fe 2O 3, GeO 2, MgO, MnO 2, MoO 3, Nd 2O 3, P 2O 5, PbO, SiO 2, SnO 2, Ta 2O 3, TiO 2, V 2O 5, ZnO and ZrO 2) were added into the intergranular regions of Nd 22Fe 71B 7 magnets and their effects on the coercivity, thermal stability and intergranular microstructure were investigated. It was found that (1) the coercivity is considerably enhanced by additions of MgO, Al 2O 3, Cr 2O 3, ZnO and CaO, but greatly decreased by ZrO 2, TiO 2, P 2O 5 and CuO; (2) the open circuit flux loss from room temperature to 200°C is decreased by additions of MgO and ZnO, slightly increased by CaO, Cr 2O 3 and Al 2O 3, and greatly increased by MnO 2, ZrO 2, TiO 2 and P 2O 5. Microstructural studies reveal that a new intergranular NdOFeM phase with composition close to 70Nd-22O-5Fe-(1-3M) (M = Mg, Zn, Ca, Mn, Zr, etc.) appears in the oxide-doped magnets. The variations in the magnetic properties 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.